Relief valve inlet/outlet lines

[sheiko]

Hi,

A basic questions:
What should be the design pressure of the inlet and outlet lines of a PSV:
1/ Inlet: the set pressure or the relieving pressure?
2/ outlet: The same pressure?

Thanks in advance

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[CJKruger]

sheiko, my recommendations are:

1) Inlet, design pressure is the same as the design pressure of the equipment that is protected (often same as set pres, and less than relieving pres).

2) Outlet, design pressure based on margin above maximum operating pres or standard value for relief system (many clients use 50 psig).

 

[sheiko]

Thanks

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[maddocks]

You may wish to consider using a higher design pressure for a certain section of outlet pipe as the pressure will be dropping. However, you really should do some backpressure calc's to see for sure. Be careful - certain valve selections depending on the size and setpoint can increase the outlet flanges to 300# ANSI and higher...

 

[CHD01]

Set pressure and inlet temperature will normally dictate what the inlet and outlet flange ratings are for a relief valve based on the selection curves provided by the relief valve vendor. It is common to match the flange rating for the discharge line to the discharge flange rating of the relief valve. However, note that the maximum outlet pressure at the valve exit will normally be much lower than inlet pressure (assuming sonic conditions apply at the valve nozzle). Outlet pressure will be equal to the superimposed backpressure (P3) plus any overpressure at the vent tip (P2) which results in noise plus the pressure drop in the line (P1-P2) due to frictional losses.

The more you learn, the less you are certain of.

 

[Kandan]

Relief valve outlet pressure should be above the maximum expected back pressure during the relief of the valve. (the maximum back pressure expected against the relief valve from all relieving scenarios..

 

[dcasto]

I've installed relief valves with ansi 600 out because the relief line went to the suction of the pump it was protecting.

 

[CHD01]

To: dcasto

Good point, yes I agree sometimes it is the destination of the discharge line that determines flange rating. Ultimately of course the engineer has to determine the max operating pressures for pipe sections and where the pipe code breaks need to occur.

One other point on a discharge on a pump suction connection. This is not the preferred choice -its better to go to a storage tank where possible. Also, for the example you mentioned , even a 600# flange matching the 600# pump suction may not be adequate if the pump discharge is a higher rating (say 900# for example) and you have a closed discharge valve by accident.

The more you learn, the less you are certain of.