# Instrumentation :How to do Valve Sizing

**URL:** <https://forumautomation.com/t/instrumentation-how-to-do-valve-sizing/370>\
**Category:** Valves\
**Created:** [March 29, 2015, 7:43am UTC](https://forumautomation.com/t/instrumentation-how-to-do-valve-sizing/370 "2015-03-29T07:43:43Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![denizen](https://forumautomation.com/letter_avatar_proxy/v4/letter/d/848f3c/32.png) [@denizen](https://forumautomation.com/u/denizen)\
**Post date:** [March 29, 2015, 7:43am UTC](https://forumautomation.com/t/instrumentation-how-to-do-valve-sizing/370/1 "2015-03-29T07:43:44Z")

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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

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