Low pressure multi PSVs Set Pressure

[simplemath]

I am working on spec on multi(3) PSVs under fire load case used for a tank (API620).

Pset = 2psig

I am asking how to specify these set pressures.

Are they all at 2psig?

I have explored all related code:
1. ASME VIII has set pressure examples. Can that be used on API 620 tank?
2. API 620 required no PSV above design pressure but not specify multi PSV and fire case.

Which one should be used under this case?
Thanks

 

[pleckner]

You do not fall within the scope of ASME Section VIII, Div. 1. You must follow the rules of API 620.

There is no mention of set pressure variations when using more than one relief valve for a single relieving event (as there is in ASME Section VIII, Div 1). Nor have I seen any interpretations that might add more information. Therefore we can only conclude based on what is said within the document that you cannot set these valves any higher than design pressure and I see no reason why you would need or want to set the other valves for any pressure other than design.

API 620 also recommends you talk to a vendor when sizing. These PSVs are not sized the same way you might size a PSV using ASME Section VIII, Div 1 because you will more than likely NOT achieve choked flow at these low pressures.

 

[JimCasey]

Expanding on what pleckner said: Since flow is not choked at these low pressure ratios, the valves do not "pop" to full opening. Weight-loaded vents such as Groth, Protectoseal, Varec, etc. need at least 50% overpressure to develop rated flow.

Pilot-loaded valves DO open fully at less than 10% overpressure and may be appropriate for what you are trying to do. Here is a link to the Groth valve (I am not affiliated with Groth)

http://www.hydrocarbononline.com/Co...1E-EDDE-11D2-94BD-00A0C9B3BDF2}&VNETCOOKIE=NO

I once used an air-loaded valve in combination with a pressure switch and a solenoid valve. When the PS got to sepoint, the contacts OPENED, de-energizing the SV. The SV Dumped the air in the valve and the valve opened.

Back to your original question, I would stagger the set pressures very slightly so that one valve takes the normal venting duties and the other valves are there to contribute capacity for fire-case.

 

[simplemath]

Thanks JimCasey and pleckner.

I tried to stagger set pressure on these 3 PSVs. But API-620 9.7.1 states that Pset in no case should exceed the design pressure.(5.3.1).So I have no choice but to set all at design pressure.

Thanks again

 

[Latexman]

No, you could set one at 0.25 psig, one at 0.5 psig, and one at 1 psig. None exceed the design pressure.

The above example used arbitrary numbers. In your case you should use meaningful set pressures.

Good luck,
Latexman