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 can be misapplied to pumping applications by people not properly trained in their application or who do not fully understand their applications.
Convenient Pump Station Upgrades
Wastewater pump stations need upgrading periodically. Sometimes it is because of a degrading of infrastructure and other times it is to increase the capacity of the pump station because of an increase in population, and hence an increase in flow into the pump station.
An increase in flow is often accompanied by an increase in pressure because of additional friction losses in the rising main. Because of this, larger pumps are sometimes needed, and these larger pumps may not be able to be accommodated by the existing wet well. Here is where the use of self-priming pumps can offer a more convenient and more cost-effective solution.
Self-priming centrifugal pumps are used extensively in the USA and Europe in sewage pumping stations, and can be very effectively used in Australia. The Gorman-Rupp brand of wastewater pumps can deliver flows from 5 L/s to 150 L/s and deliver pressures to 90 metres with a single pump, so are hydraulically suitable for the vast majority of pumping stations in Australia.
When a pump station needs upgrading and the use of submersible pumps would require the construction of a new wet well, self-priming pumps can be mounted at ground level, with only the suction lines inside the wet well, enabling the re-use of the existing wet well, delivering substantial savings in capital costs. By going down this path, designers will also be setting asset owners up with equipment that is much safer to operate and much easier to maintain.
Because the self-priming pump station is located at ground level, the pumps, valves and all mechanical equipment is easily accessed for monitoring and maintenance. The discharge of the new station will link up to the existing rising main, but the “old” valve vault can be dispensed with, reducing another hazardous work area. One operator can now safely access and maintain all the equipment at the station without the need for cranes, crane trucks or confines spaces equipment.
Self-priming pumps are also a good option for upgrading wet well/dry well arrangements. A new self-priming station can be positioned [at ground level] directly above the old wet well, and suction lines “dropped” into this wet well. Old dry wells and motor gallery areas can be decommissioned, bringing everything associated with the pump station to the surface for greatly easier access.
More information on pump station upgrades may be obtained from Hydro Innovations at info@hydroinnovations.com.au
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 in Australia may be able to generate a vacuum in excess of these figures [as can Gorman Rupp pumps], they cannot physically pump on these lifts because of limitations in NPSH 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.
