Watch out with variable speed pumping

Pump curves suggest rethinking the usual control strategy

By Cecil L. Smith

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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.

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.

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.

Flow regulation with a control valve
The flow through a pump is determined by the following curves:

  • Pump performance curve (or just “pump curve”). This curve, provided by the pump manufacturer, relates pump flow, pump head and pump speed.
  • System performance curve (or just “system curve”). 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:
    • Static head. This is the difference in head across the flow system when flow is zero. It includes pressure 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)

Figure 5. As speed increases, the impact of speed changes on flow decreases.

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.

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  • This is great! Thank you, Cecil!

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