If you get the sizing of your industrial valves wrong, it could be a costly error, but there are a few easy checks to ensure you order the correct size.
Work Out Your Flow Rate and Cv
Cv, the flow coefficient, is a measure for the amount of a fluid that a valve can open, at a given differential pressure. The Cv needed for a certain application can be calculated from the required flow rate, the density of the medium to be controlled and the allowed pressure drop. The Cv of the valve to be ordered has to be slightly above the Cv needed for the application. This information on Cv for given valves is provided by the manufacturers in Cv tables.
Measure Pipe Bore and Connection Type
Check the bore of the end connection of the pipe against the nominal bore of the pipe and make a note of the end connections (flanged, screwed, etc.) as different types of end connections are not interchangeable even if the valve is of the correct nominal connection size.
Check Pressure and Temperature Ratings
Valves have a pressure-temperature rating which forms part of their name (e.g. PN16, ANSI Class 900 etc.). This must be greater than the service conditions with some margin for error. Under UK law for safe operation of a pressure system, the operating pressure and highest temporary process pressure must be within the limits of the rating of the valve.
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Avoid Oversizing
Large valves may impede operation near closed positions. Normally a valve should be sized to operate between 25% to 75% open position under normal flow conditions.
All of these checks can be made before ordering the valves to avoid massive problems in the long run.
