7 replies to this topic

[WMJ]

A centrifugal pump is provided with a minimum flow recycle line that routed back to the suction line. The minimum flow recyle line is provided with a flow control valve. The pump discharge is around 60 barg whereas the suction line is only rated for 19 bar (150# class)upstream of pump suction valve. The question is; it possible if the flow control valve on recycle line fail open/stuck open then the suction line will be overpressure and relief valve is required to protect the line? Inlet line of pumps is from a separation vessel and have an check valve on the line from vessel to pumps. This check valve is located upstream of recyle line tie-in to suction line.

In my experiecne;if the flow control valve on the minimum flow recycle line fail open/stuck open and there is a danger to the suction line to get overpressurized (exceeding 150# limit), then it will be better to install PIT (Pressure Indicating Transmitter) at the downstream of the flow control valve (150#) which will shutdown the pump on exceeding the suction line pressure above 150#.

i have also sketched a diagram that will help to understand the location for PIT.

Attached Files

•  OverPressure Protection.pdf   70.4KB   98 downloads

Edited by WMJ, 27 May 2010 - 04:53 AM.

[sheiko]

I believe the piping back to and including the suction isolation valve should be rated for discharge pressure.

[Qalander (Chem)]

I believe the piping back to and including the suction isolation valve should be rated for discharge pressure.

Dear Sheiko, I caution you, since -1 rating I received on similar pretext of suction pipeline by..........? still unjustified by whosoever gave!

[sheiko]

[Dear Sheiko, I caution you, since -1 rating I received on similar pretext of suction pipeline by..........? still unjustified by whosoever gave!

Sorry it was a mistake. I give you back a positive 1.

Edited by sheiko, 28 May 2010 - 03:38 AM.

[Qalander (Chem)]

Thanks for kind gesture indeed!

Assuming blocked discharge and pump recycle valve fail open, the flowrates will depends on backpressure thru the minimum flow line. This is the normal suction pressure, friction loss thru pump recycle line and pressure drop across minimum flow control valve. That means no overpressure scenario forsee. Any inputs?

Yes you're right. The function of the check valve is to avoid back flow, when the recycle valve fail open, the liquid cycle in a blocked system, the sunction line class depends on suction pressure. If the upstream pressure is 19bar, the pressure after check valve is about 19bar, the minimum flow line is to keep the pump normally working without cavitation.
No need to add PSV on minimum flow line. But the PSV after minimum flow line and downstream check valve should be considered.