Don’t Judge a Valve by its Cv

Cv (or Kv) flow factors are published by valve manufacturers to enable flow or pressure drop calculations to be performed without having to analyse the individual design from first principles.

These figures are calculated from experimental data based on water flow at a set pressure drop across the valve. For other liquids, conversion factors can be applied to produce a revised factor for that media.

Control valve manufacturers have a wealth of data regarding valve and trim characteristics for a given media and condition, they are able to tailor a valve to suit the application. For simpler or less critical valves such as PRV’s, solenoid valves or filter-regulators, we may have graphs of flow curves to work from but more often only a Cv or Kv figure.

What’s the difference?
Cv is the American unit based on 1psi pressure drop and flow in US gallons per minute but can also be given in Imperial gallons which if not recognised can cause errors in subsequent calculations. Kv is based on metric units of 1bar pressure drop and flow in litres /min or m³/h. Conversions can be made between these factors quite simply to make it easier working with your normal units.

What about gases?
If working on a design for air or gas, water based flow factors have to be adjusted to deal with a compressible medium and the issues of critical flow where the velocity will not increase above the speed of sound regardless of pressure drop. This is highly complex and should result in a separate flow factor for critical and sub-critical flows but most valves only have one Cv published. Knowing whether it is a water based figure, based on a specific gas flow test or from a gas flow test and then converted back to the equivalent base water figure is a minefield of potential errors.

This can be sufficient when working with on/ off valves, but any kind of regulating valve is very difficult to equate the valve position with a single flow factor figure to produce a reliable expectation of practical flow performance, or compare directly with a similar valve from a different manufacturer.

Some manufacturers qualify their flow factors with a statement that the factor is based on a particular inlet and outlet pressure setting and pressure drop. This is fine if the pressures match your needs but not if you want to compare with another manufacturer that uses different pressures for their factors and therefore it is not possible to make a direct comparison.

Why isn’t there a standard?
There are standards that dictate how to measure water or air flow and determine flow factors but as yet there is no standard or regulation that dictates how the information is portrayed. How the published flow factor is determined is down to the individual manufacturer and will most likely be a selected average figure which is not representative of performance of the valve across its full operational stroke.

Manufacturers marketing departments may want to offer valves with the highest flow factor possible to suggest that their valve is more efficient than a competitor and unless they publish exactly how the figures were determined, it makes it very difficult to compare like for like.

All manufacturers have procedures that they follow but until a standard is published to tie the methods together and produce consistent figures, we have to interpret the valve figures as best we can.

Is there a better way to get accurate figures?
Yes, if you or the manufacturer has the capability of performing CFD (Computational Fluid Dynamics) analysis for a specific valve given your media and performance requirements; but even if you had this accurate information, will you have the same level of detail for all the components in the system to reasonably expect a repeatable result in practice?

If you can get copies of flow curves from the manufacturer or third party test house for each of your valve choices it will be easier to estimate likely performance than relying on a Cv or Kv figure and converting for your media.

Many application specifications call for minimum pressure drop based on design figures for the installation. In practice, the pressures and flow rates can deviate from this just by the pipeline design and how it has been installed, without considering the performance implications of all the other elements in the system. The inevitable result is that the engineer needs to allow a bigger safety margin “just in case” which can lead to over-sizing of valves or piping and subsequent problems of maintaining media velocity.

When valve performance is critical and the available data is limited, the best way of understanding the suitability for a given media or system, is to get one and run a practical test. The result will either confirm or deny capability but at least you have some real world figures to work from if you need to look for an alternative. It may even be that the best one for your application is not the one with the highest published Cv for its size.

Tel: 01443 772 500
Email: sales@reddragonvalves.co.uk
Web: www.reddragonvalves.co.uk


Published: 18th February 2016

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