Self Priming Pumps for Your Self Priming Lift Station
Self-primers are handy tools for many different pumping applications, and Romtec Utilities engineers complete lift station systems with self-priming pumps. Let’s take a look at some of the technical aspects of self-priming pumps.
First off, the term “self-priming” is a bit of a misnomer. Self-priming pumps do require an initial prime to be supplied manually after installation or anytime after the pump has been drained. After that, the pumps will re-prime themselves for each subsequent cycle, even if they inlet line drains back into the wet well.
Self-primers are generally regarded as centrifugal pumps because this pump type is susceptible to air-binding, when air in the pump prevents suction. As a result, centrifugal pumps that avoid getting air bound are valuable to distinguish. Positive displacement pumps, however, would be more accurately described as self-priming pumps because they can pump air at a near equivalent efficiency to water, but because they don’t experience air-binding problems as often, self-priming isn’t an interesting feature of positive displacement pumps.
The way (centrifugal) self-priming pumps work is with an external volute casing that contains the pump’s impeller system. The impeller portion of a self-priming pump is almost identical to a normal centrifugal pump. The volute casing holds a consistent water level that is above the pump’s datum line – a level above the “eye” of the impeller– which creates a Net Positive Suction Head (NPSH).
The NPSH creates lower air pressure at the impeller as the pump begins to operate, drawing water in from the volute casing and forcing air out of a valve. This movement of water is possible because atmospheric pressure –that is the weight of the air in the earth’s atmosphere– remains at a near constant psi relative to elevation. The atmospheric pressure pushes against the influent in the wet well, which, in turn, is drawn toward the decreasing pressure at the impeller. This process continues until air is vacated from the pump and the vacuum reaches close to 100% with zero psi of air pressure. Then the pumping cycle can begin in earnest.
Volute priming is ideal for situations where self-priming pumps are needed with the ability to pump solids. These self-priming pumps can include any type of impeller, whether open, closed, or solids-handling.
The volute priming configuration is typical of how a self-priming pump would operate, but there are other situations where a pump needs to be continuously priming because of air mixed with the influent. This is a common scenario in many industrial pumping applications, where mining processes or petrochemicals maintain a constant air mixture that lowers the efficiency of a pump’s curve. Air mixture -or cavitation- is bad for the pump’s efficiency and can result in frequent air-binding.
In this situation, a diffuse priming system might be used. Diffuse priming operates with a volute casing also, but it uses a slightly different pump casing, with stationary impeller-like ridges called “peelers” that are oriented perpendicular to the rotating impeller. Each of these peelers can be thought of as a cutwater on a normal centrifugal pump. Air bubbles separate from the water at the peelers and diffuse to a valve at the top of the volute.
This system increases the pumping efficiency of the self-primer by continuously removing air from the water at each peeler and thus lowering the psi. For water with a constant air mixture, this type of self-priming pump will operate more efficiently for longer periods of time and with reduced wear to the pump motor. Diffuse priming is less effective for solids due to the high opportunity for binding with the peelers.
Self-priming pumps are useful for many industrial applications because they can perform the same tasks as other centrifugal pumps without having to be submerged, which means easier maintenance and a longer life-span. Above-grade pumps can also eliminate confined spaces that require special permits and in-depth training.
Romtec Utilities can supply self-priming pumps for complete lift station systems. Now that you’ve heard a brief explanation of how they operate, call us and see if self-priming pumps can help your project!