Overpressure on PRV, Stagnation or Static.
Overpressure on PRV, Stagnation or Static.
(OP)
I would like to clarify if the overpressure in a relief valve to have rated capacity is Stagnation or Static.
A Farris 2600 vapor trim needs 10% overpressure to obtain the rated capacity. To check the 10% overpressure I compare Stagnation pressure at the inlet flange of the RV to set pressure of the RV.
¿Is is correct?
Thanks in advance.
A Farris 2600 vapor trim needs 10% overpressure to obtain the rated capacity. To check the 10% overpressure I compare Stagnation pressure at the inlet flange of the RV to set pressure of the RV.
¿Is is correct?
Thanks in advance.





RE: Overpressure on PRV, Stagnation or Static.
Stagnation pressure is the static pressure at a stagnation point (velocity is truly zero).
Good luck,
Latexman
RE: Overpressure on PRV, Stagnation or Static.
I expected to hear Stagnation.
I thought it was Stag because of when I do the pressure drop calculation in this case:
Farris 2600 orifice T directly connected to a vessel (100mm pipe + flange). P set: 8.8 barg. Capacity 90 t/h. Fluid: steam.
The velocity at the inlet of the RV is very high (160 m/s). So the static pressure is quite low, almost equal to set pressure of RV.
I would need to lower the set pressure of the valve (to a value lower than MAWP of vessel) in order to solve it.
Thank you.
RE: Overpressure on PRV, Stagnation or Static.
Plus, you have the sizing methodology wrong. Sorry I didn't see that before. You don't calculate the overpressure at the PSV. The allowed overpressure is 10% for process upset or 21% for fire. That determines the sizing pressure. The sizing pressure is usually either 1.1 x MAWP or 1.21 x MAWP. Then, you have to design for an acceptable inlet pressure drop (3% usually) and outlet pressure drop (depends on PSV type).
Is this your first PSV design?
Good luck,
Latexman
RE: Overpressure on PRV, Stagnation or Static.
Sorry, I mean a pipe as short as possible, 100mm.
8" diameter (same as RV inlet).
I´ve already installed some other RV´s.
I will design for 1.1 MAWP but also check the 10% overpressure at valve inlet to check that the valve will have rated capacity. I also check the inlet pressure drop (<3%) and outlet pressure drop (<40%).
Kind Regards
RE: Overpressure on PRV, Stagnation or Static.
Good luck,
Latexman