Chemical Processing - May 2008 - (Page 38)

Flow s Fl 50 0 0 5 0 0 0 0 3 3 6 Torque, ft.-lbf 9 12 12 15 15 6Torque, ft.-lbf 9 Variable 3Speed Torque, ft.-lbf 9 Pumping 0 6 Torque, ft.-lbf 0 3 6 9 12 12 15 15 >> Flow versus speed for a flat pump curve 250 >> Flow versus torque for a flat pump curve 250 250 200 200 150 150 100 100 50 50 0 0 0 0 3 3 6 6 Torque, ft.-lbf 9 Torque, ft.-lbf 9 12 12 15 15 250 200 150 150 100 100 50 50 0 2,000 0 2,000 2,400 2,400 2,800 3,200 3,600 3,600 4,000 4,000 Flow,gpm Flow,gpm 2,800 speed,3,200 Pump rpm 200 Flow, gpm Flow, gpm Pump speed, rpm Figure 9. High sensitivity of flow to speed at low rpm could lead to cycling. Figure 10. Nearly linear relationship offers advantage for control. dress this problem, conservative tuning usually can accommodate a change in sensitivity of 3:1. But it’s possible that the change in sensitivity could far exceed 3:1. At low flows, the pump curve in our example exhibits a significant sensitivity of head to flow. However, the pump curve for some pumps is basically flat. For such pumps, the increase in sensitivity would be much larger than 3:1 and could even lead to instabilities in the loop that outputs to the pump speed. Torque Pump curves traditionally are drawn with pump speed as a parameter. This might imply that the flow through the pump is best controlled by varying the pump speed, but it isn’t necessarily the case. Changing the torque may make more sense. Fortunately, VSDs can control either of these variables. When the input signal to the drive electronics adjusts pump speed, the relationship between flow through the pump and pump speed is important. When the input signal to the drive electronics adjusts pump speed, the relationship between flow through the pump and pump speed is important. This can be computed from the pump curve and system curve (Figure 6). The departure from linearity is noticeable — and is consistent with the changes in the sensitivity of flow to speed (Figure 5). When the input signal to the drive electronics ad38 • May 2008 justs torque, the relationship between flow through the pump and torque is important. This also can be computed from the pump curve and system curve (Figure 7). The graph exhibits only a slight departure from linearity, making it preferable for PID control as that control equation is linear. Figure 8 presents the sensitivity of flow to torque for the range over which the pump could operate. The somewhat erratic nature of the graph is the result of digitizing the pump curve. The sensitivity changes only slightly over the operating range. These graphs were computed from the pump curve (Figure 5), which exhibits significant sensitivity of head to flow at low pump flows. As already mentioned, for some pumps the pump curve is flat at low flows — that is, the head is almost constant at such flows. Figure 9 shows flow as a function of speed for a pump with a flat pump curve. The departure from linearity is much greater. At low flows this graph is almost vertical, so the sensitivity of flow to the pump speed at low flows will be very large, which could lead to cycling in the flow. However, even for a pump with a flat pump curve, the relationship between flow and torque is nearly linear (Figure 10). Generally linear behavior leads to better performance from the controls. So, for a centrifugal pump with a VSD it’s usually preferable to use torque to control flow. This also seems consistent with the future directions for VSD technology. CP Cecil L. Smith is president of Cecil L. Smith, Inc., Baton Rouge, La. E-mail him at cecilsmith@cox.net. www.chemicalprocessing.com 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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