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5:13 pm
October 21, 2009
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Utilities Manager: Reliability & Energy Cost Savings In Centrifugal Pumps

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This is what it’s all about. How much energy must go into a pump to get the most out of it, and, what does a pump’s inefficiency have to do with its reliability?

H ow many pumps in your plant operate at their best efficiency point (BEP)? The answer is very important. Back in the day, when energy costs were low and dollars for new equipment not so scarce, it might not have mattered to your operations. Today, though, with energy costs on the rise and so much riding on a site’s reliability and ability to control unnecessary spending, it’s quite a different story.

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The BEP is the point on the pump curve where the brake horsepower going into the pump is the closest to the water horsepower coming out of the pump. Pumps are sized and selected to operate at their BEP, based on system design and flow-restriction characteristics. When a pump operates at its BEP—and only when it operates at its BEP—it is as mechanically stable and as energy-efficient as possible. When it’s not operating at its BEP, a pump is consuming more energy than it should, and that inefficiency will ultimately impact the equipment’s reliability.

Over time, several factors change within a pumping system. Those changes, in turn, directly impact the pump’s operating conditions, including, for example:

  • Surface roughness
  • A change in the number or design of mechanical elements that compose a system (i.e., valves, strainers, heat exchanges, filters, tees, elbows, etc.)
  • The addition, deletion or modification of a system that ties into an existing system
  • Condition of the pump (i.e., internal clearances)

Any change in the mechanical composition of a pumping system will cause a centrifugal pump to operate at a different point on its curve than where it was designed to operate. So, how does this affect the energy costs associated with the operation of these pumps?

Centrifugal pumps are not 100% efficient. In fact, as depicted in Fig. 1, the BEP for most single-stage centrifugal units is only somewhere between 80% and 85% of the shutoff head. You are going to have to look at your individual pump curve to get the exact number for your pump. However, when selecting a new pump for an application, it is important for the pump efficiency percentage to be a component in the evaluation process—and for the required operating parameter to match the pump’s BEP.

Utilizing a simple formula and concept that operating a 1 hp motor, 24 hours a day, 7 days a week, for a full year, and paying 4.5¢ (cents) per kilowatt hour, a 1 hp motor will cost approximately $450 annually to operate, from an energy standpoint.

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It’s important to remember that there will be energy costs associated with centrifugal pumps that cannot be recovered.

As discussed earlier in this article, the BEP is the point on the pump curve where the brake horsepower going into the pump is the closest to the water horsepower coming out of the pump. When a pump’s performance waivers, it indicates that the unit is no longer operating at its BEP. Thus, the objective (and cost-savings opportunities) relating to energy consumption is the value associated with the difference between the current operating point and a pump’s BEP.

Real-world savings
Here’s an example of an actual cost summary report compiled after a plant survey of pumping equipment.

  • The survey considered a total of 58 pumps at a steel-manufacturing facility with motor horsepower ranging from 30 to 1000 (see Table I).
  • The site confirmed 4.5¢ per kilowatt hour charged for energy. That equates to energy costs of approximately $9,225,000 per year.
  • More importantly, the survey revealed cost-saving opportunities ranging from $655,875 to $1,311,750 per year when the pumps’ energy costs at BEP were compared with energy costs of them running 10% and 20% outside the BEP.

Let’s ask that question again: How many pumps at your site operate at their BEP? A simple way to determine the answer includes monitoring amp draws, monitoring suction and discharge pressures and reviewing the pump curve.

Amp draws relate directly to flow, and pressure relates directly to total dynamic head on a pump curve. Amps can be converted to horsepower. Pressure can be converted to total dynamic head. Horsepower and total dynamic head are reference points on a pump curve that identify operating parameters at a given point, which can be compared with the pump’s BEP.

If you’re not doing these things, start now—or have a consultant do it for you. It will be well worth the time and effort, in more ways than one. UM


Colleen Reeves is vice president of business development for Dubric, Inc., a company specializing in equipment reliability. Based in Comstock Park, MI, Dubric offers a variety
of pump-related services for end-users across North America, including, among other things, pump-engineering and improvement programs, rebuilds, repairs and training.
Telephone: (800) 848-0022; e-mail: creeves@dubric.com; Internet: www.dubric.com

For more info, enter 181 at www.MT-freeinfo.com

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