If the suction pressure is equal to or less than the vapour pressure of the liquid, some of the liquid inside the pump changes into a vapour, a phenomenon called cavitation. Generally in a centrifugal pump, when the liquid enters the impeller, at that time due to the impeller, the velocity of the liquid becomes very high, due to which the value of the pressure head decreases.
Sometimes such conditions occur that the inlet pressure of the pump becomes so low that the vapour pressure of the liquid exceeds it and at this time the liquid becomes vaporized and the pump stops working, this phenomenon is called cavitation.
Note:- To prevent cavitation, do not exceed the suction lift of one limit.
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The Precautions Against Cavitation
(1) By reducing the pumping rate.
(2) By not using bend to reduce friction loss in the suction line.
(3) By changing the Impeller design.
If that section
Velocity head + Pressure head > Vapour Pressure of liquid
v²/ρg + P/ρg > Pv/ρg
Then cavitation will not happen.
Types of Fittings in Mechanical Engineering
Different types of accessories are used in the construction of a pipeline, which is called pipe fittings. How to join the pipe pieces depends on the size and wall thickness of the pipe. If the wall is thick, the pieces are joined by threaded fittings or by welding. If the wall is thin, it is joined by soldering.
(1) Screwed Fittings
When helical fittings are to be used, a bangle is made from outside on the ends of the pipe, this work is done with the tool of making the bangle. Some of the bangles that are on the maximum bangle from the end of the pipe are kept incomplete so that the fitting is strengthened by joining the joint.
(2) Flange Fittings
Pipes longer than two inches are joined by flanges. A flange is a ring of metal that can be bolted together and held crosswise between their faces. The flange in the pipe is either screwed or welded.
Equivalent Length
It is the length of a pipe that has such a friction head loss that it is equal to the friction head loss generated due to fitting in the pipe.
Equivalent Diameter
It is four times the hydraulic radius and is denoted by D. It is used for the calculation of friction in straight and channels whose cross-section area is non-circular, it is used in Reynolds number and friction factor.
Entry Length
The length of a pipe that is required to reach the boundary layers in its centre and to establish a fully developed flow is called entry length.