Watch out with variable speed pumping

Related articles Achieve optimum centrifugal pump performance Tackle Tough Services for Centrifugal Pumps with Care Consider changing the speed not the pump

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.