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Process Control

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SRI RAMAKRISHNA ENGINEERING COLLEGE
[Educational Service : SNR Sons Charitable Trust]
[Autonomous Institution, Accredited by NAAC with ‘A’ Grade]
[Approved by AICTE and Permanently Affiliated to Anna University, Chennai]
[ISO 9001:2015 Certified and all eligible programmes Accredited by NBA]
VATTAMALAIPALAYAM, N.G.G.O. COLONY POST, COIMBATORE – 641 022.
DEPARTMENT OF ELECTRONICS AND INSTRUMENTATION ENGINEERING
12EI2602 - Process Control
(Answer Key)
Part – A
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
b) e-TS
b) Mercury in glass thermometer kept in boiling water.
a) kc is reduced
a) Kc (1+(1+Ts))
b) ISE
c) Frequency
c) Pneumatic
c) 3
b) faster
a) Feedforward
Part –B
11. Two tank in series, non-interactive system
12. Problem:
13. .
14. Compare Pneumatic and Electric Actuators.
15.
Part –C
16. i) Transfer Function
ii) Mass balance Equation.
17. a) i) Explain the working of PI controller and Summarize its Characteristics.
The PI controller combines the behaviour of the I controller and P controller. This allows the advantages
of both controller types to be combined: fast reaction and compensation of remaining system deviation. For this
reason, the PI controller can be used for a large number of controlled systems. In addition to proportional gain,
the PI controller has a further characteristic value that indicates the behaviour of the I component: the reset time
(integral-action time).
The controller output is given by
where
is the error or deviation of actual measured value (PV) from the setpoint (SP).
A PI controller can be modelled easily in software such as Simulink or Xcos using a "flow chart" box
involving Laplace operators:
Where
Setting a value for is often a trade-off between decreasing overshoot and increasing settling time. The lack of
derivative action may make the system more steady in the steady state in the case of noisy data. This is because
derivative action is more sensitive to higher-frequency terms in the inputs. Without derivative action, a PIcontrolled system is less responsive to real (non-noise) and relatively fast alterations in state and so the system
will be slower to reach setpoint and slower to respond to perturbations than a well-tuned PID system may be.
17. a) ii)
17. b) i)
ii)
18. a) i) Compare feed forward controller with feedback controller with an example. Also bring out
its merits and demerits
Sr.no
Point
Difference
1
of
Feedback control system
Feed Forward Control system
Definition
Systems in which corrective action is
taken after disturbances affect the output
Systems in which corrective action is
taken before disturbances affect the
output
2
Necessary
requirement
Not required
Measurable Disturbance or noise
3
Corrective
action
Corrective action taken after
disturbance occurs on the output.
4
Block Diagram
5
6
the
Corrective action taken before the
actual disturbance occurs on the output.
Control
Variable
adjustment
Variables are adjusted depending on
errors.
Variables are adjusted based on prior
knowledge and predictions.
Example
Use of roll sensor as feedback element in
ship stabilization system.
Use of flowmeter as feed forward block
in temperature control systems.
ii) Discuss briefly the control valve sizing
b) i) With an example explain the various design procedures to tune a controller using Ziegler Nichols
tuning. Discuss the design criteria considered for controller settlings.
Ziegler-Nichols Controller Tuning Example
The Ziegler-Nichols method uses a closed controller loop & requires the following steps:
• Bring system to steady state operation.
• Put on P control. Introduce a set point change and vary gain until system oscillates continuously. This
frequency is CO and M is the amplitude ratio.
• Compute the following:
The original Z-N tuning settings are given in the following table.
These controller settings were developed to give a 1/4 decay ratio. However, other settings have been
recommended that are closer to critically damped control (so that oscillations do not propagate downstream).
These PID controller settings are shown in the following table.
General 3rd Order Overdamped System Example
As an example, let's assume we have a 3rd order overdamped system with negligible dynamics in the final control
& measurement elements. Then:
ii) Draw the diagram of electronic proportional controller and explain how gain of the system can be
changed.
19 a) i) List and explain the procedure of control valve sizing.
The style of control valve is usually determined by the user’s requirements, past
experiences or plant preference. Valve selection can be a tricky process, but sizing the valve can be
even more difficult. Valves are often incorrectly specified at the time of installation.
The most important variables to consider when sizing a valve include:







Medium will the valve control
Effects will specific gravity and viscosity have on the valve size
The inlet pressure be under maximum load demand.
The inlet temperature
Pressure drop (differential) will exist across the valve under maximum load demand
Maximum capacity should the valve handle
The maximum pressure differential for closing the valve
ii) Define Cavitation and explain the procedure to prevent them.
To understand how to prevent pump cavitation, it’s important to have a good understanding of what the
problem is and how it arises. There are several types of cavitation which we’ll discuss below, but the process is
similar.
Cavitation Defined: Cavitation is the formation and accumulation of bubbles around a pump impeller. This tends
to form in liquids of any viscosity as they are being transported through and around a pump system. When each
of these tiny bubbles collapses or bursts, it creates a high energy shock wave inside the liquid. Imagine throwing
a stone into a pond. The circular ripples which are created in this process are similar to cavitation bubbles
exploding. The difference here is that due to the sheer number of bubbles creating these shock waves, the impeller
and other pump components can be eroded over time.
How to prevent cavitation due to vaporization
1. Lower the temperature.
2. Raise the liquid level in the suction vessel.
3. Change the pump.
4. Reduce motor RPM if possible.
5. Increase the diameter of the eye of the impeller.
6. Use an impeller inducer.
7. Use two lower capacity pumps in parallel
b) i) Define cascade control and explain how it is implemented in heat exchanger process?
Cascade control design considers the likely disturbances and tailors the control system to the disturbance(s) that
strongly degrades performance. Cascade control uses an additional, "secondary" measured process input variable
that has the important characteristic that it indicates the occurrence of the key disturbance.
For the stirred-tank heat exchanger, all measured variables are shown in Figure 14.1. The secondary variable is
selected to be the heating oil flow, because it responds in a predictable way to the disturbances in the oil pressure,
which is not measured in this case. The control objective (tight control of the outlet temperature) and the final
element are unchanged. The manner in which the additional measurement is used is shown in Figure 14.2.
The control system employs two feedback controllers, both of which can use the standard PID controller
algorithm. The important feature in the cascade structure is the way in which the controllers are connected. The
output of the exit temperature controller adjusts the set point of the flow controller in the cascade structure; that
is, the secondary controller set point is equal to the primary controller output.
Thus, the secondary flow control loop is essentially the manipulated variable for the primary temperature
controller. The net feedback effect is the same for single-loop or cascade control; in either case, the heating oil
valve is adjusted ultimately by the feedback. Therefore, the ability to control the exit temperature has not been
changed with cascade.
A few important features of the cascade structure should be emphasized. First, the flow controller is
much faster than the temperature controller. The improvement results from the much shorter dead time in the
secondary loop than in the original single-loop system;
ii) Draw the block diagram of model reference adaptive control and explain.
When the process parameters are unknown or change in time, in order to achieve and to maintain the desired
performance, an adaptive control scheme known as Model Reference Adaptive Control (MRAC) is implemented.
Here, a reference model which is the realization of the process with desired performances. This scheme is based
on the observation that the difference between the output of the process and the output of the reference model
(called as the process-model error) is a measure of the difference between the real and the desired
performance. TheAdjustment Mechanism directly adjusts the parameters of the controller in real time in order to
force the process-model error to zero. MRAC is also known as Direct Adaptive Control because of the direct
adjustment of the controller parameters using the model output.
block diagram of model reference adaptive control (MRAC)
20 a) Draw the distillation equipment and briefly describe its components.
In the distillation process, a feed mixture is separated by volatility or boiling point into 2 or sometimes more
component streams. Energy in the form of heat is introduced at the bottom of the column in the reboiler and
removed at the top of the column in the overhead condenser.
Also explain any two control schemes employed in distillation column.





Reboiler and Steam Control
Condenser and Pressure Control
Analyser Control
Temperature Control
Feed Preheat Control
Reboiler and Steam Control
Condenser and Pressure Control
Analyser Control
b) What is the use of distillation column in process industries? Explain driefly the various control methods
used in distillation column.
Distillation is a natural process using temperature variation to separate a liquid and one or more other ingredients
that are mixed together. Distillation is widely used in industries that refine oil, desalinate water, create liquor,
beer and wine, and produce many chemical products used in homes and factories
Control of Distillation Columns
Degrees of Freedom Analysis
to determine the number of control degrees of freedom in a distillation column. There are two equivalent
procedures based on the equation C.D.F. = Total No. of Streams - No. of Phases Present + 1
All we have to do is count all the streams in the process. Separately count the total number of extra phases i.e. add
up all occurrences of phases greater than one in all units. The number of control degrees of freedom is the
difference between these two numbers.
Controlling Pressure in Distillation
In a distillation column it is usually necessary to regulate the pressure in some way. Below there are five different
methods described for doing this.





Vent to Atmosphere
Cooling Water
Flooded Condenser - 1
Flooded Condenser - 2
Partial Condenser
Controlling Tops Composition in Distillation
As well as pressure, the other parameter most likely to be controlled is the composition of the tops product. The
reason is that the final product will most probably come from the top of the column and it is important to know
its composition. Again, as with pressure, there are many different ways of controlling the tops composition. Three
methods are described below.



Reflux Rate
Reflux Ratio
Distillate Rate
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