Chemical Processing - May 2008 - (Page 37)

Variable Speed Pumping >> 250 200 150 100 50 0 0 Pump flow versus torque 250 >> Sensitivity of flow to torque 25 Flow sensitivity to torque, gpm/ft.lbf 25 20 15 10 5 0 0 0 3 3 69 9 Torque, ft.-lbf Torque, ft.-lbf 6 12 12 15 15 Flow sensitivity to torque, gpm/ft.lbf 0 3 3 9 9 Torque, ft.-lbf Torque, ft.-lbf 6 6 12 12 15 15 250 Flow,gpm 50 200 150 100 50 0 20 15 10 5 0 Flow,gpm Figure 7. More linear relationship better matches the PID control equation. Flow,gpm Figure 8. Value remains relatively constant over the entire operating range. 250 250Nmin = N (HS/H )½ (4) C C0 where Nmin is minimum pump speed for flow, rpm; HS is static head (from the system curve); and HC0 is head at 200 200 speed NC and zero flow (from the pump curve), ft. For our example, NC is 3,450 rpm, HS is 60 ft., and 150 150 H is 127 ft. The minimum pump speed is 2,370 rpm. C0 So, the VSD must operate over the range of 2,370 rpm 100 100 to 3,450 rpm. The minimum speed is 69% of the maximum speed; the turndown ratio is 1.46:1. VSDs are easily capable of this. 50 50 Figure 4 illustrates propagation of variance from static head to flow for pump speeds of 3,450 rpm and 0 0 2,450 rpm. At 3,450 rpm, a static head of 60 ±1 ft. 2,400 2,800 2,000 2,000 3,200 3,200 3,600 3,600 4,000 4,000 gives a2,400 of 2,800 ±2 gal/min (just like for the fully flow 127 Pump rpm rpm speed, Pump speed, open control valve). However, at 2,450 rpm, a static 250 Flow, gpm dependent of flow. For such pump curves, the propagation of variance from static head to pump flow would 200 200 be much larger. Pumps with a flat pump curve at low flows likely 150 150 will experience cycling between no flow and some positive flow. Avoid them if you want a VSD to perform smoothly at low flows. Pumps with pump curves such 100 100 as the one shown in Figure 6 probably wouldn’t exhibit the cycling. 50 Flow,gpm Flow, gpm Pumps with a flat pump curve at low flows likely will experience cycling between no flow and some positive flow. head of 60 ±1 ft. gives a flow of 19 ±6 gal/min. As the pump speed decreases, the variance in flow increases by a factor of three. Even with this increase in variance, the pump probably would still perform satisfactorily. At low flows, the pump curve in Figure 4 exhibits a significant sensitivity of head to flow. This isn’t the case for all pumps. For some, the pump curve at low flows is essentially flat (slope is zero) — that is, at low flows, pump head is inwww.chemicalprocessing.com Sensitivity of flow to pump speed 0 0 It’s also important to3 understand the change in12 flow 03 69 9 12 0 6 15 produced by a given change in pump speed. Torque, ft.-lbf Torque, ft.-lbf Figure 5 presents the sensitivity of flow to pump speed for the range of speeds over which the pump could operate. (The somewhat erratic nature of the graph is the result of digitizing the pump curve.) It shows that the sensitivity increases as the pump speed decreases — that is, at low pump speeds, a change in speed has a larger effect on flow than the same change at high speeds. Such a change in sensitivity affects the performance of the control loop (flow, level, temperature or other) whose output determines the pump speed. As the sensitivity increases, more oscillations can be expected in that loop. So, to use the same tuning at low flows and at high flows, either tune the loop under a low flow condition or conservatively tune the loop at high flows. The results are essentially the same — the controller is tuned with a lower gain. For the controller to function at low flows, performance is sacrificed at high flows. While techniques such as scheduled tuning could adMay 2008 • 37 15 http://www.chemicalprocessing.com

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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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