Article 67: Did you Adjust Your Pump Data for the Motor Efficiency?
The Duke is Back - part 4 of 4
(This is a continuation of Parts 1 thru 3 (articles 55, 56, 66)
Last month we left our group at the Nasty Creek plant discussing the correct clearances between the wear rings of a pump. It was expected to be about 0.023”, and Charlie Beans was getting his team get the data and review the impact on the efficiency measurements which Bob Sellers and Jim Makem looked at in June. Below is a continuation of the story started in June.
“Ok, guys,” – plant mechanic Rusty Hammer, pointed at the whiteboard of the conference room, where he, plant manager Charlie Beans, maintenance supervisor Jim Makem, and storeroom manager Grady Cricket, gathered to review the data, and to compare it with what Sandy Mixup, an engineer from the Duck Pump company, told them last month, - “I’ve got the measurements, - and it ain’t 0.023”! I got nearly triple of that! It is a huge clearance, - almost seventy thousands! And it is not a used rotor at all – this is a spare rotor we got at the store room. I do not know if it is similar to the one Bob tested in the field in June, but if so, we got a problem!”
Charlie opened a copy of a Pumps & Systems article (March 2007) which Sandy gave him at the last meeting:
“Let’s see… The article talks about a smaller pump (16x18 size), while our unit is a lot bigger (30x36), but if we assume the results are roughly applicable, we would expect, according to the article, about a 4% efficiency degradation with rings clearances double the size. Our case is triple the size, so should we assume, say, about a 6% degradation or so? I am not a hydraulics guru, but just trying to make some sense with these numbers, - would that be a reasonable ballpark guess?”
“I think that makes sense, Charlie”, - said Jim, - “We can get a more accurate calculations from Sandy to verify, but for now, that is probably close enough estimate. So, as Jim told me in June, the pump he measured with his PREMS-2A unit, gave him a performance drop of about 13.5%, which, according to the calculations, for our pump, was about $125K per year wasted. If the real number is 6%, that would be roughly a $60K energy loss. In either case, we are wasting money due to flow recirculating thru the incorrect clearance. Like you said, we are still to determine if the pump Bob measured is similar to the spare rotor we just reviewed, but so far it looks to me it has nothing to do with actual wear, but some sort of a screw-up the Duck Pump company did with the brand new clearances when they delivered the pump.”
Charlie asked Jim to email the findings to Bob and Sandy:
“Let’s also get Bob come back and do his measurements on the other pumps as well. We got five pumps at the main booster line, and usually run only one or two, three at the most, depending on the demand, with the other units essentially an installed spares, and in anticipation for a possible expansion. Which one did Bob measure with his PREMS-2A unit, Jim?”
“I am not sure, Charlie. I think it was unit #1, but I will ask him that also in my email. It’s funny I did not pay much interest tom his testing proposal that day, but now that we discovered this issue thanks to that, I am glad I agreed, and a good thing he did it”
As emails went back and forth, it was decided to PREMS-2A test the rest of the pumps, which Bob did the following week. With the unit being portable, he was able to test all five units in one day, with the following results:
Unit 1 (same as tested originally): 13.5% off (below) on efficiency
Unit 2 and 4: 7% below
Unit 5: 5% below
After the tests were completed, the group reconvened at the conference room, to review Bob’s data.
“Well, Bob, we are glad you helped with the measurements, an impressive tool you have. But, that does not seem to point on any problems on our part. As I told you earlier, we are pumping clean water, so not much wear would have been expected. And now, from the rings examination, it looks like you guys shipped us some wrong rings! Good thing we did not go ahead and went ahead with your $143.6K quote to repair!”, - Jim was both upset that the Duck Pump company caused them lost energy, but at the same time was pleased the data was now clear, and he appreciated Jim’s honesty trying to be as helpful as possible.
“I am really sorry folks with the mix-up”, - Bob was visibly upset, - “I have no idea why the rings were shipped oversized like that, and will definitely find out from Sandy and our production folks, but I also must tell you why the numbers I got were more than the calculation we all did”
“Well, I did not think about this myself initially, - things were kind of developed rapidly, - but once I my head a bit more time to spin on this, it occurred to me that what I actually measured was a combined efficiency of the entire unit – pump PLUS motor, but my comparison was against only a pump efficiency as our factory test always show”
“OK Bob, can you break it down to us in some simple way without too much math?”
“Sure. The power I measured with the PREMS-2A was to the wires of the motor. Motor “eats” some of the power, due to its own inefficiency. The nameplates of these motors show 94% efficiency, and our pumps are 88.8%. Then resultant OVERALL system efficiency is thus 0.888 x 0.94 = 0.83 = 83%. My PREMS-2A unit measured actual 75.3%, and so it is not as huge of a difference as I originally thought (13.5%), but 83-75.3 = 7.7% - which is closer to the 6% efficiency which Jim calculated. That does not excuse the fact that we shipped you the wrong rings (at least on some of the pumps), but at least it is not as bad I we thought.”
“I am glad you liked my calculations, Bob! I never thought I would ever need to know all this stuff, - but now that we did it, it was kind of fun. Besides, we give you a break, - you are a good man, - tried hard, and we appreciate it. Let’s see, however, how will your production guys explain the mixed up rings anyway. But for now, you are back in favors. Come on – I will buy you lunch!”
A few days later, Sandy confirmed a mistake in production and wring rings shipped. Duck Pump promised to replace the rings, free of charge, during the next overhaul. Ironically, she said, no one was able to come up with the theory why pump #5 was apparently operated even better than the factory test (given the adjustment for the motor efficiency as pointed by Bob). That, she added, would remain a mystery, and hopefully a shine to Bob’s crown to discover the truth, - whichever way it turned out.
1. Nelson, E., Maintenance and Troubleshooting of Single-Stage Centrifugal Pumps, TX A&M Pump Symposium, 1984
2. PREMS-2A Pumps Reliability and Efficiency Monitoring System, rev. 2A, March 2015:
www.pumpingmachinery.com/pump_school/pump_school.htm (PVA module 10 and 11)