Saturday, July 23, 2005

Done By:

Name: Pankaj

Trainee No: OMTI 041








For most of the process control system, the final control element is a control valve and these valves are typically driven by motors which are commonly called Actuators. There is almost unlimited number of hardware variations encountered in process control valves and Actuators.

The control valve plays a very important part in the automatic control of modern plants, which depends on the correct distribution and control of flowing liquids or gases.


It is the Final Control element most commonly used in process
Industries to regulate the material flow in a process. The Control Valve manipulates the flowing fluid to compensate the load disturbance and
keep the regulated process variable like pressure, flow temperature,
level, etc. As close as possible to the desired set point.

Flow Characteristic:

Quick Opening:

The valve sensitivity (change in flow for a given change in valve position) decreases with increasing flow.

Linear Type:

The valve sensitivity is more or less constant throughout the flow range. Control valve plug has linear flow characteristics when the flow rate through is proportional to the lift.

Equal Percentage:

Control valve plug has equal percentage characteristics if at any plug position, the same percentage of change in flow takes place for the same amount of plug movement.

Control Valve Construction

Valve Body

The main pressure boundary of the valve that also provides the pipe connecting ends, the fluid flow passageway, and supports the seating surfaces and the valve closure member.


A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve.


The portion of the valve that contains the packing box and stem seal and can guide the stem. It provides the attachment of the actuator to the valve body.


The structure that rigidly connects the actuator power unit to the valve .

Types of Control Valve:

1. Linear Motion

Globe Valves

 Single Ported

• Single Ported Globe
• Angle
• Split
• Needle

 Double Ported

 Three Way

2. Rotary Motion

 Ball Valve

 Butterfly Valve

Control Valve Packing

Gland Packing

The part of the bonnet assembly used to seal against leakage around the closure member stem.

• Teflon.
• Glass loaded Teflon.
• Carbon loaded Teflon.
• Graphite.
• Asbestos with lubricants
like graphite.

Selection criteria

• Temperature.
• Service.
• Pressure.


A fluid-powered or electrically powered device that supplies force and motion to a Valve Closure Member.

Types of Actuator

 Diaphragm Actuator

 Piston Actuator

 Electrical Actuator


Is a fluid (usually pneumatic) pressure-operated, spring-opposed diaphragm assembly, which positions the valve stem in response to an input signal.

Types Of Diaphragm Actuator:

 AIR-TO-OPEN: An increase in air pressure to the Actuator is required to cause the valve to open. This is another way of saying the valve is Fail Closed or Normally Closed.

 AIR-TO-CLOSE: An increase in air pressure to the Actuator is required to cause the valve to close. This is another way of saying the valve is Fail Open or Normally Open.

The Advantages and Disadvantages of Diaphragm Actuator:


• Lowest Cost.
• Can throttle with out Positioner.
• Simplicity.
• Inherent fail safe action.
• Low supply pressure required.
• Adjustability.
• Easily maintained


• Limited output capability.
• Large size and weight.


A fluid-powered, normally pneumatic device in which the fluid acts upon a movable cylindrical member, the piston, to provide linear motion to the actuator stem. These units are spring or air opposed and operates at higher supply pressures than a Spring Return Actuator.


Also known as an Electro-Mechanical Actuator uses an electrically operated motor-driven gear train or screw to position the actuator stem. The actuator may respond to either a digital or analog electrical signal.

Advantage and Disadvantages of ELECTRIC MOTOR:


• Compactness.
• Very high stiffness.
• High output capability.


• High cost.
• Lack of fail safe action.
• Limited duty cycle


A device that is mounted on the actuator to complement the actuator's function and make it a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and limit switches.

Control Valve Accessories:

 Hand Wheel

 Air filter Regulator

 Positioner

 Booster

 Limit Switch

 Solenoid Valve

 Current to Pneumatic (I/P) Converter

 Air Lock Relay

Hand Wheel

Top-mounted hand wheel for a Reverse-acting diaphragm Actuator. This unit can be used as an adjustable travel stop to limit travel in the downward direction or to manually close push-down-to-open valves.

Supply Pressure Regulator

Supply pressure regulators commonly called air sets, reduce plant air supply to valve positioners and other control equipment. Common reduced-air-supply pressures are 20, 35 and 60 psig. The regulator mounts integrally to the positioner, or nipple-mounts or bolts to the actuator.


A position controller that is mechanically connected to a moving part of a final control element or its actuator and that automatically adjusts its output to the actuator to maintain a desired position in proportion to the input signal. It ensures proper positioning of the valve and provide fast response. The Positioner uses an auxiliary air supply and feedback cam to position the valve plug in accordance with the air signal from the controller.

Volume Booster

A stand-alone relay is often referred to as a volume booster or simply booster because it boosts, or amplifies, the volume of air supplied to the actuator.


• Quick response.
• Greater stability.
• Low air consumption.
• Higher accuracy.

Air Lock Relay

Is used for applications requiring control valve to hold its position in case of air failure.

Limit switches

Operate discrete inputs to a distributed control system, signal lights, small solenoid valves, electric relays, or alarms.

Solenoid Valve

3-way solenoid valves are used to operate pneumatic control valves from the electric on-off controller and in fail safe systems.


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