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Subject Title: Chemical Process Instrumentation & Control Subject code: 17561
Page 1 of 30

Important Instructions to examiners:

1) The answers should be examined by key words and not as word-to-word as given in the
model answer scheme.
2) The model answer and the answer written by candidate may vary but the examiner may try
to assess the understanding level of the candidate.
3) The language errors such as grammatical, spelling errors should not be given more
Importance (Not applicable for subject English and Communication Skills.
4) While assessing figures, examiner may give credit for principal components indicated in the
figure. The figures drawn by candidate and model answer may vary. The examiner may give credit for
any equivalent figure drawn.
5) Credits may be given step wise for numerical problems. In some cases, the assumed constant
values may vary and there may be some difference in the candidate’s answers and model
answer.
6) In case of some questions credit may be given by judgement on part of examiner of relevant
answer based on candidate’s understanding.
7) For programming language papers, credit may be given to any other program based on
equivalent concept.
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Q No. Answer Marks
1a Attempt any THREE 12
1a-i Definition:
Static characteristics: Static characteristics are those that must be considered 1
when the instrument is used to measure a condition not varying with time.
Dynamic characteristics: Dynamic characteristics are those that must be
considered when the instrument is used to measure a condition varying with 1
time.
List of static characteristics (any four)
Calibration, accuracy, precision, repeatability, drift, sensitivity, resolution, dead ½ mark
zone, static error. each
1a-ii Principle of radiation pyrometer: 2
According to Stefan Boltzmann’s law, the intensity of radiant energy emitted
by a hot target varies as the fourth power of its absolute temperature.
Φb = σAT4
σ –Stefan Boltzmann constant.
T – Absolute temperature.
A - Area
Diagram of optical pyrometer:
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1a-iii Sight glass method for level measurement:

Sight glass instrument consists of graduated tube of toughened glass which is

connected to the exterior of the tank at the bottom. The liquid level in the sight
glass matches the level of liquid in the tank. As the liquid level in the tank rises
and falls, the liquid level in the sight glass also rises and falls accordingly. Thus
by measuring the level in the sight glass, the level of the liquid in the tank is
measured.
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Methods of level measurement of liquids (any four):

1.Float type level indicator method ½ mark
2. Hydro static methods each
3. Differential methods
4. Electrical methods
5. Radiation methods
6. ultrasonic methods
1a-iv Principle of ultrasonic flow meter : 2
Measurement of flow rate is determined by the variation in parameters of
ultrasonic oscillations. These devices measure flow by measuring the time
taken for ultrasonic wave to transverse a pipe section , both with and against the
flow of liquid within the pipe.
Explanation:
It consists of two transducers, A and B, inserted into a pipe line, and working
both as transmitter and receiver. The ultrasonic waves are transmitted from 2
transducer A to transducer B and vice versa. An electronic oscillator is
connected to supply ultrasonic waves alternately to A or B which is working as
transmitter through a change over switch, when the detector is connected
simultaneously to B or A which is working as receiver. The detector measure
the transit time from upstream to downstream transducer and vice versa.
1b Attempt any ONE 6
1b-i McLeod gauge:
Working:
To operate the gauge, the piston is first withdrawn, causing the level of mercury
in the lower part of the gauge to fall below the level of the junction between the
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two tubes. The unknown pressure source is connected to the gauge from where 3
it also flows and fills the bulb and capillary. Next, the piston is pushed in,
moving the mercury level up to block the junction. At this stage,, the fluid in
the capillary and the bulb is at pressure P. Further movement of the piston
compresses the fluid in the tube and the mercury level is raised till it reaches the
zero reference point in R. Measurement of the height above the mercury
column in the capillary allows the calculation of the compressed volume of the
fluid.
The expression for calculating the unknown pressure is
P = Aρgy2 / V
Where A is capillary area
ρ is density of fluid
y is height above the mercury column in capillary
Diagram:

3
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1b-ii Cascade control:

Block diagram

Description:
In a cascade control system, there is one manipulated variable and more than
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one measurement. It employs 2 feedback controllers, with the output of the

master (primary) controller changing the set point of the slave (or secondary)
controller. It eliminates the effect of disturbances and improves the dynamic
response of control loop. The feedback controller attempts to maintain the
process variable at its set point in response to all the disturbances and ensures
3
zero steady state offset for step like disturbances. Cascade control system
considers the likely disturbances and tune the control system to the disturbances
that strongly degrades the performance. It uses an additional secondary
measured process input variable that has the important characteristics of
indicating occurrence of the key disturbances.
2 Attempt any FOUR 16
2-a Comparison between open loop and closed loop control system. 1 mark
Sr Open loop control system Closed loop control system each for
No. any four
1 Feedback doesn’t exists Feedback exists points
2 Output measurement is not Output measurement is
necessary necessary
3 Any change in output has Changes in output affects the
no effect on input input
4 Error detector is absent Error detector is present
5 Inaccurate and unreliable Highly accurate and reliable
6 Highly sensitive to Less sensitive to disturbance
disturbance
7 Highly sensitive to Less sensitive to environmental
environmental changes changes
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8 Simple in construction and Complicated in construction and

cheap hence costly
9 Highly affected by non- Reduced effect of non-linearity
linearities
2-b C type Bourdon tube pressure gauge
Diagram

Working:
When the fluid under pressure enters the bourdon tube, its cross section tends to
become more and more circular that causes straightening of the tube. Since one
end of the tube is fixed, straightening of the tube causes the free end to deflect,
which is called as tip travel. The amount of tip travel for given rise in pressure
is a function of tube length, wall thickness, cross section and elastic modulus of 2
the tube material. Sector and pinion converts the amplified tip travel into
proportional rotary motion of the pointer connected to the pinion. The pointer
deflection can be read on the scale calibrated in terms of pressure.
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2-c Factors to be considered for the selection of control valve : 1 mark

each for
1. The first step in control valve selection involves collecting all relevant
any 4
data and completing the ISA Form S20.50. The piping size must be set
points
prior to valve sizing, and determining the supply pressure may require
specifying a pump
2. The size of the valve is required; select the smallest valve Cv that
satisfies the maximum Cv requirement at 90% opening. While
performing these calculations, checks should be made regarding
flashing, cavitation, sonic flow and Reynolds number to ensure that the
proper equation and correction factors are used. As many difficulties
occur due to oversized valves as to undersized valves. Adding lots of
“safety factors” will result in a valve that is nearly closed during normal
operation and has poor rangeability.
3. The trim characteristic is selected to provide good performance; goals
are usually linear control loop behavior along with acceptable
rangeability.
4. The valve body can be selected. The valve size is either equal to the
pipe size or slightly less, for example, a 3-inch pipe with a 2-inch globe
valve body. When the valve size is smaller than the process piping, an
inlet reducer and outlet expander are required to make connections to
the process piping.
5. The actuator is now selected to provide sufficient force to position the
stem and plug.

6. Finally, auxiliaries can be added to enhance performance. A booster

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can be increase the volume of the pneumatic signal for long pneumatic
lines and large actuators. A positioner can be applied for slow feedback
loops with large valves or valves with high actuator force or friction. A
hand wheel is needed if manual operation of the valve is expected.
2-d Elements of Computer Aided Process Control hardware:
Computer aided process control hardware consists of four basic parts or
subsystems.
1. Central Processing Unit(CPU)
2. Storage device 1
3. Input/ Output device
4. Bus interface
The Central Processing Unit (CPU) consists of control unit, arithmetic logic
unit (ALU), main memory( Primary storage) and general purpose registers.
Computer fetches data from primary memory under the command of control
unit. ALU performs arithmetic & logical operations on the data and transfers it
to primary storage. The processed data is further transferred to input/output 3
devices (I/O) as per the requirements of application program.
Storage: They are of three types-
1.Main storage or immediate access storage
2.Auxiliary or secondary memory
3.Cache memory
Input/output devices: It is the sub system through which the CPU
communicates with the outside world. The input-output (I/O) devices of process
control computers are divided into three types.
(1) Operator I/O devices: These are used to communicate with the operators
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(people). Process operators uses devices such as keyboards, push button,

switches etc to input data or command to the computer and receive
information from computer via devices such as VDU(Visual Display
Unit),LED (Light Emitting Diode), numerical display etc.
(2) Process I/O devices : These devices communicate between CPU and
plant devices such as sensors, limit switches etc for input and control
valves, motor starters etc for output, through ADC and DAC
subsystems.
(3) Computer I/O devices. These devices directly communicate with the
CPU for data and information exchange with the peripheral devices.
Bus interface: A bus is an electronic pathway in the computer that provides a
communication path for data flow between the CPU and its memory and
amongst the CPU and the various peripheral devices connected to the computer.
2-e Application of PLC:
1) PLC can be a vital part of industrial automation as it produces on/off
2
voltage outputs to actuate elements such as electric motors, solenoids
etc.
2) It can also be used in sequential controllers used for periodical on/off of
fans, heaters and light switches.
Application Of DCS:
1) DCS are designed for continuous process where the control signal is
2
analog rather than discrete.
2) It is a powerful integrated control system having capabilities such as,
data acquisition, advanced process control and batch control capabilities
for various industrial environments such as cement factory, oil refinery,
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power plant etc.

2-f Difference between single seated and double seated valve 1 mark
each
Single seated valve Double seated valve
1. Only one plug is present 1. Two plugs
2. Valve can be completely 2. It cannot be fully closed.
closed. Therefore flow can be Therefore flow cannot be
completely stopped. completely stopped.
3. Force required to operate the valve 3. Force required to move the valve
against the upward thrust is large. is comparatively less.
4. Suitable for small flow rates. 4. Suitable for large flow rates
3 Attempt any FOUR 16
3-a Resistance temperature detector:
Description:
The resistance of certain metals changes with temperature change. Resistance
thermometer utilizes this characteristic. With the increase of temperature, the
electrical resistances of certain metals increases in direct proportion to the rise 2
of temperature. Therefore, if the electrical resistance of a wire of a known and
calibrated material is measured, the temperature of the wire can be determined.
Platinum, copper and nickel are generally used in resistance thermometers.
Construction of practical resistance thermometer is shown in fig. The
resistance element is surrounded by a porcelain insulator which prevents short
circuits between wire & the metal sheath. Two leads are attached to each side of
the platinum wire. When this instrument is placed in a liquid or a gas medium
whose temperature is to be measured, the change in temperature causes the
platinum wire inside the sheath to heat or cool, resulting in a proportional
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change in the wire’s resistance. This change in resistance can be directly

calibrated to indicate the temperature.

3-b Principle of air purge system: 4

When liquid is held in a tank, then it exerts equal pressure on the walls of the
tank. Such a pressure is due to the weight of liquid present above a certain
reference point or base and is called hydrostatic head or pressure.
Air purge system works on measuring the pressure required to force a gas into a
liquid at a point beneath the surface. This method uses a source of clean gas or
air and is connected through a restriction to a bubble tube immersed at a fixed
depth in to the vessel.
3-c Construction and working of bellows
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Bellows are made of an alloy (phosphor bronze) which is ductile, has high
strength and retains its properties over long use, i.e has very little hysteresis
effect. In the above figure pressure is applied to one side of the bellow and the
resulting deflection is counter balance by a spring. This arrangement indicates
the gauge pressure. Spring opposed bellow elements are very sensitive and are
quite useful in working signaling and tripping devices because of the
considerable amount of movement for a given change in pressure. It is made of
metallic bellows enclosed in a shell which is connected to pressure source.
Pressure acting on the outside of the bellow compresses the bellows and moves
its free end against the opposing force of the spring. A rod resting on the bellow
transmits the motion to a pointer.
3-d Construction and working of thermal flow meter:
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4
It consists of an electric immersion heater for the heating of flowing fluid. Two
thermocouples (or resistance thermometers) T1 and T2 are placed at each side
of the heater. The thermocouple T1 measures the temperature of fluid before it
is heated, while the thermocouple T2 measures the temperature so after. The
power supply to the heater equals the heat transferred to the fluid, i.e. Q, and is
measured by a wattmeter. Thus by measuring the values of Q, T1 and T2 the
flow rate W of liquid is determined from the equation
W=Q/Cp(T2-T1)
Where
Q=heat transfer
W= mass flow rate of fluid
Cp= specific heat of fluid
T1=initial temperature of the fluid after heat has been transferred
T2=final temperature after heating the fluid.
3-e Comparison between P,PI,PD and PID controller (4points): 1 mark
each
P PI PD PID
1.High maximum High maximum smallest A compromise
deviation deviation maximum between PI and
deviation PD
2.Moderate long period of Smallest A compromise
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period of oscillation oscillation between PI and

oscillation PD
3.Maximum No offset Offset smaller A compromise
offset than P type. between PI and
PD
4. Considerable Large time Shortest time
time required for required for required.
oscillation to oscillation to
stop. stop compared to
P type..
5.Mathematical Mathematical Mathematical Mathematical
expression expression expression expression
m= Kpe

4a Attempt any THREE 12

4a-i Thermocouple
Principle:
Seebeck effect: Seebeck discovered that when there is temperature difference
between two junctions of thermocouple ,an emf is developed between the
2
junctions. This emf causes electric current to flow through thermocouple
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circuit. This is called thermo electric effect by which thermal energy is

converted to electrical energy.
Peltier effect: It is defined as the change in heat content when 1 coulomb of
charge crosses the junction.
Diagram

4a-ii Bimetallic thermometer

Principle:
When heated different solids expand differently depending on their 1
coefficient of thermal expansion.
Diagram

2
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Working:
Bimetallic strip consists of twobstrips of metal such as invar and brass
welded together, each strip made from a metal having a different coefficient
of thermal expansion. Whenever the welded strip is heated, the two metals
change length in accordance with their individual rates of thermal expansion. 1
The two metals expand to different lengths as the temperature rises. This
forces the bimetallic strip to bend towards the side with low coefficient of
thermal expansion as shown in Fig above. If one end of the bimetallic strip is
fixed so that it cannot move, the distance the other end bends is directly
proportional to the square of the length of the metal strip, as well as to the
total change in temperature, and is inversely proportional to the thickness of
the metal. The movement of the bimetallic strip is utilized to deflect a pointer
over a calibrated scale.
4a-iii Difference between variable head meter and variable area meter: 1 mark
(any four ) each

Variable head meter Variable area meter

1. Area of flow is constant and Pressure drop is constant and area of
pressure drop varies with flow rate flow varies with flow rate
2. Cannot give volumetric flow rate Can give volumetric flow rate
directly directly
3. Relatively cheap costly
4. Simple in construction complex
5. Need straight pipe before and after Does not need
the meter
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4a-iv Principle of positive displacement meter

As the liquid flows through the meter, it separates the flow of liquid into
separate known volumetric increments which are counted and totaled. The sum 1
of the increments gives the measurement of the total volume of liquid passed
through the meter.
OR
These meters have two chambers of known volumetric capacity and they are
arranged so that when one chamber is being filled, the other is being emptied.
For measuring the total flow over a certain period, the fluid is continuously
filled and emptied from the chamber and then the number of times the chamber
is being filled and emptied in that period is counted which when multiplied by
the volumetric capacity of the chamber gives the total flow.
Advantages of rotating vane meter (any two)
1. It allows low pressure loss
2. It has relatively high temperature and pressure rating 2
3. It has a good accuracy
4. It is available in numerous construction material
Application (any one):
1. It is most widely used in the petroleum industry and is used for such
services as gasoline and crude oil metering. 1
2. Cooling water monitoring
3. General mechanical engineering
4. Waste water treatment
5. All heavy goods industry
6. Chemical industry
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4b Attempt any ONE 6

4b-i Types of control valve:
1. Based on number of plugs:
Control valves can be classified as single seated valve and double seated
valve
2. Based on action: 4
Control valves operated through pneumatic actuators can be either air to
open or air to close
3. Based on flow characteristics
Control valves can be classified as quick opening valve, linear opening
valve , equal percentage valve
4. Based on construction
1. Angle Valve
2. Globe valve
3. Diaphragm Valve
4. Butterfly valve
5. Rotary valve
6. Ball valve
7. Sliding cylinder valve
Function of valve actuator: it is that portion of the valve that responds to the
applied signal and results in the movement of the stem due to which the flow 2
rate of fluid changes. It consists of diaphragm, stem and diaphragm returning
spring

4b-ii PLC architecture:

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Diagram

OR 3

Explanation:
PLCs are industrially hardened microcomputers that perform discrete or
continuous control functions in a variety of processing plant and factory
environments.
A PLC architecture consists of the following main units.
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1. Power supply: Power supply unit converts power line voltages to those
required by the solid state components.
2. Input / Output system: Inputs are real world signals of sensors. These
signals can be Analog or Digital, low or high frequency, continuous or
momentary. Outputs can be of discrete, register or analog.
3. Central Processing Unit (CPU): It performs the tasks necessary to fulfill
the PLC functions such as scanning, I/O bus traffic control, program execution,
peripheral and external device communications, and data handling and self-
diagnostics.
4. Memory Unit: This is the library where the application program, input data, 3
as well as output data are being stored.
5. Programmer Unit: Programmer unit provides an interface between the PLC
and user during program development, start-up and trouble shooting.
5 Attempt any FOUR 16
5-a Working of turbine flow meter: 4
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The flow of liquid past the wheel causes the wheel to rotate at a rate which is
proportional to the velocity of the fluid. This is achieved by fabricating the
turbine blades from a ferromagnetic material and placing a permanent magnet
and coil inside the meter housing. A voltage pulse is induced in the coil as each
blade on the turbine wheel moves past it and these pulses are measured by a
pulse counter.
5-b Nuclear radiation method (or) radiation method 1
Explanation:

It consists of a radioactive source such as minute quantity of capsulated

radioactive isotope like cobalt60 fixed either inside or outside the vessel, 3
radiation receiving element fixed to the side of the vessel directly across the
source along with the indicator. As the liquid level inside the vessel changes,
the amount and intensity of radioactive radiations received by the receiver
changes. Larger the level of liquid inside the vessel, smaller is the intensity of
radiation and vice versa.
5-c Working of capacitance level indicator 4
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It consists of two conductors separated from each other by dielectric material

between them. There is an insulated capacitance probe fixed near and parallel to
tank wall such that the probe and metal tank wall acts as conductors with
conducting liquid as the dielectric medium. These two conductors are
connected to capacitance detecting element. As the liqid level changes, the
dielectric constant changes due to which capacitance changes. Thus any change
in liquid level can be measured in terms of change in capacitance.
5-d Working of LVDT 4
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When the pressure inside the bellows changes, its free end gets deflected along
with the movable core. When the core is symmetrically positioned between the
two secondary coils, the magnetic coupling of the core with both the secondary
coils is equal. In this position, equal but opposite emfs are developed in the coil,
and hence the net voltage between two secondary coils is zero. When core takes
any other position, the magnetic coupling with each secondary coil is different,
that induces different voltages in the secondary coils. Hence some unbalance
voltage is produced between the coils that depends upon the position of the core
which in turn depends upon the pressure fed inside the bellows.

5-e 1atm= 1 bar= 105 Pa= 760mm of Hg = 1 kgf/ cm2

i) 20atm= 2 x106pa 1
ii) 20atm =20bar 1
iii) 20atm =15200 mm of Hg 1
iv) 20atm = 20Kgf/cm2 1
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6 Attempt any TWO 16

6-a Working of pneumatic PID controller.

It consists of a nozzle flapper assembly and a relay. As the input error increases
baffle is moved towards the nozzle increasing the control output through the
relay. This change in output pressure is applied to the bellows further closing
the nozzle and increasing the output to the maximum. The nozzle back pressure
iscontrolled by the nozzle flapper distance. A derivative restriction is
introduced into the line leading to the feedback bellows. The addition of an 5
integral (reset) bellows and the addition of an adjustable restriction (integral
restriction) calibrated in time units, provide reset or integral control action.
Reset or integral action increases the gain of the controller.
Greater the restriction imposed upon the flow of air to the feedback bellows,
greater will be the pressure drop across the restriction and greater will be the
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increase of pressure due to derivative action. The rate at which integral action is
applied depends on the rate at which air flows through the integral restriction.
By causing both positive and negative feedback to lag the output pressure, both
rate and reset action may be obtained which is known as PID control action.
6-b Valve Characteristics:
The relation between stem position, plug position and rate of flow is described
in terms of flow characteristics of valve. Two types of valve characteristics are
there –Inherent and Installed or effective.

Inherent flow characteristics are plotted when constant pressure drop is

maintained across the valve. There are two different inherent flow
characteristics- linear and equal percent.
Linear Opening characteristics: Linear characteristics valve has linear
relation between valve opening and flow rate at constant pressure drop
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WINTER-17 EXAMINATION
Model Answer
_____________________________________________________________________________________________________
Subject Title: Chemical Process Instrumentation & Control Subject code: 17561
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Q = by
Q- Flow rate at constant pressure drop
b - constant 2
y - valve opening / valve stem travel
Generally used
• For slow process
• When more than 40% of the system pressure drop occurs across the
valve.
Equal Percentage characteristics : In equal percentage valve equal increment
of the stem travels give equal % change of the existing flow
Q = beay
Q= Flow rate at constant pressure drop
a& b = constant
e = base of natural logarithms 2
y = valve opening / valve stem travel
Generally used
• For fast processes
• When high rangeability is required
At heat exchangers where an increase in product rate requires much
greater increase in heating and cooling medium.
Installed flow characteristics are plotted when the differential pressure across
the valve changes. 2
Quick opening – In this there is maximum flow for minimum travel
It is approximately linear when the flow rate is less but beyond 30% the
flow increases rapidly with valve opening
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WINTER-17 EXAMINATION
Model Answer
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Subject Title: Chemical Process Instrumentation & Control Subject code: 17561
Page 29 of 30

It gives approximately 90% flow at 30% travel

• For on – off control
When maximum valve capacity must be obtained quickly
6-c Distributed control system:
Block diagram:

Explanation:
In DCS equipment is separated in functional area and is installed in different
work areas of a process plant. The plant operator monitors and manipulates the
set-points of the process parameter from central control room.
4
Controlling portion of the DCS, distributed at various location performs
following two function at each location.
1. Measurement of analog variable and discrete inputs
2. Generation of output signals to actuators that can change process condition
In Figure above the operator console in the conrol room is connected through a
data highway to several distributed system components.
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WINTER-17 EXAMINATION
Model Answer
_____________________________________________________________________________________________________
Subject Title: Chemical Process Instrumentation & Control Subject code: 17561
Page 30 of 30

A DCS consist of the following modules:

1 Operator stations that use microprocessor based CRT display and keyboard
communication with control device and displays
2 Remote multifunction microprocessor based controllers (PLCs)
3 A digital data link (data highway) that connects the multifunction
controllers with the central operator stations.
The first priority of DCS is to provide operator interfacing and real time process
control. DCS has flexibility of implementation of sequential control and
integration among the various types of control.