Control & measurement of process parameters:
The instrumentation system of the thermal plant have the two parts – measurement &
control
Measurment parts deals with the mearsurment of the different parameters of the process
deploying different sensors which is mainly known as Primary instruments and brings it in
the notice of the operating personnel displaying it on the indicator or storing it on the data
acquisition system(DAS) .Indicator ,DAS,& recorders are known as Secondary instruments.
Control parts takes care for automatic/manual control of different parameters of the
process. Setter, error generator, controller (PI & PID controllers) , amplifiers, positioner,
actuator, electrical to pneumatic converter etc. is the main component of the control part.
Control of process parameters:
The controls in the thermal power station is can be pneumatic or electronic. In the modern
power stations electronic control system are widely used.
The modern control system have the following components
Transducer: it converts the process variable, which is to be controlled is converted into
equivalent electrical signal.
Setter: it sets the desire value, which is to be maintained, of the process variable.
Error detector: the actual value, which is to be received from the transducer, of the process
variable is compared with the pre-set value, which is received from the setter, and
deviation, if any sent to the controller as output.
Controller: the error signal is amplified and on the basis of the amplitude and polarity of the
error signal, corrective signal is sent to the auto/manual station.
Auto/manual station: practically, it is a change over switch or selector switch. If selected for
auto then the corrective signal of the controller is directly goes to the final control element
and if selected for manual then the operating personnel has to adjust the corrective signal
for the final control element manually as and when deviation occurs.
Final control element: it is directly coupled with the control valve or control vane or control
damper and operates as per the auto signal or manual signal which received from the
auto/manual station. Practically, it is the derive of the control valve or control vane or
control damper of the process.
Valve or vane or damper: practically, these are the gate of the different parts of the process
whose opening is controlled for the controlling of the process parameter.it is mechanically
coupled with the final control element and driven by final control element.
Position indicator: basically, it is either voltmeter or ammeter whose scale is calibrated
from 0% to 100%. It shows the actual position of the control valve or control vane or
control damper of the process.
Measurements of process parameters:
Flow measurement: flow can be measured by flow rate and floe volume.
Flow rate is integrated velocity of the individual stream lines making a total velocity profile
across the conduit.
Flow volume is the total volume of fluid which has passed through the conduit over a given
period. In thermal power plant generally flow volume is measured.
There are different flow meters which is generally used in the thermal power plant.
Float switch:
How Float Switches Work
The purpose of a float switch is to open or close a circuit as the level of a liquid rises or
falls. Most float switches are “normally closed,” meaning the two wires coming from the
top of the switch complete a circuit when the float is at its low point, resting on its bottom
clip (for example, when a tank is dry).
To complete a circuit, float switches utilize a magnetic reed switch, which consists of two
contacts sealed in a glass tube. When a magnet comes close to the two contacts, they
become attracted to each other and touch, allowing current to pass through. When the
magnet moves away, the contacts demagnetize and separate (breaking the circuit), as
shown below.
In a float switch, the magnetic reed switch is hermetically sealed in a stem, most often
made from plastic or stainless steel. The float encases a sealed magnet, which moves up
and down the length of the stem as a fluid level rises and falls. As the magnet passes by the
contacts in the encased reed switch, they touch and complete a circuit between the two
lead wires, as shown in the cutaway below.
The most common types of differential pressure flowmeters are:

Orifice Plates

Flow Nozzles

Venturi Tubes

Variable Area - Rotameters
Orifice Plate
With an orifice plate, the fluid flow is measured through the difference in pressure from the
upstream side to the downstream side of a partially obstructed pipe. The plate obstructing
the flow offers a precisely measured obstruction that narrows the pipe and forces the
flowing fluid to constrict.
The orifice plates are simple, cheap and can be delivered for almost any application in any
material.
The TurnDown Rate for orifice plates are less than 5:1. Their accuracy are poor at low flow
rates. A high accuracy depend on an orifice plate in good shape, with a sharp edge to the
upstream side. Wear reduces the accuracy.
Venturi Tube
Due to simplicity and dependability, the Venturi tube flowmeter is often used in
applications where it's necessary with higher TurnDown Rates, or lower pressure drops,
than the orifice plate can provide.
In the Venturi Tube the fluid flow rate is measured by reducing the cross sectional flow
area in the flow path, generating a pressure difference. After the constricted area, the fluid
is passes through a pressure recovery exit section, where up to 80% of the differential
pressure generated at the constricted area, is recovered.
With proper instrumentation and flow calibrating, the Venturi Tube flow rate can be
reduced to about 10% of its full scale range with proper accuracy. This provides a
TurnDown Rate 10:1.
Flow Nozzles
Flow nozzles are often used as measuring elements for air and gas flow in industrial
applications.
The flow nozzle is relative simple and cheap, and available for many applications in many
materials.
The TurnDown Rate and accuracy can be compared with the orifice plate.
Variable Area Flowmeter or Rota meter
The Rota meter consists of a vertically oriented glass (or plastic) tube with a larger end at
the top, and a metering float which is free to move within the tube. Fluid flow causes the
float to rise in the tube as the upward pressure differential and buoyancy of the fluid
overcome the effect of gravity.
The float rises until the annular area between the float and tube increases sufficiently to
allow a state of dynamic equilibrium between the upward differential pressure and
buoyancy factors, and downward gravity factors.
The height of the float is an indication of the flow rate. The tube can be calibrated and
graduated in appropriate flow units.
The Rota meter meter typically have a TurnDown Ratio up to 12:1. The accuracy may be as
good as 1% of full scale rating.
Magnetic floats can be used for alarm and signal transmission functions.
Open Channel Flowmeters
A common method of measuring flow through an open channel is to measure the height of
the liquid as it passes over an obstruction as a flume or weir in the channel.
Common used is the Sharp-Crested Weir, the V-Notch Weir, the Cipolletti weir, the
Rectangular-Notch Weir, the Parshall Flume or Venturi Flume.
Ultrasonic Level Measurement
Introduction
Level measurement can be performed via ultrasonic or sonic technology too. Ultrasonic level
measurement devices basically employ sound waves for detection of liquid level. They usually work over
the frequency range between 20 kHz to 200 kHz.
Operating Principle
The working principle of a typical Ultrasonic level sensor is illustrated in the figure below.
In this design, the level sensor is located at the top of the tank in such a way that it sends out the sound
waves in the form of bursts in downward direction to the fluid in the tank under level measurement. As
soon as the directed sound waves hits the surface of the fluid, sound echoes gets reflected and returned
back to the sensor.
The time taken by the sound wave to return back is directly proportional to the distance between the
piezo electric sensor and the material in the tank. This time duration is measured by the sensor which is
then further used to calculate the level of liquid in the tank. The speed of the sound waves can sometimes
be affected due to variations in temperature for which appropriate compensations need to be provided in
the sensor design. In general, the medium over the fluid’s surface is air. However, one can employ a
blanket of nitrogen or any other vapor also.
Types of Ultrasonic Level Sensors
Ultrasonic level sensors are available in two basic types which are explained below:
Non-Contact Ultrasonic Sensors
This category of ultrasonic level sensors consists of:
1.
2.
3.
4.
An analog signal processor
A microprocessor
Binary coded decimal i.e. BCD range switches
An output driver circuit
In these devices, the microprocessor generates the gate signal and pulses and directs them to the
ultrasonic sensor via the analog signal processor. The sensor then transmits a beam of ultrasonic waves
to the surface of the fluid. It also receives the reflected echoes from the fluid surface and sends them
back to the microprocessor. The microprocessor keeps on receiving echoes of sound waves and
performs calculations to determine distance between the sensor and the fluid surface and hence detects
the fluid level.
Contact Ultrasonic Sensors
These types of ultrasonic sensors are primarily used to detect fluid level at a specific point only. These
level measurement devices consume less energy and basically include:


A sensor installed in the process field and
An integrated solid state amplifier
They consist of no movable parts and hence do not need any calibration. “Typically, they are equipped
with terminal blocks for connection of a power source and external control devices. The ultrasonic signal
crosses a one-half inch gap in the sensor, controlling relay switches when the gap contains liquid. The
sensing level is midway along the gap for horizontally mounted sensors, at the top of the gap for vertically
mounted sensors.”[1] When the level of the fluid drops below the sensing level, the strength of the
ultrasonic signal gets reduced. This eventually brings the relay to its former position.
These ultrasonic sensors find their application in vessels or pipes where they are used for automatic
action of pumps, solenoid valves, and high or low level alarms. In these areas, two sensors need to be
employed: one for filling and emptying tanks and the other one for measuring fluid volumes. They are
suitable for use with mostly all kinds of fluids. Their performance does not get easily influenced by
coatings, clinging droplets, foam or vapor etc but sometimes, highly aerated or viscous fluids can create
trouble by choking the sensor gap.
Main Features
Key features of ultrasonic level measurement devices are listed below:




“These sensors use frequencies in the tens of kilohertz range; transit times are ~6 ms/m. The speed
of sound (340 m/s in air at 15°C (1115 fps at 60°F) depends on the mixture of gases in the
headspace and their temperature.”[2]
The speed of sound waves traveling via the medium which is normally air is prone to get affected by
changes in the working temperature. In order to compensate for these changes in temperature and
resulting changes in sound wave speed, the level measurement system must include a temperature
sensing device. This will help in correct distance calculations and hence accurate level detection
results.
In cases where heavy foam is found on the surface of the process fluid, the use of ultrasonic level
measurement techniques are usually avoided since this foam work as a sound absorbent.
Consequently, the sound wave will get scattered resulting in non reception of the exact signal by the
sensor. This will cause improper functioning of the measurement system.
Excessive surface turbulence of the fluid can result in wide fluctuations in level measurement results.
To avoid this issue, one may employ a damping correction or a response delay with the device.





Good level measurement requires that the reflected echo from the fluid surface returns back in a
straight line to the sensor. Besides, it calls for proper installation of ultrasonic transmitter over the
tank. The transmitter should be mounted in such a way that the inner composition of the vessel or
tank doesn’t get in the way of the signal.
In level measurement fields where sound waves get influenced by factors like foam and vapor etc.,
one can connect a beam guide to the sensor for improving performance of these devices.
Ultrasonic level measurement technique proves to be quite costly when employed for point level
measurement applications.
In case of fluids which are less viscous, one can execute point level measurement via a technique
called ultrasonic gap technique. In this method, “a transmit crystal is activated on one side of a
“measurement gap” and a receive crystal listens on the opposite side. The signal from the receive
crystal is analyzed for the presence or absence of tank contents in the measurement gap.” [3]
To overcome barricades encountered in the vessel or tanks, a technique called tank mapping has
been introduced. “Tank mapping lets the operator take a "sonic snapshot" of an empty vessel. The
transducer transmits a sound burst and the echo is recorded as a signature of the tank. Any
obstructions in the vessel will send an echo and create a profile. Later on, this signature or profile is
locked into the ultrasonic unit’s memory so it will not respond to echoes created by these
obstructions.”[4]
Advantages
Major advantages offered by ultrasonic level measurement technique are mentioned below:



Ultrasonic level sensors are usually non contact type i.e. they do not make any contact with the
process fluid under level detection.
Besides, they consist of fixed components only hence require less maintenance
They are usually mounted at the top of the vessel due to which they are less likely to offer leakage
problems as compared to entirely wetted means.
Disadvantages
Ultrasonic level measurement technique can not be suitably applied in all fields since use of ultrasonic
level sensors includes few setbacks too. Many factors exist which have the tendency to influence the
returned echo signal back to the sensor. Some of them include:





Materials like powders etc.
Heavy vapors
Surface turmoil
Foam
Ambient noise and temperature
Besides, ultrasonic level measurement devices do not work satisfactorily in areas involving vacuum or
high pressure conditions.
Closed
Tank
Level
Measurement
with
a
DP
Transmitter
As shown below, in a close tank level measurement installation, if a gas phase exists on
top of the liquid, the gas pressure must be compensated for.
This is because a change in the gas pressure will cause a change in the level
transmitter,LT, output. Furthermore, the pressure exerted by the gas may be so high
that the pressure due to the column of liquid becomes insignificant. A pressurized close
tank application requires a reference connection. A reference connection is a
connection from the transmitter (LT) low side to the top of the tank. Its purpose is to
compensate and balance out the pressure above the liquid so that a true level
measurement can be made. If there were no reference or compensating connection,
the transmitter would indicate the level plus the tank pressure:
From the diagram above, P(high) = P(gas) + H*S.G and
P(low) = P(gas)
Differential Pressure, ΔP = P(high) –P(low) = H * S.G
As seen from the calculation above, the effect of the gas pressure is cancelled and
only the pressure due to the hydrostatic head of the liquid is sensed. The low side
transmitter piping that provides the reference connection is called a dry leg
condition. This connection will remain empty if the gas above the liquid does not
condense. In this closed tank application with a dry leg condition, if the level
transmitter is mounted at the base of the tank, then it can be calibrated with the 0%
level = 4mA and the 100% level = 20mA. Note that ΔP is measured in inches of water
colum (in wc).
If condensation of the gas above the liquid does occur, then the low side of the
transmitter piping will accumulate liquid leading to potential errors in the
measurement. To prevent this from happening, the low side of the transmitter piping
is purposely filled with a convenient reference liquid (either the process liquid or
some compatible material) to eliminate this potential error. This is called a wet leg
condition.
What is a Pressure Switch?
This is a device designed to monitor a process pressure and provide an output when a
set pressure (setpoint) is reached. A pressure switch does this by applying the process
pressure to a diaphragm or piston to generate a force which is compared to that of a
pre-compressed range spring.
A pressure switch is used to detect the presence of fluid pressure. Most pressure
switches use a diaphragm or bellow as the sensing element. The movement of this
sensing element is used to actuate one or more switch contacts to indicate an alarm or
initiate a control action.
Pressure switches have different designs with different sensing elements. One of the
most common is the one with diaphragms or bellows as the sensing elements. The one
I will discuss here uses a piston as the pressure sensing element. In any case, the
operating principle for this piston type is the same with a diaphragm or bellow type
pressure switch.
Parts of a Pressure Switch:
The basic parts of a typical pressure switch are shown in the schematic diagram below:
The following basic parts can be identified on the sectional view of the pressure
switch:

Micro-switch

Insulated trip button

Operating pin

Trip setting nut

Range spring

Operating piston
Switch case or housing
How a Differential Pressure Switch Works
The Differential Pressure Switch just like the pressure switch is a simple electromechanical device that operates on the basic principles of Levers and opposing
forces. They are mainly used for sensing a difference in pressure between two points
in a plant or system. Three essential elements in the switch are used in various
combinations to manufacture hundreds of variants of differential and pressure
switches to suit a variety of industrial applications. These elements are:
(a) Sensing element made either of bellows or diaphragm (metallic or elastomeric)
(b) A stable spring to determine the range set point and
(c) A snap-acting micro-switch available in a wide variety (SPDT, DPDT etc)
Operating Principle:
A differential pressure switch is designed to sense a difference in pressure between
two pressure sources in the plant for control purposes. When the pressures from two
different sources in a process are connected across the sensing diaphragm, metallic or
elastomeric as the case may be, the pressure difference creates a force which then
overcomes that of a pre-tensioned spring and in the process moves a balancing arm or
mechanism to effect the minimal movement required to actuate the micro-switch(es)
of the switch.
High and low pressures are applied on either side of the specially contoured
sensing diaphragm. This design feature helps to eliminate errors due to a difference
in area which is often a common problem present in twin element pressure
differential switches.
A particular design of the differential pressure switch is described below to illustrate
the principle of operation. Please note that there are various variants of the switch
from different manufacturers but the basic principle of operation remains the same.
As shown in the diagram above, the pressure ports for high process pressure and low
process pressure are separated by an elastic diaphragm. The difference in pressure
that exist between the two ports causes axial movement (measuring travel) of the
diaphragm against the measuring range spring. The differential pressure, which is
proportional to the measuring travel, is transmitted by means of a connecting
rod with little friction to the plungers of the micro-switch. The microswitch contains the electrical contacts of the switch. The electrical contacts of the
switch will actuate depending on theswitch points or setpoints. Overpressure
protection is provided by contoured metal bolsters for the elastic diaphragm.
The adjustment of the switch point or setpoint is made by setpoint
screws accessible from the front of the differential pressure switch case. The
graduated scales enable a relatively accurate adjustment of the switch points and
indicate the setpoint that is momentarily adjusted.
In conclusion, the differential pressure switch essentially works on the basis of a
difference in pressure between the high and low pressure ports. This difference in
pressure is then converted into axial movement that is used to actuate the contacts
of a micro-switch depending on the setpoint(s) of the differential pressure switch.