8.5. Calculating Cv for liquids
The equation for the flow coefficient (Cv) in non-laminar liquid flow is:
Cv = Valve sizing coefficient
FP = Piping geometry factor
q = Flow rate, gpm
ΔPa = Allowable pressure drop across the valve for sizing, psi
Gf = Specific gravity at flowing temperature
The following steps should be used to compute the correct Cv, body size and trim number (Figures 8.3–8.5):
Step 1: Calculate actual pressure drop
The allowable pressure drop, ΔPa, across the valve for calculating Cv, is the smaller of the actual ΔP from Eqn 3.2 and choked ΔPch from Eqn 3.3.
Step 2: Check for choked flow, ...