Chemical Processing - May 2008 - (Page 34)

Variable Speed Pumping A pump/control-valve combination provides smooth flow even at very small valve openings, although in some cases protection against low flow is required. However, with variable speed pumping, flow isn’t always smooth at reduced speeds due to either the pump or the drive. Here, we’ll focus on the pump — specifically, how certain aspects of the pump performance curves can lead to variations in flow at reduced speeds — because this requires the expertise of people in the process group, not the electrical department. Most electrical departments can provide a good analysis of the drive but aren’t familiar with pump performance curves. measured; instead the output of the level, temperature or other controller is connected directly to the control valve. Often some modifications are made to reduce costs. We’ll assume a flow controller here, but the points generally apply to all configurations. Because pressure measurements are less expensive than flow ones, a measurement of either discharge pressure or pump differential pressure is substituted for the pump flow measurement. The success of this approach depends on the nature of the pump performance curves; it only works if the pressures significantly change with pump flow. >> 150 120 Head, ft. Head, ft. 90 60 30 0 0 Flow regulation with a control valve 150 120 90 60 30 0 0 50 Pump curve Pump curve 3,450 rpm3,450 rpm System curve for 75% System curve for 75% valve open control open control valve System curve System curve for 100% for 100% open control valve open control valve >> Propagation of variance for control valve 150 120 90 60 30 0 Head, ft. 150 120 90 Static Head 61 ft60 59 ft System curve for 75% System curve for 75% valve open control open control valve System curve System curve for 100% for 100% open control valve open control valve Head, ft. Static Head 61 ft 59 ft 30 0 0 0 50 50 100 125 129 Pump curve Pump curve 3,450 rpm3,450 rpm 125 129 175 175 179 179 177 150 127 100 50 100 150 200 Flow, gpmFlow, gpm 127 177 200 250 250 100 150 150 200 Flow, gpm Flow, gpm 200 250 250 Figure 1. As the valve closes, friction head increases, leading to lower flow. 150 120 Head, ft. 90 60 30 0 0 150 Pump curve Constant speed pump Pump curve 3,450 rpm3,450 rpm Low flow protection for a centrifugal pump with a con120 stant speed drive normally entails recirculation back to either an upstream vessel or to the suction of the pump. System 90 2,850 rpm2,850 rpm System flow curve A proven approach is to measure the curve through the pump and send the value to a controller that manipu60 lates a control valve in the recirculation line. The set point for the controller is the minimum 2,370 rpm2,370 rpm required pump flow. Under normal operating condi30 tions, flow should exceed this level; so the controller 98 will close the valve in the recirculation line. However, 98 0 177 should flow drop below the minimum, the controller 177 150 0 50 50 100 200 250 100 150 200 250 will quickly open the control valve to provide the necFlow, gpm Flow, gpm essary flow. A control valve also is required in the line to the process to provide appropriate flow. In some configurations, this flow loop is the inner loop for a level-toflow cascade, a temperature-to-flow cascade or other 0.5 cascade arrangement. In some cases, the flow isn’t 34 0.4 • May 2008 Figure 2. The variance in head leads to twice as much variance in flow. 150 150 Head, ft. Head, ft. Head, ft. Pump avoided by inserting a fixed The control valvecurve be curve Pump can 3,450 rpm3,450 rpm orifice into the120 recirculation piping. This provides some 120 recirculation at all times, even when the process flow exceeds the minimum necessary for low flow protection. System curve System curve 90 90 It requires extra energy Static possibly, a larger pump. and, Static Head 61 ft Head 61 ft 0.5 d, gpm/rpm gpm/rpm 0.4 Variable speed 60 pumping 59 ft 60 59 ft 2,450 RPM In contrast, with a VSD a flow measurement is used to 2,450 RPM adjust a speed 30 torque controller included with the or 30 drive electronics. 25 Vendors 13 always 13 savings 175 179 25 stress possible energy 175 179 with a 0 0 VSD. However, especially in industries such as specialty 177 200 0 50 50 100 150 100 150 200 chemicals, 0few drives are large enough to provide suf- 250 Flow, gpmFlow, gpm ficient energy savings to justify the additional cost for them. However, a VSD offers other potential benefits. It may make the following equipment unnecessary: 1. sensor/transmitter for pump flow; 250 2. recirculation piping; 250 200 200 www.chemicalprocessing.com 177 250 http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - May 2008

From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor WirelessHART signals a change at plants Avoid costly fabrication mistakes Watch out with variable speed pumping Hot cutover boosts control system migration Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - May 2008

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