CAVITATION IN PUMPS AND IMPORTANCE OF CALCULATING NET POSITIVE SUCTION HEAD IN PREVENTING IT

by: Siavosh Nasiri K

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Pressure drop on propeller vane Formation of vapor bubbles Implosion of bubbles Damaging the vanes Causing disorder in continuity Even stopping pump discharge

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Damage to propeller vane due to Cavitation

Cavitation

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There is a corresponding pressure in liquid saturation in front of each temperature

Evaporation due to decrease in pressure:

Cavitation

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Cavitation

Vapor bubbles flow with liquid to other points with higher pressure

Collapse (implosion) of vapor bubbles Generating high-pressure pulses Erosion of pump vanes Dangerous and damaging to pumps in

short period of time

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Cavitation

Major pump parts Cavitation and Implosion

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Cavitation

Implosion of bubbles

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Potential Cavitation Areas

Cavitation

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Cavitation

Symptoms:

1. Reduction of volumetric flow rate (Débit)

2. Decrease in efficiency (Rendement) of the pump

3. Vibration 4. A special discontinues noise: similar to

sound of some bullets hitting a metal surface!

10 Cavitation

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NPSH

Total pressure of fluid in suction eye of pump compared to evaporation pressure of fluid in pumping temperature

Unit: SI: metre Imperial: foot

Types: NPSHAV

NPSHreq

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NPSH

NPSHAV or NPSHA :actual pressure of the fluid in the suction eye of the pump

 NPSHreq: Minimum pressure needed in the suction eye of the pump in order to prevent cavitation (presented by Manufacturer)

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Pump Factory Head-Capacity Diagram

NPSH

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NPSH

To reduce the NPSHreq: pump with less circular velocity more pumps or using a double-suction eyed

pump specially designed suction-eye propeller

(only specific volumetric flow rates)

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NPSH

Ways to increase NPSHA:

putting suction source or total system under pressure to increase pump suction pressure

if fluid temp. is high, feed source should be on a higher position than pump and under pressure

low fluid velocity reducing the losses in suction pipe

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NPSH

Maximum suction height (Zs max):

maximum altitude that pump can be installed upper than free surface of the liquid

if Zs max < pump altitude Cavitation