Variable Speed Pumps

We often get asked for a pump to achieve multiple duties points. This is often seen as minimum, normal and maximum flow rates with corresponding calculated heads. This can make it quite tricky to efficiently select a centrifugal pump, as a fixed speed curve will increase flow whilst decreasing generated head.

In reality the pump may very rarely see the maximum duty point and so it very rarely runs at BEP (Best efficiency point). Generally speaking, as the flow rate requirement in a system increases, so does the friction loss. The system can switch for a static dominated system, to a friction dominated system for higher flow rates. (Unless, the system has been installed with over-sized pipework to accommodate high flow rates).

There seems to be a habit in the industry to size the pump to be able to achieve the maximum duty point, with the pump running left of curve for normal and minimum flow rates. This can lead to reduced service life and eventual premature failure. To achieve the lower flow rates, the pump ha to be “throttled back” with an actuator or valve to walk it back on its curve. This leads to unnecessary energy consumption, as the pressure required to force past the partially closed valve is wasted energy. Or, the generated head above the required system head, is all wasted energy.

An energy efficient way to achieve multiple duty points with a centrifugal pump is to use an Inverter to reduce the running speed. It is generally appropriate to reduce pump running speed to 30Hz without the need for additional forced ventilation.

Although running left of curve uses less power with centrifugal pumps, it’s still higher than it would be if the pump speed was reduced to provide same flow rate at the system head. This means no artificial head is needed to throttle the pump back and this will serve to reduce frictional wear on the pump and valves, and increase bearing life too.

Remember, for a reduction of 10% in speed and flow, will give a 33% reduction in energy consumption. See our other articles on pump affinity laws for more information.

Inverters can also provide various types of motor and pump protection features like overcurrent protection and dry-run protection to name just a couple.

Our AVF range of pumps with canned motor design provides an extremely efficient solutions and is designed for operation with an inverter. Perfect for multiple duty points and energy saving during lower flow operation.