Chemical Process Dynamics and Control

Feedback Control

Eng. Misael González Macias

Regulatory and Servo Control

• Regulatory: systems designed to compensate for DISTURBANCES.

• Servo: the controlled variable must follow the SET POINT.

Basic Components of Control Systems

• Sensors and Transmitters.

• Controllers.

• Final Elements Control: Control Valves

Sensors and Transmitters• RangeConsider a pressure sensor/transmitter that has

been calibrated to measure a process pressure between the values of 20 psig and 50 psig. Then, the Range is 20-50 psig.

• SpanIs the difference between the high and low values

of the range. For this case the span is 30 psig.

• ZeroThe low value of the range. The zero is 20 psig.

Sensors and Transmitters

ST(s)

Process Variable

PV(s)

Transmitter Output

TO(s)

Sensors and Transmitters• Gain of a sensor/transmitter

– Consider an electronic pressure transmitter with a range of 0-200 psig with electronical signal output of 4 to 20 mA.

– Consider an neumatic temperature transmitter with a range of 100-300 °F with neumatic signal output of 3 to 15 psig.

Control ValvesA control valve acts as a

variable restriction in a process pipe.

Safety

•air-to-open or fail-closed valve.•air-to-close or fail-open.

Control Valves

ExampleControl valve

Control ValvesThe Control Valve Actuator

Transducer percent controller output

Control ValvesControl Valve Capacity and Sizing•Liquid service.

– F: liquid flow, GPM.– ΔP: pressure drop across the valve, psi.– G: specific gravity of liquid at flowing

conditions.

•Compressible Flow– Gas flow. - Steam

flow

Control ValvesControl Valve Capacity and Sizing

– Q: gas flow, scfh (scfh = ft3/h at standard conditions of 14.7 psia and 60°F)

– G: gas specific gravity with respect to air, calculated by dividing the molecular weight of the gas by 29, the average molecular weight of air.

– T: temperature at the valve inlet, R=°F + 460.

– Cf: critical flow factor. The numerical value for this factor ranges between 0.6 and 0.95. (Figure C-44 C. A. Smith and A. B. Corripio, Control Automático de Procesos, Limusa.)

– P1: pressure at the valve inlet, psia.

– W: gas flow, lb/h.

– TSH: degrees of superheat, °F

Control ValvesControl Valve Characteristics

Concept of Feedback Control

Processm

d

y

Processm

d

yFinal ControlElement

Sensor/transmitter

Controllerysp

ym

ce

Open loop

Closed loop

Controller Mechanism

-

+

Typical Feedback Control Systems

Flow Control

Typical Feedback Control Systems

Pressure Control Liquid-Level Control

Typical Feedback Control Systems

Temperature Control Liquid-Level Control

Representation of Feedback Loops

FC for flow controlPC for pressure controlLC for liquid-level controlTC for temperature controlCC for composition control

FT for flow transmiserPC for pressure transmiserLC for liquid-level transmiserTC for temperature transmiserCC for composition transmiser

Representation of Feedback Loops

ExampleControl valves on a flash drum

See also Apendix A of A. Smith and A. B. Corripio, Control Automático de Procesos, Limusa

Types of Feedback ControllerError

Proportional Controller (P)

Proportional Integral Controller (PI)

Proportional Integral Derivative (PID)

percent controller output(50 %)

Types of Feedback ControllerProportional Controller (P)

Types of Feedback ControllerProportional Integral Controller (PI)

Types of Feedback ControllerProportional Integral Derivative Controller (PID)