Self Priming Pumps: Where to use them, why to use them, and the misconceptions
Self priming centrifugal pumps are often overlooked because they are not as commonly used as either standard centrifugal pumps or submersible pumps, and not understood by many who specify pumps. They are also misapplied or not properly applied to pumping applications by people not properly trained in their application.
What is a self priming pump?
A self priming pump is like any other centrifugal pump, but it is designed with an external casing that “floods” the inner pump or volute. The casing is filled with liquid [usually the same as the liquid to be pumped] and the pump is ready to go. When the impeller rotates in the casing, a low pressure area is formed at the eye of the impeller. Because the pressure is lower than atmosphere, water is pushed up the suction pipe by atmospheric pressure, forcing all the air in the suction line into the pump. The pump mixes the air with the recirculating water in the casing. The air separates from the liquid and is discharged from the casing. When all the air in the suction line has been displaced, the pump is dynamic and delivers flow like any other centrifugal pump.
Where to use them?
Self priming centrifugal pumps can be applied to any application that standard centrifugal or submersible pumps are applied, within their hydraulic limitations. Applications from water, fuels, effluent or “grey water”, right through to raw sewage, industrial wastewater and sewage sludge can be effectively handled by self priming centrifugal pumps. Provided they are within their hydraulic limits, and they are manufactured with the right features for their intended duty, self priming pumps can deliver the safest and most cost effective of pump solutions.
Why use self-priming pumps?
Self priming pumps are mounted above the liquid source [generally at “ground level”], so less infrastructure is required to set them up. And because they are located at ground level, they are easier to access and much easier to maintain and repair. As there is no mechanical equipment in the wet well, there is no need to open wet well lids, and there is no need for hoists or cranes [as in the case of submersible pumps], making them a much safer option for operators.
How to apply self priming pumps
Firstly, the right type of pump needs to be selected for the duty [fuel pumps for fuel applications, solids handling pumps for wastewater applications etc].
The right size pump for head and flow then needs to be selected.
Then, an NPSH calculation needs to be done to ensure there is enough atmospheric pressure to support the intended flow, and the speed of the pump needs to be fast enough to prime the pump.
Selection of the suction line size is also important. It needs to be large enough to support the desired flow, and small enough to keep priming times to a minimum. It is best to avoid long suction lines, but if there is no choice, keeping part of it full during priming cycles will help [talk to Hydro Innovations about the “P Trap” design].
What are the limitations of self priming pumps?
Self priming centrifugal pumps are limited in their suction lift [usually a maximum of 7.6m]. Some Claims by some that 8.0 or even 8.5 metre suction lifts are possible, are misleading. Although some pumps may be able to generate a vacuum in excess of these figures [as can Gorman-Rupp self priming pumps], they cannot physically pump on these lifts because of limitations in NPSHr of the pump, the temperature of the liquid, and friction losses in the suction line.
A single pump is also limited to a flow rate of about 300 litres per second. And heads above 90 metres are also difficult to achieve for true self priming pumps.
Misconceptions about the use of self priming pumps
Some of the misconceptions about self priming pumps are as follows:-
- Self priming pumps need to be re-primed with water after a pumping cycle.This is not the case with “guaranteed re-primers”, like Gorman-Rupp’s Super T, Super U, and Ultra V. These pumps only need to be charged with liquid once, and they retain enough liquid in their casings to guarantee re-priming each and every time they are called upon to pump.
- Self priming pumps are inefficient.In most cases the hydraulic efficiency of a self priming pump will not be as high as that of a standard centrifugal pump or submersible pump. If engineers looked no further than this, self priming pumps would never be used, and asset owners would pay a lot more for infrastructure costs and/or the maintenance of these pumps for the life of the asset. But as many engineers and asset owners have found, the cost savings in infrastructure and maintenance can far outweigh the savings one may achieve in selecting a slightly more hydraulically efficient pump.
- Self priming pumps are too expensive.It is true that self priming pumps are more expensive than standard centrifugal pumps. This is because there is a lot more material [because of the need for an outer casing], more components and more machining. This should to be weighed up against the capital cost savings [no dry well], the ease of access and the ease of maintenance offered by a self priming pump.Also, some self priming pumps are more expensive than others. Again the facts need to be weighed up. A pump purchased that has been engineered and produced by a company that has been manufacturing pumps for over 80 years [like Gorman-Rupp] can be considered as an investment in reliability. Pumps manufactured by companies attempting to copy market leaders will invariably not be able to meet efficiency, quality, repeatability and reliability standards of the market leaders, and purchasers can rightly consider these as costs.