Instrumentation Engineering : Process control, THE GATE ACADEMY

PROCESS CONTROL

For

Instrumentation Engineering

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Syllabus Process Control

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Syllabus for Process Control

Feedback principles. Signal flow graphs. Transient Response, steady state-errors. Routh and

Nyquist criteria. Bode plot, root loci. Time delay systems. Phase and gain margin. State space

representation of systems. Mechanical, hydraulic and pneumatic system components. Synchro

pair, servo and step motors. On-off, cascade, P, P-I, P-I-D, feed forward and derivative controller,

Fuzzy controllers.

Analysis of GATE Papers

(Process Control)

Year Percentage of marks Overall Percentage

2013 4.00

3.24%

2012 0.00

2011 0.00

2010 2.00

2009 5.00

2008 4.00

2007 4.00

2006 3.33

2005 4.66

2004 2.66

2003 6.00

Contents Process Control

THE GATE ACADEMY PVT.LTD. H.O.: #74, Keshava Krupa (third Floor), 30th Cross, 10th Main, Jayanagar 4th Block, Bangalore-11 : 080-65700750, [email protected] © Copyright reserved. Web: www.thegateacademy.com Page 1

CC OO NN TT EE NN TT SS

Chapters Page No. #1. Introduction, Building Block’s 1 – 25

Process Characteristics 1 – 2

Terminology 3 – 12

Dynamics of Simple System 12 – 16

Interacting and Non-Interacting System 16 – 19

Process Degree of Freedom 19 – 20

Assigment 21 – 22

Answer Keys 23

Explanations 23 – 25

#2. Modes of Controller Operation 26 – 69 Control Parameter Range 26 – 27

Reverse and Direct Action of Controllers 27

Modes of Control Actions 27 - 30

Compostie controller modes 30 – 58

Valves 58 – 61

Assignment 62 – 64

Answer Keys 65


#3. Tuning of Controllers 70 – 83 Process Reaction Curve Method 70 – 72

Quarter Amplitude Criteria 73

Ziegler-Nicols Tuning Method 73 – 74

Controller Tuning From Routh Array Method 74 – 76


Answer Keys 80


#4. Process Control System Configurations 84 – 92 Feed forward and Feed Back control 84

Feed Forward Control 85

Cascade Control Configuration 86 – 87

Ratio Control Configuration 87 – 88


Answer Keys 91


Contents Process Control


#Module Test 93 – 117

Test Questions 93 – 105

Answer Keys 106


#Reference Books 118

Chapter-1 Process Control


CHAPTER 1

Introduction, Building blocks of Process control

Introduction

Instrumentation may be defined as an art of using instruments to measure the physical and chemical properties and changes of matter.

Since it is not possible to measure these changes directly, certain variable like temperature, humidity, level etc are measured which effects these changes of properties of matter.

Thus instrumentation mainly deals with measurement and control of these variables The team “process” refers to physical or chemical change of matter or an ensemble of

environment where a particular variable is measured and controlled. “Process Control” refers to regulation and manipulation of variables influencing the conduct

of a process in such a way as to obtain a product of desired quality and quantity in an efficient manner.

Process Characteristics

To understand the characteristics of process control system, consider a simple problem where the objective is to maintain the level of the liquid in the tank at a height ‘h’ (as shown in fig 1.1) irrespective of disturbance that tend to deviate it from desired level.

Figure 1.1

h

(Control Value )

Level measuring instrument

Human

operator

(Outlet)

(Inlet )



The tank has inlet flow-rate q and Outlet flow rate q which is controlled by a control value. To achieve the objective or to convert it into a control system, a human operator is placed

near the control valve as shown. The operator has desired level in his mind also known as set point. He constantly monitors

the level measuring instrument. If the level is above the desired level (set point) he performs an operation on control value

by opening it till the level reaches the set point and fixes the control value at this position which makes outlet flow rate equal to inlet flow rate.

Similarly, if level goes down the desired level, he performs an operation on control value by closing it.

Thus, the control system shown in fig 1.1 is manual control system. This can be converted in to an “automatic control system” by: Replacing human operator

with AUTOMATIC CONTROLLER as shown in fig 1.2.

Automatic Level Control System

Figure 1.2

Block Diagram of A Control System

Figure 1.3

Measurement

Automatic

Controller

Final Control

Element

Process

+ -

Controller Block

Output

Set Point

LT

h

Final Control Element

LT – level transmitter

(Inlet )

Automatic

Controller

Set-Point



Terminology

1. Process: As defined earlier, it is an environment where a particular variable is measured, monitored and controlled.

2. Process Variables: They are (a) Controlled variable (or) dynamic variable. (b) Manipulating variables (c) Load variables (d) flow variables (e)across variables

2(a). Controlled (or) dynamic variable: It is the variable which is measured, monitored and controlled.

Ex : Level in the tank is an example of controlled variable.

2(b). Manipulated variable: It is the variable which is manipulated to make the controlled variable remain at set point value.

Ex: In the above level control system, the output flow-rate is the manipulated variable.

2(c). Load variable: It refers to all the other variables that affect the controlled variable to deviate from set point value.

Ex: In the above example, changes in inlet flow rate also change the level in the tank, so it constitutes to load variable.

2(d). Flow Variable: Any variable which is measured along the process loop.

Ex:- Flow in a loop, current in a loop, heat transfer.

2(e). Across Variable: Any variable which is always measured as a differential value or with

reference to some point.

Ex:- Voltage (potential difference), Differential pressure, Temperature, etc.

Process Flow Variable Across Variable Relationship

Electrical N/w I Current V Voltage v = i ∙ R R = electrical Resistance

Flow Process Q Fluid flow rate

h Pressure head

h = q ∙ R R = Flow Resistance

Useful Conversations

Pressure 14.7 psi = 1 atm

1 N/ = 1 Pa (=1 Pascal)

Pa = 1 bar



1 Psi = 0.07 bar

1 gallon = 3.78 liters 3.8 liters

Process Load

The set of all load variables that cause a change or deviation of controlled variable from set point are known as process load.

Process Equation

For the example in Fig1.2, process equation can be written, which describes the process mathematically. Here the liquid level ‘h’ is a function of changes in inlet flow rate and outlet flow rate. A simple process equation is

‘ ’ = (

Measurement

It is the determination of magnitude of a variable i.e. controlled variable.

Controller

It is an automatic device having an output that varies to regulate the controlled variable. A controller performs operations such as comparison, evaluation and operation upon the next stage to meet the demands and control objectives.

Final Control Element

The device that directly controls the value of manipulated variables in a process control loop is the final control element.

Often the control element is control value.

Elements of Process Control

The different elements with which a mathematical model may be formulated for a process are

A. Resistance element. B. Capacitance element. C. Time constant element. D. Dead time element.

The mathematical model thus obtained is known as Transfer function. A transfer function is defined as ratio of L T of output to L. T of input under the assumption

that all initial conditions are set to zero. [L.T Laplace Transform]

i.e T(s = (

( | (

o p i p

T Transfer function



(A) Resistance Type Element Resistance may be defined as any obstruction or constriction which can be

incorporated by external means also, (or) the ability to resist the transfer of mass or energy is called resistance.

Example: Consider the operation of capillary flow system shown in fig 1.4. Two capillary tubes are inserted at the inlet and outlet sections of the pipeline where the fluid is flowing.

The difference in the head or height (h) between the rise of fluid in both capillary tubes is known as “Resistance” effect.

Figure 1.4

Resistance = cross Variable

Flow Variable

The flow-head equation for fig 1.4 may be written as: (head) h ∝ q ( flow-rate)

∴ h = R q ------------- (

Thus if q = input and h =output (1) represents a linear relation between input and output. Applying L.T to equation (1)

H(s)=R Q (s)

Electrical resistance, thermal resistance, in mechanical or rotational systems, the spring elements etc are all other resistance elements.

(B) Capacitance Element Capacitance is the ability to store.

Example is a tank with inlet as show in fig (1.5)

R Q(s) H(s)

h

Capillarly (1) Capillarly (2)

Fluid

flow rate

(q)

Pipeline



Figure 1.5

The flow of fluid into the tank is the input and the level of the liquid in the tank (h) is the output.

The change in the level of liquid in the tank (

) is directly propositional to inlet flow rate

i. e dh

dt ∝ q

c

= q ------ (2)

C = capacitance Applying L.T to equation (2) CsH(s) = Q (s)

∴H(s

(s =

Cs

Hence the transfer function representation of capacitance elements is

Response of capacitance elements is obtained by integration of equation (2)

∫

d = ∫

= (

)t.

(

)

(h)

t

Q(S) H(S)

h q

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