Control Valve Leakage Calculation 3

[Pruk]

I just read an article about control valve leakage calculation details below.

Cv max = 182, Flow rate at 100% open = 50 MMScfd
The valve is ANSI class IV specification. So design leakage will be 0.01 % at 50 psi(roughly).

Leakage at 50 psi = 0.01/100 * 50 MMScfd = 0.005 MMScfd

If inlet pressure is 1800 psi.
Then, the maximum leakage at 1800 psi differential
= 0.005 * sqrt(1800/50)
= 0.03 MMScfd

I'm not sure why they have to use sqrt in the equation. If you have any ideas please share them.
Thanks

 

[786392]

Are you sure that this was not dealing with something else(i.e.pneumatic air leakage etc.)?
If sure then; is it to do with what fluid(gas or liquid)and what is the Control perspective?

Best Regards
Qalander(Chem)

 

[JimCasey]

FCI 70.2 (Was once called ANSI B16.104) Class IV specifies allowable leakage up to 0.01% of rated Cv when tested on air or water at nominally 50 psi differential to atmosphere. This is at ambient temperature.

Extrapolating the leakrate to other pressures or other fluids is not meaningful.

If a valve leaks 1 scfh at 50 psi on air, you can't predict what it will leak at, for example, 1000 psi on steam.

70.2 is a standard that allows comparison of fit and finish, but not much else.

 

[Pruk]

As JimCasey say that 70.2 doesn't allows us to compare to real process. So, if we like to know leakrate at other pressure, how can we compute then?
In this calculation, he tried to compute optimum flare rate. So he have to know normal leakrate from all control valves which connected to flare knockout drum at operating condition. The problem is I never seen any standard talk about control valve leakrate at operating pressure. Please feel free to share your ideas.

 

[NGiLuzzu]

For more information, please take also a look at thread408-76110 and thread408-122060 within this website.
Up-to-date reference standards are now FCI-70-2-2006 of April 2006 and equivalent IEC 60534, Part 4, 3^rd Edition of June 2006.

The latter, in a NOTE to paragraph 4.2 about Seat leakage test, explicitly says that "this part of the standard cannot be used as a basis for predicting leakage when the control valve is installed under actual operating conditions".
A similar statement should be present also in the "American version" of the standard, in paragraph 2.2, if I remember well...


For the particular application, I would suggest to contact the Valve Manufacturer and ask for their own flow calculation method and/or computer program, for instance.


Hope this helps, 'NGL