Obviously, everyone wants their equipment to last a long time and not break down or wear out. Unfortunately, sooner or later, everything does. The issue is how to extend the life of a pump, and to evaluate if the existing machinery operates at the conditions conducive to long life.
When a pump operates at flow away from BEP (best efficiency point), its efficiency is low - the lower the flow, the lower the efficiency. A centrifugal pump, operating at 80% efficiency at BEP, will operate at perhaps 30% efficiency at half flow. The larger the pump, the more this translates to losses.
Consider, as an example, a 1,000-hp motor, which runs continuously at a wastewater treatment facility, 24 hours a day, 365 days a year. At an estimated energy cost of say $0.07, these 1000 hp (746 kW), will consume 746 x 24 x 365 x 0.07 = $457,447 per year. If this happens at 30% pump efficiency, then, if the pump efficiency can be improved or restored to the original peak of 80%, then the energy cost would be only 457,447 x (30/80) = $171,542. The difference is $285,905 per year.
You can substitute your actual numbers, but the point is, it is not a small matter, but a big dollar value, and an unhappy pump.
Why do pumps operate off-peak? Several reasons. We find that often the flow usage decreased through the years due to plant downsizing - less energy demand, workforce reduction, surrounding areas industry moved out, etc. Or, sometimes the pumps were oversized originally "just in case" but, with running experience, a proper flow is lower then at best efficiency flow point. The extra capacity is thus not needed, so the pump is throttled, and that is at big efficiency loss.
Of course, the simplest thing to stop wasting these dollars is to get a smaller, better sized-for-proper-flow pump. Pump manufacturers can do it, why not? But it is an expensive "solution." Not only because of the new pump purchase cost, but the associated changes in piping can be, as you know, can, and often are, a nightmare and prohibitively expensive.
Would it not be much better to keep the same casing, same piping, and just change over the impeller? Of course it is. A new impeller design can often be fitted into same casing, no piping changes, no hustle. The pump BEP point is essentially "shifted" to the left - smack on where the operating point is - and the efficiency is again restored to 80% (or close to), instead of the 30%, as assumed in our example.
The cost of the impeller change? Not much comparing to the whole new pump. Price? Often a 3-4 months payback.
So, why does not everyone do this, if so obvious?
First of all, the concept of fitting-in a specially-designed impeller into the existing casing is not always that obvious, and the inertia and a habit of running the pumps "to the ground" is often a fact of life. The pump does not “cry out loud” that it is being abused and, unfortunately, just runs inefficiently. No one measures the dollars being burned away by the "wasted-money-meter" at the pump, the lost dollars are hidden in the overall electric bill. Accountants see them and want to make the bills smaller but accountants are not hydraulic designers, and the pumps are left "until (maybe) next year… and the next…".
Some of these wasted dollars are passed on to the consumers, via energy surcharge or product price but, when the surcharges reach certain point, it is not so easy to keep rising the rates indefinitely to recover the wasted energy. Besides, at a given surcharge to the public why not save on energy anyway? Changing out the impeller is not a very difficult thing to do, especially if there is no piping changes required.
Another reason for inefficiently running pumps is that big pump companies are not in business retrofitted impeller designs and sizing them to right conditions. To do that means spending time and having their engineers involved. Designing impellers is easy, but you have to have people that not only know how, but have time to do that. Even large and reputable pump manufacturers are not in business of keeping a huge staff of hydraulics engineers looking out for customers to fix problems with a, say, $10K impeller, when they can sell a complete pump for $50K or more. Piping changes are not pump manufacturers problem as these expenses come out from the users pockets.
This is why some companies, involved in upgrades of the existing pumping equipment have been successful doing such retrofits. Sometime the retrofitted impellers are made from metal, and at other instances they can also be machined from solid blocks of structural engineered composites. Some of these composites are as strong as steel, good to 300 deg. F, and superior from cavitation standpoint: better then bronze or stainless steel. Handling to 15% of solids particulate is not an issue and resistance to most chemicals is excellent. Seawater, brine and brackish water do not attack the composites: cooling water pumps, river raw water intake, etc. are examples of excellent success of applications. Even for chemical plants excellent opportunities for most acids, caustics and other nasty chemicals. With 80% lighter then metal, these composites improve pump rotor dynamics, and reduce shaft deflections, solving seal leak problems, improving bearings life, and saving couplings from failing.
Talk to these companies. They can provide testimony from their own experience. Your operators will get a better pump and easier to operate. Your mechanics will thank you for 80% parts weight reduction. And your management will love the money saved.
Lev Nelik, Ph.D., P.E., Apics is President of Pumping Machinery, LLC and can be reached at DrPump@PumpingMachinery.com