EI2352 - Process Control - 2 Marks With Answers

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SNS College of Technology, Coimbatore – 35 Department of Electronics and Instrumentation Engineering

EI 2352 – Process Control 2Marks Questions and Answers III Year, 6th Sem, EIE

UNIT I - INTRODUCTION 9

Need for process control – mathematical model of first order level, pressure and

thermal processes – higher order process – interacting and non-interacting systems

– continuous and batch processes – self-regulation – servo and regulator

operations.

UNIT II - CONTROL ACTIONS AND CONTROLLERS 9

Basic control actions – characteristics of on-off, proportional, single-speed

floating, integral and derivative control modes – P+I, P+D and P+I+D control

modes – pneumatic and electronic controllers to realize various control actions.

UNIT III - OPTIMUM CONTROLLER SETTINGS 9

Evaluation criteria – IAE, ISE, ITAE and ¼ decay ratio – determination of

optimum settings for mathematically described processes using time response and

frequency response – Tuning – Process reaction curve method – Ziegler Nichols

method – Damped oscillation method.

UNIT IV - MULTILOOP CONTROL 9

Feed-forward control – ratio control- cascade control – inferential control – split-

range control – introduction to multivariable control – examples from distillation

column and boiler systems.

UNIT V - FINAL CONTROL ELEMENT 9

I/P converter – pneumatic and electric actuators – valve positioner – control valves

– characteristics of control valves – inherent and installed characteristics – valve

body – commercial valve bodies – control valve sizing – cavitation and flashing –

selection criteria.

L=45 T = 15 TOTAL : 60 PERIODS

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EI 2352 – Process Control 2 Marks Question and Answers


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UNIT-I INTRODUCTION

1. Define controlled variable, manipulated variable and load variable in process control.

Controlled variable: it is the quantity or condition of the control system which is directly

measured and controlled.

Load variable: the load variables of a process are all other independent variables except the

control variable and manipulated variable.

Manipulated variable: it is the quantity or condition which is varied by the automatic

controller so as to affect the value of control variable.

2. Distinguish between batch process and continuous process.

Batch process Continuous process

1. A process in which the materials or work

are stationary at one physical location while

being treated. Eg. Thermal type process.

2. This is suitable for different kinds for

product

3. The quantity of product is less

4. The control system is simple.

5. The Process variables are lumped

A process in which the materials or work

flows more or less continuously through a

plant apparatus while being treated. Eg.

Storage vessel control.

Suitable for one or two products

Quantity of product is large

The control system is complicated.

The pv is distributed over the entire system.

3. Define degrees of freedom.

The Degree of freedom is defined as the independent variable that must be specified in order

ro difine the process completely. The number of degree of freedom can be found by the

equation

f = V – E Where

V = Number of independent variable describing a process

E - Number of independent equation physically relating the V variables.

4. What are the different mathematical models used in process control?

1. Experimental approach: In this case the physical equipment of the chemical process is

available and the various values of input (disturbance, manipulated variable) are change and

through appropriate measuring devices the outputs of process change with time. Such a

procedure is time and effort consuming and it is usually quite costly because a large number

of such experiments have to be performed.

2.Theoretical approach:

This is given in terms of mathematical equations (differential, algebraic) whose solution

yields the dynamic or static behavior of the chemical process that is examined.

5. What is meant by self-regulation?

Self regulation of a process is defined as the process is one in which either inflow and

outflow is dependent to the controlled variable. Most of the causes the flow is self regulating

because of its steady state is increased by increasing the outflow.

6. What is non-self regulation? Give an example.

A non-self regulating process is one in which both inflow and outflows are independent of

the controlled variable this type of process has no steady state gain. The example of the non

self process is a simple liquid level system with constant outflow.



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7. Define the term resistance in liquid level system.

The resistance is defined as the ratio between the rate of change of head to the rate of change

of flow. This may be classified based on flow, turbulent or laminar resistance. The turbulent

resistance is not linear but laminar resistance is linear.

8. Calculate the thermal capacitance of a 30-gal water tank.

The thermal capacitance is defined as C = WP

Where W – weight of the water, gal. P – Specific heat of water, Btu/lb deg-1

i.e C = 30 lb x 8.3 Btu/ deg lb-1 = 249 Btu / deg.

9. Distinguish between servo and regulator operation of control system.

Servo problem Regulator problem

1. The set point is variable and load

disturbance are kept constant.

2. This method is desired by operator.

3. Tracking of missiles and automatic

machining are examples of this type.

the load disturbance is variable and set point

is kept constant

this may happen any time in the system

controlling of temperature and flow rate are

examples of the type.

10. Enlist the distinguishing features of process control systems.

i.) Increase in quantity of number of products.

ii.) Improved in quality of products.

iii.) Saving in processing materials.

iv.) Saving in plant equipment.

v.) Decrease in human drudgery.

11. Distinguish between parameters and variables of a process.

Process parameters Process variables

1. This is the quantity which is known

and it is constant.

2. This is supplied by external world.

3. This has definite value ( set point,

Controller parameters )

This is quantity to be measured in the

process.

This is developed in the process itself.

This does not have definite value

12. List any four objectives of process control.

i.) Safety

ii.) Product specification.

iii.) Environmental regulations.

iv.) Operational constraints

v.) Economics.

13. Define interacting system and give an example.

The term interacting is often referred as loading. When two tanks are connected at same

datum level if level of any one tank increases simultaneously another tank head also

increase.

14. A tank operating at 10 ft head, 5 lpm outflow through a valve and has a cross section

area of 10 sq. ft. calculate the time constant.

Given h = 10 ft. Q = 5 lpm, A = 10 ft2.

Time constant T = AR but R = h / q = 10 / 5 = 2

T = 10 x 2 = 20 min.



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15. A thermometer having a time constant of 1 min and is initially at 50 0C. it is immersed

in a bath and maintained at 1000C at t = 0. Determine the temperature reading at t = 1.2

min..

T = 1 min , A = 500C

Y(t) = A ( 1- exp ( - t / T) + 50 = 84.940C

16. What are the assumptions made to get the mathematical model of an non interacting

system?

a. The liquid to be of constant density

b. The tanks to have uniform cross sectional area.

c. The flow resistance to be linear.

17. How the mathematical modeling of higher order process obtained?

The mathematical modeling of higher order process are obtained in three ways

a. N first order processes in series

b. Processes with dead time

c. Processes with inverse response.

18. What is the need for process automation?

Process automation is employed to maintain a controlled variable at a desired value. The

main advantage of automatic control is that a machine can perform the task more rapidly and

consistently than a human being.

19. Difference between batch process and continuous process?

Batch process:

A process in which the materials or work are stationary at one physical location while being

treated. Eg. Thermal type process.

Continuous process:

A process in which the materials or work flows more or less continuously through a plant

apparatus while being treated. EgStorage vessel control.

20.list any four objectives of process control.

A. safety

B. Production specifications.

C. Environmental regulations.

D. Operational constraints

E. Economics

21. What is the significance of "degree of freedom"?

Degree of freedom is the independent variables that must be specified in order to define the

process completely. Consequently the desired control of a process will be achieved when and

only when all the degree of freedom has been specified. A good understanding of degree of

freedom is inherent in a process and they are very crucial for the design of effective

controllers.

22. What are the 2 approaches employed tops obtain the mathematical model of a process?

Experimental approach and theoretical approach

Experimental approach: in this case the physical equipment of the chemical process is

available and the various values of input (disturbance, manipulated variable) are change and



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through appropriate measuring devices the outputs (temperature, pressure, flow rate,

concentration) of chemical process change with time. Such a procedure is time and effort

consuming and it is usually quite costly because a large number of such experiments have to

be performed.

Theoretical approach: to design the control system for a chemical process before the process

has been constructed, we cannot rely on the experimental procedure, so a different

representation of the chemical process is needed in order to study its dynamic behavior. This

is given in terms of mathematical equations (differential, algebraic) whose solution yields the

dynamic or static behavior of the chemical process that is examined.

23.How does the control of unit operation affect the quality of product?

Unit operations play an important role in the development of chemical process. They provide

building blocks, or process segments, that can be analysed and designed individually and

then combined into larger processing system.

24.Define process variable, load variable and manipulated variable.

Process variable: It is the quantity or condition of the control system which is directly

measured and controlled.

Load variable: the load variables of a process are all other independent variables except the

control variable and manipulated variable.

Manipulated variable: it is the quantity or condition which is varied by the automatic

controller so as to affect the value of control variable.

UNIT II - CONTROL ACTIONS AND CONTROLLERS

1. What are the basic control actions in process control?

The basic control actions used in process control is

a).On – off control b) Proportional control c).Proportional – Integral control

d).Proportional - Integral - Derivative control

2. Define proportional band.

Proportional band (PB) is defined as the error ( expressed as a percentage of the range of

measured variable ) required to move the valve from fully closed to fully open. The Pb and

Proportional gain (Kp) is given by PB = 100 / Kp.

3. Define reset time

The time required for the output of a proportional – Integral controller to change an amount

equal to the amount of proportional response provided by a step change of actuating signal.

4. Define differential gap. Why is it introduced in a process?

A differential gap in two-position control causes the manipulated variable to maintain its

previous value until the controlled variables has moved slightly beyond the set point. In

actual operation it is the same as hysteresis. A differential gap is caused in the two-position

controller if small friction exists at the bearing on the float arm.

5. What are the applications of on-off control?

The on – off control are used in electric heater , thermostats, rapid operation switches,

electrical contacts, simple water level control and etc.



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6. What is meant by offset? How can you eliminate the offset in a process?

The difference between the set point and the steady state value of the controlled variable is

referred as offset. This is commonly happened when the proportional only controller is used.

This can be eliminated by adding integral term in the controller.

7. What is meant by integral wind up?

The reset action in a PI controller causes its output to go an changing as long as the error is

non-zero For various reasons the error cannot be eliminated quickly in many situations and

therefore if the time interval is long enough, develops due to reset action leading finally to

saturation known as the reset action or integral windup.

8. What is anticipatory control? Why is it called?

The derivative control action is anticipatory control. .With the presence of the derivative term

(de / dt), the PID controller anticipates what the error will be in the immediate future and

applies a control action which is proportional to the current rate of change in the error. Due to

this property the derivative control action is sometimes referred to as anticipatory control

9. What are the advantages and disadvantages of PI control?

Advantages :

1. It removes or reduces the steady state error without the need for manual reset.

2. it removes or reduces the steady state error without the need for manual reset.

Disadvantages :

1. It may lead to oscillatory response of increasing or decreasing amplitude

which is undesirable and the system may become unstable.

2. Constant steady state error.

10. What is meant by single speed floating control?

If the error is zero the output will not change but remains (floats) at whatever setting it is,

when the error goes to zero. The output of the control element changes at a fixed rate when

the error exceed the neutral zone

11. Write the transfer function of PID controller and sketch its step response.

The transfer function PID controller is

U(s) / E(s) = Kp ( 1 +( 1/ Tis ) + Tds)

12. Derivative control cannot be used alone. Justify your answer.

When the error is constant the derivative action is zero. The derivative action Anticipates

future errors and introduces appropriate Action. When the process has noise the derivative

control amplifies the noise and makes the noisy one. It introduces a stabilizing effect on the

Closed –loop control response of a process

13. Explain the principle of multi-position control.

In the Two position (On – Off) controller the control action is only in two states. In the multi

position control several intermediate position are used. This control mode is used to reduce

the cycling and overshoot and undershoot. This mode is represented as

P = Pi e > | ei | i = 1,2,………n

14. What are the main disadvantages of a proportional controller?

The major drawback or proportional controller is offset. When the proportional gain is

increase the controller will oscillate which tends to unstable system.



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15. Recommend control modes for flow process and thermal process.

The flow process has small time constant which gives fast response so the PI controller is

suitable the thermal process is sluggish in nature so derivative action should be present in

controller mode. So PID controller is suitable for thermal process.

16. Distinguish between Direct and reverse action of a controller.

Direct action controller Reverse action controller

Use to produce and maintain the

desired quality of the product

The process may be necessary when direct

control is difficult to accomplish.

17. What is dead time?

Dead time is a fixed interval of time between the change of an input to an element and the

beginning of response to the input.

18. Define decay ratio.

The ratio of the amounts above the ultimate value of two successive peaks is called as decay

ratio. The decay ratio is expressed as, Decay ratio = exp ( -2ζπ / 1 – ζ2)

19. Give the optimum settings for PID controller.

The optimum settings for PID controller by Z –N method is

Kc = Ku / 2 I = Pu / 2 D = Pu / 8

By C – C method,

Kc = / K td ( 4/3 + td / 4 )

I = td ( ( 32 + 6td )/ / (13 + 8td/)

D = 4td / ( 11 + 2td / )

20. What do you meant by optimum controller settings?

Having decided the type of controller for a given system, and given the performance

criteria for the closed loop response(eg., minimum peak error, ¼ decay ratio, IAE etc.)

optimum controller settings are the best suited values of the controller(Kp, Ti, Td) which

satisfies the performance criteria set by the designer and selecting these values is called

controller tuning.

21. What are the advantages and disadvantages of 2-position control?

Advantages: Easy to design , Low cost

Disadvantages: Cycling, FCE is subjected to high frequency of oscillation

22. Discuss how the stability is affected for integral and derivative control?

Integral control: as the order of the system increases, response becomes more sluggish

increasing the integral control action and the response of the closed loop system becomes

more sensitive. But such trends lead to instability of the closed loop response.

Derivative control: derivative control action does not change the order of the response. As kc

increases the effective time constant increases and the response becomes progressively

slower which means the derivative control produces more robust behavior by the controlled

process.

23. What are the advantages, disadvantages and applications of PD controller?

Advantages: Offset can be reduced without reducing settling time.

Disadvantages: Offset is not eliminated.



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At steady state PD acts as a P controller (i.e.) Steady state error is not eliminated

Applications: Used in large delay process / processes with large dead time.

24. What is the need for feed forward control? Briefly explain feed forward control with

example.

Feed forward control: Feed forward control configuration measures the disturbance

directly and takes control action to eliminate its impact on the process output. Therefore;

feed forward controllers have the theoretical potential for perfect control.

Example: Feed forward control of a heat exchanger: the objective is to keep the exit

temperature constant by manipulating the steam pressure. There are two principal

disturbances that are measured for feed forward control: liquid flow rate and liquid inlet

temperature.

25. Explain two-position control with a suitable example. Where is it used?

Two position control (on-off control): Two-position control is a position type controller

action in which the manipulated variable is quickly changed to either maximum or

minimum value depending upon whether the controlled variable is greater or less than the

set point. The minimum value of the controlled variable is usually zero (off).

Example: (electric-level control) - A float in the vessel operates an electric switch, which

controls power to a solenoid valve. When the liquid level rises, the switch contacts are

closed the solenoid valve closes, and the inflow is cut off. When the liquid level falls, the

switch contacts are opened, the solenoid valve opens, and the inflow resumes.It is widely

employed in home-heating systems and domestic water heaters.

26. What is meant by differential gap? What are its effects? Is it a desirable factor?

A differential gap in two-position control causes the manipulated variable to maintain its

previous value until the controlled variables has moved slightly beyond the set point. In

actual operation it is the same as hysteresis. A differential gap is caused in the two-

position controller if small friction exists at the bearing on the float arm.

27. Why is integral control action recommended for zero and lower order process?

Integral control actions are recommended because a minimum value of deviation is

obtained without encountering excessive oscillation and also there is no offset or the

offset is reduced.

28. What is the equation of a pid controller? What are the advantages and disadvantages of

PID control actions?

Equation of a pid controller = KP e(t) + KI + KD

KI = (KP) (1/I) e(t)

KD = (KP) (d ) de/ dt

P-control

Adv: it amplifies the error signal which increases the loop gain. This improves the

steady state tracking accuracy, disturbance signal rejection and relative stability. It

makes the system less sensitive to parameter variations.

Disadv: constant steady state error.

I-control.

Adv: it removes or reduces the steady state error without the need for manual reset.

Disadv: it may lead to oscillatory response of increasing or decreasing amplitude

which is undesirable and the system may become unstable.



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D-control.

Adv: controller gives 90 degree phase shift.

Settling time decreases.

Disadv: No control action for steady error.

Not suitable for noisyt processes.

29. Give the advantages, disadvantages and applications of three-mode control:

Three-mode control is PID control

Advantages: Stability due to p action, Ability to remove offset due to p, Ability to

remove peak error and large settling time due to d action. It is the best control if

properly tuned.

Disadvantages: Tuning is difficult, more components are used and it has a complex

circuit.

Applications: Since it combines P, Iand D modes of control, it can be used in all

control applications except where d action is not acceptable. Opening or closing

restrictions can implement any composite action.

30. Design an electronic p-controller with a proportional gain 5.

Given: kp = 5

Kp = r2/r1

5 = r2/r1

Ans: r2 = 10kΩ and r1 = 2Kω

31. Draw the circuit of electronic pi controller that –gives a proportional gain of 4 and

reset

time of 20ms?

Kp=r2/r1 ,=20=r2c,R2c=20,C=1μf

Vout=(r2/r1)ve+(r2/r1)1/r2c integral 0 to t (vedt+vout(0))

R2c=20,R2=20,Kp = r2/r1 =4,20/4=r1,R1 =5K,R2= 20K,C=1μf

UNIT III – OPTIMUM CONTROLLER SETTINGS

1. What is meant by tuning in process control?

To adjust the controller parameters is referred as tuning. The controller may tune using the

simple criteria such as ¼ decay ratio, minimum setting time, and minimum error. Using time

integral performance such as ISE, IAE, ITAE the controller may tune. The popular method of

controller tuning is Process reaction curve and Z-N method.

2. What is meant by ultimate period Pu of a controlled process?

The ultimate period Pu of sustained cycling is defined as Pu = 2π / phase cross over

frequency.

3. What is a process reaction curve?

This is a plot drawn between the measurement output and time when the closed loop

system is disconnected between the controller and final control element and is manually

operated with step change.

4. State the relation between maximum overshoot and decay ratio.

The relation between maximum overshoot and decay ratio is, decay ratio = (overshoot) 2.



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5. Give the optimum controller settings for a PID controller based on Zeigler-Nicholas

method.

The optimum controller settings for a PID controller based on Zeigler-Nicholas method is the

proportional gain Kc = Ku / 1.7 , Integral time I = Pu / 2 & Derivative time D = Pu / 8

where the Ultimate gain Ku = inverse of the amplitude ratio

Ultimate period of oscillation Pu = 2π / phase cross over frequency

6. What is ITAE and when to go for it?

ITAE is integral of the time weighted Absolute error and it is defined as ITAE = t | e(t) |

dt.

To suppress errors that persist for long times, the ITAE criterion will tune the controllers

better because the presence of large t amplifies the effect of even small errors in the value of

the integral.

7. What is ISE and when to go for it?

ISE is integral of the square error and it is defined as ISE = | e2(t) dt.

To suppress large errors the ISE is better, because the error is squared and thus contributes

more to the value of the integral.

8. What is IAE and when to go for it?

IAE is integral of the Absolute error and it is defined as IAE = | e(t) | dt.

To suppress small errors IAE is better than ISE because when square small numbers ( < 1)

they become even smaller.

9. Write Ziegler Nichols tuning formulae.

Kc I ( min) D ( min)

P only Ku / 2

P-I Ku / 2 Pu / 1.2

P-I-D Ku / 2 Pu / 2 Pu / 8

where Ku = ultimate gain, Pu = ultimate period of oscillation

10. Why is it necessary to choose controller settings that satisfy both gain margin and phase

margin?

It is necessary to choose controller settings that satisfy both gain margin &phase margin in

order to avoid unstable behavior by the closed loop of a process

11. What is tuning a controller based on quarter – decay ratio?

It’s the tuning of parameters of the controller so as to obtain a step change in the load (error).

The response gives decay ratio of ¼.it is defined as the ratio of successive amplitude of the

peaks decay ratio=C/A.

12. Write the Cohen – Coon controller settings PID controller.

For the PID controllers the parameters are,

Kc = / K td ( 4/3 + td / 4 )

I = td ( ( 32 + 6td )/ / (13 + 8td/)

D = 4td / ( 11 + 2td / )

where ratio of steady state output and slope of the sigmoidal response

K = ratio of steady state output to the steady state input



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td – time elapsed until the system responded.

13. Write the Cohen – Coon controller settings PI controller.

For the PI controllers the parameters are,

Kc = / K td ( 0.9 + td / 12)

I = td ( (30 + 3td / ) / (9 + 20td/)

14. When do you go Process reaction curve method for controller tuning.

1. For the multi capacity processes whose response is sigmoid

2. Process with very short time delay

3. The process whose response is overdamped

15. What is cycling?

An important mode of dynamic variable error is the oscillation of an error about zero.

This means the variables is cycling above or below zero set point.

16. Briefly explain about Damped oscillation method

Using only proportional action and starting with low gain adjust the gain adjusted until the

transient response of the closed loop shows a decay ratio of 1 / 4. The optimum setting of

damped oscillation method is more accurate than ultimate method.

17. Under what circumstances derivative control action is added or removed in a

controller?

The derivative action is recommended for temperature and composition control where there

is a sluggish and Multi capacity processes. The derivative action Anticipates future errors and

introduces appropriate Action.It introduces a stabilizing effect on the Closed –loop control

response of a process.

18. Explain the continuous cycling method.

The continuous cycling method or on-off control is a position type control action in

which the mv is quickly changed to either a max or min value depending on whether the

controlled variable is greater or less than the set point.The equations for two-position

control are:

m = m1 when e > 0: m = m0 when e < 0: Where, m = mv

m1 = max value of mv (on)

m0 = min value of mv (off)

19. Why derivative mode of control is not recommended for a noisy process?

The derivative control mode is not recommended for a noisy process because even when

the pv settles down at the set point the derivative control gives the control action for

noises that are at higher frequencies, so the pv moves around the set point.

20. Merits and demerits of single seated and double seated control value?

Single -seated control value

Merits:

It has only one port opening between seat and plug

Simple in construction.

Can be shut-off to provide zero flow.

Demerits :

large force acts a value because of differential head acting across port and seat area.



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double-seated control value

Advantages: Net force acting on value stem is small

Disadvantages: Cannot be shut off tightly because of differential pressure and

temperature expansion of value plug and valve body.

21. What is tuning a controller based on quarter – decay ratio?

It’s the tuning of parameters of the controller so as to obtain a step change in the load

(error). The response gives decay ratio of ¼.it is defined as the ratio of successive

amplitude of the peaks decay ratio=C/A.

.

UNIT IV – MULTILOOP CONTROL

1. What is ratio control? Where is it needed?

Ratio control is special type of feed forward control where two disturbances are measured

and held in a constant ratio to each other. It is used to control the flow rates of the two

streams

2. What are the advantages and disadvantages of feedback and feed forward controllers?

Advantages : 1. Acts before the effect of a disturbance has been felt by a system.

2. Good for slow systems.

3. Does not introduce instability in closed loop response.

Disadvantages:1. Requires identification of all possible disturbances and their direct

measurement

2. Cannot cope with unmeasured disturbance.

3. Sensitive to process parameter variations.

4. Requires good knowledge of the process model.

3. What are the advantages and disadvantages of cascade control?

The Cascade control is useful in reducing the effect of a load disturbance that moves through

the control system slowly. This type of control gives very high performances than

conventional control. the draw back of this type of control is the two loop should be tuned

properly with fine tuning methods.

4. Define and explain the concept of feed forward control.

Feed forward control configuration measures the disturbance directly and takes control action

to eliminate its impact on the process output. Therefore; feed forward controllers have the

theoretical potential for perfect control.

5. What kind of processes stand to benefit the most from feed forward control? Why?

Feed forward control configuration measures the disturbance directly and takes control action

to eliminate its impact on the process output. Therefore; feed forward controllers have the

theoretical potential for perfect control.

6. Briefly explain about multivariable control.

When many inputs (manipulated variables) and many controlled variable (measured outputs)

are present in a loop the multi variable control is suitable. The variables may be interacting,

interconnecting and decoupling. This may be controlled by this method.

7. State the conditions for the cascade control to be effective.



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Cascade control is useful in reducing the effect of a load disturbance that moves through the

control system slowly since the inner loop has the effect of reducing the lag in the outer loop

with the result that the cascade system results more quickly with a higher frequency of

response. For cascade control process of the inner loop should be faster than the outer loop.

For cascade control to be effective the control action of the inner loop is often proportional

with the gain set to a high value. The action of the primary controller is generally PI or PID.

8. Give the applications of cascade control.

The cascade control are used in CSTR’s ( Continuous Stirrer Tank Reactors ), distillation

column, value position control, boilers and etc.

9. The combination of feed forward and feedback is more effective than feed forward or

feedback control. Say true or false.

True. The feed forward control take care of all disturbances affecting the system and the

feedback control, controls the measured parameters.

10. Ratio control is a specific case of feed forward control. Say true or false.

True. The Ratio control is a special type of feed forward control.

11. What is the advantage of cascade control over conventional control?

The cascade control has two loops. When any load changes the inner loop corrected before

they affected the primary loop. This control gives high performance when the load is

frequency changes. The tuning of the control is easy compared to conventional feedback

control.

12. What are the differences between Feed Forward and Feedback controllers?

Feed forward control Feed forward control

1) It is useful for slow process.

2) It does not introduce instability in the

closed loop response.

3) It requires identification of all

possible disturbances and their direct

measurement.

4) It is sensitive to process parameters

It is unsatisfactory for slow processes

It may create instability in the closed loop

response

It does not require identification and

measurement of any disturbance.

It is insensitive to parameter changes

13. Mention an application where Ratio control is used.

The ratio control are used in the air fuel mixture control in combustion, reflux control of

distillation columns, mixture of hot and cooled water in heat exchangers and etc.

14. State any two conditions under which the cascade control is much effective?

The cascade control is recommended whenever high performance is mandatory in the face of

frequent load changes, where the secondary part of the process contains an undue amount of

phase lag or non-linearity.

15. Differentiate between feedback and feedforward control.

Feed Back control: It is useful for slow process.

It does not introduce instability in the closed loop response.

It requires identification of all possible disturbances and their direct

measurement.

It is sensitive to process parameters.



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Feed forward control: It does not require identification and measurement of any disturbance.

It may create instability in the closed loop response.

It is unsatisfactory for slow processes

It is insensitive to parameter changes.

16. When cascade control will give improved performance than conventional feedback

control?

The cascade control is recommended whenever high performance is mandatory in the face of

frequent load changes or where the secondary part of the process contains an undue amount

of phase lag or non-linearity.

17. When the split range control is needed in a process?

In a split –range control we can control a single Process output by coordinating the actions of

several Manipulated variables, all of which have the same effect on the controlled output.

The split range control are used, when the additional safety and operational optimality

whenever necessary.

18. What is inferential control? Give an example:

Inferential control uses secondary measurements to adjust the PV, as CV cannot be

measured. An estimator in the inferential control computes the estimate of values of

unmeasured cv from material and energy balance and the measured outputs.

Eg. Distillation column fig 2.2c pg. 17 Stephanopoulos

19. How is the controlled variable of inner loop chosen in a cascade control system?

The inner loop in a cascade control is the secondary loop. When the process has a variable

that need not be controlled, but whose value affects the CV, then we have a secondary

controller so as to control that value so that they don’t affect the CV.

20. Describe split-range controller with an example.

In a split –range control we can control a single Process output by coordinating the actions of

several Manipulated variables, all of which have the same effect on the controlled output.

Eg: in a split range control of the pressure in a steam Header, several boilers discharge steam

in a common Steam header and from there to the process needs .Here instead of controlling

the steam flow from each boiler, the firing rate and steam production rate at each boiler is

controlled .The control objective is to maintain constant pressure in steam header when steam

demand at various processing units increase.

21. Explain how the quality of control is improved when Feed forward control is added with

feed back control?

The combined form of the two will retain the superior performance of feed forward control

and the insensitivity of the feed back control to uncertainties and inaccuracies .The deviations

caused by the feed forward control will be corrected by the Feed back controller as it directly

monitors the behavior of the controlled processes.

22. Give the Objectives of ratio control, with eg.

Ratio control is special type of feed forward control where two disturbances are measured

and held in a constant ratio to each other. It is used to control the flow rates of the two

streams.



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23. What is inferential control? How is it implemented?

Inferential control uses secondary measurements (because the controlled variables cannot be

measured) to adjust the values of the manipulated variables. The estimator uses the values of

the available measured outputs, together with the material and energy balances that govern

the process, to compute mathematically (estimate) the values of the unmeasured controlled

variables. The controller to adjust the values of the manipulated variables, in turn, uses these

estimates.

24. Under what conditions feed forward is added with feedback?

Feed forward is added with feedback because of the following:

Feed forward requires the identification of all possible disturbances and their

measurement. Any changes in parameters of a process by a feed forward controller

cannot be compensated because their impact cannot be detected.

Feed forward control requires a very good model for the process.

Feedback control is insensitive to the above 3 drawbacks but has poor performance

and possibly unstable.

25. Discuss the situations to select a cascade control and discuss when it will become

effective.

Cascade control is useful in reducing the effect of a load disturbance that moves through the

control system slowly since the inner loop has the effect of reducing the lag in the outer loop

with the result that the cascade system results more quickly with a higher frequency of

response. For cascade control process of the inner loop should be faster than the outer loop.

For cascade control to be effective the control action of the inner loop is often proportional

with the gain set to a high value. The action of the primary controller is generally PI or PID.

26. Why are fuel and air sent at a specified ratio into a combustion chamber?

Fuel & air are sent at a specified ratio into a combustion chamber in order to obtain

complete combustion. (i.e., if the inflow increases the air ratio also increases & hence the

input is min. & o/p is max).

27. What is the significance of the sizing of control valves?

The proper sizing of the control valve is important because of the effect on the operation

of automatic controllers. if the control valve is oversize, for eg, the valve must operate at

low lift and the minimum controllable flow is too large. In addition, the lower part of the

flow-lift characteristics is most likely to be non-uniform in shape. On the other hand if

the control valve is undersize, the maximum flow desired for a process may not be

provided.

28. What are the advantages and disadvantages of feed forward control?

Adv: acts before the effect of a disturbance has been felt by a system, Good for slow systems

and does not introduce instability in closed loop response.

Disadv: requires identification of all possible disturbances and their direct measurement,

Cannot cope with unmeasured disturbance, Sensitive to process parameter variations and

requires good knowledge of the process model.

29. When is inferential control used?

Inferential control is used when the controlled output is unmeasured in the presence of

unmeasured disturbances. Since this is a rare case, inferential control can only be used.



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30. Write down the principle of distillation.

The distillation columns is use to separate different distillate and bottom products with

different temperature and different concentration.

31. Name the different types of distillation.

The types of distillation are Binary distillation column and Multi compound distillation

column.

32. What is distillation?

Distillation separates a mixture on the basis of a difference in composition between a liquid

and the vapor formed from the liquid. In the process industry, distillation is widely used to

isolate and purify volatile materials.

33. What is purpose of the tower or column in distillation equipment?

The tower or column has two purposes: First, it separates a feed into a vapor portion which

ascends the column and a liquid which descends the column. Second, it achieves intimate

mixing between the two counter-current flowing phases.

34. List some of the variables which can be manipulated when controlling a distillation

column.

a. Column pressure

b. Feed flow rate

c. Feed composition(or feed quality)

d. Heat added(boil-up)

e. Bottom product flow rate

f. Heat removed(reflux)

g. Distillate product flow rate

35. What is the different ways in product composition control in distillation column?

The different ways in product composition control in distillation column are feed forward

control, feed back control, and inferential control.

UNIT V – FINAL CONTROL ELEMENT

1. What is the function of valve positioner?

The valve positioner are use to minimize the effect of lag in large-capacity actuators, stem

friction due to tight stuffing boxes, friction due to viscous or gummy fluids, process line in

pressure changes.

2. What is the function of an actuator? What are the different types of actuators?

An Actuator is used to translate the output signal of the automatic controller into a position of

a member exerting large power and often it is employed as a power amplifying mechanism.

Different types of actuators used in control valve are pneumatic actuators, hydraulic

actuators, electro-pneumatic actuators, and electric motor actuators.



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3. What are the advantages and disadvantages of pneumatic actuator over other

actuators?

The pneumatic actuator is used in wide range of pressure. The pneumatic signal is easily

available which can transmit quite long distance without and transmission losses. No wear

and tear problem is needed as in hydraulic actuators. The main drawback in pneumatic

actuators is it requires signal conversion when the process is automated. This type of

actuators is dependable and difficult in construction.

4. Define range-ability of a control valve.

The range ability of a control valve is the ratio of maximum controllable flow to minimum

controllable flow.

5. What are the advantages and disadvantages of rotary type motion valves over linear

stem motion type valves?

The rotary type stem motion valve is providing high capacity flow with minimum pressure

drop. They are used to handle slurries or fibrous materials. They require minimum space for

installation and they are used in low pressure services. The rotating type valves have low

leakage tendency and the range ability is limited.

6. What are the different types of process parameters to be considered in selection of

control valves?

Different types of process parameters to be considered in selection of control valves are the

pressure drop across the value, range ability, flow rate coefficient, control valve size and etc.

7. What are the different types of factors to be considered in control valve sizing?

The proper sizing of the control valve is important because of the effect on the operation of

automatic controllers. if the control valve is oversize, for eg, the valve must operate at low

lift and the minimum controllable flow is too large. In addition, the lower part of the flow-lift

characteristics is most likely to be non-uniform in shape. On the other hand if the control

valve is undersize, the maximum flow desired for a process may not be provided.

8. Differentiate flashing and cavitations in a control valve.

In a control valve when the pressure at venacontracta goes below the vapour pressure and

also at the pressure is below the liquid vapour pressure. So the fluid enters the port as a liquid

& comes out as a vapour. This phenomenon is called Flashing. It occurs in a valve when the

pressure drop across the orifice first results in the pressure is being lowered to below the

liquid’s vapour pressure and then recovering to above vapour pressure. This

pressure recovery causes on implosion or collapse of the vapour bubbles formed at the

venacontracta. This Phenomenon is called Cavitation.

9. What are the different types of flow-characteristics of a control valve?

The flow lift characteristics of a control valve fall into three approximate categories

a. Decreasing sensitivity type

b. Linear type

c. Increasing sensitivity type.

10. What do you mean by Flashing?

In a control valve when the pressure at venacontracta goes below the vapour pressure and

also at the pressure is below the liquid vapour pressure. So the fluid enters the port as a liquid



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& comes out as a vapour. This phenomenon is called Flashing.

11. Suggest suitable control valves for use on slurries and fluids with suspended particles.

Gate valves, ball valves and Pinch valves are suitable for control of slurries and fluid with

solid particles

12. When a Butterfly valve is used?

The butterfly valve is most often used in sizes from 4 to 60 inch for the control of air and gas.

It is also used for liquid flow if the pressure differential is not large.

13. Relate valve flow coefficient and liquid flow rate.

For control valve the flow rate is given by m = Ka 2g(h1 – h2 )

where m – flw rate ft3 / sec

K1 – a flow coefficient

a – area of control valve port, ft2.

g = acceleration sue to gravity, ft/ sec2.

h1 – upstream static head of flowing fluid, ft

h2 – downstream static head of flowing fluid, ft

14. Mention the two distinct characteristics of an equal percentage valve.

The equal %valve has increasing sensitivity and linear Characteristics. When the valve

pressure drop is small or when the process gain decreases with increasing flow this valve can

be used.

15. What is the function of the spring in a control valve?

The spring is used to bring back the actuator in static position. The spring develop Inertia and

static force which may use to get the force balance in control valve.

16. What are I/P and P/I converter? State the stand and valves for P and I in

instrumentation practice.

I/P and P/I converter are signal converters which are use to convert current to pneumatic and

vise versa in process system. The standard Pneumatic value is 3 – 15 psi and the current is 4

– 20 mA DC.

17. Why installed characteristics of control valve are different from inherent

characteristics?

The inherent characteristics of control valve are desired by the manufacture with standards,

but the installed characteristics are change when the control valve is in usage. Because the

control value may apply many applications its coefficient may not be same for all conditions.

18. What is meant by cavitation in a control valve?

It occurs in a valve when the pressure drop across the orifice first results in the

pressure being lowered to below the liquid’s vapour pressure and then recovering to

above vapour pressure. This pressure recovery causes on implosion or collapse of the vapour

bubbles formed at the venacontracta. This Phenomena is called Cavitation

19. Why is equal % valve mostly used in process industries?

The equal %valve has increasing sensitivity and linear Characteristics. When the valve

pressure drop is small or when the process gain decreases with increasing flow this valve can

be used.



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20. What is meant by cavitation and flashing in a control valve?

Flashing: In a control valve when the pressure at venacontracta goes below the liquid vapour

pressure the fluid enters the port as a liquid & comes out as a vapour.

Cavitation: It occurs in a valve when the pressure drop across the orifice first results in the

pressure is being lowered to below the liquid’s vapour pressure and then recovering to

above vapour pressure. This pressure recovery causes on implosion or collapse of the vapour

bubbles formed at the venacontracta.

21. What is self regulation?

It is the ability of a control valve to reach a specific value for nominal changes in the load

variable without any controller.

22. What are the different types of actuators used in control valve?

Different types of actuators used in control valve are pneumatic actuators, hydraulic

actuators, electro-pneumatic actuators, and electric motor actuators.

23. List the merits and demerits of using a positioner in a control valve?

Merits: Hysteresis is reduced and linearity is improved, Actuator can handle higher static

forces and speed of response is improved.

Demerits: Does not improve the ability of actuator to handle inertia or thrust forces. Requires

maintenance.

24. Derive the expression for flow through an equal percentage valve?

The expression for flow is, [ m / M] ={ R(x/X) - 1}

where, m = outlet flow rate, M = desired maximum outlet flow rate

R = rangeability , x / X = ratio of lift

25. What is “equal percentage” in the equal percentage valve?

The equal percentage characteristics is so named because for equal increments of stem travel

at constant pressure drop, an equal percentage change in existing flow occurs (i.e.) The same

% change increase in flow will occur when the stem position changes from 40% to 50% of

travel.

26. The upstream pressure of control value is 4kg/cm² and the downstream pressure

varies 51kg/cm². The port diameter of the control value is 5 cm. Find the actuator

force required?

. Given : pı =4kg/cm² , p2= .51 kg/cm² ,d= 5cm , a= π/4*d^2 ,

Ans: F=(4-.51)*π/4*8^2= 68.5kn

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EI2352 - Process Control - 2 Marks With Answers

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