Pump energy savings with VFDs

Tests reveal the economics of applying variable-frequency drives in centrifugal pump systems.

By Michael Offik, P.E., Frank Stauble and Roger Turley

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Centrifugal pumps are the most likely pump style to provide a favorable return based on energy savings when applied with a variable-frequency drive (VFD). To help illustrate this, we conducted benchmark testing to document various head and flow scenarios and their corresponding effect on energy savings. We explored the relationship of static and friction head in the energy efficiency equation and the effect of motor, pump and VFD efficiencies. The result is a reference point for plant engineers and maintenance personnel to select the best prospects for maximizing efficiency and energy savings.

While most centrifugal pumps operate at the fixed flow established by the hard-piped “free system” needs, many systems require variable flow to meet changing process demands. The two most common methods for controlling variable pump system output are a control valve (throttling) and a variable speed drive.

Controlling the flow with a throttling valve is like modulating the speed of a car using only the brake pedal. You set the accelerator pedal at a fixed point and use the brake to change speed. The engine works at nearly the same rate, but applying the brake restricts the work output by changing the resistance of the drive train. At low speed, the engine strains, the brakes overheat and reliability suffers — while consuming fuel at a nearly constant rate. Of course, this is a silly way to control your car, but most varying pumping systems are controlled in an analogous manner. The pump speed is fixed and a control valve adds system resistance, changing the system curve and thus restricting the output of the pumping system — while consuming nearly the same amount of energy.

Using variable-speed control, on the other hand, can be compared with the way people drive cars, changing vehicle speed by changing the engine’s output. Variable-speed pumping uses the same principle. Instead of changing the system resistance to modulate flow, the pump speed changes. This shifts the pump’s head-capacity (HQ) curve to alter the point at which it crosses the system curve. Variable-speed control changes the energy input rather than relying on a valve to strip system energy. The result is often a dramatic energy savings.

While a throttled pump consumes slightly less power than it would running free, it continues to rotate at the same speed, thus maintaining high velocity in the mechanical seal and bearings, and velocity directly determines bearing and mechanical seal life. Moving the operation of a centrifugal pump equipped with a constant pressure volute (the most common centrifugal pump type) away from BEP alters the hydraulic balance between the volute and impeller. The pump develops ever-increasing radial thrust loads, which increases radial forces that produce high bearing loads and shaft deflection. That affects mechanical seal alignment and, therefore, reduces bearing and mechanical seal life.

Centrifugal pumps and the affinity laws

Energy usage decreases with throttling as shown by the valve throttling curve in Figure 1. However, speed reduction results in a more significant energy reduction. The larger the flow reduction from the free operating point, the larger the energy savings. The advantage is that centrifugal pumps performance follows the affinity laws. Flow rate is directly proportional to pump speed. The differential pressure is directly proportional to the square of the pump speed. Power usage is directly proportional to the cube of the pump speed.

For example, reducing speed by 50% requires only 12.5% of the power needed at full speed. Determine the new operating point by using the affinity laws to generate a new pump curve and finding where it intersects the system curve. Adjust the flow, head and power at several points along the original pump curve to find the new curve.

Follow the energy

System curves combine the effects of both static and frictional head. Static head is the height to which fluid is being pumped plus the surface pressure at the outlet less the height of the supply tank and its surface pressure. Frictional head is the frictional pressure loss in the pipe, fittings and valves.

In systems exhibiting only frictional head loss, flow rate can be reduced by slowing the pump. The power savings mount as the pump slows. In systems with high static head, the operational flow point is continuously moving toward the pump minimum flow as speed drops. A minimum operating speed is required to overcome the static pressure difference, Therefore, the energy savings are limited.

Because systems with only friction head provides the most likely energy-saving scenario and those with greater static head provide the least, making a decision on VFD or throttling appears simple. However, most applications fall somewhere in the middle, making the economics less clear.

Running the test

To develop some energy savings guidelines, we configured a 40-hp, pump/motor/drive system and measured energy consumption for various static-versus-friction head scenarios. We then compared the various scenarios to each other and to the throttled base case. Figure 2 shows the test configuration.

These tests used a two-pole, 3,560 rpm, 40-hp, totally enclosed fan-cooled motor with a NEMA nominal nameplate efficiency of 94.1% matched with a pump having a 3-in. suction, 2-inch discharge and 8-inch impeller. The VFD was rated at 40-hp. Both the drive and motor are three-phase 460 VAC. Figure 3 shows the system curves for the five-test regimen that recorded the input power and power factor as a function of flow rate.

The first test used no VFD. The pump was throttled to move along the pump curve. The VFD was used in the remaining tests to vary the flow rate and move along one of the four system curves. The system curves represent 0, 60, 140 and 210 feet of static head. Each curve intersects the pump curve at approximately 340 gpm.

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