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Control Valves

Presented by:Syed Waqas HaiderProcess Engineer

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Definition

Control Valves

control conditions (flow, pressure, temperature, and liquid level )

open or close in response to signals received from controllers

Controller compare a "setpoint" to a "process variable" (value provided by sensors)

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Types

Control Valves

Rotary Motion

Butterfly

valves

Ball vales

Plug valves

Linear Motion

Globe Valves

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Control Valves Parts

Three main parts

Valve's actuatorValve's positionerValve's body

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ActuatorThe purpose of the valve actuator is to

accurately locate the valve plug in a position dictated by the control signal

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Types of ActuatorsPneumatic Valve Actuators: Adjust the position of the valve by converting air pressure into rotary motion or linear motion. Piston and Diaphragm Actuators

are examples of Pneumatic Actuators

Electric Valve Actuators: An electric actuator is powered by motor that converts electrical energy to

mechanical torque

Hydraulic actuator: Consists of a cylinder or fluid motor that uses hydraulic power to facilitate

mechanical operation. The mechanical motion gives an output in terms of linear, rotary or oscillatory

motion

according to their supply

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Types of Actuators

Multi-turn actuatorgives torque for at least one full

revolutionPart-turn actuator

gives torque for less than one full revolutionLinear actuator

opens and closes valves that can be operated via linear force--sometimes called rising stem valves

according to their movement

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PositionerPositioners are instruments that help improve control by accurately positioning a control valve actuator in response to a control signal

Positioners receive an input signal either pneumatically or electronically and provide output power to an actuator

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Positioner Working

Some of the mechanisms

i. Force Balance Positionersii. Motion balance Positionersiii. Electronic Positioners

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Force Balance Positioners

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Motion balance Positioners

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Electronic Positioners

Sensor to detect valve position

Micro-Processor Error = Position - Signal

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Valve Bodies

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Flow characteristics of Control valves

Quick Opening: Quick open plugs are used for on-off applications designed to produce maximum flow quickly

Linear: produces equal changes in flow per unit of valve stroke regardless of plug position, used where valve pressure drop is a major portion of the total system pressure drop

Equal Percentage: change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made, generally used for pressure control