What is pump priming?
A centrifugal pump priming is done when the passageways of the pump are filled with the liquid to be pumped. The liquid replaces the air, gas, or vapor in the passage ways. This maybe done manually or automatically. Centrifugal pumps are usually filled with the liquid to be pumped before start-up of centrifugal pump. When filled with liquid, the pump is said to be primed. Pumps have been developed to start with air in the casing and then be primed. This procedure is unusual with low-specific-speed pumps but is sometimes done with propeller pumps. In many installations, the pump is at a lower elevation than the supply and remains primed at all times. This is customary for pumps of high specific speed and all such pumps requiring a positive suction head to avoid cavitations. (NPSHa>NPSHr)
Priming the pump
- 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 from pump suction if the valve seats tightly. Foot valves were very commonly used in early installations of centrifugal pumps. Except for certain applications, their use is now much less common.
- A better method is to close a valve in the discharge line and prime by evacuating air from the highest point of the pump casing. Many types of vacuum pumps are available for this service.
- A priming chamber is a tank that holds enough liquid to keep the pump submerged until pumping action can be initiated.
- Self-priming pumps usually incorporate some form of priming chamber in the pump casing.
Prevention of un-primed pump operation
A form of float switch in a chamber connected with the suction line is used. If the level in the chamber is above the impeller eye of the pump, the float switch control allows the pump to operate. If the liquid falls below a safe level, the float switch acts through the control to stop the pump, to prevent its being started, to sound an alarm, or to light a warning lamp. Such a valve and switch are illustrated in the following Figure
Importance of pump priming
Priming reduces the risk of pump damage during start-up as it prevents the dry. For reliable operation, pumps must first be primed; that is, air or gases expelled from the suction and impeller eye area, and replaced with liquid. This is not a problem when the pump is submerged (submersible or vertical sump pumps) or when liquid supply is above the pump. However, when suction pressure is negative, air must be evacuated to accomplish pump priming. The self-priming pump is designed to insure that a sufficient quantity of liquid to re-prime is always retained in the priming chamber see the following figure.