What Happens When a Pump No Longer Operates at Optimum Conditions?
What are the implications of pumps operating “to the left” of the best efficiency point (BEP)? Low efficiency, high radial loads, noise, vibration - become a real problem when that happens. Damage to the seal, shaft, couplings and poor reliability are a real and direct result of such operation. And – tremendous waste of energy. But, have you actually thought of a magnitude of such wasted energy, for a typical pump? Consider, for example, a 8x10-17 size (10”suction, 17” impeller):
Let’s assume this pump operates at 2000 gpm, 280 feet head, instead of a best efficiency peak point of 4000 gpm. The efficiency at the actual operating point is only 70% instead of the potentially achievable 83% by this pump.
The horsepower at the operating point is roughly 225 hp (168 KW). At $0.07 per kw-hrs cost, the yearly energy bill is:
168 kw x 24 hrs x 360 days x 0.07 = $101,516
At restored efficiency, this would be: 101516 x (70/83) = $85,616
The nest savings would be: 101516 – 85616 = $15,900 !
For larger sizes, the energy savings could be even greater.
As you can see, the net savings depend on how far back away from the Best Efficiency Point the pump operates. Unfortunately, this problem exists in all too many actual installations in the field. Many pumps, procured and installed years ago, often no longer operate at the originally intended hydraulic conditions. As operating conditions change, the pump is simply throttled further and further away from the BEP. The result – dollars literally “burned”, and washed down the drain.
Replacing such pump is not the best answer. First of all, a smaller pump may still not (and usually does not) have the hydraulics sized to hit the operating point “dead on”. It may help somewhat, but an expensive approach. The user choice is limited only to the pump sizes available, as standard, from the pump manufacturer’s catalog, and even with a large number of sizes in the catalog, it is virtually impossible to cover each and every set of operating conditions. So, the user is forced to settle for the “second best”, but not the optimum. Even more prohibitive are economics and feasibility of piping change, to accommodate a proposed pump downsizing. Piping changes alone can often cost more then a pump.
A better solution is to have a new impeller, custom-designed and sized exactly for your operating conditions. By doing that, a pump performance will essentially “shift” or “slide” to exactly where the Best Efficiency Point is, - and the net losses become zero. Such approach is effective, and the investment is minimal, with a payback typically less then a year. The evaluation process for such program usually starts with a review of the existing operating conditions, flow analysis, and design optimization to evaluate potential energy savings for a specific plant installation case.
For more information on energy optimization contact the author at www.PumpingMachinery.com or call 770-310-0866
Dr. Nelik has 30 years experience with pumps and pumping equipment. He is a Registered Professional, with over fifty publications on pumps and related equipment. He is a President of Pumping Machinery, LLC company, specializing in pump consulting, training, and equipment troubleshooting. He teaches pump training courses, and consults on pumps operations and troubleshooting, engineering aspects of centrifugal and positive displacement pumps, maintenance methods to improve reliability, energy savings, and optimize pump-to-system operation.