6 replies to this topic

[process101]

Hi every one
Hope all of you are doing good. I have a few questions? What will happened if your suction pressure jump off the max. suction pressure.
Scenario (1)
If you have a pump. your PSV is installed on inlet line of this pump. Due to some reasons your pump start producing more pressure and your PSV relieve all the pressure in your inlet Suction line to the same pump. It will increase its suction pressure. does it cause any disturbance in the system or not if it does then What will be the consequences?
Scenario (1)
I saw in some P&IDs that PSV is installed before Control Valve and the discharge line of the PSV is going into same pipe but after Control Valve. My Question Does it Cause any Disturbance which I am sure it will. But Why the process engineer design this way?
I hope you guys understand what I am try to say.

[JoeWong]

process101,

For 1st scenario (1), are you saying the PSV is installed on pump discharge and relieve to pump suction ?

For 2nd scenario (1), can you tell the disturbance that you expected when you say "Does it Cause any Disturbance which I am sure it will." ?
My opinion for this scenario is ....In the event the Control valve (CV) failed to close position, the forward flow will increase pressure upstream of CV. A PSV will release the pressure by crossing the CV.

[process101]

Thank allot for your reply
For Scenario (1) yes The PSV is Installed on Pump Discharge and relieve To Pump Suction.
Can you also explain that why PD pump comes with PSV. I mean PSV INLET and OUTLET both goes to PD pump?

For Scenario (2) I agree with your point but PSV has 150 PSIG set point and it release the pressure in same pipe just by crossing it. AND I think that This high pressure will increase the flow rate in the system that will cause disturbance and it might open other PSV in the System. thats What I am Concerned about. But If you have some good point please let me know.

Thank you very much Joe for your help

[djack77494]

A PSV installed in a pump's discharge line and routed back to the pump's suction line is a very common configuration. It guards against generating overpressure due to a blocked outlet. If I read your concerns correctly, you seem to be wondering about the pump's suction. It will not "pressure up" under a relieving event because the fluid will merely be travelling in a circle from the discharge, to the suction, through the pump, and back to the discharge. The PSV will take a pressure drop that very nearly matches the presure gain through the pump. THe end result is that the suction side of the pump will be at about whatever pressure it would "normally see" (i.e. Suction Vessel Pressure + Static Head). You will not find a well-designed PD pump that does not have a PSV just because the pump will easily develop destructive pressures if the discharge is blocked.

[JoeWong]

process101,

Doug has clearly explained what will happen...
In additional, one of the point you should not miss is that the PSV outlet pressure is about same as pump suction pressure which is basically the suction static head plus maximum operating pressure of the suction vessel (when PSV open).

Nevertheless this is common in many configuration. But common does not mean 100% good for all application. When fluid passing the the PSV, large pressure drop across PSV induced temporary bubble formation. Normally the bubble should quickly collapse downstream of PSV. However, just by chance this bubbles get into the pump, it will collapse in the pump. Bubble collapse may damage the pump internal + impeller.

Second problem is...when the fluid in circulation (pump suction --> pump discharge --> PSV --> pump suction) and external power get into the pump to move the fluid. But within this close envelop, power continuously input into this envelop. Where does it goes ? Part of it release as noise and vibration and remaining converted to heat. The fluid temperature will increase and subsequently continuous cavitation in the pump.

[process101]

Thank you very much for all your valuable comments.
Can some one also explain the 2nd Scenario. where your outlet of PSV is Connect to pipe line after control valve.

[djack77494]

Can some one also explain the 2nd Scenario. where your outlet of PSV is Connect to pipe line after control valve.

I think this was incorrectly labelled Scenario (1) in your original post, leading to some confusion.

If you wish to safeguard a pump system against the possibility of a blocked outlet condition, AND you have a certain path available to you except for the fact that a control valve sits in that path, then you could chose to "jump over" the control valve with a PSV. Then, even if your controls force the control valve closed, the PSV will allow the normal discharge flow to continue on to (what I presume is) the normal destination. There are some disadvantages to this configuration, but it can be the best solution under some circumstances.