Should I Use the Published Cv for Flow Sizing?

Tuesday, October 17th, 2023

Should the catalog Cv value be used to size valves and regulators?  This is a common question.  Cv is suitable for sizing valves, but it is not recommended for regulators. This blog post will briefly explain why Cv should not be used to size gas regulators and link to more detailed analysis and reference materials.

Let’s start with defining the flow coefficient or Cv.  Cv is defined as the flow of water (in gallons per minute) through a fully open device with 1 psi difference inlet to outlet. There are formulas and procedures for testing with gases rather than water and published standards, if you would like a detailed explanation of SEMI Cv measurement standards for valves, please see this product note titled Valve Flow Coefficient (Cv) per SEMI F32. However, the standards, such as SEMI, are open to some interpretation and judgement when it comes to pressure regulators.  Cv is typically measured with the regulator fully open. Unfortunately, there is little consistency in testing between regulator manufacturers which makes comparison of Cv for different devices a challenge. Regulators from different manufacturers might have identical flow curves but very different Cv ratings due to differences in testing and regulator design.

Direct regulator sizing formulas for flow capacity generally rely upon a Cv value for the calculation. The formulas also generally do not account for gas properties beyond density such as the gas thermal properties. These factors limit the usefulness of such formulas.

A flow curve plots outlet pressure on the Y-axis and flow on the X-axis.  Flow curves are the best way to compare regulator performance. One needs to make sure that the test gas, inlet, and outlet pressures are the same for curves being compared.  As flow increases, the outlet pressure “droops”. Droop (drop in outlet pressure as flow increases), is a fundamental concept in understanding regulator performance. Please see our regulator training section for a detailed explanation of droop and other fundamental regulator concepts.

How can I get flow curves?   AP Tech will provide you with a custom flow curve per request.  Please inquire with your local AP Tech distributor or directly with the factory.   Information needed is model, fitting size, gas type, maximum and minimum inlet pressure, and outlet pressure setpoint.   Very limited flow curves are also on data sheets.

AP Tech also publishes a compendium of flow curves  based on nitrogen gas service for various models and inlet and outlet pressures.   To convert to a different gas, AP Tech publishes common specialty gas data tables to compare relative flow capacity for N2 and other gases. However, your custom flow curve will be converted to your specific gas, so there is no need to calculate conversions.

Finally, another option for determining what size regulator should be used is our 208 guide. This guide has all the common specialty gases for source and distribution applications by flow rate.  You can select regulators (and valves) based on gas by flow rate.

For a much more detailed discussion about this topic, please refer to product note #455 titled Understanding a Pressure Regulator Cv Value