3 replies to this topic

[raisie]

Hey guys,

I am studying a system which has a PRV at the discharge of a positive displacement pump.
The discharge of the PRV is sent back to the suction line of the pump.
The pump has no control whatsoever. 
I was wondering, if the PRV relieves flow to the suction while the pump keeps receiving flow from the main source,
there will be a sudden increase of flow to the pump.
What will happen to the head and other pump parameters?

Thanks in advance!

[samayaraj]

Raisie,

 

In positive displacement pump, flow is nearly constant for a fixed rotation of shaft. When the discharge valve is closed, PRV will pop up and send back to suction. This will be like re-circulation and the flow in discharge will remains constant and the pressure will rise upto PRV set pressure.

 

 

#Samayaraj

Edited by samayaraj, 22 January 2015 - 11:43 AM.

[Art Montemayor]

This topic has been discussed and commented many times - since the Forums first began.

 

Sending a recycle stream (whether from a PSV or from a control valve) from the discharge side back to the suction nozzle or pipe side of a positive discharge pump is probably one of the worse things you could do for the pump and for your operation.  It is nothing but a cheap, quick, and bad engineering attempt to handle a high-pressure, usually 2-phase stream.  The discharge of a positive displacement pump should always - in mine and many engineers' minds - be sent directly back to the suction source (tank or drum).  This is safest and most efficient way to handle any recyle around a pumping device.  

 

The resulting unstable hydraulic effects of a free-expanding liquid (usually warm or hot) are bad for any pumping situation - especially for a PD pump.

[fallah]

raisie,

 

As Art well mentioned it's a bad engineering practice sending back the PRV discharge flow to the PD pump suction line; but if you need to know the situation in blocked discharge conditions in a PD pump in which the PRV discharge connected to relevant suction line, the pump wouldn't take any flow from the suction drum and the flow recirculates inside a closed loop. The flowrate wouldn't be changed anymore but the pump head would be the head corresponding to the PRV set pressure...