Pressure Control Valve On Pump Discharge

Dear All,
Recently i was reviewing P&ID for emergency product transfer pumps. I saw a PCV on the minimum flow header of centrifugal pump (diesel driven) with outlet of pcv connected back to suction. This looked a little strange to me. As far as i know, in order to keep pump running on minimum flow to avoid NPSH problem you need a FCV or RO ( Flow orifice).
Pump discharge pressure is 26 barg
Pcv set pressure is 31 barg
Pump shutoff is 32 barg

I would like to know opinion of experts here, if this system will work or not?

Control valves can be used to regulate minimum flow and may offer a more economical solution for large pumps. Why not control the total discharge flow instead of the pressure? Pressure control may work in 'normal' conditions, but what if the impeller becomes worn? What if the fluid density drops? I would stick with flow control to reduce the system hazards.

Recently i was reviewing P&ID for emergency product transfer pumps. I saw a PCV on the minimum flow header of centrifugal pump (diesel driven) with outlet of pcv connected back to suction. This looked a little strange to me. As far as i know, in order to keep pump running on minimum flow to avoid NPSH problem you need a FCV or RO ( Flow orifice).
Pump discharge pressure is 26 barg
Pcv set pressure is 31 barg
Pump shutoff is 32 barg

I would like to know opinion of experts here, if this system will work or not?

 

muddu4,

 

The pumping system you described is similar to the fire fighting pumps system in which there is no flow control and it's adequate having adequate pressure at discharge header. In such systems the minimum flow line is equipped with a PCV to conduct the flow to the suction line/tank at a pressure corresponding to the specified minimum flow to avoid going to the unstable flow/shut off conditions. Then the system will work but with having no flow control at relevant discharge line/header...

Dear Fallah,

The service here is Motor gasoline. During emergency situation (fire) the product from Depot tanks will be emptied and sent to refinery storage tanks which are located at approx 12 kms from existing storage Depot.

When the consumption at refinery end is reduced, FCV if present, will operate to recirculate flow inorder to protect pump. Whereas with current design (PCV) i am not sure how minimum flow would be maintained to protect pump. Pump would experience very high velocities which resut in excessive vibrations,in turn it will affect NPSH and further it might cavitate your pump as well.

The service here is Motor gasoline. During emergency situation (fire) the product from Depot tanks will be emptied and sent to refinery storage tanks which are located at approx 12 kms from existing storage Depot.

When the consumption at refinery end is reduced, FCV if present, will operate to recirculate flow inorder to protect pump. Whereas with current design (PCV) i am not sure how minimum flow would be maintained to protect pump. Pump would experience very high velocities which resut in excessive vibrations,in turn it will affect NPSH and further it might cavitate your pump as well.

 

muddu4,

 

In applying PCV as pump protection device in minimum flow line, as i mentioned, the set pressure is the corresponding pressure of the minimum flow. A simple sketch of the system would help to submit more proper answer to your query.

Dear Fallah,

 

I am Attaching P&I diagram for better clarity.

 

Note : GO service code is for Gas Oil

muddu4,

 

The PID shows no problem for pump protection against block discharge; especially because there is a PSV in discharge line as a back up for PCV in the case of stuck closed. Is there any other issue(s)?

PSV will never pop-up as pump shutoff pressure is 32 barg whereas psv set pressure is 40 barg, piping is designed for 36 barg.

Only issue is pump curve being too flat, very less margin is available between pcv set point corresponding to minimum flow. Do you think system would have worked better if we had FCV?

PSV will never pop-up as pump shutoff pressure is 32 barg whereas psv set pressure is 40 barg, piping is designed for 36 barg.

Only issue is pump curve being too flat, very less margin is available between pcv set point corresponding to minimum flow. Do you think system would have worked better if we had FCV?

 

muddu4,

 

Then what is the role of the PSV? Are you certain the pump is centrifugal type? The symbol implied it might be positive displacement type...please recheck and report...

 

As per your statements, the pump will be started on emergency (fire) situation to empty the Depot tank hence needs maximum flow without any flow control. Then, the pump might be operated near the end of curve and having FCV on minimum flow line necessitates getting the signal from DCS/PLC while in fire case they might be damaged and being out of service. For this reason considering a self regulation PCV can be the best choice in safety viewpoint for this matter rather than a DCS dependent FCV.

 

Dear Fallah,

Its the result of poor engineering. Now since everything is completed including procurememt of PSV we have decide to go with system as it is.
Symbol is as per company standard, Yes it is centrifugal pump.
I agree with your statement that during fire scenario DCS/PLC might be down and PCV is best choice as far as safety is concern.

But once the flow is reduced at user end will this PCV help to avoid cavitation problem due to pump flow falling beyond minimum flow?

May be i need to have a word with Pump Vendor.

But once the flow is reduced at user end will this PCV help to avoid cavitation problem due to pump flow falling beyond minimum flow?

May be i need to have a word with Pump Vendor.

 

muddu4,

 

Yes it can do the job avoiding pump damage due to unstable condition and catastrophic thermal effects of operation below minimum stable flow, provided that the PCV set point had been properly selected and the PCV/minimum flow line sizes to be such that can handle the minimum flow corresponding to the PCV set pressure in the case of complete discharge blockage.