Sanjeev,
Theoretically if the system is operating above its design conditions it is
not OK. The truth is that all valves, flanges etc. can take lots more
than their rating but you operating above the rating puts you in a
difficult legal situation in the event of a failure.
I expect the orifice is there because centrifugal pumps must operate on
their curves. If the downstream pressure is below design the flow rate
increased dramatically, so does the power consumption. In fact you could
burn the motor out. Your pump is most probably salvaged from somewhere.
It most probably has too high a head, so they put an orifice plate in to
match the system resistance with the one it is designed for.
Yes changing to water does affect the pump. I do not have the formulas
here but I think the head will increase by the ration of the product sg to
the design sg. I think the power will go up by the square of cube of that
ratio.
The important pressure for you to watch is the pump shut in pressure. For
goodness sake get hold of the pump curve and calculate what the shut in
pressure will be when the pumped fluid is water. THAT is the pressure you
should be worrying about. I agree with previous comments that you should
ensure that an adequate relief and shutdown are installed.
BUT also worry about the power consumption of the motor. You should be
able to measure what that is at current operating conditions and then
estimate what it will be at the worst operating conditions using those
simple relationships we all learned at college. The pump will most
probably always hold out but overpowering the motor will cause a fire and
that is the last thing you want.
Gordon Reddek
Specialist Mechanical Engineer
Alcan Engineering, Level 3, 443 Queen St, Brisbane, Qld 4001, Australia.
Tel: +61 7 3328 6424
Fax: +61 7 3328 6990
Email: gordon.reddek@alcan.com
"sanjeev gupta" <sanjeev.gupta@eil.co.in>
29/07/2004 01:33 PM
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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has been replaced by the centrifugal type.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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Received on Thu Jul 29 19:14:00 2004
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