KORBA SIMULATOR
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KORBA SIMULATOR
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FOREWORD
Power is the single most important necessity for common people and industrial development of
the nation. Electricity can bring a sea change in the quality of life style. Gamut of operating the
power plant specially large thermal units having very sophisticated technology and complex
control, need to be managed and experience shared by the trained and developed human
resources.
The Simulators are computer based training tool that are modeled mathematically to provide
practical on-job training at real time environment, improve retentivity levels to more than 75%,
tuned considerably for high confidence level, over and above the training is completely risk free.
The Simulators have been developed to function as the replica of power plant (200MW of Korba
unit) and they give the feeling of operating real power plant in a clean and pleasing environment
without making use of the auxiliary equipments. NTPC firmly believes that the engineers and
the officials operating or having intensions to manage the power plant should be trained
regularly through Simulators having features as above.
The operation manuals provide the adequate reference information to augment systematic
hands-on training. The operational manuals of 200MW Simulator in two volumes (Vol-I & Vol-II)
should prove valuable to all the participants of the Simulator Institute, be they the fresh
executives under the Executive Trainee schemes of the Company or the experienced power plant
engineers particularly operating the power stations or working in the power projects under
construction and commissioning phase. It will provide a direct appreciation of basics of thermal
power plant operations and encourage them to take on such responsibilities far more sincerely
and effectively. The manuals in your hand have been revised suitably based on the feedbacks
received from various participants who have undergone training in our Simulator Institute.
The revised volume I & II bring together the information from manuals of original equipment
manufacturers, theory and course materials & texts from Instruments and Control
suppliers/manufactures, efficiency related power plant literatures, water treatment and
chemical plants etc.
I appreciate the time spent in making the manuals and the exhaustive efforts in bringing these
out within the shortest time by the Simulator In-charge, Senior Managers, Faculty Members and
the office staff of the Central Simulator Training Institute, KORBA.
I hope the readers of the operational manuals will find the contents stimulating and helpful in
understanding and managing thermal power plants especially in the operation activities.
I believe that in spite of all sincere efforts and care, some areas of improvement might have
remained. The suggestions and comments are welcome.
General Manager
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Few words from HOD Simulator & EDC
It has been our endeavour to ensure that the persons responsible for operation of
Power Plant should have accessibility to all technical information as presently
only running the machine is not enough, but running it efficiently and
economically will always give an edge over other power utilities.
One of the thrust areas of NTPC management has been skill up gradation and
imparting every possible knowledge to concerned employees, which will take
the organization into brighter tomorrow.
Training gets the utmost priority in our organization. With a view to make the
learning easy and spontaneous, need was felt to consolidate the 200MW Power
Plant Operation manual in the soft form.
At present, Computer and E-Communication have become our essentialities and
we have adapted ourselves to it.
As a first step towards making our operation engineers equipped with knowledge
of operational aspect of power plant and using, it as a knowledge-refreshing tool
and making the information available in a CD was thought of.
I am pleased to present this CD, the operational manuals of 200MW Simulator in
two volumes (Volume I & II), to the personnel who are associated with power
plant activities.
I sincerely hope that the contents are going to be helpful in upgrading the
knowledge and skill of Power Plant employees.
Dy. General Manager
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CONTENTS
CHAPTER NO.
TOPIC
1.
FEATURES OF THE SIMULATOR
2.
DATA ACQUISITION SYSTEM (DAS)
3.
CONDENSATE AND FEED WATER SYSTEM
4.
5.
6.
7-13
15-25
CONDENSATE EXTRACTION PUMP
29-31
BOILER FEED PUMP
33-40
BOILER SYSTEM
BOILER: GENERAL
43-54
AIR PRE-HEATER
55-57
ID FAN
59-62
FD FAN
63-64
PA FAN
65-66
PULVERISER
67-69
HP AND LP BYPASS SYSTEM
HP BYPASS
73-78
LP BYPASS
79-86
TURBINE SYSTEM
STEAM TURBINE: GENERAL
7.
PAGE NO
89-100
TURBINE GOVERNING SYSTEM
101-138
TURBINE PROTECTION
139-152
AUTOMATIC TURBINE TEST
153-164
TURBINE STRESS EVALUATOR
165-178
GENERATOR SYSTEM
GENERATOR AND AUXILIARIES
181-194
STATIC EXCITATION
195-205
GENERATOR PROTECTION
207-228
8.
MEASUREMENT AND CONTROL
229-338
9.
BOILER WATER CHEMISTRY
339-350
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FEATURES OF THE SIMULATOR
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FEATURES OF THE SIMULATOR
Studies on technical feasibility and economical viability of fossil power plants have
led to the construction of gigantic sized units equipped with sophisticated control
schemes. Increased protection and safety for reliable and efficient operation of these
units at the same time ensuring the maximum availability has thrust tremendous
responsibility on the operating personnel. The operator is the key to efficiency and
safety. As our power plant operation becomes more sophisticated with complex
controls, the problem of running a plant profitably and safely becomes critical.
Conventional on the job training hitherto imparted has become impracticable and
inefficient to develop the skills of a reliable, confident and efficient operator
demanded by these modern units. Training simulator plays a vital role by not only
training the operator but also by tuning his reflexes in a real time environment.
BENEFITS OF TRAINING SIMULATOR
Power plant simulator is an effective training tool, with which the actual
characteristics of a power plant can be generated through real time execution of
mathematical models of various systems on a computer. The trainee operator quickly
gains experience in normal, abnormal and emergency operation of power plant
through Simulator Training. Operator confidence is increased, resulting in improved
efficiency of power plant operating personnel, better equipped to respond to problems
and emergencies. The hands-on training in a highly realistic environment provided
by the training Simulator cannot be substituted by any other form of training. A welltrained operator runs a plant safely and expensive downtime caused by operator
error is significantly reduced. In a highly automated plant, refresher training on
Simulator also helps experienced operators to maintain a high level of proficiency.
THE PROCESS OF SIMULATION
The Simulator creates a realistic representation of any process in an interacting
manner. All the engineering systems of power plant are programmed in a computer
in the form of mathematical models. The main hardware elements of the simulator
system consist of two Main Computers, Shared Memory, Input-Output System and
the UCB panel.
The computer used for the process simulation is called the Master computer. It is
used for computation of various simulation parameters. All the processes and
interlocks of the unit are defined by the math models and are iterated by the
computer. The inputs from the UCB panel are scanned at a very fast rate and
transmitted to the Master computer by the input-output system through a highspeed data link. The computer in accordance with the math models calculates the
output parameters and the effects are displayed on the panel in the form of lampoutputs, annunciations, meter/recorder indications etc and updated dynamically.
Simulation is based on predicted plant design data. It displays the parameters and
provides the necessary alarms or protective system action when plant limits are
approached or exceeded. Emergency conditions can be inserted at any time during
an exercise or prior to the start of the exercise. The simulator responds dynamically
to all changes in the process from within or imposed from outside.
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Simulator design includes equipments, instrumentation and controls. This enables
an operator to function in all modes of the specified coal fired power plant operation
including normal, transient and emergency operating conditions except where
specifically noted. Responses resulting from operator actions, automatic plant
controls and inherent operating characteristics are copied realistically so that the
operator cannot observe any difference (within limits of performance criteria) between
the simulator control room indicators and those of the actual power station.
The computer used for DAS is called the Slave computer. The data needed for
computation of DAS parameters is stored in a common memory called shared
memory. The DAS computer has access to this memory and uses this data for
calculation of various parameters. All the important parameters are displayed and
updated dynamically on various CRTs by means of CRT controllers. This computer
also executes several other programs, which provide various facilities like Mimic
diagrams, bar charts, group displays of DAS points, video trends etc to the trainee
operator.
The simulation software is structured and organised in well-defined modules and
levels. The various modules are: Computer system software, Simulation executive
software and Application software. The system software consists of the operating
system (MPX-32, Rev 1.5C) and utilities like text editors, file manager, debugger etc.
System level services like management of computer memory, processor time etc is
provided by the operating system. The simulation executive software controls the rate
of execution of math models and helps in debugging process by tracking the
execution sequence. The Application software is further consists of plant simulation
software, Instructor station software and DAS software. The Plant simulation
software consists of various math-models and subroutines, which are written using
FORTRAN-77 and Assembly. The instructor station software enables the operation of
instruction station through which simulation is initialised and various facilities of
the same become available. The DAS software enables the functioning of its various
facilities and features.
200MW SIMULATOR: COMPUTER SYSTEM
Computer
32 bit, GOULD SEL - 32/77
Supplier
GOULD /ENCORE COMPUTER CORP., USA.
Software
MPX - 32 Rev. 1.5 C
Supplier
GOULD / ENCORE COMPUTER CORP., USA
KORBA SIMULATOR
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HARDWARE FEATURES OF THE SIMULATOR
Full size Replica Control Room
The control panel exactly resembles that of the actual plant. (200MW: unit-I of
Korba).
All the Switches, Push Buttons, Indicating Lamps and Instruments,
Recorders, Annunciations are located precisely at the same position on the simulator
control panel as in the real plant.
Computer Complex
The heart of the Simulator is the Computer and its associated software. Two 32-bit
Computers (GOULD SEL-32/77) are the driving force behind the 200MW Simulator.
One Computer is used for the simulation of plant system and the other is for Data
Acquisition purpose.
Instructor Station
This is the place from where the instructor is able to control the training process. He
can create a number of plant conditions, inject malfunctions, monitor and analyze
the trainee’s performance. Several functions are available to the instructor by which
he can utilise the training potential to the Simulator to a maximum.
Computer Interface
This consists of an Input / Output System by means of which data can be
transferred from the Computer to the control Panel and vice versa at extremely high
speeds.
SCOPE OF SIMULATION
Following are the systems covered in simulation:
• Condensate and Feed Water System
• Air and Flue Gas System
• Fuel System (Oil and Coal)
• Furnace Safeguard and Supervisory System (FSSS)
• Steam Generator System
• Turbine System
• Automatic Turbine Run-up System (ARTS)
• Cooling Water System
• Electrical Unit Distribution System
• Hydrogen and Seal Oil System
• Analog Control System (ACS)
• Main Generator and Auxiliaries System
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SIMULATOR OPERATIONAL FACILITIES
From a remote console, the instructor can implement following training features:
•
Initial condition
•
Snapshot
•
Remote operator functions
•
Freeze/Run
•
Malfunction activation and removal
•
Programmable Response Time (Real time, Slow time and Fast time)
•
Backtrack
•
Record/Replay
•
Remote control
•
Computer Assisted Exercise (CAE)
•
Trainee Proficiency Review (TPR)
Initial Condition/Snapshot
It is a programmed status of the plant from where simulation is to start. There are
fifteen initial conditions, which can be chosen for starting simulation. The feature of
snapshot allows storing the plant status at a given instant during simulator
operation for later use as initial condition.
Remote Operator Functions
The instructor serves as an auxiliary operator in providing the operation of manual
valves etc. located outside the main control room and other controls not provided on
UCB panel.
Freeze/Run
This feature allows all dynamic actions to be suspended during the simulation status
remaining intact. This gives the instructor time to discuss the frozen simulated plant
condition.
Malfunction Activation and Removal
Malfunctions simulate fault conditions, which can occur within the plant. Instructor
introduces them in the process from the console or hand-held remote transmitter.
Programmable Response Time
Normally simulator runs in one to one correspondence with real time. Instructor can
select either slow time or fast time. Real time expansion is slow time; simulation
provides an apparent increase in time for fast changing phases of plant operation
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such as feed water process, unit trip sequences, characterised by short time
constants. When activated all math models are called at one tenth their normal rate
causing all apparent operations to slow to one tenth real time rate.
Real time compression is fast time simulation providing an apparent decrease of time
intervals for less dynamic phases of plant operation such as turbine warm up which
usually takes long intervals. Some of the models run at ten times the normal rate
under this condition.
Backtrack
In automatic snapshot of the simulator, plant status condition will occur at a time
interval of one minute for a period of 60 minutes. This feature allows the instructor
to select any one of these past 60 selected set of conditions and initialise the
simulator at that specific backtrack time. This feature is known as backtrack.
Record/Replay
This feature permits the status of the control panel displays and indications to be
saved during operation for future replaying by instructor. Up to one hour of
simulator can be saved.
Computer Assisted Exercise (CAE)
CAE permits the instructor to develop and store training scenarios, including
malfunctions and remote functions, for uniformity of performance testing.
Trainee Proficiency Review (TPR)
TPR permits automatic monitoring of instructor selected parameters for their
deviations during operation, above or below the selected/set limits of safety and
efficiency and also the time for which the parameter remained out of contact can also
be computed and recorded.
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DATA ACQUISITION SYSTEM (DAS)
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DATA ACQUISITION SYSTEM (DAS)
Data Acquisition System (DAS) in the case of 200 MW units is for monitoring of
process data. DAS system is not used here for process control. This means that the
person on desk can get readily available information about the different process
parameters on different display devices as well as on the printers but he cannot use
the DAS system for the control of any parameter.
PROCESS INPUTS
Process inputs to the DAS system are fundamentally of two types: Analog Inputs and
Digital Inputs.
Analog Inputs
The types of analog inputs are as follows:
• 0-10 volt analog inputs.
• 4-20 mA inputs.
• Thermocouple inputs.
• RTD inputs.
• All other inputs of analog nature as may vary from plant to plant.
Digital Inputs
Digital inputs have only two states. Any input with only two states namely
OPEN/CLOSE, ON/OFF, TRIP/ NORMAL, HIGH/LOW etc falls in this category.
ANALOG INPUTS
The number of analog inputs in the case of Simulator is of the order of 650. These
are the direct inputs coming from the process. In addition to these process inputs,
there are some more inputs of analog type, which are not directly coming from the
process but are derived from the inputs coming from the process. These inputs are
called Calculated Inputs.
Miscellaneous Calculation Inputs
Calculated inputs are of two types. One type of calculation is mostly averaging or
differentials. Say for instance, we are measuring the casing temperature for BFP and
these points are directly coming to the DAS. We can calculate the difference between
the upper and lowercasing temperature to get the casing differential temperature.
Also we can add the coal flow of all the mills per hour and get the total coal flow per
hour. We can also add the economiser outlet FW temperature left and right and
divide by two to get the average economiser outlet FW temperature.
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DATA ACQUISITION SYSTEM (DAS): BLOCK DIAGRAM
Thus we can derive the following three derived inputs which can be treated as DAS
inputs but which are not directly coming from the process. These are:
1.
BFP casing differential temp.
2.
Tons of coal fired/hour.
3.
Economiser outlet FW temp.
This type of calculation is called Miscellaneous Calculation.
Performance Calculation Inputs
This consists calculation Inputs
•
Terminal temperature difference of different heaters.
•
Excess air percentage.
•
Turbine efficiency.
•
Boiler efficiency by different methods.
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•
Cycle efficiency.
•
Heat rate deviation from standard.
The total number of calculated input in the case of analog point is approximately
200. This includes both miscellaneous calculation and performance calculation.
ANALOG SCANNING, ALARMING
The analog inputs coming from the process are scanned by the computer at various
rates depending upon the criticality of the parameter and the computer capability.
The different rates of scanning of analog inputs are 1 sec., 2 sec, 12 sec, 30 sec and
60 sec corresponding to each analog input, there are two limits, one high and the
other low. It is of course not necessary that each and every analog input will have
both high as well as low limits. There are some inputs, which have only high limits
and not low limits and vice versa. If a high or low limit is kept at a defined value then
if the analog input varies very near to that limit and oscillate, this will cause the
alarm to appear, on different display devices at one time and vanish at the next time.
To avoid this, each point having alarm associated with it is provided with a dead
band. The alarm appears whenever the value goes beyond high or low limit but the
alarm stays so long as the value does not come below the dead band value.
All analog points which are having alarms have three types of alarms, both for high
as well as low. They are 1 HI, 2 HI, 3 HI or 1 LO, 2LO, 3LO. The alarm described
above is 1HI or 1LO, 2HI or 2LO. Alarm comes when the value increases beyond 1HI
or 1LI.
The value between 2 HI & 1 HI or 1 LO & 2 LO is called repeat increment. The 3HI or
3LO alarm comes when the value deviated further from the normal value. Beyond
this (3HI or 3LO) there is a digital status, which will cause tripping of the particular
device if of course, tripping is provided for that device.
Variable Limit Alarms
However, there may be analog point whose alarm value is dependent on the load.
That means the alarm value will change depending upon the MW generated. For that
we have variable limit of alarms. This load dependent variable limit, calculations are
also part of miscellaneous calculations, which has already been discussed.
Alarm Cutouts
Let us consider a case where BFP-C is not running & BFP-C flow is 0. This is a
condition where BFP flow low alarm will appear, even though this is not an alarm.
So some means are required to avoid these alarms. For that, there are cut out
equations, which will see the digital status of the equipment before displaying the
alarm. The alarm will be displayed only when that equipment is running. This is
more important for mills, feeders, BFPs, etc.
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PERFORMANCE CALCULATION
The performance calculation points are treated in a different way than miscellaneous
calculation points. The miscellaneous calculation points are scanned all the time at
the same rate as the normal process points. For performance calculation, past ten
minutes value of the points that are used for the performance calculation is stored in
the memory. It is not required to store each and every scanned value of past ten
minutes but a number of values. Whenever the operator asks for the log of
performance calculation points, these values are averaged and this averaged value is
used for the calculation.
For example, if we want the boiler efficiency, then all the input scanned points
required for this calculation are averaged for last ten minutes and this averaged
value is then used for the calculation. To elaborate, take the example of boiler
efficiency measurement by input, output method.
The inputs for this calculation are:
1. FW Flow.
2. Reheat spray flow.
3. Cold reheat flow.
4. Tons of coal fired per hour.
5. Heavy oil supply flow.
6. Heavy oil return flow
Except point 4, all in the above are process points. Point 4 is a miscellaneous
calculation point is obtained by adding the coal flow through each feeder.
The scanned values of the above points are stored and whenever the calculation is
demanded, first the averaging of past ten minutes is done for all the above points.
In addition to the above, we need the following information:
Blow down flow.
Heating value of coal.
Heating value of fuel oil.
Heat added other than chemical.
SH outlet enthalpy (Function of SH outlet temp. and pressure).
FW enthalpy at economiser inlet (function of FW temp. and pressure).
RH outlet enthalpy (function of RH outlet steam temp. and pressure)
HPT exhaust enthalpy (function of RH outlet steam temp. and pressure)
Blow down enthalpy (function of drum pressure).
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The first four are the constant point, which are entered by the engineer and remains
constant unless changed and the computer does enthalpy calculation from the steam
table entered.
However, for performance calculation points, it should be noted that all the above
calculations, which are done on the basis of performance test code, are valid only
from 30% to 100% load. So the calculation is not done below 30% load.
DIGITAL INPUTS
The number of digital inputs is of the order of 1300. These are the process inputs
coming directly from various equipments like pipes, ducts etc. Since the digital
points are having two states, the scanning of these points are much faster than the
analog points, All digital points under normal circumstances are scanned at every
one see. Whereas number of analog points scanned per sec. is of the order of 110. Of
the process points there are some two hundred inputs, which are high-resolution
digital inputs. These inputs are different from other inputs in that if anyone of the
above inputs goes to the alarm states then all high-resolution digital inputs are
scanned at a much higher rate of 5-milli sec. This scanning goes on until there is no
change in status of the high-resolution digital inputs for a period of two minutes.
After that it gives a sequence of events recording on printer. In the digital, it is not
necessary that all inputs will have an alarm associated with it. For instance, a BFP
off is not an alarm state. In general, the set point for digital is set at a value slightly
higher than the corresponding analog value so that the engineer comes to know
about alarm in advance and takes necessary action.
The different type states in the case of digital points are HI-NORMAL, LOW-NORMAL,
HIHI-NOT HIHI, LOLO-NOT LOLO, TRIP-NORMAL, ON-OFF.
GENERAL DESCRIPTION OF SIMULATOR DAS
Any DAS point in the case of NTPC 200 MW unit consists of five characters, two
alphabets followed by three numerics. The example is:
SF005, ET005, FT003,
TV501
The first two characters represent the system. In the above, S represents the steam
generator point, E represents electrical point, F represents feed water point and T
represents turbine point. The second letter represents the parameter measured. In
the above case (SF005), F represents vibration. The next three numeric characters
represent whether it is an analog point or a digital point. In NTPC philosophy, any
points from 001 to 449 are analog points and 500 to 999 are digital points. Thus in
the above SF005 and FL003 are analog points where as ET501 are digital points.
Any points whether analog or digital; if the first two characters are KC or KV then
they are calculated points. If the first two characters are KC, then they are
miscellaneous calculation points. If it is KV, it is performance calculation point,
which is only analog point. All the points, which start with KN, are constant points,
which are used for performance calculation. The AV points are averaged values of the
process points, which are used in the performance calculation.
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CAPABILITIES OF THE SIMULATOR DAS SYSTEM
The Simulator DAS system is having the following capabilities:
a.
Point detail of any point digital or analog.
b.
Review of analog or digital points.
c.
Group display of any point analog/digital
d.
Turbine message display
e.
Display graphics of various systems
f.
Acknowledge of alarm
In the DAS system, for the output purpose, there are four colour CRT'
s and two
printers. The CRT on the unit controller'
s desk is called operator console-1 or in
short OPCON 1. The one on the UCB section 2 i.e. on the middle of the UCB panel is
called OPCON 2 and the one on UCB section 1 of is called the UTILITY CRT. There is
a fourth CRT in UCB section 3, called as ALARM CRT. There are two keyboards. One
is on the unit controller'
s desk and the other with the OPCON 2 CRT i.e. section 2.
The alarm CRT is dedicated to alarms. The analog points, which cross its normal
limits or the digital points, which are having an alarm, will blink as long as alarm is
not acknowledged. The new alarm will come on the top of the first page. In total there
are eight pages of alarms. There are previous page and next page buttons through
which we can go to any page of alarm. But if while going to previous page or next
page, a new alarm comes, it reverts back to the first page. New alarm appears on the
top of the first page pushing the previous alarms down the page. The number of
alarms in each page is 20. But if all eight pages are full with alarms and new alarm
comes, the last alarm in the last page vanishes creating space for the subsequent
alarm to come down so that the latest alarm appears on the top of the first page.
When the alarm acknowledge button on any of the two keyboards is pressed, then all
alarms, which are flashing, get acknowledged and the flashing stops. If an alarm
returns normal, then its colour changes to green. When the acknowledge button is
pressed, the alarms which returned to normal value vanishes. The alarm compress
button on the keyboard can be used to compress the empty space.
It may be noted that other than compressing, acknowledging, next page and previous
page, there is no other control of the alarm CRT. That means this CRT is dedicated
for alarms only and no other display is possible on this CRT. The other CRT'
s can be
used for any display. With the latest DAS software modifications, alarm CRT can be
used for display. However, if it is a communication between operator & CRT, that
communication is possible only through OPCON 1 and OPCON 2 and not through
any other CRT.
GRAPHICS
The system can display the P&ID diagram in the form of graphics with dynamic
capabilities. Dynamic capabilities mean, the value and the status of the graphic
displayed are updated continuously and it is not necessary to call the graphic every
time to get the latest information. In the graphics arrangement, status of various
equipments, pipe ducts, valves, dampers etc are shown by different colour. For
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example colour of the ducts are yellow when there is flow of air through it and it is
half intensity white when there is no flow. The colour of a PA Fan is green when off,
red when running and white when tripped. Thus, just by seeing the colour, the
status of equipment can be ascertained. A valve open is shown in red colour and a
valve closed is shown by green colour. For selecting graphics, first one will have to
select a system. There are ten systems here, namely: boiler air, boiler steam, feed
water, boiler gas, condenser, circulating water, turbine, coal mill, water steam cycle,
boiler water.
FUNCTION MENU
The functions of this menu are:
•
To update the time of the Computer
•
To bring any alarm page from alarm CRT to any other CRT.
•
Display points of different groups on CRT.
•
To display analog parameter in the form of bar chart.
•
To assign any analog parameter to the trend recorders.
•
To enable or display post trip log.
•
To display any analog parameter in the form X-Y plot.
Update Time
This is a communication format by which the operator can enter present date and
time. The operator in the ‘fill in the blanks’ format can insert the present date and
time.
Alarm Paging Supervisor
Using the function menu, we can go to this supervisor through which by fill in the
blanks format the operator can select any page of alarm CRT to appear on any other
CRT.
Log Supervisor
By this we can get a point out of a log group in the printer. A log consists of some 6
to 7 groups each having some 20 points. Example for the logs are: Boiler run-up log,
Turbine run-up log, hourly log, summary log, Turbine Generator diagnostic log,
performance log etc. Boiler start-up and turbine start-up logs are automatic in the
sense that they start automatically. When boiler is lighted up, collections for boiler
start-up log start and it gives printout after desired number of collections. The same
is the case with turbine run up, which starts when the turbine rolling starts.
Most of the logs are automatic which causes these logs to come after a specified
event or after a specified time interval. Through log supervisor, we can assign any
point to a group and also we can assign any group to any log. We can, instead of
taking the log on printer, get the display on CRT.
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Group Supervisor
There are a number of points (16), which are assigned to a group. In all we have 40
groups. From function menu through group supervisor, we display any group on the
selected CRT. This is an important function because we can display both analog and
digital parameters on the CRT, which will dynamically update. We can assign any
point to any group or a number of points to the same group.
Bar Chart Supervisor
The bar chart supervisor is to display bars in the form of horizontal or vertical, of a
group of points, which are of similar type if it is a vertical bar and or any type if it is a
horizontal bar.
We can go to bar chart supervisor through function menu. The vertical bars are
included in the graphics in the case of Simulator DAS whereas horizontal bars are
under chart supervisor. In one page there can be 8 particular variables. Under
normal conditions the bars are green, if the value exceeds beyond high or low limit
then the colour change to red and dead band is shown in yellow colour.
Trend Pen Supervisor
The recorders on the Panel are assigned to some particular analog variables. The
DAS gives a facility to assign any analog variable required to be assigned to any of
the nine pens. In all nine analog variables can be assigned to all the nine pens at a
time. The assignment may be changed as and when required by the operator through
man machine communication format.
Post Trip Supervision
Whenever a unit is started, enabling post trip log through this supervisor, will give a
print out of all points of the post trip log when the unit trips. Post trip log is having
some 100 analog points, which are important ones and which gives a clear picture of
parameters changed from 3 minutes prior to tripping to 5 minutes after tripping. All
the points, which are coming in this log, are of such type that any time 5 values of all
the variables of last 5 minutes are always available. When a trip occurs considering
that to be zeroth time values of all these variables are collected up to a time five
minute after tripping. Then the printout of post trip log starts. Thus post trip log
print out comes only after trip occurs and it gives ten values for all variable starting
from five minutes before tripping five minutes after tripping. Post trip log points are
all analog points.
X-Y Plot Supervisor
This supervisor is to display a process variable with respect to time and with respect
to total generation. Each display consists of seven points plotted with respect to time.
The plot of the variable is for past ten minutes. In each display gross generated MW
is plotted. Each plot is identified from the other by the colour because each variable
plot colour is different from other.
KORBA SIMULATOR
24
POINT REVIEW FUNCTION
Unlike the group supervisor and the log supervisor the point review gives the points
display in the alphanumeric sequences. This gives one-shot value of the points in
either CRT or printer. Here values of both analog and digital, in the same review and
in the same sequence can be obtained.
The values of all points, scan analog & digital points, calculated analog & digital
points and constant points along with the limit set for all scan and calculated analog
points can be seen in the display and in the printer.
POINT DETAIL
The point details function is to get the value information about a particular point.
The point detail of a point gives much more information, but they are not relevant to
the operator.
Step Next Point
When we are in point details then pressing step next point button causes the display
of the next point in analog/digital in alphanumeric sequence. There are some more
keys: OPCON 2, Utility CRT. These keys are for selecting the particular CRT to which
subsequent instructions are to go.
TURBINE MESSAGE DISPLAY
When turbine is rolled, pressing this button gives the criteria not satisfied in a step
with description. This is an added feature and included because the ATRS console in
KWU turbine does not give the description of the criteria in the step. The status
changes dynamically in the display as in the case of group review and this can be
displayed in any of the three CRTs.
SYSTEM MENU
This is also another means to display graphics. Instead of selecting the system by the
keys say mill or boiler air etc., we can go to system menu and select the system and
then through display list or display diagram we can display graphics. This gives a
brief idea of DAS system in general and the DAS system in the simulator. The
keyboard operation described pertains to the simulator DAS system that may vary
from system to system.
ANALOG SCANNING, ALARMING
The analog inputs coming from the process are scanned by the computer at various
rates depending upon the criticality of the parameter and the computer capability.
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25
KORBA SIMULATOR
26
CONDENSATE
AND
FEED WATER SYSTEM
KORBA SIMULATOR
27
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28
CONDENSATE EXTRACTION PUMP (CEP)
INTRODUCTION
The condensate extraction pump (CEP) is a centrifugal, vertical pump, consisting of
the pump body, the can, the distributor housing and the driver lantern. A rising
main of length depending upon NPSH available, is also provided. The pump body is
arranged vertically in the can and is attached to the distributor body with the rising
main. The rotor is guided in bearings lubricated by the fluid pumped, is suspended
from the support bearing, which is located in the bearing pedestal in the driver
lantern. The shaft exit in the driver lantern is sealed off by one packed stuffing box.
Casing
It is split on right to the shaft and consists of suction rings and 4 no. of guide vane
housing. Casing components are bolted together and sealed off from one another by
'
O' rings. For internal sealing of individual stages, the casing components are
provided with exchangeable casing wear rings in the arc of impeller necks. In each
guide vane casing, a bearing bush is installed to guide the shaft of pump.
Rotor
The pump impellers are radially fixed on the shaft by keys. The impellers are fixed in
position axially by the bearing sleeves and are attached to the shaft by means of
impeller nut. Impellers are single entry type, semi-axial and hydraulically balanced
by means of balance holes in the shroud and throttle sections at suction and
discharge side. A thrust bearing located in the motor stool absorbs residual axial
thrusts.
Bearings
In each guide vane housing the shaft is guided by a plain bearing. These bearings do
not absorb any axial forces. Pump bearings consist of bearing sleeve, rotating with
the shaft and bearing bush, mounted in guide vane housing. The intermediate shaft
is guided in bearing spider and shaft sleeve. The arrangement of bearing corresponds
to the bearings of pump shaft. They are lubricated by condensate itself. A combined
thrust and radial bearing is installed as support bearing to absorb residual thrust.
Axial load is transmitted to the distributor casing via the thrust bearing plate, the
thrust bearing and bearing housing. A radial bearing attached to the bearing is
installed in an enclosed housing and is splash lubricated by oil filled in the
enclosure. Built-in cooling coils in the bath and cooling water control oil temp.
Shaft sealing
The drive shaft passage in the distributor casing is sealed off by packed stuffing box
with lantern ring. During operation, the packed stuffing box reduces the leakage flow
in the clearance between shaft protecting sleeve and stuffing box housing. The shaftprotecting sleeve is sealed off from shaft by '
O'rings. To prevent air entry during
standstill operation or during reduced pressure operation as well as for cooling
stuffing box sealing water is fed into the stuffing box via lantern ring. Connecting line
to suction relieves the shaft-sealing chamber.
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29
CEP CONNECTIONS
Coupling
The pump shaft coupling is connected to the driver shaft by a rigid clamp coupling. A
flexible claw coupling is used to transmit the torque from driver to drive shaft.
Venting/sealing/cooling fluid lines
A vent line connects the suction compartment to the top of suction vessel, with an
isolating valve. This is to vent the pump off any gas formation, which might interfere
with smooth running of pump. From the pressure relief line a partial flow is
branched off as sealing fluid.
Pump Lubrication System
Motor bearings are grease lubricated. The pump bearing is a thrust-cum-journal
bearing sub-merged in an oil bath. The oil is cooled by clarified water.
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30
TECHNICAL DATA
Pump
Design
Type
WKTA 200/4
Speed
1480 rpm.
3
Discharge Capacity
Overload
1480
3
Head
610 m /hr.
190 M
710 m /hr
170 M
Power
402 KW
429 KW
Temperature of medium handled
Water at 40 oC
3.5 M
NPSH
Motor
Power
470 KW
Supply
6.6 KV, 3-phase, 50 Hz, PF=0.85 lag.
Current
51 Amps.
Speed
1480 rpm.
Class of Insulation
F
CONDENSATE SYSTEM
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32
BOILER FEED PUMP (BFP)
INTRODUCTION
The weir type FK8D30 pressure stage pump is an eight stage horizontal centrifugal
pump of the barrel casing design. The pump internals are designed as a cartridge,
which can be easily removed for maintenance without disturbing the suction and
discharge pipe work, or the alignment of the pump and the turbo coupling.
The pump shaft is sealed at the drive end and non-drive end by Crane mechanical
seals, each seal being flushed by water in a closed circuit and the water is circulated
by the action of the seal retaining ring. The flushing water is cooled by passing
through seal coolers, (two coolers per seal, one working and one standby), and each
seal cooler being circulated with clarified cooling water. The rotating assembly is
supported by plain white metal lined journal bearings and axially located by a
Glacier double tilting pad thrust bearing.
BFP CONNECTIONS
KORBA SIMULATOR
33
TECHNICAL DATA
BOILER FEED PUMP
Manufacturer
: Weir Pumps Ltd
Pump Serial Number
: 11723-001/9
Type
: FK8D30
No of stages
: 8
Direction of rotation viewed at drive end
: Anti-clockwise
Liquid pumped
: Boiler feed water
S.G. at suction temperature
Suction temperature oC
Suction pressure, Kg/cm
2
Discharge pressure, Kg/cm
2
Differential pressure, Kg/cm
Differential, m
2
NPSHA above impeller eye, m
3
Design
Duty
0.904
0.906
163
161
14.42
15.32
212.66
188.86
198.24
173.54
2196.8
1918.9
96.8
101.7
381
Flow rate, m /h
Speed, RPM
4750
Power, KW
2542
BFP MOTOR
Design
Output
3500 KW
Current
361 A
Supply
6.6 KV, 3 phase, 50 Hz.
Power factor
0.8 - 0.85
Efficiency
95.5%
Speed
1485 rpm.
Type of cooling
Closed air circulation
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2018
34
BFP DESCRIPTION
Pump Casing
It consists of forged steel barrel with welded suction discharge branches and
mounting feet. A suction guide closes the drive end of casing and it is located on the
inner casing by a spigot. For prevention of leakage between suction annulus and
barrel casing a MS gasket is located between the suction guide spigot and casing
inner face. An '
O' ring is also provided on the periphery of suction guide for
preventing leakages. Leakage between suction annulus and the drive end is
prevented by an '
O'ring and gasket located on the insert ring, secured to the casing
by studs/nuts. A discharge cover closes the non-drive end with an '
O'ring secured to
base plate pedestals by spacer pieces, washers and holding down bolts, allowing
expansion. Transverse key on the drive end feet and longitude keys under the casing
transfer moments and thrust to the base plate, allowing for expansion.
Discharge cover
It also forms the balance chamber; which do the non-drive end water jacket and
mechanical seal-housing close. Discharge cover is in close fit in casing bore and is
held in place by a ring of studs/nuts. A spring disc is located between the last stage
diffuser and discharge cover balance drum bush to provide force required to hold
ring section assembly in place against the drive end of the barrel before start up.
After starting the discharge pressure assists the spring disc in holding the ring
section in place. Last stage diffuser can slide freely over the balance drum bush
which is shrunk on to the discharge cover bore to minimise flow of liquid to balance
chamber.
Two radial holes drilled through the periphery provide outlet connections for
balancing chamber lead off to pump suction. Two similar holes are provided for
cooling water connection for water jacket. Non-drive end bearing housing is attached
to bearing bracket secured to outer face of discharge cover by stud/nuts and dowel
pins.
Suction Guide
It closes the drive end of casing and forms the suction annulus. Ring section
assembly, the discharge cover and the spring disc hold suction guide against an
internal shoulder in the casing. The drive end water jacket and mechanical seal
housing close the suction guide.
Two tapped holes are provided in the suction guide for cooling water connections to
water jacket. The drive end bearing housing to be attached to bearing jacket secured
to the outer face of suction guide by studs, nuts and dowel pins.
Ring Section Assembly
The ring section assembly consists of seven ring sections which locate one to another
by spigots and are secured to each other by socket head screws in counter-bored
holes, sealing being effected by metal to metal joint faces and '
O'rings with back-up
rings located in grooves in the ring section spigots. Diffusers are dowels and spigot
KORBA SIMULATOR
35
located to ring sections and to the suction guide, and the last stage diffuser is
secured to the last stage ring section with socket head screws in counter bored holes.
Packing rings are shrunk into bore of ring section and grub screws locate diffusers.
These prevent recirculation of pumped liquid between stages. Ring sections and
diffusers form transfer passages from the impeller outlet of one stage of the pump to
other impeller inlet of next stage.
Rotating Assembly
Dynamically balanced rotating assembly consists of shaft, impellers, abutment rings,
keys rotating parts of mechanical seals, shaft nuts, balance drum, thrust collar and
pump half coupling. Chromium plated shaft at each end is supported by journal
bearings and its diameter increases in increments from the non drive end towards
drive end to facilitate fitting and removal of impellers.
Impellers are of single entry shrouded inlet type and are keyed and shrunk onto the
shaft, keys per impeller are being alternately fitted on diametrically opposite sides of
the shaft to maintain balance. The hub of each impeller butts against a split
abutment ring fitted in a groove in the shaft.
The balance drum is keyed and shrunk onto the shaft and held in position against
shaft locating shoulder by balance drum nut and lock washer. Inner end of drum is
recess and the bore of recess is a close fit over the last stage impeller hub. Provisions
are made to inject oil for removal of drum and tapped holes are provided for
withdrawal. The rotating parts of mechanical seals are fitted to the shaft where it
passes through the seal housing. Seal sleeves are keyed to shaft and are clamped in
position by seal sleeve nuts and lock nuts. Thrust collar is keyed to the non-drive
end of shaft and is secured against a shoulder on the shaft by the thrust collar nut
locked by lock washer.
Mechanical Seals
They comprise a seal body assembly secured to seal housing, which contains rotating
components of seal. Each seal consists of rotating tungsten carbide seat mounted in
a carrier, running against a stationary carbon face. Contact between the face and
seal is maintained by hydraulic pressure during running and by spring pressure
during start-up. Other leakage paths are sealed with '
O'rings. The seal is designed to
recirculate the pumped product through seal water coolers to maintain acceptable
temperature in the region of seal face.
Journal and thrust bearing
The rotating assembly is supported at each end of the shaft by a white metal lined
journal bearing and a tilting pad double thrust bearing mounted at non-drive end of
pump carries the residual thrust. Journal bearing shells are of mild steel, white
metal lined, thin wall type and are split on horizontal plane through the shaft axis.
Thrust bearing is fitted in non-drive end bearing housing and has eight white metal
line tilting pads, held in split ring positioned on each side of thrust collar. The carrier
rings are prevented from rotating along with shaft by dowel pins in each ring, which
engage in slots in the bearing top half. Thrust pads are retained on the carrier rings
by special pad stops screwed into the rings.
KORBA SIMULATOR
36
A split floating oil-sealing ring is located in the groove in the thrust bearing housing
to restrict the escape of lubricating oil from the thrust-bearing chamber. To ensure
thrust bearing remains flooded, an orifice is fitted at the oil outlet.
Hydraulic balance
Due to differential pressures acting on the impeller the rotating assembly is
subjected to axial thrusts. The balance drum located at the non-drive end is designed
to keep these forces neutralised and only the residual thrust remains, which is taken
up by thrust bearing.
The main components of hydraulic balancing arrangement are the balance chamber
machined in discharge cover, the balance drum secured to the shaft and balance
drum bush fitted in the bore of discharge cover. The thrust caused by the suction
pressure acting on the area inside the wear ring on inlet side of each impeller is
overcome by much greater thrust caused by the discharge pressure acting on the
equivalent area on the outlet side of each impeller. The resultant thrust is therefore
towards drive end of pump. Thrust force varies with load on the pump but hydraulic
balance arrangement will reduce its effect enabling residual thrust to be taken by
fitting pads of thrust bearing.
The hydraulic balance arrangement operates as follows.
The pumped feed water passes from last stage of the pump between the balance
drum and the bush and enters the balance chamber at a pressure approximately
equal to the suction pressure. Two ports in the discharge cover allow the product to
be piped back to the pump suction side. The pressure differential across the balance
drum is therefore equal to that across the impellers. The cross sectional area of the
balance drum is sized to give a small residual thrust towards the drive end of the
pump.
Flexible coupling
Between the turbo coupling (hydro-coupling) and pump shaft, a flexible coupling,
consisting of two hubs flexibly connected through laminated steel elements to a
tubular spacer, is provided. This can accommodate a certain amount of
misalignment between turbo couplings and pump shafts to which hubs are fitted.
KORBA SIMULATOR
37
FEED WATER SYSTEM
DEAERATOR
SYSTEM
KORBA SIMULATOR
38
BFP BOOSTER PUMP
Introduction
Each pump set consists of a weir type booster pump directly driven from one end of
the shaft of an electric driven motor and a weir type pressure stage pump (Main
Pump) driven from the opposite end of motor shaft through a VOITH type variable
speed turbo-coupling. The drive is transmitted, in each case through a Torsiflex
spacer type flexible coupling, each coupling being enclosed in a split, fabricated
guard.
The bearings in booster and main pump and in motor are lubricated from the forced
lubricating oil system incorporated in the turbo coupling. Each pump set is supplied
with a metallic suction strainer, a NRV on main pump discharge pipe and minimum
flow recirculation system comprising a pneumatic valve and a non-return valve. Each
pump, motor and turbo-coupling is mounted on its own base plate and on a common
grillage.
The pump set is provided in each case, with instrument panel and instrumentation
for monitoring feed water pressure, temperatures, bearing temperatures, lub oil
pressure etc. The booster pump is a single stage horizontal, axial split casing type,
having the suction and discharge lines on casing bottom half, thus allowing the
pump internals to be removed without disturbing suction and discharge pipe work
on the alignment between the pump and driving motor.
The pump shaft is sealed at drive and non-drive end by Crane mechanical seals,
which are cooled by a supply of clarified water. The rotating assembly is supported
by plain white metal lined journal bearings and axially located by a glacier double
fitting pad thrust bearing.
TECHNICAL DATA
PUMP
Manufacturer
Weir Pumps Ltd
Pump Serial Number
11723-010/018
Type
FA1F56
Direction of rotation viewed at drive end
Anti-clockwise
Liquid pumped
Boiler feed water
S.G. at suction temperature
Suction temperature oC
Suction pressure, Kg/cm
2
Discharge pressure, Kg/cm
2
Differential pressure, Kg/cm
KORBA SIMULATOR
2
Design
0.904
Duty
0.906
163
161
7.263
7.07
15.118
16.02
7.855
7.95
39
Differential, m
87
88
NPSHA above impeller eye, m
16.85
16.0
381.6
346.2
3
Flow rate, m /h
3
Leak-off flow, m /h
Efficiency, %
95.4
Speed, RPM
1485
Power, KW
104.5
78
75.5
99.4
DESCRIPTION
Pump Casing
The cast steel pump casing is of double volute type, split on horizontal centre line.
The bottom half pump casing has the suction and discharge branches and support
feet cast integrally. A flanged air vent connection is provided on top half casing for
initial venting of air during line-up. Connections are also provided on suction and
discharge pipes for pressure gauges and drain.
Rotating Assembly
The dynamically balanced assembly consists of the shaft, impeller, nuts, keys, seal
sleeves, thrust collar, rotating part of mechanical seals and pump coupling. The
double entry impeller is keyed to the shaft and is located axially.
Journal & Thrust Bearings
The rotating assembly is supported at each end of the shaft by a white metal lined
journal bearing and residual axial thrust is taken up by a tilting pad double thrust
bearing mounted at the non-drive end of the pump. The bearings are supplied with
lubricating oil from forced lubrication oil system.
Mechanical Seals
The drive and non-drive end stuffing boxes are fitted with mechanical seals located
within seal cooling jackets to prevent feed water escaping along the shaft. Clarified
water flow is maintained through cooling water jackets.
KORBA SIMULATOR
40
BOILER SYSTEM
KORBA SIMULATOR
41
KORBA SIMULATOR
42
BOILER: GENERAL
DESCRIPTION
Steam generator is radiant reheat, dry bottom, natural circulation, single drum, semi
outdoor type, direct fired, balance draft, top supported type, has provision for firing
coal as the principal fuel and is of Combustion Engineering, USA design.
Super-Heater / Re-Heater Section
The super heater steam system has mainly three sections, the low temperature
superheater (LTSH) the radiant platen superheater and final superheater. Two
numbers of de-superheater have been provided in between the LTSH and platen
superheater (in the connection links) for controlling the superheated steam
temperature over a wide load range. The complete second pass of the boiler up to
economiser has been covered with steam cooled superheater wall sections.
The complete reheater is in one section; which has been located in the horizontal
pass of boiler, in between platen and final superheater sections. An emergency
reheater de-superheating unit has been provided at the inlet of reheater.
Flue Gas heat recovery System
The economiser is non-steaming continuous finned type arranged between the LTSH
and air heater section. The boiler has two numbers regenerative air heaters of the trisector type for the last stage of heat recovery. The flue gas occupies (1800), i.e., half
of the portion. 120o is occupied by the secondary air and the rest (i.e. 600), is
occupied by primary air. Two steam coil air pre-heaters are also provided in each of
FD Fan discharge ducting to heat up the secondary air prior to entering
LUNGSTORM Air Pre-heaters. The SCAPHs are to be charged to maintain cold end
temperature of air heater to avoid cold end corrosion.
Draught System
The draught system includes two induced draught fans, forced draught fans,
ignitors, scanner air fans and steam coil air pre-heaters. The forced draught system
provides the air required for combustion of fuel, and induced draught system expels
the flue gases through stages maintaining balance draught. This system also
supplies air to scanner cooling and for lighting up ignitors. Two axial flow reaction
type forced draught fans are provided to supply the necessary secondary air for
combustion. Two axial flow impulse type induced draught fans are provided to
evacuate the flue gases from boiler. Four electrostatic precipitators are provided in
each flue gas path, to remove the fly ash from flue gas before it enters ID Fans. ESP
passes comprises seven collecting zones. In addition to above, the system includes
various ducts and dampers required for maintaining the desired flow, pressure of air
or the flue gas depending on demand.
KORBA SIMULATOR
43
GENERAL ARRANGEMENT OF BOILER
KORBA SIMULATOR
44
KORBA SIMULATOR
45
1.
ECONOMISER
7.
PLATEN SUPER HEATER
2.
BOILER DRUM
8.
FINAL SUPER HEATER
3.
DOWN COMERS
9.
REHEATER
4.
WATER WALLS
10.
BURNERS
5.
WATER WALL PLATEN
11.
IGNOTORS
6.
PRIMARY SUPER HEATER
12.
FRS (FEED REGULATING STATION)
FUEL FIRING SYSTEM
The boiler has direct pulverised coal firing system, which comprises of raw
coalbunkers, R.C Feeders, Bowl Mills, discharge piping, coal nozzles with tilting
tangential firing system, primary air fans and seal air fans. Each mill supplies the
pulverized coal to all the four corners of an elevation. Thus there are six tiers of coal
burners and in all twenty four coal burners. The entire burner assembly for all four
KORBA SIMULATOR
46
corners can be tilted in the vertical plane (+ 30o) by a burner tilting arrangement;
basically for controlling the steam temperature and particularly the hot reheat
temperatures.
To ensure increased safety, reliability and care in operation, the fuel firing system is
equipped with Furnace Safeguard Supervisory System which facilitates single
operator to start, stop and control the complete firing system from remote control
panels. In the bowl mill, pre-crushed coal is pulverised to desired fineness and is
further directed by the primary air. Cold and hot primary air dampers are provided to
regulate the flow/ temperature of the primary air. Boiler is equipped with
sophisticated flame sensing scanners mounted on all the four corners at three
different elevations. In order to cool these scanners, scanner air fans are provided.
LOCATION OF RADIANT AND CONVECTION SUPER-HEATER
ASH DISPOSAL SYSTEM
The bottom ash handling system for each unit is capable of removing bottom ash at a
rate not less than 15 T/Hr. and conveys it to trenches in slurry form. The ash
removal is done continuously. Both side slag baths are provided with continuously
moving feeders for transferring the wet slag ash to the respective clinker grinders and
is then discharged with the aid of the high-pressure water jets. Fly ash is collected in
each of the ESP, air heaters, and economiser and stack hopper. The flushing
equipment serves to mix the ash with low-pressure water and discharge the ash in
the form of slurry into the ash slurry pit for further disposal by means of slurry
pumps.
KORBA SIMULATOR
47
BOILER DRUM AND DRUM INTERNALS
FURNACE TEMPERATURE PROBE
It is an electro-mechanical equipment for positioning a thermo-couple element in the
furnace gas steam for temperature measurement. The thermo-couple is fixed to the
tip of a lance tube, which travels into and out of the gas passage. The lance travels
approximately two meters per minute while extending and retracting. The thermocouple can be retracted manually in case of an emergency.
Flue gas temperature in the area just before platen superheater or reheater elements
at the exit of furnace can be critical during boiler start-ups before steam circulation
for the cooling of SH and RH tube material is sufficiently established. By using the
furnace temperature probe, continuous measurement of gas temperature is possible
and thereby the danger over-heating of tubes can be reduced. For this reason, such
probes are known as basic start-up probes.
The furnace temperature probe can also be used to obtain gas temperature during
low load operation of boiler. This standard furnace probe is equipped with ChromelAlumel thermo-couple installed in a lance tube (with air cooling). Model TFP-1E for
travel from 1.5 to 7.3 m has a 76 mm outer diameter (OD) lance Model FTP-11E for
travel from 7.4 to 12.2 m has a 108 mm OD lance. Both model probes are available
with or without air-cooling.
KORBA SIMULATOR
48
The probe may be operated in furnace gas temperature up to 537 oC (1000 o F) and
for very short period of time in gas temperature as high as 565 oC (1050 oF) without
air-cooling. The temperature probe can be used to measure furnace gas temperature
up to max. 815 oC (1500 oF) with lance cooled by air.
ARRANGEMENT OF BOILER AUXILIARIES
1. COAL BUNKER
6. BURNER
11. ESP
2. COAL FEEDER
7. FD FAN
12. ID FAN
3. COAL MILL
8. WIND BOX
13. STACK
4. PA FAN
9. SCANNER AIR FAN
14. SEAL AIR FAN
5. AIR PRE-HEATER
10. IGNITOR FAN
KORBA SIMULATOR
49
BOILER TECHNICAL SPECIFICATIONS
General:
Manufacture
BHEL, CE (USA), Design
Radiant, reheat, natural circulation, single
drum, semi-out-door, balanced draft and
direct fired.
Type
Type of firing
Tilting, tangential
Type of SH
Pendent, platen, horizontal
Total Aux. power
5400 KW at 91% MCR
Min. mill load with oil support
Total water content of boiler
40% MCR
321.5 T (including RH)
Furnace
Type
Fusion welded walls.
Water walls
Surface area
Front Wall (EPRS)
618 cm2
Side walls (EPRS)
757 cm2
Real Walls (EPRS)
620 cm2
Roof (EPRS)
122 cm2
Total heat surface (EPRS)
2117 cm2
Tube material
SA-120, Gr. A1
Outer Diameter x Thickness
Design metal temp.
63.5 mm x 6.3 mm
40 oC.
Resident time of fuel particles in furnace
2.5 Sec.
Drum
Material
SA-229
Elevation of Drum
53340 mm
Overall length
15000 mm approx.
Shell thickness
170/135 mm (Bi-hickness)
Design metal temp.
354 oC
Permissible max. Differential Temp between any parts of drum.
Normal Operation
55 oC
Accelerated start
55 oC
Water capacity with MCR condition between
normal & lowest water level permitted
KORBA SIMULATOR
25 Sec
50
Super Heater
Heating surface
LTSH
6490 Sq.m.
PLATEN SH
810 Sq.m.
FINAL SH
823 Sq.m
No of stages
3
Material
LT SH
SA210 GrA1, SA209 T1 SA213 T11
PLATEN SH
SA213 T11, SA213 TB
FINAL SH
SA 213 T22, SA213 TB, 347 HH
Type of flow
LT SH
Counter
Platen SH
Parallel
Final
Parallel
Maximum Gas side Temperature
LT SH
4900C
Platen SH
5700C
Final SH
5890C
SHH Specification
Material
SA210 Gr. B, SA335 P12, SA335 P22.
Design Pres.
176.8 Kg/cm
Total Weight
2
68 Ton
Super Heater Attemperator
Type
Spray type mixing
Stages
1
Position of spray in steam circuit
LTSH→Attemperation→Platen SH
SH temperature between 60% and
535 oC
100% MCR load
Maximum Spray
KORBA SIMULATOR
15,000 Kg/hr. at 60 % MCR
51
Reheater
Total heating area
2630 m2
Number of stages
1
Material
SA 269T, SA233T T22, SA213TP 304 H
Max gas side metal temp.
585 oC.
RH Headers Material : Inlet
SA106 Gr. 3
Outlet
SA 335 P22
Design Pressure
46.0 Kg/cm2
Design metal temperature: Inlet
550 oC
550 oC
Outlet
Reheater Temperature Control
Angle of tilt
+ 30 degree
Type of tilting
Power tilting Cylinder
540 oC
RH steam temp. 60% - 100% MCR
RH Emergency Temp. Control
Type
Spray type mixing
Stages
1
Position in steam circuit
In CRH line
Material
SA - 106 Gr. B
Maximum water flow
22 T/Hr.
Economiser
Material
SA 210 Gr. A1
Outer Diameter x Thickness
44.5 mm x 4.5 mm
Maximum gas side temp.
295 oC
Headers
Material
SA-106 Gr. B
Design Pressure
181.1 Kg/cm
Design metal temp.
310 oC
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Boiler Parameters
Flow (T/Hr)
T/Hr
Conti.
Load
120 MW
T/Hr
HPHs out &
Load
200 MW
T/Hr
670.0
603.7
402.0
547.0
598.2
537.7
363.7
540.6
670.0
600.7
394.8
519.7
3.0
7.2
27.3
179.9
167.3
121.5
193.4
57.4
70.0
56.5
43.9
611.4
533.7
371.8
557.1
870.5
792.9
571.7
816.2
144.4
131.6
90.4
135.4
MCR
NCR
200 MW
T/Hr
Superheater outlet
Reheater outlet
Description
Steam
Water
Feed Water
Spray
Air
Fuel
(Coal)
TEMPERATURE OF STEAM, WATER, AIR AND GAS (in 0C)
Description
MCR
NCR
200 MW
Cont.
load
120 MW
HPHs out &
Load 200 MW
0C
0C
0C
0C
Steam:
a. Sat temp in drum
349
348
344
346
b. LTSH outlet
426
421
417
435
c. SH platen outlet
520
520
523
518
d. Final S/H outlet
540
540
540
540
e. R/H inlet
344
339
328
345
f. R/H outlet
540
540
540
540
243
241
223
164
b. Economiser outlet 386
284
270
234
Water:
a. Economiser Inlet
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Air:
a. Ambient
50
50
50
50
b. APH outlet (prim.)
325
318
297
282
c. APH outlet (sec.)
318
313
294
277
a. S/H platen outlet
1135
1132
1080
1129
b. R/H front inlet
(Furnace exit)
1024
1025
945
1008
c. R/H rear inlet
922
917
837
907
d. Final S/H inlet
758
750
682
747
Gas:
e. LTSH inlet
671
661
603
660
f. Economiser inlet
470
462
433
467
g. APH inlet
354
343
312
307
h. APH outlet
(Corrected)
136
134
124
121
OXYGEN, CARBON DIOXIDE (Dry Vol.) and EXCESS AIR (in %)
Description
a.
b.
c.
d.
e.
f.
Oxygen in gas at Eco. outlet
(by dry. vol.)
Oxygen in gas at APH outlet
(by dry vol.)
Max leakage of air across
APH in %
Total air to gas leakage in
T/Hr.
CO2 in gas at Eco outlet (by
dry vol)
Excess air in gas Eco outlet
KORBA SIMULATOR
NCR
200 MW
Cont. load
120 MW
HPHs out &
Load 200 MW
%
%
%
3.87
3.87
4.69
4.3
5.56
5.56
6.88
6.01
8.13
8.8
11.5
8.6
77.3
76.2
71.4
76.6
14.94
14.94
14.22
14.56
22
22
28
25
MCR
54
AIR PRE-HEATER
DESCRIPTION
The boiler is provided with two number of tri-sector type re-generative air pre-heaters
by which the primary and secondary air heating is done, utilising the waste heat
from flue gases. Each air heater is capable of meeting 60% maximum continuous
rating of steam generator.
Air pre-heater consists mainly of rotor housing, cylindrical cellular rotor, guide and
support bearings, oil systems for guide and support bearings, soot blower system,
stationary washing devices, auxiliary air motor drive with over-running clutch, air
and gas duct access doors etc.
The heating elements of specially formed plates from the baskets, which are,
arranged compactly in three layers and within twelve sectors shaped compartments
of radially divided cylindrical shell called rotor. The housing surrounding the rotor, is
provided with duct connections at both ends and is adequately sealed by radial,
circumferential and axial sealing members forming passages, for secondary air,
primary air and flue gases. The complete rotor is supported by a thrust bearing at
the bottom and guided by the radial bearing at the upper end.
A pinion attached to the low speed shaft of power driven reduction gear engages a pin
rack, mounted on the rotor shell. An air motor is connected at auxiliary high-speed
shaft extension of drive unit. The air motor ensures the continued operation of the
air pre-heater, even if power to electric motor is interrupted. It may also be used to
control speed of the rotor during water washing of heating surface.
BEARING LUBRICATION
Support bearing sump is kept filled up with lubrication oil for flood lubrication of
Mitchell type thrust bearing. Oil circulating system is provided to supply support
bearings with a bath of continuously cleaned oil at proper viscosity. To accomplish
this, the bearing oil supply is circulated by means of a motor driven screw pump
through an external filtering system. Guide bearing is a double row cylindrical roller
type. It is lubricated and cooled by oil filled in the bearing housing.
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AIR PREHEATER EXPLODED VIEW
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TECHNICAL DATA
Air Heater
Number per boiler / size
:
2 Nos. / 27 VI (T) 80" (72 o)
Max operating temp 0C
:
365 oC
Max air leakage %
:
9.3 %
Bearing guide & support
:
Radial / SPH Roller thrust.
Effective heating surface.
:
9000 m2 (per heater)
Gas flow area.
:
23.9 m2
Airflow area.
:
21.6 m2
Speed of air heater
:
1.42 RPM
Length hot end/cold end
:
864/305 mm
Material hot/cold end
:
Corten '
A'
/Corten '
A'
Total Wt. of elements
:
130000 Kg / heater.
Material-shaft
:
Carbon steel.
Material Seals
:
Carbon steel '
A'
:
11.0 KW
:
1500 RPM
:
5.0 HP
Rotor
Motor
Power
Speed
Power of Air Motor
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INDUCED DRAUGHT FAN
DESCRIPTION OF FAN
Each I.D. Fan is provided with inlet regulating vanes (IGVs) for controlling the
loading on fans and inlet and outlet shut-off dampers for isolation to facilitate startup and maintenance of fan. Flue gas interconnection is provided with dampers before
Electrostatic Precipitator in order to maintain balanced flow through both the air preheater and second pass when only one I.D. Fan is running.
ID Fan mainly consists of a suction chamber, inlet vane control assembly, impeller,
outlet guide vane assembly, diffuser bearings and flexible coupling.
Suction Chamber
The suction chamber is of welded sheet steel construction and is split horizontally for
easy assembly and dismantling. A manhole is provided in the suction chamber for
checking up the inlet of the fan.
Inlet Guide Vane Control
Flue gas entering suction chamber passes through the number of inlet axial aerofoil
vanes before reaching impeller. Inlet guide vanes adjust the angle at inlet with
respect to impeller blade depending on the inlet vane angle setting. The axial inlet
vanes fixed to individual shafts, which are connected by means of angular joints to a
central ring. The ring is guided by a set of roller and spring assemblies. A control
lever is connected to the ring, which is operated by pneumatic power cylinder. The
inlet vane control assembly is split to facilitate handling and dismantling.
Impeller
The impeller body is welded sheet steel construction; with welded on, non-profiled,
solid blades. The impeller is dynamically balanced at the works. It is bolted to the
flange welded on the hollow shaft. The impeller casing is of undivided type by the
conical connection piece connected to the casing is split horizontally such that the
top half can be removed for removal of the impeller. A peephole is provided in the
casing for checking the wear on impeller. The impeller is supported in between the
bearings.
Outlet Guide Assembly
The outlet guides are fixed in between the core of the diffuser and the casing. These
guide vanes serve to direct the flow axially and to stabilise the drift flow caused in the
impeller. The outlet blades for fans handling dust-laden gases are of removable type
from outside. During operation of the fan, these blades can be replaced one by one.
Diffuser
Diffuser is of welded sheet steel construction with a core inside. The core of the
diffuser houses the inner bearing, which is supported by all the outlet blades. The
core of the diffuser is provided with a manhole with access from diffuser casing so
that the bearing can be checked even during the operation of the fan. For fans
handling hot gases, the diffuser cores are insulated inside. The lubrication pipe as
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well as the thermometer for the inner bearing is brought outside through the core for
easy access.
Bearings
The bearings are self-aligning roller type. The flanged bearing on the impeller side is
the fixed bearing and the outer bearing is the expansion bearing. Both bearings are
grease lubricated and the lubrication points are available on diffuser casing for inner
bearing. A grease quantity control ring is provided in each bearing discharge the
surplus amount of grease. Contact less thermometers are provided for indicating the
bearing temperatures and for initiating alarm/tripping signal when bearing
temperature rises to 95oC/105oC respectively.
Flexible coupling
ID Fan rotor shaft is directly coupled to the motor by flexible pin type coupling (with
rubber bushing inserts).
FLUE GAS SYSTEM
ID FAN OIL CIRCULATION SYSTEM
This Oil Circulation System is designed and manufactured to cool the bearings of
fans, which normally operate at high speeds. Oil is drawn from a reservoir tank by
means of Trocholdai driven by electric motors. Oil flows to the bearings to be cooled,
through suction filters, oil coolers and pressure filters. Oil level indicator, sight
glasses, breather, instrument panel are mounted on to the tank. Pressure gauges,
pressure switches, thermometers and valves are provided at all important points.
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This is a complete interlocked system with stand-by motor, pump, oil coolers, filters
and automatic pressure control devices. Once all the instruments are set to the
required value, this system will run with little supervision.
DESCRIPTION OF FAN MOTOR
ID Fan motor is 3-phase squirrel cage induction motor having closed circuit aircooling system. Air within the motor is circulated by means of internal centrifugal
fans and centrifugal action of rotor itself. Rotor support bearings are hydrodynamic
ring assisted oil lubricated type. Each motor is provided with a lub oil system for
circulating and externally cooling of lub oil.
Contact less thermometers are provided for indicating bearing temperature; also for
initiating alarm/tripping signal when bearing temperature goes high.
FAN LUBRICATION SYSTEM
Each motor of ID Fan is provided with an independent lubricating oil system. The
bearings of the ID Fan motor are ring lubricated and hence do not require any force
lubrication.
TECHNICAL DATA
Fan
Type and size
:
Axial impulse AN 2806
Orientation
:
Horizontal
Medium handled
:
Flue gas
Location
:
Ground level.
No of fans / boiler
:
2
Capacity
:
225 m3/sec
Total head
:
356 mm wcl.
Temperature of medium
:
136 0C.
Specific weight of medium
:
7966 Kg/cm .
Speed
:
740 RPM
Type of fan regulation
:
Inlet Guide Vane (IGV)
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Motor
Type
: Squirrel cage inductor motor
Rated power
: 1100 KW
Rated voltage
: 6.6 KV
Rated frequency
: 50 Hz
No of phases
: 3
Lubricating system
: Forced oil lubrication
Bearing type
: Hydrodynamic ring assisted bearing.
Speed
: 740 RPM
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FORCED DRAUGHT FAN
FAN DESCRIPTION
Forced draught fan may be operated in partial load range without affecting
considerable economic efficiency.
The rotor is accommodated in cylindrical roller bearings. In addition an inclined ball
bearing at the drive side absorbs the axial thrust. For controlling the bearing
temperature, there are contact tele-thermometers connected to signalling
instruments.
An oil hydraulic servomotor flanged to the impeller and rotating with it, adjusts the
blades during operation. This results in a closed flux of force between adjusting
forces and oil pressure, so that no forces are released to the outside (bearings,
housing, foundation). The servomotor consists of piston; cylinder and The FD fan
consists of the following components:
•
Silencer
•
Inlet bend
•
Fan housing
•
Impeller with blades & blade pitch control mechanism.
•
Guide wheel casing with guide vanes and diffuser.
FD FAN CONNECTIONS
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The inlet bend is executed as inlet nozzle at its impeller end. The unit is driven from
the suction side. In the core of the inlet bend, the shaft is accommodated in a
specially designed bearing housing. The impeller is mounted in over-hung position
on the shaft. The critical speed of the latter is well above the operating speed. The
fans control device. The hydraulic servomotor is controlled by a pneumatic power
cylinder, which in turn gets command from UCB via E/P converter.
FAN MOTOR DESCRIPTION
FD Fan is a 3-phase squirrel cage induction motor having closed circuit air-cooling
system. Air within the motor enclosure is made to circulate by the help of internal
centrifugal action of rotor itself. Contact less thermometers are provided for
indicating bearing temperature also for initiating alarm/tripping signal when bearing
temperature goes high.
TECHNICAL DATA
Fan
Fan type
:
Axial reaction type
Fan orientation
:
Horizontal
Location
:
Ground level
Medium handled
:
Air
No of fans/boiler
:
Two
Type of fan regulation
:
Blade pitch control.
Lubrication system
:
Forced oil lubrication
Capacity
:
Total head developed
:
105 m /sec.
510 mm wcl
Temperature of medium
:
50 0C.
Specific weight
:
1.619 Kg/m of medium
Flow (reserve)
:
26 %
Pressure (reserve)
:
50 %
Rating
:
750 KW
Voltage
:
6.6 KV Three-phase
Speed
:
1480 rpm.
Lubrication System
:
Grease lubrication
3
3
Motor
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PRIMARY AIR FAN
FAN DESCRIPTION
Each boiler is provided with 2 Nos. of PA fans, each fan being capable of catering
total air requirement of 3 mills. Fans are of radial type with single entry and
horizontal orientation. It takes suction from atmosphere through a double entry
silencer.
Fan is coupled to the driving motors directly through a rubber-bushing coupling. The
fan rotor is placed in two cylindrical roller anti-friction bearing and is provided with a
double row inclined ball bearing to take up the axial thrust. All the three bearings
are housed in a single housing, which is filled with oil. Silencer, provided at PA Fan
suction to damp the noise level, is supported as a separate structure and bolted
directly to the fan suction.
Regulating the inlet guide vane unit arranged in the suction side controls fan
loading. The axial inlet guide vane assembly of the fan consists of a number of
aerofoil inlet vanes fixed to individual shafts, which are connected by means of
angular joints to a central ring. The ring is guided to rotating position by a set of
roller and spring assemblies. A control lever is connected to the ring, which can be
operated by a pneumatic actuator. For monitoring the fan bearing temperature,
indicators are provided for each of the bearings.
PRIMARY AIR AND SEAL AIR SYSTEM
Hot air from air pre-heaters outlet is connected to a common hot air duct from where
toppings are taken for individual mills. Hot air shut-off gate and control dampers are
provided in the branch line to each mill. Cold air from both the PA Fans discharge is
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led directly into common cold air duct from where tapping are given to individual mill
for tempering air, to hot air gates for sealing, to feeders for bearing sealing and to
mill discharge pipes for sealing and purging. Isolating gate (hand operated) and
regulating dampers are provided in the branch lines of cold air to each mill. Hand
operated isolating valve are provided in feeder sealing line and solenoid operated
isolating valves are provided in sealing air line to pulverisers discharge piping.
Primary air fans are provided with isolating dampers at discharge. A portion of the
air discharge by the fans is heated up in the air pre-heater and the remaining air is
sent directly as cold air.
Both the PA Fan discharge ducting is interconnected before the APHs through
interconnecting ducting. Each APH is provided with isolating dampers at primary air
inlet and outlet.
The interconnecting ducts provide the flexibility of operating the PA Fan in
combination with any APH and it makes it possible to distribute the primary airflow
to both the APH when only one PA Fan is running.
TECHNICAL DATA
Fan
Fan type
: Single suction, radial fan
Fan orientation
: Horizontal
Medium handled
: Air
Location
: Ground level.
No of fans/boiler
: Two
Fan regulation
: Inlet guide vane control
Capacity
Total head developed
: 75 m3/sec.
: 1187 mm. of water column
Temperature of medium:
: 50 0C
Specific weight of medium
Speed
: 1.019 Kg/cm2
: 1480 rpm
Type of fan regulation
: Inlet guide vanes control.
Lubrication system
:
Type
:
Motor
Forced oil circulation system having
5 Lit/min capacity for lubrication
Rated Power
3-phase, air-cooled, Squirrel cage,
induction motor
: 1250 KW
Voltage
: 6600 V
Speed
: 1480 RPM
Lubrication
: Grease lubricated.
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PULVERISER
DESCRIPTION
The bowl mill consists essentially of a reduction gear box, mill side and liner
assembly forming air and mill reject chamber, revolving bowl and scrapper, separator
body with separator body liner assembly, grinding rolls and journal assembly,
pressure spring assembly, classifier, multi port outlet assembly, central feed pipe
and separating inner cone.
Pre-crushed coal is fed by the RC Feeder through central feed pipe into the revolving
bowl of the bowl mill. Centrifugal force feeds the coal uniformly between the bullring
and independently rotating spring-loaded rolls to travel through the outer periphery
of the bowl. The springs, which load the rolls, impart the pressure necessary for
grinding. The partially pulverised coal continues up over the edge of the bowl due to
centrifugal force.
Hot and cold primary air mixed in the dustings enter the mill side housing below the
bowl and is directed upwards past the bowl around the separator body liners which
carry pulverised coal upwards into the deflector openings at the top of the inner
cone, then out through the venturi and multi port outlet assembly. As air passes
upward around the bowl, it picks up the partially pulverised coal. The heavier strike
the separator body liners and are returned to the bowl immediately for further
grinding. The lighter particles are carried up through the deflector opening impart
the spinning action to the material with the degree of spin set by the angle of opening
of the blades, determining the size of the pulverised coal.
Any tramp iron or dense foreign material in the raw coal feed which is difficult to
grind, if carried over to the top of the bowl, is dropped out through the air stream to
the lower part of the mill side housing. Pivoted scrappers attached to the bowl hub
sweep the tramp iron or other material around to the tramp iron spout through
normally open pyrite hopper first by closing the inner gate and opening the outer
gate of the hopper.
The motor is coupled directly to worm shaft of the reduction gear, which rotates the
bowl at a reduced speed and transmits the total power required for pulverizing the
coal.
LUBRICATION SYSTEM
Pulveriser and roller bearings are oil lubricated. Pulveriser radial bearings receive oil
supplied by the helical pump mounted on bottom of lower half of mill journal in the
oil bath. Rollers are filled with oil independently. Worm shaft reduction gear is
dipped in oil bath. Motor is grease lubricated
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BOWL MILL
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Pulveriser Specifications
Air flow per mill
:
60 T/Hr
Air temperature at mill inlet
:
260 0C
Mill outlet temperature
:
77 0C
Coal flow per mill
:
36 T/Hr
Fineness of coal milled
:
70 % through 200 mesh
Primary air pressure inlet/outlet
:
650/244 mm wcl
COAL MILL ARRANGEMENT
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HP AND LP
BYPASS SYSTEM
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HIGH PRESSURE (HP) BYPASS SYSTEM
The HP Bypass system in coordination with LP Bypass enables boiler operation and
loading independent of the turbine. This allows quick raising of steam parameters to
a level acceptable to turbine for rolling during start up. Steam is bypassed from main
steam line to cold reheat line through HP-Bypass and from hot reheat line to
condenser through LP bypass. The HP bypass valve can handle a maximum of 60%
of the full load turbine steam flow.
The possible phases of operation of HP bypass station can broadly be classified as
follows:
•
Boiler start-up with TG set at standstill.
•
Raising of steam parameters to a level acceptable for TG rolling at a relatively
faster rate than otherwise is possible.
•
Turbine loading while steam flow gets transferred to the turbine.
•
Parallel operation with turbine on load rejection.
•
Allowing boiler operation following turbine trip, provided boiler load < 60%.
•
Preventing safety valves opening at raised steam pressures.
Description
The HP Bypass system consists of two parallel branches that divert steam from the
M.S. line to CRH line. The steam pressure on the valve upstream side can be
maintained at the desired level. The steam is de-superheated in order to keep the
steam temperatures in cold reheat line within limits, below 345 oC.
The M.S. pressure ahead of the turbine is maintained by two nos. of pressure
reducing valves BP-1 and BP-2 with valve mounted electro-hydraulic actuator. The
steam temperatures downstream of the HP-Bypass station is maintained by 2 nos.
of spray water temperatures control valves BPE-I and BPE-2 with valve
mounted electro-hydraulic actuators. The spray water is available from the BFP
discharge line. There is also a spray water pressure control valve with valve mounted
electro-hydraulic actuator.
HP Bypass System (Hydraulics)
Oil Supply unit
The oil supply unit provides the hydraulic actuation energy for the complete
actuating system, and functions as follows:
An axial-piston oil pump draws the oil through a suction strainer and pumps it
through a pressure filter and via a non-return valve into the accumulator. A safety
relief valve protects the system against over pressure. The accumulator is of the
bladder type and consists of a steel pressure vessel containing a nitrogen filled
rubber bladder, which separates the oil from the gas. The accumulator supplies
pressurised oil to the system and covers the entire peak supply requirement. The oil
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pump therefore, is sized only for the mean supply requirement and it is switched off
when the accumulator is fully charged. From the accumulator the oil is fed through
the supply manifold with the pressure reducing valve and the pressure is set and
controlled. The pressure switch monitors the oil pressure in the accumulator and
provides the signals to switch on the oil pump.
Servo valve
HP BYPASS SYSTEM
The two-stage servo valve is actuated by the torque motor, which is controlled from
an analogue-positioning amplifier or from a manual desk control. The torque motor
moves the control fork (of the servo valve) and operates the pilot stage (1st stage),
which controls the position of the control piston (2nd stage). A mechanical override
acting directly on the control piston permits local manual operation of the valve.
Blocking Unit
The electro hydraulically pilot-operated blocking unit is mounted between the servo
valve and the actuator. It closes off both ports to the actuator if electrically deenergized or with insufficient oil pressure, and holds the piston of the actuator
(disregarding some leakage drift) in its last position. A mechanical override on the
blocking unit permits local manual de-blocking.
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Actuator
The actuator consists of a double acting cylinder with piston and piston rod. An
intermediate yoke connects this cylinder with the valve, and a solid coupling
connects the valve stem with the piston rod. A feedback transmitter unit is mounted
onto the coupling yoke and is connected to the valve stem by a linkage system.
Technical Data
Oil Pump (Type)
OV 16
OV 32
Oil Supply
12
24
Litre/min
Speed
1500
1500
RPM
Power
4
7.5
KW
Voltage
380
380
V
Frequency
50
50
Hz
Phase
3
3
No load speed
1500
1500
RPM
Oil Tank volume
45
70
Litres
Useable volume
20
50
Litres
Motor
Oil Tank
Hydraulic Accumulator (Standard)
Nominal volume
10 lit
Pressure rating approx.
200 bar
Ambient temperature
15 oC
65 oC
min.
max.
Operating gas
Nitrogen
Bladder material
Perbunan (Synthetic Rubber)
Available Oil Pressures
The controlled system pressure (set with the 25 to 120 bar
pressure
reducing
valve)
The maximum
oil pressure
(limited with the 50 to 180 bar
pressure relief valve)
Pressure Switch
4 micro-switches for the set points:
Pump motor - on
Pump motor - off
Pressure too low
Pressure too high
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Electrical Rating:
20
Amp at 488 V AC
10
Amp at 125 V AC
0.25
Amp at 250 V DC
0.5
Amp at 125 V DC
Mode of Operation
The HP bypass system is intended to ensure reheater protection, minimum super
heater safety valve lifting under emergency conditions, adequate steam in CRH for
auxiliary steam consumers, if taken from CRH, to retain the boiler under fire in case
of turbine load rejections and to follow boiler control system during certain operation.
The control system is designed to maintain the steam pressure ahead of bypass valve
to the given set value. The pressure set point can be adjusted from UCB. The steam
temperatures at the downstream of valves are automatically controlled to the given
set value. The temperature set point can be adjusted from UCB.
Operation of the HP bypass station is manipulated by the pressure and temperature
set points and is independent of LP bypass operation. Depending upon the initial
pressure condition at the time of boiler firing, the pressure set point is to be adjusted
to a value equal to the steam pressure ahead of bypass valve minus a bias pressure.
This will result in opening of the valves. The pressure controller would then maintain
the set pressure by allowing a flow matching with the steam flow. As the firing rate
increases, the set point needs to be manipulated in the same manner to allow flow
sufficient through RH. This however, shall be possible till the maximum flow
capability of the valve is reached at any particular pressure and temperature at
upstream. Upon reaching the target steam parameter for turbine rolling, the boilerfiring rate will be maintained at that level.
Before admission of steam into turbine, the HP bypass shall be set to maintain the
relevant steam pressure ahead of valves plus a bias pressure. Consequent upon
steam admission in the turbine, the pressure ahead of bypass valve would tend to
fall in view of constant firing rate.
This would result in proportional closing of bypass valve during pressure controller
action. The process shall continue till the set pressure up stream is reached. After
this, further loading of the set can be achieved by increasing the firing rate.
Thereafter, the bypass set point shall be raised to live steam pressure plus a bias
pressure of 2-5 ata. With this, the HP bypass station would automatically open and
balance the discrepancy between steam generation and consumption, acting out of
load rejection under normal operation of the unit.
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HP BYPASS: ELECTRO HYDRAULIC SERVO SYSTEM
The control loop for the steam temperature at downstream of HP bypass valve can be
operated by modulating set point as required for different mode of start-ups governed
by boiler/turbine characteristic as well as warm-up requirements of steam piping.
Pressure Control
The signal for the HP bypass station is sensed from the main steam and converted to
proportional current signal by transmitters. The actual pressure is displayed at the
desk by indicators. The set point can be varied from the desk by a push button
module and is indicated on the console itself.
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77
Temperature Control
The control positioners for the spray valves are designed in the same way as those for
the steam valves. In addition PI controllers are connected up to the control
positioners. The temperature signal from transmitters is compared at the PI
controllers with the common temp. set point. According to particular control
deviation the PI controller forms a rated signal for the control positioners of the
associated spray control valves.
The electro-hydraulic actuators make it possible to attain short positioning time for
the spray water control valves and then allow the temperature control to intervene
fast enough in the event of quick opening of the HP bypass valves. To offset the time
delay of temp measurement and to achieve favourable conditions when reaching on
the spray water-cooling system rapid adjustment to temp input of the injection valve
controller by the associated bypass valves positioning monitor. Thus, independent of
the temperature signal, a certain amount of water is injected during the opening of
the bypass valve. Manual operation of the bypass spray water temperature control
valve is effected by means of push button modules. The valve position and the
control deviation are indicated on the desk. In order to ensure proper spray cooling
on BP-1 and BP-2 (under different steam flow rates), the spray water control valves
BPE-1 and BPE-2 are reset to a constant pressure feed water supply through the BD
Valve.
Interlocks for the HP Bypass System
HP Bypass valve BP-1 or BP-2 opening less than 2% will automatically close the
spray water pressure control valve (BD valve).
If opening of either of the bypass valves BP-1 or BP-2 is above 2%, the control of
spray water pressure control valves and temperature control valves BPE1 &
BPE2 shall be changed to '
AUTO'mode irrespective of their initial conditions.
If BP valve position drops < 2% open, it will receive auto close command to
ensure positive shut-off.
If the steam temperature downstream of the BP valves becomes 380oC, the
closing signal for these valves are initiated accompanied with an alarm. In this
case, the BP controller will transfer itself from AUTO to MANUAL.
The following will activate the '
Fast Opening'Signal:
•
Generator Circuit Breaker Open.
•
Turbine Load Shedding Relay operated.
• HP BP Pressure controller deviation more than (+) 10%.
• Depressing of the '
FAST OPEN'push button.
KORBA SIMULATOR
78
LOW PRESSURE (LP) BYPASS SYSTEM
Low Pressure bypass system enables to establish an alternative pass for dumping the
steam from reheater outlet directly into condenser at suitable steam parameters.
The controls for LP BYPASS system are essentially a combination of electrical and
hydraulic system. Electro-hydraulic converter provides the necessary link between
hydraulic actuators and the electrical system. The control of LP bypass system is
hooked up by the same control, which is used for turbine governing system. The LP
bypass valves are two in number. The double shut-off arrangement separates the
reheater from the condenser during normal operation. In addition to these, two steam
pressure control valves, four injection water valves are provided for de-superheating
purposes. This injection water is taken from condensate extraction pump discharge.
LP BYPASS SYSTEM
Set Point Formation
Two set points, the fixed set point and the variable set point are formed for the LP
Bypass controller, the effective set point under any set of operating conditions being
the greater of the two.
The fixed set point can be set manually from the control panel to a point between 0 120 % of the maximum LP Bypass pressure with the aid of a motorised set point
adjuster. It can also be regulated automatically by means of the '
Automatic Control
KORBA SIMULATOR
79
Interface'during the start-up phase and is normally used to set the lower limit for
pressure set point. The pressure upstream of the H.P. blading, required for reference
variable set point formation, is measured by a pressure transducer and transmitted to
a matching amplifier which sets the characteristic for the reference variable as a
function of the pressure upstream of H.P. blading i.e. throttle pressure.
LPBP EHC POSITION
Vs VARIOUS VALVE OPENING
PRESSURE CONTROL FOR LP BYPASS SYSTEM
The reheat steam pressure before interceptor valve is the controlling variable for the
LP bypass system. Control of this parameter can be done in the '
MANUAL'mode by
changing the electro-hydraulic controller (EHC) output as required by means of the
OPEN/CLOSE push buttons located on the control module. In the '
auto'mode, the
controller matches the hot reheat pressure with the effective set point (either FIXED
or VARIABLE) by modulating the LP Bypass control valves as necessary. A tracking
controller is provided so that the control mode (manual or auto) not in service
automatically follows the effective controller. This facilitates bump less changeover,
between the modes. But when charging over from '
MANUAL'to '
AUTO'care must be
taken for matching the set point and actual value, otherwise, a jerk in the system will
be felt due to the error present (which the AUTO controller tries to bring to zero).
KORBA SIMULATOR
80
LP BYPASS CONTROLLER
AUTOMATIC CONTROL INTERFACE DEVICE (ACI)
During the start-up, it is intended to avoid a very high level of set point. For this
purpose, the Automatic Control Interface Device has been introduced. For the
Automatic Control Interface to come in action, it must be switched on by means of the
ON/OFF push button provided on the control panel. Also the Bypass controller must
be in auto mode. When the Automatic Control Interface is switched ON, it brings the
fixed set point down to 3 Kg/cm2, in case the actual reheat pressure is below 3
Kg/cm2. When the actual reheat pressure exceeds 3 Kg/cm2 the ACI opens the LP
Bypass control valves + 25% and they remain locked in 25% position up to a reheat
pressure of 12 Kg/cm2. During this time, the fixed set point tracks the actual reheat
pressure so that the output of LP Bypass "auto" Controller is zero. Once the ACI has
brought the fixed set point 12 Kg/cm2, it gets automatically switched off. The fixed set
point remains static at 12 Kg/cm2 and the LP Bypass controller modulates the control
valve to maintain this set pressure. Any change in the reheat pressure can now be
brought only by manually varying the fixed set point to the desired value.
Two Stage Water Injection
To prevent undue overloading of condensate pumps under normal shutdown/start-up
conditions, the injection water demanded from CEPs is staggered in two stages. This
arrangement opens the injection valves (INV-2, 4) via the pressure switch (LPPS),
solenoid valve (SVV) & slide valve SV-2/4 when the steam pressure upstream at the
expansion orifice exceeds value corresponding to 45% of maximum bypass flow.
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81
Protective Closing of LP Bypass System (Condenser Back-up Protection)
The LP Bypass valves will close automatically under the following conditions to
prevent damage to the condenser.
•
Condenser vacuum is low (> 0.4 Kg/cm2 abs)
•
Spray water pressure is low (< 10 Kg/cm2 or both condensate pumps off).
•
Condenser wall temperature is high (> 90oC).
•
The steam pressure downstream of LP BP is greater than 19 Kg/cm2.
High exhaust hood temperature will automatically switch on the exhaust hood spray
water. In case of condenser wall temperature protection operation, the '
RESET
BYPASS TRIP'
-Pushbutton for solenoids SV-1 and SV-2 are to be depressed to reset
the TRIP command.
LP Bypass Control (Hydraulic)
Due to difference between set and actual HRH pressure the electro-hydraulic LP
bypass governor generates a proportional signal voltage in the moving coil of the
converter (EHC). With increasing signal voltage, the jet pipe of the converter moves
towards right and the amplifier piston (KA-08) moves down. A feedback mechanism
stabilises the amplifier piston for a given voltage change. The sleeves (KA04) of followup piston valves (KA02/KA03) also move down increasing the signal oil Pressure of
water injection Valves, there by opening them, in the beginning of control operation.
LP BYPASS CONTROL SYSTEM
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82
LP bypass stop valves (LPSV-1, 2) open up with a slight time delay after injection
valves are opened; due to rising oil pressure in follow-up pistons KA02 (assuming
piston KA07 of bypass limiting regulator is in upper position). LPBP Steam control
valves (LPCV-1, 2) open up due to hydraulic feedback between actuator pistons and
pilot valves (PV-1, 2).
1.
2.
3.
4.
5.
6.
Electric LP bypass governor
Plunger coil measuring system
Jet pipe
Adjusting spring
Adjusting screw
Jet pipe regulator
a. Control fluid
a1. Control fluid under control piston of differential
pressure relay
a2. Control fluid above control piston of differential
pressure relay
Electro Hydraulic Converter for LP Bypass
LP bypass limiting regulator (LPLR) has priority over (EHC). As soon as condensate at
required pressure is available with sufficient vacuum in condenser, its jet pipe swings
to right and its piston KA07 moves to upper position. This increases the signal oil
pressure in KA02 (follow-up pistons), releasing steam Stop Valves and Control Valves
to open. In case of condensate water pressure low and condenser pressure high the
reverse action takes place and the spring of KA02 is de-tensioned to such an extent
that LP bypass valves are unable to open, Refer to Figure.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Jet pipe
Jet pipe regulator
Adjusting spring
Adjusting screw
Corrugated measuring system
Adjusting spring
Corrugated measuring
Corrugated measuring
Adjusting spring
a. Control fluid
a1. Control fluid above control piston
of limit pressure amplifier
a2. Control fluidunder control piston
of limit pressure amplifier
k. Condensate from hydraulic
pressure switch of injection water
pressure monitor
l. Vacuum signal from bypass steam
piping behind bypass control
valve
LP Byapss Limiting Regulator
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83
LP BYPASS PROTECTIONS
Low Vacuum Safety Device
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Capnut
Adjusting csrew
Cover
Compression spring
Diaphragm
Valve
Valve sleeve
Casting
Can
Lever
a Bypass signal oil from converter
a1 Signal oil to bypass valve
c Oil drain
l Vacuum from condenser
A low vacuum safety device is installed in the signal oil line from follow-up piston
KA02 to bypass valves'pilots (PV-1, 2) and (PV-3,4) If vacuum drops below a preset
value; the valve of the safety device moves downwards due to increasing pressure
above it. The valve thus blocks off the signal oil line and opens the oil between itself
and PV-1, 2 & 3, 4 to drain, closing the LP bypass stop and control valves. As vacuum
increases, bypass operation is restored in reverse sequence when the preset vacuum
has built up.
Low Injection Water Pressure Protection
A pressure switch (WPS) is installed in the signal oil line from KA02 to PV-1, 2 & PV3,4 of bypass valves, to protect the condenser in the event of water injection failing. If
the injection water pressure drops below a preset value, the valve of the pressure
switch (WPS) moves down, blocking off the signal oil line and de-pressuring the oil
between itself and PV-1, 2 & PV 3, 4. The LP bypass valves are thus closed, due to low
condensate water pressure. Bypass operation is restored in the reverse sequence
when injection water pressure becomes normal.
KORBA SIMULATOR
84
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Hood
Bellos
Pushrod
Knife edge lever
Cam shaft
Compression spring
Fitted key
Shaft
Scale
Cylindrical pin
Nozzle
Slide valve
Valve bushing
Compression spring
Bearing bushing
Torsion spring
Lever
a Control oil
a1 Control oil
a2 Control oil to pilot valve of
bypass valve
c Return flow
l Injection water pressure
Low Injection Water Pressure Protection
High Condenser Wall Temperature Protection
At a preset condenser wall temperature the two thermocouples mounted in steam
dome opposite bypass steam inlet transmit a switching pulse to the associated
solenoid valves (SOLV-1, 2).
1. Solenoid
2. Compression spring
3. Solenoid valve
4. Compression spring
5. Solenoid
6. Compression spring
7. Main valve
8. Compression spring
9. Limit switch for injection
a Control oil
b Signal oil to Stop and Control valve
operator of bypass SV/CV
c Drain
The solenoid valves block off the depressive signal oil and close bypass valves in the
event of high condenser wall temperatures. The bypass valves can be opened from the
control room manually only after the solenoids are manually reset after the
temperature has become normal.
KORBA SIMULATOR
85
KORBA SIMULATOR
86
TURBINE SYSTEM
KORBA SIMULATOR
87
KORBA SIMULATOR
88
STEAM TURBINE: GENERAL
DESCRIPTION
210MW capacity turbines at Korba station are of Kraft Werk Union (KWU-Germany)
design and supplied by BHEL. The turbine is condensing, tandem compounded,
horizontal, reheat type, single shaft machine. In has got separate high pressure,
intermediate and low-pressure parts. The HP part is a single cylinder and IP & LP
parts are double flow cylinders. The turbine rotors are rigidly coupled with each
other and with generator rotor.
HP turbine has throttle control. The steam is admitted through two combined stop
and control valves. The lines leading from HPT exhaust to reheater have got two cold
reheat swing check NRVs. The steam from reheater has got two cold reheat swing
check NRVs. The steam from reheater is admitted to IP turbine through two
combined stop and control valves. Two crossover pipes connect IP and LP cylinder.
210 MW
KWU TURBINE
Blading
The entire turbine is provided with reaction blading. The moving blades of HPT, LPT
and front rows of LPT have inverted T roots and are shrouded. The last stages of LPT
are twisted; drop forged moving blades with fir-tree roots. Highly stressed guide
blades of HPT and IPT have inverted T roots. The other guide blades have inverted Lroots with riveted shrouding.
Bearings
The TG unit is mounted on six bearings HPT rotor is mounted on two bearings, a
double wedged journal bearing at the front and combined thrust/journal bearing
adjacent to front IP rotor coupling. IP and LP rotors have self-adjusting circular
journal bearings. The bearing pedestals of LPT are fixed on base plates where as HPT
front and rear bearing pedestals are free to move axially.
Pedestals at machine level support the brackets at the sides of HPT. In axial
KORBA SIMULATOR
89
direction, HP & IP parts are connected with the pedestals by means of a casing guide.
Radial expansion is not restricted. HP & IP casings with their bearing pedestals move
forward from LPT front pedestal on thermal expansion.
HP TURBINE
1. TURBINE ROTOR
2. OUTER SEAL RING
3. BARREL CASING
4. GUIDE BLADE CARRIER
5. THREADED RING
6. CASING COVER
HP TURBINE SECTIONAL VIEW
HP Turbine is of double cylinder construction. Outer casing is barrel type without
any axial/radial flanges. This kind of design prevents any mass accumulation and
thermal stresses. Also perfect rotational symmetry permits moderate wall thickness
of nearly equal strength at all sections. The inner casing is axially split and
kinematically supported by outer casing. It carries the guide blades. The space
between casings is filled with the main steam. Because of low differential pressure,
flanges and connecting bolts are smaller in size. Barrel design facilitates flexibility of
operation in the form of short start-up times and higher rate of load changes even at
high steam temperature conditions.
KORBA SIMULATOR
90
IP TURBINE
1. TURBINE ROTOR
2. OUTER CASING
3. OUTER CASING
4. INNER CASING
5. INNER CASING
6. EXTRACTION NOZZLE
7. INLET NOZZLE
IP TURBINE SECTIONAL VIEW
IP Turbine is of double flow construction. Attached to axially split out casing is an
inner casing axially split, kinematically supported and carrying the guide blades. The
hot reheat steam enters the inner casing through top and bottom centre.
Arrangement of inner casing confines high inlet steam condition to admission breach
of the casing. The joint of outer casing is subjected to lower pressure/temperature at
the exhaust. Refer to Figure.
KORBA SIMULATOR
91
LP TURBINE
Double flow LP turbine is of three-shell design. All shells are axially split and are of
rigid welded construction. The inner shell taking the first rows of guide blades is
attached kinematically in the middle shell. Independent of outer shell, middle shell is
supported at four points on longitudinal beams. Two rings carrying the last guide
blade rows are also attached to the middle shell. Refer to Figure.
1. OUTER CASING
2. OUTER SHELL
3. INNER SHELL
4. INNER SHELL
5. OUTER SHELL
6. DIFFUSER
7. OUTER CASING
LP TURBINE SECTIONAL VIEW
Fixed Points (Turbine Expansions)
a. Bearing housing between IP and LP
b. Rear bearing housing of LP turbine
c. Longitudinal beam of LP turbine
d. Thrust bearing.
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92
Front/rear housing of HPT can slide on base plates. Any lateral movements
perpendicular to machine axis are prevented by fitted keys. Bearing housings are
connected to HP-IP casings by guides, which ensure central position of casings while
axially expanding and moving.
The LPT casing is located in centre area of longitudinal beam by fitted keys cast in
the foundation cross beams. Axial movements are not restricted. The outer casing of
LP turbine expands from its fixed points towards generator. Bellows expansion
couplings take the differences in expansion between the outer casing and fixed
bearing housing. Hence HPT rotor & casing expands towards bearing no (1) while IPT
rotor expands towards generator. The LPT rotor expands towards generator. The
magnitude of this expansion is reduced by the amount by which the thrust bearing is
moved in the opposite direction due to IPT casing expansion.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
HP FRONT PEDESTAL
HP REAR PEDESTAL
LP FRONT PEDESTAL
LP REAR PEDESTAL
HPT OUTER CASING
IPT OUTER CASING
LPT OUTER CASING
HP FRONT PEDESTAL BASE PLATE
HP REAR PEDESTAL BASE PLATE
LP FRONT PEDESTAL ANCHOR POINT
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
LP REAR PEDESTAL ANCHOR POINT
LP OUTER CASING ANCHOR POINT
HPT INNER CASING
IPT INNER CASING
LP INNER OUTER CASING
LP INNER OUTER CASING
HP INNER CASING ANCHOR POINT
IP INNER CASING ANCHOR POINT
LP INNER –OUTER CASING ANCHOR POINT
LP INNER –INNER CASING ANCHOR POINT
TURBINE ANCHOR POINTS AND EXPANSIONS
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93
Front/rear housing of HPT can slide on base plates. Any lateral movements
perpendicular to machine axis are prevented by fitted keys. Bearing housings are
connected to HP-IP casings by guides, which ensure central position of casings while
axially expanding and moving. The LPT casing is located in centre area of
longitudinal beam by fitted keys cast in the foundation cross beams. Axial
movements are not restricted. The outer casing of LP turbine expands from its fixed
points towards generator. Bellows expansion couplings take the differences in
expansion between the outer casing and fixed bearing housing. Hence HPT rotor &
casing expands towards bearing no (1) while IPT rotor expands towards generator.
The LPT rotor expands towards generator. The magnitude of this expansion is
reduced by the amount by which the thrust bearing is moved in the opposite
direction due to IPT casing expansion.
Turbine Oil Supply
In the 200MW KWU turbines, single oil is used for lubrication of bearings, control oil
for governing and hydraulic turbine turning gear. During start-ups, auxiliary oil
pump (2 Nos.) supplies the control oil. Once the turbine speed crosses 90% of rated
speed, the main oil pump (MOP) takes over. It draws oil from main oil tank. The
lubricating oil passes through oil cooler (2 nos.) before can be supplied to the
bearing. Under emergency, a DC oil pump can supply lub oil. Before the turbine is
turned or barred, the Jacking Oil Pump (2 nos.) supplies high-pressure oil to jack-up
the TG shaft to prevent boundary lubrication in bearing. Refer to the figure.
TURBINE LUBRICATING OIL SYSTEM
The oil systems and related sub-loop controls (SLCs) can be started or stopped
automatically by means of SGC oil sub-group of automatic control system. The
various logics and SLCs under SGC oil are given in the ATRS section.
KORBA SIMULATOR
94
MAIN OIL PUMP
The main oil pump is situated in the front bearing pedestal and supplies the entire
turbine with lubricating oil and control oil, which is connected to the governing rack.
1.
2.
3.
4.
5.
6.
7.
8.
Threaded ring
Pump casing, upper
Journal Bearing
Oil pipe
Bearing bushing
Seal ring
Impeller
Feather key
9.
10.
11.
12.
13.
14.
15.
16.
Feather key
Journal + Thrust Brg
Ring
Vent pipe
Oil inlet vessel
Hyd. Speed Xter
Oil line
Turbine shaft
17.
18.
19.
20.
21.
22.
23.
Coupling
Elect. Speed Xter
Permanent Magnet
Pump shaft
Spacer sleeve
Pump casing, lower
Oil tube
TURBINE TURNING GEAR
The turbine is equipped with a hydraulic turning gear assembly comprising two rows
of moving blades mounted on the coupling between IP and LP rotors. The oil under
pressure supplied by the AOP strikes against the hydraulic turbine blades and
rotates the shaft at 110 rpm (220 rpm under full vacuum condition).
In addition, provisions for manual barring in the event of failure of hydraulic turning
gear, have also been made. A gear, machined of the turning gear wheel, engages with
a Ratchet & Pawl arrangement operated by a lever and bar attachment.
KORBA SIMULATOR
95
HYDRAULIC BARRING GEAR AND MECHANICAL BARRING GEAR
TURBINE GLAND SEALING
Turbine shaft glands are sealed with auxiliary steam supplied by an electro2
hydraulically controlled seal steam pressure control valve. A pressure of 0.01 Kg/cm
(g) is maintained in the seals. Above a load of 80 MW the turbine becomes selfsealing. The leak off steam from HPT/IPT glands is used for sealing LPT glands. The
steam pressure in the header is then maintained constant by means of a leak-off
control valve, which is also controlled by the same electro-hydraulic controller,
controlling seal steam pressure control valve. The last stage leak-off of all shaft seals
is sent to the gland steam cooler for regenerative feed heating. Refer the Figure.
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96
TURBINE SEAL STEAM SYSTEM
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97
TURBINE SPECIFICATIONS
Type: Three cylinders reheat condensing turbine having:
i. Single flow HP turbine with 25 reaction stages.
ii. Double flow IP turbine with 20 reaction stages per flow.
iii. Double flow LP turbines with 8 reaction stages per flow.
Rated Parameters
Nominal rating
: 210 MW
Peak loading (without HP heaters)
: 229 MW
Rated speed.
: 3000 RPM
Main steam flow at full load
(With HP heaters in service).
: 630 tons/hr.
Main steam pressure/ temperature at
full load.
: 147.1 kg/cm2. 535 oC.
HRH pressure/ temp at full load.
: 34.23 kg/cm2. 535 oC.
Permissible SH / RH temp variations.
o
: 543 C. (Long time value but keeping
within annual mean 535oC.)
: 549 oC. (400 hours per annum)
o
: 536 C. (80 hours per annum & max.
15 min in individual case)
: 76 mm Hg with CW inlet temp 33 oC.
Condenser pressure.
STEAM TEMPERATURE
80-hr/
annum
maximum.
per
15 min., in
individual
cases
oC
Rated value
Annual
mean value
Long
time
value keeping
400h
within annual
annum
mean value
oC
oC
oC
Initial steam
535
543
549
563
IPT SV Inlet
535
543
549
563
HPT exhaust
343
359
Extraction 6
343
359
KORBA SIMULATOR
500 + (special
425
case)
500 + (special
425
case)
98
Extraction 5
433
438
473
Extraction 4
316
326
366
Extraction 3
200
211
255
Extraction 2
107
127
167
Extraction 1
62
82
127
LPT exhaust
49
70
100
70
* Long-time operation: Upper limit value permissible without time limit
Valid only for the no-load period with high reheat pressure after trip-out from fullload operation. For the individual case approx. 15 min. Provision for this is that the
turbine is immediately reloaded or the boiler immediately reduced to minimum load
if no-load operation is maintained.
Permissible differential temperature - No time limitation
between parallel steam supply lines - Short time period
:
:
17 K.
28 K.
In the hottest line the limitations indicated for initial steam and reheat temperature
must not be exceeded.
Turbine Extractions (Pressure/ Temperature) at 200 MW
Extraction
Pres. (bar)
Temp.
1.
Extraction No. 6 (from HPT exhaust)
39.23
343
2.
Extraction No. 5 (from 11 the stage IPT)
16.75
433
3.
Extraction No. 4 (from IPT exhaust)
7.06
136
4.
Extraction No. 3 (from 3rd stage LPT)
2.37
200
5.
Extraction No. 2 (from 5th stage LPT)
0.858
107
6.
Extraction No. 1 (from 7th stage LPT)
0.216
62
KORBA SIMULATOR
0
C.
99
Alarm and Limiting Values of some Important Parameters
Parameters
Alarm value
Limit value
HPT Diff. Expansion.
+4.5 mm
+5.5 mm
- 2.5 mm
- 3.5 mm
+5.0 mm
+ 6.0 mm
-2.0 mm
- 3.0 mm
+25.0 mm
+30.0 mm
-5.0 mm
- 7.0 mm
HPT exhaust casing temperature
480 oC
500 oC
LPT outer casing metal temperature
90 oC
110 oC
IPT Diff. Expansion.
LPT Diff. Expansion.
Metal temp diff. between upper & lower casing
+/- 30 oC
(HPT front middle, IPT front, rear).
+/- 45 oC
Turbine Bearing Metal Temperature
76 oC
Maxm Oil Temperature before coolers
Whose normal operating temp is 75 oC
90 oC
120 oC
Whose normal operating temp is 85 oC
100 oC
120 oC
Turbine bearing housing vibration
35 microns
45 microns
Turbine absolute shaft vibration
30 microns
200 microns
Condenser vacuum (absolute)
120 mm Hg
200 mm Hg
Turbine axial shift
±0.3 mm
±0.6 mm
Turbine over speed
51.5 Hz
55.5 Hz
KORBA SIMULATOR
100
TURBINE GOVERNING SYSTEM
In order to maintain the synchronous speed under changing load/grid or steam
conditions, the KWU turbine supplied by BHEL at NTPC Korba is equipped with
electro-hydraulic governor; fully backed-up by a hydraulic governor. The measuring
and processing of electrical signal offer the advantages such as flexibility, dynamic
stability and simple representation of complicated functional systems. The
integration of electrical and hydraulic system is an excellent combination with
following advantages:
•
Exact load-frequency droop with high sensitivity.
•
Avoids over speeding of turbine during load throw offs.
•
Adjustment of droop in fine steps, even during on-load operation.
Elements of Governing System
The main elements of the governing system and the brief description of their
functions are as follows:
•
Remote trip solenoids (RTS).
•
Main trip valves (Turbine trip gear).
•
Starting and Load limit device.
•
Speeder Gear (Hydraulic Governor).
•
Aux. follow-up piston valves.
•
Hydraulic amplifier.
•
Follow-up piston valves.
•
Electro-Hydraulic Converter (EHC).
•
Sequence trimming device.
•
Solenoids for load shedding relay.
•
Test valve.
•
Extraction valve relay.
•
Oil shutoff valve.
•
Hydraulic protective devices.
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101
REMOTE TRIP SOLENOIDS (RTS)
The remote trip solenoid operated valves are two in number and form a part of
turbine protection circuit. During the normal operation of the turbine, these
solenoids remain de-energised. In this condition, the control oil from the governing
rack is free to pass through them to the main trip valves. The solenoids gets
energised whenever any electrical trip command is initiated or turbine is tripped
manually from local or UCB. Under energised condition the down stream oil supply
after the remote trip solenoids gets connected to drain and the upstream will be
blocked. By resetting Unit Trip Relays (UTR) from UCB, these solenoids can be reset.
Refer to Figure.
REMOTE TRIP SOLENOIDS
MAIN TRIP VALVES
The main trip valves (two in numbers) are the main trip gear of the turbine protective
circuit. All turbine tripping take place through these valves. The control oil from
remote trip solenoids is supplied to them.
Under normal conditions, this oil flows into two different circuits, called as the Trip
Oil and Auxiliary Trip Oil. The Trip Oil is supplied to the Stop Valves (of HP Turbine
and IP Turbine), Auxiliary Secondary Oil circuit and Secondary Oil circuits. The
Auxiliary Trip Oil flows in a closed loop formed by main trip valves and turbine
hydraulic protective devices (Over Speed trip device, Low Vacuum trip device and
Thrust Bearing trip device).
The construction of main trip valves is such that when aux. trip oil pressure is
adequate, it holds the valves' spools in open condition against the spring force.
Whenever control oil pressure drops or any of the hydraulic protective devices are
actuated, the main trip valves are tripped. Under tripped condition, trip oil pressure
is drained rapidly through the main valves; closing turbine stop and control valves.
Refer to the figure below.
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MAIN TRIP VALVES
STARTING AND LOAD LIMIT DEVICE
The starting and load limit device is used for resetting the turbine after tripping, for
opening the stop valves and releasing the control valves for opening. The starting
device consists of a pilot valve that can be operated either manually by means of a
hand wheel or by means of a motor from remote. It has got port connections with the
control oil, start-up oil and auxiliary start-up oil circuits. The starting device can
mechanically act upon the hydraulic governor bellows by means of a lever and link
arrangement.
Before start-up, the pilot valve is brought to its bottom limit position by reducing the
starting device to 0% position. This causes the hydraulic governor bellows to be
compressed thus blocking the build-up of secondary oil pressure. This is known as
control valve close position. With the valve in the bottom limit position (starting
device = 0%) control oil flows into the auxiliary start-up circuit (to reset trip gear and
protective devices) and into the start-up oil circuit (to reset turbine stop valves). A
build-up of oil pressure in these circuits can be observed, while bringing the starting
device to zero position. When the pilot valve i.e. the starting device position is raised,
the start-up oil and auxiliary start-up oil circuits are drained. This opens the stop
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valves; ESVs open at 42% and IVs open at 56% positions of the starting device.
Further raising of the starting device release hydraulic governor bellows which is in
equilibrium with hydraulic governor'
s spring tension and primary oil pressure
(turbine speed), and raises the aux. sec. oil pressure; closing the aux. follow-up
drains of hydraulic governor.
STARTING DEVICE ACTING ON SPEEDER GEAR
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SPEEDER GEAR
The speeder gear is an assembly of a bellow and a spring, the tension of which can
be adjusted manually from UCB by an electric motor or locally by a hand wheel. The
bellow compression depends upon the position of the starting device and the speeder
gear position, which alters the spring tension on the top of the bellow. The bellow is
also subjected to the primary oil pressure, which is the feedback signal for actual
turbine speed. The zero position of speeder gear corresponds to 2800 rpm i.e.
hydraulic governor comes into action after 2800 RPM. The bellow and spring
assembly is rigidly linked to the sleeves of the auxiliary follow-up piston valves. The
position of the sleeve changes with the equilibrium position of the bellow.
SPEEDER GEAR
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HYDRAULIC SPEED TRANSMITTER
The
hydraulic
speed
transmitter runs in the MOP
bearing and operates on the
principle of a centrifugal
pump. The variation of
pressure in the discharge line
is proportional to the square
of the machine speed. This
primary oil pressure acts as
the control impulse for the
hydraulic speed governor.
The transmitter is supplied
with control oil via an oil
reservoir. An annular groove
in the speed transmitter
ensures that its inside is
always covered with a thin
layer of oil to maintain a
uniform
initial
pressure.
Excess oil drains into the
bearing pedestal.
CURVE SHOWING TURBINE SPEED Vs PRIMARY OIL
PRESSURE
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AUXILIARY FOLLOW-UP PISTON VALVES
Two
Auxiliary
Follow-up
pistons are connected in
parallel and the trip oil is
supplied to them through
orifice. The sleeves of these
valves are attached to the
speeder gear bellow link. The
position
of
the
sleeve
determines the draining rate
of trip oil through the ports.
Accordingly the trip oil
pressure
downstream
of
these valves changes. Oil
downstream
of
auxiliary
follow-up pistons circuit is
termed
as
AUXILIARY
SECONDARY OIL. Hence,
aux. follow-up piston valves
can be said to control
auxiliary
secondary
oil
pressure.
SEQUENCE TRIMMING DEVICE
The function of the sequence trimming device or HP/IP TRIM DEVICE is to prevent
any excessive HP turbine exhaust temperature due to churning. It changes response
2
of main and reheat control valves. When the reheat pressure is more than 32 Kg/cm
and load less than 20% the IP turbine tends to get loaded more than HP turbine. The
steam flow through HP turbine tends to fall to very minimum, causing a lot of
churning and excessive exhaust temperature. The trim device operates at this
moment trimming the IP turbine control valve. The control valves of HPT open more
to maintain flow of steam, reducing the HPT exhaust temperature.
It consists of a spring-loaded piston assembly, which is supported by control oil
pressure from beneath, under normal conditions. The control oil is supplied via an
energised solenoid valve. When the turbine loads is less then 40 MW and hot reheat
2
pressure is more than 32 kg/cm the solenoid valve gets de-energised cutting out the
control oil supply to the trim device.
The trim device trips under spring pressure. The trim device is connected to the
follow-up piston valves of IP control valves by means of a lever. Upon tripping, the
trim device alters the spring tension of follow-up pistons of IP pistons control valves,
draining the secondary oil. The IP control valves openings are trimmed down.
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HYDRAULIC AMPLIFIER
Hydraulic Amplifier consists of a pilot valve and an amplifier piston. The position of
the pilot valve spool depends upon the aux. secondary oil pressure. Depending upon
the pilot spool position, the control oil is admitted either to the top or the bottom of
the amplifier piston. The other side of amplifier is connected to the drain. The
movements of the amplifier piston are transformed into rotation of a Camshaft
through a piston rod and a lever assembly. A feedback linkage mechanism stabilises
the system for one particular aux. secondary oil pressure.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Amplifier piston
Follow-up piston
Sleeve
Shaft
Lever
Feedback lever
Pilot valve
Compression spring
Adjusting screw
a : Control oil
b : Secondary oil
b1 : Aux. Sec oil
c : Return oil
HYDRAULIC AMPLIFIER
SOLENOIDS FOR LOAD SHEDDING RELAY
A pair of solenoid valves has been incorporated in the IP Sec oil line on control valves
and Aux Sec. oil line, in order to prevent the turbine from reaching high speed in the
event of sudden turbine load throw-off. The control valves are operated (closed) by
the load-shedding relay when the rate of load reduction exceeds a certain value. The
solenoid drains the IPCV secondary oil directly. Direct draining of IP Sec oil circuit
causes the reheat valves to close without any significant delay. The HP control valves
are closed due to draining of aux. secondary oil before the hydraulic amplifier, by the
second solenoid valve. The extraction stops valves controlled by IP secondary oil
acting through extraction valves relays also get closed. After an adjustable time delay
(approx. 2 seconds) the solenoid valves are re-closed and secondary oil pressure
corresponding to reduce load builds-up in the HP and IP turbine secondary oil lines.
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FOLLOW-UP PISTON VALVES
The trip oil is supplied to the follow up piston valves through orifices and flows in the
secondary oil piping to control valves. The secondary oil pressure depends upon
position of sleeves of follow-up piston valves; which determines the amount of
drainage of trip oil.
FOLLOW-UP PISTON VALVES
There are in all twelve follow-up piston valves. Six of them are associated with
hydraulic amplifier and six of them with EHC in the governing system.
The follow-up piston valves constitute a minimum value gate for both the governors.
This means the governor with lower reference set point, is effectively in control. This
is also termed as HYDRAULIC MINIMUM SELECTION of governors.
The drain port openings of follow-up pistons of hydraulic amplifier depends on
auxiliary secondary oil pressure, upstream of aux. follow-up pistons; and that of
electro hydraulic converter, on the piston of pilot spool valve of the elector-hydraulic
converter (i.e. EHC output).
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TEST VALVE
1. Bolt
2. Hand wheel
3. Spindle
4. Cover
5. Oil Seal
6. Bushing
7. O-ring
8. Valve Cover
9. Valve Body
10. Trip Oil
11. Piston sleeve
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
EXTRACTION N.R.VS AND
Trip Oil
Piston valve
Spring plate
Spring
Spacer
Bottom cover
Trip oil
Drain
Trip oil
Startup oil
Each of the HP and IP stop
valves' servomotors receives
trip
oil
through
their
associated test valves. The test
valves have got port openings
for trip oil as well as start-up
oil. The test valves facilitate
supply of trip oil pressure
beneath the servomotor disc.
(Stop valve open condition,
under normal operation). For
the purpose of resetting stop
valves after a tripping, startup oil pressure is supplied to
the associated test valves,
which moves their spool
downwards against the spring
force. In their bottom most
position the trip oil pressure
starts building up above the
stop valve servomotor piston
while the trip oil beneath the
disc gets connected to drain.
When start-up oil pressure is
reduced the test valve moves
up draining trip oil above the
servomotor
piston
and
building the trip oil pressure
below the disc, thus opening
the stop valve. A hand wheel
is also provided for manual
operation of test valves.
EXTRACTION VALVE RELAY
Four pair of swing check valves are provided in the extraction lines to the feed
heaters (LP Heaters No: 2,3, Deaerator and HPH No: 5) to prevent back flow of
condensed steam into the turbine from heaters on account of high levels in the
heaters. There are two NRVs provided in each of these extraction lines and is force
closing type. Both these valves are free-swinging check type, however the first valve is
equipped with an actuator. In case of flow reversals, both the valves are closed
automatically. The actuator assists the fast closing of the first valve.
The mechanical design of force-closed valves is such that they are brought into freeswinging position by means of trip oil. They are open as soon as differential pressure
is sufficient. If the trip oil pressure falls, the spring force closes the valve when steam
pressure either falls or is lowered (reduced load).
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The extraction valve relay, its changeover valve and its solenoid valve control the trip
oil to each of the actuators of force closing type valves. Extraction valve relay
actuates the FCNRVs in proportion to secondary oil pressure. By suitable adjustment
of its spring, the secondary oil pressure at which the FCNRVs will be released for
opening can be set. However, swing check FCNRVs will also open without the release
action, also if the steam pressure is more than the spring force. But in this case the
pressure loss shall be more leading to loss of efficiency.
In case of turbine trip or sudden load reduction, by energising the associated
solenoid valve, draining of trip oil pressure through extraction valve relay assists
closing movements of FCNRVs. In both the cases the actuator is devoid of trip oil and
its spring force closes the NRV. Extraction (4) FCNRV solenoid is also energised
additionally by lower differential pressure in the extraction line.
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b
:
Control Oil
c
:
Return Oil
:
Trip Oil
:
Trip Oil
b1
:
Secondary Oil
x
b2
:
Secondary Oil
x1
COLD REHEAT SWING CHECK VALVE
Two numbers of swing check valves are provided on the CRH lines from which the
steam is drawn for HPH-6. Their pilot valves via their rotary servomotor in proportion
to secondary oil pressure operate the CRH NRVs. They open out fully when main
control valves open up corresponding to 5-10% of maximum turbine out-put. Only
when the control valves are closed to this threshold again, the NRVs return into
steam flow by the hydraulic actuator, so that when the steam flow ceases in the
normal direction, they are closed by the torque of rotary servomotor. Even when the
pressure of secondary oil has not built up sufficiently, NRVs can be opened up like
safety valves when the upstream pressure rises above the downstream side pressure
by one bar.
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VACUUM BREAKER
The function of the vacuum breakers is to cause an increase in condenser pressure
by conducting atmospheric air into the condenser together with the steam flowing
from the LP Bypass. When the pressure in the condenser increases, the ventilation
of the turbine balding is increased, which causes the turboset to slow down so that
the running down time of the turboset and the time needed for passing through
critical speeds are shortened.
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HYDRAULIC AND ELECTRO-HYDRAULIC GOVERNING OF TURBINES
Power produced by any power plant is sent out on utility grid (Transmission line and
control equipments) together with power from other plants through process of
synchronization with the grid and to distribution systems and then to the consumer.
Control of system frequency on the grid or interconnected grid/pool is a major
responsibility of load dispatchers. When a Turbo-generator is connected to grid, the
speed of each machine in the grid remains same to all other machines connected to
the grid. When an increase of load is required, more steam is admitted by
opening/controlling the steam control valves. A basic understanding of turbine speed
governors is necessary to maintain the central control of system parameters like
speed, frequency, load, system voltages etc.
In the paragraphs that follow, the turbine governing has been explained using
theoretical information, figures and descriptions of governing systems.
All turbines are equipped with speed governors. The purpose of the governor is to
sense the instantaneous speed of the turbine in revolutions per minute, and to
transmit a signal to the turbine control valves to open or close and maintain the
desired speed. Most governors do not hold absolutely constant speed as load
changes, but are designed to permit the speed to drop as the load is increased. As
load is increased on the generator, the turbine speed tends to slow down. The speed
governor spins slower (control arm moves toward “LOW” position), which results in
the control mechanism in increasing steam flow to the turbine (control valve opens).
The governors therefore control the steam supply to the turbine as well as ensure
maximum safety of the machine and to the operating people when the turbine is on
load.
Basically, the governors perform functions such as: •
Parallel operation/working of machines with other turbine-generators
connected together in a grid.
•
Output of each individual unit is controllable due to governing actions.
•
The governor enables the electrical grid system to be to some extent selfcompensating to changes in load demand.
•
The governor enables the turbine-generators not connected together, in a
grid, run as single unit. (Before synchronisation), and also enables speed of
turbine, kept under control.
•
The governor controls the rise in speed of all turbines irrespective of duty,
in instances of losing its’ electrical loads.
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Turbine Governor System type-1
Governors of the turbines basically control the steam flow to the turbine. The
governor usually takes the form of spring-loaded weights mounted on a shaft
assembly that is driven by a worm & worm wheel from end of the H.P. shaft.
The weights, which are held by springs, tend to move outwards due to centrifugal
force and this movement is dependent upon the speed of the turbine shaft. The
movement of the weights is arranged to operate on oil relay valve and this valve
through an oil pressure relay system, opens or closes valves that admit steam to the
turbine. When an increase of load is required, more steam is admitted to the turbine
by opening the steam valves.
Simple turbine governor type-2
The governor (A) is driven from the turbine shaft. An arm pivoted at (B) has attached
to it, the governor weights and a moveable sleeve (C). Sleeve (C) is connected to a
floating lever (D) to which is attached the spindle (E) of the pilot relay valve and the
spindle (F) of the main steam valve.
If the turbine shaft speed increases, the governor weight will move outwards causing
sleeve C to lift; this also tilts floating lever (D). These movements uncover the port (G)
of the pilot valve thereby allowing oil pressure to act on the top of the power piston
(H). At the same time port (I) in the pilot valve, allows oil to drain from the bottom (J)
of the power piston. Due to this operation, the steam valve will move towards the
closed position, thus admitting less steam to the machine. During installation and
also afterwards, the governor springs are adjusted periodically, so as to keep the
range at which the governor operates between limits.
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Loading on the machine is done/carried out by operating the hand wheel (K) thus
opening the steam valve. The hand wheel (K) is normally on remote operation from
the control panel by means of a reversible motor known as the “speeder motor”. Such
governors do not use the electro-hydraulic governors, which control the operation by
electrical interfacing units i.e. the electro-hydraulic converter. For detailed working of
Governor, the drawing as shown below should be referred.
The percentage of control valve opening on each turbine depends upon the electrical
output from that individual T.G, and in turn the entire system at the same speed
(frequency). The system frequency decreases, as more electrical load is required. To
regain the previous frequency/speed, the amount of fuel fed to the steam generator is
increased adequately. Since with more customer load on the system, the frequency
tends to decrease then the governors on all the system turbines need to operate (to
open) the control valves to admit more steam to Turbine and allow the system to
supply the extra load.
Mechanical –Hydraulic System Block Diagram:
The speed acts on the radial spring governor, this in turn, affects the hydraulic relay
and also, the anticipatory derivative system (acceleration component). Local or
remote adjustment on the speeder gear output is algebraically summed to act with
the speed component, thus the gain that is also regulated by local adjustment of
governor reputation through the pilot oil regulating valve, passes through a
minimum selector that has been provided with another signal of locally/remotely
controlled load limiting device; minimum signal thus obtained from here is acted
upon the Auxiliary and main relays of governor valves of H.P and I.P control valves
and the pressure switching & relaying that effects to operate the release and bled
steam check valve. The feedback signal of S.V pressure, vacuum unloading gear and
anti-motoring device act on check valve and also for differential pressure switching (it
compares the minimum selector O/P as explained above); this forms the speeder gear
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116
runback as the feedback also. H.P and I.P control valves’ position are derived for
valve offset adjustments.
The figure below shows the block diagram of mechanical-hydraulic system.
The hydraulic oil used in the governor system is at a pressure up to 20 Bar. Better
control can be achieved by increasing this pressure (more than 35 Kg/cm2 pressure)
but this leads to leaks and fires. For this reason some turbines in use today utilize
the Fire Resistant Fluid (F.R.F) system and thus the pressures can be increased
without the risk of fires.
Turbine bearings are lubricated with oil at between 0.3 and l.4 bar pressure
depending upon the make and type of machine. A high-pressure oil pump normally
supplies this oil and then pressure of oil is reduced as above.
Emergency governors (often referred as the Over speed Governor): The emergency governor is the final line of defense to protect the turbine from
dangerous over speeds. This device, when actuated rapidly closes all valves
associated with steam supply to the turbine. Emergency governors are normally set
to operate instantaneously if turbine speed reaches 110% of rated (3300 rpm on a
two pole turbine generator) or higher speeds. The emergency governor shuts off the
steam supply in the event of rotor speed increasing by more than 10% above its
normal speed. A sliding bolt or an eccentric ring is attached to the shaft. These are
held in position by means of a retaining spring.
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The bolt or the ring flies out of the normal position .In doing so, it operates a trip and
releases the relay oil pressure, which is holding the emergency, valve open. The
emergency valve then shuts off the steam supply .
The emergency governor is tested at periods by deliberately over-speeding the
machine when the load has been taken off. Each of the twin bolts or rings is operated
in turn. The one not being tested is made inoperative by a selector lever.
Droop of Turbo-generators:
Speed regulations of turbine also called the Droop, (or the proportional band), is
defined as the amount of speed change from no load to full load divided by the rated
speed. Turbine Droop can be set in turbines either mechanically or electrically (In
KWU turbines the provision of droop is made to range from 2.5% to 8.0% and to
match the grid frequency, chosen setting is 5%). If the governor speed regulation is
required to be set at 5% then for a 3000 rpm turbine, the control valves will be open
wide at a speed of 2925 rpm or 2½ % below 3000 rpm. And likewise in other side of
50 Hz frequencies, the control valves will be fully closed, at a speed of 3075 rpm, or
2½ % above 3000 rpm. The droop setting in electronic system of EHG has been
incorporated in a module connected in series which receives input as the load
controller/comparator
forming
the
error
(MV-DV),
and
the
droop
corrected/incorporated signal is fed to the final load controller module of the load
control loop.
The amount of the inherent decrease in speed from no load to full load is called
speed regulation, droop, or proportional band. The Droop is necessary in the control
system in order to sense a change in speed and thus to reposition the valves. In KWU
turbine (of SSTPS droop is set at 5%, i.e. = ±2.5% from 3000 rpm, or 50 Hz
KORBA SIMULATOR
118
frequency), the droop is set such that a biased zone is maintained from 3000 r.p.m to
3075 rpm. Beyond this speed until 3225 rpm, the droop gets affected automatically
for unloading.
Most grids operate automatically, to sense a change in system frequency as load goes
up or down and to provide continuous signal to the controlled generating units in
order to maintain the desired 50 Hz system frequencies. If the cost of generation at
given moment on the grid is such that a load of 100 MW should be generated by that
unit, that is the load that the automatic control will attempt to maintain The
frequency bias of all controlling turbine generators on the grid is added up to
determine the system frequency bias.
In order to view the economical loading on the sets connected in parallel an example
of a single unit can be considered for understanding the cost controlled situation. If
the cost of generation at given moment on the grid is such that a load of 100MW
should be generated by that unit, that is the load that the automatic control will
attempt to maintain. The frequency bias of all controlling turbine generators on the
grid is added up, to determine the system frequency bias.
Our single unit example was being cost controlled to provide 100MW and it went to
104MW when system frequency dropped 1/10th of a cycle. With a 0 bias setting, as
soon as the load increased to 104MW, the cost control would close the control valves
to restore 100MW. At this point, the cost control is acting to oppose frequency
correction back to 50 Hz.
Further, let us review the frequency effects and the frequency bias on a particular
unit , if it has been set to 4 MW per 0.1 Hz deviations. As soon as the system
frequency drops to 49.9 Hz, the cost signal representing desired generation from this
unit changes from 100 MW to 104 MW, under the added influence of frequency bias.
If we can again assume that the turbine governor would again have picked up 4 MW,
no control action occur to reduce generation back to 100 MW and system frequency
should return to 50 Hz.
Of course, if no automatic load frequency control is being used, then the dispatcher
must manually direct an increase or decrease in generation from the units under his
control, in order to restore system frequency to 50 Hz. In this case, the dispatcher
“corrects” system frequency in order to provide the correct frequency on a 24-hour
basis. This is usually done fairly close to midnight of each day. Instrumentation will
advice him how far above or below 50 Hz the system has been operating for the past
24 hours. Knowing his system frequency bias, the dispatcher can then order more or
less load to be generated for a given period in order to restore system frequency to an
average of 50 Hz for the past 24 hours. This phenomenon is particularly important
for controlling system frequency specially in view of controlling power generation with
ABT.
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Transient speed rise (TSR):
When load rejection takes place,
8-
settling down to steady state value
6-
TSR gives the % speed rise on full
4-
load throw-off
2-
TSR
speed shoots up temporarily before
O-
Steady state
Time
Steady State Regulation:
nmax.
It is defined as the Ratio of % speed
nmin.
change (from no load to Full load) to
the nominal rated speed.
%Regulation=100x(nmax–
min)/nnom
0%
Load
100%
Load Frequency Control is shown in the figure below; it shows the single turbogenerator system supplying an isolated load. Main component are;
1. Fly ball Speed governor system
2. Hydraulic Amplifier
3. Linkage Mechanism
4. Speed changer
Increase in frequency f causes the fly balls to move outwards so that B moves
downwards by a proportional amount k2’ f. The net movement of C is therefore yC
= k1 kC PC + k2 f and movement D, yD= k3 yC + k4 yE. The movement
yD depending upon its sign opens one of the ports of the pilot valve admitting highpressure oil into the cylinder thus moving the main piston & opening the steam valve
by yE.
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In KWU turbines, the stop valve & control valve (one set) share a common body. The
piston of the servomotor is subjected to disc spring force in the close direction and
Hydraulic pressure in the opening direction. Hydraulic Governor controls the steam
supply by operating the control valves. The fluid pressure under the piston
determines the position of the valve; this is controlled by pilot valve of the turbine
governor & the secondary fluid oil system.
Electro-Hydraulic Governor (EHG)
Electro-Hydraulic Governor (EHG) works in parallel with Hydraulic governor at all
times of requirements. Basically the Electro-Hydraulic Converter (EHC) is the
connecting element between the electrical and hydraulic parts of the turbine
governing control system for carrying out the Electro-Hydraulic Governing of the
turbine.
The Electro-Hydraulic Governor (EHG) is beneficial in:•
•
•
•
Offering the flexibility, dynamic stability, dependability, excellent
operational reliability, Low transients and steady-state speed deviations at
all instances.
Maintaining exact load frequency droop with high sensitivity.
Providing reliable operation at times of grid isolation conditions.
Operating the turbo-generator Safely in conjunction with TSE.
In KWU turbines, Electro-Hydraulic Governing has been achieved through various
electronic / selector modules configured in four modes of controls:
KORBA SIMULATOR
121
•
•
•
•
Admission Control mode,
Speed Control mode,
Load Control mode
Pressure Control mode.
The Hydraulic governor and the EHG system have been designed such that the
governor with lower set point takes over or assumes the system control, as such
normally, the set point of the Hydraulic Governor must be set above that of the
Electro-Hydraulic Governor when EHG is effective. In cases, when EHG fails to cause
shut-off, the set point that is, affected is that of Hydraulic Governor.
In such situations the Tracking Device provides a revised set point of 5-10% above
the EHG set point and it causes increase in small load when the control is
transferred to Hydraulic-Governor. The tracking device is either switched on or off
manually but when EHG failure or turbine trip occurs, the tracking device is
switched off automatically thus tracking under faulted operation mode is prevented
or prohibited. More details on tracking actions are covered in the follow-up circuits of
the speed/load control modes.
Electro Hydraulic Converter details:
Electro Hydraulic Converter converts the electrical signal in to the hydraulic signals
and large positioning forces are generated in control valves. The electrical signal from
governor control circuit operates the sleeve and pilot valve spool; this regulates the
trip fluid drain. Under steady state condition pilot is at central position; in deflected
position, the control oil is admitted above or below the amplifier piston. The motion
of the amplifier piston is transmitted via a lever to a camshaft, which actuates the
sleeves of follow-up piston valves, causing secondary oil pressure to change. The
speed, load, and pressure signals are measured and converted into conditioned
signal in electronic modules.
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Admission Valve (spool) Controller
Admission Valve (spool) Controller also referred as the position controller is Common
for all three modes of EHG, and it supplies the operating current for driving the
plunger coil. The Position controller loop uses a PID control mode for processing
outputs that provide the driving current signal to the plunger and regulate the oil
drains of HP/IP control valves (CV) ; thereby it controls steam supply into the
turbine.
The current in the plunger coil is increased for closing the HP /IP CV and vice versa
for opening of the HP /IP Control Valve. The reference signal therefore works in
reverse manner (rise in the coil current for low reference condition). By using two Nos
of differential transformer (housed in EHC), feedback signal from the valve lift is
derived to ensure proper stationing of plunger spool.
Whenever current through the plunger coil gets interrupted or the electrical feedback
circuit gets faulted, the reference value of the Hydraulic controller determines the
actual valve position. Although the force to the plunger coil and to the control sleeve
is, considerably smaller, but the regulating signal to the secondary auxiliary oil flow
as transformed is quite large. The figure below gives various connections and
modules used in EHG.
Control Transfer of various controllers:
Three selectors have been used for specific functioning Speed controller output (hrnc)
and the load controller output (hrpc) are passed through a Maximum selector (MAXKORBA SIMULATOR
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1) and the selected signal passes to a minimum selector (MIN-1) in such a fashion
that at times of over-speeding of turbine (during load throw-off situations), the input
to the minimum selector: MIN–1: takes care of transient condition of the load throwoff and is sufficient to check the turbine from over speeding. (During sudden load
throw-off, over speeding of turbine is effected and since 10.5 V is generated by a
potentiometer that gets algebraically summated with hrnc then it outputs voltage
which is less than that of the speed/load signal as selected from the MAX-1:)
The signal from the Minimum selector: MIN–1: passes through another Minimum
selector: MIN–2: that receives the Pressure Controller output (hrPrc) signal as
explained in pressure controller loop. Finally through the last minimum selector:
MIN–2:, the control signal connects the Admission Valve (spool) Controller loop which
outputs the driving current for the EHC plunger coil.
Operation of EHG in various modes
Start-up
Switching the supplies ON and setting the speed/load setter to zero puts the EHG in
Operational condition. The hydraulic speed control eqpt and the start up eqpt of the
hydraulic controller are set in upper end position. The actual speed is sensed since
turbine already is in barring gear and by slow rising of speed reference the speed
controller works /is in service; the turbine speed is then brought up situation for
synchronising TG with grid using speed controller.
Operation under load
Load controller can be taken in service after turbine is synchronised to control load
in quick response and high linearity either as per LDC/AFDC or using various
modes/sub loops explained in Load control. Frequency change is selected via the
integral action load controller to corresponding droop values and a sensitivity of 5
Milli-Hz is obtained which meets the operational requirements of the present day
large grid. The output signal of the speed controllers is automatically matched to the
output signal of load controller from rated power on down to station load. The speed
controller then remains in standby mode only and stands ready to provide station
load in of load shading.
Shutdown
During normal shut down operation, the load controller is set to zero value. After the
speed controller has assumed control of TG set, the unit can be disconnected from
the grid.
Load shedding
In case of load shedding i.e. sudden separation of the generator, from the grid, the
output signal of the load controller is immediately reduced to value below that of
speed controller. Consequently due to minimum selection, the speed controller
assumes control and returns turbine back to the set reference speed.
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This reference speed practically coincides with the rated speed, since the speed
controller is set to provide the station load during the start of operation under load.
This provision improves the dynamic response of the closing of the main steam stop
valve /control valve and keeps the turbo set speed from rising along the droop
characteristic. An additional effect is the reduction of the speed oscillations. In case
of automatic reclosing of the generator CB, the reduction of the load controller
output signal below the speed controller output signal below the speed controller
output signal is cancelled and the initially selected load level restored.
Speed Control Mode
Speed Control Mode works during
•
•
•
•
•
•
Rolling (start-up or shut-down of the turbine),
Speeding up of turbine until synchronisation,
For effecting block loading & full loading of TG set at exceptional emergency
situations House-loading operation during fast load throw-off
For Controlling the TG set during rapid/large frequency fluctuations.
Regulating during Over-speeding;(When the speed of the TG set rises slightly
above synchronous speed, the control action in speed control mode quickly
reduces the turbine speed very close to synchronous speed)
During load shedding with subsequent operation of the TG set in an isolated
grid situation, (The speed controller assumes continuous TG set control in
such situations)
Speed reference signal (nR) is varied (In the range of 0-3600 rpm):
•
•
•
Manually by Raise/Lower push buttons (using motorized potentiometer,
By the synchronizer (when selected) or
By follow up signal (explained separately). The speed reference (nR) can not be
raised when follow-up condition exists and dn/dt is less than monitoring (in
this situation lowering of nR gets slowed down.
The reference nR is varied in the range of 0-3600 rpm and for minute operation
during synchronizing, above the speed of 2800 rpm; a reducing gear lowers the speed
of the motorized potentiometer to ¼ rate for exact speed adjustment. The speed
reference (nR) cannot be raised when follow-up condition exists and dn/dt is less than
monitoring rather in this condition raising or lowering phenomena of nR gets slowed
down when the speed reference (nR) is less than 2800. Two indicators have been
provided in UCB panel for monitoring speeds; of narrow range (2700-3300) and wide
range (0-3600).
The Time-dependent speed reference signal ( nRTD )
The Time-dependent speed reference signal (nRTD) also referred as nR lim. influences
the speed reference nR considerably. During start-up of turbine this nRTD allows
rising of the turbine speed at the highest permissible rate consistent with the
conservative operation as decided by the TSE computed margin signal introduced
between a DC amplifiers. The Integrator module performs this function rising with
time like a ramp. The slope of the integrator ramp can be adjusted over a wide range
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and is optimized during commissioning. Fast mode or the stop action facility, modify
the final nR .The output nRTD of the integrator module, is transmitted to the speed
controller and displayed on the desk in the range 0-3600 rpm.
Quantum of Follow-up signal is the difference between actual speed (nact) and offset
of 120 rpm and is effected (switched automatically) if load controller is operative,
final load reference (hrpc) is more than final speed reference (hrnc) by 10% and
frequency is between 49-51 Hz OR if turbine is tripped (time elapsed) and speed
reference (nr) is equal to actual speed (nact) minus 60 rpm. During follow up, the
quantum of the follow-up signal is derived from the actual less the off-set (60-120
rpm) speed reference (nR) and difference is further added or subtracted as per the
magnitude to cause change in speed reference (nR ).
‘Blocking ‘ or the ‘Stop nRTD ‘ of the speed signal is generated by an AND module in
conditions i) speed >2850rpm, ii) nR is more than nRTD by 300 rpm and iii) an OR ed
output of many conditions as given below:1. TSE influence gets faulted (goes out of order or switched off) or EHC fault
condition appears AND turbine speed is more than 2950rpm. .
2. During the transition of control from electric to Hydraulic, the speed reference
signal becomes less than actual speed and if is more than 50 rpm, i.e. (nR nact ) < 50 rpm.;
3. If nR > nRTD; pressure controller is in action OR Generator breaker not ON.
This Block signal stops the integration (further) function of time dependent speed
reference integrator, it blocks the already generated nRTD , and thus the speed
controller input signal remains stay-put during stop action.
During rolling of the turbine, if between the speeds of 600-2820 rpm the rate of
speed rise is very low i.e. less than 100 rpm per minute, then the dn/dt is less than
monitoring signal appears to alarm the operator; it also blocks any further rise in
speed and brings back the speed reference to 600 rpm. dn/dt less than monitoring
alarms the operator and takes care of low acceleration rate in turbine during rolling
by suitable output from the speed reference setting module, and at the critical
speeds (between 600-2829 rpm) of the turbine.
The dn/dt is less than monitoring is derived from an AND gate module, its conditions
are i) nR is more than 600 rpm, ii) nact is less than 2850 rpm, iii) MSV is open (>0%),
vi) speed controller is selected & in action, v) Generator breaker is not on and a
feedback signal of dn/dt <108 rpm per rpm (0.03 sec –2).
Refer block diag. for details.
Actual speed (nact) Measurement:
Actual speed nact is acquired by three digital speed pickups (Hall probes) in the form
of pulses /frequency. Channel-2 is utilized while other two-channel pick-up remains
redundant; electronically switching ensures no affect in channel in service and also a
full - proof monitoring. The selected sensed speed channel signal is further divided
into three measuring signals (f/v of 0-60 Hz, low range 0-6 Hz & full range 0-60 Hz
and a quartz frequency standard) for various other applications in the EHG and
other circuits.
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The difference of actual speed and time dependent speed signals (nact - ‘nRTD) form
the input error of the Speed controller which outputs control signal (in the path as
explained in selection section) through the selection modules for driving the EHC and
finally establishing the EHG.
ACTUAL SPEED MEASUREMENT / FORMATION
The speed controller is poportional+derivative (P-D) action controller, with sloping
characteristics. During steady-state condition, the speed controller outputs for better
load sharing by more nos. of turbo-generators connected in the grid as compared
with purely mechanical and Hydraulic Governor run T-G sets. Due to proportional
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control, a control error (off-set) is obvious in the speed reference but it does not
matter much. An identical speed at synchronising point is possible to be achieved
due to a Pre-feed function of pressure.
No load correction of speed is achieved by a feed forward signal that is obtained from
Boiler pressure controller during synchronising the T.G. set.
Load Control Mode
Load reference value pR is generated by means of a reference value setter module as
described in speed control mode and is derived manually by the operator adjusted
(lower/raise) values by means of a remote driven motorized potentiometer, Load
controller is switched ON for bringing load controller in service., it can also be varied
by the Automatic dispatch control (When switched ON ) ; it is also termed as pR ADS
,and is basically the MW demand generated by the Coordinated Master Control
loop(CMC). The load demand signal is restricted within upper and lower MW loading
limits as detailed in CMC loop description.
ADC influence ‘ON’ appears when there is NO ADC fault if is selected. In case if, CMC
or ADC gets faulted, it is automatically switched OFF; at this situation, matching or
the follow up is automatically taking place and loading of the TG set is subsequently
made in standby basis.
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Load reference pR as obtained from the potentiometer in voltage signal, is fed to the
high gain amplifier whose gain is adjusted by the rate of loading dp/dt or the load
margins (In order to load the turbo-generator at highest possible rate consistent to
permissible level of thermal stress, the upper/lower load margins are computed by
TSE, explained separately.). The Load gradient (load rate dp/dt.) is selected either by
On/Off push button or by follow-up command, for inclusion and it either modifies
the high gain amplifier slope in both negative and positive sides of the amplifier or
the TSE computed margins as explained above modify the high gain amplifier
through minimum selector. Upper release margin can result reduction of generated
power and lower release margin can result unloading.
Time dependent reference signal also referred as ‘pRLIM’’:
The Time dependent reference signal also referred as ‘pRLIM’’ is generated through a
high gain amplifier and an integrator functioning in fast, normal and stop modes. It
follows the ramp characteristic. The proportional leg of the response of the pRLIM can
be adjusted between 0-20% of MCR power of the TG Set. The response of the pRLIM is
purely integral, if the rate of rise of the pRLIM is limited to the load gradient selected,
and at this situation the proportional channel is switched off.
This pRLIM rises during start-up at a rate (Mw/min ) selected through load gradient
setter until final value. The pRLIM module is continuously allowing matching of the
actual power output as long as the generator breaker is open (not synchronized) and
ensures smooth transition of speed (during start-up) to load controller (after
synchronization).
The characteristic of pRLIM is linear 0-10v rising in 04 minutes. And it (pRLIM) acts
directly on the load controller without any intermediate control device. At conditions
when TG is not synchronised, power error (Pr-Pact) signal governs the follow-up
/tracking as explained below.
Tracking or the follow-up conditions in the load controller:
When the generator C.B. is on but the speed controller has taken over (due to
conditions of follow-up) and speed controller remains in action until load controller
signal (pr) < (nr) of speed controller; then tracking gets released as soon as (pr) = (nr)
and
When load shedding is less than station auxiliary power (p act < station load) and
Mw error reaches to more than 5 % or the generator circuit breaker is not made on;
the load controller output, tracks to speed signal.
Stop signal in load control:
A ‘Blocking’ or STOP command gets initiated at conditions shown below then the
integrator stops further integrating and pRLIM (the load demand) remains steady until
the blocking signals are cleared or restored. The block conditions are met at
conditions as given below: •
TSE switch is ON (selected) and it goes out of order (got faulted); it is enabled
again after the TSE is reset and it becomes O.k..
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•
The load reference signal has been raised and the pressure controller
goes/switches to limit pressure mode of operation.
•
The STOP set point binary condition is/gets introduced.
A highly and sensitive linear response with respect to power grid frequency is effected
by having the additional load reference component pR∆f (this can be set as low as
possibly up to 5 milli Hz); it can be included for operating the load controller with
frequency influence included in the system.
This frequency influence was being excluded in the system sometimes in nineties,
because the units were been operated at very high differentials of frequencies when
frequency used to rise to approx 52 Hz at off-peak hours (in night hours) or
reduce/decline as low as 47.5 Hz at peak load hours. But now due to insistence by
Load Dispatch Center (LDC) to regulate the grid frequency at very tight margin and in
order to run systems on ABT mode, the frequency influence inclusion have become
mandatory. This is being referred as FGMO operation of units.
The load reference thus derived is fed to a minimum selector, which also is fed with
the load limiter output. In order to restrict loading, Load limiter is preselected; the
Load limit value (due to plant conditions) can be adjusted by the load limiter
potentiometers and can also be seen on control desk. Even the reduction in grid
frequency cannot cause the TG set to exceed the preset power level due to load
limiting.
The output signal from the load limiter at the minimum selector in the form of the
pR , that is the sum of all reference values acting on the load controller as reference
signal or the desired value.
Actual Load signal is acquired threefold by means of the load measuring device and
transmitted to the controller comparator module but in case the signals of the three
parallel cannels deviate by more than 5% an alarm ‘ACTUAL LOAD SIGNAL FAULTY’
along with group alarm of ‘Turbine Controller Faulty’ appears.
The difference of the actual measured power signal (pact) and the pR form the input
of the load controller that outputs control signal and passes through a selection
module for driving the EHC as explained in the admission control and selection
diagram.
Load Controller consists of two plug-in modules first one to accommodate isolated
grid detection and the second to accommodate dynamic loading of the generators &
to housing the tracking module. Load Controller is a proportional (P) + integral (I)
controller to take care of small changes of load in Proportional mode and large
changes in Integral mode operation. Due to this addition, the response of the
controller is proportional for small changes of the load reference value but for the
large changes of reference value proportional plus integral mode refines the system
operation.
In order to effect smooth transition from speed controller to Load Controller
(Generator breaker open condition i.e. turbine not synchronised with grid) pR is
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compared continuously with pact and control signal is matched ensuring bump less
switching.
During the time the speed controller is in control for start-up, shut-down or no load
operation of TG set, the starting time constant of the TG set remains in dominating
whereas during synchronised operation, the transfer functions of generator and the
power grid become of vital importance for controller optimisation.
Pressure Control Mode
The pressure controller controls the turbine load with respect to the main steam
throttle pressure and prevents the large pressure drop during fast loading (Quick
load increase). The actual steam throttle pressure is measured in turbine area and
pressure reference is derived from CMC loop, after comparison the deviated control
signal (hRprc) is fed to the Proportional +Integral (PI) action type Pressure controller
and its final output is fed to the minimum selector-2 as described earlier in speed
controller and load controller loops.
The Pressure Controller functions in two modes of operation:
• Initial pressure mode
• Limit pressure mode
Initial Pressure Mode:
In Initial pressure mode of operation, constant initial pressure (turbine inlet throttle
pressure) is maintained and acts in proportional to pressure setting by minimizing
the pressure error (Actual-Ref) even up to zero value. The power delivered by the TG
set is determined by the boiler capability up to a maximum of power level as set by
load controller; increase of load above this is blocked thus, because it is connected to
a minimum selector. The difference pressure ∆p between the reference and actual
value, is controlled up to a value of 10 Kg/cm2 which is equal to the pressure drop of
steam flow from boiler to the turbine control valve; it therefore ensures natural
differential pressure of the steam flowing from boiler to the turbine. A preset
potentiometer equivalent to this pressure generates negative voltage to the controller
input and it biases the pressure differential ∆p thus in the controller.
Limit pressure mode:
Limit pressure mode uses the boiler storage capacity and is effected either by push
button or gets automatically selected as soon as the pressure deviates to 10 Kg/cm2
from normal running pressure to operate the controller in Limit pressure mode. This
deviation of 10 Kg/cm2 pressure signal already subtracted in the in the input of
Pressure controller, as described in the initial pressure mode, gets neutralized by
automatic switching.
Introducing the Limit Pressure Operation is therefore possible to regulate boiler
pressure beyond a pre-set pressure of main steam and load in small or quick
variations, and is controlled until pre-set pressure is reached that is not possible in
normal frequency based load control.
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In fact the normal ∆p from boiler to turbine (as explained in initial pressure mode) is
not persisting either due to increase in pressure at turbine side due to load throw,
vacuum drop, extractions’ closure etc or due to pressure drop in boiler side by nonavailability/reduced loading of mills, of coal feeders or any other causes. At this
situation due to equating/reducing of differential pressure ∆p , an alarm is generated
so as to warn/alarm the operator of the discrepancy. When Limit pressure engaged
alarm appears, the stop signal in the load control loop is also generated for blocking
the pRLIM signal from increasing/reducing.
All the three controllers are operative in such a manner that the governing of T.G is
ensured full proof and speeding or loading of the T.G. is best maintained as per the
pressure in the system and Boiler or turbine follow mode is achieved with full
reliability and safety.
Co-ordinated Master Control (CMC) ensures co-ordination between Boiler & Turbine.
The Co-ordinated Master Control (500 Mw) block diagram has been given below, we
find that the Unit master receives load demand signal from load dispatcher (ALC). A
GNI computer/SPCM is provided with the system to decide target value Z0, Run
Back load limits & load rate required for proper generation, Boiler master controller,
Turbine load set-point etc., through which the CMC is ensured. The load demand
signal as generated in CMC, for turbine control reaches to point ‘D’ of EHG block
diagram (refer the load control mode) and is switched for inclusion to operate the
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EHG in coordination with grid dispatch ADC demand. Boiler Follow or Turbine follow
modes are decided by switching suitably and loaded TG operation is achieved as
explained in details of CMC mode of integrated control in C&I , ACS section.
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TURBINE PROTECTION SYSTEM
Turbine protection system performs to cover the following functions: a. Protection of turbine from inadmissible operating conditions.
b. In case of plant failures, protection against subsequent damages.
c. It restricts occurring failures to minimum.
Standard turbine protection system comprises the following:
Mechanical/hydraulic turbine protection.
Electrical turbine protections.
BLOCK DIAGRAM OF TURBINE PROTECTION AND ATT
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Mechanical Hydraulic Turbine Protection
The design of mechanical hydraulic protection equipment is in accordance
with hydraulic break current principle and consists of following:
a. Two manual trip devices (main trip valves)
b. Two speed monitors (over speed trip device)
c. One hydraulic low vacuum device
d. Two solenoid valves for trip initiation (remote solenoid valves)
As explained earlier, turbine stop and control valves are tripped to close position if
the trip oil pressure is reduced below the minimum value. The main trip valves allow
rapid draining of trip oil in case they are operated either manually or automatically
by the reduction of aux. trip oil pressure. Aux. trip oil pressure can be drained
because of actuation of hydraulic low vacuum trip device, over speed trip device or
thrust bearing trip device. The principle of functioning of individual hydraulic trip
devices is explained in details under the chapter of Automatic Turbine Testing
System.
Remote trip solenoids act as interfaces between mechanical hydraulic and electrohydraulic protection equipment of turbine. Upon receiving the electrical trip
command, the solenoids get energised and close the valves. Thus control oil supply
to main trip values is cut off leading to their closure.
Electrical Hydraulic Turbine Protection
Electrical turbine trip equipments comprise two-channel redundancy and function
on operating current principle. All electrical trip criteria act on the two remote trip
solenoid valves to energise the solenoids.
The electro-hydraulic turbine protection equipment features - Two solenoid operated valves for trip initiation (Remote trip solenoids).
- Emergency trip contactor cabinet containing trip channels 1 and 2
- Monitors with signal conditioning
- One substitute channel to ensure uninterrupted transmission of eventual
turbine trip signals during testing by ATT.
The remote trip solenoids (RTS) have already been described. Operation of any one
channel causes energising both solenoid-operated valves leading to turbine trip
eventually. Transmitters that cause a trip in the case of any electrical tripping signal
are conditioned and monitored via binary signal conditioning of the ATT system or
via the central analog/binary signal conditioning.
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TURBINE PROTECTION FOR 200MW KWU SETS
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Turbine Trip Actuation Circuits
The turbine protection system is sub divided into two parts:
a. Protective circuits for the standard turbine protection equipments or criteria.
b. Protective criteria from other areas.
Standard criteria are specified by the turbine manufacturer and are responsible for
full protection of turbine under various specific conditions, which are:
1.
Manual tripping devices (Turbine trip gear local operating lever)
2.
Speed monitors (over speed trip devices)
3.
Thrust bearing trip device
4.
Hydraulic low vacuum trip device
5.
Electrical low vacuum trip device
6.
Lub oil pressure protection
7.
Fire protection
8.
Manual turbine tripping (electrical UCB switch)
Protection criteria from other areas are as follows:
•
Boiler trip (MFR)
•
Boiler drum level very high ( > + 225 mm wcl )
•
Main steam temperature trip ( < 480 o C )
•
Trip from functional group control (ATRS shut-down programme)
•
Generator trip
Like low vacuum tripping (electrical) the low steam temperature protection also
comprises '
Arming'and '
Disarming'features to facilitate re-start of turbine, under
low main steam temperature conditions.
Over Speed Trip Device
Two hydraulically operated over speed trips are provided to protect the turbine
against over speeding in the event of load coincident with failure of speed governor.
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OVER SPEED TRIP DEVICE
1.
2.
3.
4.
5.
Bearing pedestal
Spindle
Spring
Piston
Piston body
6.
7.
8.
9.
10.
Spring
Pawl
Over speed trip bolt
Shaft journal
Limit switch
c: Return Oil
u: Auxiliary Stratup Oil
x: Auxiliary Trip Oil
When the preset over speed is reached, the eccentric fly bolt activates the piston and
limit switch via a pawl. This connects the auxiliary trip oil to drain thereby
depressurising it. The loss of auxiliary trip medium pressure causes the main trip
valve to drop, which in turn causes the trip oil pressure to collapse.
Low Vacuum Trip Device
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In the hydraulic low vacuum trip device, a compression spring set to a specific
tension pushes downwards against diaphragm, the topside of which is subject to the
vacuum. If the vacuum is too weak to counteract the spring tension, the spring
moves valve 6 downwards. The pressure beneath valve is thereby dispersed and the
auxiliary trip medium circuit is connected to drain. The resultant depressurisation of
the auxiliary trip oil actuates main trip valves MAX51 AA 005 and MAX51 AA 006
thereby closing all turbine valves.
The electrical tripping on low vacuum occurs through a pressure switch on the
vacuum line to mechanical hydraulic low vacuum trip device also at the same
condenser pressure. When turbine is started up again, this pressure switch is
interlocked against a second pressure switch, which monitors this condition and
prevents continuation of tripping initiation when condenser pressure is high.
Thrust Bearing Trip Device
The function of the thrust bearing trip is to monitor the shaft position in the bearing
pedestal and, if a fault occurs, to depressurize the auxiliary trip medium and thus
the trip oil in the shortest possible time, thereby tripping the turbine.
1. Compression
spring
2. Bearing pedestal
3. Piston
4. Valve body
5. Turbine shaft
6. Pawl
7. Torsion spring
8. Piston
9. Compression
spring
10. Limit switch
11. Knob
a:
c:
u:
x:
Test Oil
Return Oil
Aux. Startup Oil
Aux. Trip Oil
The two rows of tripping cams, which are arranged on opposite sides of turbine shaft,
have a specific clearance, equivalent to the permissible shaft displacement, relative to
pawl of the thrust-bearing trip. If the axial displacement of the shaft exceeds the
permissible limit, the cams engage pawl, which releases a piston to depressurise the
auxiliary trip oil and at the same time to actuate limit switch.
Electrical tripping of turbine is achieved by fire protection along with
closure/stoppage of total control oil supply to turbine governing system by tripping
the emergency stop valve on the control oil line. The fire protection trip is achieved by
manual Pushbutton in UCB or automatically by very low MOT level (- 150 mm below
the normal working level '
O'
). Please refer to the associated logics at the end of this
chapter. Also fire protection-1 (automatic actuation) gets bypassed if the barring gear
valve is '
not closed'
.
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FIRE PROTECTION-1 CHANNEL-1
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FIRE PROTECTION-1 CHANNEL-2
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FIRE PROTECTION-2 CHANNEL-1
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FIRE PROTECTION-2 CHANNEL-2
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FIRE PROTECTION OIL TANK LEVEL MONITOR
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AUTOMATIC TURBINE TESTING (ATT)
INTRODUCTION
Under the present crunch of power crisis, the economy dictates long intervals
between turbine overhauls and less frequent shutdowns. This warrants testing of
equipments and protection devices at regular intervals, during normal operation.
The steam stop valves and control valves along with all the protective devices on the
turbine must be always maintained in serviceable condition for the safety and
reliability. The stop and control valves can be tested manually from the location but
this test does not cover all components involved in a tripping. Also, manual testing
always poses a risk of mal-operation on the part of the operator, which might result
in loss of generation or damage to machine components.
These disadvantages are fully avoided with the Automatic Turbine Test.
A fully automatic sequence for testing all the safety devices has been incorporated
which ensures that the testing does not cause any unintentional shutdown and also
provides full protection to turbine during testing.
SALIENT FEATURES
The Automatic Turbine Tester is distinguishable by following features:
Individual testing of each protective device and stop/control valve assembly.
Automatic functional protective substitute devices that protect turbine during
ATT.
Only its pretest is carried out without any faults i.e. if the substitute circuit is
healthy, the main test begins.
Monitoring of all programme steps for execution within a predefined time.
Interruption if the running time of any programme step is exceeded or if tripping
is initiated.
Automatic re-setting of test programme after a fault
Full protection of turbine provided by special test safety devices.
Automatic Turbine Testing extends into trip oil piping network where total reduction
of trip oil pressure due to actuation of any protective device, is the criteria for the
satisfactory functioning of devices.
During testing, general alarm or the cause of tripping is also initiated so that this
part of alarm annunciation system also gets tested. Also, during testing, two
electrically formed values of 3300 rpm take over protection of turbine against over
speed.
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The testing system or ATT is sub divided in two functional sub-groups.
Each sub-group contains the device and all associated transmission elements for
initiation of a trip.
AUTOMATIC TESTING OF PROTECTIVE DEVICES
ATT sub group for protective devices covers the following devices.
1. Remote trip solenoid-1.
2. Remote trip solenoid-2.
3. Over speed trip device.
4. Hydraulic low vacuum trip device.
5. Thrust bearing trip device.
During normal operation, protective devices act on the stop/control valves via the
main trip valves. Whenever any tripping condition (hydraulic/electrical) occurs, the
protective device concerned is actuated. It drains the control/aux. trip oil, closing the
main trip valves. The closure of main Trip Gear drains the trip oil, causing
stop/control valves to close.
During testing, trip oil circuit is isolated and changed over to control oil by means of
test solenoid valves and the changeover valve. This control oil in trip circuit prevents
any actual tripping of the machine. However, all alarm/annunciation are activates as
in case of an actual tripping. Refer Fig.
ATT for protective devices broadly incorporates the following sub programmes:
a. Preliminary test programme.
b. Hydraulic test circuit establishment.
c. Main test programme.
d. Reset programme.
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ATT SAFETY DEVICES
Preliminary Test
In preliminary test programme, the substitute circuit elements and the circuit are
tested for their healthiness; the turbine is fully protected against any inadvertent
tripping during ATT. If any fault is present; further testing is inhibited. During
preliminary test, following steps are performed.
KORBA SIMULATOR
155
Test solenoids (TSX) become energised.
Build-up of control oil pressure upstream of changeover valve is monitored.
Test solenoids de-energised one by one & drop of control oil pressure is
monitored.
If all steps are executed within a specified time period pre-test is said to be
successfully.
Hydraulic Test Circuit Establishment
If no fault is present during preliminary test; command is automatically given to
establish hydraulic test circuit (substitute circuit). The hydraulic test circuit is
responsible for the supply of control oil in trip oil circuits. The test solenoids valves
are again energised building up the control oil pressure upstream of changeover
valve. At this moment another solenoid (SVX) gets energised, draining control oil and
creating differential pressure across the changeover valve, it assumes upper (test)
position and annunciation is flashed to this effect. With changeover valve in its test
position, control oil flows in the trip oil piping. After successful establishment of
hydraulic test circuit command goes to initiate the main test, in which individual
devices can be checked.
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156
Main Test
During main test programme, the associated hydraulic test signal transmitter with
the exception of remote trip solenoids provides the necessary signal to actuate
protective devices. The protective device under test operates and drains the aux. trip
oil. Turbine trip gear (main trip valves) is closed after trip oil pressure drains and
associated alarms flash.
Reset Programme
The resetting programme automatically starts after the main test is over. The reset
solenoid valves energise and supply control oil in aux. start-up oil circuit to reset
main trip valves and protective devices, which have tripped from their normal
positions. Once they return to their normal position, trip oil and aux. trip oil
pressure can be built-up and monitored. If oil pressure is satisfactory, reset
solenoids along with test solenoid valves and SVX get de-energised, deactivating
hydraulic test circuit and resetting circuit.
TESTING OF PROTECTIVE DEVICES
The main trip valves and remote trip solenoid valves have already been discussed in
previous chapters; hence the remaining ones will be taken up here.
Over speed Trip Device
Trip consists of two eccentric bolts fitted on the shaft with centre of gravity displaced
from the shaft axis. They are held in position against centrifugal force by springs
whose tensions can be adjusted corresponding to 110% - 111% over speed. When
over speed occurs, the fly weights (bolts) fly out due to centrifugal force and strike
against the pawl and valves, draining aux. trip oil pressure and tripping the turbine.
KORBA SIMULATOR
157
HYDRAULIC TEST SIGNAL
TRANSMITTER (HTT) FOR
OVER SPEED TRIP DEVICE
During
ATT,
the
associated
hydraulic
test signal transmitter
I.
II.
III.
IV.
V.
Control Oil
Test Oil
Aux. Trip Oil
Aux. Startup Oil
Drain Oil
1.
2.
3.
4.
Limit switch (normal)
Limit switch (test)
Valve for Test Oil
Actuator
(HTT) becomes '
on'
; spool
valve slowly moves down
to gradually build-up test
oil
(control)
pressure
beneath the flyweights.
At pre-defined test oil
pressure fly weight one
and two operate to
actuate individual pawl
and spool arrangements
bringing
in
the
associated alarm. For resetting, spool moves-up
and
when
test
oil
pressure is fully drained,
aux. start up oil (control
oil from '
reset'solenoids)
pressure
resets
the
devices to their normal
position.
Low Vacuum Trip Device
With deterioration of vacuum, pressure builds-up over the diaphragm, the spool valve
move down, causing valve also to move toward lower position. The aux. trip oil
pressure drains, tripping main trip valves and the turbine stop/control valves. During
ATT, after hydraulic test circuit is established, the HTT (Hydraulic Test signal
Transmitter) gets energised and connects the space above diaphragm to atmospheric
pressure through an orifice. The device operates, bringing in the associated alarm. As
soon as reset programme starts, HTT is de-energised and vacuum trip device is
automatically reset, Field adjustment facilities and checks have been provided when
turbine is stationary and there is no vacuum in the condenser.
Thrust Bearing Trip Device
This device operates in case of excessive axial shift ( >0.6 mm) or excessive thrust
pad wear. Two rows of tripping cams on the shaft engage with the pawl) under high
axial shift condition. Valves spool moves up draining aux. trip oil and tripping the
trip gear and turbine. During ATT, associated ATT solenoid is energised and test oil
pressure is supplied to test piston valve. The piston rod actuates the pawl and spool
valve assembly, bringing in the associated alarms. During resetting, HTT is deenergised and aux. start-up oil (control oil) resets the device back into normal
position.
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158
AUTOMATIC TESTING OF STOP/CONTROL VALVES
The combined stop/control valves are final control elements of the turbine governing
system. They must be maintained in absolutely workable condition for safety and
reliability of turbine. All the four stop and control valve assemblies are tested
individually.
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159
During ATT of stop/control valves, they are actually closed. In order to prevent large
fluctuations of initial pressure or load on the machine, it is essential that Electro
Hydraulic Governor is in service and machine load is less than 160 MW and load
controller is '
ACTIVE'
.
As soon as test programme is initiated, the positioner motor of control valve
servomotor'
s pilot starts. The control oil supply pressure beneath the servomotor
piston drops and control valve starts closing. After the control valve is fully closed,
command goes to energise solenoid valve (1). The trip oil pressure drains beneath the
disc of stop valve servomotor piston. The stop valve closes. After the stop valve is
closed, automatically a command goes to energise another solenoid (2). This supplies
trip oil to the test valve such that test valve moves down gradually to admit trip oil
pressure above the servomotor piston.
As soon as piston sits on the disc, there is a sudden rise in trip oil pressure, which is
sensed by pressure switches. After these solenoids (1) & (2) de-energise, the test valve
moves up to admit trip oil beneath the disc and connecting the space on top of the
piston to drain. This pressure difference causes the stop valve to open. Once the stop
valve is opened, next command goes to the positioner motor to move in reverse
direction; opening the control valve.
All along this test, the other control valves are operated by the governor, so as to
keep the load and pressure reasonably constant. Should any turbine trip occur
during the test, all solenoids are de-energised and tripping takes place in the usual
manner.
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160
TESTING SCHEDULE
All important turbine components must be tested at regular intervals. The operating
reliability and availability can only meet the high requirements if testing is
undertaken at the scheduled times, as recommended below.
Testing Intervals
Tests are scheduled according to the following Testing Interval Categories.
1. Testing Interval 0
Fortnightly
2. Testing Interval I
Quarterly
3. Testing Interval II
Six-monthly
4. Testing Interval III
Annually
5. Testing Interval IV
After operation interruptions more than 12
month
6. Testing Interval V
After or during overhauls
Testing Interval Category 0 applies to devices, which can be tested automatically
without interrupting operation. The tables show the allocation of the Testing Interval
Categories to the test.
Controller
System /
Device
Test
Test Conditions
Operation
Turbine
controller
Load
shedding
relay
Bypass
controller
Pressure
controller
Oil temp
controller
Function
Adjustment
Test Interval
0
I
II
III
x
x
x
X*
Standstill
x
x
x
Load rejection
Operation
Function
V
Load Rejection
Standstill
Function
IV
x
X*
x
x
Load rejection
x
X*
Adjustment Standstill
x
x
Function
Operation
x
x
x
Function
Operation
x
x
x
X*: Recommended; not required by manufacturer
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161
Sub loop Control of Pumps
System /
Device
Oil Pumps
Test
Test
Conditions
Test Interval
O
I
II
Function
Shutdown
x
Start-up
Pressure
Operation
x
III
IV
V
x
x
Valves
System /
Device
Test
Test Interval
Test
Conditions
O
I
II
III
Standstill
Stop Valves
Freedom of
movement
Leak Test
Operation
Freedom of
movement
Leak Test
V
x
x
x
x
x
x
x
x
x
x
x
x
x
Operation ATT x
Shutdown
Start-up
/
x
Standstill
Control
Valves
IV
Operation
x
Operation ATT x
Shutdown
/
Start-up
LP Bypass
Control
Valves
Freedom of
movement
Standstill
Extraction
Valves
Freedom of
movement
Operation
Safety valves
Actuating
valves
Operation/
Standstill
Vacuum
breaker
Function
Standstill
x
x
x
x
x
x
IV
V
x
x
x
x
Protection and Safety Devices
System /
Device
Test
Main Trip
Valves (Gear) Function
Remote Trip
Solenoids
Function
KORBA SIMULATOR
Test
Conditions
Test Interval
O
Standstill
Operation
ATT
Standstill
Operation
ATT
I
II
III
x
x
162
Over Speed
Trips
Function
Actuating
value
Hydraulic
Function
Low Vacuum Actuating
Trip
value
Electrical
Function
Low Vacuum Actuating
Trip
value
x
Function
LP Bypass
Condenser
Protection
Function
Actuating
value
Reverse
Power
Protection
Fire
Protection
x
x
Rated speed
x
ATT
Standstill
Operation
ATT
x
x
Standstill
ATT
x
x
Standstill
Thrust
Bearing Trip
x
x
x
x
Standstill
x
x
Function
Shutdown
x
x
Function
Shutdown
x
x
Safety
Function
Devices for
Actuating
Reverse Flow value
Low Lub Oil
Pressure
Protection
Device
Over speed
after
load
operation
Rated speed
Function
Actuating
value
Protection
Devices
Too high
Steam
Pr.
Function
Actuating
value
Too low
Steam
Pr.
Function
Actuating
value
KORBA SIMULATOR
Operation
x
x
Standstill
x
x
x
Standstill
x
x
Standstill
x
x
163
Alarms and Measuring Devices
System /
Device
Alarms for all
system
Digital Signal
Transmitter
Test
Conditions
Test
Function
Operation
Actuating
Value
Standstill
Function
Actuating
Value
Standstill
Speed
Test Interval
O
I
II
III
IV
V
x
x
x
Operation
x
x
x
Measuring Devices
Pressure
x
x
Temperature
x
x
x
x
Expansion
Vibration
Oil Level
Accuracy
of
indication
Standstill
Valve
Position
x
x
x
x
x
x
x
x
x
Measurement of Important Operating Parameters
System / Device
Steam Temperature.
Steam Flow
Internal Efficiency
Condenser Leak
Tightness
Bearing Metal Temp.
Test
Conditions
Test Interval
O
Long Term
Monitoring
Steam Pressure
Test
I
II
III
IV
V
x
x
x
x
x
Operation
x
x
Expansion
x
x
Vibration
x
x
Oil Levels
x
x
Oil Pressure
x
x
Oil Temperature
x
x
KORBA SIMULATOR
164
TURBINE STRESS EVALUATOR (TSE)
SIGNIFICANCE OF TURBINE STRESS MONITORING
It is important for the operator to know how quickly his turbine can be started up
and what changes in load he can make without the fear of over-stressing the turbine
components; thereby causing excessive fatigue. Whenever steam inlet temperature
changes within the turbine, the metal temp. follows the steam temp. with a certain
delay. This causes differential thermal expansions within the turbine casing and
shaft & corresponding stress in the metal. Thermal over-stressing can reduce useful
operation life of turbine and its components. Turbine Stress Evaluator measures and
calculates the relevant temperature values and evaluates them in an analog
computing circuit and determines the allowable conditions of operation so that
useful life of the turbine shall not be unduly reduced. Thus it allows the operation of
the turbine at the highest possible rates of load/speed change while limiting the
stresses within permissible values. The results of TSE, which are the appropriate
operating instructions, are displayed by means of an indicating instrument.
TASK OF TSE
If the turbine is to be operated so that there is no undesirable material fatigue, these
thermal stresses must be kept within acceptable limits. The optimum balance
between longevity on one hand and material flexibility of operation on the other is
achieved when the permissible range of material stress can be utilised to the fullest
extent.
The turbine stress evaluator provides the basis of continuously calculating
permissible values for desired changes in operating conditions at all times and under
all operating states and by displaying temperature margins, within which the
speed/load can be changed during loading/unloading of the machine. Signals from
the TSE are also fed to the speed and load reference limiter of the turbine controller
for use in set point and gradient (speed and load) control.
MEASURED VALUE ACQUISITION AND PROCESSING
Wall temperature sensors (thermocouples) are used to sense the temperature of
various turbine components and these signals are given to the TSE as the input
temperature signal. The wall temperature sensors measure temperature of the
surface, which is in contact with steam (Ts, at 95% depth) and mean wall
temperature in the middle section of the components (Tm, at 55% depth). The
thermal stress on the individual components can be ascertained from difference
between the two temperatures of the component.
Specially designed wall temperature sensors are used on combined stop and control
valves of HP turbine for measuring these temperatures. These sensors have two
measuring points. These comprise a screwed sleeve containing a measuring insert.
The screwed sleeves are inserted in a through hole in the wall of casing and welded
on outside. It is made of the material having temperature characteristics similar to
those of casing. This ensures good thermal contact and same thermal gradients
through temperature sensor as surrounding wall.
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165
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166
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167
Shaft temperature simulation
If the thermal stress in rotor is to be monitored, the surface temperature on the
inside of casing surrounding the rotor is measured by a single thermocouple at a
point where the dynamic behaviour of temp of the shaft corresponds to that of
casing. It is taken as the surface temperature of shaft itself. The corresponding mean
shaft temp is simulated, values derived from the measured surface temperature,
depending upon machine load, steam temperature and time lapsed.
The mean internal (mid metal) shaft temperature can be calculated with an adequate
degree of accuracy by means of the following mathematical equation.
Tm = Ts [ 1- (0.692 e -t/T1 + 0.131 e -t/T2 + 0.177 e -t/Tk ) ]
Where,
Ts
Tm
t
:
:
:
Surface Temperature
Mid metal Temperature
Time in minutes
T1
T2
Tk
:
:
:
2408.31
457.08
56.62
Time constants
Various constants used in the above equation are derived from the shaft diameter
and thermal diffusivity of the rotor material. The solution of this equation is realised
by means of three integrators and one summing amplifier.
Normally 5 measuring points feed the TSE. First two measuring points are located in
the body of combined Stop/Control valves are called ADMISSION sensors. The next
two are located in the HPT cylinder adjacent to the first drum stage and are called
HPT wall temp sensors. The last measuring point is in the flange of IPT cylinder inner
casing, before the last drum stage to represent the surface temp of the shaft.
FUNCTIONING OF TSE
The mV output from thermocouples is fed into the signal conditioning cabinet where
the transducers give out 4-20 mA signals as temp signals. The measured values are
processed in an analog computing circuit with 3 channels namely ADMISSION, HP
TURBINE and IP TURBINE channels. Each computing channel determines ∆Ta
between surface and mean (mid) temperatures. The thermal stress is proportional to
this temp difference. The calculated temp difference is compared against the
permissible mean temp difference ∆Tp, which is derived from function generator for
each computing channel. The difference between ∆Tp and ∆Ta is called MARGIN.
Comparing ∆Ta against ∆Tp on the positive side, we get upper margin and that on
the negative side we get lower margin.
The smallest of the respective upper and lower temperature margins calculated for
Admission and Turbine area, are selected as representative margins and are
displayed by TSE indicator and used for further processing.
Till the machine load Pact remains < 2% PMCR, the TSE display selection remains in
the ADMISSION / SPEED mode, in which the actual speed and temperature margins
either form admission or turbine channels, as sleeted, are displayed. At Pact >2%
PMCR , the TSE changes over to TURBINE or LOAD mode in which the actual load
and load upper and lower margins are indicated. The load margins are calculated
from the available temperature margins and changes in the casing temp differential.
KORBA SIMULATOR
168
KORBA SIMULATOR
169
TSE BLOCK DIAGRAM (200MW KORBA UNITS)
TSE INDICATOR
The indicator is divided vertically into two sections one for starting and one for load
operation. It comprises of three discs.
- One Circular Scale disc, partly calibrated in Speed
- and
Two Load
partly coloured glass discs.
These discs are controlled by means of three electrical servomotors equipped with
feedback potentiometers. The circular scale is controlled by the actual value for
either Speed or Load. The upper red coloured disc is controlled by the upper
available margin either for the temperature or the load. The lower red coloured disc is
controlled by the lower available margin either for the temperature or the load. The
module for control of the potentiometer-equipped servomotor is located in the TSE
indicator and receives three impressed currents of ± 1 mA for the computing circuits.
Power supply for the TSE indicator is ± 24 V.
Before the generator is synchronised, or the load is below 2% MCR the actual speed
and temperature margins are displayed on the left side of the display. After the
generator is synchronised and load is greater than 2%, the actual load and load
margins are displayed on the right side. The effective section is illuminated according
to the operating mode.
KORBA SIMULATOR
170
The row of alarm windows located across the top of the two section of the indicator
shows which computing channel is on line. The red window in the middle is the fault
alarm. Illumination of the appropriate chosen window indicates whether the
displayed margins are being supplied by admission or turbine channel. Additional
LEDs located above and below are the symbols for HP and IP turbine in the turbine
related window and indicate from which turbine upper margin (Upper LED) and the
lower margin (Lower LED) is originated. The appropriate LEDs then show a red light.
During start-up, speed and temperature margins are displayed on the left hand
section. Speed is indicated on a circular moving scale. The upper and lower
temperature margins are covered on the screen. These screens have two zones in
different colour; the red area represents a warning or prohibited range, while the
white indicates the permissible zone of operation.
During load operation, the right hand section displays the actual load and the load
margins. The actual load is shown on a rotating disc marked in MW. Two rotating red
discs indicate the permissible load range. The turbine is operating within the
permissible stress as long as the actual temperatures, load values are located within
the transparent region between the discs.
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171
The opaque red section between the discs covers the prohibited range. The upper
boundary of the transparent sector indicates the upper margins (for start-up and
increasing load); the lower boundary indicates lower margins (for decreasing load and
shut-down).
Changing Section of the TSE Indicator
During speed operation, the TSE indicator can be switched manually by means of
two push buttons on the control desk to indicate the respective margin either from
the admission or from the turbine channels as desired.
During speed operation, if the non-illuminated green alarm window flashes, this
indicate that the temp margins originating from one of the channels related to the
flashing window has been reduced to less than 100K. The flashing lamp instructs the
operator to changeover to the range with the smaller margins. When changeover has
been affected, the green alarm window changes the flashing to steady light. When the
unit is synchronised, the TSE indicator automatically changes over to the right hand
section, which displays the load margins calculated from the temperature margins.
Two red discs indicate the permissible load change.
KORBA SIMULATOR
172
TSE INDICATOR SELECTION
KORBA SIMULATOR
173
IMPORTANCE OF THE MARGINS
The temperature margin is a measure of the degree of thermal stresses, which a
turbo-set can be subjected to, during rapid increase in speed during synchronisation
etc. The load margin is the greatest step change in load based on the instantaneous
stress condition, which the turbine can withstand without being over-stressed.
If the margin is consumed, this means that the component is being stressed to its
permissible limit. The condition is indicated by the relative edge of the red disc
horizontal position. Further, increase in speed or load should then only be made at a
rate, which will enable the disc to maintain their position. In this way optimum startup or load change is achieved without over-stressing the component.
If the indicated actual load becomes covered by the upper or lower red discs, the
component material is being subjected to excessive stresses which means an
intensified effect on the material fatigue. If the excessive stresses are to be reduced,
the steam temperatures are to be brought closer to the turbine temperature that is to
say, if the upper margin is exceeded the steam temperature must be reduced and
conversely, the lower margin is exceeded the steam temperature is to be increased.
Any reduction in steam flow leads to less effective heat transfer and thus tends to
reduce temperature differential. In many cases, it is sufficient to wait or to reduce the
rate of change of load and temperature until an adequate margin is obtained. All
counter measures must be directed towards protecting the component, which is in
the greatest danger of overstressing. Before synchronising is carried out, it is
necessary to have temperature margins of more than 300K available, so that the
minimum load on set can be achieved immediately after synchronisation.
Load Margin Calculation
The upper and lower load margins are computed by using the respective temperature
margins of the turbine computing channels. The upper load margin represents the
minimum value determined from the individually computed available upper margins
of HPT and IPT.
dPu
=
Minimum value out of dPuH and dPuII(M)
Where,
dPuH
dPuI
=
dPuI + dPuII(H)
= dTuHT min . (A + B. Pact)
dPuII(H) =
dPuII(M) =
dTuHT min. . (V2 + W2. Pact)
eH (TH – Tm HT min)
dTuMT . (V1 + W1. Pact)
eM (TM – Tm MT)
KORBA SIMULATOR
174
Values of various constants are:
A
=
0.110
TM
=
5.833
V2
= 1.296
B
=
0.090
TH
=
8.250
W1
= 0.090
eH
=
3.000
V1
=
0.315
W2
= -0.150
eM =
6.000
The load lower margin available is calculated as follows:
dPl = dTlT min . (C + D. Pact)
Where,
A = 2.725
B = -0.150
TSE TEST
Panel Testing
A test programme for the
Turbine Stress Evaluator is
available for testing the
correct
functioning
of
individual
computing
channels, from the input
amplifiers to the display
unit. For this purpose,
fixed
voltages
are
introduced
into
the
computing circuit through
relays, instead of the
measured temperature and
load signals. Testing can
be done only if '
Enable'or
‘Release’ signals from EHC
(electro hydraulic turbine
controller)
and
FGA
(functional
group
automatic
control)
are
present.
If the TSE is functioning
correctly,
the
indicator
must show specific known
values for each computing
channels. If there is a
deviation
from
the
tolerance value, it indicates
that
there
is
some
fault/error
in
the
evaluator.
KORBA SIMULATOR
175
The following table can be used while performing the TSE test. The buttons needed to
be depressed, for testing of each category, are shown in shaded.
Sl. No.
Selector Pushbutton
Test Programme
Computing
Pushbuttons
MSV
MCV
Channel
(ADMISSION/TURBINE)
Initial condition
HPC HPS
IPS
Turbine
Rolling
Turbine on
Load
1
Admission
HP Stop Valve
A
A
2
Admission
HP Control Valve
A
A
3
Turbine
HPT Casing (Rolling)
T
T
4
Turbine
HPT Shaft (Rolling)
T
T
5
Turbine
IPT Shaft (Rolling)
T
6
Turbine
HPT Casing (Load)
T
7
Turbine
HPT Shaft (Load)
T
8
Turbine
IPT Shaft (Load)
T
For example, if it is required to start HPT Casing (Sl. No:3) test while the machine is
on load, the pushbuttons A and HPC are to be pressed simultaneously.
Test Results for 200MW TSE
MSV
MCV
HPS
HPC
IPS
Tu
30 K
21 K
96 K
60 K
104 K
Tl
79 K
99 K
6K
13 K
13 K
Pu
230 MW
200 MW
157 MW
Pact
100 MW
100 MW
100 MW
Pl
79 MW
50 MW
49 MW
TSE influence can be switched off from the EHTC control cabinets and under such
conditions turbine should be operated in accordance with the recommendations of
the manufacturer within permissible temperature differences.
KORBA SIMULATOR
176
Dynamic Test (Monitoring) of TSE
This facility has been provided for continuous monitoring of the healthiness of all the
input signals, computational values, and output signals. This testing is automatically
carried out all the time. For this test, a testing device is incorporated in all the
signals, which monitors the rate of change of those signal values. If any signal
changes at an unrealistic rate, then those devices generate a TSE fault alarm.
Consequently PRTD setter will freeze avoiding the erroneous values entering into the
load controller loop. PRTD can be reset and made free after TSE influence is
switched off.
TSE Output Signals
1) To ATRS
a) From Step No. 14 to Step No. 15.
If TSE upper margin is less then 300 K, further speed rise from 600 rpm to
3000 rpm is not possible.
b) SGC Turbine start up programme gets switched off if TSE upper margin is less
than 00K with turbine speed > 600 rpm < 2800 rpm while rolling.
c) Switching off TSE influence will make SGC turbine programme off
Fault in TSE does not make SGC Turbine programme off.
2) EHC
a) Speed Controller
Only upper TSE margin is used.
Lower TSE Margin is not used, as coasting down is natural. The time
dependent speed reference signal (NRTD) allows rising of turbine speed at the
highest permissible rate consistent with the conservative operation as decided
by TSE computed speed margin signal introduced D.C. amplifier.
b) Blocking or ‘stop NRTD’ of the speed signal if
Speed > 2850 rpm
NR > nRTD by 300 rpm
AND
TSE Influence sets faulted.
KORBA SIMULATOR
177
c) Load Controller
Both lower and upper margins used.
These margins determine the gradient at which PRTD varies.
-ve upper margin can unload the machine whereas reduced lower margin can
prevent turbine from deloading.
Load signal gets blocked in case TSE going out of order when influence is on.
3) CMC
Unit load rate (set in CMC module) is going to Guided Target Indicator gets
compressed with TSE margin.
Minimum of TSE lower margin and unit load rate considering NO RUNBACK
situation goes to GNI which ultimately gives the rate at which the unit should be
unloaded.
Maximum of TSE upper margin and UNIT load Rate goes to GNI which ultimately
gives us the rate at which the unit should be loaded.
KORBA SIMULATOR
178
GENERATOR SYSTEM
KORBA SIMULATOR
179
KORBA SIMULATOR
180
GENERATOR AND AUXILIARIES
DESCRIPTION
The 200MW generator is a three phase, horizontally mounted two-pole cylindrical
rotor type, synchronous machine driven by steam turbine. The stator winding is
cooled by de-mineralised water flowing through the hollow conductor while the rotor
winding is cooled by hydrogen gas maintained inside the machine. Fans mounted on
the generator rotor facilitate circulation of hydrogen inside the machine. Four coolers
mounted inside the machine cool the hydrogen gas. The generator winding is
provided with epoxy thermo-setting type insulation. The machine is provided with
completely static thyristor controlled excitation system, fed from terminals of the
machine.
Hydrogen being a light gas with good heat carry away capacity is used for cooling the
rotor winding, rotor and stator core. Two hydrogen driers are provided to facilitate
moisture removal. Hydrogen is circulated through them via the fans in dry condition.
Normally one drier is kept in service and other is on regeneration.
Four hydrogen coolers are provided to cool the hot gas to maintain the cold gas
temperature at 40oC. Liquid Level Detectors (LLDs) are provided to indicate liquid in
generator casing. This provision is to indicate leakage of oil or water inside the
generator. It can be drained through drain valve.
H2 gas purity is to be maintained of very high order i.e. more than 97%.
STATOR WATER-COOLING SYSTEM
The stator winding of the generator is cooled by de-mineralised water circulating
through hollow conductors of stator winding bars in a closed loop. The cooling water
system consists of 2x100% duty AC motor driven pumps, 2x100% duty water
coolers, 2x100% duty mechanical filters, 1x100% duty magnetic filter, expansion
tank, polishing unit and ejector system.
The stator water pump drive the water through coolers, filters and winding and
finally discharges into the expansion tank situated at a height of about 5m above the
TG floor. It is maintained at a vacuum of about 250 mm Hg by using water ejectors.
A gas trap is provided in the system to detect any traces of hydrogen gas leaking into
the stator water system.
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WATER PATH OF STATOR WINDING AND TERMINALS
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SEAL OIL SYSTEM
To prevent leakage of hydrogen from generator housing, ring type shell seals are
provided at both ends of the generator. During normal operation the AC seal oil
pump draws the seal oil from the seal oil tank and feeds it into the shaft seals via
2x100% capacity coolers and 2x100% capacity filters. The differential pressure
2
regulator maintains seal oil pressure differential of 1.3 Kg/cm over the hydrogen
pressure irrespective of the value of hydrogen pressure. The seal oil is supplied to the
shaft seals into the annular groove of seal ring via the passage in the seal ring
carrier. The clearance between shaft and seal ring is such that frictional losses and
seal oil temperature rise are minimum. Oil film is of sufficient thickness to provide
proper sealing. Higher-pressure ring relief oil is fed in the annular groove in the
airside seal ring carrier.
Thus gas and oil pressure acting on the seal ring are balanced and friction between
seal ring and seal ring carrier is minimized. The seal ring is free to adjust its position
according to shaft position.
Airside seal oil is directly returned to the seal oil tank via a float valve. The oil
drained towards the hydrogen side is first collected in pre-chamber and then passed
to the intermediate oil tank in order to separate any trace of hydrogen present in seal
oil. The oil from this tank also is returned to the seal oil tank via a float valve. Any
possible traces of gases or vapour etc. are removed by vacuum pump from top of the
seal oil tank.
In case of failure of DPRV-A or AC as well as DC seal oil pumps failure, DPRV-B will
come into service and governing oil is used as seal oil.
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GENERATOR SEAL OIL SYSTEM
SPECIFICATIONS OF THE GENERATOR
Rated parameters:
Maximum continuous KVA rating
247, 000 KVA
Maximum continuous KW rating
210, 000 KW
Rated Terminal Voltage
15, 750 V
Rated Stator Current
9050 A
Rated Power Factor
0.85 Lag
Excitation voltage at MCR condition
310 V
Excitation current at MCR condition
2600 A
Excitation voltage at no load
102 V
Excitation current at no load
917 A
Rated speed
3000 RPM
Rated frequency
50 Hz.
Stator winding resistance per phase at 20 oC.
Rotor winding resistance per phase at 20 oC.
0.00155 Ω
0.0895 Ω
Efficiency at MCR condition
98.49 %
Short circuit ratio
0.49
Rise in voltage with 100% load throw off
22.40 KV (without AVR)
Negative phase sequence current capability
1
Direction of rotation when viewed from slip ring
Anti clockwise
Phase connection
Double star
No of terminal brought out
9 (6 neutral, 3 phase)
Generator gas volume
56 m
Nominal pressure of hydrogen
3.5 kg/cm
Permissible variation of gas Pressure
Nominal temperature of cold gas
± 0.2 kg/cm
40 oC. (Alarm)
Purity of hydrogen
> 97 %
Relative humidity of H2 at nominal pressure
2
2
t <8
3
2
2
60 %
Max temperature of cooling water outlet
36 oC.
43 oC.
Hot gas temperature
75 oC (Alarm)
Nominal gauge pressure at winding inlet
3.09 Kg/cm
Max temperature of Stator Water at winding inlet
36 oC
Max temperature of Stator Water at winding outlet
70 oC
Max temperature of cooling water inlet
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Stator water flow
3
Normal
27 ± 3 m /hr
Alarm
21 m /hr
Trip
13 m /hr
3
3
Stator water conductivity
Normal
< 5.0 micro mho/cm
High
13.3 micro mho/cm
Trip
20.0 micro mho/cm
Stator water expansion tank Vacuum
200-300 mm of WCL
Auto start of standby SW pump
2.4 Kg/cm
2
Nominal consumption of cooling water
At 35 oC.
At 37 oC.
95 m /hr
At 40 oC.
130 m /hr
3
3
110 m /hr
3
Safety Valve release (A.C. seal oil pump)
9 Kg/cm
2
Safety valve release (D.C. seal oil pump)
9 Kg/cm
2
Seal oil temperature after Seal oil cooler
Normal
20 - 40 0C.
Alarm
45 0C.
Seal oil outlet temperature
Normal
40 0C.
Alarm
65 0C.
Differential pressure across duplex filter
Normal
0.4 Kg/cm
2
Alarm
0.6 Kg/cm
2
2
Seal oil pressure at Turbine and Slip ring end
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2
(0.9 Kg/cm static head).
187
Permissible temperature rating of Generator:
High
V. High
1.
Generator bearing and seal Babbitt temperature
75 0C.
85 0C.
2.
Generator bearing oil outlet temperature
60 0C.
70 0C.
3.
Stator winding temperature
75 0C.
105 0C.
4.
Rotor winding temperature
110 0C.
115 0C.
5.
Stator core temperature
95 0C.
6.
Hot gas temperature
75 0C.
7.
Cold gas temperature
44 0C.
OPERATION LIMITS
Capability of the Generator
The generator is capable of delivering 247 MVA continuously at 15.75 KV terminal
2
voltage and stator current 9050 A, at a Hydrogen pressure (g) of 3.5 Kg/cm . The
cold gas temperature not to exceed 44 0C and distillate temperature at inlet of stator
winding not to exceed 45 0C. Output of the generator at various lagging and leading
power factors are as per the generator capability curve.
Variation of Terminal Voltage
Generator can develop rated power at rated power factor when the terminal voltage
changes within ± 5% of the rated value i.e. 14.96 KV to 16.54 KV. The stator current
should accordingly be changed within limits of 5% ± i.e. 8600 A to 9500 A.
Permissible voltage of operation and corresponding values of the MVA outputs of
stator currents are given in Table - 1.
During continuous operation of the generator at 110% of the rated value stator
current should not increase beyond 9500 A corresponding to 105% of the rated
value.
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TABLE – I
Terminal
17.32
voltage in KV
Output
in
217
MVA
Stator current
7.24
in KA
17.17
17.01
16.85
16.7
16.54
15.75
14.96
224.7
231
237
242
247
247
247
7.56
7.92
8.14
8.37
8.6
9.05
9.5
During continuous operation of the generator at 110% of the rated value stator
current should not increase beyond 9500 A corresponding to 105% of the rated
value.
Frequency Deviation
The Generator can be operated continuously at rated output with a frequency
variation of ±5% of the rated value i.e. 47.5 Hz. to 53.5 Hz. However, the performance
of the generator with frequency variation is limited by the turbine capability.
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Temperature of the Coolants
If the temperature of the cooled hydrogen or inlet water to gas coolers increases
beyond the rated value, the unloading of the generator has to be carried out as per
the given curves. The operation of the generator with cold gas temperature more than
55 0C is not permitted. Operation of the generator with cold gas temperature below
20 0C is not recommended.
Similarly, if cold distillate temperature at inlet of stator windings increases beyond
the rated value, unloading of the generator has to be carried out as per the curves.
The operation of the generator with cold distillate temperature more than 48 0 C is
not permitted. Operation of the generator with cold distillate temperature below 35 0
C is not recommended.
UNLOADING SCHEDULE DUE TO HIGH DISTILLATE TEMPERATURE
Overloading
Under normal conditions, the generator can be over loaded for short duration.
Permissible values of short time over loads of Stator Current Vs Time in minutes and
Rotor Current Vs Time in seconds are given in Table - II and Table - III respectively.
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TABLE – II
Stator current in KA
13.57
12.67
12.22
11.76
11.31
10.86
10.41
9.95
Time in minutes
1
2
3
4
5
6
15
60
TABLE - III
Rotor current in KA
5.2
3.9
3.12
2.73
Time in seconds
20
60
240
360
Operation under Unbalanced Load
The turbo-generator is capable of operating continuously on an unbalanced system
loading provided that continuous negative phase sequence current during this period
shall not exceed 5% of the rated stator current i.e. 452.5 A. It implies that maximum
difference between limit current is about 10% of the rated value. At the same time
current of maximum loaded phase should not exceed the permissible value for
normal conditions of operation of turbo-generator under balanced loading. If the
unbalance exceeds the above permissible limits, measures shall be taken
immediately to eliminate or reduce the extent of unbalance within 3 to 5 minutes. In
case it is not possible to achieve this, the machine has to be run down and tripped.
If negative sequence current reaches a value of 20-25% of the rated value trip-relay
will operate and the generator will be automatically tripped.
Asynchronous Operation
Asynchronous Operation of the generator on field failure is allowed depending upon
the permissible degree of the voltage dip and acceptability of the system from the
stability point of view.
During field failure, the field suppression shall be cut out from the circuit and active
load on the generator shall be decreased to 60% of the rated value within 30 seconds
and to 40% in the following 1.5 minutes. The generator can operate at 40% rated
load asynchronously for a total period of 15 minutes from the instant of excitation
failure. Within this steps should be taken to establish the reasons of field failure to
restore normalcy or the set should be switched over to reserve excitation.
Unsymmetrical Short Circuit Performance
The duration of unbalanced operation should be such that the product of square of
2
negative sequence component of current I 2 expressed in terms of per unit value of
2
stator current and its duration in seconds does not exceeds 8 (I 2 t < 8).
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NEGATIVE SEQUENCE CURRENT CAPABILITY CURVE
The permissible value of negative sequence current and the corresponding durations
are given in Table - IV.
TABLE – IV
Duration of short circuit in seconds
1.2
5
10.9
Negative sequence Current
2.5
1.25
0.9
Operation at Reduced Hydrogen Pressure
Continuous operation of the turbo-generator with lower hydrogen pressure than the
2
rated value of 3.5 Kg/cm is not permitted. However, during emergency, the
generator can be run at reduced hydrogen pressure with reduced load for a short
duration as given in Table - V.
TABLE - V
H2 Pres. kg/cm2 (g)
Generator Load (MW)
Duration of Operation
3.0
200
Continuous
2.0
115
Continuous
1.0
30
Continuous
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Within this time action should be taken to restore the hydrogen pressure to normal
value. Operation of generator with air-cooling is NOT PERMITTED.
Capacity of the Generator with One Cooler out of Service
The generator can deliver 185 MW continuously when one gas cooler is out of service.
The operation of the generator with more than one cooler out of service is not
permitted. Refer to Fig
Motoring Action
Motoring of the turbo-generator is permissible within the limitation of the turbine.
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STATIC EXCITATION SYSTEM
Description
Static Excitation System is used in most of the 200 MW Generator sets. The AC
power is tapped off from the generator terminal, stepped down and rectified by fully
controlled thyristor bridges and then fed to generator field as excitation power, to
control the generator-output voltage. A high control speed is achieved by using an
inertia free control and power electronic system. Any deviation in generator terminal
voltage is sensed by an error detector and causes the voltage regulator to advance or
retard the firing angle of thyristor thereby controlling the field excitation.
The static excitation system consists of:
i.
Rectifier Transformer
ii.
Thyristor Converter
iii. Automatic Voltage Regulator
iv.
Field flashing Circuit
v.
Field breaker and field discharge equipment.
Rectifier Transformer
The power transformer gets input supply from the generator output terminals. The
secondary is connected to the Thyristor Bridge, which delivers a variable DC output
to the generator field. Normally it is a dry type transformer.
Thyristor Converters
The converter is assembled in one or more numbers of cubicles depending on the
number of thyristor bridges connected in parallel. The number of bridges is so
designed that in case one bridge fails during operation, the remaining bridges will
have adequate capacity to feed the generator field for full load output. Fans mounted
on the top of the cubicles cool the thyristor bridges.
Field Flashing Circuit
Since it is difficult to start the excitation system with the residual voltage at nominal
speed, a field flashing circuit is provided to overcome this problem. Initially the
station auxiliary supply of 415 volts is stepped down by a small transformer and
then rectified in a rectifier bridge and supplied to the generator field. As soon as the
generator output builds up to 40%, the excitation system starts working smoothly. A
back-up battery supply is given in parallel to field flashing output.
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FIELD BREAKER CUBICLE
For rapid de-excitation of synchronous machine and complete isolation of the field
from the Thyristor Bridge, a field breaker is provided. In case of electrical faults, the
field breaker provides protection by isolating DC source from the field. The magnetic
field energy is dissipated through a field discharge resistance.
AUTOMATIC VOLTAGE REGULATOR
It is the heart of excitation system. It consists of the following components:
ERROR DETECTOR AND AMPLIFIER
The generator terminal voltage is stepped down by three phases PTs and fed to the
Automatic Voltage Regulator (AVR). The AC input thus obtained is rectified and
compared against a highly stabilized reference value and any difference is amplified
in different stages of amplification. For parallel running of generators compounding
feature is provided. Three CTs sensing the current in the generator terminal feed
proportional current across the variable resistors in the AVR. The voltage thus
obtained across the resistor can be added vectorially either for compounding purpose
or for transformer droop compensation.
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GRID CONTROL UNIT
The output of the AVR is fed to a grid control unit. It gets its synchronous AC
reference through a filter circuit and generates a row of pulses whose position
depends on the DC input from the AVR i.e. the pulse position varies continuously as
a function of the control voltage. The pulse limit for rectifier and inverter (deexcitation) operation can be adjusted independent of each other by potentiometer
provided on the front side of unit. Six double pulses displaced by 60o from one
another are generated at this output.
Two relays are provided, by exciting which, these pulses can be either blocked
completely or shifted to inverter mode of operation.
PULSE AMPLIFIER
The pulse output of this grid control unit is amplified further at an intermediate
stage of amplification. This is also known as pulse coupling stage. It has also DC
power supply unit which operates from a three-phase 380V supply and delivers +
15V, -15V, +5V and a coarse stabilised voltage UL.
A built-in relay is provided which can be used for blocking a 6-pulse channel. In a
two-channel system, energising and de-energising the relay affect the changeover.
PULSE FINAL STAGE
The unit receives input pulse from the previous stage i.e. pulse amplifier
(intermediate stage) and transmits them through pulse transformer to the gates of
the thyristor. The step pulse at the output ensures simultaneous firing of several
thyristor in parallel. A built- in power supply provides the required DC supply (+ 15V
+5V & UL) to the final amplifiers.
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Each thyristor bridge has its own final pulse stage. Therefore, even if a thyristor
bridge fails with its final pulse stage, the remaining thyristor bridges can continue to
provide full load output and thereby ensure (n-2) operation.
Pulses can be blocked with an internal relay provided in this unit. Pulses are blocked
in case of
Failure of one or more thyristor fuses.
-
Failure of power supply of the final stage.
-
Failure of the converter fans.
MANUAL CONTROL CHANNEL
A separate manual control channel is provided where the controlling DC signal is
taken from a stabilized DC voltage through a motor operated potentiometer. The DC
signal is fed to a separate grid control unit whose output pulses after being amplified
at an intermediate stage can be fed to final pulse stage. When one channel is working
generating the required pulses, the other remains blocked. Therefore, blocking or
releasing the pulses of corresponding intermediate stage affects a changeover
between auto and manual control.
A pulse supervision unit detects spurious pulses or loss of pulses on the pulse bus
bar and transfers control from '
Auto'to '
Manual'channel.
FOLLOW-UP UNIT
To ensure a smooth changeover from '
Auto'to '
Manual'control it is necessary that
the position of the pulses in both the channels should be identical. A pulse
comparison unit detects any difference in the position of the pulses and with the help
of a follow-up unit in actuates motor operated potentiometer on the manual channel
to turn in direction so as to eliminate the difference. However, while transferring
control from manual to auto, any difference in the two control levels can be visually
checked on the balance meter provided on the control panel and after matching the
two signals the changeover can be done.
LIMIT CONTROLLERS
When a generator is running in parallel with the power network it is essential to
maintain it in synchronism without exceeding the maximum permissible load on the
machine and also without the protection system tripping. So it is necessary to
influence the voltage regulator by suitable means to limit the over-excitation and
under-excitation.
The following limiters are normally used in the static excitation system.
a.
Stator Current Limiter
b.
Rotor Current Limiter
c.
Rotor Angle Limiter
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Stator/Rotor current Limiters limit the control voltage to a value corresponding to
the permissible excitation in over excited operation while rotor angle and stator
current limiter limits the control voltage to value corresponding to permissible
excitation in under excited operation.
STATOR CURRENT LIMITER
This unit functions in conjunction with an integrator unit, which provides the
necessary dead time and the gradient that can be adjusted by potentiometers. The
regulator consists of a measuring converter, two comparator, two PID regulators and
a DC power pack. A discriminator in the circuit differentiates between inductive and
capacitive current. The positive and negative signals processed by two separate
amplifiers are brought to the output stage and only that output which had to take
care of the limitations is made effective. The inductive current limit is affected
through the integrator while the capacitive current gets directly on the AVR output.
ROTOR CURRENT LIMITER
The unit basically comprises an actual value converter, a limiter with adjustable PID
characteristics, a reference value, dv/dt sensor and a signaling unit. The field
current is measured as the AC input side of the thyristor converter and is converted
into proportional DC voltage. The signal is compared with an adjustable reference,
amplified, and with necessary time lapse, fed to the voltage regulator input. The limit
is reduced during (n-2) operation through a relay switched circuit. Also, during a
fault condition (when dv/dt is large and -ve), the limit is raised (field forcing limit).
ROTOR ANGLE LIMITER
This unit limits the load angle i.e. the angle between the voltage of the network
centre and the rotor voltage. The limiter is fed with generator terminal voltage &
current and through a simulation circuit derives the rotor voltage & grid voltage &
hence the load angle. It comprises an actual value converter, a limiting amplifier with
adjustable PID characteristics and a reference value unit. The limiting regulator
operates as soon as the DC value exceeds the reference value.
SLIP STABILISING UNIT
The slip stabilising unit is used for the suppression of rotor oscillations of the
alternator through the additional influence of excitation. The slip as well as
acceleration signals needed for the stabilisation are derived from active power
delivered by the alternator. Both the signals are amplified and summed up to
influence the excitation of the synchronous machine through AVR in a manner so
as to suppress the rotor oscillation.
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EXCITATION SYSTEM PROTECTION
Excitation Transformer Protection
The protection unit for the excitation transformer is normally mounted on the
swing frame of the regulator cubicle. It consists of two over current relays, with
adjustable ranges. The current supply for the relays is made through a DC
converter, which receives its input supply from battery through a filter circuit.
Besides over-current protection a dry type rectifier transformer is embedded with
temperature dependent resistance at the low voltage windings. With rise in
temperature the resistance value changes sharply after a certain level. This
change with one resistor is used for 'warning'and with another for '
tripping'
.
Converter Protection
Fuses
Each thyristor in the converter is connected with a fast acting semiconductor fuse
to protect in case of over-current.
Resistor/Capacitor
Network across to each thyristor for protection against hole storage effect.
Airflow Monitoring
Since converters are air-cooled by fans the airflow is monitored by airflow relays.
Redundancy
The thyristor bridges are designed such that in case of failure of one, the
remaining bridges will be adequate to provide full load output with field flashing.
Isolator
Isolators are provided on the input and output side of the converters to enable
replacement of defective thyristor under running load.
AVR Protection
All D.C. power supplies receive their input AC supply through Miniature Circuit
Breaker with thermal overload relays. Failure of protection and control voltage, as
also Regulation Supply, result in tripping of the Field Breaker. Failure of Auto
Power Supply / Regulation supply result in to Manual channel change over.
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Power chart of a Turbogenerator (Solid rotor design)
SIGNIFICANCE OF MACHINE CAPABILITY DIAGRAM
Capability diagram of the generator gives the safe operating domains. It gives the
basic information regarding the limiting zones of the operation so that limiter can be
set / commissioned suitably for safe operation of unit.
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EXCITATION SYSTEM PARAMETERS ( KORBA STAGE-1 )
GENERATOR
MVA
MW
VOLTAGE
STATOR CURRENT
POWER FACTOR
SPEED
247
210
15.75 KV
9050 A
0.85 LAG
3000 RPM
EXCITATION SYSTEM
Ifn
:
2512 A
Vfn
: 300 V
Ifmax
:
3000 A (contin.)
Ifo
: 917A
Vfo
:
120 V
Field resistance
:
0.0896 Ω
Field forcing
:
1.4 Ifn
(n-2) limitation
:
0.65 Ifn, 1.1 Ifn
α min
:
30o
α max
:
72o
Over voltage setting
:
2.5 KV
Rotor Earth fault
:
5 K Ohm, 2.2 K Ohm.
Range of control of AVR
:
Field flashing off
:
30% (85% - 115%)
Blocking of Ch. pulses upto 30% voltage
70% V
Excitation transformer
:
( 3900 A, 620 V for 10 sec )
15.75 KV/575 V, 2500 KVA,
CTs
Generator
:
10,000/5 A
Excitation Transformer
:
200/1 A, 2500/5 A
DCCT
:
3000/2 A
PTs
:
15750/110 V
DCPT
:
600V/20 mA
Field flashing transformer
:
5 KVA, 415/35 V
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Field Breaker closing
1
GE2 Open
2
386GX Reset
Field Flashing
:
Up to 70% voltage within 20 Secs + RPM Relay (2950 rpm)
Field Breaker Tripping Protections
1
Class A
30Z → 86G→ 386 GX → Field Breaker Trip
a.
Transformer Over Current Instantaneous
b.
Transformer Over Current delayed 2nd stage
c.
Rotor Over Voltage
d.
48V supply fail
e.
Three or more (≥/3) failed
i.
ii.
Fan fail
a.
Air Flow Relay
b.
Fan supply
Pulse final stage power supply failed
iii. Thyristor fuse failed
iv.
2
Isolator open.
Class B
30F → UTR → LFPR(32GI) → 32GI→ 2/32G1 → 86G → 386G → FB Trip
3
a.
Rotor Earth Fault, stage-II
b
Regulator supply fuse fail
c
Manual Channel fail
d
Thyristor Fan supply fuse fail
e
Transformer Temp. HI - HI
Direct Trip
Trip the F.B. directly, Gen trips through field failure relay.
a
Field Flashing disturbed
b
Switch in Test Position
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4
Automatic Changeover to Manual
a
Supply A1 fail
b
Supply A fail
c
Excitation transformer O/C delayed Stage-I
d
Excitation transformer temp HI
e
Auto Channel pulse failure
f
AVR PT Fuse Failure
Alarms
a
Protective change over to manual
b
AVR Fault
c
FB Tripped due to AVR fail
d
Limiters in Action
e
Loss of Control voltage
f
Rotor E/F
Limiters
a
Stator Current Limiter
b
Rotor current limiter
c
Rotor Angle Limiter
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GENERATOR PROTECTION
Description
The core of an electrical power system is the generator. During abnormal operating
conditions certain components of the generator are subjected to increased stress and
therefore, could fail, referred to as faults. It can be either internal fault or external
fault depending upon whether they are inside or outside of the machine. The
machine with fault must be tripped immediately. The corrective measures against
generator'
s abnormal operation are taken care by stubborn protective system.
Task of the protective system:
- Detect abnormal condition or defect.
- Limit its scope by switching to isolate the defect.
- Alarm the operating staff.
- Unload and/or trip the machine immediately.
Requirement of Protective Device:
- Selectivity: Only that part of the installation actually containing fault should be
disconnected.
- Safety against faulty tripping: There should be no trip when there is no fault.
- Reliability: The device must always act within the required time.
- Sensitivity: Lowest signal input value at which the protective device must act.
- Tripping time:
There should be clearly a distinction between the tripping time of the device,
considering the circumstances such as current and total tripping time for the fault.
The total fault clearing time now is of the order of 100 (mill sec.)
Protective Devices
The choice of the protective equipment for a generator requires precise knowledge of
the stress to which the generator is subjected to during services in order that
preventive measure may be devised for avoiding inadmissible stress. Important
stresses include the electrical voltages to which insulation is exposed, the
mechanical forces affecting various parts of the machine and effects of the
temperature rise.
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Types of Protections
The details of the protective circuits of a 200 MW turbo-generator are given in the
above fig. The several faults occurring in generator can be either electrical or
mechanical in nature. As such generator protection is broadly segregated into two
parts i.e. electrical protection-mostly Class-A type and mechanical protection-mostly
Class-B type.
ELECTRICAL PROTECTION
1.
Differential Protection:
a.
Generator Differential
b. UAT Differential
c.
Overhead Line Differential
d. G.T. Restricted Earth Fault, Main
e.
2.
Overall Differential
Earth fault protection:
a.
Stator Earth Fault
b. Stator Earth Fault, stand by
c.
Rotor Earth Fault
3.
Stator Interturn Fault
4.
Negative Phase Sequence Current
5.
Generator Backup Impedance
6.
Loss of Excitation
7.
Pole Slipping
8.
Over Voltage
9.
Over Fluxing
10. Low Forward Power
11. Reverse Power
12. Generator Local Breaker Backup (LBB)
13. Generator Transformer Protections
a.
Buchholz Protection
b. PRD Protection
c.
Winding Temperature High
d. Oil Temperature High
e.
Fire Protection
14. UAT Protection
15. Bus Bar Protection
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DIFFERENTIAL PROTECTION
a. GENERATOR DIFFERENTIAL
A direct short circuit between different phases of the winding causes a severe fault
current flow through the windings and results in extensive damages. As a result
there is a distinct difference between the current at the neutral and terminal ends of
the particular winding. This difference is detected by differential relay.
The current entering and leaving the protected object are determined by current
transformers and compared by relays by means of a differential circuit as shown in
the figure.
A fault inside the protected zone is fed from either one side or both sides depending
upon the current sources present, thus producing a difference current in the
differential circuit. If this differential current exceeds a set percentage of the current
flowing in the protected object, the relay picks up.
The relays used is designated 87 G and is CAG 34.5 amp type. It is set to operate
10% (0.5 amp) relay current which corresponds to 1000 amp fault current.
b. UATS DIFFERENTIAL
Since UATs are connected directly to the stator windings, it has been provided with a
biased differential protection in a similar circulating current scheme. The relays are
designated 87 UAT and 87 UTB are DTH 31 type.
c. G.T. OVERHEAD LINE DIFFERENTIAL
The 400 KV bushings of the generator transformer are connected to the switchyard
double moose conductor overhead line. Any fault occurring on these lines is detected
by overhead line differential protection.
The relay designated 87 L is of CAG 34 type 1 amp.
G.T. RESTRICTED EARTH FAULT
The H.V. winding of the generator transformer is star connected and the neutral is
solidly earthed. This protection meant for complete protection of H.V. winding of
generator transformer. The delta side of the generator transformer is considered as a
part of the generator and its earth fault would cause the earth fault current to flow
toward the generator neutral and be detected as generator earth fault.
The relay is designated as 64 GT and is CAG 14 type one amp. and a high impedance
definite current attracted armature type.
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d. G.T. OVERALL DIFFERENTIAL
Since Generator Transformer is directly connected to the stator winding, it would be
proper to include the transformer windings associated bus ducts including those for
UAT HV side and conductors in a similar circulating current protection scheme. The
relay is designated 87 GT and is of DTH 32 type 5 amp which is a biased differential
type relay.
Biased setting of 30% is used to prevent the relay operation in case of a through fault
when the current transformer may saturate and produce an erroneous secondary
current.
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EARTH FAULT PROTECTIONS
a. STATOR EARTH FAULT (Main)
The generator neutral is earthed through the primary winding of neutral grounding
transformer of the rating 50 KVA, 15.75/0.24 KV ratios. The secondary winding of
the transformer is shorted through loading resistance of 0.42 Ω. For an earth fault in
the generator the E/F current flows in the primary of the neutral grounding
transformer. As a result a voltage across the resistor is developed which activates
stator E/F sensing relay.
The reason for this kind of protection is due to mechanical damages resulting from
the insulation fatigue, creepage of the conductor bases, vibration of the conductors
or other fittings of the cooling systems. The earth fault relay designated is VDG 14
type 64 G1. The relay has a inverse definite minimum time characteristics.
Generally 5% Generator winding starting from neutral point remains unprotected
because a fault in these portions will generate too low a voltage for relay operation.
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Stator Earth Fault (Main)
b. STATOR STANDBY EARTH FAULT
The relay is connected across an open delta of the generator PT secondary windings.
When there is no E/F, the sum of the phase voltages of the generator and hence the
voltage across the relay is zero. The voltage across the point a & b will assume a
positive value when one phase voltage of the generator drops because of earth fault
on that phase.
The relay is 64 G2, VDG 14 type. It has a inverse time voltage characteristics.
c. ROTOR EARTH FAULT
Ground leakage in the rotor circuit of a generator does not adversely affect operation,
if it occurs only at one point. Danger arises if a second fault occurs causing the
current to be diverted in part at least, from the intervening turns, which can burn
the conductor causing severe damage to rotor. If a large portion of winding is
shorted, the field flux pattern may change causing the flux concentration at one pole
and wide dispensation at the other. The attractive force, which is proportional to the
square of the flux density, will be stronger at one pole than the other which will
cause high vibration and may damage the bearings and may sufficiently displace
rotor thereby fouling the stator.
Rotor E/F protection is provided by monitoring the I.R value of the rotor winding
I.R value < 5.5 KΩ : Alarm ,
I.R value < 2.2 KΩ : Trip
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STATOR INTER-TURN FAULT
When leakage occurs between the turns in the same phase of a winding the induced
voltage is reduced and there will be a voltage difference between the centre of the
terminal voltage triangle and the neutral of the machine. Therefore, in a generator
having one winding per phase, a voltage transformer is connected between each
phase terminal and the neutral of the winding, the secondary transformer leads
being connected in open-delta, when inter-turn leakage occurs at the ends of the
open delta, it is detected by a polarised voltage relay.
For generators having several parallel windings per phase, the neutral ends are
connected together to form, as many neutrals as there are parallel windings per
phase. These neutral are then joined through current transformer to current relays,
or through voltage transformer to voltage relay. If an inter-turn fault occurs in the
machine, the current transformer carries a transient current or alternatively voltage
transformer produce a voltage thereby picking up relay and tripping the Generator.
The relay is designated 50 GI, is a CAG 14 type 5 amp attracted armature, definite
current operated type.
NEGATIVE PHASE SEQUENCE
A three phase balanced load produces a reaction field, which is constant and rotates
synchronously with the rotor field system. Any unbalanced condition could be
resolved into positive, negative and zero sequence components. The positive sequence
component is similar to the balanced load. The zero sequence components do not
produce armature reaction. The negative sequence component is similar to that of
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positive sequence but the resulting reaction field rotates in the opposite direction.
Hence the flux produced by negative phase sequence current cuts the rotor at double
the rotational speed thereby inducing double frequency currents. As a result eddy
currents produced are very large and cause severe heating of the rotor windings
particularly damper windings.
For any current conditions in the three phases the amount of unbalance can be
determined from the values of negative sequence components I2 of current by the
method of symmetrical components.
The degree of unbalance is taken to the value of the negative sequence current
component expressed, as percentage of rated current. The losses in the rotor are
2
proportional to the square of the degree of unbalance. This generator has I2 t = 8
2
characteristics indicating that within I2 t < 8, the generator is capable of
withstanding but beyond it there is time delay. The time delay has to be matched to
the machine negative sequence current withstands capability. The relay used is
designated 46G and is of solid-state design and CTNM type.
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GENERATOR BACK-UP IMPEDANCE PROTECTION
Three-phase zone impedance is provided for the back-up protection of generator
against external three phases and phase to phase faults in 400 KV systems. The zone
of impedance relay should be extended beyond 400 KV switchyard and it should be
connected to trip the generator after a time delay of 1 to 1.5 seconds so that the
generator is tripped only when 400 KV protections has not cleared the fault even in
the second zone. The relay used is designated 21G.
LOSS OF EXCITATION
Failure of the field system leads to losing of Synchronism and resulting in running
above synchronous speed. It acts as an induction generator, the main flux being
produced by wattless stator current drawn from the system. Operation as an
induction generator necessitates the flow of slip frequency current in the rotor;
damper winding, and slot wedges, excitation under these conditions requires a large
reactive component which approaches the value of rated output of the machine.
Since rotor would get over heated due to slip frequency current, the machine should
not run more than a few seconds without excitation. Also, it could overload the grid,
which may not be able to supply the required excitation MVAR. When loss of
excitation is accompanied by under voltage it will initiate Class-A trip. Otherwise
Class-B trip if the grid is able to sustain the voltage dip. The relay used is designated
40 G YCGF type.
POLE SLIPPING
The asynchronous operation of the machine while the excitation is still intact unlike
loss of excitation causes severe shock to both machine and grid due to violent
oscillations in both active power and reactive power. Because of this the machine
may fall out of step or usually known as pole slipping trip.
The oscillation may disappear in few seconds; in that case it is not desirable to trip
the machine. If however the angular displacement of the rotor exceeds the stability
limit the rotor will slip a pole pitch. If this disturbance has been sufficiently reduced
by the time this has occurred, the machine may regain synchronism, but if it does
not, it must be isolated from the system.
The swing curves can be detected by an impedance relay. The relay has two
measuring elements set at two values near the independence as seen by the relay. As
the impedance seen by the relay changes it comes in the operating zone of the two
relays one after the other. The sequential operation is observed by auxiliary relays.
Since the system faults would suddenly change the system impedance both the
relays shall operate within 55 ms. however, during pole slipping, the two elements
would operate sequentially and a trip command is given when both have operated.
The relay can be set to be in operation for swings up to +90O corresponding to the
stability limits of the unit. The relay used is 98G and is of solid-state design of ZTO
type. In order to discriminate against swings on the grid, the tripping is through an
impedance relay (98 GY) set with a reach up to the 400 KV yard.
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OVER VOLTAGE
The generator winding is rated for 15.75 KV at terminal, sustained over voltage
would unduly stress the winding insulation and may lead to failure.
To protect the machine against over voltage the protection relay senses the voltage at
the secondary of the bus duct PTs. The relay is set to operate at 10% rise in the
terminal voltage. A time delay of 3 seconds is provided to take care of transient over
voltage arising from line charging, switching capacitive faults etc. The relay used is
designated 59 G & is VTU 12 type 110V A.C.
G.T. OVER FLUXING
The iron core of the generator transformer carries the flux to produce required emf. If
the flux increases unduly, the magnetic circuits of the generator and G.T become
over saturated resulting in high magnetising current. This in turn leads to higher
iron losses, which will increase the winding temp. of the transformer. Since core can
be damaged because of this overheating, protection has to be provided against it. The
flux is dependent on ratio of voltage & frequency.
The condition of over fluxing could arise in case the voltage at the machine terminal
rises or its frequency drops or both occurring simultaneously. Practically this
condition will arise if the machine AVR misbehaves thereby unduly increasing the
voltage even when the grid frequency is low.
The relay used is 99 GT and is GTT21 type which senses v/f ratio at the secondary
of the bus duct P.T. and gives alarm and trip signals at different time delay. The
adopted setting for relay is v/f = 1.2 P.U. i.e. 20% higher than rated v/f ratio.
Alarm is at 0.5 - 1 sec & trip at 12 sec in v/f relay & generates AVR '
Raise'block.
Surge voltage originating from lines because of switching or atmospheric disturbance
is dealt with directly by lighting arrestor and surge diverters.
LOW FORWARD POWER PROTECTION
When a generator, synchronised with the grid, loses its driving force the generator
remains in synchronism. The generator should be isolated from the grid after the
steam flow ceases and the flow of power to grid reduces to minimum i.e. the point
when the generator starts drawing power from the grid and acts as motor. When the
load on generator drops to less than 0.5 percent, generator low forward power relay
gets energised and with turbine tripped or stop valves closed, trips the generator with
a time delay of 2 seconds.
This is a protection to trip generator on other than electrical faults. Also this
protection is used for a few electrical faults where generator trip can be delayed.
However, provision for time lag unit is there to prevent undesired operation from
transient power reversal.
The power relay used is designated 32 G1 and is a WCD type.
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REVERSE POWER PROTECTION
The generator must be disconnected from the grid as soon as turbine stop/control
valves have closed, completely shutting off the steam. Continued full speed turbine
rotation causes lot of turbulence of the trapped steam, which results in increase of
temperature. Thus turbine will be subjected to excessive thermal overstress,
vibration and distortion. So there is a back-up arrangement to trip the generator if it
does not trip within 2 seconds i.e. on L.F.P. protection. This is known as Reverse
power Protection which acts in two stages.
1st stage reverse power relay operates after 5 seconds time delay and includes stop
valve closing/turbine trip. 2nd stage Reverse Power Relay acts after 60 seconds time
delay which trips the generator irrespective of either stop valve closing or turbine
trip. This acts as a final back up to L.F.P. protection. The power relay designated is
32 G2 and is also WCD type.
LOCAL BREAKER BACK-UP PROTECTION (LBB)
This is a protection against the main Gen. C.B. failure, which may occur due to
(i)
Mechanical failure
(ii)
Trip circuits not healthy.
Hence, this protection acts as a back up to the main generator by tripping all the
breakers connected to that particular bus.
Relay sensing
•
D.C. to the relay extended through trip command (either 86 G or 286 G or
B/B protection trip).
•
Over current element senses actual fault persisting.
When both the above conditions are satisfied LBB protection acts with a timer (0.2
secs.) to trip all other breakers connected to that bus. The LBB protection initiates
bus bar protection. (Refer to the bus-bar protection scheme). The relay used is
designated as 50Z.
GENERATOR TRANSFORMER PROTECTIONS
G.T. BUCHHOLZ OPERATION
Any internal fault in generator transformer will result into rapid increase in the
winding temperature resulting in vaporisation of oil (dissociation of oil) accompanied
by generation of gas. The generated gas is utilised for relay operation.
The relay is a gas-operated device arranged in the pipeline between the transformer
tank and separate oil conservator. The vessel is full of oil. It contains two floats b1
and b2 which are to be hinged and to be pressed by their buoyancy against two
stops. If gas bubbles are generated in the transformer due to fault, they will rise and
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get trapped in the upper part of the relay chamber thereby displacing the oil and
lowering the float b1. This sinks and eventually closes an external contact, which
operates an alarm.
If the rate of generation of gas is small the lower float b2 is unaffected. When the fault
is dangerous and gas production is violent the sudden displacement of oil along with
the pipe tilts the float b2 and causes a second contact to be closed and making the
trip-circuit and operating the main switches on both HV & LV sides. Gas is not
produced until temperature exceeds about 150oC, so momentary overload of
transformer does not affect the relay unless the transformer is really hot. Also
insufficient oil level in Buchholz relay could lead to same operation.
THERMAL OVERLOAD PROTECTION
Vapour pressure thermometers or resistance temperature detectors are used for this
purpose. The transformer winding temperature and the oil temperature are
continuously monitored; when the temperature reaches a certain value it will give
indication. Then the load on the transformer is to be reduced. If the temperature
rises still further tripping will take place.
FIRE PROTECTION
Sprinkler system is utilised to protect the transformers from fire hazards. Sprinkler
installation comprising of a system of interconnected pipes into which sprinkler
heads are fitted on a definite basis of distribution. Sprinkler heads are so
constructed that the heat arising from fire will cause them to rupture. Generally the
sprinkler/system consists of a compressed air line and a water line. Sprinkler heads
are provided in the compressed air line.
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The compressed air line will always be kept in charged condition. When the sprinkler
head ruptures, the pressure in the water header will open to send the water into the
water header, from his water will be sprinkled on to the transformer.
BUS BAR PROTECTION
This is a protection against 400 KV bus faults. This protection trips all the feeders
connected to the faulted bus zone; it must be reliable and discriminatory so as to
(i)
trip only the faulted bus section (zone)
(ii)
not to operate for external faults.
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The current differential senses the fault through high impedance voltage relay (Type
FTG) to reduce chances of mal-operation on external faults due to CT saturation.
All CTs in that particular zone are parallel with proper polarity to obtain the current
differential, which is fed to the relay. Sensing of the particular zone is made through
isolator contact status relay (type VAJC).
400 KV yard has 6 bus zones (4 main and 2 transfer zone) as shown in figure below.
Section I
Bus 1 (A)
X
Bus 3 (D)
Bus 2 (B)
X
Bus 4 (E)
Section II
Trans. Bus 1(C)
X
Bus 3 (D)
Each feeder has one common CT for main zone bus protection. The current is
switched into appropriate zone (zone A, B, C etc.) through isolator status relay
contacts. The operation of the relays is used to energize B/B protection trip buses.
The bus bar protection trip D.C. relays (97) are connected to these trip buses again
through respective isolator status contacts.
In addition to each of the main zones there is an overall check zone to increase
reliability of the whole system. This zone covers the whole of 400 KV yard and uses a
separate CT core to reduce chances of mal-operation due to CT saturation, loose
connection, shorting etc.
The A.C current scheme is similar to main zone except it is not routed through any
isolator selection. The D.C trip circuit is not complete unless the check protection
also operates i.e. for any Bus bar protection trip to occur, both or any one of the
main zones and check zone must operate. Also a supervision relay '
95'connected in
the A.C. scheme is provided. This is set at a lower value so that it can sense shorting
/ opening of one CT circuit current at normal operating value. This provides an
alarm and also isolates the B/B protection scheme.
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GENERATOR PROTECTION
GT Fire Protection, 30G
GT Pressure Release Valve, 30 B
GT Buchholz Protection, 30C
Bus Bar Zone ‘C’ Protection, 30 D
LBB Protection, 30 E
Excitation System Fault, 30 Z
Generator Diff Protection, 87 GX
Gen. Over head feeder Diff Protection, 87 LX
UAT A Diff Protection, 87 UTX
UAT B Diff Protection, 87UT BX
GEN. Over voltage, 59 GC
GEN. Inter-Turn Fault, 50 GIX
GEN. Pole Slipping protection, 98 GX
Stator E/F Protection, 64 GIX
GT Restricted E/F Protection, 64 GTX
Low Forward Power Protection, 32 G1
UTR-A
UTR-B
GT Block Overall Diff Protection 87 GTX
GEN. Stator stand-by E/F Protection 64 G2X
UT-A Buchholz Trip 30 K
UT-A Buchholz Trip 30 N
UT-A Fire Protection 30 L
UT-B Fire Protection 30 P
Gen Negative Sequence Protection 46 T
LBB Protection 50 ZX
From Relay 86 G
UAT-A Back-Up Over current Protection 51 UTAX
UAT-B Back-Up Over current Protection 51 UTBX
GT Over-Fluxing, 99 GTTX
Gen. Back-up Impedance Protection, 21G
Gen. Reverse Power Protection 32G
Low Forward Power Protection, 32 G1
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GENERATOR PROTECTION
Gen. Backup Impedance Protection, 21G
Trip Relay 86 G
Trip Relay 286 G
GT E/F Protection
TRIP
RELAY
386 G
GEN CIRCUIT BREAKER TRIP
TRANSFER BUS COUPLER BREAKER TRIP
UAT-A WINDING TEMPERATURE 2nd STAGE
UAT-A OIL TEMPERATURE 2nd STAGE
TRIP RELAY 86 G , TRIP RELAY 286G
6.6 KV UNIT
BUA-A TRIP
UAT-B WINDING TEMPERATURE 2nd STAGE
UAT-B OIL TEMPERATURE 2nd STAGE
TRIP RELAY 86 G, TRIP RELAY 286G
6.6 KV UNIT
BUA-B TRIP
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MEASUREMENTS
AND
CONTROL
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INSTRUMENTS AND MEASUREMENTS
Measurement is the source of important and necessary information for any
continuous process. Measurement is essential tool and it supports to monitor the
operational parameters of any system or the process that performs. Knowledge of
measurement is essential for designing any process. In any of the process, the
Measurement systems are usually of two types 1) Mechanical and 2) Power type. In
making most engineering measurements, we require the assistance of some form of
measuring system and measurement by direct comparison (is less general than
indirect) or by indirect comparison. Instruments are the measuring means and these
are vital exactly like body nerves or the brain in human organs.
The basic measuring element or the combination of elements, '
sense'or '
measure'the
quantity that sets the limit for the '
integrity' of the measured value. Measuring
systems are studied, utilized and designed such that, the output signals truthfully
represent the state, condition or characteristic of the system in study. The accuracy
of indicated or displayed value may be less if the output signal of the basic
measuring element is handled by some intermediate systems. The accuracy with
which the condition of a system is controlled by an automatic control system has
similar accuracy as measured system must have. Computer control of an industrial
process demands that the measurement is fast enough to result in real-time
instrumentation and control.
Signal processing on output signals bring them into a form or description compatible
with the computer. The output signals of the primary measuring systems are
converted into proportionate analog electrical signals, and into digital signals.
Digital-signal processing techniques are used for developing sophisticated
instrumentation systems with multifunction capability.
Measuring element provides an output signal that is displayed and recorded in both
analog and digital fashion. Digital signals can be transmitted to a distant and remote
station, with greater accuracy and integrity than that of analog signals. Electronic
data handling systems tackle the signals for display, and recording. Data acquisition
systems (DAS) are used for displaying and subsequent operation on the data derived
from all the basic measuring systems.
The data is transmitted over a long distance; it is reduced and reconstructed to the
original form. The data handling operations constitute acquisition, transmission and
reduction of the data. Data logging is done by computers, which have provisions for
storing data. In analogue signal processing the amplitude is increased or a power
through some form of amplification is utilized.
In the Figure shown below, various stages of operations carried out on the controls
and measurement system has been depicted. Control function is generated after the
measured value is compared with the desired or reference value in the form of
standard output signals for use in automatic control. Measured data is acquired and
pooled at a central location in complex system for display, record or decision-making.
Instrumentation system encompasses the entire data handling, starting with the
basic function of measurement or releasing the control function signals. Automatic
data processing and automatic computation use the logical components in the
modern sophisticated systems. The instrumentation system is formed from
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application of all the basic measuring systems as well as the processing of the
measured data. One has to understand fully the system and select/use
instrumentation for processing data.
In power plants, measurements of the parameters viz. the steam pressure,
temperature and flow, air & flue gas flow; its quality, and characteristics in
combustion chambers (Boilers), the vibration, frequencies, voltages or the current
amplitudes of the turbines, and generators etc. are essentially required for keeping
proper control/check on operation activities. The chain of devices converts the basic
form of input into analogue form or into a digital form that represents the output as
known function of the input. The measuring instrument senses, converts and finally
presents an analogue or the digital output in the form of display and displacement in
a scale or a chart.
The first contact that a measuring system has with the quantity to be measured is
through the input sample accepted by the detecting element that senses or detects
the input signal and then transduces into an analogous form.
Acquiring the reliable measurements and then correctly interpreting its meaning
invariably leads one nearer & nearer to the desired solution. In making most
engineering measurements, we require the assistance of some form of measuring
system and measurement by direct comparison (is less general than indirect) or by
indirect comparison. Normally Mechanical devices function as Primary
Detector/Transducer and the Electrical device serve as a Secondary Transducer.
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MEASURING SYSTEM: INPUT-OUTPUT FUNCTIONING
The input/output relationship and the elements have been shown in the functional
diagram of measuring systems. Input quantity in the form of sensation is used to
drive the primary sensing element as shown, the transducer then converts it in
secondary signal, it is then conditioned & converted suitably; additional power
source is given for the transducer/primary sensor operation. The signal then
becomes the desired data which is transmitted & telemetered then further processed
for driving the output instruments like display indicators, recorders CRT displays,
limit monitors and other applications.
In the subsequent diagram the input-output relationship in instrumentation has
been shown. The input quantities are classified as the Desired input, the Interfering
input, and the Modifying inputs. These inputs are then amplified/suitably
conditioned through amplifier modules as shown. Finally the output components due
to interfering input and the output component due to the desired input with
modifying components are added, to give adequate output.
MEASURING SYSTEM FUNCTIONAL DIAGRAM
INPUT-OUTPUT FUNCTIONAL DIAGRAM
INSTRUMENTATION AMPLIFIERS
Instrumentation Amplifiers are used to boost the amplitude of the signal, buffer the
signal, convert the signal current into voltage and separate differential signal,
unwanted common mode signal etc The amplifiers are mostly used in the
applications of thermocouples, Strain gauge bridges and biological electrodes
connected in Wheatstone bridge configurations as shown. The operational amplifiers
are intended primarily for amplification of voltage signal derived from transducers’
circuit with accurately adjusted gain values; any variation in gain affects the
accuracy of measure of primary quantity.
The Instrument amplifiers amplify the low level signals superimposed with common
mode voltage and have the characteristics like low drift, high input impendence, high
linearity, high CMRR, high noise rejection capability etc.
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The instrument amplifiers are the improved version of differential amplifiers (FET
input op-amps. connected in high CMRR using non-inverting configuration), its Net
gain; A = (1+2R2/R1) (R4/R3); the potentiometer R1 can give the required gain. Full
scale voltage level out of the amplifier is achieved either by sample and hold circuits,
analogue multiplexer, A/D converters etc.
The Wheatstone bridge is connected with the differential amplifier having voltage
Vequivalent and the resistances at the input terminals of the Op-amp become unequal.
The bridge is not perfectly balanced and error Vo appears due to Common-Mode
input at the amplifier (voltage EBD decides the error) used as instrument amplifier.
Differential voltage ediff appears across Rg and creates an imbalance in the current
flowing through the transistors Q1 and Q2.The current mirror consisting of Q3 & Q4
forces the imbalance current to flow through Rs.The output voltage is Rs/Rg. The
amplifiers are available with programmable gains
Basically, the voltage sensitive measuring instruments have very high input
impedance so loading must be avoided. All electrical measuring instruments
sensitive to current can be used for voltage application (calibrated in voltage scale) by
adding suitable resistors in series.
WHEATSTONE BRIDGE CONFIGURATION
INSTRUMENT AMPLIFIERS (DIFF VOLTAGE)
INSTRUMENT TRANSMITTERS/DETECTORS
A detector senses the input information, and the transducer puts it in to a more
convenient form. The Instrument transducers convert the mechanical input to an
analogue electrical output for further processing it in readable/usable form. The
sensor or the detector detects the Physical quantities as “signal” by mechanical
elastic members or by electrical means. Therefore the transducers require electrical
power supply particularly for dynamic measurements, driving and transferring the
signal into usable form.
The transduction implies energy conversion, and the transducers may be genuine
energy converters (called ‘active’ transducers), in some cases they require auxiliary
energy source and are therefore energy controllers (called ‘passive’ transducers). The
transducer, sensor or detector is the device that measures the physical quantity by
electrical or other means. The measuring system consists of a primary sensing
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element and a transducer that converts the energy sampled into proportional
/corresponding to the input energy signal. Information about the mechanical is
usually obtained as displacement (seismic mass converts acceleration to force or
movement). A transducer is the first element in the measurement of mechanical
quantity (termed as primary sensing element); it requires long-term stability for
input-output relationship. A transducer has the size, weight and shape, and it
responses to rapid changes in measurand and its electrical output impedance.
The transducer/detector elements are classified according to the device or the
attachments (Class-1: Detector, Class-2: Detector with Single Transducer and class3: Detector with two transducers) etc. The detectors/ transducers are further
classified in two categories as per their functioning;
i)
ii)
Passive transducers and
Active transducers.
Passive transducers depend on material characteristics & physical configuration and
the conductance (inverse of resistance), inductances, capacitance etc. so they need
external supplies for effecting changes in signal voltage, current or frequency.
Active transducers are self-generating and are used for measuring the velocity,
temperature, light and force; they normally do not require the auxiliary electrical
supplies for driving signal. The active transducers involve conversion of energy from
one form to another, and in most cases this conversion operates in both directions.
According to signal handling, the transducers are further categorised as:
a) The Analogue transducers and b) Digital transducers
Analogue transducers provide outputs in analogue form and they need A/D
converters for digital application. The outputs of Analogue Transducers express the
measured quantity as amplitude that is continuous with time (analogue signal).
Digital Transducers (The direct digital transducers) provide digital output signal
directly in the form of rectangular pulses of constant duration and amplitude. In the
indirect digital type transducers the output signals are sinusoidal with the frequency
related to measurand and it works in combination with digital frequency measuring
system. The digital encoder for linear and angular displacement is used in direct
digital transducers. The digital encoders are either incremental encoder or absolute
encoder type.
Due to enormous research works, the transducers have been miniaturized too much;
these characteristics have led designers to use transducers invariably in all process
industries. The digital signals are in the form of pulses, or in the form of sinusoidal
time sequence of 1-0 (binary output), but are not essentially dependent on signal
amplitude. Digital signal can be transmitted to long distances and it does not get
affected due to amplitude change, phase shift (in analogue signal), and it is
manipulated without error in electronic processing circuit. Analogue transducers are
becoming obsolete presently because of easy refined and cheap technique deployed in
digital 1-0 binary system.
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Many quantities e.g. pulse count, determined frequency and encoded position,
accurately with high resolution are measured in one or the other form of digital
transudation. Frequency outputs include amplitude of frequency converters (voltage
oscillators). The digital resonators measure the temperature (even that of very small
mass). Periodic time is very easily converted into binary signal so when the frequency
of transducer is very high then signal is mixed or multiplied with transparent high
frequency signal and there is no difference of frequency. Velocity, temperature, light
intensity and force can be transduced with devices that are self-generating and do
not require auxiliary electrical power supplies.
Measurement of electrical quantity normally does not require transducers; since the
primary sensing element is filter or rectifier. The filter networks are common in
instruments and filter characteristics are obtained by the use of the feedback of the
output signal. Mostly, the electrical elements convert mechanical displacement into
voltage thus it normally performs as secondary transducer. Some of the inherent
compatibilities of mechno-electrical transducers are as under:
1. Amplification or attenuation may be easily obtained.
2. Mass-inertia effects are minimized.
3. The effects of friction are minimized.
4. Output with sufficient power for control is provided.
5. Remote indication or recording is feasible.
In the figures shown below are the transmitters that detect the mechanical forces like
the ones given in the mechanical elastic members or the proving rings.
The Electric transmitter, shown below, is Foxboro make, electronic force balance
transmitter that receives the force through the force bar or the vector flexure. Force
is applied via a flexure to the lower end of the force bar, which pivots on the
diaphragm seal (It isolates the process from measuring system). Corresponding force
bar transmits the effective angle for span adjustment. The force on vector assembly
causes movement on the apex.
The detector armature (ferrite disc) moves with detector coil and modifies the
coupling between the coils, connected to an oscillator circuit and applied to feedback
motor to balance the initial disturbing force. This current flowing through the
feedback coil is used as the output signal of the transmitter. Vector flexure is used
for fine-tuning and larger changes by effective gain change of oscillator as shown
here.
The D.P. transmitter senses the differential pressure or Pressure signal that is
converted into force-balance system for measuring the gauge or absolute pressures
as shown in both two figures. Force developed by the primary element is applied via a
flexure to one end of the force bar joined together with a thin circular diaphragm
serving as flexure. The framework carries the Zero-adjustment spring/ feedback
bellows, pneumatic relays etc.
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The force-balance mechanism functions as gauge pressure transmitter, if the lowpressure connection is open to the atmosphere. It also functions as a high-range D.P.
transmitter, if both high-and-low-pressure sensing signals are connected to pressure
connections of the obstruction elements (discussed in detail in flow measurement
section). It can function as a Target flow transmitter, if the primary & secondary
elements (forming an integral unit) is particularly used for measuring high viscosity
liquids. i.e. asphalt, tar, slurry at pressures up to 100 bars.
Similar arrangements can be used as Target flow meters for gas flow. In that case the
liquid impinging on the target will be brought to rest so that the pressure increases
by V2/2g and the Force on target is given by F=(K. V2.A1)/2. The Force is balanced
through the force bar by the air pressure in the bellow so that a 0.2 – 1.0 bar signal
proportional to the square root of flow is obtained. Overall accuracy varies from 0.5%
and repeatability of 0.1%. The pneumatic system is common for all detectors such as
DP , pressure, target flow transmitter etc applications.
ELECTRICAL TRANSMITTER
PNEUMATIC D.P.TRANSMITTER
PNEUMAT. PRESSURE TXR
Capacitive Transducers
The Capacitive Transducers work on the principle of variation of capacitance due to
cutting of dielectric and change by plate (area) for transducer operation. The
Capacitance C = 0.244 KA (N-1)/d, where, K = die-electric constant, A =Plate area (in
2), N = Number of plates, D=separation of plate surface in inch etc. It is evident from
above that the capacity effect can be obtained either by change in the dielectric
constant or change of area. The change of dielectric constant is used in measurement
of level in a container of liquid hydrogen or similar chemical element.
The capacitive device detects liquid level even though the reference in dielectric
constant between the liquid and vapour state is as low as 0.5. The changing distance
between blades of capacitance is undoubtedly more commonly employed for using
capacity pickups. The capacitance between the diaphragms & the electrode output is
used in a measure of relative position of diaphragms and the distance change
between itself and the electrode.
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Geometrical variation
Single
Permittivity variation
V=dielectric/metal
O/p C
Liquid level gauge
Differentia
l o/p
C± C
and
C± C
W=dielectric/metal
VARIOUS TYPES OF CAPACITIVE SENSORS USING DIFFERENT CHARACTERISTICS
Capacitive Pressure Transducers: Metallic diaphragm/member forms the movable
plate of the transducer. The capacitance change due to deformation of the clamped
diaphragm can be obtained. Final fractional change in capacitance is given by:
C/C0 ={(1-µ 2 ).R4. P}/16.E.dt3
Thus differential pressure can be related with the change in capacitance as above.
In a Cylindrical electrode type pressure transducer, the pressure is continuously
measured under flowing conditions. The walls of the metallic pipe are used as the
outer electrode and a solid cylindrical rod running along the pipe serves as the inner
electrode and its capacitance is related by equation: C0= (2 . 0 r.L)/ln (r2/n),
Where L= length of central electrode, r2= Inner radius of pipe, ln= natural log,
r1=central electrode radius, and r =dielectric constant of the fluid
Such transducers are used for indication of pressure under static or slow varying
conditions. Bridge circuit is used, the standard capacitor under static & std pressure
is kept as reference in the bridge arm.
Capacitive Displacement transducer is a standard proximity type sensor in which the
movable plate/electrode functions as conductive surface of the object in the vicinity
of the fixed plate.
Electrostatic instruments (for measuring sufficiently high DC voltage): work on the
principle of variable capacity (movable plate), it draws certain amount of energy in
the initial stage for setting up the electric field otherwise Electrostatic instruments
does not draw any power for AC (rms) measurement; it draws the current however.
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Typical Primary Detector-Transducer Elements & their Operations
Type
I. Mechanical
A. Contacting spindle, pin or finger
B. Elastic member
1. Proving ring
2. Bourdon tube
3.Bellows
4.Disphragm
5. Spring
C. Mass
1. Seismic mass
2.Pendulum scale
3. Manometer
D. Thermal
1. Thermocouple
2. Bi-metal (includes Hg in glass)
3. Temperature-stik
E. Hydro-pneumatic
1. Static
a) Float
b) Hydrometer
2. Dynamic
a) Orifice
b) Venturi
c) Pitot Tube
d) Vanes
e) Turbines
II Electrical
A. Resistive
1. Contacting
2. Variable-length conductor
3. Variable area of conductor
4. Variable dimensions of conductor
5. Variable resistivity of conductor
B. Inductive
1. Variable coil dimensions
2. Variable air gap
3. Changing core material
4. Changing coil positions
5. Changing core positions
6. Moving coil
7. Moving permanent magnet
8. Moving core
C. Capacitive
1. Changing air gap
2. Changing plate areas
3. Changing dielectric
D. Electronic, Piezoelectric,
E Photoelectric
F. Streaming potential
KORBA SIMULATOR
Operation
Displacement to displacement
Force to displacement
Pressure to displacement
Pressure to displacement
Pressure to displacement
Force to displacement
Forcing function displacement
Force to displacement
Pressure to displacement
Temperature to electric current
Temperature to displacement
Temperature to phase
Fluid level to displacement
Specific gravity to displacement
Velocity to pressure
Velocity to pressure
Velocity to pressure
Velocity to force
Linear to angular velocity
Displacement to resistance change
Displacement to resistance change
Displacement to resistance change
Strain to resistance change
Temp. to resistance change
Displacement to inductance change
Displacement to inductance change
Displacement to inductance change
Displacement to inductance change
Displacement to inductance change
Velocity to inductance change
Velocity to inductance change
Velocity to inductance change
Displacement to capacitance change
Displacement to capacitance change
Displacement to capacitance change
Displacement to current, voltage
Light intensity to voltage
Flow to voltage
239
In view of statistical treatment of data some characteristics of the instruments &
related defining terms or terms often applied during working, are given below:
Information: The media handled in the process is termed as Information. The
information is extracted from extraneous mass input energy by conversion,
transudations, filtering process etc.
Data is the elemental bits of information in numerical form obtained by experimental
means.
True or actual value (Va) is the actual magnitude of an input signal to a measuring
system. The true value of any physical quantity is obtained by experimental methods
as far as possible nearest to the true value; the error of measurement is the
difference between the true value and the obtained result. Indicated value (Vi) is the
magnitude of raw or directly recorded data by the measuring system.
Transfer efficiency (ηT) = Iout / Iin (information) gives the ratio of the input
information [Iin] received & output information [Iout] delivered by the pick-up.
[Normally ηT < 1,but it is desired to have this value as high as possible.)
Correction improves the worth of the result and it is the revision in the form of
either an additive factor or a multiplier or both applied to the indicated value.
Result (Vr) is obtained by making all known corrections to the indicated value and is
derived from Vr = A Vi + B, where A, B are the corrections.
Discrepancy is the difference between two indicated values or results determined
from a supposedly fixed true value.
Error is the actual difference between the true value and the result and is derived
from Error = Va – Vr; the value of the error is never really known. Errors can be the
systematic or fixed, random or accidental, illegitimate etc and Error is according to
the nature and type of activity & techniques used like Human & Experimental
Errors, Loading/ judgmental, Variation of conditions, Definition, Blunders or
mistakes, Computational, Chaotic etc. Maximum Error is the value by which the
result differs from the true value and measured value (associated with small
systematic error) and is given by Vr (max or min)] for the true value or the actual
value. Certain errors may add directly whereas other may not.
A study of error propagation must include consideration of the interrelationship of
the various types of error. ei =√ (e1)2 + (e2)2 + (en)2 where ei = overall independent
error And e1,en = independent errors.(It’s relationship is often employed for summing
equally weighted, independent errors).
Accuracy is expressed in percentage based either on the actual scale reading or on
full-scale reading: and it is derived from Accuracy = Vr (max or min) – Va.;
Percent accuracy (scale) = {Vr (max or min) – VA] / Va} X 100 and
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Percent accuracy (full scale)={Vr (max or min) – Va] / Vfs} X 100,here Vfs is the full
scale reading; these are based on scale reading and the full scale readings
respectively; The full-scale accuracies have particular settings and scale in use.
Precision shows the degree of agreement between repeated results – measurement
with small random error. Precise data have small dispersion (spread or scatter), but
may be far from the true value.
Uncertainty informs possible error or what one thinks may be the range of error.
Uncertainty differs from accuracy, for; although accuracy may not actually be
known, it is a definite concrete number for a given situation. Uncertainty, on the
other hand, is the region in which one believes (or guesses) the error to be. This
relation may be described in terms of an analogous idea, a limit dimension.
Propagation of uncertainty is the manner in which the uncertainties affect result.
Range is the difference between the largest and the smallest result. Scale Range of
any instrument is defined as the largest and the smallest reading of the instrument.
Frequency range defines the range of frequencies over which measurement may be
taken with specified accuracy.
Deviation: Deviation or the Residual is the difference between a single result & mean
of many results is termed as. Percent standard deviation is the ratio of the standard
deviation to the mean.
Divisions are marked on a scale; the set of marks or division forms an Index scale
and the division moved is the index reading.
Linearity: The measurement should response to linear variation (maintain linearity);
In order to account for small error in read out system the percentage linearity is
maintained as small as possible Percentage linearity is given by: 100 x (maximum
deviation of parameter/ Full-scale deviation).
Fidelity of any system is defined as the ability of the system to reproduce the output
in the same form as the input. Ideally a system should have 100% fidelity and the
output appears in the same form as the input. There should not be any distortion.
The fidelity refers to the situation that there is no time lag or phase difference
between output and input.
Response Time: The time required by instrument to settle to its final steady step
position after application of the input is defined as the Response Time of an
instrument. Whereas the Speed of Response defines the quickness with which a
instrument responds to a change in the quantity being measured.
Measuring Lag: The delay in the response of the instruments to change in
correspondence with the measured quantity is given by Measuring Lag. The
measuring lag becomes important where high-speed measurements are required and
the time lag is required to be reduced to minimum.
Dead time is defined as value before the instrument begins to move after the
measured quantity has been changed. Dead Zone is defined as the largest change of
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the quantity being measured to which the instrument does not respond. The Dead
zone is also termed as backlash or the Hysteresis.
Overshoots: When an input is applied to an instrument, the pointer does not
immediately come to rest at its steady state position but moves beyond it or in other
words overshoots its steady position. It is desirable to have a little overshoots but an
excessive overshoots is undesirable.
Steady-state periodic quantity has definite time-cycle whereas the magnitude/time
variations of transient quantity do not repeat.
Mechanical quantities, in addition to its inherent defining characteristic and
distinctive time-amplitude properties, are Static and Dynamic (steady-state &
transient). Static system is non-changing in characteristic and is most easily
measured. Dynamic inputs require system components, which are sufficiently fast
acting to faithfully follow the inputs. Dynamic system refers to the situation when a
system does not settle to its equilibrium or steady state condition after the
application of the driving force In order to analyse the dynamic behaviour of the
system the STEP, RAMP, SINUSOIDAL functions are imposed and results noted for
its analysis.
Loading: Energy is always taken from the signal source by the measuring system,
which means that the information source must always be changed by the act of
measurement. This effect is referred as Loading; the smaller is the loading on the
signal source of the measuring system, the better is the system. In measuring
systems primarily electrical elements sense input & detect the process parameters;
the loading of the signal source is almost exclusively a function of the detector. The
recorders or the output indicators (being the intermediate modifying devices) receive
most of the driving energy from sources other than the signal source, without
draining an undue amount of energy from the signal.
Measuring Systems: Mechanical Elastic Members and Proving rings
Elastic members are the mechanical sensing elements and these are used to change
forces into displacements. The Pressure-measuring devices use elastic members of
one type or another. The force output from the diaphragm, the bellows, or the
Bourdon tube are all based on elastic deformations brought about by the force
resulting from pressure summation. The mechanical displacements are usually small
in pressure-measuring devices, as with force measurement, (except for the
manometer) and so secondary transducers are used for providing interpretable
outputs. All instruments available in market make use of properties of either
mechanical elastics members or the proving ring characteristics. The driving forces
or the movements/functioning of transmitters depend upon the developed force or
displacements caused by the mechanical elastic members or proving rings.
There are many forms of elastic members for converting mechanical sensation of
pressure/flow/level parameters. They fall into one or a combination of following three
categories:
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(1) Direct tension or compression
(2) Bending moment application
(3) Torsion application
The Bourdon tube is usually referred as Primary Detector–Transducer for sensing
the pressure; it gives output in the form of displacement to drive the linkage chain of
the elements. In this case, there is no secondary chain in signal form. But when the
displacement from the Bourdon tube is used to move the core of the Differential
Transformer for outputting voltage (pressure to displacement and then to voltage in a
pressure transducer application), it performs the function of a Secondary
Transducer.
Secondary Transducer: the ordinary Dial Indicator, in which the spindle acts as a
detector through its contact with the signal source functions as Secondary
Transducer. Another example of Secondary Transducer is the compression type force
measuring load cell.
Manometer is an elastic member device whose deflection is proportional to the force.
The Torque meters usually, although not always, make use of elastic torsion
members. The elastic torsion member twists in proportion to the applied torque, and
the deformation is used as a measure of torque. Here again, secondary transducer
elements are employed to provide a usable output.
Mechanical Proving rings are some form of the mechanical springs & stiff sensing
elements and are widely used in instruments as secondary force standards for
calibrating testing and weighing machines. Micrometer or dial gages are often used
for measuring the deflections; it can accommodate direct tension or compression
members. Strain gages are secondary transducers that measure the deflections.
MECHANICAL ELASTIC MEMBERS
MECHANICAL PROVING RINGS
Pressure Measurement
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The pressure measurement employs three types/methods as given below:
1. Balancing unknown pressure against pressure produced by a column of
known density liquid.
2. Allowing the unknown pressure and measuring the resultant force either
directly or indirectly.
3. Allowing the unknown pressure and measuring the resultant stress / strain
acting on elastic member of known area
Process of balancing a column liquid in U tube of known density; thus gauge
pressure = atmospheric pressure + hp and
P = hm (A/a+1)(p1-p2) also p = p0 / 1+ (T-T0); =coefficient of cubic expansion.
Bourdon Tube: Bourdon tube is a mechanical elastic member as shown above. The
simplest form of Bourdon tube comprises a tube of oval cross-section bent into a
circle. One end is sealed and attached via an adjustable connection link to the lower
end of a pivoted quadrant. The upper part of the quadrant is the toothed segment,
which engages in the teeth of the central pinion, which carries the pointer that moves
with respect to a fixed scale.
Backlash between the quadrant and pinion is minimized by using a delicate
hairspring. The other end of the tube is open so that the pressure to be measured
can be applied via the block to which it is fixed and which also carries the pressure
connection and provides the datum for measurement of the deflection. Bourdon Tube
If the internal pressure exceeds the external pressure the shape of the tube changes
from oval towards circular with the result that it becomes straighter. The movement
of the free end drives the pointer mechanism so that the pointer moves with respect
of the scale. If the internal pressure is less than the external pressure, the free end
of the tube moves towards the block, causing the pointer to move in the opposite
direction.
Diaphragm pressure elements: Two basic categories of diaphragm elements
comprise of i) stiff metallic diaphragms and ii) slack diaphragms. The unknown
pressure is applied to the underside of the diaphragm and resultant movement of the
center of the diaphragm is transmitted through a linkage to drive the pointer as in
the Bourdon gauge.
Bellow elements for pressure measurement: The spring rate or modulus of
compression of bellow varies directly modulus of elasticity of the material from which
it is formed and proportionally to the third power of the valve thickness. The spring
rate of bellow varies directly as the modulus of elasticity of material from which it is
formed.
Bellow and diaphragm sensors are used for measuring differential pressure as given
in the figure below. The bellows are replacing diaphragms since the modulus of the
elasticity of the material is better to have good valve thickness.
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The diaphragm elements are made up of corrugated diaphragms with spacing ring
stretch welded at central hole. This ensures the safety of the diaphragms when
excess pressure is acted upon.
Dead weight tester is the simplest technique for determining the pressure by
measuring the force i.e. generated when it acts on a known area. The calibration of
each instrument confirms the accuracy and correctness of measurement; dead
weight tester is one such type of testing and calibration tool. Use of standard
pressure gauges in parallel to the gauge to be tested is another method of calibration.
BELLOW TYPE D.P. SENSOR
Flow
DIAPHRAGM TYPE DIFF. PRESSURE SENSOR
Measurement
In flow metering, the Primary method includes the weight or volume tanks, burettes
positive displacement category sensors; whereas, the Secondary devices in flow
applications e.g. the ventury, flow nozzle, orifices, and variable area meters fall in the
obstruction category and total/static pressure & direction of sensing probes fall in
velocity probes category. In flow metering, the flowing medium e.g. liquid, gaseous,
granular solid and the type of flow e.g. laminar, turbulent, steady state or transient
etc. decide the type and size of sensor.
In flow measurement, the upstream pressure P1 and the downstream pressure P2
are obtained by suitable tapping provided on either side of the obstruction; the
differential pressure P = (P1-P2) is used to obtain the theoretical value of the mean
flow velocity V2 =E (2 P/ ) , where E = velocity of approach. In flow measurement,
the pressure change is measured and a measure of velocity is obtained. Fluid velocity
is measured by reference to aero or hydrodynamic principle.
The basic principle of flow sensing using velocity of stream is given by V= (2g h) &
the differential pressure is P= h g . Here the,
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Flow Relation formulae can be also reffered, i.e; for an incompressible fluid,
Flow (Q)=A1V1=A2V2 where Velocity (V)= (2g h) and A=area of cross section; and (P1P2) =V22 2g[1-(A2/A1)2 ] , the Velocity of approach (M)= 1/[1-(A2/A1)2 ;
Discharge co-efficient (Cd) = Qactual/Qideal and the Flow-coefficient (K)=CM. Finally the
flow is derived by Q = Cd. E. A2 = K.A2 .V2,
Actual volume flow rate is calculated from the mean flow velocity V2, the area
of opening offered by obstruction is A2 (= d2/4) and K= flow coefficient
(=CdE); and Reynolds Number Rd for a pipe is Rd=D V/µ, in this, D=pipe dia,
V= fluid velocity, =mass density, µ= absolute viscosity etc.
The discharge coefficients and the flow coefficients are determined experimentally for
each size and version of obstruction element and provided by the manufacturers.
The Discharge coefficients vary with the flow conditions as determined by the
Reynolds number, and the accuracy of the flow rate determination depends on the
application of the correct coefficient. Details of the obstruction elements have been
given below for proper understanding flow measurements. Obstruction elements e.g.
the orifice plate, ventury tube, flow-nozzle etc are used for the measurement of mean
flow velocity as also the flow rate. There appears considerable disturbance to the flow
pattern and consequent pressure loss due to obstruction.
All the measurements involving obstruction elements have their useful range
restricted from one-third to the full-scale value of velocity because of the relationship
between velocity and differential pressure.
The Ventury consists of three parts: entrance cone, throat, and exit as shown and
the location of the taps are also shown in the figure. The Ventury is an obstruction
element and it creates a total loss of pressure equal to 10-20% of the differential
pressure across it.
The flow nozzle as shown is a one-piece obstruction element, which can be welded
into the pipe for the measurement of high flow velocity of water or steam at high
pressure.
The Dahl tube is also an obstruction element and it produces longer pressure
differential with lower pressure loss as compared to the venturi. The Dahl tube given
in figure consists of a short length of parallel lead in pipe followed by the converging
upstream cone and the diverging upstream cone. A circumferential gap is kept
between the two cones and the down stream pressure is tapped at the location of the
gap.
The Orifice plate is the simplest and cheapest obstruction element, with its’
coefficient of discharge being the lowest at 0.6. Orifice is a thin metallic disc arranged
concentric with the pipe in most cases. It may be eccentrically located when intended
for use with fluids containing small traces of particulate matter. The segmental
orifice plate has a hole that is partly circular located below its centre. The three types
are shown in figure. The edge of the orifice plate on the upstream face should be
sharp as rounding or burring faces considerably affect the flow rate. The dimensions
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of one of the sharp-edged versions are shown in figure. The size of the opening is
designed so as to produce approximately the maximum pressure difference at the
maximum, rate of flow. Depending on fluid type its low rate, the orifices are made of
stainless steel, nickel, gunmetal, ebonite or plastics.
Pitot tube named/derived from its inventor, Henry Pitot, is used for measuring flow
since in a horizontal pipeline, flowing fluid coincide with velocity vector which varies
from zero at the wall and maximum at the centre. The Pitot tube is having a small
opening facing the fluid flow direction. The tube is used to determine pressures
resulting from total flow-rate rather than change of rate and it determines the impact
pressure.
Pitot tubes are also used for gas flow measurements. The Pitot tube is combined with
static opening. Proper alignment of the tube with the flow direction and YAW-ANGLE
(probe axis and the flow stream line at the pressure opening) of Pitot tubes need to be
properly aligned with flow direction and the Yaw – angle should be zero.
The pressure built up in the Pitot tube is higher than the free stream (static)
pressure. The excess pressure h is termed as impact pressure. Excess pressure P2P1 = p = g h and velocity at Pitot tube mouth is V1 = (2g h); finally V1 = (2hm
m g/ ) ,where hm = diff. in levels of the manometer m = mass density of the liquid,
= mass density of fluid in motion. Figure below show, the obstruction type flow
sensors and their flow pattern.
FLOW METERING SENSORS AND DEVICES
FLOW CHARACTERISTICS
Flow Measurements using Rota meters:
The Rotameter flow system is versatile in that it can be designed to measure the flow
rates of liquids widely ranging in their viscosities and volume flow rates as low as 0.1
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cc/min. The mass flow rate, however, is made totally independent of density, by
making the density of the float double that of the fluid. The float is made hollow or of
solid light plastic material. To make the float independent of viscosity drag, the
length of the float is made small while ensuring that the Reynolds number of the flow
is not greater than 2000. In the Rota meter float systems, the tube is made of
borosilicate. The guiding spindle or shaft carrying the float at the lower end passes
through upper part of the tube, with upper end serving as the index on the scale.
Rotameters are designed on the basis of the types (two) of flow:
i) The vertical flow
ii) The horizontal flow.
Vertical flow type Rota meter-float instruments are of constant head variable area
type flow meters /rotors. The Rota meter-float system employs a float kept in a fluid
stream so that its’ position is a measure of the velocity of the fluid. The measuring
fluid is passed through a tapered glass tube as shown in figure. The float obstructs
the flow and the fluid flows through the annular clearance between the float and the
inner wall of the tube. The float comes to rest, when the forces working on the float
due to upward fluid flow, is balanced by the weight and buoyancy forces.
The clearance between the float and the tapered glass varies in area with the position
of the float, and it sets up a differential pressure across the top and bottom surfaces
of the float. The float always assumes a position for each velocity holding the
differential pressure constant. The position of the float is always hold the D.P and
thus is calibrated in terms of the fluid velocity. The flow rate and differential pressure
are given by:
Flow rate Q=ACd = {2 Vfg ( f – )}/Af , and the Differential pressure P = (Pb -Pt) = {2
Vfg ( f – )}/Af ;here A is area; Cd is the discharge coefficient.
In Horizontal flow type Rota meter-float systems; the tube is in
position and the float is backed by spring as shown. The spring force
to the displacement of the float and is not a constant. Hence the
nonlinearly related to displacement. The secondary transducer and
circuitry are adjusted to correct this non-linearity and the final signal
to the velocity.
the horizontal
is proportional
flow rates are
the associated
is proportional
Turbine Type Flow Meters as shown has axially mounted freely rotating turbine
rotor; its axis coincide with pipe centreline and flow direction. The fluid in motion
impinges on the rotor blades, and torque is developed on each blade/wing. The rotor
rotates in proportion to the fluid velocity. At steady state, the flow rate is proportional
to the angular velocity .The rotor is hinged rigidly inside the meter. Electromagnetic
transducer nowadays measures the speed of the rotor and associated digital read out
is obtained as against the older methods of using traditional counters with gearing
mechanisms.
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HORIZONTAL FLOW ROTAMETER
VERTICAL FLOW ROTAMETER
Magnetic Flow Detector: When the conducting liquid flows through a magnetic field
an e.m.f is generated which is proportional to the rate of flow of the liquid; the e.m.f.
is used to drive an indicating or controlling equipment. This type of sensor is able to
overcome the inaccuracies caused due to pressure drop and any restriction in flow of
slurries that may clog and no change materially with time. Basic advantages of
magnetic flow meters (although resistivity is limited in certain applications) are the
density variation, not immune to fluid viscosity, no affect due to suspended solids,
bi-directional flow sensing and fast response inductance of the coils, this waveform
can not be entirely achieved
Electromagnetic flow Detector: It is based on Faradays Laws of electromagnetic
induction. According to this when conducting fluid passes through a pipe of nonconducting and non-magnetic material it can be treated as equivalent to a set of the
moving parallel straight conductors lying in a plane perpendicular to direction of
motion.
EMF E = BDV where B = magnetic flux density, Wb/m2 D = pipe diameter in mtr, V
= velocity of flow, m/s. Maximum EMF is induced when the electrodes placed across
the diameter of the pipe and the direction of flow makes the fluid medium remain in
continuous contact with the element between the electrodes; equivalent shortcircuiting effect is observed and the actual voltage across the electrodes becomes less
than B.D.C. Such type of flow meters do not obstruct the flow and can measure the
velocity of flow of slurry and corrosive liquids and are insensitive to the viscosity,
density and temperature. Electromagnetic flow meters are suitable for measuring a
wide variety of liquids such as dirty liquids, pastes, acids, slurries, alkali, etc.
Temperature, pressure, density, viscosity, conductivity etc. have no bad effects.
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In the figure of the waveform as shown below, the square wave excitation is used;
D.C supply to the coil is switched on/off at 2.5 Hz frequency with polarity revers al
every cycle.
At a) Shows the ideal current waveform for pulsed D.C excitation but because of the
inductance of the coils, this waveform can not be entirely achieved .
at (b) Shows the excitation from constant current source. and
at (c) the signal produced at the measuring electrode; signal is sampled at five points
during each measuring cycle and true value of flow is achieved.
MAGNETIC FLOW DET
ELECTRO MAG. FOW DET
WAVEFORM IN THE COIL
Doppler Effect & Ultrasonic Flow Detectors
According to Doppler effect, the frequency of sound changes (this indicates the speed)
if its source/reflector moves relative to the listener or the monitor. Two piezo electric
crystals (one a transmitter and the other a receiver) are potted/fitted on pipe wall as
shown in RHS bottom. Velocity is proportional to the frequency and thus V=C (F2F1)/(2F1 COS ).
In this type, the flow stream must have some discontinuities and pipeline be
acoustically transmissive. In Ultrasonic system the time of flight of the sound wave
between the two points get modified by the velocity of flowing medium and difference
between the flight time is to be directly proportional to the flow velocity.
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Ultrasonic Flow Sensor
Ultrasonic Sensors mounting in pipe
Measurement of Level systems
The practical method of knowing the level of contents (volume); presence of
substances, the level sensors are needed for detecting these levels and provide
proportional read out of the level with respect of chosen datum. Position of the
sensor head is such that the problems of turbulence created by the contents flowing
in and out to the tank is subsided. Corrosion effects caused by the components of
the sensing system, high temperatures, abrasion in granular materials may cause
undue friction and change the mass of floats and level affected. Level measurements,
associate with possible errors of i) surface tension effects in the sensing media, ii)
turbulence occurring at the sensor due to material flow, iii) changes of the mass of
float due to sediment built upon the float sensors, and iv) temperature and pressure
changes to the contents.
Level Measuring systems are generally of following types:
1.Side gauges
2.Float driven instruments, up-thrust buoyancy
3.Capacitance type probes
4.Pressure sensing
5.Microwave and ultrasonic time transit method
Levels in tanks are measured by using pressure of column of liquid at constant
density
and is obtained as gh; gh = ( mhm – y)g; m = density of mercury The
mercury levels in the two legs of the manometer are adjusted initially to be at the
same height as the bottom of the container, then h = K hm Normally, a condensing
chamber is provided at the top of the second tube of the manometer and is filled with
the same liquid as in the container.
Level sensing by Air bubbler system: In this method air pressure, built up due to
admission of air through a tube into the container of a liquid, is measured. The air is
supplied past a regulating valve so that it just escapes from the bottom end of the
tube as bubbles. Then the pressure in the tube equals the pressure due to the head
of liquid, h, above the lower end of the tube, and is given by P = h g.
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Level measuring system type using Horizontal U-tube/manometer: In this type the
weight of the tube and its liquid contents are measured by any weight or force
measuring transducer, by using null-balance principle. The level measuring system
as shown gives h= h1 – hm ( m/ -1) and h = K hm as h1 is held constant.
Alternatively, any other transducer such as the diaphragm type can be used in place
of the manometer for sensing the pressure difference between the top and bottom of
the closed container, while retaining the condenser. If the liquid is of constant
density, then the weight of the closed container is obtained by means of load cells.
Using containers of uniform cross-section, the level of the liquid can be ascertained
from the weight.
Level measuring system using Hydro pneumatic devices: These are based on the
ordinary float idea, or that of the hydrometer. The simple float is used primarily as a
liquid-level detector and makes no allowance for change in density of the supporting
liquid, it being assumed that the float is always immersed to the same depth. On the
other hand, the hydrometer uses the depth of immersion as a means for detecting
variations in specific gravity of the supporting liquid. Both these level sensors of
course, function at static conditions.
Level instruments using magnetic sensors are installed in the tanks needing tripping,
alarming and lamp indications for detecting high/low level states of the liquids in
tanks. Magnetic indicators (as shown in the figure) use floats to follow the liquid
surface using mechanical linkages for operating remotely located read out devices.
Level Sensing By Hydra-Step: The Hydra step is self-validating by the continuous
comparison between adjacent channels type remote display unit. It is used in place
of direct gauge glass for level measurement of boiler drum. This is also accepted as
replacement of direct gauge-glass by Boiler Inspectors. It is observed that
approximately 15 seconds are only available for tripping down the boilers in cases
when level of the high capacity (200MW or more capacity) boiler drums reach beyond
its normal operating levels, thus, the Hydra step is the fastest gauge glass to support
operating people for safe operation. Necessary precautions are taken to maintain the
security of the hydra step such that plus minus 1 step tolerance is eliminated in
acknowledging the drum level in the gauge glasses. Schmitt trigger circuit is utilized
for creating logic matrix and relaying.
The Hydra step operates on the philosophy of continuous comparison between
adjacent channels by the logic matrix and it ensures that the indication presented to
the operator has been fully verified. The difference of standard resistivity is sensed
and indicated as steam (red colour, at higher resistivity) and as water (green colour,
at lower resistivity) through logic matrix obtained by Hydra step cells which are
mounted at the pre-decided locations of the gauge glass unit. The measurable
resistivity is prominent at 100 bar to 183 bar or more pressures. At this pressure
and sufficient temperature (200 to 360O C) the resistivity of water is above one mega
ohm and of steam is 100-mega ohm. The cells are energized at 10 volt 15Hz and 10
micro amps. The voltage drops from 4.9 volt to 2.9 volt as water level increases.
Level Sensing by Constant Head Unit/Chamber: Almost all type of transmitters used
for level sensing of high temperature/pressure vessels, are provided with a constant
head chamber/unit in the sensing/impulse lines of the transmitters. The CHU
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ensures the connectivity of impulse lines at normal room temperature. When the
transmitter is charged for the first time, the impulse lines of a heated vessel/CHU
gets filled with air and condensate liquid. CHU is vented for a few minutes. The air
gets vented after a few minutes of charging and the condensate liquid remains in
CHU that comes in contact with atmospheric temperature which after an hour or so,
gets cooled to atmospheric temperature thus by this time the impulse line connecting
the transmitter has attained the normal room temperature. The process of venting is
adopted for flow/pressure/level applications also in order to keep the input
connections to the transmitters cool. Mounting arrangement is shown below.
MAGNETIC FLOAT LEVEL SENSOR
HYDRASTEP LEVEL INDICATOR
CONSTANT HEAD UNIT CONNECTION
MERCURY FILLED U TUBE LEVEL SENSOR
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Photovoltaic or barrier cell is a semi conductor device in which proper processing
produces a insulating barrier layer between the semiconductor and the metal layer T.
If light is incident on the barrier layer, an e.m.f is developed across the base plate
and the top metal layer, with the base plate being the positive terminal. A thin film of
gold or platinum is deposited on crystalline selenium with a base plate of iron.
Presently, cells with silicon and germanium for semiconductor layers are available.
Since all the energy of the current is derived from light source and the cell acts as an
energy converter, the current, though of measurable magnitude, represents very
small power. The short circuit current is proportional to the area of the cell and
increases linearly with luminance as shown. The sensitivity of cell is of the order of 1
mA/lm. The open circuit voltage increases in approximately logarithmic fashion, as
shown and is independent of the cell area. When the incident light flux is increased
by 1000 times, the resistance of the cell falls by 100 times.
The cell is considered as a source of current and used with external circuits of very
lower resistance. If amplification is desired, amplifiers with very low input impedance
are used. The spectral response of the selenium is almost similar to that of the
human eye and extends from about 250 nm to 750 nm, with a maximum response at
about 570 nm. The response of the germanium cell is primarily in the infrared region
responding to wavelengths from 250 nm (near ultraviolet) to 2000 nm, with a
maximum sensitivity around 1500 nm. The silicon cell has its threshold wavelength
at 1200 nm. All the above cells are also sensitive to X-rays, and rays and gamma
radiation.
Photo Electric Transducers: The P. E. Transducers cell behave as a light control
variable so the output is obtained which is proportional to the intensity of the light
source. The Photocells find its application in measurement as a strain gage, dew
point control, edge and tension controls etc and in mechanical measurements
including simple counting where the interruption of beam of light could be implied.
Light Sensitive Detectors, Photo Sensors or Photo Cells are some other forms of
Photo Electric Transducers category of instruments. The older version of Electronic
type photocells consists of a cathode/anode combination within an evacuated glass
or quartz envelope; light impingement freezes electrons to flow thereby causing a flow
of small current. The photoconductive cells consist of thin film of material such
selenium, metallic sulphide/germanium coated between electrodes on a glass plate.
In the figure below the constructional feature of photocell has been shown, as to how
light energy impinges on the cells, the base plate, the semiconductor layer and the
emf.output is derived in a photocell. The curve shows the curve of light flux versus
emf, relative spectral sensitivity versus wavelength ( ) and the light flux versus the
short circuit current Io at impedance loads as shown.
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PHOTOCELLS ‘ CHARACTERISTICS
LIGHT FLUX VERSUS Eo and Io
Load Cells: The Load Cells are basically the strain gauges. Load cell measures the
deformation produced by the force or the weight. Load cell consists of a short column
or strut with strain gauge attached thereto and the force reflects of strains the block
for sensing the input, In this the load is converted into deflection analogous to the
weight or force. It yields a measure of the quantity in hopper or feed of dry or liquid
materials; forces up to 5 MN can be measured.
An accelerometer works by the inertia of a concentrated mass; this serves to measure
the characteristics of dynamic motion, i.e., displacement, velocity, acceleration, and
frequency, through application of Newton’s laws of motion. The pendulum, or any
simple mechanically vibrating member, may serve as a time or frequency transducer,
chopping the passage of time into discrete bits.
A Load cell converts force into movement against the reaction of a spring. The
movement is then measured by a displacement transducer and converted into
electrical form. Combined actuator transducer (CAT) is used for automatic optical
instruments having a torque motor and a miniature preamplifier. In Electronic Force
Balance system the displacement is caused by applied force and is sensed by
displacement transducer; its output is fed to servo amplifier to give an output
current through restoring coil.
Various forms of load cell, Vibration pickup, and combined actuator transducer have
been drawn in the figures below.
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COMBINED ACTUATOR TRANSDUCER
LOAD CELL AND ITS CIRCUIT
VIBRATION PICKUP
Hall generators and Hall effect sensors: Hall Effect Sensor mainly comprises of
Hall generator. The electrical characteristics as per Hall (scientist) depend upon the
material chemistry and features like Hall mobility, drift mobility and the relationship
of drift and microscopic mobility to conductivity. If a magnetic field ‘B’ is applied to a
flat or round conductor, which is carrying input current ‘Ic’ in the direction
perpendicular to Ic, a Potential difference VH proportional to the applied magnetic
field B appears in the direction perpendicular to both Ic and B. The potential
difference VH = K. IC. B
Hall generator is a four terminal solid-state device and its’ output voltage is
proportional to the normal magnetic field and magnitude of input current I in. As the
conversion takes place of input or load current to the output voltage (Hall Voltage) in
magnetic field generated by the input current around the current line, it provides
total isolation between sensing circuit and the current line being sensed. In electrical
engineering applications
Hall output Eh= K . IC . If . e (off set) Where K = product sensitivity in 9 mv/ A), IC =
control current, If = load current (Amp) and e (offset) = zero current (off set 9 mv) at
If = 0 If Ic is kept constant Eh (hall output) is linearly proportional to the current
being sensed.
Hall effect sensor contains four building blocks viz. hall generator, the magnetic core,
the amplifier and temperature compensated constant current source. Output of the
Hall sensor does not appear as a current but it appears as a voltage or electromotive
force generated by magnetic field. The output voltage of the Hall effect device is
proportional to the control current ‘Ic’ or ‘Iin’, therefore it is necessary to operate Hall
current sensor by constant current regulation. AC/D.C flux are sensed with no
limitations by Hall sensors.
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HALL EFFECT DEVICES
Optical Pyrometer: It works on the principle of Thermal radiation. Such detector
avoids physical contact of the system at more than 1400OC temperature,
thermometer elements get damaged / distorted (rather not possible to be mounted).
Instrument based on measurement of radiant energy emitted by the hot body are
developed and used to estimate the temperature of the body these are known as
“Pyrometers” also.
Radiation is classified into several regions / bands depending on its characteristics
and wavelength. The radiation detectors are thin strip of blackened metal foil of
platinum and usually called bolo meter, a change in the resistance of the foil is
indicative of the temperature of the hot body. Energy levels of 3x10-8 w can be
detected; the time constant is 3 msec. approx. Total radiation pyrometer follows the
Stefen-Boltzman’s law Et= R T4 Where T= temp. in 0 K R= Stefan’s Const.
Photo Electric Transducers:It covers, Light Sensitive Detectors, Photo Sensors or
Photo Cells. The Electronic type transducer consists of combined cathode/anode
within an evacuated glass or quartz envelope; light impingement freezes electrons to
flow thereby causing a flow of small current. The photoconductive cells consist of
thin film of material such as selenium, metallic sulphide/ germanium coated
between electrodes on a glass plate.
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OPTICAL PYROMETER
PHOTO ELECTRIC TRANSDUCER
Piezo Electric Transducers: Certain materials possess the ability to generate an
electrical potential when subjected to mechanical strength or dimension changes
when subjected to voltage. This effect is known as Piezo Electric Effect. Quartz is
undoubtedly the most stable Piezo-Electric substance and although its output is
quite low but it is used almost universally in electronic oscillators. The Quartz is
ground to the shape of a rectangular or square plate and firmly held between two
electrodes contacting its faces.
Piezo Electric Transducers are used for force & pressure, temperature and
acceleration etc. measurements. Forces from 1N to 200 KN with linearity
approaching =-1%, outputs sensitivity of 125 mV per k Pa for 2.5 mm thick and 10
cm2 area of the crystal are available. Acceleration transducer may be about 4 mm dia
and 10 mm long having 2 gram weight that can operate up to 2000C. Crystal also
gives very linear & sensitive correspondence between resonant frequency and
temperature and allows measurement of absolute and differential temperature. The
effect is in crystals; the deformation causes the displacement of internal charges and
equal opposite charge on opposite side of the crystal. Potential difference Vo=Q/CP.
Mechanical resonance and mounting orientation decides the frequency limits. Range
2 Hz –1MHz, temp. 3500c.
PIEZO ELECTRIC SUBSTANCE
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Radioactive gauges
Radioactive vacuum gauge is based on the effect of ionization of the gas, whose
pressure is under measurement by -rays emitted from a radioactive source kept
inside the gauge. The valves are of sufficient thickness and of a material, which does
not allow radiation leakage. The number of ions formed is proportional to the gas
pressure as long as the range of the -rays exceeds the dimensions of the chamber.
The ion current does not increase any further when the -particles emitted are totally
absorbed by the gas inside. The ion current is proportional to the pressure and the
range pressure is the largest with this gauge. These gauges can be used from 10-4
torr to 10-3 torr.
In the Radioactive thickness gauge (shielded source type) shielded source of nuclear
radiation and radiation detector is used as the source of radiation is shielded except
in the direction required for absorption or penetration through the material whose
thickness is under measurement. In the Radioactive thickness gauge (Back-scattered
detector type), the amount of back-scattered radiation from the test sheet and the
backing material (different atomic number) depends on the amount of scattering and
absorption of radiation in the test sheet & the backing material and varies with the
test sheet thickness.
Gamma and X-rays are highly penetrating and are used for heavy metals and thick
specimens. Beta particles are much less penetrating and hence suitable for
measurements of thickness of metallic foils and thin deposits of metals on paper,
rubber or plastics. The -rays are used only for very think foils of a few microns
thickness. Gamma and X-rays are normally composed of radiation of different
wavelengths and hence thickness gauges have to be calibrated for each radiation and
for each material. The thickness gauges are fast and need not be calibrated everyday.
No mechanical contact between the gauge and test piece is necessary.
Radioactive Level Gauge has column of liquid, the radioactive source & detector.
Ionisation Transducers:
Ionisation Transducer consists of a glass envelope with two internal electrodes filled
by gas or gas under reduced pressure. The circuit shown indicates the radio
frequency power source ionizes the gas in the transducer in the field from two
external electrodes Space charges created and DC output signal furnishes the
potential of the electrode. Either C1 or C2 varies to balance the electric field and
produce an output.
The length of the path determines the intensity of radiation received by the detector.
As is the case with sheet materials, the characteristic is non-linear. Where linearity
is required, a good number of strip sources and strip detectors may be located along
the sides but on opposite faces so that the total output of the detectors when added
makes up the output signal.
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RADIOACTIVE VACUUM GAUGEs
RADIO ACTIVE DETECTOR (scatter)
RADIOACTIVE THICKNESS GAUGE
IONISATION TYPE SENSORS
ELECTRICAL SENSORS AND SYSTEMS IN MEASUREMENT:
The electrical element transforms the analogue displacement into Voltage or the
current through the passive elements i.e. resistors, inductors, capacitors.
The variable resistance (e.g. ordinary switch, sliding contact/potentiometer, wire
resistance strain gauge, thermisters, thermocouple etc.) or The variable inductance
(single/double coil self inductive, simple two/three coil mutual inductive, variable
reluctance in moving iron/moving coil/moving magnet etc.) effect changes in
transducer applications. Variation of permeance of magnetic circuit causes a change
in the flux and voltage is developed due to expanding or collapsing of the flux. The
variable reluctance is generated when the magnetic lines of flux emanated from the
permanent magnets of the systems, are cut by the turns of the coil, this principle is
used for transducer operation and thus the relative motion is incorporated into the
device.
The active pick-ups e.g. tacho-generator, thermocouple, photovoltaic diode and
piezoelectric crystal, produce voltage outputs related to non-electrical inputs, without
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the need for separate voltage supplies and transducer bridges. However, more often
than not, they are coupled to instrumentation amplifiers, which do require power
supplies. The piezoelectric substances, and the thermocouples with thermoelectric
characteristics fall in active sensor category.
Digital Tachometers, Counters & Electronic Frequency Meters
Tachometers produce voltage & frequency proportional to the shaft speed; one type
uses the voltage level and the other one-frequency waveform. The output may be
single or three phase (preferred for lower speeds, in which effect of ripple is
minimized, it reduces oscillation at lower speed). Signal is rectified & smoothened in
case of voltage level type tachometers. In such case the system is very dependent on
the impedances of the components in the circuit so has poor accuracy. The toothed
wheel and magnetic probe which considers the frequency level type, used universally
nowadays by using disk of non-magnetic material that eliminates the influence of
shaft magnetization. The voltage type was normally working on analogue signals
generation principle.
In Digital tachometers, the shape of the output signal is similar to the shape of the
pulses those are counted over a given time and the time between the pulses is
measured.
In linear function tachometer, Pulse counting mechanism is employed to give digital
outputs for converting the rotational speed. The capacitor system is excited by a high
frequency source so that the output is effectively a pulse-modulated signal. The
incremental encoders are used for the measurement of either linear velocity over
limited distances or rotational speeds. Pulses are generated due to the capacitance
change between a probe plate and serrated rotor; its rate is proportional to rotor
speed. The pulse counting method provides measurement of the average speed only
and the accuracy depends on the clock period and the resolution varying with speed,
so very low speed measurement is not possible
The clock provides the pulses to open the gate for the prescribed period to set the
counter before each count and simultaneously update the digital output. Sensing
the direction of motion and indication of the same is possible. In case of inverse
function tachometer the clock each incremental pulse of encoder gates pulses.
Control logic is used to reset the counter and update the output at each pulse from
the incremental and encoder disc. The instantaneous speed of rotation at any instant
during revolution is reckoned from the counter and the transient in speed are
detected.
Electro Mechanical Transducers (Tachometers)
Angular speed of rotating shaft measurement is very important particularly in the
automatic speed control system. The tachometers are used for analogue indication of
speed and DC and AC speedometers using voltmeters. The dc tachometers converts
shaft rotation (proportional to speed) signal into an electrical signal. Permanent
magnet stator and wound rotor principle is used for dc generator (tachometer). The
voltage relationship is given by:
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E.M.F, Eg = (NPΦ
ΦΩ)/a ; where N = total number of conductors of the armature
P = number of field poles
Φ = total magnetic flux per pole, Wb
Ω = speed of rotation, rad/s
a = No. of parallel paths in armature winding
For desired high resolution of the order of ±0.1% to ±0.01%, feedback- using the dc
tacho-generator or the ac pulse-generating tachometer type measuring system is
used.
In eddy current drag-cup type tachometer, the eddy current is set up in the cup that
interacts through the magnetic field in a manner as to follow the magnet and
therefore torque developed is proportional to the magnet and cup. Thus the angular
deflection is proportional to the angular speed of rotation in steady state condition.
COUNTERS TYPE FREQUENCY METER
ELECTROMECHANICAL TACHOMETERS
Linear Variable Differential Transformer (LVDT): is basically a Differential
Transformer that uses the principle of variable inductance. LVDT provides an output
a-c voltage proportional to the displacement of a core passing through the windings
enclosures. LVDT is a mutual inductance device that generally makes use of three
coils. The core displacement results in a proportional output on either side of the
null position within limits. In general, the linear range is primarily dependent on the
length of the secondary coils. While the output voltage magnitudes are ideally the
same for equal core displacements on either wide of null balance, the phase relation
existing between power source and output changes 1800 through null.
It is therefore possible to distinguish between outputs resulting from displacements
on each side of null, through phase determination or by use of phase-sensitive circuit
arrangement. The core movement (length change) causes the inductance variations.
For stability and performance improvement, addition of balanced, differential, phase
sensitive detection systems are used. The input voltage of LVDT is limited by the
current-carrying ability of the primary coil. The sensitivity increases with increased
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number of turns on the coils in a LVDT. There is a limit, however, determined by the
solenoid effect on the core. When utmost sensitivity is desired and the transformer
output is amplified, it may be necessary to use external circuitry to improve the null
balance condition Exciting frequency, sometimes referred to as carrier frequency,
limits the dynamic response of a transformer. The LVDT sensitivity is directly
proportional to exciting voltage and, as indicated above, also it increases with
frequency.
The LVDT is used in primary detector to convert mechanical force into a proportional
electrical voltage. It is also relatively insensitive to high or low temperatures or to
temperature changes and it provides comparatively high output, often usable without
intermediate amplification. It is reusable and the cost is also reasonable. A voltage
dividing potentiometer of sufficiently high resistance is placed across the transformer
secondary output to avoid appreciable loading.
In the figures below the process of inductance change due to core movement, the
core movement causing the differential transformation, the force balance type LVDT
sensing, and complete detector using LVDT have been shown.
FORCE BALANCE TYPE LVDT
LVDT SENSING, DIFF. Xter, INST DETECTOR
LVDT FUNCTIONING
Permanent magnet moving coil type measuring system makes use of the driving
force/the torque generated by the D.C current flowing in the coil. Such meters are
also used for measuring ac current and of frequency higher than 150 Hz as well as
dc current. The electrometer amplifiers are used for extremely low current
measurements, by use of Transfer Standard in which a thermocouple and a small
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heater coil enclosed in an evacuated glass bulb, is used. Bridge rectifier circuit is
used for measuring the Average value of current) by unbalance current flow through
a permanent magnet moving coil instrument.
Measurement and displays of Temperature Systems
In simple thermometer, or the bimetal strip thermometer the Temperature detection
is based on differential expansion of two different materials. Hot Wire Resistance
Transducers use transfer of heat through conduction by using resistive element in
physical contact with the system. The Resistance thermometer and the thermocouple
are categorised as the secondary transducers. The temperature is determined in
thermocouples from thermoelectric properties of materials or combinations of
materials; whereas the variation of resistance gives the temperature figures of
substances from resistance thermometers detectors.
Potentiometers consist of resistance element provided with a movable pot/contact,
in translation or rotation or combination or helical motion in multi-turn rotation. The
resistance element is driven by a.c or d.c. excitation and the output voltage is ideally
linear function of the input displacement.
The Potentiometer network is used for measuring very precise & accurate value of
voltage; small e.m.f is obtained from active transducers (Thermocouple etc.); this
e.m.f. is compared with standard cell configured in detection of unbalance current
through slide wire and the potentiometer resistance coil. Potentiometer N/W are used
for Commercial versions of slow varying transients & faster voltages/current in
applications like range-change, standardization and read out of unknown voltage
through recording instrument.
Potentiometers are available with varying design & provisions. Manual balancing is
possible by galvanometer. Automatic balancing of the potentiometer requires high
gain amplifier and a servomotor. Nowadays the Zener diode reference is used to
provide the voltage for standardization in place of standard cells. The circuit using
potentiometer has been shown in the figure below.
Resistance Temperature Detector (RTD) is a resistance transducer that changes
its value due to change in temperature at the sensing source. RTD is a wire element
that is wound on a firmer, (in form of a coil) and in a sheath or the protecting tube to
achieve small size and improve thermal conductivity.
The information required for selection of the RTD sensors is; the sensitivity of the
RTD, response (slow or fast) time (0.5 – 5 sec) giving the thermal conductivity,
good/poor thermal contact of sensor with the medium etc. Platinum-metal sensor for
use as RTD, is quite sensitive (having 0.004/0C sensitivity), easily repeatable but is
certainly costlier than the Nickel sensor (having 0.005/0C sensitivity). The Platinum
resistance temperature detectors (RTD) enable achievement of very high resolution of
the order of ± 0.0001k and high sensitivity. The variation of platinum resistance is
too much, as is clear from the fact that the resistance of value 7000 Ω at 2000 K can
fall down to even 6 Ω at 6 K.
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Linear fractional change in resistance
and resistance versus temperature values
decide the sensitivity of the RTD. Since resistance is given by Ro= ρ0.l/A, so, any of
these three items/figures (i.e. specific resistance, length, area.) can be varied for
obtaining resistance change of the RTD.Also, RT = Ro (1+α0∆T); new value of RT gets
changed in correspondence with temperature due to variation of initial resistance
(Ro), difference of temperature (∆T), temperature co-efficient (α0). The
increase/decrease of resistance is linear. Tables of RTD resistances to corresponding
temperatures are available widely for calibration, testing and confirmation of
resistance versus temperatures.
In the given circuit, the RTD is connected in a bridge circuit in which a null condition
is detected by a galvanometer or an amplifier for driving an indicator or the recorder.
A compensation line in the R3 leg of the bridge is required when the lead length is
long enough.
RESISTANCE TEMPERATURE DETECTOR
POTENTIOMETERIC RECORDER CIRCUIT
Thermocouple Temperature sensor produces an e.m.f that is proportional to the
temperature of the junction of two dissimilar metals. A thermocouple is a junction of
two dissimilar metal wires usually joined to a third metal wire. The e.m.f. is almost
linear and repeatable with change in temperature. Many metals and alloys exhibit
the thermal electric effect but only a few metals are used for temperature sensors.
The generated voltage is fed to operational amplifiers of high input impedance to
avoid loading. The amplifier output (Vout) is used in indicators, controllers or
recorders. Ice bath is generally used, for making the reference junction. Otherwise
reference junction is at triple point of water apparatus, kept at 0.01± 0.0005OC
temperature.
All thermocouples are encapsulated by stainless steel sheath or hard brass/copper
tubes and are protected from contamination and mechanical strengths. Most of the
thermocouples can be used in oxidizing environment up to 7500C and reducing
environment up to 10000C. The quality of wire produced from batch to batch may
vary slightly with the result that the e.m.f produced may not exactly conform to the
values made available from standard tables. In all cases, the polarity of the
thermocouple material mentioned first is positive for temperatures greater than the
reference junction temperature.
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Platinum metal is stable and platinum-rhodium thermocouple is invariably used, as
primary standard for temp between 630.50C and 1063OC.Its sensitivity is only about
6 µV/0C, and is used up to 15000C. Constantan (Ni 40%, Cu 60%) is another alloy
that is used with copper, iron or chromel (Ni 90%, Cr 10%). Copper constantan T/C
has the maximum sensitivity of 6µV/0C and is useful for the range from -20000C to
+4000C. Iron/constantan thermocouple is most widely used for industrial
applications for the range of temperatures from -1500C to + 10000C. Chromel Alumel
(Ni 94%, Mn 3%, Al 2%, Si 1%) is another widely used thermocouple for temperatures
from -2000C to +13000C, and is preferred for use in non-reducing environment at
temperatures between 7000C and 13000C.
The circuit below shows connection of T/C in Op-amp input through cold junction
compensation by using two T/C in both leads. In order to obtain a higher output
emf, two or more thermocouples can be connected in series as shown in figure and
for measurement of average temperature, a parallel connection as shown in figure
may be used. Whenever compensation for variation of cold junction temperature is
required, a wheatstone bridge circuit as shown in figure is used with one of its arms
having a resistor of nickel wire or a thermistor and located near the cold junction.
For accuracy, it is better to calibrate each thermocouple and then use it for
measurement of temperature. The temp./e.m.f. Characteristics of thermocouples are
also shown below.
Semi conductor temperature sensor is also used for temperature measurement
and it works on temperature sensitivity of semiconductors (i.e. geranium or silicon).
Silicon has +ve temperature co-efficient (0.7% per OC) and possesses linearity of ±
0.5% in its operating range of –65OC – 200OC. Similarly germanium crystal doped
with Arsenic, Gallium or Antimony are used for low and cryogenic temperatures in
the range of 1K-35K.
Thermistors are semiconductor compound temperature elements and are thermally
sensitive as resistors, relatively small in size, having low thermal capacity & high
speed of response and possess large resistance values. Thermisters depict resistivity
of 10-1 to 109 Ω-cm. Depending on their composition; the thermisters have limited
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application in the range 100oC to +300oC. Thermistors are used often for
compensating electrical circuit to ambient temperature changes. Thermistors possess
high value of temperature co-efficient compared to wire wound resistors and are
mostly used in integrated circuits for obtaining change in resistances at desired
ranges.
PNEUMATIC INSTRUMENTATION
Pneumatic instruments find its application in the preliminary system of
measurements & control in process industries since inception of industrialization.
Pneumatic systems have many features and characteristics e.g. it is most versatile
but simple, very cheap, robust, needs less maintenance support, fast in operation,
high torque and thrust etc. The pneumatic instruments work on the principle of
accurate conversion of mechanical movement to a proportional pneumatic signal by
use of flapper nozzle mechanism working on air pressure.
The flapper nozzle works on force- balance principle and it uses negative feed back
which is used to oppose the force of the measuring element and the feedback bellows
expand till the force is balanced and there is no further increase in output pressure.
Air can exhaust faster than passing through a restrictor, resulting in increased gauge
pressure reading. The orifice (nozzle) is around 3 times larger in size to that of orifice
(restrictor). When the flapper is positioned such as to seal off the nozzle, the pressure
builds up equal to the supply air pressure. The actual movement of the flapper is
very small about 0.02 mm only. Small change in supply pressure does not affect the
output but the flapper movement is so small that even the slightest amount of wear
on pivots or linkages affects the system performance.
Electro-pneumatic converters (E/P) convert electrical signal in pneumatic form for
operating/driving valves, dampers or the power cylinders. The high processing speed
of electrical and electronics signals is contained in UCB/ACS panels while the
fastness of pneumatic drive operation is interfaced by use of E/P converters. The E/P
converters receive electrical signals and control the pneumatic air operated power
cylinder, valve or systems.
The Fielden E/P converter is a force balance device without feedback; setting up of
the nozzle is critical in this E/P converter because of the lack of feedback. The device
is supplied with air at 1.5 bar pressure and has restrictor and nozzle size ratios
similar to a conventional flapper nozzle system i.e. 3:1.The beam is pivoted at one
end whilst the other end is attached to a permanent magnet, the plug of the primary
valve is also connected to the beam. The spring tension and the primary valve plug
relative to the nozzle seat is adjusted for its’ Zero adjustment.
E/P converters use low-level electric current (4-20 mA) signal for conversion to
pneumatic air pressure in 3-15 p.s.i. or 0.2 –1.0 kg/cm2 range; conversion is done by
the flapper-nozzle process as explained earlier. From the diagram, it is clear that
current flows through the coil to produce a force that pulls the flapper down and
close–off the gap. Adjustments of the springs and perhaps the position relative to the
pivot to which they are attached allows the unit to be calibrated in range i.e. 4 mA for
0.2 kg/cm2 & 20 mA for 1.0 kg/cm2. Current is applied to the coil and a magnetic
field is set up, (the strength of which depends upon the value of the current) the
permanent magnet is forced down which brings the primary valve closer to its seat,
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pressure builds up and forces the diaphragm down which seals off the exhaust valve
and opens the secondary valve, resulting in an increase in output pressure. Excess
pressure is vented through the exhaust, resulting in a drop in pressure. The magnet
is provided with Oil damping for smooth operation.
PNEUMATIC INSTRUMENTS
TARGET TYPE TXR WITH AIR RELAY
Air relays (Booster relays): Booster Air relays are used to supply large volume of air
in the connecting pipe work and for the improvement in system response
In Continuous bleed type air relays, the air continuously escapes via the vent. The
rate of leakage determines the backpressure & the output pressure increases as the
nozzle pressure increases in direct acting type and vice-versa in reverse acting type.
i.e p out increases as the p nozzle decreases.
In Non-bleed type air relays, the nozzle pressure is applied to the exterior of the
large outer bellows and the control line pressure is exerted on the interior of the
small bellows when the forces between the two are equal; the balance condition
exists. This type of relay is used to provide a positive closing force. Any increase in
primary nozzle pressure overcomes the spring pressure and tend to reduce the
response time of relay.
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Three Port Air relay consists of a two-lobe spool running in a surface ground
cylinder, compressed air can be switched to the outlet by the application of a force to
the spool. The force can be removed and the outlet will remain connected to the air
supply. The application of a second force will return the spool to its original
condition, main air will then be isolated and the air to the outlet will exhaust from
third port.
Five Port Relays is same as that of three port relays in its’ basic construction, the
only difference being the use of a three- lobe spool. Compressed air can now be
routed through the relay whilst at the same time a signal can be exhausted through
it. The direction of force determines the routing of supply and exhaust.
Continuous Bleed Type Relay
Continuous Non-Bleed Type Relay
PNEUMATIC POWER CYLINDERS
Power cylinders are single acting or double acting as per their functions.
Single Acting Cylinder is the simplest type of power cylinder. In this type, air is used
to make the piston unit out stroke or extend (+). Once the pressure has been
removed, the return or in stroke (-) is achieved by mechanical means, in this case a
spring. The cylinder can be air to extend type (application of a signal will push the
piston out) or air to retract type (signal application pushes the piston in). Electropneumatic Solenoid converts electrical signal into mechanical rectilinear motion i.e.
in a straight line. The solenoid consists of a coil and plunger. The plunger is free
standing and is spring loaded. The coil operates on AC/DC specified supply
voltage/current. The force of full or push is important parameter in deciding solenoid
specifications. Solenoids are of continuous or intermittent rating and of single coil or
double coils with specific duty cycle type i.e. percentage on total time is specified for
particular type of application. SCR are normally used to activate the solenoid coil.
Normally, the Solenoid operated power cylinders are single acting or air to open or air
to close type and are termed as Shut off cylinders.
Double Acting Cylinder is different than single acting in operation point of view that
the power cylinder requires air pressure to move in both directions. In the double
acting cylinder, if air is applied to Port-1 (with Port-2 open to exhaust) the piston will
outstroke (+); and if air is applied to port-2 (with port-1 open to exhaust), the piston
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will in stroke (-); (+)&(-) i.e up & down indicate cylinder movement. All regulating
power cylinders are generally double acting.
The Power Cylinder out strokes under the action of applied pressure, air is
displaced from the other side of the piston to atmosphere through the main port and
needle valve. When the cushioning boss enters the cushioning seal, the main port is
blocked off, air can, therefore, only escape through the needle valve at a much slower
rate, thereby, causing the piston to slow down for the premium period of travel. This
results in the cushioning effects on high-pressure systems. Piston speeds can be in
the order of 450 mm/sec applied to Port-2 (with Port-1 open to exhaust) the piston
travels up. Impact forces at the ends of the stroke can be great. In order that damage
may not be caused by sudden contact between the fast moving piston and the
cylinder end housing, some form of buffer or cushioning can be used. This does not
limit the piston travel but allows gradual deceleration in the last 25 mm or so of
travel.
In Kent Mark IV Pneumatic Power Cylinder, the Electronic controller output acts
onto the E/P converter that converts the signal current (4-20 mA) into proportional
air (0.2-1.0 Kg/cm2) pressure and is passed on to input bellows; it effects bellows to
expand. As, the pilot valve spool is attached to the bellows via a connecting link; this
unbalances the spool of the three port pilot valve, which is also supplied with driving
air of supply pressure regulated as per the size and thrust from the power cylinder.
Assuming an increase in the controller output, the Driving Air through the 3-port
pilot valve is then admitted to the top of the piston and piston begins to move down.
In doing so it takes the cam with it, as the cam moves down the bell crank lever
turns about its pivot and through the spring opposes the movement of the bellows
and restores the spool of the pilot valve to its original position.
This way the system is the back in equilibrium. An equalizing valve is included to
enable manual positioning of the piston. Wherever direct regulation by pneumatic
system, is needed the signal air is directly sent to the bellows by a pneumatic
regulator and the process repeats same as through E/P converter, explained earlier.
Refer figures
In Bailey Pneumatic Power Cylinder, the controller output acts on the bellows the
spool assuming an increase in the controller output the bellows will expand and put
the pilot beam up against the restraining force of the spring. This unbalances the
pilot valve and causes air to be admitted to the top of the piston. The piston,
therefore, begins to move down, this results in the positioner drive arm turn the cam,
which puts more tension on the spring and so restores the pilot beam to its original
position. Refer figure for details
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Kent Mark-4 Pneumatic power Cylinder
Bailey Pneumatic power Cylinder
Pneumatic (Valve) Positioners are fitted in pneumatic power cylinders and also in the
pneumatic valves. These operate generally on a pilot valve principle. Since a
pneumatic actuator requires a large power input to produce a large power output, it
is necessary that sufficient quantity of pressurized air is made available to the large
sized diaphragms or the power cylinder. Positioners are used for supporting the pilot
valves. The pilot valve is attached to the pilot beam at one end whilst the other end is
anchored and pivoted.
Diaphragm Control Valves: A control valve must be capable of responding smoothly
and rapidly to small changes in the controller output pressure signal. The quality of
control will be impaired if any force, for example, that due to friction of working
parts, oppose the movement of the spindle and the valve plug. The Diaphragm valves
contain flexible diaphragms. The diaphragm virtually seals the chamber into two
parts, the upper sections receiving the pneumatic signals from the controller via the
air input. The input signals deflect the diaphragm, which is fixed to the thrust plate.
The spindle attached to the thrust plate extends downwards into the body of the
valve.
There are three basic types of the valves:
Quick Opening type diaphragm valves are used predominantly for full ON or full OFF
control applications. A relatively small motion of valve stem results in maximum
possible flow-rate through the valve; 90% of flow rate with only a 30% stem travel.
Linear Type diaphragm valves have flow rate that varies linearly with the stem
position. The flow relation is Q/Qmax = S/Smax; flow rate versus stem positions. Equal
Percentage type of valves has equal percentage change in stem position that
produces an equivalent change in flow type characteristic.
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The primary function of a Pneumatic Valve Positioner is to ensure that the control
valve plug position is always directly proportional to the value of the controller
output pressure, regardless of gland friction, actuator hysteresis, off-balance of
forces on the valve plug etc. This is achieved by incorporating a feed back lever that
acts in opposition to the movement to the input.
Since the spindle is connected to valve plugs, there is provision of automatically
adjusting the orifice area in response to action from the controller and thereby
altering the block valves (Diaphragm type valves). The deflection is opposed by the
range spring whose stiffness constant and rating determines the extent of travel of
the spindle for a given pressure range and effective diaphragm area.
AIR RELAY & CONTROL VALVE SET-UP
PNEUMATIC DIAPHRAGM CONTROL VALVE
This output pressure continues to increase until the valve spindle moves; mechanical
feedback then restores the equilibrium. This force applied to move the valve spindle
is sufficient to overcome the effect of all forces, no matter what the origin, which tend
to oppose the spindle movement. Without the positioner the slight change in
controller output signal may have been too small to initiate any corrective action. The
matching of input signal range to valve travel range is achieved by changing the ratio
of bellows/nozzle distance to feedback arm/nozzle distance.
The controller output signal does not directly actuate the valve stem but is fed to a
bellows unit. Assume that the system is in equilibrium and then the controller
output increases slightly. The flapper is moved towards the nozzle and the relay
output pressure begins to increase.
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Pneumatic Controllers
Pneumatic Proportional action control; The principle of nozzle-flapper applies with
the proportional controllers also. The input (0.2- 1.0 kg/cm2 air) changes then the
input bellows expand/contract and that in turn forces the flapper to move the nozzle
appropriately and balances the forces on the flapper/nozzle, the controller output
pressure (range same as above) changes in the nozzle piping. The standard response
of the pneumatic controllers is the Gain Kp =(x1/x2)(A1/A2); where A1, A2= effective
area of the input & feedback bellows; x1, x2= Level arm of input and feedback lever
arm. The proportional gain Kp can be changed by changing x1; since the bellows are
having fixed geometry, the gain Kp is similar to the gain factor of an operational
amplifiers.
Pneumatic Integral action control; is achieved by using a variable restrictor in
feedback loop (termed as integral bellow) and fitted in opposition to the proportional
bellow.
Pneumatic Derivative action control; is achieved by replacing the integral bellow
with a spring and a variable restrictor in between the proportional bellow and the
nozzle loop.
Pneumatic Proportional Plus Integral Plus Derivative Control; It is obtained by
suitably adjusting the proportional gain chamber bellows, integral and derivative
action control module restrictors. The response curve drawn below shows that the
final output of PID controller reduces the recovery time to approximately half to that
of proportional action and the damped oscillation is resulted nearly at the set point
only. The proportional plus integral action control mode slows down the process and
thus the recovery time is increased quite more although the offset due to
proportional action is reduced fully.
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PNEUMATIC PROPORTIONAL + INTEGRAL+ DERIVATIVE CONTROLLER
Electrical Actuators or the Pneumatic Actuators translate the control signal in to the
driving force for dampers/valves.. Pneumatic, Electrical and Hydraulic actuators are
classified according to the medium of application. The Electrical actuators are either
the solenoids operated or the electrical motor driven actuators for driving the valve /
dampers of mechanical types. The Hydraulic actuators use pressurized oil to operate
the valve and the principle of its working is similar to the pneumatic actuators,
except hydraulic pump units for pressuring, blocking units and filters etc are used.
Electrical Motors are used for driving the actuators by producing the continuous
rotation of motors. The style and size of the motor are decided on the basis of
starting torque, rotation torque, rotational speed etc. The normal AC induction
motors or the stepper motors are used for actuator application. The rate of rotation
is determined by the AC line frequency and the rotor continues in rotation & in
phase with line frequency. General details of electrical motors can be referred from
other electrical equipments
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Electrical Interferences (random or periodic) create problems for control and
measurements. The power levels of interferences are extremely low but they become
significant for elimination. There are two modes of interferences: i) Differential mode
is in series of interference that causes one signal lead (wire) in potential relative to
the second signal and
ii) Common mode interference appears between both signal leads and ground and
causes the potential of both sides of the signal transmission circuit to be changed
simultaneously and by the same amount relative to ground.
Interference is usually coupled by conduction through metallic, resistive and
capacity links or by radiation via stray capacitance and mutual induction effects. The
conductance-coupling path is always stray.
Grounding of the transducer is achieved by mounting its case and through the
mains power lead. The ground potential differences cause common mode interference
current which reaches the signal circuit through the conductor. Ground connection
comprise the most common and most troublesome sources in measuring system
which can be eliminated by providing single point grounding preferably at the source
end.
Electrostatic coupling is resulted due to capacitance between two objects and so
such interference is associated with capacity leakage paths to ground. Reduction of
circuit impedance (of signal) is the effective way of controlling the interference.
Screening & shielding is second very important method for controlling capacitive
coupled common mode interference. Screens or guard shields are situated and
connected in such a way that interference current are returned to their sources
without entering the guarded signal path. Coaxial, double coaxial lead and the twin
conductor with screens are used for screening. At high frequencies care should be
taken to match the impedance of the cable to that of connectors to avoid reflection
and distortion of the signal.
Signal conditioning circuits such as amplifiers may require external power, but be
screened from the power supply. Common mode rejection can be used to reject the
common voltage and provide an amplified signal for a grounded indicator. A
conductor carrying AC is surrounded by an alternating magnetic field and voltages
are induced in any conducting material position within the magnetic field. They
require magnetic shielding to provide high permeability path for diverting any
interference of the magnetic flux. Electro-static screens provide the protection from
electro-magnetic interference. Addition of interference signals is avoided.
Where cable screens are used, earth continuity of screens must be maintained
through the installations with the earthling at one point only i.e. in the control room.
At the field end the cable screen should be cut back and tapped/floating from earth.
Intrinsically earth system be earthed in its own earth bar, static earthling be
connected to common plant earth.
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SHIELDING& SCREENING
INSTRUMENT EARTHING FOR COMPUTERS
Signal conditioning (Data Acquisition) and Signal processing
Signal conditioning (Data Acquisition) and Signal processing are defined as the
excitation and amplification system for the Passive transducers and the amplifiers for
the Active transducers so as to bring up the level of transducer to a sufficient
quantum for conversion, processing, indicating and recording.
Since the current carrying lines are also the power delivery lines to energise the
transducer so, laying only 2 wires to connect the transducers does local signal
conditioning and signal conditioned measurement system to the rest of the loop.
Following three factors are important for conditioning:
The load impedance should remain between 0-1000 ohms
The interchange-ability of the process controllers; as the controller sees a 4-20
milliamp input signal and
The measurement of power supply
If the input is a variable resistance and a bridge or divider is used, the effect of nonlinearity with resistance in output voltage and the effect of current through the
resistive sensor are considered. Any possible loading of voltage sources by the signal
conditioning be always considered, since such loading is a direct error for the
system.
The potentiometers, strain gauges, resistance thermometers, inductive and capacitive
transducers require excitation whereas thermocouples, techo-generators, inductive
pickups, piezoelectric crystals produce their own voltage which only requires
amplification and therefore are termed as active transducers.
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The DC amplifiers need differential inputs in CMRR mode. They are easy to be
calibrated at low frequencies and have ability to recover rapidly from an overload
condition. Carrier type AC conditioning systems for use with variable resistance or
variable inductance transducers require carrier frequencies.
Active filters can be used to reject the frequency and prevent overloading of the AC
amplifier. DC systems are generally used for common resistance transducers (strain
gauges) while AC systems are essential for the variable transducers and systems that
have long lead lengths for transducers to the signal conditioning equipments.
After signal conditioning physical quantities such as pressure, temperature, strain
and positions are transformed into an electrical voltage or current of sufficient level
for further processing in electronic circuits and indicators or the recorders.
Almost all transducers are followed by signal-conditioning equipment. Resistance
transducers are often placed in a D.C. Wheatstone bridge, while push-pull reactive
transducers are placed in special a.c. bridges such as the Blumlein Bridge,
producing a.c. carrier systems. The outputs of these bridges and signals from active
transducers are usually amplified. Feedback around D.C. Operational amplifiers are
used to obtain well-defined gain, and balanced differential inputs give high rejection
to common-mode signals.
Special amplifiers can provide particular requirements such as very low drift,
isolation, and operation with high–impedance transducers. After amplification signal
may be sampled, quantized, multiplexed and converted to digital form for storage,
computation or transmission. Special care is needed to avoid addition of interference
to the signal, and special equipment grounding, electrostatic screening and electromagnetic shielding may be necessary.
Signal Conditioning – Design guidelines
The guidelines as given below can be important in Signal Conditioning and
designing. Thus enough information is made available to address an issue properly
and exercise good technical judgment. Mostly, the entire system is developed, from
selecting the sensor to designing the signal conditioning. Since the sensor is selected
from what is available, the actual design is really for the signal conditioning.
Guidelines for analog signal conditioning design:
1.
Define the measurement objective.
a) Parameter: What is the nature of the measured variable: pressure,
temperature, flow, level, voltage, current, resistance etc.?
b) Range: What is the range of the measurement: 100 to 200OC, 45 to 85 psi, 2 to
4 V, etc.?
c) Accuracy: What is the required accuracy: 5% FS, 3% of reading, etc.?
d) Linearity: Must the measurement output be linear?
e) Noise: What is the noise level and frequency spectrum of the measurement
environment?
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2.
Select a sensor (if applicable)
a) Parameter: What is the nature of the sensor output: resistance, voltage, etc.?
b) Transfer function: What is the relationship between the sensor output and the
measured variable: linear, graphical, equation, accuracy, etc.?
c) Time response: What is the time response of the sensor: first-order time
constant, second-order damping, and frequency?
d) Range: What is the range of sensor parameter output for the given
measurement range?
e) Power: What is the Power specification of the sensor: resistive dissipation
maximum, current draw, etc.?
3.
Design the analogue signal conditioning (S/C)
a) Parameter: What is the nature of the desired output? The most common is
voltage, but current and frequency are sometimes specified. In the latter
cases, conversion to voltage is still often a first step.
b) Range: What is the desired range of the output parameter (e.g. 0 to 5 V, 4 to
20 mA, 5 to 10 kHz)?
c) Input impedance: What input impedance should the S/c present to the input
signal source? This is very important in preventing loading of a voltage input.
d) Output impedance: As offered at short-circuiting on loaded circuit?
OPERATIONAL AMPLIFIERS (OP-AMP)
An Operational amplifier is the name given to the electronic amplifier that is
employed to analogue computers for performing mathematical operations.
Sophisticated Integrated Circuit (IC) chips fabricated on single chip of
semiconductors ( Si, Ge ) are the basic elements of the Op-Amp.
An Op-Amp is direct coupled, high gain voltage amplifier, designed to amplify the
signals over a large/ wide frequency range. It has two input terminals and one
output terminal and a gain of at least 10 5. The ideal Op-Amp is a linear device in
that the output is directly proportional to the inputs for all values of input voltage.
Practically the relationship
v0 =- Kvi holds true where K =zo/zi; An Op-Amp is
basically the differential amplifier which responds to the difference in the voltages
applied to the positive and negative input terminals.
Basic Op-amp application and the circuits are given in paragraphs that follow:
In the Non- Inverting configuration, an Op-amplifier is connected in single ended
input and the input impedance is high by virtue of the feedback, the output signal
appears in same sign of the input signal. We use the resistors as the input &
feedback impedances as shown in the fig. In Laplace representation form VI (s) =
V1(s) - (Rf/R1) V1(s) and the output is given by ( vi ) = v1 - (R1/ R1+ Rf) vo
In the Inverting mode which uses an Op-amplifier as a sign inverter, thus the
output signal appears in sign inverted of the input signal; resistors are used in the
input & feedback impedances as shown. In Laplace representation form Vo (s) = (Rf/R1)V1(s) and the output is given by ( vo ) = - (Rf/R1). v1.
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Basic Op-amp.
Non-inverting Op-amp.
Inverting Op-amp.
In the Voltage follower configuration, the output voltage follows the input. This is
basically the more refined version of emitter follower or the source follower; its gain
becomes 1, and so the Circuit gain Av = (vo)/(vi) =1. Since the feedback resistor Rf =0
and input resistor R1 is α (open circuit). It also performs the action of an Impedance
Transformer.
In the current to voltage converters, the current [Is] is introduced into the (–) ve
terminal of the Op-amp, an equal current in the feed back resistor is setup and since
the amplifier input voltage (v1) is practically zero, thus the source current has been
converted into a voltage, and the output voltage is ( vo ) = -Is.Rf
In the voltage to current converters, the voltage source of high internal resistance
drives a low-resistance load; no current flows into the op-amp terminals & no voltage
drop across the resistor thus full voltage appears between the + terminal and ground;
the source voltage gets converted into a load current. The Load current IL and voltage
source Vs is related: IL= Vs/R1
Voltage follower
Current to voltage convertor.
Voltage to current convr.
In the Integrator, the input signal (v1) is integrated to obtain the output signal (vo).
In integrating, the initial conditions are added and are initially biased by a dc voltage.
A capacitor is used as the feedback impedance and a resistor is used as the input
impedance In Laplace representation form Vo (s) = - V1(s). (1/RiCS) and the output
vo =1/ (R1 C). v1 dt
In the Differentiator, the input signal ( v1 ) gets differentiated to obtain the output
signal (vo), the resistance and the capacitors in the circuit are inter-changed to form
the differentiator as shown in the figure; in Laplace representation form Vo(s)=Vi(s).RiCS and the output vo = - R C1. (d v1/ dt)
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In the Summing Op-amp, several inputs can be summed up by using resistors in
the input as given in Inverting configuration and feedback impedances of the high
gain d.c op-amp. The output voltage (vo) of the multi-input voltages is given by (vo) =
K {(v1) + (v2) + (vn)} = where K=Ro/Rz
Integrator Op-amp
Differentiator Op-amp
Summing Op-amp.
In the Waveform generators, Triangular wave, Square-wave, pulse train, etc. can be
generated by the use of one Op-Amp and a pair of back to back Zenor diodes and
some filter elements, input may be a simple sinusoidal signal. While designing the
op-amps for typical waveform generation, an appropriate type as per required
application of op-amp along with the power supply and the feedback network is
selected.
Comparator & Clipper Op- Amp.
Waveform Generator Op- Amp.
Square wave Generator Op- Amp.
Triangular-wave Generator Op- Amp.
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TURBINE SUPERVISORY EQUIPMENTS
The advantage of equipping turbines with supervisory instrumentation was
recognised some forty or more years ago and at that time it was envisaged that such
instrumentation would provide accurate and easily interpreted indications of the
mechanical behaviour and working clearances of steam turbine generating plant.
However, in the context, of turbine start up the records obtained are peculiar to the
units concerned and are of particular value in providing a record of past operation on
which to base future procedures.
The supervisory equipment supplied with large steam turbines involves the
application of various machine-mounted detectors to provide the necessary
information on temperature differentials, running clearances and dynamic balance of
the turbine/shaft and to ensure safe start up, operation and shut down within
minimum time.
The detectors of measurement of parameters mounted outside of the steam space e.g.
shaft eccentricity; axial differential expansion and vibration all give little or no
information on the situation inside of the cylinders. Nevertheless, turbine supervisory
equipment are regarded as essential in providing a warning of imminent trouble, as
well as an indication and permanent record of the behaviour of turbine generating
plant during start up and running on line; the thermal stresses, expansions and
vibrations should not become excessive such that turbines are safe and defect free.
Under changing conditions, the rate of heating or cooling of the rotating and
stationary parts of steam turbines is not the same and differential expansions and
contractions occur, which for example, tend to close the small axial clearances
between the rotating and stationary blades.
Furthermore, surfaces exposed to the live steam cause change in temperature more
rapidly than the rest of the turbine and if temperature differences are not kept within
satisfactory limits, plastic strains with cracking of the affected parts may result in
the long term. Reliability and repeatability are of overriding importance in
establishing the confidence of operatives in turbine supervisory equipment. A few of
the turbovisory instruments have been described in the paragraphs that follows:
Differential Expansion Measurement: Historically, the measurement of axial
differential expansion between turbine rotors and cylinders has been significant as
an operating parameter. With the increased length of machines, however, the
methods of measurement based on the use of both double taper and straight radial
collars on the shaft have sometimes presented major problems in the realisation of
acceptable accuracy. To deal with this situation a system-employing eddy current
probe has been developed which does not suffer from drift. Such detectors are
mounted on the bearing keep so as to present the probes appropriately to the
periphery of the specially machined collar on the turbine shaft. Changes of shaft
behaviour /attitude and eccentricity have no effect on the accuracy of measurement
so long as the probe gap remains within its working range.
Vibration Detection and Measurement: Pedestal vibration analysis can give
information of the shaft condition (whether cracked/may develop crack within some
period). The vibration is measured in Amplitude, frequency & Phase. In a vibration
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detector the voltage is induced across the end of a conductor that moves through a
magnetic field. The induced voltage is proportional to the relative velocity between the
coil and magnetic field and it depends on the coil length, magnetic strength and
velocity of passing magnetic field. Permanent magnet is firmly attached to the case; it
follows the vibratory motion. Coil assembly is supported by a spring suspension
system and combined mass is designed to have low natural frequency. Induced
voltage is proportional to vibration velocity (measured so) and displacement can be
obtained by electronically integrating the signal. Shaft vibration is defined as the
dynamic movement of the shaft around the eccentricity locus. And by this
measurement routine operational monitoring of changes in vibrational behaviour and
radial clearances of the machine can be ascertained.
Differential Expansion Detector
Vibration Measurement Detector
Eccentricity
Detector
Measurement
The Vibration measurement can be made in three basic quantities as under;
1. Amplitude: It refers to the level of vibration; it can be further represented by
i)
Displacement which is the total distance the vibrating part moves in a given
direction, its unit of measurement is micrometer (m-6)
ii)
Velocity refers to the speed of the part at which it moves at any instant during
the vibration cycle ( 0 to max or peak); measured in r.m.s & its unit is mm
/sec
Acceleration refers to the rate of change of vibration velocity at any instant
during the vibration cycle; it is measured in r.m.s and its unit is mm/sec/sec
iii)
2. Frequency: Vibration in rotating machines occurs as a result of imbalance in the
forces generated or acted upon in the M/C and the frequency is in multiples of
shaft rotational speed which ranges from 10 Hz (close to 1st critical speed of
500MW T.G sets) to 750 Hz (new revised figures reached are 5-1000 Hz)
3. Phase: It refers to the angle of unbalance and is given in radian/degree.
Normally the Piezo-electric (earlier explained) type sensors are used as vibration
pick-up .As explained earlier, small variation in construction of vibration pick-up i.e.
the spring stiffness and in magnet characteristics etc gives rise to output voltage
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variations. Magnetic effects are picked up in considerable quantum due to the a.c.
generator vicinity and may create noise, but this can be overcome by incorporating
magnetic screen with the instrument.
Depending upon the overall sensitivity, vibration instrument may give an output
reading when subjected to high cross axis excitation. On a turbine pedestal for
example, the Horizontal and Vertical vibration may be very similar in magnitude, in
this circumstances the transducer with high transverse (cross-axis) sensitivity could
produce an output which is significantly different from the actual vibration level in
the required direction.(Cross-axis figure should not exceed 10 %)
While going for Vibration Measurement, common practice is followed to measure
bearing pedestal vibration which, though dependent to some degree on the mass of
the pedestal and the rigidity of the foundations, is particularly sensitive to out of
balance forces on the turbine shaft at the higher speeds.
Vibration amplitude for a certain level of severity must get smaller and smaller and
the alarm level changed accordingly. It provides signals directly proportional to
acceleration and offers particular advantages of robustness and resistance to
environmental conditions. The responses to imbalance, thermal bends, rubs &
unloading of bearings are more clearly observed by measurements of shaft
movements within the bearings rather than by the vibration of pedestal in the steam
turbines. It is measured as the diameter of the locus traced by the shaft centre; the
change in radial air gap within the cylinder is inferred from eccentricity.
Shaft eccentricity equipment indicates the amplitude of shaft vibrations relative to
the pedestal, in one direction, at several points along the rotor, all of them outside
the steam space and close to a bearing.
In the limit, rubbing may occur with the resultant localised heating possibly leading
to serious damage. The detection of shaft eccentricity is particularly valuable at the
lower speeds in establishing the straightness of a turbine shaft but the measurement
is often complicated by phenomena of shaft whirl, which can arise from effects
originating at the bearings. For shaft eccentricity measurement an eddy-current
probe is presented to the periphery of the shaft at a defined longitudinal distance
from the centre line of the bearing.
As permanent, continuously operating monitoring equipment, this is extremely
useful, but for indicating the onset of shaft vibration trouble on a particular turbine
it is not so reliable. At all times, the shafts of steam turbines must conform within
close limits to the natural deflections due to their own weights. If the temperature
distribution in a shaft is not symmetrical about it’s axis differential expansion will
cause the shaft to bend and if the speed is raised centrifugal forces will aggravate the
condition. Normally the Non-contacting type proximity sensors are used which
contains transducer, connecting cable and electronic processing units; these
transducers operate on eddy current or the variable reluctance principle. LVDT has
been explained
Thrust Position (Shift) Measurement: The primary purpose of thrust position
measurement of the turbine shaft is to provide an accurate indication of the onset of
thrust bearing failure. With such an indication giving advanced warning the damage
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to the machine can be limited to only the loss of the thrust bearing. The
measurement is made, as near to the thrust bearing as possible since the greater the
distance between the thrust collar and the points of measurement, the greater will be
the inaccuracy. This is brought about by thermal growth and various pressure
strains on the shaft and the turbine casing. If possible the thrust probe should
observe the thrust collar directly. The probe should be installable, removable, and its
gap adjustable from the outside of the bearing housing with the machine in operation
however, this is not always possible.
Shaft Eccentricity Measurement: Eccentricity is defined as the out of centre
Turbine Rotor &Casing Expansion Measurement: The front bearing housing of the
HP and IP turbines can slide on their base plates in an axial direction. Fitted keys
prevent any lateral movement perpendicular to the machine axis. The bearing
housings are connected to the HP and IP turbine casings by guides which ensure
that the turbine casings remain in their central position while at the same time axial
movement takes place. Thus the origin of the cumulative expansion of the casings is
at the front bearing housing of the LP turbine.
The casings of the LP turbine are separately and axially located by fitted keys at the
front supports of their longitudinal beam members on the base plates. Free lateral
expansion is allowed. The centre guides for these casings are recessed in the
foundation crossbeams. There is no restriction on axial movement of the casings. At
the rear supports of the longitudinal beam members the casing is free to expand
horizontally in any direction.
Hence, when there is a temperature rise, the outer casing of the LP turbine expands
from its fixed points towards the generator. Differences in expansion between the
outer casing and the fixed bearing housings to which the housings for the shift
glands are attached are taken by bellows type expansion joints.
The IP and HP casings expand towards the front bearing housing of the HP turbine
as the fix point of the turbine casing on the foundation is at the bearing housing
between the IP and LP turbines. Also the fixed point of L.P Turbine is there at the
bearing casing housing front base plate support of each longitudinal beam member
of the LP turbine; from these points the members and the LP outer casing bolted to
them expand towards the generator.
The thrust bearing is incorporated in the front bearing housing of the IP turbine.
Since this bearing housing is free to slide on the base plate the shafting system
moves with it. Seen from this point, both the rotor and casing of the HP turbine
expand towards the front bearing of the HP turbine. The rotor and casing of the LP
turbine expand towards the generator in a similar manner.
The movement and expansion of individual casings is measured to ensure that the
alignment between pedestals is maintained within acceptable limits. In turbines
when the HP and IP casings heat up, they expand in the direction towards HP end,
so each LP casing expands towards the generator end. Rotor expansion takes place
in the direction away from the location point within the thrust block assembly. The
rotors can, either expand within the clearances provided at the pedestals or can be
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keyed to the casing to move axially with the expanding rotor depending on the
turbine designs.
Rotor Temperatures: The HP and IP rotors may be subjected to high thermal
stresses due to temperature gradients through the rotor assemblies. These
conditions arise during start-up when high temperature steam flows over relatively
cool rotors. Several methods are adopted to predict the temperature gradient during
turbine run-up. One way is to rely on heat transfer equations written into the
computer software, other way is to make use of measurements taken in the HP and
IP steam pipes located in positions within the pipe work in such a way that their
response over short periods is the same as that of the rotor. Metal temperatures are
displayed as individual temperatures or differential temperatures from combinations
of individual or average temperatures.
CHEMICAL MEASURING INSTRUMENTS
Chemical control of water systems plays an important part in the operation of
modern power plant. Without it plant would deteriorate rapidly and major failures
would soon result leading to loss of availability. The basic aim of chemical control is
to maintain an environment in all parts of the system, and the material used for all
conditions of operation. This is achieved by minimizing the impurity ingress,
removing impurities & corrosion products etc. thereby maintaining high water purity
& Chemical conditioning. Chemical control of the feed water system is achieved by
addition of ammonia and hydrazine concentrations in the feed water.
pH measurements provide indication of adequacy of ammonia dosing & necessity of
continuous hydrazine monitoring.
The electro chemical technique is adopted for measuring the concentration of a
component in solution in terms of effect it has on the electrical properties of the cell.
The effect therefore forms the basis of the electrochemical transduction of the active
type and the e.m.f. developed between the two electrodes of the cell constituted for
this purpose is used to signify the concentration level. A galvanic cell is used as a
converter of chemical energy into electrical energy, whereas an electrolytic cell is
supplied with electrical energy from an external source. At the exact point at which
the galvanic e.m.f is balanced by the applied e.m.f., no current flows through the cell,
and under these conditions, the potential of each electrode reflects the composition
of the solution.
Chemical monitoring instrumentation monitors the chemical conditions of the
system during operation and facilitates chemical control of the water treatment
plant, the condensate polishing plant etc. and alarms the operator of any marked
departure from acceptable conditions and provides information for troubleshooting.
Some of important application of chemical instrumentations at various plant
locations are shown and described in short in pages below.
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The Make up Water Treatment Plant. Conductivity (before & after cat-ion resin)
reactive silica and sodium measurements are done for finding out the performance of
individual items of plant. It gives warning of resin bed exhaustion and ensures
required water quality. In addition pH equipment is often installed at sumps etc. to
ensure the neutrality of acid and alkali spent by re-generants prior to disposal.
The Condenser: Direct conductivity to facilitate in finding the condenser leak
locations by means of probes installed at various locations within the shell.
The Condenser Extraction Pump: Direct and after cat-ion conductivity (supplemented
by sodium in some instances) to provide warning of condenser leakage and the
Dissolved oxygen to ensure adequacy of oxygen removal at the condenser.
The Condensate Polishing Plant: Conductivity (before and after cat-ion resin) and
reactive silica to facilitate plant operation and check on outlet water quality in
addition to this chloride and sodium are monitored.
Downstream of chemical dosing points: PH as a check on adequacy of dosing.
Deaerator Inlet / Outlet: Occasional need for on-line monitoring of dissolved oxygen
to check on deaerator performance.
Final Feed water line: Conductivity, dissolved oxygen, pH, sodium and reactive silica
as a check on the final feed water quality and its acceptability for feeding to the
boiler. During commissioning and subsequent start-ups it may be considered
beneficial to continuously measure the total iron (i.e. corrosion product) inventory to
the boiler. In addition continuously monitoring of ammonia and hydrazine
concentrations in the feed water since chemical control of the feed system is achieved
by addition of these chemicals.
Boiler Water (Drum type system): Conductivity (before and after cat-ion resin)
chloride and sodium to ensure correct alkaline conditions in the bulk are being
maintained. pH measurements provide an indication of the adequacy of ammonia
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dosing and the necessity for continuous monitoring of hydrazine other than to
economise on its use is (or has been in the past) debatable. This situation may
change in the future as a result of excess hydrazine possibly being linked with
erosion / corrosion problems currently being investigated.
Main Steam: Conductivity (before and after cat-ion resin) and reactive silica to obtain
a measure of salt carry over to the turbine blades. The latter may only be required
during early operation to establish a relationship between silica in steam and boiler
water once this relationship has been established then monitoring the silica in the
boiler water may be adequate.
Electro Chemical Transducers
The concentration of a component in solution is measured in terms of effect it has on
the electrical properties of the cell. The effect of the component on an electrode
introduced into the solution forms the basis of the electrochemical transducer of the
active type and the e.m.f developed between the two electrodes of the cell constituted
for this purpose is used to signify the concentration level. Electrochemical
transducers enable the presentation of ionic potentials into a suitable form so that
output signals are representing the bio-chemical phenomena (in which both
membrane barriers and metal electrolyte interfaces are used). The electrochemical
cells are of two types.
1) Galvanic 2) Electrolytic.
A galvanic cell is used as a converter of chemical energy into electrical energy,
whereas an electrolytic cell is supplied with electrical energy from an external source.
At the exact point at which the galvanic e.m.f is balanced by the applied e.m.f, no
current flows through the cell, and under these conditions, the potential of each
electrode reflects the composition of the solution. The theory of electrodes and the
principles that govern their design are a little involved and require an understanding
of electrochemistry.
Measurement of ion concentration in electrolytes pose problems because an electrode
is selective to desired ion species and its concentration in the solution and reference
electrode remain independent of the variation of the electrolyte surrounding the
indicator electrolyte and separating the electrolyte. The metal used for electrode is
mercury, which is in contact with a paste containing equal weights of calomel and
potassium chloride. A column of saturated KCL solution over the paste enables
contact with the test solution through a salt bridge of KCL whose concentration is
kept about 3.8 mol per litre. Contact with mercury pool is by means of a platinum
wire which may be amalgamated Ag/AgCl or Calomel (Mercurous chloride) electrode
is used for pH measurement of water. The porous plug or the salt bridge serves the
function of connection between the reference and indicator electrode without allowing
the solutions to mix with each other. However the passage of ion takes place through
the unequal rates of diffusion develop some junction potential.
Electrochemical transducers enable the presentation of ionic potentials into a
suitable form so that output signals represent the bio-chemical phenomena (in which
both membrane barriers and metal electrolyte interfaces are used) for pH
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measurement of water the metal used for electrode is mercury, which is in contact
with a paste containing equal weights of calomel and potassium chloride. A column
of saturated KCL solution over the paste enables contact with the test solution
through a salt bridge of Kcl whose concentration is kept about 3.8 mol per litres.
Contact with mercury pool is by means of a platinum wire, which may be
amalgamated Ag/AgCl or Calomel (Mercurous chloride) electrode. The porous plug or
the salt bridge serves the function of connection between the reference and indicator
electrode without allowing the solutions to mix with each other. However the passage
of ion takes place through the unequal rates of diffusion develop some junction
potential.
Measurement of oxygen
Although the concentration of oxygen in gases is the parameter of widest interest it
should be remembered the most if not all, methods of measurement actually respond
to oxygen partial pressure. Since instruments read directly in concentration units
they must be calibrated at the pressure of measurement.
Gas Absorption Methods
These methods are dependant upon the oxygen in a measured volumetric sample of
gas being absorbed by a chemical solution, normally alkaline pyrogallol solution. The
resulting reduction in volume, measured at atmosphere pressure, indicates the
volume of oxygen originally present and allows the concentration, (by volume) to be
calculated. Apparatus for measuring oxygen by this method, along with other gases
such as carbon monoxide and methane, are typically those referred to as the Orsat,
Haldane, Bone and Wheeler and Schholander gas analysis apparatus. Of these the
Orsat is the most common and although somewhat crude it still finds use in the
laboratory and as a portable means of gas analysis on plant. The limitations of this
type of apparatus are obvious and the accuracy of measurement is very dependent
upon the skill of the operator.
Paramagnetic Analysers
Two of the electrons in the outer shell of the oxygen molecule are unpaired. The
magnetic moment is therefore not neutralized and as a result, the oxygen molecule is
strongly paramagnetic i.e. attracted by a magnetic field. (Most other gases are either
unaffected by a magnetic field or exhibit weak diamagnetic properties i.e. they are
repelled) There are two-three types of instrument for measuring oxygen, which make
use of this property.
Magnetic Wind Analyser utilizes the principle of paramagnetic property of O2 as hown
in the diagram the sample gas enters at the bottom and passes upwards through the
side tubes. Located in the cross tube is a heated filament which forms one arm of a
Wheatstone bridge. Oxygen present in the sample gas is attracted from the left hand
tube into the cross-tube by the magnetic field. On entering the cross-tube it is heated
by the filament and this has the effect of reducing its paramagnetic susceptibility.
The heated gas is pushed along the cross-tube by cold gas entering at the left thus
creating a flow of gas along the cross tube proportional to the partial pressure of
oxygen. The flow of gas cools the filament so changing its resistance. This change is
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resistance unbalances the Wheatstone bridge giving a signal, which can be related to
the oxygen concentration.
Quincke Type Analyser: In this analyzer a continuous steam of nitrogen gas passes
over two sections of a Wheatstone bridge. Flows are adjusted so that the heat losses
over the two sections are the same and the Wheatstone bridge is balanced. The right
hand stream of nitrogen passes through a magnetic field whilst the left hand stream
passes through a similar volume, but without any magnetic field, into a common
outlet. Sample gas is introduced upstream of this outlet and mixes with the nitrogen
as it emerges from the two arms. The presence of oxygen in the sample gas generates
a small backpressure on the right hand flow pressure distorts the flow pattern of
nitrogen around the arms of the Wheatstone bridge which becomes unbalanced and
gives a signal proportional to this oxygen content. It requires a continuous flow of
nitrogen and accurate alignment to ensure that there is no gravitational chimney
effect.
Magneto-Dynamic Analysers also make use of the paramagnetic properties of oxygen
but in a different way. A diamagnetic body located in a non-uniform magnetic field
will be repelled by that field, the forces acting upon it being dependant upon the
strength of the pole pieces and the magnetic susceptibility of the gas which
surrounds it. The presence of a paramagnetic gas such as oxygen increases that
susceptibility (and therefore the forces acting upon the diamagnetic body) in
proportion to the oxygen partial pressure.
This is the basis of the Magneto-Dynamic Analyser. The first measuring cell to take
advantage of this effect was that developed by Pauling and known as the Pauling
Dumb-bell system. The Pauling cell consists of two diamagnetic spheres of glass
filled with nitrogen and mounted at the ends of a bar to form a dumb-bell. This
dumb-bell is mounted horizontally on a vertical quartz fibre torsion suspension.
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The whole measuring cell operates inside a strong non-uniform magnetic field. The
spheres are repelled from the strongest part of the field and so rotate the suspension
until the force produced by the twist of the suspension is equal to the force acting on
the spheres. A change in the oxygen content of the gas inside the cell changes the
force acting on the dumb-bell causing it to take up a new position. This can be
measured wither directly e.g. using a lamp/mirror system or electromagnetic
feedback can be used to restore the dumb-bell to the zero position as in the Munday
cell.
The current required to maintain the dumb-bell in the zero position is a measure of
the magnetic susceptibility of the gas present in the cell and therefore the oxygen
partial pressure. This cell is the basis of the Servomex range of oxygen analysers.
With magneto-dynamic analysers only the magnetic susceptibility of the gas is being
measured and the reading is unaffected by changes in the carrier gas composition.
The response is linear with oxygen partial pressure and two-point calibration usually
with air and nitrogen (to set the zero of the instrument) is sufficient. Calibration and
measurement at the same pressure enables a direct readout in concentration units
to be obtained.
The High Temperature Ceramic Sensor
The High Temperature Ceramic Sensor is in principle an oxygen concentration cell
comprising two electrically conducting; chemically inert electrodes (usually platinum)
attached to either side of a solid electrolyte closed ended tube. This is shown
schematically. The tube is completely gas tight and made of a ceramic (usually
calcium stabilized zirconium oxide), which, at the temperature of operation, conducts
electricity by means of oxygen ions.
The potential difference across the cell is given by the Nerst equation i.e.
E = 2.303 RT log( P1/ P2) /RTvolts , P1 & P2 oxygen partial pressure at electrodes F
P2,, E is the potential difference; R is the gas constant (1.987 cal /deg /mole) T is the
absolute temperature. F is Faradays constant (23060 cal/v)
Thus, provided that the oxygen partial pressure is known at one electrode, then the
potential difference between the two electrodes enables the unknown oxygen partial
pressure to be determined at the other electrode.
In practice air is invariably used as a reference gas and is allowed, or directed to
come into contact with the one electrode whilst the other electrode is exposed to the
sample gas. The high temperature ceramic sensor holds over a very wide range of
oxygen partial pressure allowing measurements to be made in the percent oxygen
range on the one hand through ppm concentrations to oxygen range partial pressure
as low as 10-25 atm on the other. Oxygen reacts with combustible gases in the sample
at above 600OC.
The platinum outer electrode of the sensor acts as a catalyst in this respect and the
output will therefore be a measure of the residual oxygen. The ceramics used for
sensing leads to situation of even operating at a lower temperature.
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The measurement of oxygen in flue gas is an important aid to the combustion control
of oil and coal-fired plant. The immediate problem that has to be overcome in this
application, however, is the wet dirty and potentially corrosive nature of the gas to be
analysed. The direct oxygen probe based on the high temperature ceramic sensor
and manufactured by Kent and Westinghouse has the advantage that it can be
installed directly into the flue gas ducting. Not only does this avoid the complication
of a gas sampling system but high temperature operation ensures freedom from
“dewing out” of water on the cell surfaces.
Filtration of particulate matter can be more readily achieved and a further advantage
is that oxygen reading on a “wet” basis can be obtained directly. A fast instrument
response is also a feature of the Kent probe system as shown. For direct oxygen read
out on a “wet” basis sampling system and measuring cell must be maintained above
the dew point. A steam ejection sampling system of the type as shown is also
employed to provide a sample suitable for oxygen measurements using a
paramagnetic analyser
CONTROL SYSTEMS
In any industrial operation, controlling the process parameters like pressure, flow,
Level temperature, humidity, viscosity etc., and automatic control becomes essential;
it has become the important and the integral part of the power plant (any process).
Automatic controls provide means for attaining optimal performance of the dynamic
system, improve the quality and economizes the cost, expand the production rate,
reduces the repetitive manual operation. All above thus calls for good understanding
of the field. As the efficient power- plant-operation requires handling many inputs
and outputs, systems are becoming more complex and lot many equations are being
incorporated to cope/ be in pace with the advancements in control field, the
designing of control system must be such as to use complex computation through
the electronic analog, digital and hybrid computers and on-line computers for
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operation support. Some terminologies appearing frequently are given below to
describe the intricacy of control systems: Plant: A set of machine parts functioning together to perform a particular operation
is considered as a Plant.
Process: Any system comprised of changing and dynamic variables usually involved
in manufacturing and production operation describes a Process.
Systems: A combination of components that act together, but not limited to physical
ones rather the abstract and the dynamic phenomena forms a System. The System
implies to biological, economic, physical etc.
Internal & External Disturbances: The signal that affect the output of the system,
is termed as the Internal Disturbances whereas the External Disturbances refers to
the disturbance generated outside the system although partly is an input only.
Unpredictable disturbances are designated as unknown parameter beforehand; with
predictable or known disturbances, it is always possible to include compensation
within the system so that measurements are unnecessary.
Servomechanism: is a feedback control system in which the output is some
mechanical position, velocity, or acceleration etc.; in control system, the terms
servomechanism and position are synonymous.
Control Lag: The control system has Control Lag (it refers to the time) associated
with its operation that must be compared to the process lag. Due to the control lag,
when a controlled variable experiences a sudden change, the process-control loop
reacts by outputting a command to the final control element to adopt a new value to
compensate for the detected change. Control lag refers to the time for the processcontrol loop to make necessary adjustment to the final control element.
Dead Time: Another time variable associated with process control that is both the
function of the process-control system and the process is Dead Time. This is the
elapsed time between the instant a deviation (error) occurs and the corrective action
first occurs. The dead times can have a very profound effect on the performance of
control operations on a process.
Cycling: The dynamic behaviour of the variable error under various modes of
control, leads to an oscillation of the error about zero. This means the variable is
cycling above and below the set point value. If cycling continues indefinitely, this
leads to the steady state cycling; in such situation we are interested in both the peak
amplitude of the error and the period of the oscillation. We have cyclic transient error
if the cycling amplitude decays to zero, We are interested in resolving and controlling
the initial error, the period of cyclic oscillation, decay time for the error to reach etc.
Feedback control: A feedback control in a control system is one which tends to
maintain a prescribed relationship between the reference input( set point decided any
where within the range) and the output by comparing these two signals and using
the difference as an error input to the controller.
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Proportional Band (PB) is the change in proportional input that is required to
produce full-scale change in output. E.g. The PB is 10 % if the 10% change in error
causes 100 % change in output.
Dynamic Response of control Devices is the dynamic behaviour, in response to a
sudden change in load etc.; this is required to evaluate the range of regulation and
tuning of the controller performance. Often, the changes are random and therefore
unpredictable except on a statistical basis. One has to find complete response to a
step function or alternatively the frequency response for steady-state sinusoidal
inputs.
Automatic regulating system employs feedback in which the reference input or the
desired output is either constant or slowly varying with time Controllers are meant to
maintain the actual output at the desired value in the presence of disturbances of
automatic regulating system. Transducers are used to measure variables and convert
them to electrical form. Computers or comparators are used in determining the
difference between reference and feedback variables. Controllers and amplifiers are
used to develop signals to actuate the controlled system. Motors are used to provide
the ‘muscle’ for system control.
Process control is essential requirement of any industry. Automatic control of
pressure and of electric quantities such as voltage, current and frequency,
programmed controls of temperature by heating furnaces according to a preset
program etc. are often used in Process control systems. It should be noted that most
Process control systems include servomechanisms as an integral part. Despite their
great variety, control systems can be analysed into a few basically similar
components. Working of the Watt’s Flywheel governor is the best example to
understand the philosophy of the primitive process control system installed in the
process industries.
CONTROL SYSTEMS: MODES OF CONTROL ACTIONS
The Control Systems work in two modes as explained below:
Open Loop Control Mode: In this mode, the output has no effect upon the control
action; the output is neither measured nor fed back for compensation with the input.
For each reference input, there corresponds a fixed operating condition as the o/p is
not compared with the input and the system accuracy depends on calibration. It
operates on a time basis that is why it is termed as open loop system
Closed loop control mode: is the mode in which the output signal has a direct effect
upon the control action. System requires a feed back signal. Error signal between
the input and the feedback signal is fed to the Controller so as to output to the
system for reducing/neutralising the error and bringing the output of the system to a
desired value. In a closed loop system use of feed back signal reduces the system
error. In comparison to an open loop system, the closed loop is capable of greater
accuracy over a wider range of conditions even when less precise control elements
are used.
In Watt’s flyball governor, it being perhaps the first automatic control device, the
governor of the engine drives a rotating spindle carrying the flyweights. The engine
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speeds up until the centrifugal force of the flyweights overcome the force of the speed
adjusting spring and partially closes the throttle valve. An increase in load on the
engine momentarily reduces its speed, reduces the centrifugal force of flyweights,
allows the throttle valve to open, and accelerates the engine until the set speed is
reached.
Block diagram of a fly ball governor
Load Frequency Control:
The figure below shows the single turbo-generator system supplying an isolated load.
The system consists of components:
Fly ball Speed governor system senses the change in speed (frequency); as the
speed increases, the fly balls move outwards and the point B on linkage
mechanism moves downwards and reverse happens in decreasing of speed.
Hydraulic Amplifier has a pilot valve and main piston arrangements. Low power
level pilot valve movement is converted into high power level piston valve
movement. For opening /closing the steam valve against high-pressure steam.
Linkage Mechanism: ABC is a rigid link pivoted at B and CDE is another rigid link
pivoted at D. This link mechanism provides a movement to the control valve in
proportion to change in speed. It also provides a feedback from the steam valve
movement (link4).
Speed changer: It provides a steady power output setting for the turbine. Its
downward movement opens the upper pilot valve so that more steam is admitted
to the turbine under steady conditions (hence more steady power output). The
reverse happens for upward movement of speed changer.
The point A on the linkage mechanism is moved downward by yA then yA = kC PC ;
PC is commanded increase in power which sets into motion a sequence of events
like pilot valve moving upward, high pressure oil flowing under top causing main
piston moving downward, the steam valve opening causing increase in turbine,
generator speed and frequency goes up. Increase in frequency f causes the fly balls
to move outwards so that B moves downwards by a proportional amount k2’ f. The
net movement of C is therefore yC = k1 kC PC + k2 f and movement D, yD= k3
yC + k4 yE. The movement yD depending upon its sign opens one of the ports of
the pilot valve admitting high-pressure oil into the cylinder thus moving the main
piston & opening the steam valve by yE.
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Certain justifiable simplifying assumptions, which can be made, are:
Inertial reaction forces of main piston and steam valve are negligible compared to the
forces exerted on the piston by high-pressure oil.
Because of (i) above the rate of oil admitted to the cylinder is proportional to port
opening yD.
The volume of oil admitted to the cylinder is thus proportional to the time integral of
yD. The movement of yE is obtained by dividing the oil volume by the area of the
cross-section of the piston. Thus yE = k5 (- yD) dt. (i)
From the schematic diagram (left side), it is observed that a positive movement yD
creates a negative (upward) movement yE accounting for the negative sign in (i)
above.
Load frequency and excitation voltage control:
This control is non-interactive for small changes and can be modelled and analysed
independently. Furthermore, excitation voltage control is fast acting in which the
major time constant encountered is that of the generator field; while the power
frequency control is slow acting with major time constant contributed by the turbine
and generator moment of inertia; this time constant is much larger than that of the
generator field.
Thus, the transients in excitation voltage control vanish much faster and do not
affect the dynamics of power frequency control. Changes in load demand can be
identified as: (i) slow varying changes in mean demand, and (ii) fast random
variations around the mean. The regulators be designed to function insensitive to
fast random changes, otherwise the system will be prone to hunting resulting in
excessive wear and tear of rotating machines and control equipment.
Schematic Diagram of Load– Frequency Control
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Load–Frequency & Excitation voltage Control
295
CONTROL SYSTEMS: CONTROLLERS
Controllers process data that are input from the sensor, apply the logic of control,
and cause an output action. The output signal may be transmitted either to the
controlled device or to other logical control functions. The controller compares input
from sensors with a set of instructions such as set point, throttling range, and action
and then produces an output signal. The control logic usually consists of a control
response along with other logical decisions that are unique to the specific control
application.
The control response conveys as to how the controller functions. Control responses
are characterized as:
•
•
•
•
•
•
Two-Position Control
Floating control
Proportional (P Only) control
Proportional Plus Integral (PI) control
Proportional Plus Derivative (PD) control
Proportional Plus Integral Plus Derivative (PID) control
Controlled Device or Output
A controlled device responds to the signal from a controller or control logic in a way
that changes the condition of the controlled medium or the state of the end device.
These end devices include valve operators; damper operators, electric relays, fans,
pumps, compressors, and variable speed drives for fans and pumps applications.
Two-Position Control
In a two-position control sequence, the controller compares an analog or variable
input with instructions and generates a digital (two-position) output. The
instructions involve definitions of upper or lower limits. The output changes value as
the input crosses the limits.
There are no standards for defining limits. Common terms for limits include set point
and differential. Set point indicates the value where output pulls-in, energizes, or is
true. The output changes back or drops out after the input value crosses through the
value equal to the difference between the set point and the differential. Two-position
control functions as a simple switch. Two-position control can be used in basic
control loops for temperature control or for limit control such as freeze stats or
outside air temperature limits.
Floating Control
A floating control response produces two digital outputs based on changing variable
input. One output increases the signal to the controlled device while the other output
decreases the signal to the controlled device. The control response also involves an
upper and lower limit with the output changing as the variable input crosses these
limits.
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FLOATING CONTROL RESPONSE
According to actions of the control systems the controllers are categorized in either
Two-position (On/Off) or Proportional (‘P’ action) or Integral (‘I’ action) or Derivative
(‘D’ action) and combination of all above, e.g P, P+I, P+D, P+I+D etc.
Proportional Action Control Mode
A proportional control response produces an analog or variable output change in
proportion to a varying input. In proportional control, a unique value of the
measured variable corresponds to full travel of the controlled device and a unique
value corresponds to zero travel on the controlled device. The change in the
measured variable that causes the controlled device to move from fully closed to fully
open be called the throttling range.
The loop controls within this throttling range, assuming that the system has the
capacity to meet the requirements. The type of action dictates the slope of the control
response. In direct-acting proportional control response, the output will rise with an
increase in the measured variable. In a reverse-acting response, the output will
decrease as the measured variable increases. Offset is defined as the difference
between the control point and the desired condition.
A proportional controller utilizes the principle of negative feedback. The relationship
of the gain Kp of the amplifier (the proportional sensitivity) of the control can be
obtained by the controller output, which is directly proportional to the deviation
(error) of controlled variable (input). The controller output is given by:-p=(100/P.B).e
+ po,where e is the error, po is the bias or the offset(initial condition is po)and Kp is
proportional gain and is equal to 100/P.B.(proportional band, is denoted as P.B, it is
inverse of gain Kp) .P.B.is to be chosen such that oscillation is minimum, P.B. is
increased to damp the oscillation& attenuate the input cycle.
Desired value must differ from the measured value and the measured value is
controlled to a proportional desired value by off setting the set value. In proportional
action output p = po at error=0 and is constant If the deviation (error) exceeds the
Proportional Band then the output may exceed the range of correcting element; thus
large gain is required then. In proportional controllers:
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i)
ii)
iii)
The output is a constant equal to output po for the error = zero
If there is error, for every 1% of error a correction of Kp percent is added to or
subtracted from po,
There is a band of error about zero of magnitude PB within which the output
is not saturated at 0 or 100%.
In the Op-amp circuit, the resistance R1 varies the proportional const and so the P.B.
The electronic proportional controllers are basically the amplifier, which receive small
voltage signals and output higher voltage/power signals.
The controller output eo = K. (ei –eo.R2/R1)
K .R2/R1 > 1 hold good
Normally Laplace transform, is obtained for the dynamic response of the controller
from transfer functions and the overall transfer function defines the system behavior.
Gain G(s) = Eo(s)/Ei(s), Kp= Ri/Ro, Proportional Constant
Prop. Control response
Prp Cont characterstic
Prop. Control set-up
The diagram below gives the arrangement of control system in which the valve
responsible for the boiler combustion say oil/coal valve is regulated to control the boiler
outlet/steam pressure. The desired steam pressure (Desired Value) is applied to the
controller which also receives the feedback signal of actual steam pressure (Measured
value), an error is created (e=M.V.-D.V) thus, which is amplified by gain factor K to a
value suiting to the valve regulation.
Proportional Control Set up
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Error Vs. Controller Output at varying gains
298
Effect of Controller Output for gain K=1
Effect of Controller Output for gain K=2
The controller output varies according to the gain (proportional gain constant
Kp=1,Kp=2,Kp=4) as shown in the figure. The effects of gain Kp =1 and the gain Kp
=2 has been shown in another sketch from which it can be inferred that the Off-set
values (Deviation from desired output) can be reduced by increasing values of Kp but
can never be neutralised since the proportional action mode generates Off-set (as is
clear from the equation). Since the inverse of Kp is the Proportional Band (P.B), so
with Kp=1,P.B=100%and Kp=2, P.B=50% and the pressure oscillates with rise of
Kpat reduced offset than before when Kp=1. ‘Kp’ is the gain of the proportional
controller, which can be adjusted. System performance of control actions can be
predicted on the basis of Step response.
Integral Action (Reset Action) Control Mode
In integral action controller, the output increases at a rate proportional to the control
variable error and so the control output is the integral or the error over time with a
gain factor (integral gain) the output p(t) =(1/KI) e.dt and the Transfer function is
given by. P(s)/E(s)= Ki/s. When the error is zero the output is fixed at the value that
integral term had when the error occurred.
The integral time (TI) adjusts integral action while a change in value of the Kp affects
the proportional and integral parts of the control action If the error is not zero the
proportional term contributes a correction and the integral term begins to increase or
decrease the accumulated value depending on the sign of the error and the direct or
reverse action. In Integral mode the value of the controller output p(t) changes at a
rate proportional to the actuating error signal as given above. For zero (0) value of e(t)
the value of p(t) remains stationary.
The integral time Ti is adjustable and this Ti affects the integral control action. The
integral component removes any standing error as achieved by I action alone;
although it effects to sluggish the controller response. The inverse of the integral time
is termed as the Reset rate, which is measured in, repeats per minute. In the OpAmplifier circuit, the R (variable) and C decide the integral time and can be set as
desired.
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Derivative Action (Rate Or Anticipatory) Control Mode
In derivative control (also termed anticipatory), the controller output is directly
proportional to the rate of change of control variable;. Since it is time dependant at
deviation (error) of zero value the output is zero and there is no output. In Derivative
(rate) mode the output p = Kp. KD d ep/ dt where KD = derivative gain constant d
ep/dt = rate of change of error. If error changes by an amount ep in a period of time
TD, then change in output is equal to change in output as a result of proportional
action. The derivative action responds only to the rate of change of error. In the ckt
combination of R1& C in ratio of R2 can be set for the derivative time.
The graph below shows the effects of modes of controls (P, PI, PID),it is observed that
the recovery time in proportional mode if is X then due to addition of Integral mode(in
PI) the recovery time of process error is increased a lot say 2 times X, and if further
derivative action is added (in PID) , the recovery time of the process error is reduced a
lot say ½ X.The Proportional action controller maintains the output at an offset
whereas the Proportional plus Integral or the Proportional plus Integral plus
Derivative controllers maintain the output at set value line as is clear from the graph.
Curves showing the Proportional, Integral, Derivative Action Control Modes
Proportional Plus Integral Control (Pi) Mode
PI control measures offset or error over time. The error is integrated, and a final
adjustment is made to the output signal from a proportional part of this model. PI
control response will work in the control loop to reduce the offset to zero. A well setup PI control loop will operate in a narrow band close to the set point and not over
the entire throttling range. PI control loops do not perform well when set points are
dynamic, sudden load changes occur, or the throttling range is small.
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Prop. + Int. response
P+Icontroller O/P for Ramp Error
Proportional + Integral Contr Set up
The proportional gain and the integral action time play important role in (PI) control
system; its Output is p =Kp. ep +Kp.KI ep(t) dt and Transfer function is P (s)/ E(s) =
Kp(1+1/ TIs) In PI mode one to correspondence of the proportional mode is available
and the integral mode eliminates the inherent off set The integral function provides
the required new controller output and error goes to zero after load change. The
integral term can not become negative thus it will saturate at zero if the error and
action to drive the area to net is negative value. The system is useful in frequent and
large load changing type processes. The effect of the integral action shifts the whole
proportional band.
The output saturates whenever the error exceeds the
Proportional Band limit.
The P+I control set up shown below is similar to the proportional control set up with
addition of integral unit. The Step change effect is shown in the curve by which it is
clear that the error is almost neutralised due to P+I action and the valve opening is
also less fluctuating. The ramp change is also shown and the combined effect is
drawn in next curve, which gives that integral effect improves the controller response
although process speed is slowed down.
Proportional + Integral Control Set up
P + I Controller Output for Step Error
Proportional Plus Derivative Control (PD) Mode
In this mode the cascaded use of proportional and the derivative modes is made. The
effect of derivative action is moving the controller in relation to the error rate change.
The controller output is given by p = Kp.(1 + . KD d ep /dt)
The transfer function of the PD is P(s)/E(s) = Kp (1+TDs)
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Option of proportional controller cannot be eliminated, however it can handle past
process load changes as long as the load change off set error is accepted. The setup
in electronic circuit has been drawn for the P+D control .a ramp change of the load
has been shown from where it can be inferred that with D action the correcting
signal is anticipatory in nature and it is created at times when change takes place
but in opposite direction of the change process, which is due to the anticipatory
characteristic of derivative control.
P + D Controller O/P for Ramp Error.
Electronic P + D controller set-up
Proportional (+) Integral (+) Derivative Control (PID) Mode
PID control adds a predictive element to the control response. In addition to
proportional and integral calculation, the controller will compute the derivative or
slope of the control response. This calculation dampens a control response that is
returning to set point so quickly that it would overshoot the set point. PID control
should be selectively applied to control loops.
The combination of all three modes i.e. the PID mode eliminates the offset of the
proportional mode and still provides fast response. Each of the three constants (Kp,
KI ,KD) can be varied independently without affecting the effective value of the other
two i.e. the integral and the derivative Controller output p is given by P(t) = Kp..ep +
Kp.KI ep(t)dt + Kp.KD dep/dt + po
The transfer function of PID controllers is: P(s) = Kp (1+TDs +1/TIs) E(s)
The P+I+D control set up has been shown which is further addition of D in the P+I
set up. The control characteristic due to result of typical error change is shown .from
that it can be observed that the combined effect gives result at very short
duration(refer the recovery time been reduced tremendously due to all three control
mode action as shown in earlier curve)
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Proportional + Integral Control Set up
P+I+D Controller O/P for Step: Error
Electronic P + I + D controller
P+I+D Controller O/P for Ramp Error
Various modes of controls can be achieved by using Op-Amps, capacitors &
resistances.
The controller output in proportional mode is P: KP = R2/R1, Proportional plus Integral
mode is PI: KI= 1/R2.C (Output inverters are included for eliminating instability and
expected fast variable time change). Proportional plus derivative mode is PD: KP=
R2/(R1+R3) and KD= R3.C. (Offset error of P action remains and D term provides the
rate action) and in Proportional plus Integral plus Derivative mode the gains KP is
obtained by R2/R1, KI by 1/RI.CI and KD by RD.CD etc.
Bump less transfer: - If p’ is the signal value corresponding to the position value of
the correcting element and p is the output signal so p’= p be met by automatic
control. In manual control p varies as per deviation (error) and p’ varies
independently.. To transfer from manual to auto control, the p’ = p be ensured. If it
is achieved; the correcting element will respond and make large disturbances
minimized, thereby Bump less transfer will be possible in Manual to auto operation.
PI, PD & PID modes of control in Electronic circuits have been shown below:
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DIRECT/DISTRIBUTED DIGITAL CONTROL SYSTEM (DDCS)
Direct digital control (DDC) systems consist of networked microprocessor-based
controllers connected to analog and digital devices, which either sense information or
control components of a system. Control logic initiates and sequences operations
programmed with software stored in the hardware.
Analog-to-digital (A/D) converters transform analog electrical values into digital
information for the microprocessor. The analog sensors may be thought of as
transducers that convert a thermodynamic property to an electrical property. Most
systems include stand-alone or remote controllers with software to eliminate the
need for continuous communication.
A personal computer workstation primarily monitors status and stores back-up
copies of the programs, records alarms and trending functions, and archives
historical information to operate the energy management system (EMS). Relatively
complex strategies and energy management functions are normally available at low
levels in the system architecture. Electric-to-pneumatic transducers (E/P converters)
can provide pneumatic actuation.
Controlling a system by DDC, involves three distinct steps:
•
•
•
Measure a variable and collect data
Process the data with other information
Cause a control action
A control loop comprises three main components: sensor, controller, and controlled
device. These three components or functions interact to control a medium. Figure 1
shows air temperature as the controlled medium. The sensor measures air
temperature and outputs data; the controller processes the data; and the controlled
device causes an action.
Similar figures could illustrate, the pneumatic or electronic control systems, where
the controller is a separate and distinct piece of hardware. In a DDC system, the
controller "function" takes place in software, as shown in figure 2. The double lines
indicate the boundary in and out of the DDC controller.
Figure 1. Basic control loop
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Figure 2. DDC control loop
304
Algorithms describing the mathematics of the software handle the controller
function. Various modes of control have been described earlier in details. However
the sensor, points, data classifications etc. have been elaborately explained below.
Sensor
Sensors measure the controlled medium or other control input in an accurate and
repeatable manner. HVAC sensors may measure temperature, pressure, or humidity.
Some sensors may measure other relative temperatures, time of day, electrical
demand condition, or other variables that affect the controller logic. Other sensors
input data that influence the control logic or safety, including airflow, water flow, and
current, fire, smoke, or high/low temperature limit. Sensors are an extremely
important part of the control system and can be a weak link in the chain of control.
Points
The word point is a common term used to describe data storage locations within a
DDC system. The data can come from sensors or from software calculations and
logic. The data can also be sent to controlled devices or software calculations and
logic. Each data storage location has a unique means of identification or addressing.
DDC data can be classified three different ways-by types, flow, and source.
Data Classifications
Data types are digital, analog, and accumulating. Digital data may also be called
discrete or binary. The value of the data is an integer, a 0 or 1, and usually
represents the state or status of a set of contacts. Analog data is represented by
numeric or decimal number (usually defining a varying electrical input that is a
function of temperature, relative humidity, pressure, or some other variable).
Pulse input data are the accumulating data and are represented by a numeric or
decimal number; the resulting sum is stored. Data flow refers to whether data go to
or from the DDC component or logic. Input points describe data used as input
information and output points describe data that are output information.
Points can be classified as external when the data are received from an external
device or sent to an external device. External points are sometimes referred to as
hardware points. Internal points represent data that are created by the logic of the
control software. Other terms used to describe these points are:
•
•
•
•
Virtual points
Numeric points
Data points
Software points
Global or indirect points are terms used to describe data that are transmitted on the
network for use by other controllers.
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Common Point Types
Analog input points normally imply an external point and represent a value that
varies over time. Common analog inputs for process applications include
temperature, pressure, relative humidity, carbon dioxide, or airflow. Analog output
points are control signals for modulating valve positions, damper positions, or drive
speed.
Digital inputs for process applications are usually status indicators, such as whether
or not a motor is running, or for fans, pumps, motors, and lighting contactors. A
temperature high limit is considered a digital input because, although it is
monitoring an analog value related to temperature, the information that is
transmitted to the controller is a digital condition indicating whether or not the
temperature has exceeded a threshold. Digital outputs usually control relays that
command motors or other devices to turn on or off.
Software Characteristics
There are many different software programs used in DDC systems. DDC systems
include operating software, configuration software for the points or system
architecture, programming software, and software for alarms. The characteristics of
the programming software can be an important difference in various DDC systems.
There are three common approaches used to program the logic of DDC systems:
•
•
•
Line programming
Template or menu-based programming
Graphical or block programming
Line programming-based systems use software languages, similar to Basic or
FORTRAN with the process subroutines. Familiarity with computer programming is
helpful in understanding and writing logic for any process applications.
Menu-driven, database, or template/tabular programming makes use of templates
for common process system logical functions. These templates contain the detailed
parameters necessary for each logical program block to function. How one block is
connected to another or where its data comes from is known as data flow and is
programmed in each template.
Graphical or block programming is an extension of tabular programming. Graphical
symbols connected by "data flow" lines represent the individual function blocks.
Symbols similar to electrical schematics and pneumatic control diagrams depict the
process. Graphical diagrams are created and the detailed data are entered in
background menus or screens.
Architecture
System architecture is the map or layout of the system used to describe the overall
local area network (LAN) structure. This map will show where the operator interfaces
with the system and may remotely communicate with the system. The network, or
LAN, is the media that connects multiple devices. This network media allows the
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devices to communicate, share, display and print information, and store data. The
most basic task of the system architecture is to connect the DDC controllers so that
information can be shared between them.
LAN Communication
Communications between devices on a network can be characterized as peer-to-peer
or polling. On a peer-to-peer LAN, each device can share information with any other
device on the LAN without going through a communication manager.
The controllers on the peer-to-peer LAN may be primary controllers, they may be
secondary controllers, or they may be a mix. The type of controllers that use the
peer-to-peer LAN will vary with different manufacturers.
In a polling controller LAN, the individual controllers cannot pass information
directly to one of the other controllers. Data flow from one controller to the interface
and then from the interface to the other controller.
The interface device manages communication between the polling LAN controllers
and with higher levels in the system architecture. This same device may also
supplement the capability of polling LAN controllers by providing the following
functions:
Clock function
Buffer for trend data, alarms, messages
Higher order software support
Many systems combine the communications of a peer-to-peer network with a polling
network. In this case, the interface communicates in a peer-to-peer fashion with the
devices on the peer-to-peer LAN. The polling LAN-based devices can receive data from
the peer-to-peer devices but data must flow through the interface.
Controller
Classification
Many DDC manufacturers make two distinct levels of controllers. Some make only
one. These levels describe where the controller resides within the system architecture
on the control network. Knowing the difference between these controllers is
important because the appropriate controller is application dependent. Many
specifications do not distinguish between the various types of controllers.
Higher-end controllers normally reside on a higher-level network and communicate
in a peer-to-peer fashion. These are called primary controllers. Peer-to-peer, means
that the controllers can share information to other peer-to-peer devices without going
through an intermediary device. (Called a supervisory interface). Such controllers
have more memory, more sophisticated CPUs, higher resolution A/D converters,
more accurate clocks, and can store more complex control strategies as well as
trends, schedules, and alarms.
Manufacturers also make lower level controllers that normally reside on a lower levelpolling network. These controllers have more limited memory and processing
capabilities and must use a supervisory interface device to communicate with all
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other devices. There are many different designs. Some are designed for terminal
applications like variable air volume boxes or fan coil units. Others may be used for
air handling systems with simple to moderately complex sequences of operation.
These terminal controllers are usually configured for the number of points required
for that application.
Some of these controllers use a free form of programming, which requires a complete
set of custom programming, while others have application specific programs for
typical applications. These programs have selectable parameters that can be set up
for each individual application. Since these controllers have more limited memories,
they usually do not store historical information (such as trends) and rely on the
supervisory interface for this function.
The secondary polling networks are configured such that one supervisory interface
can monitor a limited number of controllers. This limitation varies by manufacturer.
A large number of controllers on a secondary controller network can negatively affect
the number of trends that can be practically used, the amount of data that can be
processed, and the speed of transmission over the network. How many is too many
on this secondary network? This varies and depends on the manufacturer, the speed
of their network, and the application in question.
In a process control applications, these lower-level controllers may be adequate for
many simple systems, but a primary controller is more appropriate for critical
applications. How does one specify these distinctly different controllers? First, the
engineer must define requirements of various types of controllers and their
corresponding interfaces (both network and operator). Once defined, the engineer can
dictate which controller should be used on various applications.
The process control systems have undergone dramatic changes. They have evolved
from pneumatic controls through various generations of EMS and DDC systems to
current generation distributed DDC. At this time the computer industry trend of
increasing processing power and memory at lower cost is quickly influencing DDC
controllers. The advent of open protocols, the increased availability and use of
site/building/campus networks, and the interfacing of DDC systems to the Internet
have increased the complexity of these systems.
During the last 20 years (relatively short time), we moved from a non-proprietary
communication protocol to one that has been very proprietary. In addition, the
control logic that was distributed to single function hardware components (receiver
controllers, and switching relays) now resides in software. These are significant
changes to a critical subsystem any process systems, which is vital to the
performance and basic operation of industrial units.
The DDC system is the "brain" of the process industry dictating the position of every
damper and valve in a system and determining which fans, pumps, and auxiliaries
run and at what speed or capacity. There is a tremendous need for all involved in the
process to increase their knowledge of control systems. Many product lines are
presented in generically described layers. Vendors' proprietary controllers and
interface devices are placed on these layers, allowing a user to compare similar
systems. The user can penetrate this architecture diagram.
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The software adopts manual mode when the plant is being regulated manually,
independently of the microcomputer. Match mode is adopted for two successive
execution cycles, when transferring from Auto to Manual or when BAD data is
detected; it allows the software to be ‘initialised’ to the prevailing plant state. Autonormal is the mode adopted when the plant is under unrestrained automatic control.
Auto-constraint mode may be adopted when automatic control has to be suspended
because a plant limit has been reached. The development of the so-called computer
on a chip in the form of a single integrated circuit (IC) microprocessor has given
considerable impetus to the use of DDC.
Above information provides theoretical background of DDCS used in a small process
but in power plant many computers and related systems are grouped to achieve
complete
plant
operation
by
use
of
hybrid
controls
deploying
pneumatic/electronic/hydraulic units. Since our simulators are uniquely using
Analogue Control System, only preliminary information has been given as above.
In another type of Direct Digital Control System (DDCS) as employed by CEGB in
their power plants, the computer performs all the functions of error detection and
controller action; digital logic circuits are integral part of the loop. DDC have the
capacity to control multivariable processes with interactions between elements. The
keyboard functions as the communication medium; VDU terminal uses plain
language instruction. The computers in DDCS are referred as control centre.
The mainframe computer is the central point for accepting information transmitted
from the process; in our case the power generating plant. It processes and displays
the result to an operator through Data Processing System (DPS). The computer has
the potential of loop control. The plant signals are sampled and calculations are
repeated at intervals and treated mathematically.
The performing close measured values for the control system is derived by
processing large number of detectors / sensors of the plant parameters which are
separate from the electronic control system In process where hybrid type control is
utilized both DDCS and electronic controls are used controlling carry out the
programming but since they do not have the information of control engineering in
detail it becomes difficult to communicate the control requirement with sufficient
precision in programming.
As such several commercial hardware and software packages are marketed which
use high level languages and the control engineers can write their control software
directly; software packages are written suiting to a particular process not
particularly for the power plant The minicomputer and microprocessors plus the
user-friendly operating systems have been developed which software engineers can
utilize relatively straight forward. The fig. Below shows the DDCS logic diagram.
The software used by an individual control centre besides in the memory the control
centre and all signal associated with that software are connected of to the input /
output hardware of the control centre.
Presently data processing (DP) is carried out in systems having mainframe
computers. The computers used are mostly the microcomputer-using CUTLASS
language is typically developed by CEGB, UK for DDC application.
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The CUTLASS, therefore, has the facility for carrying out arithmetic (co-efficient, time
constants etc.) and logical manipulation from data, including IF statement, SubRoutines and FOR loops for calling up items of data arranged in arrays. All data types
can have the status BAD either because they are indeterminate or because the user
has used an IF statement to reject unacceptable data item. The CUTLASS DDC
language recognizes four alternatives of control mode-manual, match, auto-normal
and auto-constraint.
The software adopts manual mode when the plant is being regulated manually,
independently of the microcomputer. Match mode is adopted for two successive
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execution cycles, when transferring from Auto to Manual or when BAD data is
detected; it allows the software to be ‘initialized’ to the prevailing plant state. Autonormal is the mode adopted when the plant is under unrestrained automatic control.
Auto-constraint mode may be adopted when automatic control has to be suspended
because a plant limit has been reached. The development of the so-called computer
on a chip in the form of a single integrated circuit (IC) microprocessor has given
considerable impetus to the use of DDC.
MULTIPLEXING
Multiplexing is the process of sharing a single transmission channel with more than
one input.
There are two main types: i) Time-division multiplexer (TDM) and
ii) Frequency-division multiplexer (FDM).
Time-division multiplex (TDM) System
The TDM multiplexer consists of a number of switches in the transmitter such that
each analogue input (V1, V2….Vn) is connected to the transmission channel in turn.
If the multiplexer is differential, two switches are provided for each input signal, as
actually shown in figure; if the multiplexer is single-ended, only one switch per input
is provided and common ground return completes the circuit for all input signals.
A control unit whose function is to provide the signals, which select the various
switches in the multiplexer and provide timing for the multiplexer and other
associated equipment, controls the switches. Often the switches are selected in
sequential order under the control of a ring counter circuit in the control unit.
A similar set of multiplexer switches is required at a receiver, and the synchronizing
signal in the transmitter data is used to synchronize the receiver and transmitter
switches so that data channels are isolated. A major difficulty with the TDM system is
that each input is measured at a different time, and these values are not valid for
comparison if there is appreciable change in magnitude of the inputs between
samples. Sample and hold units are used to overcome this problem.
Each signal input to the TDM system is connected via a sample and hold switch to a
capacitor. All inputs are sampled at the same time by closing these switches, and
when the switches open the input voltage are held on the capacitors. The multiplexer
can now select each hold input in turn.
The analogue switches maybe electromechanical or solid-state switches.
Electromechanical devices such as dry-reed and mercury-wetted contact relays have
sampling rates generally limited to about 250 samples per second. Solid-state
switches such as bipolar or field effect transistors (FET’s) can be used at sampling
rates in excess of 100 000 samples per second. The limiting factor to sampling rates
is the presence of switching transients.
The switch characteristics can contribute errors in the signals; thus leakage current,
bipolar transistor ‘on’ offset voltage and FET ‘on’ resistance can cause error. In
theory, the sampling rate required to accurately transmit a signal whose highest
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frequency component is fHz is 2f samples per second. In practice, due to limitations
in the reconstruction filters, it is necessary for the sample rate to be, say, 5f samples
per second. Thus, for example if f=10 Hz and 100 data channels are to be sampled,
the sampling rate must be at least 5000 samples per second. The time between
samples in 200 µs, & each input is read at intervals of 20 ms.
Generally TDM is used for may channels of data containing low frequency
information as obtained from temperature, pressure, loading strain and voltage
monitors.
Frequency-division multiplex (FDM) system
A schematic diagram of AM frequency-division multiplex system is given above.
Each input modulates an assigned subcarrier at comparatively low frequencies (540kHz). The AM-FDM system permits a number of data signals to be simultaneously
sent over a common transmission channel. Data usually are sent by a FDM are
vibration, acoustic noise, acceleration and dynamic strain. Such a system is well
adapted to analogue signals, as all input and output voltages are continuous,
whereas a TDM system has discontinuous outputs and is well adapted to digital
signals, especially when sample and hold circuits are used.
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PROGRAMMABLE LOGIC CONTROLLERS (PLC)
The Discrete state relates to each event in the sequence and specified conditions of
all operating units of the process. Set off conditions is described as a discrete state
of the whole system. An event in the system is defined by a particular state of the
system. An event lasts for as long as the input variable remains in the same state
and the output variable is left in the assigned state. In a discrete state process each
event is described by a unique specification of the hardware& it must be carefully
defined in terms of the nature of two states & its relation with the process. The
sequence of events can be described in narrative fashion (flow-chart, or Boolean
equations)
SWITCES are the primary input elements in the discrete-state control system;
Normally Open (NO) or Normally Close (NC) states are activated from many sources.
Different types of switches are symbolized as shown in logic diagrams. Ladder
diagrams are used for designing and describing the sequence of the process events.
Computer based method of control also termed as programmable logic control uses
relay control system and ladder diagrams. There are only discrete number of
possible states of input and output values e.g. for three input and three output
variables 26=64 possible states shall be there.
In a process control the discrete state situations are involved in operations some of
these are in series/parallel. Some of the events involve the discrete setting state in
the plant e.g. valve open or close, motor on or off etc. Other events involve regulation
of continuous variable over time or the duration of an event e.g. maintaining
temperature at a set point for a given time.
Continuous control for controlling flow of liquid through a valve into the tank and
some unspecified flow out of the tank can be referred as an example of continuous
variable regulation. The controller operates according to some mode of control in
order to maintain the level against variations induced from external, Thus if outflow
increases the control system will increase the opening of the input valve to
compensate by increasing input flow rate and the tank level is regulated. Here both
levels and the valve setting vary over a range. Even if the controller is operating in
on / off mode there is still variable regulation although the level will now oscillate as
the input valve is opened and close to compensate for output flow variation.
If the same example is studied in discrete state control logic then the level and valve
setting are discrete because they take two values. This means that the valves can
only be opened or closed and the level is either above or below the specified level.
There is no continuous measurement or output over a range because the variables,
level measurement, input valve setting and output valve setting are of two state
quantities and these form the discrete state control. In PLC system the sequence of
event must be described that will direct the system through the operations to
provide the desired end result. Narrative statement as to what must happen during
the process operation and what events must occur to achieve the object is required
for such requirement.
In systems, which run continuously a start up, or initialization phase and running
phase are typically there it is important to realize that with relay. Control each
RUNG of the ladder is evaluated simultaneously and continuously influences
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consequences are immediate which is not there in the computer based
programmable controllers; the relays are replaced by software, that are able to
accommodate and make changes in programmed sequence.
Time delay actions and counters, solid state devices for controlling high power
AC/DC in response to low level commands by use of SCRs and TRIACs are part of
the PLC/software. The processor (being the main hardware of the computer)
executes programme to perform the operations according to ladder diagram or
binary commands, performs arithmetic and logic operations on input variables,
functions under a permanent supervisory O.S for directing the overall operation
from data input / output and executes the user programmes.
In PLC the processor is a serial machine, which performs one operation at a time
and sequentially sample input to evaluate the required programme. Another main
hardware of the PLC is the input module, which examines the state of physical
status and other output devices to put their state into a suitable form acceptable to
the processor. It converts the input into binary 1 or 0 state. The other main
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hardware of the PLC is the output module. The output modules supply AC power to
external devices viz., motor, light, solenoid etc. at suitable driving power. It also
accepts binary 1 or 0 input from the processor and uses this to turn on or off the
output devices.
The PLC operation considers the simultaneous selections of ladder diagram and
relay sequencers in two modes, the input scan mode and the execution mode. The
scan time is around 20 mS and it depends upon the clock frequency of the
processor.
Programming unit, RAM, ROM, time relay & counters, programming diagram
interpretation, addressing and programming units are important hardware in the
PLC. As the technology of computers and PLC advances, many new features are
included such as computerized control of continuous processes and DDC as a builtin feature of the PLC. The ladder diagram includes rungs for such control system
with specified proportional, integral, derivative gains.
ANALOGUE CONTROL SYSTEM (ACS)
The Analogue Control System (ACS) has been supplied by M/s. H & B, West
Germany. Power supplies to transmitters are derived from various supply modules
placed in the cubicle. Eight no. of cubicles cater the total requirement of ACS. All
the control modules have the facility of galvanic isolation of the order of 250 volt
between input and output terminals. Transmitters output is to 4 to 20 mA or –2V to
10 V as per the requirement.
Electronic modules e.g. AV95 (Power Supply), AW02 (Galvanic Isolation), AV96
(Power Supply), XU01 (Computing), XP01 (Comparator), XK12 (Limit Value), XN01
(Max./Min Selector), RS01 (Controller), XM02 (Analogue Memory), HA01 (Interlock
Selection), HS01 (Relay Module), RL01 (Controller), RK01 (Controller), RK04 (Low
Range Controller), XM13 (Memory), LL02 (And Logic), LL03 (Logic Unit), VV01
(Signaling), LT01 (Relay Unit), HV02 (Amplifier) etc. have been used for various
application in the ACS system. The detailed testing, checks, operation, maintenance
can be referred from the C&I manuals on the above systems.
Establishing Automatic Operation requires certain steps like operate the plant such
that the measure variable is within the limits to that of the set values, ensure the
plant stability and safety prior to switching over to automatic operation, record the
plant parameters before automatic operation is switched, connect the recorder and
assign the correct colour for recording the measured, desired value etc. Operate the
manual switch and regulator such that the position follows demand at the
appropriate speed when a demand changes made. Record the final setting of the
controller in tabular form. For more details refer the designing of signal conditioning
as given in earlier chapters.
In 200MW Thermal Power Plant Korba, following controls have been incorporated.
•
•
Co-ordinated Master Control (Firing Rate; Fuel flow & Air flow).
Drum level & Feed regulating station differential pressure control
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•
•
•
•
•
•
•
•
•
•
Feed pump re-circulation flow control
Dearator Level & Pressure control.
Hot well level & Re-circulation control
S/H Steam & Reheat steam temperature control.
Heavy fuel oil control.
Furnace pressure control
Air flow /F.D.Fan control.
Primary Air Pressure Control.
Pulverisers A, B, C, D, E, F, Temperature / Air flow control.
Mill Feeder A, B, C, D, E, F, Speed control.
CO-ORDINATED MASTER CONTROL (CMC) OF THERMAL POWER PLANT
The power flow in the grid from various generating units needs to be controlled and
regulated as per the consumer demand and the capability of generating units. Net
power flow in the grid is certainly zero for maintaining the energy/heat throughput
to the generating units and energy/power sent out to the system. This results
complete energy balance.
As the boilers at the thermal generating units supply steam to the Turbo-generators,
the balance of heat input to boilers with that of the power generated by the TG set
need to be regulated & controlled by grid managing load dispatch center authority
(LDC). If power is generated as per the instructions of the LDC within the limits of
generating capability of each unit then the economic and efficient loading of the
boilers can be ensured.
Let us look at to the block diagram of CMC we find that in addition to LDC, the
automatic operation of power plant is linked with the load demand, frequency
correction, load rates, load limits, unit capabilities (Runback limits), turbine load
index, throttle pressure control, turbine generator control, boiler master (Fuel & Air
flow control). Feed water flow control is not directly linked with the CMC (as shown
in block diagram) in our 200MW sets as the three-element control of Boiler drum
control fully takes care of steam flow/load index. Similarly the Gas flow control is
delinked in our plant, as actual value signal is furnace suction to wind-box Diff
Press as explained in I.D.Fan control loop.
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BLOCK DIAGRAM OF CO-ORDINATE MASTER (INTEGRATED) CONTROL
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The Co-ordinated Master Control System (CMC) operates in four modes as given
below.
•
Co-Ordinated Mode
•
Turbine Follow Mode
•
Boiler Follow Mode
•
Runback Mode
CMC- VARIOUS MODES SELECTION BY SWITCHES
CO-ORDINATED MASTER CONTROL MODE
The objective of the CMC is to enhance the response of the boiler and turbine control
systems under all operating conditions while maintaining the outputs of the turbine,
boiler and all major plant auxiliaries within the safe operating limits. The CMC
receives unit megawatt demand signal from the load dispatch centre & throttle
pressure signal. The unit master station translates the load demand signals within
maximum / minimum limits as per unit requirement for the boiler and turbine
control system. The generated load is controlled through regulation of turbine
control valve by EHC signal as received from GNI output through selectors explained
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in CMC loop details (as described in pages that follow). The functioning of CMC in
general & in nutshell can become clear from the block diagram given below.
BLOCK DIAGRAM OF COORDINATOR MASTER CONTROL
TURBINE FOLLOW MODE
In the turbine follow mode, Boiler is manually controlled from the Boiler Master A/M
station and the turbine remains in initial pressure mode controlling the throttle
pressure. In turbine follow mode, the actual unit load as selected by energizing of
relay/switch T1, becomes unit load demand signal and the unit load demand signal
as generated from Unit Master/ALDC gets by passed. In the turbine follow mode, the
steam flow to turbine becomes the demand signal for boiler master. (since the steam
flow to turbine is measured by using a computed signal of turbine first stage
pressure and main steam temperature corrected signal as obtained from curve
plotter ; there is no direct measurement of steam flow).
The Boiler master station also receives the error signal from steam throttle pressure
comparator (actual – desired pressure; this difference is kept within -+ 3 kg/cm 2)
and the boiler master outputs the signal that is termed as firing rate demand signal
to the fuel and air control loops as required for efficient boiler operation and
pressurizing. The turbine load is controlled through regulation of turbine control
valve by EHC when it is operative/selected in Load control mode and also the CMC
has been selected to modify the turbine load. Block diagram shown below is clearly
giving turbine/ boiler operation considering the reference signals i.e boiler is
controlled by the throttle pressure and the turbine is controlled by its own output
i.e. megawatt signal.
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BOILER FOLLOW MODE
In Boiler follow mode the Throttle pressure is maintained constant, when put in auto
mode of operation and the turbine remains manually operable in limit pressure
mode through load controller. In Boiler Follow Mode the throttle pressure deviation
becomes the demand signal of the boiler-firing rate through boiler master selected in
auto mode. In Boiler follow Mode the actual unit load as selected by energizing the
relay T4 becomes the load demand by passing the Unit Master load demand signal
and the guided tracking integrator( GNI) sends this signal for further processing and
outputting for boiler control. The load signal derived from the export bus is used for
controlling the turbine control valve movement. (Refer the block diagram BD-2 for
details), thus the turbine is manually controlled through the load controller and in
Limit pressure mode. Block diagram shown below is clearly giving turbine/ boiler
operation considering the reference signals i.e boiler is controlled by the megawatt
signal and the turbine is controlled by the boiler’s output i.e. throttle pressure.
TURBINE FOLLOW MODE
BOILER FOLLOW MODE
RUNBACK MODE
The unit capability in terms of load generation is calculated by the run back
condition based on the availability/healthiness of the various plant auxiliaries. Each
one of the major auxiliary viz; ID Fan, FD Fan, BFP, CEP etc. contribute to the unit
loading and the contribution percentage of each auxiliary has been pre-decided thus
due to non-availability of the particular auxiliary, the load of the unit shall be
reduced equal to the contributing capacity of the particular auxiliary.
Let us consider the situations that, if two no. of FD fans are required for full 210MW
operation of the unit and if one of the FD fan is tripped or not available due to some
major problem, then the unit load shall be brought down to say approx.140MW
since available one FD fan can support unit operation for this load, similarly if one
mill is not available because only three mills out of 4 mills are required for full load
operation then the unit load shall be reduced to 155MW i.e. 210 – 55 = 155MW etc.
The unloading rate (run back limit) means the percentage at which the load set point
has to be reduced due to non-availability of a particular auxiliary. This unloading
rate is set by potentiometers mounted on the electronic module HA-02 and through
the minimum selector electronic module XK-01. The load demand of the unit is
another signal available to this minimum selector. Hence the output signal from the
XK-01 becomes the load set point guidance signal designated as down gradient (GD)
of the GNI.
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Further the unit load rate setter outputs, the signal as desired for setting the loading
rate and is compared with the lower load margin (decided by TSE) and the minimum
of above two signal is fed to the maximum selector XN-01 which also receives the
run back rate (unloading rate of the unit) and the maximum of the two is fed to GNI
in subtractive port and another parallel signal of the unit load rate is compared with
upper load margin (decided by TSE) and the minimum of these two is fed to additive
port of the GNI. (Refer the block diagram BD-1 for details). The run back
requirement of the unit is suitably processed at the GNI as described above.
In the Runback mode, the automatic feed back (GNI tracking) signal to GNI
bypasses the load limit selectors & the boiler master PID Controller because of
changed contact of relay T2 as the relay T2 has been energized due to selection in
runback mode and the boiler firing rate signal from GNI directly controls the boiler
operation as per the run back requirement explained above.
COORDINATED MASTER CONTROL (CMC)
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COORDINATED MASTER CONTROL (CMC): detailed loop description
A coordinated master panel insert is provided as a means for operator to supervise
the operation of coordinated master control system. This panel contains the
following items & functions:
• Unit master station
• Max/Min load setters
• Unit load rate/change setter
• Boiler master A/M station
• Throttle pressure set point
• C.V. Correction setter
Selection push buttons for boiler follow, turbine follow and coordinated mode
•
Indicators for Turbine on CMC, Limit Pressure, Initial Pressure, Run Back
•
Digital indicators for Demand Load, Generated Load. The following
interconnection signals between Boiler Control and turbine control are provided
to ensure proper functioning.
•
Analogue Signal: Actual load, Frequency deviation, Diff. Temperature, Limit
Pressure, Initial Pressure, Diff.Temperature (U min, Lt min), Actual Pressure.
Deviation Wp. Reference load. Binary Signal: Coordinated Mode, Turbine follow
mode, Boiler follow mode Turbine on CMC, EHC fault.
The Co-ordinated Master control (CMC) has been provided to simultaneously control
both turbine and boiler in cohesion with grid demand. The Unit Master station with
provisions of auto / manual selection, is the main module in the CMC loop. The Unit
Master Station receives the Mw load demand of the unit from the automatic load
dispatch centre (ALC) also termed as load dispatchers’ demand in the form of
Coordinated Master Control (CMC Load demand) and Unit master station outputs
the CMC load demand when this is selected in AUTO MODE operation.
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CMC- FORMATION OF LOAD INDEX AND SIGNAL DISTRIBUTION
Unit Master Station, when put in manual mode of operation, outputs the Mw load
demand signal, as generated by/ desired by the unit operator suiting to the plant
requirement.
Guided Target Integrator (GNI
The Guided Target Integrator (GNI) a computer module computes the target signal,
the frequency correction signal, the desired unit load rate, as compared with upper
and lower power margin generated from turbine stress evaluator (refer TSE chapter
for more details). The unit load demand signal as explained above becomes the
target value (Zo) of GNI; in addition a fixed 15% (K. δF) signal is added directly to
the output of the GNI. For K. δF within the band of +/-15% the correction is added
to the GNI output (Ao). This signal (Ao) is subjected to a MAX/MIN cut off by the
presets load limits and minimum cut off by the unit capability signal decided solely
on the healthiness/availability of the coal pulverisers, PA fans, ID fans, CEP, BFP
etc.
The unit capability signal forms the Run Back Limit signal for controlling the unit
load at times when full no. of auxiliaries are not available. Unit load rates,
upper/lower load margins, runback rate, all modify the GNI output as explained in
runback mode. Three selectors/ switching relays T1 , T2 and T4 are energized for
running the controls in boiler follow, turbine follow, or coordinated mode of
operation.
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CMC-GUIDED TARGET INTEGRATOR INPUTS & LOAD DEMAND
The frequency correction signal ensures frequency deviation K. δF > A pre-selected
value (15%) is added directly to the output of GNI (Ao) to obtain an immediate
change in the load set point to the boiler, and the signal gets slowed down as the
target value of the GNI is corrected and the output runs towards the target with a
selected gradient. The target value Zo of the GNI is limited by the Maximum of the
turbine load signal and the runback limit signal. The Electronic Module XU 02 with
catalogued program through X 203 works as a GNI, which is the heart of the load
set point guidance sub-loop
CMC-GUIDED TARGET INTEGRATOR INPUTS & OUTPUTS
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While the unit load demand thus generated as above is used for directly the boiler
master and the firing rate control, it is also further sent to turbine side for
controlling the turbine generator load through electro hydraulic governor.
The frequency influence is incorporated in CMC loop by suitable setting in the
function generator module. The frequency deviation signal modifies the Unit load
demand generated by the GNI module, through a proportional module P and is
further compared with run back limit signal in a minimum selector. Frequency
corrected Mw load demand signal finally forms the input of GNI module for further
processing. Final load demand signal is passed on for signalling to the system
(healthiness) and also indicated in the dual point indicators.
The unit load demand received either from LDC or set by the operator from the unit
master is also modified by the deviation in the system frequency. The correction
provides a change in unit load demand equivalent to the expected change in MW
output due to any deviation in system frequency. System frequency deviations
within a small range (±5%) are passed directly and larger deviations are routed
through the GNI, influencing the electrical load demand signal.
BOILER MASTER CONTROLLER
The boiler master controller is provided to maintain the throttle pressure constant
during various plant conditions. The output of this controller is used as a demand
signal to the boiler firing rate and air controls. The signals steam flow to turbine
(1st stage pr.) in case of Boiler Follow Mode and electrical load demand signal in
case of coordinated mode and turbine follow mode are used as a feed forward signal.
Under transient operating conditions, the throttle pressure is allowed to vary within
a small range to increase turbine output by utilizing storage capacity of the boiler
and to raise or to lower the energy level of boiler by over or under firing. However,
any larger variations in throttle pressure shall restrict the turbine output till boiler
has produced additional output to match the increased demand.
An additional digital indicator has been provided to monitor the Mw load demand in
megawatt scale / display. Two nos. of maximum limit and minimum limit value
setters, are available to limit the Mw load demand within limited range as desired
(Max – 250 MW & Min 100 MW) and output of the unit master is within these two
limits. The frequency deviation K. δF > +/-15% corrects the MW load demand signal
that forms input to the GNI.
FIRING RATE CONTROL
The firing rate demand signal generated from the coordinated control system is
divided into two controls loops/ sub-systems of Fuel flow control and Airflow control
to establish the fuel and air flow control systems.
The firing rate demand signal to the fuel master stream is compared with total air
flow in a minimum selector card 15 and the lower of the two signals form the set
point and also the feed forward signal to fuel control stream.
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Also the same firing rate demand signal to the Airflow control stream is compared
with total fuel flow signal in a minimum selector card 15 and the lower of the two
signals form the set point and also the feed forward signal to Airflow control
FUEL FLOW CONTROL (COAL FEEDER SPEED CONTROL)
The Coal feeder speed signal is 4 to 20 mA and is obtained from coal feeder speed
sensors. There are two redundant speed signals for each feeder, one already
isolated, and for the other an isolator card is provided. The isolated feeder signal for
each feeder is given to signal monitor and distributor card 30 from where 4 to 20 mA
decoupled signal is sent for DAS and for various indicators.
A provision is made for selecting either of the two speed signals to be used, one for
control and the other for switching the feeder speed control to manual upon failure
of the signals in use. This is achieved by using the 24-volt release signal from signal
monitor card 30 and analog switch 51.
The supply and return oil flows are measured by flow meters in the heavy fuel oil
supply and return lines. The oil flow to the burners is the difference between the
two measurement signals of HFO flow to burners and the return oil back to the HFO
tanks. The sum of the feeder speed and the oil flow becomes the Total fuel flow
signal (Actual value) in fuel master loop.
Calorific value Cv correction is included to modify the fuel flow signal as per the
quality of the coal received, this signal is not other than the multiplying constant
adjusted at the Cv potentiometer in the derived fuel flow for comparison with the
firing rate demand as desired value signal.
The fuel master controller has the provision for receiving the Auto release signal
from (master controller logic card 02) the feeder speed controller to ensure that
unless at least one of the slave feeder controllers is in Auto, the fuel master
controller shall not operate in Auto. A uniform dynamic response of the system
outputs for the entire load range is maintained and achieved by automatically
changing the appropriate control system gain, by Proportional-amplifiers card 16
and analog switch 51 as pulverizes are taken in and out of service.
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CMC : FIRING RATE CONTROL (COAL FEEDER-TYPICAL SCHEME)
A separate controller controls the heavy fuel oil header pressure by comparing the
measured value with the set point adjusted by the operator. The output signal
reaches to the heavy fuel oil flow control valve. A/M station MA1012 (Oil flow) ,
MA1040 (Fuel master), MA 1016, MA 1020, MA 1024, MA1028, MA1042,
MA1036.(Coal feeder speeds) have been used The signal monitors(for feeder on Auto,
set minimum, speed>30%) and system output contacts are provided for each feeder.
Release for totalizing circuit, Release feeder speed to Auto, Set feeder speed to
minimum; signals are provided for each feeder.
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CMC- FIRING RATE : FUEL FLOW /COAL FEEDER SPEED CONTROL
AIR FLOW CONTROL (F.D.FAN CONTROL)
The secondary airflow is measured with two redundant flow transmitter s for left and
right side. Each signal is pressure and temperature compensated in computer card
22.The excess air controller 03 via the proportional adjuster 07 influences the set
point signal. The set point for the excess air Controller is developed by function
generator card 12, according to the firing rate demand Signal as a load
characteristic and compared to the measured oxygen value in a PI-step-Controller
03.
To ensure a minimum airflow for furnace purge and low load operation a maxselector 15 is provided. Cross limiting by comparison of the master Signal with total
fuel and air flow to ensure excess air condition under all circumstances is included
in the air flow control. On load pick up, the airflow will be increased first and on
load drop, the fuel flow must be decreased first.
Provision is also made for selecting either of the two oxygen analysers to be used for
control & for switching the oxygen control to manual upon failure of the oxygen
content measured signal in use with the help of Signal monitoring card 30 & Analogswitch 51and for selecting either of the redundant secondary airflow signals or for
rejecting FD blade pitch control to manual upon failure of the Signals in use. A
uniform dynamic response of the output for the entire load range is maintained by
automatically changing the appropriate control system gains in P-amplifier card 16
as and when F.D. fans are taken in and out of service. A/M station (MA1001) and
(MA1002) for FD fan A /B with provision for manual biasing and (MA10013) for
Excess air control has been provided.
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CMC- FIRING RATE: SECONDARY AIR FLOW CONTROL
The interlocks STOP +y/-y in case of furnace pressure too high or to low from DRG6
and MAN-transfer in case of failure of the corresponding ID fan is provided. Signal
monitor (SMD 019) for fuel air deviation high & very high and SMD 020 for
airflowless than 30% & less than 40% and the system input contacts of ‘CLOSE’ &
‘OPEN’ and system output contacts of Lock-in position Signal to final pneumatic
actuator in case of Controller failure for both F.D. fans A & B are provided.
PULVERISERS TEMPERATURE
& AIR FLOW CONTROL
The pulveriser temperature control loop has been used to operate the hot and cold
air dampers in tandem and in a pre-determined relationship, such that adjustments
in the ratio of hot and cold air result in a minimum disturbance to primary airflow.
It means the hot air damper opens and the cold air damper closes to maintain the
required temperature of the PA to mills. Flow sensing tubes (pilot tube) in each
primary air duct sense and measure the primary airflow to each Pulveriser; the
temperature is compensated by a computer card 22, which also performs the square
root extractor function. The measured primary airflow Signal is applied to the PIController 04 and is compared with the desired set point. The Controller 04
generates a control Signal to position both the hot and cold air damper together for
the regulation of primary airflow. The measured temperature Signal is applied to the
PID Controller 04, which generates a control Signal to position both the hot and cold
air damper to regulate the temperature. Signal monitors and alarm contacts have
been provided for temperature low and high.
An interlock is provided to automatically open the cold air damper and close hot air
damper on Pulverisers trip conditions. Lock-in position is used for cold and hot air
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damper Controller failures. Auto Release for cold and hot air dampers, Open. for
cold air damper, Close for hot air damper, Set To Min. for cold air damper etc. are
available for interlock and other requirements.
PULVERISER ( MILL ) TEMPERATURE CONTROL
PRIMARY AIR PRESSURE CONTROLLER
Primary air to all Pulverisers is maintained by controlling the primary air pressure
(and so the air quantity) at the common discharge duct of the two PA fans A and B.
The primary air pressure is measured and the output signal is given to Signal
monitoring and distributing card 30. The output Signal from this Signal monitoring
card is compared in a ‘P’ amplifier card 16 with the desired pressure set point from
the A/M station MA-1010 and the resulting error is amplified and processed in the
Controllers for finally driving the E/P converters of the power cylinders for
regulating the positions of the PA fan A/B inlet dampers/vanes. A uniform dynamic
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response is maintained by automatically changing the appropriate control system
gain depending on the number of fans in AUTO.
An interlock is provided on both Controllers to automatically close the inlet vanes on
start command of PA fans. Lock-in position Signal for pneumatic actuator of PA fan
A and B and the CLOSE Signal for PA fan A and B are made available in this control
loop for required applications.
PRIMARY AIR PRESSURE CONTROL
BOILER FURNACE PRESSURE CONTROL
Boiler furnace pressure is controlled by positioning the inlet vanes of the two
Induced Draft fans. Two redundant furnace pressure transmitters have been used.
The two transmitters are connected through separate taps on the furnace. A
comparison network is provided to supervise the outputs of the transmitters with
Signal monitors card 17.This comparison network automatically selects the draft
control system to manual and also actuates an alarm contact upon detection of high
deviation between the outputs of the two transmitters. Furnace pressure is
measured and compared to the desired set point in P-amplifiers card 16 and error is
amplified and processed to drive the E/P converters of the inlet vane position power
cylinders. A uniform dynamic response of the system for the entire load range of the
unit is maintained by automatically changing the appropriate control system gain in
proportional amplifier card 16, depending upon the number of fans in Auto. The
furnace pressure controller also receives the signal of total airflow as a feed forward
Signal for an immediate interaction in case of any change in the airflow.
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One cabinet mounted switch is provided for selection of one of the transmitters to be
used for control. A second cabinet mounted switch is provided to detect the monitor
when it is necessary to operate with one transmitter out of service. A/M stations
MA-1004 & 1006 have been used for ID Fan A/B vane drives. Pressure switches for
high and low furnace pressure and one monitor card 17 is provided for interlock &
protection Signal for alarm on High differential between pressure transmitters
&Defeat is provided in the control loop.
An interlock is provided to block the automatic control from increasing the induced
draft fan inlet vanes and decreasing the forced draft fan bled pitches when furnace
pressure is excessively low. Conversely, when furnace pressure is excessively high,
induced draft fan decreases and forced draft fan increase is blocked.
Interlocks are provided to automatically close or open the ID fan inlet vanes.
BOILER FURNACE PRESSURE ( SUCTION ) CONTROL
BOILER DRUM LEVEL (FEED WATER FLOW & STEAM FLOW) CONTROL
The feed water flow to the boiler is controlled by the low range and high ranges FWControl valves that maintain the drum level as desired. The boiler drum level is
measured on the left and right side. The boiler drum level signal is pressure
compensated in computer card 22. The left or right drum level signal is compared
with the set point in the PI-Controller 02 for the FW low range control valve and in
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the PI-PI Controller 23 for the FW high range control valve. Provisions are there for
selection of the level transmitters (mounted in right and left sides) and also
automatic transfer to Auto for the low range to high range Controller in case of Feed
Water -flow less than 20% or larger 30% conditions. Following output contacts are
provided for alarm/signaling:Drum pressure left. L/LL
Drum pressure right.L/LL
High-Diff.of right/left Alarm
Drum pressure left L/H
SM-D021; Drum level left.
H/HH SM-D023
SM-D024 Drum level left.
SM-D024
SM-D025 (switch defeat)
SM-D036 Drum pressure right L/H SM-D037
The feed water flow is measured in both pipes doubly by orifice/flow elements,
which create differential pressure (Delta-Pi) proportional to the feed water flow. The
‘DP’ signals are temperature compensated and square root extracted in computer
card 22. Total FW-flow thus obtained, becomes the feed back signal to the FW high
range valve controller for further processing.
BOILER DRUM LEVEL CONTROL
The Main steam flow is measured on the left and right sides of main steam pipes. In
each case the delta-P signals are square root extracted, pressure-and temperature
compensated in the computer card 22. The two signals are added in the operational
amplifier card 16 and processed for various uses. The auxiliary steam flow is
measured and added to the signal ‘steam flow to turbine. The steam pressure
signals PT-D003 and D004 are supervised for Low and High values with signal
monitors SM D060 and D061 the difference between both signals is monitored with
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SM D062 and alarm contact is provided. The feed flow and the steam flow should
match in normal conditions
FEED WATER CONTROL VALVE DIFFERENTIAL PRESSURE CONTROL
The Feed Water control valve differential pressure control is implemented to ensure
an optimal reaction of F/W flow control by holding the differential pressure (DeltaP) above F.S valve, as constant. The Delta- P is measured and compared in an OpAmp with that of the set point value. The resulting signal reaches to a following
amplifier with provisions for gain change if two boiler feed pumps are in operation.
The control deviation signal passes to the Delta-P master Controller. Main steam
flow signal is used as a feed forward signal. The power amplifier drives the electromechanical actuators that are mounted on the hydraulic coupling of BFPs.
BOILER FEED WATER DIFFERENTIAL PRESSURE CONTROL
The Controller output is connected to the Scoop Controllers of Boiler feed pump A,
B, C .The UCB operator can adjust the scoop position set point via proportional
adjuster for balancing the scoop position controllers of individual pumps that are in
operation. The output signal of these position Controllers are going via function
generator card 12 to the power amplifier KE 01 (40) for each scoop tube of BFP
hydraulic coupling. Scoop tube can be set to minimum position during starting.
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Auto/manual stations of DP master (MA 1009), BFP scoop Position Controller A,B,C
(MA 1046, 1047, 1048) have been provided in this control loop.
SUPERHEATED STEAM TEMPERATURE CONTROL
The Superheated (S/H) steam temperature is maintained constant by controlling the
outlet steam temperature during various plant conditions with the help of spray
control valves. The S/H outlet steam temperature is measured on the left and right
side. This signal is compared with the set point in a Proportional plus Integral
controller. Additional to the outlet steam temperature signal the S/H or De -S/H
outlet steam temp. is measured on both sides and used in the control loop. Main
steam flow signal is connected as feed forward signal via a function generator card.
The individual temperature signals are monitored. Modules are there for obtaining
high-diff. Alarm between left and right side. The hand/auto stations of S/H outlet
steam temp. Controller left/right MA 1042/1043 with provision for set point
adjustment are provided in the control system. The alarm contacts are provided i.e.
S/H outlet steam temp.left High/Low (SM-D026), Low Low(SM-D066) , S/H outlet
steam temp Right H/L (SM-D067), Right LL(SM-D027) De S/H outlet steam
temp.left H/L (SM-D028) right H/L (SM-D029) High diff. Alarm of S/H outlet temp.(.
SM-D031)
SUPERHEATED TEMPERATURE CONTROL
REHEAT STEAM TEMPERATURE. CONTROL
The R/H steam temperature control maintains the R/H outlet steam temperature
constant during various plant conditions. The R/H temperature is measured on the
left and right side and supervised for high/low values. Average temp. of both left and
right side forms the measured variable and it is compared with the set point in the
burner master Controller. The controller output signal drives the I/P converter of
the four burner tilt power cylinders. The control deviation of the burner master
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controller is also connected to the R/H spray master controller. Main steam flow
signal is used as a feed forward signal. In case of high negative control deviation,
that means the R/H outlet temp. is too high, the Reheat spray control valves (left
and right) are opening to avoid further increase.
Under normal operation the R/H spray control valves are closed The interaction is
suppressed by a fixed input signal ( i.e. 5%). Provisions are made for balancing the
left/right R/H spray valve. The position of the R/H spray control valves are
monitored and contacts below minimum are provided for opening or closing of
additional. Block valves. Interlocks for the position controller are included. The
position of the burner tilts is measured and a network is included to produce the
signals i.e. Position out of synchronization (SM-D071), Nozzle tilt horizontal (SMd071) , R/H outlet temperature left& right H/L (SM-D032&33) , High diff. Temp
Alarm between left/right SM-D034, R/H spray valve position left/right below MIN.
SM-D042/044 alarms.
DEAERATOR LEVEL AND CYCLE MAKE-UP CONTROL
The deaerator level is measured by level transmitter and compared with the set point
value in Controller card. The Controller output signal is used to position the D.M.
make up control value. Deaerator level is maintained by modulating the D.M. make
up control valve (condensate surge-tank outlet line) and thus regulates the make up
water to condenser.
CYCLE MAKE-UP AND DEAERATOR LEVEL CONTROL
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DEAERATOR PRESSURE CONTROL
CONDENSER HOT WELL LEVEL AND MINIMUM RE-CIRCULATION
The hot well level is measured by level transmitter and compared with the set point
value in Controller card. The Controller output signal is applied to the pneumatic
converters of the control valves MC-27 and MC-33. Condenser hot well level is
maintained by regulating the condensate re-circulation flow control valve MC-33 to
full open condition by diverting the condensate water to the condenser up to the
condition that a minimum flow of 210 T/H flows through condensate extraction
pumps. As soon as the CEP flow increases beyond this, the condensate flow control
valve MC-27 starts opening and simultaneously the MC-33 starts closing until MC –
27 opens to 40%, by this time MC-33 is fully closed and the hotwell level is further
maintained through modulation of control valve MC-27.
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CONDENSER HOTWELL LEVEL CONTROL
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BOILER WATER CHEMISTRY
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BOILER WATER CHEMISTRY
It is important to maintain proper quality of feed water and boiler water for trouble
free operation of boilers. The quality requirements become more stringent for highpressure boilers, as they are designed to closer tolerances. Since the boiler, water is
primary output; it is not possible to achieve desirable steam purity for trouble free
operation of boiler and turbine, without proper control of boiler water chemistry. In
spite of good water management the internal surfaces in a boiler become dirty over a
period of operation. It is essential to periodically clean the boiler during overhauls
etc.
The principal objective of boiler water treatment is to prevent.
a. Scaling
b. Corrosion
c.
Steam contamination
SCALING
Water contains many impurities like dissolved salts and/or suspended matter. The
suspended impurities such as biological growth, mud and bacterial growth can be
removed easily as compared to dissolved solids. These dissolved impurities are
insoluble at elevated temperatures. When temperature rises, solubility of these
dissolved salts decreases and some precipitation occurs locally. These precipitations
are sticky in nature and form coating on the metallic surface. This is called scaling. It
can also be described as a continuous, adherent layer of foreign material formed on
the waterside of a surface through which heat is exchanged. Scales are objectionable
because of their heat insulating effect.
The co-efficient of conductivity of several metals and some compounds of which
scales are made of, are tabulated below:
CONDUCTIVITY OF METALS
Metal
3
x 10 µ mho/cm
Copper
Carbon steel
Bessemer steel
Scales
Aluminium oxide fused
Calcium Carbonate
Ferric oxide
Calcium sulphate
Magnetite
Magnesium Phosphate
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920
110
98
(A12 O3)
(Ca CO3)
(Fe2 O3)
(CaSO4)
(Fe3 O4)
Mg3 (PO4)2
8.0
2.2
1.4
3.1
6.9
5.1
341
From the above table it is very clear that how insulating these scales are. It is
sometimes said that a thin layer of Ca CO3 (calcium carbonate) should be maintained
to protect the surface from corrosion. But it is impossible to lay down uniform
thickness of scale, because the scale thickness depends upon the amount of heat
being transferred, which is not same in all sections of boiler.
Any scale in the boiler, however, is absolutely undesirable. Scales and deposits are
formed because the compounds, of which they are composed, are insoluble under
high temperatures prevailing in the boiler. Certain anhydrous calcium salts
especially sulphate, decrease in solubility as temperature and pressure increase.
Similarly, solubility of Ca CO4 decreases rapidly with increasing temperature
producing extremely hard, adherent coating on boiler tubes, especially in locations
where heat flux is high. Accumulation in boiler drums is most often in the form of
mud or sludge. When oil is present as a contamination in boiler water, loose scales
may form particularly in water tubes. Oil serves as a nucleus and binder of scaling at
hot spots. The '
oil balls'found in steam drum and water wall headers are typical
formation in turbine flow sections.
Prevention of Scaling
The most effective method for prevention of scaling is to eliminate scale-forming
elements from the feed water, or to transform them by some means into an
innocuous form. That is the reason why dematerialised water is used in the system.
Demineralises can produce water quality with nearly zero hardness. All ionised salts
are removed in these processes, which greatly minimise the potential for boiler
deposits, corrosion and turbine fouling. On-line removal of scales forming salts is
done by phosphate treatment. These salts are inevitably present is the boiler water in
the form of residual hardness even after demineralisation.
The tri-sodium phosphate, which is used for phosphate treatment, tends to increase
the pH value while di-sodium phosphate formed as a by-product; is a neutral salt.
Tri-sodium phosphate reacts with salts of Magnesium and Calcium to form sludge
(Calcium & Magnesium Phosphate). The reaction is as follows:
2NA3 PO4 + CaSO4
Na3 PO4 + H2O
Na2 HPO4 + H2O
=
=
=
Ca3 (PO4)2
NaOH
NaOH
+ 3Na2 SO4
+ Na2 HPO4
+ Na3H2PO4
The advantages of phosphate treatment are:
•
Adequate alkalinity can be maintained in the boiler water.
•
Na2 HOP4 is additionally available to form phosphate sludge.
•
Self-containing hydrolysis, hence proper control over pH (above pH =10.2
reactions reverts to left) can be observed.
•
No Problem of caustic corrosion.
The only disadvantage of phosphate treatment is Phosphate hideout.
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Phosphate Hideout
Sometimes, objections are raised against co-ordinate phosphate treatment because of
the phenomenon of the phosphate hideout. At higher loads phosphate comes out of
the solution because of its low solubility at raised temperature. But when the load is
reduced it goes back into the solution adding to the total phosphate content of water.
This may lead to excessive total dissolved solids content of boiler water in drum.
However, there is no danger in phosphate hideout. It is nothing more than a
nuisance to the operators for control of boiler water chemistry regarding (PO4) - level
control. Any phosphate salts, hidden out, are available to take care of any hardness
in water. It is never advisable to resort to any boiler water treatment with free
caustic, even in small amounts. It does much more damage to the boiler tubes than
any '
phosphate hide-out'
.
The recommended boiler water limits for phosphate & drum water pH are as follows:
Phosphate should be
<
3 - 10 ppm
pH
=
9.4 - 9.7
CORROSION
Scattered pitting in the presence of oxygen is sometimes observed in the water line in
the steam drum and in the down-corner tubes of boilers. Economiser, on account of
their high temperature, is also susceptible to corrosion by oxygen. The mechanism of
pitting in a metallic surface produced by a bubble of air is shown in the figure 10.3.
Two stages in the formation of a pit are represented by the following chemical half
reactions
Cathodic
2e- = 1/2 O2 = H2O = 2 OH
Anodic
20H = Fw = Fe (OH)2 = 2e-
It ultimately forms a cell and is continuous in nature unless surface of oxygen is
removed. Corrosion of iron and copper in condensate systems leads to formation of
porous deposits under which salts in boiler water concentrate and damage the
underlying surface of boiler steel. Then, even in absence of deposits, caustic gauging
can occur owing to the concentration of sodium hydroxide particularly in places
where the rate of heat transfer is unusually high. Other possibilities are the
corrosion of stressed metal. The severity of these effects can be controlled to some
extent by reducing the concentration of oxygen and free alkali, and by eliminating
products introduced from pre-boiler system.
Corrosion is the oxidation of metal by some oxidising agent in the environment. The
area over which the metal is oxidized is called the anode and at which the oxidising
agent is reduced is called cathode. These areas are necessarily separated but usually
are not far apart. As corrosion products, electrons flow between these areas through
the metal while ions migrate through the solution. This system constitutes an
electro-mechanical cell. In boiler the oxidation of iron is accompanied by the
reduction of hydrogen ions supplied by the hot water.
3Fe + 4H2O = Fe3O4 + 4H2)
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(A)
343
In case of acidic water
2 H+ + Fe
= Fe ++ + H2
(B)
The reaction (A) is self-limiting on account of the barrier of Fe3 O4 that forms on the
surface of the metal. The reaction (B) on the contrary, continues until the supply of
hydrogen ions is depleted in boilers. Both reactions are posed by an irreversible
potential called the hydrogen over-voltage, which is affected, by the condition of the
surface of the metal.
PREVENTION OF CORROSION
Removal of Oxygen
Oxygen is introduced into boilers dissolved in feed water. When this water enters the
steam drum, most of the oxygen flashes into steam space, producing characteristics
pitting at the water wall lines and in the vicinity of the discharge of the feed line.
In addition in high-pressure boiler, several local corrosion, pinhole failures and
pitting in the rear furnace wall tubes are attributable to attack by the dissolved
oxygen.
The concentration of dissolved oxygen in feed water should be less than 0.03 ppm
and preferably less than 0.005 ppm in water for high-pressure boilers. Cold water
saturated with air contains about 10 ppm of oxygen. This can be reduced to 0.3 to
0.7 ppm in an open heater and about 0.01 in a spray type deaerator normally used
inn power plants. The greater part of corrosive gases, and carbon dioxide and oxygen
that are dissolved in water can be removed by de-aeration. Open heaters are suitable
for low pressure but spray type deaerating heaters are commonly used. In these
units, steam heats the feed water in primary heater and also scrubs the heated
water. Hot spray flows down through a baffle arrangement against a rising flow of
steam that sweeps the liberated gases out through a vent at the top of the vessels,
while deaerated water collects in a storage section at the bottom. The vent is
equipped with a condenser through which cold feed water flows to prevent excessive
wastage of steam. The oxygen concentration of less than 0.007 ppm can be obtained
through this method. Because of volatilisation of CO2 and thermal decomposition of
bicarbonate, the pH of deaerated water is normally maintained 8.5 - 9.5.
2 HCO3
=
CO3 + CO2 + H2O
H+ + HCO3
=
CO2 + H2O
So far we have discussed the mechanical method of deaeration. Let us see how
effectively the corrosive oxygen is removed with the help of chemicals. There are two
chemicals, which are primarily used to remove oxygen.
a.
Sodium Sulphate
b.
Hydrazine
Sodium Sulphate is commonly used in boilers operating at less than 60 Kg/cm2
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while hydrazine is the reducing agent at higher pressures. The reaction of hydrazine
is as follows:
N2H4 + O2
=
N2 + 2H2O
3N2H4
=
4NH3 + N2 (at 200
0
C)
H2O
2NH3 + CO2
(NH4)2 (At 270
0
C)
Nitrogen being inert gas gets liberated and is removed as non-condensable gas.
Advantages of hydrazine treatment are
o
Low equivalent weight.
o
Does not increase dissolved solid content of drum water
Disadvantages of hydrazine treatment are
o
Vapour toxic nature.
o
Excess of hydrazine at high temp disintegrates into ammonia.
o
Concentrated solution of hydrazine is flammable.
At the economiser inlet the concentration of hydrazine is to be limited to 0.050 ppm.
The presence of porous deposits on the waterside of boiler tubes lead to serious
corrosion, especially when there is free alkali in the water. Various conditioners are
added to disperse insoluble materials and prevent the accumulation of sludge on
surface, where the heat is transferred. Recently poly-crystallites and other synthetic
polymers have come into use.
Small amount of particulate matter comprising finely divided oxides of copper often
contaminate condensate. These oxides, besides causing foaming, deposit on boiler
tubes at a rate proportional to the heat flux. The rate of deposition increases rapidly
above 55 Kg/cm2. The presence of these deposits causes over heating of the tubes
and sometimes ductile gauging. Direct reaction of steel with particles of ferric oxide is
also possible.
4Fe2O3 + Fe+ = 3Fe3O4
Monitoring Feed/Condensate Water pH
The metallic surface on the waterside of a boiler tube is naturally protected by a thin
film of magnetite formed by the action of hot water on steel.
3Fe + 4H2O =
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345
Ideally there is no further oxidation of metal after the protective layer is formed. The
minimum rate of corrosion is realised at pH value 11 to 12. At lower pH values
hydrogen ions are discharged whereas at values greater than 12, the magnetite layer
thickens, peptises to some extent and is made porous by diffusion of ions from
underlying metal. Above pH value of 13, the magnetite layer is completely destroyed.
2
With the pressure above 40 Kg/cm , hydrogen may diffuse into metal, blistering and
weakening it severely. Hydrogen atoms react with the carbon in steel to form
methane. The pressure generated in this may cause fissures along the grain
boundaries, so ideally the pH of 8.5 to 9.5 should be maintained. The re-circulation
of a small amount of alkaline boiler water through BFP has been recommended but
this can lead to plugging of feed lines in economiser, feed water heaters by insoluble
phosphate.
At lower pH values than 8.5 in the drum, the removable sludge formed by phosphate
treatment of scale forming salts becomes very sticky itself. Also at lower pH values
the silica carry over (Distribution ratio x1/pH) increases very rapidly, not to say a
rapidly increasing rate of corrosion due to pH values lower than those recommended.
STEAM CONTAMINATION
Carry over of salts in steam occur either due to mechanical or vapour carry over.
Efficient drum internals can only reduce mechanical carry over. Silica is always
carried over in vaporous form. The vaporous carry over of remaining salts mainly
2
sodium salts is significant only at pressure above 180 Kg/cm . The carry over may
occur in four types. They are:
a.
Leakage carryover.
b.
Spray or mists carry over.
c.
Priming.
d.
Foaming.
Leakage Carryover
Leakage of water droplets through seams or gasket joints of steam purifying
equipment cause this type of carry over. It is usually highly localised and may not be
detected in steam purity test unless the sampling points are very near to the leakage
point. This will be revealed in steam purity measurement by the fact; that with
increase in load condition the purity of steam will deteriorate. The change in water
level may not alter the degree of contamination.
As this type of carry over is most frequently localised, it is responsible for localised
super heater failure. If suspected, then new gasket or seal welding may be required
to eliminate this problem.
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Spray or Mist Carry-over
In this case, atomised droplets of water will be carried with the steam. This is
common in all boilers to some extent. Spray or mist carry over can be avoided by
installing steam purification equipment. If the steam purification equipment is under
designed, this will be present even after the installation of these equipments.
Priming
It is relatively unimportant in present power plant boilers and is rarely encountered.
Carry over of this type is characterised by a sudden carry over of gross quantities of
boiler water, which would show up a drastic deterioration of steam purity. It can be
caused by the variation in pressure such as large pressure drop. In this case the
water in the boiler would swell due to expansion of steam and formation of additional
steam. This action is similar to the” bumping" experienced when water is boiled in an
open beaker. It is more violent spasmodic action, resulting in the throwing of slugs of
boiler water with the steam flow.
Foaming
The important factors that affect the carry over in steam are:
1.
Drum and its internals.
2.
Water level in the drum.
3.
Boiler water concentration.
4.
Foaming and vaporous and carry over.
To achieve the quantity or purity of steam required for power and industrial units,
mechanical contrivances are provided inside the boiler drum. These are known as
drum internals. They distribute and mix feed and chemicals added to boiler water
while removing entrained moisture from steam as it leaves the drum.
The three basic effects by the internal arrangements are:
1. Centrifugal action to produce separation force, which is many times greater than
gravity.
2. To direct the steam water mixture so that the upward velocity vector is zero.
3. Provision of drainable wetted surface in which fine spray can coalesce.
Foaming is the condition resulting from the formation of bubbles on the surface of
boiler water. The foam produced may entirely fill the steam space of the boiler or may
be relatively minor depth. In either case this foaming condition causes appreciable
entrainments of boiler water with steam. Generally presence of organic matter
and/or oil will promote foaming.
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Silica Carry Over
Certain dissolved solids in the boiler water are carried away with the steam as
vapour and the internals have no control over such vaporous carry over. One of the
detrimental constituents is silica. In order to limit silica carry over, the concentration
of silica in the drum water must be limited to a specified value for a given operation
pressure range. In order to control the silica in boiler water, the most effective
method used is blow down.
Blowdown
As steam leaves the boiler, solids introduced in feed water are concentrated in the
water left behind in the drum. If this concentration were allowed to continue, the less
soluble components in the water eventually crystallize on the internal surface and in
addition, the steam would contaminate. In ideal operation, the concentration of
solids is allowed to reach the limit after which the concentrated boiler water is bled
off at such a rate that the amount of solids entering in feed water is exactly balanced
by that method in bleed stream. This process is called continuous blow down.
Pressure
Dissolved
Solids
Suspended
Matter
Alkalinity
Silica
1000 psi
500
10
50
10
1500 psi
150
3
0
3
2000 psi
50
1
0
1
Suspended solids in the presence of iron tend to collect as sludge in the lower parts
of boiler i.e. down comers or ring header. Opening the intermittent blow down can
blow out this concentrated sludge. The turbulence caused by opening the valve
disperses the sludge. So there is no point in leaving the valve open longer than 15
sec. It is a usual boiler practice that water wall headers should never be blow down
because the circulation or water through them is usually critical when the boiler is
on load.
The outlet for CBD should be below the level where the riser tubes enter the steam
drum because this is where the dissolved solids in re-circulating water are most
concentrated.
Chemicals for internal treatment (phosphate, sulphate etc.) should be introduced
above the down comer tubes to prevent sludging on the hot risers, and also to
promote mixing and reaction with saline in the entering feed water. If the chemicals
are injected near the blow down outlet, short-circuiting will occur.
Continuous blow down is the most effective way for controlling the amount of solids
in boiler water after the rate of withdrawal has been adjusted properly. If the rate of
blow down is too high, heat and water are wasted, if too low the permissible limits
will be exceeded.
KORBA SIMULATOR
348
% Blow down
(In terms feed water)
=
% Blow down
(In terms of steam)
=
Total Solid in feed
Total solid in water
Total solids in feed
x 100
Total solid-Total solid in feed
RECOMMENDED BOILER / FEED WATER LIMITS
The following are the generally recommended feed water and boiler water limits for
high pressure drum type boilers:
Total solids
50 ppb
(max.)
Total iron
10 ppb
(max.)
Total copper
10 pbb
(max.)
Total silica
20 ppb
(max.)
Total oxygen
5 ppb
(max.)
Feed water pH
9.2 to 9.4
Phosphate
5-10 ppm
Boiler water pH
9.4 to 9.7
The recommended boiler and feed water limits for 210MW Korba boiler is as follows:
Recommended Feed Water Limits
2
Operating drum pressure in Kg/cm (g)
60-100
100% & above
Hardness
Nil
Nil
pH at 25 oC (copper alloy pre-boiler system)
8.8-9.2
8.8-9.2
pH at 25 oC (copper free pre-boiler system)
9.0-9.4
9.0-9.4
Oxygen (max.) ppm
0.007
0.007
Total iron (max.) ppm
0.01
0.01
Total copper (max.) ppm
0.01
0.005
Total CO
Nil
Nil
Total silica (max.) ppm
0.02
0.02
Sp. conductivity after cation exchanger (mho/cm)
0.5
0.3
Residual Hydrazine ppm (before economiser)
0.01-0.02
0.01-0.02
Oil
Not allowed.
KORBA SIMULATOR
349
Recommended Boiler Water Limits
2
60-125
125-165
165-180
Total dissolved solids (max.) in ppm
100
50
25
Sp. Conductivity at 25oC (mho.cm) max
200
100
50
Phosphate residual ppm
5-20
5-10
3-7
pH at 25 oC
9.1-10
9.1-9.8
9.1-9.8
Silica (max.) ppm
To be controlled on the basis of
Silica distribution / Drumpressure curves to maintain less
than 0.02 ppm in steam leaving
the drum
Drum operating pressure in Kg/cm
KORBA SIMULATOR
350