PROCESS CONTROL IT62

1
PROCESS CONTROL (IT62)

• Syllabus
• Chapter 1 Controller Principles
• Chapter 2 Analog Controllers
• Chapter 3 Control Loop Characteristics
• Chapter 4 Controllers, Converters, Transmitters
  Relays
• Chapter 5 Process ID Symbols and Drawings

2

• Textbooks
• Process Control Inst. Technology (7th Edn) by
  C.D.Johnson, Pearson Education Pub.2003. For
  Chapter 1, 2 and 3
• Instrument Engineers Handbook Vol. 1-Process
  Measurement, Vol. 2-Process Control, by Bela J.
  Liptak, Chilton Book Company. For Chapter 4 5

3
COURSE OVERVIEW

• The course provides a fundamental knowledge to
  instrumentation engineer regarding the
  implementation of process control techniques in
  the process industries.
• The course also provides a basic knowledge about
  process instrumentation drawing symbols and
  diagrams.

4
COURSE OBJECTIVE

• To make instrumentation technology students to
  become familiar with process control techniques
  tools.
• At the end of this course you will be in a
  position to measure, acquire, control tune the
  process parameters in process industries.

5
INTRODUCTION TO PROCESS CONTROL

• Process
• In general, process constitutes a sequence of
  events in which a raw material will be converted
  into finished product.
• Any system composed of dynamic variables,
  usually involved in manufacturing production
  operations.

6

• Control
• The methods/techniques to force parameters or
  variables in the environment / process to have
  specific values.
• Control System
• All the elements necessary to accomplish the
  control objective i.e. regulation of some
  parameters to have specific or desired values

7

• Process Control
• It deals with the elements and methods of
  control system operations used in industry to
  control industrial processes.

8
PROCESS CONTROL PRINCIPLE

• Objective of process control is to regulate the
  value of some quantity.
• Regulation It means to maintain a quantity or
  variable at some desired value regardless of
  external influences.

9
Process An Example-Level of Liquid in a tank

• Qout gt Qin Level drops
• Qout lt Qin Level rises

10
Human Aided Control

• Definition
• Example Control of liquid level in a tank

11
Automatic Control System

• Definition
• Example Control of liquid level in a tank

12
Some Definitions Related to Process Control

• Controlled Variable The process variable
  regulated by process control loop.
• Controlling Variable The process variable
  changed by the final control element under the
  command of controller to effect regulation of
  controlled variable.
• Setpoint The desired value of a controlled
  variable in process control loop.

13

• Error The algebraic difference between the
  measured value of variable and setpoint.
• Controller The element in a process control loop
  that evaluates error of the controlled variable
  and initiates corrective action by a signal to
  controlling variable.
• Dynamic variable The process variable that can
  change from moment to moment because of unknown
  sources.

14
ELEMENTS OF PROCESS CONTROL

• Process
• Measurement
• Error Detector
• Controller
• Control Element

15
PROCESS CONTROL BLOCK DIAGRAM
p Controller output c Controlled variable b
measured value of c e Error signal r Set
point u Controlling variable Fig. Block
diagram of basic process control loop.
16
Example
Fig. Physical diagram for flow process control
system
17
Final Control Element
I/P Converter, Actuator, Valve
Controller
(Error Detector)
(Controller)
Pipe Flow
r
Process
Orifice plate, DP Cell, P/I Converter
Measurement
Fig. Block diagram for flow process control
system.
18
ANALOG DATA REPRESENTATION

• Analog Standards for representing range of
  variable in Process Control Systems
• Current Signal 4 to 20 mA
• Pneumatic Signal 3 to 15 psi (20 to 100kPa)

19
Why Current Signal?

• Current is used instead of voltage because the
  system is then less dependent on load.

Fig. Transmission of current signal is
independent of line resistance
20
FINAL CONTROL OPERATION
Fig. Elements of Final Control Operation
21
Chapter-1CONTROLLER PRINCIPLES
22
OBJECTIVES

• Define understand the process characteristics,
  process system parameters
• Describe the discontinuous and continuous
  controller modes
• Describe the composite controller modes
• Compare and differentiate discontinuous and
  continuous controller modes, and their advantages
  applications.

23
PROCESS CHARACTERISTICS

• Selection of controller modes is a function of
  process characteristics.
• Important Process Characteristics
• Process Equation
• Process Load
• Process Lag
• Self Regulation

24
Example

• Process Parameters
• TL Liquid temperature (controlled variable)
• QA Input flow rate
• QB Output flow rate
• TA Ambient Temp.
• TO Inlet temperature of liquid
• TS Steam Temp.
• QS Steam flow rate (Controlling variable)

Fig. Control of liquid temperature in a tank by
process control
25
1. Process Equation

• The process equation is a function which
  describes the process and provides the
  information about other process parameters which
  influence the controlled variable.
• For liquid temperature control the process
  equation is
• TL f (QA, QB, QS, TA, TS, TO)

26
2. Process Load

• The process load refers to set of all process
  parameters excluding the controlled variable.
• Nominal Set
• Nominal Load
• Nominal value
• Process Load Change
• Transient Load Change Example

27
3. Process Lag

• Process lag refers to the time consumed by the
  process itself to bring the controlled variable
  to setpoint value during load change. Example

4. Self-regulation

• Some processes adopt to stable value without
  being regulated via process control loop.