Relief valve sizing basis for full flow 1

[heaterguy]

What flow rate should I send to the relief valve manufacturer to have them size the valve? Please reference a specifications such as ASME, ANSI or API and the paragraph location.

Here is the situation:

1500# ANSI line
Design Pressure is 2300 psig at 167 degF
Operating Pressure is 1830 psig at 158 degF

Pressure reduction valve (max Cv=14.1)

300# ANSI line
Design Pressure is 450 psig at 167 degF
Operating Pressure is 400 psig at 82 degF

Normal operating condition is 18,200 lb/hr of fuel gas when the valve is 70% open.

The valve will be on the 300# ANSI line to protect it from over pressure.

 

[MortenA]

Refer to API 521 sec 3.10.3 and 3.10.4

This states that you mut consider the case where the valve is fully opened but that you can allow for the pressure being at 110% MAWP on the downstream side.

You should be able to this calc. yourself - or get someone with proper QA procedures to do them for you.

Best regards

Morten

 

[heaterguy]

Morten,

Very nice answer. Thank you so much.

Regards,

Heaterguy

 

[davefitz]

For the case of a 2300 psig fuel gas system discharging into a 400 psig system, there will be choked flow. To calculate the "casualty flow" for choked flow ,you need to know both the valve Cv and the valve Xt, Per ISA handbook of control valves.

Also, there are several competing codes that define what mandatory overpressure protection is required for fuel gas systems. You will need to determine which governing code is in effect, but I am pretty certain that if you provide 100% safety valve capacity then the code will be met. The only caveat is that if the pressure reducing valve is replaced or modified then the sizing of thr relief valves may need to be increased.