Valve Leakage Class and Rated Capacity

[algerrl]

Some valve specifications, such as ANSI/FCI 70-2, specify an allowable leakage rate through a valve as a percentage of rated valve capacity. The definition of "rated valve capacity" is vague. The leakage rate is measured with the valve closed so the pressure differential is the test pressure. If I use the same test pressure as the pressure differential across the full open valve, I can use the Flow Coefficient of the valve, Cv, to calculate an expected flowrate through the open valve. This flowrate is huge and will certainly choke. Is anyone aware of a better definition of rated valve capacity?

 

[bljnv]

Rated valve capacity is the capacity based on the published Cv values for a given valve.
Standards do specify leakage rate as a percentage of rated Valve capacity like 0.01 % and etc..
That means the leak across the valve in its fully closed condition at the test pressure should not exceed this limit.
Normally Cv will be rated for water and test fluid will also be water. No need to take correction on Cv to estimate the allowed leak rates. Standard ASME B16.104 refers leak rates in terms of Valve Cv.

 

[Fog1]

The way I approach it is this: -

The differential pressure is applied to the sizing equation with the capacity coefficient, Cv value, held fixed at the value specified by the manufacturer at the fully open position.

Using the sizing equations it is then possible to calculate what flow would be expected under the conditions of the test pressure applied.

Example a valve with a Cv of 150, P1 = 3.5 barg, Air at 25 C gives a maximum allowable leakage rate of slightly over 15 L/min

The rated capacity you may have gathered from this is the Cv of the valve at the fully open position.

Fog Jones