PSV pressure at outlet

[MohdYaseen2010]

PSV is set to protect the system from overpressure by releasing the build up pressure @ valve upstream & release it to downstream. My question, normally the high pressure at the PSV inlet & low pressure in the outlet, how much pressure can be expected (or calculated) at the outlet if we know the pressure at the inlet?
for example:
3X6 PSV set @ 25MPa, what will be the pressure at the outlet or what should the rating of the discharge system?

Mohd Yaseen

 

[RVAmeche]

This is why relief valves require inlet and discharge pressure drop calculations. Conventional spring-loaded valves have a maximum 3% loss on the inlet and 10% loss on the discharge (aside from external fire) which you could use as a guestimate (if it's a conventional valve). Balanced bellows valves can allow much higher backpressure on the discharge side.

The exact pressure at the discharge flange of the relief valve depends on the overpressure, inlet losses, loss across the orifice, discharge losses, discharge location (atmosphere, relief header/tank, etc), and flow rate.

 

[Latexman]

Mohd,

The PSV outlet flange pressure is calculated backwards, from the exit (atmospheric pressure, if the exit is not choked) to the PSV outlet flange. It usually has very little to do with the PSV set pressure or sizing pressure. The outlet pipe and fittings and flow sets the pressure at the PSV outlet flange.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[MohdYaseen2010]

Dears;

Thanks for your responds

Mohd Yaseen