3% Inlet dP on a PSV w/set pressure less than 30 psig

[david6245]

Hello,

I have a PSV set at vessel MAWP of 20 psig. API recommends the inlet pipe to the PSV not have a pressure loss greater than 3% of set pressure at valve capacity. However, this is only 0.6 psi. ASME allows an overpressure of 10% for vessels with MAWP greater than or equal to 30 psig and 3 psig over pressure for vessels with MAWP less than 30 psig.

If my total overpressure at the vessel at PSV capacity is less than 3 psig, can my inlet pressure drop exceed 3% or 0.6 psi?

My pressure profile would look like the following for example:

vessel P ->PSV inlet->PSV outlet -> Atmosphere

23 psig ->22 psig ->2 psig -> 0 psig

Acceptable?

 

[Latexman]

In my world for a new design, it is not acceptable. For an existing installatiion AND the blowdown setting of the PSV can be increased, it MAY be acceptable. Depends on the approval of the Chief Engineer.

Good luck,
Latexman

 

[sheiko]

I'm not sure having understood your problem because there is no cause and effect relationship between inlet line pressure drop limitation and overpressure (BTW, relative to set pressure not MAWP).
However, what, i believe, you should ensure is that inlet line pressure drop (in term of % of set pressure) keeps lower than Blowdown. Blowdown is typically 6% of set pressure. 3% inlet max. pressure drop is a rule of thumb which means that if the pressure in the inlet line does not fall down to about half the blowdown (at wich point PSV closes), PRV will not start chattering (opening and closing intempestively).

"We don't believe things because they are true, things are true because we believe them."