The Pump Guru
NPSH Part 2 of 3 – NPSHr: The Pump Taketh
Understanding the determination of Net Positive Suction Head (NPSH) and how it affects your pump can prevent annoying or catastrophic consequences for your process
Centrifugal pump design data includes a curve that provides the pump’s Required Net Positive Suction Head, NPSHr, at flows across its performance. As the pump draws liquid into its suction, it creates a low pressure zone at the inlet which becomes increasingly negative as the flow increases. Consider that the pump must pull harder as the amount of water it is pulling through the inlet increases. Like NPSHa, NPSHr is also expressed as an absolute and is always related to water as the motive liquid. NPSHr is the height (weight) of liquid column required, above the pump, for the water to remain above its vapour point:
The important thing to remember is that, even if the liquid level is below the pump, the effects of atmospheric pressure, as discussed in the previous article, will possibly result in a high enough NPSHa to allow pumping without cavitation. However, care must be taken when dealing with pumps that have an inherently high NPSHr or when dealing with liquids with a high temperature or vapour pressure.
In the next issue, we will discuss application examples that require additional considerations to avoid cavitation.
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