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Self-Priming Pumps – continued.
Why is priming needed
Before we get into how a self-priming pump works - it may be good idea to review why priming is needed in the first place. Recall that a centrifugal pump uses centrifugal force to expel water outward from its center. For a continuous operation, additional water needs to be supplied to the center of the pump to allow it to work continuously. There are two ways this can be accomplished. First, gravity feed water into the center of the pump - this is easily accomplished when the pump is located at an elevation that is lower than the pump's center point. The second method is to make use of the slight vacuum that the pump creates when it expels water from its eye to its circumference. The vacuum created in this fashion is rather small but in properly-designed pumping systems it is enough to suck in water. This is the reason to "prime" a pump. Priming allows a pump to create a slight vacuum and establish a constant supply of water to its eye.
An Example of a Self-Priming Pump
In the illustration provided here, notice the water storage section in the lower part of the pump casing. The water present in this section is enough to prime the pump for startup. The left hand side figure illustrates the priming action. During this phase, the pump impeller expels the water present in its casing - thereby creating a vacuum in the casing. The vacuum draws in water from the suction side.
This pump is designed with two volute sections. During the priming cycle, the lower volute functions as an intake and while the upper volute discharges liquid and entrained air into the separation chamber. Air is separated and expelled through the pump discharge while liquid circulates into lower volute. Once air is completely removed from suction and liquid fills the impeller eye, the pump is fully primed, and starts to work normally.