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|3. Effect of cavitation
The phenomena of pump cavitation is due to the vaporisation of the liquid pumped at the entrance of the pump impeller. Bubbles formed then collapse within the impeller when the pressure goes up again. The quick collapse of the bubbles translates in high forces that can damage the pump impeller.
Cavitation is very harmful for pumps.
Cavitation will happen if the pressure at impeller entry is below the saturation pressure of the liquid sucked. Static pressure decreases within the pump suction due to the acceleration of the fluid at the suction and pressure can go below the saturation pressure of the liquid.
Figure 1 : Pump cavitation - decrease of pressure at pump suction
Cavitation condition : Psuction - NPSH < Psat
Non Cavitation condition : Psuction - NPSH > Psat + margin
The NPSH is given by the pump
In practice, a margin must be included in the design to account for process variations (temperature...), it is to be determined by the engineer according to what is acceptable for the case studied (consider different liquid pressure, for example, which can have as effect to change the vapor pressure of the liquid and thus the pressure at which cavitation can appear), according to the calculation codes applicable, or experience - some supplier, if no other value is specified, will consider 0.5 m of margin.
The suction pressure is calculated by summing the pressure in the vessel in which the liquid is pumped with the static pressure of the liquid in the vessel minus the pressure drop due to the suction line.
Psuction = P1 + h1*spgr/10.2 - ΔPf
Note : conversion from bar to m of liquid : ΔP = H*spgr/10.2
- ΔP in bar
- H in m
- spgr in kg/m3
Figure 2 : Pump circuit
Cavitation in pumps has the following effects
- Possible reduction in pump capacity due to the vaporization,
this drop in performance can be translated by drop of head at same
flowrate or a change in the efficiency of the pump
- Wear of impeller and pump housing due to the collapse of the bubbles and the pressure associated. It can decrease pump performance and lead to mechanical failure of the pump. The elements impacted by cavitation take a very specific "pitted" aspect.