RE: Pump Shut off pressure

From: <Steve>
Date: Thu Jul 29 2004 - 08:05:00 EDT

Hi Sanjeev
if the relief valve from the old recip pump has been removed, then you have a small problem. Your flanges are rated to about 17 bar, and you have the potential for a 25% overpressure if the flow is restricted. It is quite likely that the old relief valve is undersized for centrifugal pump duty anyway.
Ignoring code/legal requirements, it wouldn’t keep me awake at night provided I put in a relief/bypass valve and a safety valve at the next shutdown (assuming 3 months max and infrequent starts), AND immediately (now) put in a pressure switch and timer to trip the pumps if the pressure stayed over system rating for more than three times the expected startup transient period. This is probably safer than the system you had with the recips as the pressure changes more gradually. Your other aspect is simply more reason to do something now, as the shutoff pressure is proportional to the sp gravity. I assume the system is in good condition.

Cheers

Steve

-----Original Message-----
From: sanjeev gupta [mailto:sanjeev.gupta@eil.co.in] Sent: Thursday, July 29, 2004 3:33 PM
To: PipingDesign@yahoogroups.com
Subject: [PipingDesign] Pump Shut off pressure

We have a typical problem. At an offshore platform existing recip. pumps has been replaced by the centrifugal type. The fluid handled is crude oil having a design temp. of 60°c. The new pumps have a shut off pressure of 21kg/cm2, but the existing piping is 150 class. There is an Restricted orifice in the discharge piping therefore the piping upstream of RO is expected to see a pressure of 18-19kg/cm2 on regular basis i.e. whenever the pumps are in operation. The pumps are designed to operate at 14 kg/cm2 and discharge of say 5m3/hr. As the downstream vessel is operating at pressure of 2-3 kg/cm2 probably this is the reason why Restricted orifice plate has been inserted into the discharge piping??.

As the pumps are for intermittent service, can the downstream piping which is 150 class can be considered as safe?? The frequency of operation of pumps is not yet known. The pipe wall thickness are not a problem but valves, flanges etc. are of 150 class.

There is another aspect of the problem. The fluid handled is crude oil which is supposed to have maximum sp. gravity of 0.8 and probably the pumps are designed accordingly. As the water cuts are increasing day by day and if say low level switch on the vessel on the suction side of the pump malfunction due to some reason, there is every possibility that the water will enter the system. Will the pumps shutoff pressure remains the same when water is being handled by the pump? What about the piping?

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