For the most comprehensive procedure in sizing pressure reducing valves, it is best to use our ValveMaster software or the Performance Charts in the Engineering Section of the OCV catalog. In the absence of these, the following procedure will get you where you need to be, and enable you to avoid the most common error in sizing PRV’s: an oversized valve.

PROCEDURE
The following procedure takes both factors (flow rate/pressure drop) into account through the use of the flow coefficient, or Cv. The theory is simple: for best results, a PRV should be sized to operate between 10% and 90% of its capacity, or in other words, between 10% and 90% of its wide open Cv. It is a four-step procedure:

Calculate Cv Minimum

    Q Minimum = Minimum anticipated flow, GPM
    S = Specific gravity of fluid (water = 1.0
    P1 = Inlet pressure at Q minimum, psi
    Ps = Desire outlet pressure, psi
   

Calculate Cv Miaximum

    Q Max. = Maximum anticipated flow, GPM
    P2 = Inlet pressure at Q maximum, psi
    Ps = Desired outlet pressure, psi
   

From the table, find the size that includes both the Cv min. and Cv max. you have calculated in either the globe or angle valve column.

From the table check that the velocity (GPM) at the calculated Q max. does not exceed 25 ft/sec.

Occasionally, the flow range is so wide that both the Cv min. and Cv max. will not fit in the proper range for any one size valve. In such cases, a parallel valve installation, with a smaller valve by passing around a larger one, should be given strong consideration. The valves should be sized so that: