Safety Relief Valves

[homerphish]

If I have a safety relief valve where the set point is at 250 psig and the maximum allowable back pressure is 52.2 psi, is the pressure at the valve discharge at 250 psig and the maximum allowable pressure drop along the line 52.2, or is the pressure at the valve discharge 52.2?

 

[1969grad]

In your case, the discharge pressure will be no more than about half of the inlet pressure or less due to sonic flow through the valve. See API RP 520 for a detail explanation.

If the discharge system backpressure exceeds 52.2 psi (assuming the number is correct) then the capacity of the relief valve will decrease unless hardware provisions are made in the valve such as a balanced bellows.

 

[israelkk]

What is the fluid type? is it a gas or a liquid?

If it a gas the 52.2 limit is not reasonable because up to ~125 psi the flow will be constant (sonic) when the upstream pressure is 250 psi. If it is a liquid then

flow=constant*SQRT(P_up-P_down)

and this will be affected if the P_down is larger then 52.2 psi.

 

[TD2K]

Given your statement "If I have a safety relief valve where the set point is at 250 psig and the maximum allowable back pressure is 52.2 psi", the pressure at the valve outlet can not exceed 52.2 psi during relief.

That said, API 520 restricts conventional relief valves where the built-up back pressure is not more than 10% of set pressure. Static backpressure can be higher than 10% but affects the pressure the relief valve opens open.

Where the built-up backpressure is greater than 10% of set pressure, then bellow relief valve or pilot operated relief valves are options.