Cavitation, in the context of trenchless technology, refers to the formation of gas bubbles in the liquid passing through a pump impeller, thus causing it to receive an inadequate supply of liquid. This phenomenon can occur during the process of a bypass pumping operation for repairing sewage lines or for laying new sewage pipelines when the flow of the liquid does not match up to the speed of the pump.
Cavitation is basically the formation of gas in a flowing liquid when subjected to low pressure at constant ambient temperature. It is a type of boiling that occurs as a result of low pressure rather than due to heat. In the case of pumps installed for removing sewage or other waste water from a manhole, sometimes the liquid is either too viscous such that the speed of the flow is reduced, or insufficient such that the head supplied to the pump is low. In either case, it results in the formation of gas bubbles as the liquid passes through the inlet of the pump. In normal conditions, the piston inside the pumping chamber meets with a column of water passing through the inlet at the same speed as the piston and thus is able to discharge normally. In the event of cavitation at the inlet of the pump, the accompanying gas bubbles do not allow complete filling of the pumping chamber with liquid. This causes a slamming action of the piston on the discharge valve, sending a shock wave through the system. This is often accompanied by a cracking noise.
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