Self-priming pumps are centrifugal pumps that retain a small volume of fluid in order to start up without requiring outside attention to clear their passages of air. Centrifugal pumps convert mechanical energy into kinetic energy by increasing the pressure of a liquid but can become air bound, a circumstance in which the pump is too full of air to operate correctly.
Self-priming pumps prevent this by keeping some fluid in the system as the pump is shut down. These pumps are constructed of aluminum, cast iron, stainless steel or plastic. Self-priming pumps are essentially the same as many other centrifugal pumps, such as axial flow or vertical pumps; the difference is in the design. Centrifugal pumps are made up of an impeller, a volute and a motor. A common use is pumping water from an underground well into a plumbing system or storage tank. Pumps are used for drainage and water transportation as well as in wastewater and sewage treatment plants. Some pumps are heavy duty and are used in commercial and industrial applications to move chemicals and corrosive liquids; the most common use, though, is with water. Self-priming pumps eliminate the need for additional pumps, vacuums and valves which can complicate the system or are impractical to use because of the pump's location; as a result, self-priming pumps are used for deep well operations or remote placements.
The simple design of centrifugal pumps allows for many variations in design and use. The motor rotates the impeller which is mounted on a shaft inside a volute. As liquid (usually water) is sucked in, it is imparted with kinetic energy; the volute slows its speed but increases the pressure which directs the liquid out of the pump. Axial flow pumps use a propeller while jet pumps feature a Venturi tube for even higher pressures; the general technique, however, is the same. In order to retain a small amount of water and become self-priming, pumps may have volutes and casings with built in reservoirs that are filled as the pumps runs. This water is recirculated through the system during the priming phase to get the pump running and then is discharged. Another option is a suction cavity above the impeller that collects water to ensure that it is always present in the system. Despite a self-priming pump's ability to prime itself, it must have water in the casing and cannot be run dry. These pumps are unable to create a vacuum without the presence of a liquid. Submersible pumps are typically self-priming because they are placed in the water source and have no difficulty obtaining water. Industrial pumps placed in factories or plants can be self-priming; some are even designed for use with dirty water and can handle solids without becoming clogged.
