True Wastewater Pump Energy Costs Revealed

During a speech at the Pump Centre Conference 2011, Stefan Abelin from Flygt urged end users to do more to understand true pump efficiency in actual field conditions. Extensive measurements in the field have shown that end users' actual energy costs are often increased by as much as 30 per cent for on-off pump operation in wastewater, compared to clean water pumping. The costs are even higher for pumps with long duty cycles or those in continuous duty operation. Little effort is being spent on measuring and understanding true pump efficiency.

There is a general lack of awareness of the key issues involved in efficient pump operations in the wastewater industry and an over reliance on minimum pump through-let size as a criterion for equipment selection. The pump industry has not yet taken on the task of developing test procedures for wastewater pumps that reflect 'real world' duty. Today the end user is left to assume that the same pump efficiency will be delivered in the actual application of the pump. While this may be true for clean water pumping, non-clog pumps have vastly varying applications that challenges pump performance.

The type and amount of soft solids being transported immediately impacts the pumps performance and the measure of actual efficiency may vary widely from the factory-tested efficiency. Sustained efficiency is defined as a pump's ability to maintain its momentary efficiency over a long period of time, when operating in its intended application. Non-clog pumps that cannot deliver sustained efficiency will have substantially higher energy costs than a modern self-cleaning pump design, often by as much as 25-40 per cent. In particular, Abelin highlighted the problem of using non-clog pumps with variable speed drives to deliver a 'soft-stop' to the pump.

The general belief is that this is good practice, but this lack of pump cycling means the pump does not benefit from the back flush that occurs each time it is stopped. The downside is that a clogged pump will not benefit from the important flush generated by a hard pump stop and the pump is less likely to regain its original efficiency. Our advice to pump users is to ensure that they are using a self-cleaning pump, such as the N-pump from Flygt, which will enable them to retain pump efficiency in actual operational conditions.

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