Fluid Handling / Motors and Drives

How to Save Energy on Pumps

Through rightsizing, use of variable-frequency drives and other strategies, pumps can save significantly on electric power.

By Pan Demetrakakes, Senior Editor

Electric power flows to pumps, and fluids flow from them. The best pump setups get more of the latter with less of the former.

Next to labor, electric power is almost always the biggest operating expense in food and beverage plants. Pumps of all kinds are ubiquitous, for propelling ingredients, finished or semi-finished product, sanitation water and other fluids – and they’re a major consumer of electricity. But their power consumption can be kept down by various strategies, ranging from simple maintenance to sophisticated energy-saving designs.

Probably the most basic, straightforward way to save energy on pumps is to make sure that they’re the right size for the application. There’s a temptation to specify pumps with more capacity than necessary, to cover all situations.

“Most manufacturers grossly oversize the pressure capabilities of the pump and intentionally sacrifice efficiency for longevity,” says Mike Dillon, president emeritus of Seepex Inc. 

Centrifugal pumps move liquid with an impeller, a rotating bladed wheel that propels liquid out an aperture. They’re the most common type of pump in industry generally. In food and beverage plants, they’re most often used with water, or fluids that flow like water.

According to one estimate, most centrifugal pumps have an overcapacity of 20-30 percent. This can lead to great inefficiency; a Finnish research study showed that industrial pump efficiency averages less than 40 percent.

One of the most common ways to match a centrifugal pump to a given application is to change the diameter of its impeller. By doing so – usually there’s about a 25 percent margin of possible variance – users can change the pressure and flow rate as needed.

Handling variety

However, setting the pump capacity becomes more challenging in applications with high variations in volume and flow. The pumps have to be able to handle peak flow – no matter how rarely it occurs – while running effectively at a lower flow rate. This is complicated by the fact that in many cases, the AC motors that power pumps run at only one speed.

A common way to regulate flow with a mono-speed pump is with valves that can be throttled as needed. But this isn’t very efficient, in energy or anything else. It’s like setting the cruise control on your car and regulating its speed just by hitting the brakes; you use more gas than you need to while wearing out the brakes.

Another alternative is simply to use several smaller pumps instead of one big one, stopping and starting them as needed. With such a setup, “each pump in itself continues to run closer to its designed efficiency level,” says Mark Lensing, NEMA pump motor product specialist at ABB.

This can save energy over the long run. The drawbacks are that the additional pumps add to the system’s initial cost and have to be turned on and off, either manually or through a feedback loop tied to the flow rate, both of which involve trouble and expense.

Changing speeds

An increasingly popular alternative to make pumps more energy-efficient without using more of them is to vary their speed. This often means using variable frequency drives to alter the current that powers them. As VFDs become smaller, cheaper and sturdier, they are being paired with AC motors in a growing number of industrial applications of all kinds, including pumps.

“You’re going to do yourself a lot of favors by slowing down the [pump] motors as opposed to throttling the actual flow,” says Richard Kirkpatrick, product manager for NEMA variable-speed AC motors at ABB.

The obvious advantage of variable-speed pumps is that they can handle peak flow situations while using less energy for longer periods of low flow.

“Any given motor/pump application, if it’s running variable speed, has got to be designed for the worst case. It’s got to be designed for the case when it’s running all out,” Kirkpatrick says. “I’ve got capacity I’m not using, but I have to, because I might need it in some extreme cases.”

Centrifugal pumps have the potential to save energy with VFDs, because they have a variable torque load – i.e., the pump applies pushing-pulling power that varies with the speed and size of the load. What this means in practical terms can be expressed in three “affinity laws” of centrifugal pump speed: flow varies linearly with pump speed; pressure varies with the square of pump speed; and power consumption varies with the cube. So when a centrifugal pump runs at half speed, it uses power at the cube of 0.5, or 12.5 percent.

“So that’s really useful. If I’m at 100% speed and I consume 100% power, if I run half speed, I only consume an eighth of the power,” Kirkpatrick says. “That’s why it’s so enticing from an energy savings standpoint to put a motor in a variable-speed mode, because you can really consume a lot less energy.”

However, while VFDs may have the potential to save energy with centrifugal pumps, that potential is mostly untapped, says Michael David, manager of pump technology for Central States Industrial, a distributor and fluid handling systems integrator.

The problem, David says, is that a centrifugal pump with a full-sized impeller, a non-overloading motor and VFD will cost twice as much as one with a simple motor starter. Few companies are willing to lay out that much in capital costs, even for a guaranteed return on investment – and with centrifugal pumps, the ROI isn’t always there. For applications that run more or less continuously, it probably will be, but for ones that run intermittently, such as with sanitation procedures, “the ROI is basically forever,” David says.

Accentuate the positive

But the situation changes with another kind of pump often used in food plants: positive-displacement. Instead of centrifugal force, these propel fluids with a direct pushing motion of some kind.

They include rotary-lobe pumps, which move fluids with meshing bulbous rotors; progressive-cavity pumps, which use augur-shaped rotors; diaphragm pumps, which use large flexible membranes that create alternating pressure when they shift; and piston pumps. Positive-displacement pumps are often used to handle ingredients and products, especially viscous ones.

Positive-displacement pumps are more likely to use variable-frequency drives. Pelle Olsson, national sales engineer with Unibloc-Pump, says that eight out of 10 lobe pump installations he oversees now feature VFDs.

Unlike centrifugal pumps, positive-displacement pumps have a direct, linear relationship between their speed and the power they draw. However, because they usually move viscous products, they’re often required to put out more torque at startup than during operation. This is especially true for certain products, like ketchup, that are more viscous when stationary than when flowing.

This disparity can result in the pump motor drawing significantly more amperage at startup, and it can lead the system designer to specify a more powerful pump than necessary, says Gordon Fenton, vice president of engineering at Seepex.

“Seepex pumps have two separate operating parameters that influence power required,” Fenton says. “The first is starting torque and the second is operating torque. Most of the time, starting torque dominates the requirement.”

To keep the pump’s motor from drawing excessive power to achieve starting torque, Fenton recommends properly calibrating the gearbox that translates the motor shaft’s rotations into torque. In general, this means increasing the gear ratio as much as possible, so that starting torque can be achieved with less power from the motor.

Another, perhaps more straightforward, way to increase energy efficiency is simply to use motors and related equipment designed to save power. Thanks to better winding and other improvements, electric motors in general have become more energy efficient, to the point where efficiency standards have become written into law, in both the U.S. and Europe.

Denmark-based Grundfos, for instance, uses motors that meet the European Union’s IE5 efficiency standard – the highest in the world. Grundfos also promotes efficiency in certain products with permanent-magnet motors, which save power by not requiring electricity to be routed to the rotor.

“The Grundfos MLE permanent magnet motor with integrated frequency drive allows you to benefit from the highest level of efficiency for electrical motors,” says Mike Kelzer, Grundfos’ industry product manager.

Pumps have great potential for energy savings, as long as all relevant aspects of the application are taken into account.

“We look at the entire width and breadth of our program to figure out what makes the best pump,” says CSI’s David. “Sometimes it’s pressure, sometimes it’s temperature, flow rates, particulates -- we have to take into account everything about the product in an effort to not choose incorrectly.”