Watch out with variable speed pumping

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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.

Figure 1. As the valve closes, friction head increases, leading to lower flow.

Here, we’ll focus on the pump — specifically, how certain aspects of the pump performance curves can lead to variations in flow at reduced speeds — because this requires the expertise of people in the process group, not the electrical department. Most electrical departments can provide a good analysis of the drive but aren’t familiar with pump performance curves.

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.

Figure 2. The variance in head leads to twice as much variance in flow.

A control valve also is required in the line to the process to provide appropriate flow. In some configurations, this flow loop is the inner loop for a level-to-flow cascade, a temperature-to-flow cascade or other cascade arrangement. In some cases, the flow isn’t measured; instead the output of the level, temperature or other controller is connected directly to the control valve.

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

Figure 3. Only pump speeds greater than 2,370 rpm provide flow.

In contrast, with a VSD a flow measurement is used to adjust a speed or torque controller included with the drive electronics.

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:

1. sensor/transmitter for pump flow
2. recirculation piping
3. control valve in the recirculation piping
4. 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.”

Figure 4. The variance in head causes triple the variation in flow.

The above only considers normal process operations. The minimum flow for the VSD will be lower than that for the constant speed drive but won’t be zero. There’re 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 shutoff is required, the control valve in the line to the process must be replaced by a block valve.