Chemical Processing - May 2008 - (Page 36)

Variable Speed Pumping Control valves regulate flow by varying friction 150 150head — that is, they work entirely through the sysSystem Figure 75%It presents syscurve for 1. tem curve, as illustrated in System curve for 75% open control valve valve open control tem curves for a fully open control valve and a control 120 120 valve that is 75% open. For the fully open valve, flow is 177 gal/min. As the control valve for 100% resistance to System Systemcloses, 100% curve curve for open control valve valve open control flow increases. This raises the friction head component 90 90 of the system curve, which makes that curve intersect the pump curve at a lower flow — at 75% open, flow 60 60 is 127 gal/min. Pump curve Small variations in both static and Pump curvehead ocfriction 3,450 30 30cur in any pumping installation. 3,450 rpm rpm These head variations cause flow variations, which also can be determined 0 0from the performance curves. 177 127 127 177 0 50 0 Suppose the static head is 60200 ft. Figure 250 50 100 100 150 150 ±1 200 250 2 shows Flow, gpm and system curves for 59 Flow, gpm 61 ft. and where they inft. tersect with the pump curve. For the fully open control valve, flow is 177 ±2 gal/min; for the 75% open control valve, flow is 127 ±2 gal/min. In each case, a head vari150 150 ance ofPump ft. leads to a flow variance of ±2 gal/min. ±1 Pump curve curve 3,450 rpm the control valve doesn’t significantly affect 3,450 Closing rpm 120 120 the propagation of variance from head to flow. But as we shall see, this isn’t necessarily the case for variable System System curve curve 2,850 90 902,850 rpm rpm speed pumping. Head, ft. Head, ft. 60 Head, ft. Head, ft. Head, ft. H(N) = HC (N/NC)2 (3) where 150 is speed, rpm; NC is speed for the pump curve N 150 System System curve for 75% curve for 75% supplied by the manufacturer, rpm;open control valve valve Q(N) is control open volumetric flow at speed N, gal/min; QC is volumetric flow at 120 120 speed NC (from the pump curve), gal/min; H(N) is head System curve for the at speed N, ft.; and HC is head at speed NC System curve for 100% (from 100% open control valve valve open control 90 90 pump curve), ft. Static Static Head Head 61 ft 59 ft With variable speed pumping, the curve Pump curve Pump 30 30 3,450 rpm 3,450 system curve is fixed but the pump rpm 125 129 curve shifts with129125 175 179 179 pump 175 speed. 0 0 0 0 50 50 100 100 150 150 200 200 250 250 Flow, to Flow, gpm Flow decreases in proportion gpmthe pump speed; head decreases in proportion to the square of the pump speed. The affinity laws also state that power decreases with the cube of the pump speed, which strengthens the 150 150 energy savings arguments curve by VSD manufacturers. Pump curve Pump Figure 3 illustrates 3,450 rpm regulating flow by varying pump 3,450 rpm 120 120 speed. The system curve is fixed. For a pump speed of 3,450 rpm, the flow through the pump is 177 gal/min; System System curve curve 90 90 for 2,850 rpm, 98 gal/min; and for 2,370 rpm, 0 gal/ Static Static Head Head min. In fact, 2,370 rpm is the minimum pump speed 61 ft 61 ft for flow60 that is, flow occurs only for speeds above — 60 59 ft 59 ft 2,370 rpm. 2,450 RPM RPM 2,450 The minimum speed to obtain flow is easily comput30 30 ed. When there’s no flow, the system head is the static head. The pump 25 curve shows the head delivered by a 175 179 13 175 179 13 25 0 pump with no 0 flow and running at speed NC. The mini177 177 0 50 100 100 150 150 200 200 mum pump0 speed to50 obtain flow is computed using the 250 250 Flow, gpm gpm Flow, affinity laws: Head, ft. Head, ft. 60 60 61 ft 59 ft Head, ft. With variable speed pumping, the system curve is fixed 2,370 rpm rpm 2,370 but the pump curve shifts with pump speed. Some ven30 30 dors provide pump curves for certain selected speeds; others provide a pump curve only for the rated speed of 98 98 a constant speed drive. In either case, the affinity laws 0 0 177 177 0 can be applied100 obtain pump curves at other speeds: 0 50 50 to 100 150 150 200 200 250 250 Flow, ) Flow, Q(N) = QC (N/NCgpm gpm (2) Flow regulation with a VSD 60 >> 0.5 Flow sensitivity to speed, gpm/rpm Flow sensitivity to speed, gpm/rpm 0.4 0.3 0.2 0.1 Sensitivity of flow to pump speed 0.5 0.4 0.3 0.2 0.1 >> Pump flow versus pump speed 250 200 150 100 50 250 200 150 100 50 Flow, gpm 0.0 0.0 2,000 2,000 2,400 2,400 2,800 2,800 3,200 3,200 3,600 3,600 4,000 4,000 Pump speed, rpm rpm Pump speed, Figure 5. As speed increases, the impact of speed changes on flow decreases. 36 • May 2008 Flow, gpm 0 0 2,000 2,000 2,400 2,400 2,800 2,800 3,200 3,200 3,600 3,600 4,000 4,000 Pump speed, rpm rpm Pump speed, Figure 6. Nonlinear relationship can hamper using PID control. 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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