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Cavitation

An analysis of Cavitation which can lead to loss of performance and physical damage to hydraulic machines.
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Overview

Cavitation is the formation of empty cavities in a liquid, followed by their immediate and sudden implosion. Cavitation is a common problem encountered in pumps and control valves; one that causes serious wear and tear and can reduce a component's time-in-service dramatically. The primary causes for their formation include vaporization at low pressure, air ingestion, flow turbulence, and internal re-circulation.

Consequences And Prevention

If the pressure in the hydraulic circuit (Pump, Turbine etc.) falls below the vapour pressure of the liquid at the prevailing temperature, then the liquid will form bubbles. These are carried down stream until they reach an area of higher pressure, where they collapse or implode.

This process has two serious consequences.

  1. If the bubbles collapse against a solid boundary (Impeller Blades, Vanes, etc.), the inrush of water causes high local impact forces which may cause damage to these surfaces. Alternatively if cavitation occurs in combination with chemical attacks within dirty water, then the surfaces may become eroded or pitted.
  2. The flow patterns will be disturbed by the presence of the bubbles. The area of line flow is reduced and eddies are formed giving rise to vibrations( If a Centrifugal Pump sounds as if it is pumping gravel, the pump is almost certainly cavitating).

Associated with this is a loss of performance and efficiency. In severe cases the pump may stop delivering.

Cavitation can be prevented by reducing the suction lift of a Pump or reducing the height of the Turbine above the Tail Race.

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The effects of suction lift of pump performance

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Draft tube design for a turbine