Control Valve (Part 1)

Definition

A final controlling element, through which a fluid passes, which adjusts the size of flow passage as directed by a signal from a controller to modify the rate of flow of the fluid.


Major components

(a) Body
(b) Actuator

Sub-components

(a) Valve body

A housing for internal parts having inlet and outlet flow connections


(b) Bonnet
The part of the body assembly which serves as a means for mounting the actuator


(c) Cage
A hollow cylindrical trim element that is a guide to align the movement of a valve plug with a seat ring


(d) Seat
The portion of the valve body where the valve plug contacts for closure


(e) Valve Plug
A movable part which provides a variable restriction in a port.


(f) Valve Plug Stem
A rod extending through the bonnet assembly to position the valve plug


(g) Trim
The internal parts of the valve which are in contact with the controlled fluid


(h) Port
A fixed opening inside the diameter of the seat ring through which the fluid passes


(i) Packing Box
The part of the bonnet assembly used to seal against leakage around the valve plug stem

Control Valve (Part 2)

Actuators

An air operated mechanical device used to open and close or modulate a valve. The actuator is designed to convert air pressure into mechanical force sufficient to operate the valve.

Actuator - Terminology

(a) Diaphragm
A flexible pressure responsive element which transmits the force to the actuator stem

(b) Actuator stem
The extension of the diaphragm plate or piston for connection to the valve plug stem

(c) Actuator spring
The spring enclosed in the yoke is to move the actuator stem in a direction opposite to that created by the diaphragm pressure

(d) Piston
A movable pressure responsive element which transmits force to the operating fluid

(e) Yoke
The structure by which the diaphragm case is supported rigidly on the bonnet.

(f) Travel indicator
A pointer to indicate the travel of the valve plug.

(g) Travel indicator scale
A graduated scale attached to the yoke for indication of the travel.

Control Valve (Part 3)

Terminology

On-Off Service
When the valve is used to start / stop the flow by being cycled to the full open or to full closed position

Modulating Service
When the valve is being used to throttle or regulate the flow by varying the opening between open and closed positions

Maximum shut-off Pressure
The pressure of the fluid flowing into the valve against which the valve will have to close

Service Temperature
The maximum and minimum temperature of the media

Supply Pressure
The plant air supply pressure available to operate a pneumatic actuator

Stem Torque
The force required at the valve stem to open or close the valve against system pressure and service conditions

Capactiy
Rate of flow through the valve under stated conditions

Fail - close
The condition wherein the valve port remains closed should the actuating power fail

Fail - open
The condition wherein the valve port remains open should the actuating power fail

Fail - last
The condition wherein the valve port remains in the last position should the actuating power fail


Control valve - end connections

Threaded

• smaller size valves
  
• isolation valves

Flanged

• common for all sizes


• most general & severe service

Welded

• leak tight applications


• gas lines

Valve Plug Guiding

Top and Bottom Guided

• Valve plug is aligned by guide bushings in the bonnet & bottom flange


• High stability and minimum deflection (high pressure drop applications)

Cage Guided

• Valve plug is guided by a cylindrical cage throughout the travel range


• Increased trim life & lower stem friction (high pressure drop applications)


• Steam, gas and clean liquid applications only

Stem Guided

• Valve plug is aligned with the seat ring by a guide bushing in the bonnet


• commonly used for throttling applications
  

Top and Port Guided

• Valve plug is aligned by a guide bushing in the bonnet or body and also the port