basic concepts of process control

BASIC CONCEPTS

MAPUA INSTITUTE OF TECHNOLOGY

School of Chemical Engineering & Chemistry

Process Control & Instrumentation

OUTLINE

• Process Control Definition & Objective

• 4 Basic Components in a Feedback Control Loop

• Open & Closed Loop Systems• Loop Components

PROCESS CONTROL

• Definition– the physical regulation of a

process to maintain a particular process variable as close as possible to a desired value.

IMPORTANT TERMS

• Controlled Variable (CV)– the process variable being controlled.– e.g., outlet temperature of a heat exchange

equipment, liquid level inside a drum, fluid flow rate in a pipeline, pressure inside a process vessel

IMPORTANT TERMS

• Set Point (SV)– the desired value of the controlled variable.– based on design operating conditions and/or product

specifications– an external input to an automatic control loop

IMPORTANT TERMS

• Process Variable (PV)– the measured value of the controlled variable.– feedback coming from a measuring element going to

the controller– actual condition of the process

IMPORTANT TERMS

• Manipulated Variable (MV)– the process variable being adjusted by the controller

to maintain the controlled variable at the set point.– often, not the same as the controlled variable– almost always, fluid flow

IMPORTANT TERMS

• Load Variable (Disturbance)– any process variable that could cause a disturbance

in the process that is outside of the control of the controller.

– another external input, aside from the set point, to an automatic control loop

4 BASIC COMPONENTS IN A FEEDBACK CONTROL

LOOP

Refining Process (Plant)1. Process

3. Controller

Set Point (SV)

Controlled Variable (CV)

Operator

2. Measuring Element 4. Final Control Element

Process Variable (PV)

Manipulated Variable (MV)

Load Variable

Controller Output

FEEDBACK CONTROL LOOP

• Open Loop– one of the component is

disconnected from the loop

– no instantaneous feedback from the process

– controller is on manual mode

• Closed Loop– continuous flow of signal

around the loop– there is instantaneous

feedback from the process

– controller is on automatic mode

OPEN LOOP

Refining Process (Plant)1. Process

3. Controller (Manual Mode)

Controller Output

Controlled Variable (CV)

Operator

4. Final Control Element

Process Variable (PV)

Manipulated Variable (MV)

Load Variable

Controller Output

Previous

Refining Process (Plant)1. Process

3. Controller

Set Point (SV)

Controlled Variable (CV)

Operator

2. Measuring Element 4. Final Control Element

Process Variable (PV)

Manipulated Variable (MV)

Load Variable

Controller Output

CLOSED LOOP

Previous

• the loop component being controlled– may be mechanical, electrical, chemical, thermal,

hydraulic, or pneumatic– follows the same laws of physics and dynamics

PROCESS

• measures the actual value of the controlled variable– provides feedback about the actual condition of the

process to the controller– consists of a Primary Element and a Transmitter

MEASURING ELEMENT

• a device that serves as the handle in the process for the controller to do the necessary adjustment in the manipulated variable (MV)– e.g.,

pneumatic spring-diaphragm actuated control valve, metering pump, compressor turbine

FINAL CONTROL ELEMENT

• a device located in the process unit out in the field that converts some intrinsic property of the process into a measurable signal.– e.g., orifice, thermocouple

Primary Element

• a device that converts the signal from the primary element into a standard instrument signal.– e.g., temperature transmitter, DP transmitter

Transmitter

• Pneumatic System– uses air signal for transmission

• Range: 3 - 15 psig• Symbol:

• Electronic System– uses current signal for transmission

• Range: 4 - 20 mA• Symbol:

Standard Instrument Signal

• Thermocouple– based on Seebeck Effect: an emf is generated

when opposite junctions of certain dissimilar pairs of wires are exposed to different temperatures.

Temperature Measurement

ISA TYPE A (+) B (-) SpanE CHROMEL CONSTANTAN 0 – 1200 OFJ IRON CONSTANTAN 0 – 1200 OFK CHROMEL ALUMEL 0 – 2000 OFT COPPER CONSTANTAN 0 – 600 OF

CONSTANTAN

THERMOCOUPLE

• Orifice Plate - DP Transmitter– based on Bernoulli’s relationship between pressure

and velocity• increase in velocity is accompanied by a

decrease in pressure, as the static head is converted into velocity head

• Actual Flow Reading = Square Root of the signal from the DP Transmitter x Full-Scale Range of the Flow Indicator

Flow Measurement

ORIFICE PLATE

DP TRANSMITTE

R

ORIFICE PLATE

FLANGE

• can be classified according to its fail-safe position - the position (either fully open or fully close) of the valve if instrument air supply fails or is cut-off

Pneumatic Control Valve

FAIL-OPEN VALVE

•fully opens upon instrument air supply failure

•also called an air-to-close valve

FAIL-CLOSE VALVE

•fully closes upon instrument air supply failure

•also called an air-to-open valve

• compares the measured value (PV) of the controlled variable with the set point (SV) and makes the necessary control adjustment in case there is a deviation, or an “ERROR” (e), between the two signals– based on feedback mechanism– can be DIRECT-ACTING or REVERSE-

ACTING

CONTROLLER

• the sign of the controller output is the same as the sign of the process variable.– controller output tends to increase (+) when the

process variable is increasing.– controller output tends to decrease (-) when the

process variable is decreasing.

Direct-Acting Controller

• the sign of the controller output is the opposite of the sign of the process variable.– controller output tends to increase (+) when the

process variable is decreasing.– controller output tends to decrease (-) when the

process variable is increasing.

Reverse-Acting Controller

SINGLE-LOOP

CONTROLLER

DISTRIBUTED CONTROL SYSTEM