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Thursday, April 1, 2010

Priming on Centrifugal Pumps

A centrifugal pump is primed when the passageways of the pump are filled with the liquid to be pumped. The liquid replaces the air, gas, or vapour in the passageways. This may be done manually or automatically.

When pump is first put into service, the passageways are filled with air. If the suction supply is above atmospheric pressure, this air will be trapped in the pump and compressed somewhat when the suction valve is opened. Priming is accompanied by venting the entrapped air out of the pump through a valve provided for this purpose.

At the rate capacity, a positive displacement pump will develop the necessary pressure to exhaust air from the chambers and from the suction piping. Centrifugal pumps can also pump air at their rated capacity, but only at pressure equivalent to the rated head of the pump. Because the specific weight of the air is approximately 1/800 that of water, a centrifugal pump can produce only 1/800 of its rate liquid pressure. For every 1 ft (1 m) water has to be raised to prime a pump, the pump must produce a discharge head of air of approximately 800 ft (m). it is therefore apparent that the head required for a conventional centrifugal pump to be self priming and to lift a large column of liquid (and in some cases to greater than the rating of the pump. Centrifugal pumps that operate with a suction lift can be primed by providing;
  1. A foot valve in the suction line.
  2. A single chamber priming tank in the suction line or a two chamber priming tank in the suction and discharge line.
  3. A priming inductor at the inlet of the suction line or
  4. Some form of vacuum producing device.
Foot Valves:

A foot valve is a form of check valve installed at the bottom, or foot of a suction line. When the pump stops and the ports of the foot valve close, the liquid cannot drain back to the suction well if the valve seat tightly. Foot valves were very commonly used in early installation of centrifugal pumps. Except for certain applications their use is now much less common.

Priming Chamber:

Single chamber primer is a tank with a bottom outlet that is level with the pump suction nozzle and directly connected to it. An inlet at the top of the tank connects with the suction line. The size of the tank must be such that the volume contained between the top of the outlet and the bottom of the inlet is approximately three times the volume of the suction pipe. When the pump is shut down, the liquid in the suction line may leak out, but the liquid in the tank below the suction inlet cannot run back to the supply. When the pump is started, it will pump this entrapped liquid out of the priming chamber, creating a vacuum in the tank. The atmosphere pressure on the supply will force the liquid up the suction line into the priming chamber.