Process Control Instrumentation II Module 1

Module 1 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 process – self-regulation – servo and regulator operation. 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.

Introduction PROCESS: An operation or series of operations on fluid or solid materials during which the materials are placed in a more useful state. The objective of a process is to convert certain raw materials into desired products using available sources of energy in the most economical way. CONTROL: means methods to force parameters in the environment to have specific values.

PROCESS may be controlled by measuring a variable representing the desired state of the product and automatically adjusting one of the other variables of the process. A desired quantity is kept at set point irrespective of external influences. AUTOMATIC PROCESS CONTROL is the maintenance of a desired value of a quantity or conditions by measuring the existing value, comparing it to the desired value and employing the difference to initiate action for reducing this difference. This requires a feedback control system which does not require human aid.

Areas - Process Control Processing industries such as petroleum, chemical, steel, power and food for the control of assembly operations, work flow, heat treating and similar variables. Goods manufacturers such as automobile parts, refrigerators and electronic equipments like television sets, radio etc. for the control of assembly operations, work flow, heat treating and similar variables. Transportation systems such as railways, airplanes, free missiles and ships. Power machines such as machine tools, compressors and pumps, prime movers, and electric power supply units for the control of position, speed and power.

Need for Process Control Increase in productivity (increase in quantity or number of products): helps to increase the efficiency of both man and machine. Improvement in quality of products by meeting the product specifications overcoming operational constraints. Improvement in the consistency of product dimensions, performance and length of service. Economical improvement by way of savings in processing raw materials, savings in energy, effective utilization of capital and human labour etc. Minimize/ suppress the influence of external disturbances on the process. Ensure the stability of the process. Optimize the performance of the process. Meet environmental regulations.

Batch & Continuous Processes Batch Process: A process in which the materials or work are stationary at one physical location while being treated. Eg:

Batch Process Advantages Disadvantages A variety of different products can be made using the plant. Slow reactions can be carried out. Can use reactants in any state including solids. Disadvantages Risk of contamination if more than one than one product made in reaction vessel. Expensive down time while reactor is being filled and emptied. Larger workforce required. Can be difficult to control highly exothermic reactions.

Batch & Continuous Processes Continuous Process: A process in which the materials or work flows more or less continuously through a plant apparatus while being manufactured or treated. Eg: Almost all chemical plant processes

Continuous Process Advantages Disadvantages More cost effective if large quantities of the chemical are being made. No expensive ‘down time’ when plant is not being used. Automated process requires less labour. Disadvantages High capital cost of setting up the plant Costs rise if plant not operated continuously.

FACTOR CONTINUOUS BATCH Cost of factory equipment High Low Rate of production High Low Shut-down times Rare Often Workforce Few people Many people needed needed Ease of automation Relatively Relatively easy difficult

Self Regulating Systems Some systems have the capability that is designed to produce continuous balance.

Refer: Krishnaswamy/ Stephanopoulos CSTR – Self Regulation

General Closed Loop System

Equation for Feedback

Servo & Regulator Operation SERVO OPERATION Set point only changes ; disturbance does not change d(s) = 0 REGULATOR OPERATION Disturbance only changes ; set point does not change y SP(s) = 0

Process Characteristics Process Equation Process Load Transient Process Lag Self Regulation

Control System Parameters Error Variable Range Control Parameter Range Control Lag Dead Time Cycling Controller Modes Control Actions (Direct & Indirect)

CONTROLLER MODES Discontinuous Control Modes Continuous Control Modes 2 position (ON/OFF control) mode Multi position mode Floating control mode Continuous Control Modes Proportional Control Integral Control Derivative Control Composite Control Modes PI PD PID

DISCONTINUOUS CONTROLLER MODES 1. Two-Position Mode (ON-OFF controller)

1. Two-Position Mode (ON-OFF controller)

Electrical Two Position Controller

Pneumatic Two Position Controller

Two Position Controllers APPLICATIONS Adapted to LARGE SCALE SYSTEMS with relatively SLOW PROCESS rates. Eg: AC in a Hall Disadvantage: Oscillation

2. Multi position Mode

2. Multi position Mode Requires more complicated Final Control element

3. Floating Control Mode (Single speed) dp/dt = ±KF

4. Multiple Speed Floating Control

Applications Well suited to self regulation processes with a very small lag or dead time.

Continuous Controller Modes Proportional Control Mode Also known as correspondence control, droop control and modulating control. Control action is proportional to Error. Kp = Proportional Gain(Proportional Sensitivity) Proportional Band is the range of error to cover the 0% to 100% controller output. PB = 100/Kp

p = Kpep +p0 Proportional Control Mode p = Controller Out put (%) ep = Error (%) p0 = Controller output with no error (%)

Proportional Control Mode

Proportional Control Mode OFFSET : Whenever a change in load occurs, the proportional control mode produces a permanent residual error. This can be minimized by a larger Kp Application : used in processes where large load change are unlikely or with moderate to small process lag times.

Integral Control (Reset Action) Mode the value of manipulated variable is changed at a rate proportional to the deviation. If deviation is doubled over a previous value, the final control element is moved twice as fast; when CV at SP the FCE remains stationary. Controller output, p0 = Controller O/P at t=0

Integral Control (Reset Action) Mode

Integral Control (Reset Action) Mode If the error is zero, the output stays fixed at a value to what it was when the error went to zero. If the error is not zero, the output will begin to increase or decrease at a rate of KI percent/ second for every one percent error. Transfer Function of Integral Control is: Integral Windup

Integral Control (Reset Action) Mode ** Offset eliminated

Derivative Control Mode Controller output depends on rate of change of error. Also known as Anticipatory Control, Rate response or lead component. The controller anticipates what the error will be in the immediate future and applies action which is proportional to current rate of change of error.

Derivative Control Mode

Derivative Control Mode Drawback: for a noisy response with almost zero error it will compute large derivatives and thus large control action, which is not needed. Not used alone. For zero or constant error, no control action. Transfer Function of Derivative mode:

COMPOSITE CONTROL MODE PI Control PD Control PID Control

Proportional – Integral Control (PI) pt(0) = Integral term value at t=0 (initial value) One-to-one correspondence of the proportional mode is available and the integral mode eliminates the inherent offset.

Proportional – Integral Control (PI)

Proportional – Integral Control (PI) Used in systems with frequent or large load changes. Disadvantage Because of the integration time, however, the process must have relatively slow changes in load to prevent oscillations induced by the integral overshoot. During start up of a batch process, the integral action causes a considerable overshoot of the error and output before settling to the operating point.

Proportional – Derivative Control (PD) Disadvantage: It cannot eliminate offset of proportional control Advantage: It can handle fast process load changes as long as the load change offset error is acceptable.

Proportional – Derivative Control (PD)

Proportional – Integral - Derivative Control (PID) (three mode controller)

Proportional – Integral - Derivative Control (PID)

ELECTRONIC CONTROLLERS TWO POSITION CONTROLLER

ELECTRONIC CONTROLLERS FLOATING TYPE CONTROLLER

ELECTRONIC CONTROLLERS PROPORTIONAL MODE CONTROLLER

ELECTRONIC CONTROLLERS INTEGRAL MODE CONTROLLER

ELECTRONIC CONTROLLERS DERIVATIVE MODE CONTROLLER

ELECTRONIC CONTROLLERS PI MODE CONTROLLER

ELECTRONIC CONTROLLERS PD MODE CONTROLLER

ELECTRONIC CONTROLLERS PID MODE CONTROLLER

PNEUMATIC CONTROLLERS PROPORTIONAL MODE CONTROLLER

PNEUMATIC CONTROLLERS PI MODE CONTROLLER

PNEUMATIC CONTROLLERS PD MODE CONTROLLER

PNEUMATIC CONTROLLERS PID MODE CONTROLLER