Self-priming centrifugal pumps: What you need to know
Most maintenance and operations personnel who work with centrifugal pumps have been warned repeatedly that starting a pump dry can scorch and permanently damage the seals or packing, and that pumping won’t begin if the suction liquid is below the pump’s suction lift. Then they encounter a “self-priming” pump and begin to question all those warnings.
Most maintenance and operations personnel who work with centrifugal pumps have been warned repeatedly that starting a pump dry can scorch and permanently damage the seals or packing, and that pumping won’t begin if the suction liquid is below the pump’s suction lift. Then they encounter a “self-priming” pump and begin to question all those warnings. So, what’s the real scoop on self-priming pumps?
In fact, no centrifugal pump is truly self-priming in suction lift situations. And while there are several ways to start a pump that’s filled with air, the first priority is always to protect the seal from overheating. This is usually accomplished by cooling and lubricating the mechanical seals and packing with a small amount of liquid that migrates between the stationary and rotating parts.
Protecting the seals
A properly primed pump will have the seal vented, ensuring a continuous supply of pumpage or flush liquid to cool and lubricate the seal. Centrifugal pumps classified as “self-priming” usually have double seals with a barrier fluid in the chamber between them. The barrier fluid floods both seals and supplies the necessary cooling and lubrication to protect them from scorching when the pump is started dry. See the “API Seal Plans” (API 683 / ISO 21049) for more information on double seals and barrier fluids, or contact your seal distributor.
Two common approaches
Assuming the seal has adequate cooling and lubrication, the next concern is whether the fluid is above (flooded suction) or below (suction lift) the pump. The problem, of course, is to create sufficient suction to lift the liquid into the pump. But centrifugal pump impellers can’t pump air. By design, they can only develop pressure differentials in pump housings with liquids, which are commonly 800 times denser than air.
Of two common solutions to this problem, the most straightforward is to evacuate the air and draw liquid into the pump using an auxiliary pumping device (see Figure 1).
This assumes the suction line is submersed in the liquid, forming an air seal. The discharge must also have an air seal, which is usually provided by using a ball or flapper check valve to prevent air from being drawn into the pump housing from the discharge line. The secondary “air pump” may be a diaphragm, piston or eductor pump, and may be electrically, mechanically or pneumatically driven.
With the pump suction and discharge sealed, the secondary pump will push the air out and draw in the liquid. When the liquid reaches the level of the impeller, the impeller will begin to pump, forcing the discharge check valve open. A pressure switch will then shut down the secondary air pump.
A second solution is to design the pump housing so that it will retain liquid when both the suction and discharge lines are drained. Suction and discharge nozzles may be located well above the impeller, creating a “tank” below that houses the impeller and volute (see Figure 2). A ball or flapper check valve on the suction or discharge may prevent siphoning of the liquid in the tank when the pump is stopped. When the pump is restarted, the fluid in the tank is sufficient to develop suction lift and draw fluid into the pump, tank and impeller, and to purge the air out the discharge. The pump is said to “digest” or pass the air. With this approach a new or rebuilt pump must be primed initially when it is installed. If the pump tank is later drained for any reason, the pump will not begin to pump. Since the self-priming feature of this pump style is only effective after an initial prime, it might be called a re-priming pump. For pumps of this nature, the user may consider freeze protection or draining during cold months.
Eugene Vogel is a pump and vibration specialist at the Electrical Apparatus Service Association Inc. (EASA).