Overpressure on PRV, Stagnation or Static.

[Pandenes]

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.

 

[Latexman]

Static pressure. A simplistic view is the huddling chamber surface area is parallel to the approaching fluid flow, so the velocity head has no effect on it. Plus, the fluid flow turns down and away from the huddling chamber, and the departing velocity head does not affect it, though the change in momentum does enhance lift on the huddling chamber. There is velocity in the PSV but because of impact angles there is no velocity head effects on the working parts.

Stagnation pressure is the static pressure at a stagnation point (velocity is truly zero).

Good luck,
Latexman

 

[Pandenes]

Oh.
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.

 

[Latexman]

A 4" (100 mm) inlet to a T nozzle PSV?? I'm used to seeing at least an 8" (200 mm) inlet to a T nozzle PSV. What size is the inlet connection on your PSV?

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

 

[Pandenes]

OK, I´ll do that.
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

 

[Latexman]

In your case, the assumed 10% overpressure is in the vessel. Then design for the proper inlet and outlet pressure drop. No need to calculate an overpressure at the PSV.

Good luck,
Latexman