Operation far away from a pump’s best efficiency point (BEP) is a common cause of pump unreliability. There are a number of reasons why a centrifugal pump may be found operating in an off-design condition: The pump may either have originally been mis-applied or the process requirements may have changed since its initial specification and installation. Two minimum flows must be satisfied on the pump curve for reliable service: 1) The minimum continuous stable flow (MCSF), which prevents internal recirculation; and 2) the minimum thermal flow, the point where rapid internal heating begins. The pump manufacturer will usually state the MCSF for its pumps in its documentation. For best overall performance, it is always best to keep the operating point above the hydraulic minimum flow. Regardless of the reason for pump misapplication, the effects of a poor hydraulic fit will typically be dismal reliability performance and poor overall system efficiency. MORE

The prices of low-voltage variable frequency drives (VFDs) have declined substantially since they were introduced, while the cost of power has increased. Normal project economics will now justify these VFDs - which go up to 375 kW (500 hp) or beyond - for pumps over the largest part of the range of application of the pumps. To efficiently evaluate and apply VFDs, process engineers need to know certain elementary things about them. They also need to know about the interactions of pumps and hydraulics when the pumps run at variable speeds. Further economic evaluation beyond the approximate method must be performed, but because low-voltage VFDs require only a small incremental capital investment, their application cannot justify a very detailed evaluation. Usually, screening level economics - as presented here - must be applied and some uncertainty must be accepted. MORE