Open Flow Path for relief

[EngChick22]

I am evaluating some exchangers to determine if we need a relief valve. The only case is fire. If we car seal open the relief valves, the exchangers can relieve back to the tower. However, there is a fail open control valve in the path. My boss says you can't relieve through a control valve since it's a restricted flow, is this correct?

 

[Latexman]

Correct. I have never relieved flow from a PSV through a CV. I have relieved a CV that fails open through a PSV though.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

Not only is a restricted flow, even if fully open it will have a resistance to flow much greater than straight pipe, but you can't guarentee that it will open even if it "fails". It might not fail and close because that's what it is supposed to do or just get stuck in position.

What exactly do you mean by "car seal open the relief valves"? Jam them open?? Ni that case just have an open vent and remove the control valve.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EngChick22]

My mistake, I meant car seal open the block valves, not relief valves. So, basically it sounds like even though the valve is fail open, you have to consider it getting closed and blocking the relief flow.

 

[LittleInch]

Correct. All that fail open means is that if it looses it's motive power, usually an air supply, or its positioner air supply, the spring will force the actuator open. However if the air supply is ok and the positioner signal is working, it could be in any position.

However even if fully open, it will have a certain CV which will cause it to have a pressure drop. What that is depends on the valve size and trim, but could be considerable. Either way you can't put it in a relief line.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way