Trimming the impeller is one of best solutions in reducing operating costs and may solve other problems such as excessive flow velocity, erosion, and noise. The process of removing the impeller, reducing its diameter on a lathe, checking the static balance, and reinstalling the impeller is not lengthy or expensive. It often can be done in an afternoon. What are some tips for trimming a pump impeller:
A system that has multiple sub-circuits must be hydronically balanced before you try to determine the correct impeller diameter. An impeller can be trimmed too much if you use an unbalanced system curve.
When selecting a pump, anticipate the need for adjusting the impeller diameter by avoiding a pump that requires either the largest- or the smallest-possible-diameter impeller for the given volute. This practice alone ought to help reduce uncertainty in the design phase because you can be confident in your ability to adjust the impeller if necessary. It doesn’t happen as often, but sometimes the system head requirement is underestimated, and an impeller that’s too small is specified. If you already have the largest-possible impeller, you have no upward wiggle room.
Trimming the impeller often reduces operating horsepower, which brings up the question of replacing the pump motor with a smaller one. Consider the cost of buying, installing, and realigning the new motor. Often the original, large motor can be left in place to operate at lower horsepower.
Despite the potential for large savings in operating costs, people still resist the idea of trimming the impeller, often citing an expected future increase in required flow as the reason. If you encounter resistance to the idea of trimming the impeller, consider using an adjustable-frequency drive and dialing-down the motor speed to simulate trimming the impeller. Then, if greater capacity is required in the future, you simply can turn the speed back-up. If an automatic control system is not required, drive costs are low enough that this can be a perfectly acceptable alternative if the building owner is strongly opposed to trimming the impeller.
Pump impeller trimming has several advantages, the first of which is a short simple payback (often the criteria driving project implementation decisions). The simple payback has been found to be less than two years for most buildings. Another advantage is that the impeller trim balancing procedure is relatively simple to implement. It is based on well understood–albeit seldomly applied–engineering principles. The implementation requires commonly available pressure measuring instruments, such as a differential pressure gage or Bourdon pressure gages with pressure test plugs. The actual impeller trimming process involves two steps: trimming the impeller and balancing it, usually with a dynamic or static balancing apparatus. The necessary equipment is commonly available in machine shops.
Finally, pump impeller trimming could be economically beneficial to large end-users such as universities, school districts, and state and federal buildings, which have a large number of pumps in use. It could also benefit existing buildings and new construction projects. Existing buildings exhibit a potential for energy savings while the benefits in new projects may provide even greater than in an existing building, since the incremental cost of the impeller trimming will be lower than in a retrofit application.
However, depending on the age of the pump and its applications, it may be time to invest in a new, more energy efficient circulator.