Actuator

Control Valve

1 February 2012 By Eng. Ahmed Ghreeb

Actuators

Control Valve


Basic Information

Valve Main components

Valve Body

Valve Bonnet

Valve stem

Valve closure member

Valve seat ring

Valve packing

Valve actuator (if it is controlled valve)

Valve Hand (if it is manual valve)

Control Valve


Basic Information

Control Valve


Actuators

Pneumatically operated control valve actuators are the most popular type in use, but electric, hydraulic, and manual actuators are also widely used. The spring-and-diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically operated piston actuators provide high stem force output for demanding service conditions. Adaptations of both spring-and-diaphragm and pneumatic piston actuators are available for direct installation on rotary-shaft control valves.

Control Valve


Diaphragm Actuators

Diaphragm Actuators

Pneumatically operated diaphragm actuators use air supply from controller, positioner, or other source.Various styles include: directacting (increasing air pressure pushes down diaphragm and extends actuator stem, figure 3-31); reverse-acting (increasing air pressure pushes up diaphragm and retracts actuator stem, figure 3-31); reversible (actuators that can be assembled for either direct or reverse action, figure 3-32); direct-acting unit for rotary valves (increasing air pressure pushes down on diaphragm, which may either open or close the valve, depending on orientation of the actuator lever on the valve shaft, figure 333).Net output thrust is the difference between diaphragm force and opposing spring force.Molded diaphragms provide linear performance and increased travels.Output thrust required and supply air pressure available dictate size.Diaphragm actuators are simple, dependable, and economical.

Control Valve


Piston Actuators

Various accessories can be incorporated to position a double-acting piston in the event of supply pressure failure. These include pneumatic trip valves and lock-up systems.Also available are hydraulic snubbers, handwheels, and units without yokes, which can be used to operate butterfly valves, louvers, and similar industrial equipment.Other versions for service on rotary-shaft control valves include a sliding seal in the lower end of the cylinder. This permits the actuator stem to move laterally as well as up and down without leakage of cylinder pressure. This feature permits direct connection of the actuator stem to the actuator lever mounted on the rotary valve shaft, thereby eliminating one joint or source of lost motion.

Figure 3-34. Control Valve withDouble-Acting Piston Actuator

Control Valve


Electrohydraulic Actuators

Electrohydraulic actuators require only electrical power to the motor and an electrical input signal from the controller (figure 3-35).Electrohydraulic actuators are ideal for isolated locations where pneumatic supply pressure is not available but where precise control of valve plug position is needed. Units are normally reversible by making minor adjustments and might be self-contained, including motor, pump, and double-acting hydraulically operated piston within a weatherproof or explosion-proof casing.

Control Valve with Double-ActingElectrohydraulic Actuator and Handwheel

Control Valve


Manual Actuators

Manual actuators are useful where automatic control is not required, but where ease of operation and good manual control is still necessary . They are often used to actuate the bypass valve in a three-valve bypass loop around control valves for manual control of the process during maintenance or shutdown of the automatic system.Manual actuators are available in various sizes for both globe-style valves and rotary-shaft valves.Dial-indicating devices are available for some models to permit accurate repositioning of the valve plug or disk.Manual actuators are much less expensive than automatic actuators.

Typical Manual Actuators

Control Valve


Electric Actuators

Traditional electric actuator designs use an electric motor and some form of gear reduction to move the valve. Through adaptation, these mechanisms have been used for continuous control with varying degrees of success. To date, electric actuators have been much more expensive than pneumatic for the same performance levels. This is an area of rapid technological change, and future designs may cause a shift towards greater use of electric actuators

Control Valve


Terms and DefinitionsANSI rating class

numerical pressure design class defined in ASME B16.5 and used for reference purposes

NOTE The ANSI rating class is designated by the word "Class" followed by a number.

bi-directional valvevalve designed for blocking the fluid in both downstream and upstream directions

bleeddrain or vent

block valvegate, plug or ball valve that blocks flow into the downstream conduit when in the closed positionNOTE Valves are either single- or double-seated, bi-directional or uni-directional.

Control Valve


Terms and Definitionsbreakaway thrustbreakaway torque

thrust or torque required for opening a valve with maximum pressure differential

by agreementagreed between manufacturer and purchaser

double- block-and- bleed (DBB) valvevalve with two seating surfaces which, in the closed position, blocks flow from both valve ends when the cavitybetween the seating surfaces is vented through a bleed connection provided on the body cavity

drive trainall parts of a valve drive between the operator and the obturator, including the obturator but excluding the operator

Control Valve


Terms and Definitions

locking devicepart or an arrangement of parts for securing a valve in the open

and/or closed position

manual actuatormanual operator

wrench (lever) or hand wheel with or without a gearbox

maximum pressure differential (MPD)maximum difference between the upstream and downstream pressure across the obturator at which the obturatormay be operated

Control Valve


Terms and Definitionsflow coefficientK V

volumetric flow rate, in cubic metres per hour, of water at a temperature between 5C (40F) and 40C (104F) passing through a valve and resulting in a pressure loss of 1 bar (14,7 psi)NOTE follows:Kv relates to the flow coefficient Cv in US gallons per minute at 15,6"C (60F) resulting in a 1 psi pressure drop as

full-opening valvevalve with an unobstructed opening capable of allowing a sphere or other internal device for the same nominal size as the valve to pass

Hand wheelwheel consisting of a rim connected to a hub, for example, by spokes, and used to operate manually a valverequiring multiple turns

Control Valve


Seat ring Retention Methods

1- Screw in Seat ring

Not suitable for high Temperature application (Leakage)

Leakage could erode the seat ring and valve body

Easy for installation and removal

Control Valve


Seat ring Retention Methods

2- Clamped Seat ring

Suitable for high Temperature application (No Loose)

Suitable for high pressure drops

Attention to bonnet bolting torque is important concern

Control Valve


Plug Balancing

1- Unbalance valve Plug

Provide tight Shut off (only one leakage path when the valve is closed)

Instable operation

Required larger actuator specially with larger sizes or high pressure drop application

Control Valve


Plug Balancing

2- Balance valve Plug

Introduce second leakage path (Between the plug and the cage

Stable operation

Small stem forces so it could use smaller actuator with larger sizes or high pressure drop application

Plug Balancing

Control Valve


Packing Options

The purpose of Packing is to create a tight seal between the

backing bore and the valve stem to prevent fluid leakage to the

atmosphere. Selection Criteria for packing include:

1-Low friction so that the actuator can stroke the valve

2- Compatibility of packing component with the process fluid.

3- Compatibility of packing parts with the service temperature.

The most used packing material are :

1- PTFE

2- Graphite

Control Valve


Packing Options

Control Valve


Shut-Off Tightness

Control valve leakage is classified with respect to how much the valve will leak when fully closed. The leakage rate across a standard double seat valve is at best Class III

The following leakage rates are taken from the British Standard BS 5793 Part 4 (IEC 60534-4).

Control Valve


Thank you.

Questions?

Slide Number 1Basic InformationBasic InformationActuatorsSlide Number 5Slide Number 6Slide Number 7Slide Number 8Electric ActuatorsTerms and DefinitionsTerms and DefinitionsTerms and DefinitionsTerms and DefinitionsSeat ring Retention MethodsSeat ring Retention MethodsPlug BalancingPlug BalancingPacking OptionsPacking OptionsPacking OptionsShut-Off Tightness Slide Number 22

Actuator

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