Inlet / Outlet Piping to PSV Pressure Drop 2

[SETXChE]

API states that the pressure drop in inlet should not be greater than 3% set pressure and pressure drop in outlet should be no greater than 10% set pressure.

When calculating this pressure drop should the piping inlet pressure drop from the vessel to the piping ex.(borda entrance, flush entrance) be included?

Also should the pipe exit (for ATM relief) or branched Tee (for flare header relief) pressure drop be included as well?

Thanks for the help!

-SETXChE

 

[LittleInch]

First question answer is yes. All pressure drop from the vessel to the inlet of the valve is included at full relief flow.

Same applies to the discharge. This should be all losses from exit of valve to flare or vent tip. Also allow for other vents going off for a header system.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SETXChE]

Thank you for the help LittleInch. You have confirmed what I was thinking. Do you know of any literature, other than API 520/521 that could lend me some guidance on the subject?

Thanks,

SETXChE

 

[LittleInch]

API 520 is pretty much the key document in terms of requirements.

B31.3 also has some requirements, but more practically based.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[prochem]

LittleInch,

The requirement for the pressure drop in psv inlet line comes from ASME Section VIII, Div 1, Appendix M and API RP-520 Part II. non-recoverable losses are mainly the line frictional loss. Please note that the static pressure loss is not included. Though 3% of set pressure criteria can used in most of the designs, the higher pressure inlet line pressure drop can be tolerated (say about 5%) by higher blowdown rate (of course with coordination with vendor) when required.

PSV outlet lines should be sized to avoid high back pressure which affects PSV capacity, resulting in relieving pressure exceeding MAWP. please note that 10% of the set pressure criteria is only for conventional relief valves.For Bellows and pilot operated PSVs the limit can be as high as 30% and 50% of the set pressure respectively.

Cheers