Cavitation in pumps and ...
Transcript of Cavitation in pumps and ...
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!
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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