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Define NPSH, Thoma's cavitation factor and suction specific speed of the pump. Explain NPSHA and NPSHR w.r.t cavitation in pump using a neat sketch

Mumbai university > mechanical engineering > sem 7 > mechanical utility systems

Marks: 8M, 10M

Year: Dec 2017

Difficulty: Medium

1 Answer
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NPSH: Net Positive Suction Head can be defined as the difference between the liquid pressure at the suction point of the pump and vapour pressure of the liquid,

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which is expressed in the terms of height of liquid column.

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Thoma’s Cavitation Factor: It is an equation which is used to measure the cavitation in a hydraulic pump installation. It is denoted by the symbol ‘σ’. The equation for Thoma’s cavitation factor is as follows.

Where,

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Suction Specific Speed(Nss): It is the index used in impeller design which relates the rotational speed of the impeller (N), its rate of discharge(Q) and the NPSH.

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NPSHR (Net positive suction head required) is the head required at the inlet of the pump for its satisfactory operation. It depends on the flow rate and can be determined as follows:

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NPSHA (Net positive suction head available) is the difference between the absolute head at the inlet of the pump and the vapour pressure head.

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Cavitation can be avoided by increasing the NPSHA or by reducing the NPSHR. The following graph illustrates the relation between NPSH, NPSHA, NPSHR and cavitation.

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