Chemical Processing - May 2008 - (Page 35)

30 0 0 30 0 0 50 Pump curve Pump curve 3,450 rpm3,450 rpm 30 0 30 125 Pump curve Pump curve 3,450 rpm3,450 rpm 50100 100 150 150 200 Flow, gpmFlow, gpm 127 127177 177 0 0 0 50 200 250 250 Variable 150 150200 200 Speed Pumping250 50100 100 Flow, gpmFlow, gpm 129 125175129 179 175 179 250 >> 150 120 90 60 30 0 0 Flow regulation with a VSD 150 Pump curve Pump curve 3,450 rpm3,450 rpm 120 >> Propagation of variance for VSD 150 120 150 Pump curve Pump curve 3,450 rpm3,450 rpm 120 Head, ft. 90 60 30 13 25 13 25 175 179 175 179 177 Head, ft. Head, ft. Head, ft. 2,850 90 rpm2,850 rpm 60 2,370 rpm2,370 rpm 30 98 98 177 System curve System curve 90 60 30 0 Static Head 61 ft 59 ft Static Head 61 ft 59 ft System curve System curve 2,450 RPM 2,450 RPM 0 0 50 50100 100 150 150 200 Flow, gpmFlow, gpm 177 0 0 0 50 50100 100 150 150 200 Flow, gpmFlow, gpm 200 250 250 200 250 177 250 Figure 3. Only pump speeds greater than 2,370 rpm provide flow. Figure 4. The variance in head causes triple the variation in flow. 3. control valve in the recirculation piping; and 4. control valve in the line to the process. The controller for pump flow also is eliminated but 0.4 this component is likely to be in the software of the 0.4 digital controls and therefore “free.” The above only considers normal process operations. 0.3 0.3 The minimum flow for the VSD will be lower than that for the constant speed drive but won’t be zero. There’re 0.2 0.2 always startup issues to be addressed and usually other considerations. For example, it may be important to prevent fluid backflows through the pump. If positive 0.1 0.1 shutoff is required, the control valve in the line to the process must be replaced by a block valve. 0.0 0.0 a item 2,000 2,400 2,800 3,200single3,600 of equip4,000 2,000 Nevertheless, eliminating even 3,600 2,400 2,800 3,200 4,000 ment, particularly one that would be made of stainless Pump speed, rpm Pump speed, rpm steel or other expensive material, may easily offset the additional VSD cost. And these savings come upfront, which appeals to project managers. A VSD also can provide maintenance savings. Control valves are high upkeep items, and handling corrosive and toxic fluids makes matters worse. Just replacing a control valve with a block valve reduces maintenance costs. From a control perspective, a VSD has another distinct advantage — it doesn’t exhibit stiction, hysteresis and other mechanical issues posed by a control valve. Today’s drives incorporate either speed control or torque control, thus providing the counterpart to the positioner advocated by control engineers for control valves. 0.5 0.5 Flow sensitivity to speed, gpm/rpm Flow sensitivity to speed, gpm/rpm Flow, gpm Flow, gpm This curve, provided by the pump manufacturer, relates 250 250 pump flow, pump head and pump speed. System performance curve (or just “system curve”). 200 200 This curve relates the head and flow of all components in the fluid flow path (the flow system) excluding the pump. Two components determine this curve: 150 150 Static head. This is the difference in head across the flow system when flow is zero. It includes pres100 100 50 50 Eliminating even a single item of equipment, particularly one made 0 0 of stainless steel, may3,200 3,200 3,600 easily3,600 4,000 offset 2,000 2,400 2,800 2,000 2,400 2,800 Pump speed, rpm Pump speed, rpm the additional VSD cost. 4,000 Flow regulation with a control valve The flow through a pump is determined by the following curves: Pump performance curve (or just “pump curve”). www.chemicalprocessing.com sure head and hydrostatic head. The pump must work against this head even at zero flow. Friction head. As flow increases, pressure drop due to the fluid’s resistance to flow increases by approximately the square of the flow. All components in the flow system contribute to friction head. Suppose friction head is known for a specific flow. (This data point should be available from design calculations.) Assuming friction head varies with the square of the flow, the system curve can be approximated by: HP = HS + HD (QP/QD)2 (1) where QP is process flow, gal/min; HP is head at process flow QP, ft; HS is static head, ft; QD is design flow, gal/ min; and HD is friction head at flow QD, ft. The operating point is always found at the point where the pump curve intersects with the system curve. May 2008 • 35 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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