The Pump Guru
NPSH Part 2 of 3 - NPSHr: The Pump Taketh

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:

  • If NPSHa < NPSHr vapour bubbles form at the impeller eye and cavitation occurs;

  • As the vapour bubbles progress along the impeller vanes, the pressure gradually (or suddenly) increases and the bubbles implode, returning to liquid;

  • This "implosion" of the bubbles is what causes the symptoms of cavitation.
    The symptoms of cavitation will exhibit themselves in a number of different ways that get progressively more noticeable, and potentially more damaging, as the gap between NPSRa and NPSHr widens. Generally, the symptoms will progress as follows:

  • Noise: a crackling or roaring sound which will worsen as flow increases;

  • Decrease in performance: the flow of the pump will diminish as some of the hydraulic work, that would have otherwise been put towards moving the liquid, is taken up in the formation and recompression of the vapour;

  • Vibration: in severe cases, the cavitation will cause vibration in the pump which will cause damage to the pump seals and bearings and to the system piping;

  • Impeller erosion: over time, the shock waves caused by the implosion of the vapour bubbles will pit and erode the surface of the impeller.

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.

The Pump Gure: Q1, 2016

Part 1 | Part 2 | Part 3

 

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