Reasons for selecting the pump performance curve
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During the selection of pumps, it is generally preferred to choose an operating point that lies on the downward slope of the head curve. Why is there also a type of pump whose performance curve has a peak? Why is it called an unstable curve, and what are the disadvantages of this? Additionally, what does the shut-off head mean at a certain rotational speed? Is this value corresponding to the point at the far left end of the curve? Then why is it necessary to ask at what rotational speed it occurs? I would appreciate it if experts could provide some guidance
First question: According to API 610, the head curve of a pump must decrease monotonically as flow rate increases; in other words, API does not recommend the presence of peaks on the performance curve, as this can lead to instability in the pump. As for why it is unstable, it is mainly because, within a certain range of outlet pressures, the same outlet pressure corresponds to two different flow rates. This means that at a particular pressure, the flow rate is highly unstable; in the worst cases, this can lead to periodic fluctuations in the flow rate. However, since one flow rate corresponds to the shaft power of one pump, this causes periodic shocks to the pump shaft, couplings, and similar components, thereby significantly shortening the lifespan of the pump shaft, couplings, and even the motor. Having understood the above, looking back at it, why is the operating point chosen on the section of the head curve where it slopes downward? This statement isn’t actually very precise; in fact, it is necessary to avoid, as much as possible, sections along the flow path where the same pressure corresponds to two different flow rates. Second question: The shut-off head of the pump varies at different rotational speeds. This value is the point at the far left end of the curve. For pumps whose head decreases monotonically with flow rate, this Shut-off head is the maximum outlet pressure of the pump. But this is not necessarily the case for the pump with a peak-loaded head curve you mentioned earlier.