Most maintenance and operations personnel who work with centrifugal pumps have been warned to never start a pump unless it is primed. They have been warned that a pump that is started when loaded with air may cause the seal or packing to be scorched and permanently damaged, and that when the suction liquid level is below the pump (suction lift), the pump would not begin to pump. Then they encounter a pump that is said to be self-priming. At this point, they begin to question if all that caution is necessary.
The real scoop
The fact is that no centrifugal pump is truly self-priming in suction lift situations. Furthermore, there actually are several approaches where a pump may be started when loaded with air. First, though, in all cases the seal must be protected from overheating. That requires more than just a cooling mechanism, since mechanical seals and packing depend on some small amount of liquid to migrate between the stationary and rotating members to lubricate them.
Protecting the seals
A properly primed pump would have the seal vented—ensuring that the seal is supplied with pumpage or flush liquid for cooling and lubrication. Centrifugal pumps classified as "self-priming" are most often equipped with double seals that have a barrier fluid in the chamber between the two seals. This barrier fluid supplies the necessary cooling and lubrication to protect the seals from scorching when the pump is started dry. (See the "API Seal Plans" for more information on double seals and barrier fluids, or contact your seal distributor.)
Assuming the seal has been provided with adequate cooling and lubrication, the concern now is whether the fluid is above (flooded suction) or below (suction lift) the pump. The problem, of course, is with suction lift. The task is to create sufficient suction to lift the liquid into the pump. The impeller can't do that. The impeller is designed to develop a pressure differential with liquid in the impeller and pump housing. Common liquids are more than 800 times as dense as air. Centrifugal pumps won't pump air.
Finding a solution
There are two common approaches to the problem. The most straightforward is to provide the pump with an auxiliary pumping device that will evacuate the air out of the pump and draw the liquid in (see Fig. 1).
It is assumed the suction line is submersed in the liquid, forming an air seal. Similarly, the discharge must also have an air seal, which is usually provided by means of a ball- or flapper-type check valve that prevents air from being drawn into the pump housing from the discharge line. The secondary "air pump" may be a diaphragm- or eductor-type unit, and electrically, mechanically or pneumatically driven.
With the pump suction and discharge sealed, the secondary pump will pump the air out and draw the liquid in. When the liquid is drawn up to the level of the impeller, the impeller begins to pump, forcing open the discharge check valve. A pressure switch will then shut down the secondary air pump.
A second approach is to build the pump housing in such a way that liquid will remain in the housing when both 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 Fig. 2). A ball or flapper check valve on 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" the air. This approach requires that on initial installation of a new or rebuilt pump, there must be an initial prime loaded into it. If, for any reason, the pump tank is drained, the unit will not begin to pump—thus, the self-priming feature of this pump is only effective after an initial prime. This style of equipment may be referred to as a "re-priming" pump.
Secondary air-pump example
For an interesting example of a pump that uses a secondary air pump for priming, click here to read an article about the drainage pumps in New Orleans, LA. They have both suction and discharge submersed, so a discharge check valve is not required. For an example of pumps that use the re-prime approach, visit the Websites of WEMCO, Gorman-Rupp, ITT Goulds or other pump manufacturers. MT