Rules of thumb for Control Valves sizing and selection

The following should be used as a guidelines, but not as design criteria :

  • In a pumped circuit, the pressure drop allocated to the control valve should be equal to 33% of the dynamic losses in the system at the rated flow, or 15 psi, whichever is greater.
  • The pressure drop allocated to a control valve in the suction or discharge line of a centrifugal compressor should be 5% of the absolute suction pressure, or 50% of the dynamic losses of the system, whichever is greater.
  • In a system where static pressure moves liquid from one pressure vessel to another, the pressure drop allocated to the valve should be 10% of the lower-terminal vessel pressure, or 50% of the system’s dynamic losses, whichever is greater.
  • Pressure drops in valves in steam lines to turbines, reboilers and process vessels should be 10% of the design absolute pressure of the steam system, or 5 psi, whichever is greater.
  • The gain on a control valve should never be less than 0.5.
  • Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10–80% range.
  • Generally, control-valve bodies are one size less than the line size. If this causes the valve body to be significantly less than the line size, which would reduce the valve’s effective Cv. then do not apply this generalization.
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