Instrumentation :How to do Valve Sizing


Step 1 - Define System

For Example :
The system is pumping water from one tank to another through a piping system with a total pressure drop of 150 psi. The fluid is water at 70 0F. Design (maximum) flowrate of 150 gpm, operating flowrate of 110 gpm, and a minimum flowrate of 25 gpm. The pipe diameter is 3 inches. At 70 0F, water has a specific gravity of 1.0.
Key Variables: Total pressure drop, design flow, operating flow, minimum flow, pipe diameter, specific gravity

Step #2: Define a maximum allowable pressure drop for the valve
The usual rule of thumb is that a valve should be designed to use 10-15% of the total pressure drop or 10 psi, whichever is greater

Step #3:Calculate the valve characteristic

Cv =Q * Root Square of (G / Delta P)

Q = Design Flow Rate
G = Specific Gravity Related to Water
Delta P = Allowable pressure across wide open Valve

Step 4 : Preliminary Selection of Valve
a)Cv Value
b)Never use a valve that is less than half the pipe size
c)Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10-80% stroke range

valve selection guide

STEP #5: Check the Cv and stroke percentage at the minimum flow

STEP #6: Check the gain across applicable flow rates

Gain = Delta flow / delta stoke travel

Gain should never be less than 0.50

Control Valve What is Valve Rangeability?