A pump performance curve is an important chart for any pump, representing the variation of differential head developed by the pump against operating flow rate through that pump.
This kind of a pump curve is provided by the pump manufacturer to guarantee a certain level of performance by the pump for the range of operating flow rates.
It is therefore used in pump performance testing to ensure that the pump actually performs as guaranteed by the manufacturer via the pump curves.
Reading a pump performance curve
In the simplest words, pump performance curve is a graph of 'differential head' developed by the pump, plotted against the operating flow rate.
When more fluid is pushed through the pump, it is generally going to develop less differential head (given the mechanical and power constraints).
The same can be observed on the pump performance curve. For any given impeller diameter, pump generates the maximum differential pressure or differential head near the shut off point, when there is very little fluid getting pushed through the pump.
As the flow rate is increased, we move to the right of this graph. And you can find that for the same impeller diameter, differential head starts to drop as the flow is increased.
But, even as the diff. head drops down the resulting output given by the pump in terms of flow rate multiplied by differential head goes up because of increasing flow rate. As this happens, the pump also consumes more and more power to push more fluid while trying to maintain similar level of differential pressure (or differential head).
As a result, there is a point of optimal efficiency where the pump can operate at highest ratio of output power / input power. This point is known as the best efficiency point for that pump and clearly visible on 'Pump Efficiency Curve' plotted on the same chart, against operating flow rate.
Values of NPSH required (NPSHr) are also provided by the pump maker, in the form of another graph plotted on the same chart.
Using pump curves in operation
Pump curves are quite useful in the pump selection, testing, operation and maintenance.
- When selecting a pump for a particular process, pump curves are essential as they specify not only the performance characteristics, but also operating flow range, efficiency characteristic, NPSH requirements etc. So the engineer can decide how suitable the pump would be for different operating scenarios.
- When the pump is procured, it has to be tested for the performance promised by the pump maker. Pump curves are used for such pump performance tests.
- During operation of the plant, conditions and process requirements may change considerably. Correspondingly the operating flow rate and differential pressure requirements also changes. Pump curves are then used to find out if the existing pump can be used in the modified operating conditions. If yes, then what would be the limitations.