What is priming of a Centrifugal Pump? |Why Priming is necessary| How priming is done?|What is Cavitation of a Centrifugal Pump?
Priming:
Priming of a centrifugal pump is defined as the operation in which the suction pipe, casing of the pump, and a portion of the delivery pipe up to the delivery valve are completely filled up from an outside source with the liquid to be raised by the pump before starting the pump. Thus the air from these parts of the pump is removed and these parts are filled with the liquid to be pumped.
Many times the water source is in a position above the pump in this case the water supply is sufficient, but when the water supply is below the pump the supply of water in the casing is not sufficient. In this case, the pump creates a vacuum inside the pump. The sealing and the pump can be damaged. In some pumps, the suction side of a priming unit is fitted to suck the air in the pump and help to get water in the pump.
            This means that when a pump is running, the air in the pump is running freely at high speed, and the head generated is in terms of meters of air. If the pump is primed with water then the pump can suck and deliver the proper amount of water to avoid this difficulty, priming is necessary.
Why is Priming necessary?
Priming is necessary because if air will be present inside the pump we know that air is compressible. Considering the liquid in the pump is water, so air has a density 1000 times less than water so what happens is the amount of centrifugal force will also decrease by 1000 times. If we go by the formula of centrifugal force [F =mv2/r and M=density/volume]. So when density decreases 1000 times force also decreases 1000 times.
So it will not be able to create suction pressure at the eye of the impeller so the liquid will not able to come up and the impeller will run dry. There will be no discharge taking place and due to this dry run of impeller it may cause overheating that can damage the pump and the pumping action will be lost. That’s why priming is necessary.
Practically how is priming carried out?
Consider using a foot valve and an outside source of liquid in order to prime the pump.
The following are the steps to priming the pump:
- During priming, open the suction valve, and keep the discharge valve closed, while the vent is open.
- Open the valve in the outside supply line and let outside source water fill the pump until only liquid escapes from the vent.
- Then close the vent and slowly open the discharge valve.
- Close the outside supply line and priming is achieved.
Priming done by using a Vacuum pump:
The vacuum priming system is used to pull water into the pump, which then allows the pump to start. It does this by creating a vacuum that extends from the top of the impeller right down to the water level of the reservoir from which the liquid is being pumped. As the vacuum removes air molecules from the pump, water is pushed up into it.
Cavitation:
Cavitation is defined as the phenomenon of the rapid formation of vapour bubbles in the suction side and subsequent collapsing of these bubbles, when the suction pressure gets below the vapour pressure of the liquid.
When the vacuum is created at the pump water is sucked in and when water flows at a higher velocity at one section comes, it vaporizes and forms vapour bubble (where kinetic energy increases and pressure decreases). When this bubble passes through low pressure region to high pressure region bubbles implodes. The formation and collapse of bubbles are going to be very rapid and when the bubbles implode they strikes the impeller, this striking action is called pitting which exert high stress on the metal of the impeller, casing, and other parts inside the pumps.
Because of this the impeller and other parts of the pump’s insight get damaged and very small holes (cavity) are made in the outer surface of the parts.
Note
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