VARIABLE-SPEED DRIVES AS FLOW-CONTROL ELEMENTS

Variable-speed drives (VSD) are often proposed as an energy-efficient alternative to throttling or recycling fluid from pump discharge to suction as a means of controlling the flowrate. The effectiveness of VSD in control is critically dependent on the interaction of the pump characteristic curve (H/Q curve) and the process characteristic curve, just as the controllability of a control valve is. This paper demonstrates a simple means of calculating the useful flow turndown of any VSD/centrifugal pump system, which is frequently much poorer than might be expected and demonstrates the variation in process gain over the useful speed range. It covers both the simplest process characteristics and some of the more complex.