Priming is the process of preparing the pump prior to operation. In particular, the pump head must be full of fluid to ensure proper functioning.
As explained in the Flow-Pressure Curves lesson, the pressure developed by centrifugal pumps depends on the density of the fluid being processed. If the pump head contains air (which is 800 times less dense than water), the pressure developed will be insufficient to draw any fluid into the pump, expel the air, and generate a flow rate.
The impeller will rotate in the pump head in a condition called “dry running” (there is no liquid surrounding the impeller). Levitronix pumps can detect dry running if it happens for more than 1.5 seconds while the rotation speed is above 6000 rpm. If dry running is detected, the controller will automatically react by slowing down the impeller to 5000 rpm and generating a warning message.
While dry running is an issue for most pump technologies, Levitronix pumps can sustain this condition for extended periods of time without damages.
Positive displacement pumps are self-priming due to the very small clearances within the pump. They displace a fixed volume of fluid (gas included) with each stroke. As a result, positive displacement pumps are able to expel the air from the suction line and generate a flow.
The video on the side shows how a peristaltic pump is able to self-prime (credit: István Gáspár).