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[immy_huss23]

I wanted to know the affect on how a psv would operate if the valve discharged into a tank that was kept under a partial vacuum? How would back pressure come into affect in this situation?

[ankur2061]

Is this a liquid service relief valve or a vapor service relief valve?

Since it is discharging into a vessel I am presuming that this a liquid service relief valve with some flashing occuring of the liquid due to the lowering of the pressure. Since the liquid is expanding from a high pressure to a lower pressure (vacuum) at the inlet to the vessel it will form a jet of liquid entering the vessel at high velocities (venturi effect). You might think of providing a baffle at the inlet connection of the relief valve tail pipe to the vessel to prevent jet impingement.

The back pressure at the relief valve outlet connection would be the sum of the superimposed back pressure (in your case the vessel vacuum) plus the variable back pressure due to the frictional pressure drop in the relief valve tail pipe for the rated relief capacity of the relief valve.

Hope this helps.

Regards,
Ankur.