by Lev Nelik

Editor's Note: While running a pump at its best efficiency point saves money, reduces downtime and improves performance, many plant managers are unaware of how their equipment is actually performing. The following real-world scenario is intended to illustrate the importance of monitoring pump efficiency.

Jim, a maintenance manager at a municipal water plant, was content. His plant was operating smoothly, and his pumps seemed to be running as designed. He was pleased to see Bob, a pump rep, walk in with a box of doughnuts in hand to talk about the efficiency of the plant's pumps.

"Jim, these are for the guys at the shop. How are the pumps doing?"

"Doing fine, Bob. That spare mechanical seal you sold us is still in my office, ready to go if anything goes bad. But so far—knock on wood—all is well."

"Good to hear, Jim. I must say, we do make good pumps. But I've noticed that it has been a long time since we've done any major repairs for you. Have you checked to see if your pumps are still efficient?"

"They are very efficient. They're pumping water nonstop."

"That's not what I mean, Jim. Do they burn too much energy?"

"Too much energy? I have no idea. All I know is that they run well—no vibrations, no leaks, no trouble for me."

Bob opened the pump catalogue.

"See, Jim, these pumps should be nearly 90 percent efficient, according to our books. But it has been a few years now, and these pumps wear. That makes them take more power than they should."

Really? Just after four years? It's basically clean water we're pumping."

"That's true, Jim. But, why not check it anyway? These are 3,000-horsepower (HP) motors. Guess how much it costs you to run them."

"Bob, all I know is that they run all day long and don't fall over. That's all I care about. And whatever we pay to run them, go talk to Charlie in accounting. I have enough to deal with."

Bob pulled out his calculator.

"Okay, but just for kicks, say your 3,000-HP motor runs nonstop. That's about 2,238 kilowatts. If you run them 24 hours for 365 days, you get—let's see—19,604,880 kilowatt-hours, which, at 10 cents per kilowatt-hour, makes it nearly $2 million!"

Impressed by the amount of money the pump uses but content after two doughnuts and a steamy cup of coffee, Jim agreed to allow Bob to measure the pumps' energy consumption using his pumps' reliability and energy savings monitoring (PREMS) technology.

This monitoring system would provide the plant with live, nonstop equipment data at any office computer.

Pulling his car to a main water booster pump, Bob and the plant mechanic, Rusty, attached instrumentation to the device. The vibration and temperature transducers had mag bases and took little time to install on the bearing housing.

For the pressure, they teed off the existing gauges. Flow and power already had output signals on the plant distributed control system (DCS) controllers, and they connected output to that. The system was up and running just before lunch.

"Hey, Jim. My system is collecting data from the instrumentation and will send the gateway signal wirelessly to the cell. From there, our software will transform it into a live pump performance curve and compare it with the original manufacturer curve. I haven't had a chance to analyze it, but if you want, we can take a quick look at it at lunch."

"Sounds good, Bob. Show me what you've got."

At lunch, Bob opened his laptop and pulled up the pump data using the wireless software. Pointing to the solid line that represents expected performance—pump head, power and efficiency curves versus flow—and comparing it with the dashed line representing the pump data gathered that morning, he explained that Rusty asked the operators to throttle the valves to force the pump to run at various flow rates. While the piping system limits the flow to about 40,000 gallons per minute (gpm), the data they took covered the entire flow range.

"Well, that was quick, Bob. You did all that in one day?"

"Sure. Thanks to Rusty, who helped with some wrenches, and the fact that you already have a magnetic flow meter, we simply connected our 4-20 mA leads to your output DCS terminals."

While the original curve indicated a best efficiency point (BEP) at 40,000 gpm, the day's data indicated a BEP at 35,000 gpm.

The data also showed lower head, which indicates lost pressure, and increased power.

"Sounds like you might have some wear—maybe rings opened up or some internal rub—which will take more power."

"How much is all that costing us?"

"Well, I added a tabulation near the curve there. See, at 40,000 gpm, you should have 255 feet of head, and you've got 227 feet. That is an 11 percent reduction in pressure. The power should be 2,900 HP, and you are actually taking 3,045 HP. Seems like you are running into a motor safety factor. I wouldn't be surprised if your motor starts tripping pretty soon."

"What? Are you kidding? This is almost a brand new motor! You sold it to us about four years ago with the pump!"

"We did, Jim. But we told your engineers at that time to upsize the motor a bit—to 3,500 HP instead of 3,000 HP—in case something like this happens. All that aside, your efficiency at 40,000 gpm is 75.3 percent versus the 88.8 percent it should be."

"Really? And?"

"That is a 13.5 percent difference. Remember what I told you: You are paying about $2 million for this pump running nonstop. Divide $2 million by 100 and you get roughly $20,000 per efficiency point. For 13.5 points, that is more than a quarter of a million dollars wasted per year if you run it nonstop."

"But we don't run nonstop. We probably run, on average, about 10 to 12 hours per day."

"That's still $125,000 per year—wasted."

Jim took off his baseball hat and scratched his head.

"Charlie sure won't be happy to hear that. But if I tell him, I know what he will ask: How much would it cost to fix it?"

"Well, based on our previous repair, you'll probably need new wear rings, a new shaft, bearings and maybe an impeller. I'd say that's probably a $100,000 job."

"And that would save us $125,000 a year? So, roughly a one-year payback?"

"That sounds right."

"Interesting. Our budget is tight this year, but let me talk to our guys on this and I'll get back to you. Thanks for coming, Bob."

In Part 2 of this series, Jim, Bob, Charlie and storeroom manager Grady Cricket evaluate the pump parts to determine which repairs are needed and what factors can dramatically affect efficiency. Read it here.

  1. Nelson, E., Maintenance and Troubleshooting of Single-Stage Centrifugal Pumps, Teas A&M Pump Symposium, 1984
  2. Nelik, L., Let's Find the True Best Efficiency Point (BEP), Pumps & Systems Magazine, May 2015
  3. PREMS-2A Pumps Reliability and Efficiency Monitoring System, rev. 2A, March (PVA module 10A)