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Priming the pump is necessary for it to draw or lift water to be pressurized and transferred.

 

But improper methods for priming a pump can cause damage to the pump or cost hours of lost time due to lack of proper methods for priming at start up.

 

Centrifugal pumps have no built-in mechanism to cause them to draw fluid. Even many positive displacement pumps require fluid in the pump to prevent damage from running dry. By carefully following basic steps, priming can easily be attained.

 

Centrifugal pumps can be divided into two groups, End Suction, which includes multistage, and Self-Priming. In addition, pumps may have a flooded suction or may be designed to lift water.

 

End Suction Pumps

 

If the system has a flooded suction, such as an elevated tank or a pressurized suction line, priming the system can be as simple as opening the valve and allowing water to flow through the pump and out of the discharge. While this may be enough to prime, it is important to verify that air is not trapped in a high point of the volute or casing. If possible, remove a plug near the top of the casing and allow any air to escape. If power is available, you may be able to run the pump to help purge any air pockets.

 

If the system requires the pump to lift water from a point below the pumping level, the pump and suction line must be purged. This typically will require a foot valve in the suction line to prevent fluid from exiting during the fill process. Purging will need to be done from a port in the top of the volute or a port plumbed into the discharge piping.

 

It is necessary to purge all air from the line by pouring slowly through the port while allowing the air to escape as the water enters. This includes piping down to the foot valve.

 

It is important that the vertical suction lift does not exceed the maximum depth for the liquid being pumped. This is based on the specific gravity of the fluid, temperature, and atmospheric pressure. For instance, the maximum for cold water at sea level is theoretically 34 ft. Consult tables and local elevations for correct information.

 

Self-Priming Pumps

 

Self-priming pumps are designed to hold prime in case the suction pipe is empty either by pumping the source empty or backflow. The pump does, however, require being primed to start with. The pump must be filled with fluid before it can pump at all.

 

The pump is designed so that the impeller “EYE” is always submerged. If air enters the pump that is primed, it will be bypassed through the discharge so that the air will not displace the fluid held within the pump. This allows for the pump to draw the air from the suction pipe and bypass to the discharge. Shallow well jet pumps often fall into this category and prevent damage to the pump itself if the well should run out of water.

 

Priming is required for centrifugal pumps. Reluctance to prime is caused by a breach in the suction side. It is easier for a pump to draw air rather than a fluid.

 

Sources of air must be eliminated. Check piping, especially fittings and joints. A small leak that may not be shown under pressure can easily leak under suction. If PVC piping is used, make sure the PVC cement is sealed entirely. If threaded joints, make sure a good thread sealant is used and applied correctly.

 

Ultimately, make sure the pipe extends to the lowest level of the fluid. Keeping the number of fittings to a minimum will prevent added piping losses. A correctly sealed suction system will prime easily.