3.6 Steps in Selecting a Control Valve

The basic steps in control valve selection are presented below.

  1. The first step in control valve selection involves collecting all relevant data and completing the ISA Form  S20.50.  The piping size must be set prior to valve sizing, and determining the supply pressure may require specifying a pump.  The novice might have to iterate on the needed piping, pump pressure and pressure drop through the piping network.

  2. Next, the size of the valve is required; select the smallest valve Cv that satisfies the maximum Cv requirement at 90% opening.  While performing these calculations, checks should be made regarding flashing, cavitation, sonic flow and Reynolds number to ensure that the proper equation and correction factors are used.  As many difficulties occur due to oversized valves as to undersized valves.  Adding lots of “safety factors” will result in a valve that is nearly closed during normal operation and has poor rangeability.

  3. The trim characteristic is selected to provide good performance; goals are usually linear control loop behavior along with acceptable rangeability.

  4. The valve body can be selected based on the features in Table 5 and the typical availability in Table 6.  Note that the valve size is either equal to the pipe size or slightly less, for example, a 3-inch pipe with a 2-inch globe valve body.   When the valve size is smaller than the process piping, an inlet reducer and outlet expander are required to make connections to the process piping.

  5. The actuator is now selected to provide sufficient force to position the stem and plug.

  6. Finally, auxiliaries can be added to enhance performance.  A booster can be increase the volume of the pneumatic signal for long pneumatic lines and large actuators.  A positioner can be applied for slow feedback loops with large valves or valves with high actuator force or friction.  A hand wheel is needed if manual operation of the valve is expected.

Table 6.   Information on Standard Commercial Control Valves1

Body Type

Size (in)

Maximum Pressure (psia)2

Temperature (F )

Capacity3
Cd = Cv/d2

Globe

1/4 to 16

50,000

cryogenic to 1200

10

Ball

1/2 to 36

  2500

up to 1400

30

Butterfly

3/4 to 200

  2500

cryogenic to -2200

20

Diaphragm

1/4 to 20

    100

-30 to 2200

22

  1. Compiled from Andrew and Williams (1980) and Driskell (1983)
  2. Higher pressures for smaller sizes and moderate temperatures.
  3. The parameter d is the valve connection diameter in inches.
For an introductory step-by-step method for selecting and sizing a control valve, select this button to be directed to a site on the WWW.
For additional information on valve selection and sizing. See Chapter 5 in this
resource
.
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