Discussion on the performance curve of centrifugal pumps
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In the characteristic curves of centrifugal pumps that we often see, the H-Q curve starts drawing from a flow rate of 0, and most power-flow curves also start from 0 flow rate. However, the NPSH-Q curve does not start from 0. Why is that? Since the N-Q curve is determined through experiments, how is the NPSH-Q curve obtained?
The NPSHr curve is determined through experiments, with values taken at the NPSH point where the head drops by 3%. That’s why the new API uses NPSH3 instead of NPSHr. As for why it doesn’t start from zero, I’m not quite sure. Wait for an expert to explain it
I think many H-Q and P-Q curves are extrapolated to 0; in reality, they haven’t been measured.
Generally, the pressure when the pump is operated under load must be measured, so I think 0 represents that value. It’s hard to say for the ones nearby.
It’s simple: when measuring the net positive suction head, the measurement starts after the pump has reached stable operation during testing, so the flow rate is not 0.
The last edit to this post was made by A fierce tiger in the heart on 2016-5-18 at 17:54. Generally, there is a considerable distance from point 0 to the minimum flow rate on the cavitation head line; isn’t the cavitation head value over that distance measured as well? Or does the pump fail to operate stably when the flow rate is lower than this? But at this point, the H-Q curve has values.
The H-Q curve becomes meaningful once the outlet valve is opened after the pump starts operating. The first thing to consider is whether this pump can achieve the designed flow rate and head. In fact, for us, the net positive suction head is a supplementary parameter; it is only necessary to measure it under specific operating conditions. It’s not possible to scrap a pump just because there’s an issue with its net positive suction head – after all, the net positive suction head is a sophisticated technical parameter. Vacuum is created to measure the cavitation margin only after the pump’s designed flow rate is met.
Does that mean then that pump manufacturers do not consider the net positive suction head at low flow rates? Why not test it?
Basically no measurement is required, unless special operating conditions dictate it (measurement is done only when there is negative pressure at the pump inlet)
Actually, Ah Lei upstairs has explained it pretty well. During the testing of NPSHr, the flow rate is kept constant while the liquid level at the pump’s inlet is gradually lowered. The pressure difference between the inlet and outlet is monitored; when this difference decreases by 3%, the inlet pressure is recorded, which represents the NPSHr in the conventional sense. This process is quite long. At zero flow rate, the pump is actually in a non-normal operating condition; the energy density of the liquid inside the pump is very high, which makes cavitation and vibration more likely to occur, conditions that can cause irreversible damage to the pump. In severe cases, shaft seizure may even happen. Therefore, it is not feasible to conduct a cavitation measurement in such situations. From a design perspective, zero-flow pressure is primarily used to determine the maximum pressure that the pump outlet equipment and systems can withstand under the most adverse operating conditions, and it is crucial for process selection. ISO 9906 specifies explicitly that a zero-flow factory test must be conducted. As for the cavitation margin at zero flow rate, its significance is not well understood, neither in terms of process design nor operation; coupled with the potential risks associated with it, it is generally not tested.