Blow off valve allowable leakage rate calculation Class IV

[DarthMalik]

Greetings gentlement.I work in quality control inspection department and I have the prime responsibility for valve testing of critical valves.We have valves following various standards but I am having problems with calculating permissible leakage rate for Class iv control valves as per FCI 70-2.As this is a new valve I only have access to instrument data sheet and valve test certificate and the plant is still in construction phase.As per the 0.01% permissible leakage criteria for class iv valves i calculated it using CVs value provided in the instrument data sheet 6850Usgal/min and 0.01% of this value is 0.685gal/min only permissible..this converts to 2.59liters per minute in metric system but testing at 3.5 bar pressure at injection point we measured a flow rate of 45Liters/min and 20Liters/minutes using dry air respectively which doesn't fall within the tolerance.I think I am doing something wrong here and I would be obliged if someone can point me in the right direction for calculating permissible leakage..My project team is waiting for the results so please help.I am attaching test report and valve datasheet for reference.

 

[goutam_freelance]

You have selected 3.5 bar as differential pressure for the test but MAWP of the valve is 2.76 bar(g). As per FCI 70.2 you should select 2.76 bar(g) as differential pressure as it is less than 4 bar(g)

 

[Doug Hunter]

You need to be clear between fluid flow data and air leak data. Is the valve normally handling fluid but is air-tested for leakage?

 

[DarthMalik]

As per the valve test certificate the test pressure was 52 PSI .We are just replicating the test conditions.I agree with you MAWP is the max pressure the valve is designed to hold is 2.76 Bar but idk wht the test conditions did the seat test pneumatic @ 4 bar sorry the test certificate is wrong

You need to be clear between fluid flow data and air leak data. Is the valve normally handling fluid but is air-tested for leakage?

The Valve is intended for gas service only.tye CV value stenciled on valve body is 6850usg/min..

 

[DarthMalik]

You have selected 3.5 bar as differential pressure for the test but MAWP of the valve is 2.76 bar(g). As per FCI 70.2 you should select 2.76 bar(g) as differential pressure as it is less than 4 bar(g)

Please check the following correct test certificate.We are just replicating the test conditions.Disregard my previous upload..Wrong valve test cert attached.

 

[DarthMalik]

I urgently need to calculate permissible leakage as per class iv..Please guide me as we have scheduled MRT for the system where this valve is being installed.

 

[goutam_freelance]

The leakage rate is calculated based on rated flow, not Cv of valve.

From the indicated data,

Design flow rate=16000 kg/h
Design inlet press=2.09 bara
Design inlet temperature=116.5 deg C
Density of air at 15 deg C and 1.01325 bar(a)=1.225 kg/m^3
Density of air at inlet conditions=1.225x(15+273.15)/(116.5+273.15)x(2.09/1.01325)=1.8686 kg/m^3
Design volume flow rate=16000/1.8686=8562.7 m^3/h
Permissible leakage rate@0.01%=8562.7*.01/100=0.856 m^3/h=14.27 l/min
It is close to 20 l/min, as you indicated.
However, please check the differential pressure for the test considering the maximum operating differential pressure.

From ANSI FC 70.2

5.1.2 Pressure of test medium shall be 3-4 bar (45-60psig) or within +/- 5 percent of the maximum operating differential pressure, whichever is less.