I would like to know that for all the AUTO START centrifugal pumps ( Open valve start up) , whether it should be designed for End of Curve operation?  That is , NPSHR should be less than NPSHA even even at its end of curve. I am guided to select a vendor who can satisfy the above condition. We have two pumps ( 1 running + 1 standby) . If one pump trips the other pumps should come in line in auto. we have individual NRV for these pumps . When I should propose to the vendors that my requirement of a centrifugal pump should be designed for the End of curve operation where the NPSHR is less than NPSHA at its end of the curve. Can U please throw some light on this.

You need to draw a system head curve on the pump H/Q performance sheet- as it is possible that the pump will not reach the end of curve on start-up as the system resistance may intersect the H/Q prior to end of curve.

If the pumps can not run to end of curve there is no need for the NPSHr to be less than the NPSHa - calling for this in a contract will only result in larger and more expensive pumps which will then run too far left of their BEP in normal operation.

However, if they can run to end of curve you need to look at the running period before they run up the H/Q curve to the normal operating point - a short period of cavitation is usually not a problem. 

It can also be pointed out that designing the pump for end of curve operation is not a free option.  At that point, the system would have a lower NPSH(available) and the pump would have a higher NPSH(required).  Selecting for this condition would tend to drive the selection to a pump with a lower NPSH(r) and a resulting higher suction specific speed.  High suction specific speed can result in a pump with poor performance at lower flows.  Only select a pump for conditions that it will actually experience for longer than a few minutes.  Other examples have come up at my plant.  In order to switch pumps from the main to the spare, both pump need to be running at the same time for a few seconds. But, we do not design our piping or our pumps for parallel operation of both pumps.  During unit start-up or shut-down, there may be very brief periods of operation at very low or very high flow.  For most services, this is not a problem and the pump design should not be changed drastically to be optimum at these unusual operating points. As I already noted, this might not be true if the pump is a Sundyne running at 20,000 rpm or a 30 stage vertical turbine.  For a typical single stage pump, short duration transients are usually not a problem.

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