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 su ction as a means of controlling the flowrate. The effectiveness of VSD in control is critically dependent on the interaction of the pump cha racteristic curve (H/Q curve) and the process characteristic curve, ju st as the controllability of a control valve is. This paper demonstrat es a simple means of calculating the useful flow turndown of any VSD/c entrifugal pump system, which is frequently much poorer than might be expected and demonstrates the variation in process gain over the usefu l speed range. It covers both the simplest process characteristics and some of the more complex.