The effluent pump is one of the critical components of a dairy farm effluent system. In most cases there are multiple pumps, which are required to do specific jobs, pumping varying levels of effluent quality.
It is essential that the correct pump be installed to have the required flow rate, pressure, velocity and overall performance.
A large number of pump types are available for the pumping of dairy effluent, these include:
- Pontoon Mounted Effluent Pumps.
- End Suction Pumps.
- Multistage Pumps.
- Progressive Cavity Pumps.
- PTO Pumps.
- Submersible Pumps.
The appropriate effluent pump should be selected taking into consideration the required pump duty (Flow and Head), and the quality of the effluent to be pumped.
Reasons for correct pump selection:
- Reduced operating and maintenance costs.
- Reliable operation.
- Reduced operational management requirement.
Key Considerations of different pumping system types
Centrifugal pumps have one or more rotating impellers. For dairy effluent they are typically:
- Pontoon mounted with a submerged pump head, long shaft and external air cooled motor.
- Shore mounted end suction pump with air cooled motor and a suction pipeline (electric or PTO driven).
- Submersible pumps with submerged liquid cooled motors typically mounted on pontoons, frame systems, guide rail or free-standing with a shore mounted winch system.
- With raw effluent large open impellers are required to minimise blockage. Often chopper/cutter systems are mounted on the impeller shaft to further reduce the risk of pump and system blockage.
- The larger the impeller vane size and tolerances the less efficient the pump is.
- Shore mounted centrifugal pumps typically have closer tolerances to achieve the required NPSH (Net positive suction head) which can result in higher risk of blockage.
- Closed/semi closed impeller, multistage and self-priming type shore mounted centrifugal pumps are much more efficient however are less tolerant of large particle size therefore should only be used after adequate effluent screening/separation.
- Centrifugal pumps are high flow and require higher kW motors. It is essential that the pipeline system is matched to pump performance to maximise operating efficiency. This is where centrifugal pumps shine but they need large pipelines 100mm+.
- Centrifugal pumps require operating as close as possible to their Best Efficiency Point (BEP). This means running at the optimum pressure and flow (not too high or too low). Pumping downhill or filling pipelines can result in motor overload or excessive current over long periods significantly reducing pump and motor life. Variable Speed Drives are required to control the motor in these situations.
- Centrifugal pumps have the lowest capital cost however supporting infrastructure and operating cost must be considered. For example, larger motors mean larger power supplies and cable size. Varied flow at different pressures often means VSD’s are needed (adding cost), floating pontoons and access ways add additional cost.
Progressive Cavity Pumps (AKA PC, Helical Rotor, Mono Pumps)
Progressive cavity (PC) effluent pumps have stormed the dairy effluent market over the past decade. This has come to fruition due to their extremely efficient performance (low power consumption) and their user friendliness. The increase in demand is due to the desire for shore mounted self-priming pumps (over pontoons) as the need for large storage capacity has become a necessity.
The desire for larger effluent areas has meant the need for higher operating pressures to get to those hard to reach places. This is where PC pumps really show their advantages.
- PC pumps are mounted on the shore and are positive displacement. This means they are completely self-priming and are not affected by airlocks or gas build up.
- PC pumps flow variation is minimal. Which makes nutrient management and uniform nutrient distribution a breeze. It does not matter if you are pumping near, far, high or low the flow will hardly vary.
- PC pumps can use up to 3x less power than equivalent performing centrifugal pumps which can result in power savings in the range of $1,000 - $5,000 per year (farm environment and system design dependent).
- PC pumps have the highest capital cost however the full solution and lifetime cost is often cheaper. Don’t be fooled by the high cost of the pump as your power supply, electrical control and supporting infrastructure will cost a lot less and the solution can be a lot more effective.
- PC pumps can pump thick long strand material without risk of blockage.
- PC pumps operate at very low RPM (350 – 450 vs centrifugal pumps at 1450 – 2900). This results in less wear. Wear parts are high cost to replace with PC pumps however the frequency is significantly less. PC pumps are more susceptible to wear with high volumes of sand and abrasive solids however this can be easily overcome with good design e.g. tank/pond inlet placement, floating suction lines and stirrer placement.