Class 1 need for process control & process terminology

ICE401: PROCESS INSTRUMENTATION

AND CONTROL

Class 1: Need for Process Control & Process Terminology

Dr. S. MeenatchisundaramEmail: [email protected]

Process Instrumentation and Control (ICE 401)

Dr. S.Meenatchisundaram, MIT, Manipal, Jan – May 2015

Introduction:

Process control is a mixture between the statistics and engineeringdiscipline that deals with the mechanism, architectures, andalgorithms for controlling a process. Some examples of controlledprocesses are:

� Controlling the temperature of a water stream by controlling theamount of steam added to the shell of a heat exchanger.

� Operating a jacketed reactor isothermally by controlling themixture of cold water and steam that flows through the jacket of ajacketed reactor.

� Maintaining a set ratio of reactants to be added to a reactor bycontrolling their flow rates.

� Controlling the height of fluid in a tank to ensure that it does notoverflow.



Process Control Background:

� The traditional role of process control in industrial operations wasto contribute to safety, minimized environmental impact, andoptimize processes by maintaining process variable near thedesired values.

� Generally, anything that requires continuous monitoring of anoperation involve the role of a process engineer.

� In years past the monitoring of these processes was done at theunit and were maintained locally by operator and engineers.

� Today many chemical / process plants have gone to fullautomation, which means that engineers and operators are helpedby DCS that communicates with the instruments in the field.



Benefits of Process Control:

� The benefits of controlling or automating process are in a numberof distinct area in the operation of a unit or chemical plant.

� Safety of workers and the community around a plant is probablyconcern number one or should be for most engineers as theybegin to design their processes.

� Chemical plants have a great potential to do severe damage ifsomething goes wrong and it is inherent the setup of processcontrol to set boundaries on specific unit so that they don’t injureor kill workers or individuals in the community.



Objectives of Control:

1. Maintain the process at the operational conditions and setpoints:

� Many processes should work at steady state conditions or in astate in which it satisfies all the benefits for a company such asbudget, yield, safety, and other quality objectives.

� In many real-life situations, a process may not always remainstatic under these conditions and therefore can cause substantiallosses to the process.

� One of the ways a process can wander away from theseconditions is by the system becoming unstable, meaning processvariables oscillate from its physical boundaries over a limited timespan.



Objectives of Control:

� An example of this would be a water tank in a heating and coolingprocess without any drainage and is being constantly filled withwater.

� The water level in the tank will continue to rise and eventuallyoverflow.

� This uncontrolled system can be controlled simply by addingcontrol valves and level sensors in the tank that can tell theengineer or technician the level of water in the tank.

� Another way a process can stray away from steady stateconditions can be due to various changes in the environmentalconditions, such as composition of a feed, temperature conditions,or flow rate.



Objectives of Control:2. Transition the process from one operational condition toanother:

� In real-life situations, engineers may change the processoperational conditions for a variety of different reasons, such ascustomer specifications or environment specifications.

� Although, transitioning a process from one operational condition toanother can be detrimental to a process, it also can be beneficialdepending on the company and consumer demands.

� Examples of why a process may be moved from one operational

set point to another:

1. Economics, 2. Product specifications, 3. Operationalconstraints, 4. Environmental regulations, 5. Consumer /Customer specifications, 6. Environmental regulations, 7. Safetyprecautions.



Definitions and Terminology:

In controlling a process there exist two types of classes of variables.

1. Input Variable – This variable shows the effect of thesurroundings on the process. It normally refers to those factorsthat influence the process. An example of this would be the flowrate of the steam through a heat exchanger that would change theamount of energy put into the process. There are effects of thesurrounding that are controllable and some that are not. These arebroken down into two types of inputs.

a. Manipulated inputs: variable in the surroundings can becontrolled by an operator or by a control system in place.

b. Disturbances: inputs that can not be controlled by an operatoror control system. There exist both measurable andimmeasurable disturbances.




2. Output variable- Also known as the control variable. These arethe variables that are process outputs that effect the surroundings.An example of this would be the amount of CO2 gas that comesout of a combustion reaction. These variables may or may not bemeasured.

As we consider a controls problem. We are able to look at twomajor control structures.

1. Single input-Single Output (SISO) - for one control (output)variable there exist one manipulate (input) variable that is used toaffect the process.

2. Multiple input-multiple output (MIMO) - There are severalcontrol (output) variable that are affected by several manipulated(input) variables used in a given process.




Cascade: A control system with 2 or more controllers, a "Master" and"Slave" loop. The output of the "Master" controller is the setpointfor the "Slave" controller.

Dead Time: The amount of time it takes for a process to startchanging after a disturbance in the system.

Derivative Control: The "D" part of a PID controller. With derivativeaction the controller output is proportional to the rate of change ofthe process variable or error.

Error: In process controls, error is defined as: Error = setpoint -process variable.

Integral Control: The "I" part of a PID controller. With integral actionthe controller output is proportional to the amount and duration ofthe error signal.




PID Controller: PID controllers are designed to eliminate the needfor continuous operator attention. They are used to automaticallyadjust system variables to reduce error and hold a processvariable close to the setpoint.

Error is defined above as the difference between setpoint andprocess variable.

Proportional Control: The "P" part of a PID controller. Withproportional action the controller output is proportional to theamount of the error signal.

Setpoint: The setpoint is where you would like a controlled processvariable to be.



Engineering

Class 1 need for process control & process terminology