Pumping applications account for more than 20% of global electric motor energy consumption. Many of the pumps that are in operation today were built in the 1960s and 70s, which is a testament to the original design and the skill of matching the pump to the operating demands of the application. A properly specified pump will operate reliably for decades, with only the minimum of repairs needed. It is essential that designers match the conditions that encompass the entire system, typically referred to as the system curve. The pump duty point (design point) is only one point on that system curve.
As operating conditions evolve over time, so the pumping application demands also change. This can include a change in flow, media characteristics or duty. However, just because a pump is capable of operating on the outer edges of the system curve, it does not mean that there will not be consequences in terms of performance or reliability.
For this reason, manufacturers establish the minimum operating flow as well as listing all of the limiting design conditions, such as net positive suction head required (NPSHR), maximum flow and others. It is important to understand the consequences of dialing back pump flows which means measuring the operating conditions and calculating the point at which the pump will be operating on the system curve.
This can be illustrated by a recent repair project that was completed by one of Sulzer’s service centers. A multi-stage boiler feed pump was brought in for repair and it was immediately clear that the pump had suffered some severe damage. An initial inspection showed that the suction-side seal, several impellers, sleeves and stationary components had been destroyed. The evidence indicated that the pump had been operated at or near shutoff flow.