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Watch out with variable speed pumping
By Cecil L. Smith
ChemicalProcessing.com
Keywords: centrifugal pump, variable speed drive, maintenance, operating point and constant speed pump
Pump curves suggest rethinking the usual control strategy
It’s increasingly popular to team a centrifugal pump with a variable speed drive (VSD) rather than couple a constant speed pump with a control valve on the discharge. However, many engineers don’t understand a VSD’s impact on flow and how that affects control.
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.
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Figure 1. As the valve closes, friction head increases, leading to lower flow. |
Constant speed pump
Low flow protection for a centrifugal pump with a constant speed drive normally entails recirculation back to either an upstream vessel or to the suction of the pump. A proven approach is to measure the flow through the pump and send the value to a controller that manipulates a control valve in the recirculation line.
The set point for the controller is the minimum required pump flow. Under normal operating conditions, flow should exceed this level; so the controller will close the valve in the recirculation line. However, should flow drop below the minimum, the controller will quickly open the control valve to provide the necessary flow.
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Figure 2. The variance in head leads to twice as much variance in flow. |
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.
The control valve can be avoided by inserting a fixed orifice into the recirculation piping. This provides some recirculation at all times, even when the process flow exceeds the minimum necessary for low flow protection. It requires extra energy and, possibly, a larger pump.
Variable speed pumping
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Figure 3. Only pump speeds greater than 2,370 rpm provide flow. |
Vendors always stress possible energy savings with a VSD. However, especially in industries such as specialty chemicals, few drives are large enough to provide sufficient energy savings to justify their additional cost.
However, a VSD offers other potential benefits. It may make the following equipment unnecessary:
- sensor/transmitter for pump flow
- recirculation piping
- control valve in the recirculation piping
- control valve in the line to the process
The controller for pump flow also is eliminated but this component is likely to be in the software of the digital controls and therefore “free.”
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Figure 4. The variance in head causes triple the variation in flow. |
Nevertheless, eliminating even a single item of equipment, particularly one that would be made of stainless 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.
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