Common Pumping Mistakes
by Jim Elsey
October 30, 2017
Where pump efficiency is taken at the rated duty point and expressed as a decimalFigure 2. Where pump efficiency is taken at the rated duty point and expressed as a decimal

The pump efficiency is expressed as a decimal in the denominator of the equation (formula) so the effect of efficiency has an inverse result. Should the efficiency increase, the BHP will reduce and vice versa. From the formula you can determine the efficiency of a pump if you know the other factors.

This is not the method used by the original equipment manufacturer (OEM) on the performance test stand.

Background

An electrical induction motor converts electrical energy to rotational mechanical energy. The pump receives that mechanical energy and converts it to pressure energy, which manifests as flow and head. As a result of earlier industry requirements and a DOE government program, motors are more efficient than they were years ago. It is now standard for a new motor to be more than 90 percent efficient, sometimes approaching 96 percent. The motor maintains that high efficiency over the upper 50 percent of its horsepower operating range.

Pumps, on the other hand, only reach peak efficiency at one operating point or at the very least a small operating range. Pump operations that depart from the BEP will experience a marked drop in efficiency in addition to other deleterious effects such as radial thrust, cavitation and recirculation. Centrifugal pumps are really only designed for one condition point of head and flow. To operate anywhere else on the curve is simply a compromise. The key to an efficient installation is to design the system and properly select the pump so that it operates at its BEP most of the time. In reality, this rarely happens and is the main point of this article.

When a pump operates there are many types of losses that preclude the pump from being highly efficient. There are mechanical losses caused by bearings, packing or seals. There are volumetric losses due to recirculation, and there are hydraulic losses due to friction, some of which dissipate as heat and noise.

Additionally, there are losses due to sudden acceleration and deceleration of the fluid (shock losses). There are losses due to pump out vanes or balance holes on the impeller. There are losses due to running clearances in wear rings, throttle bushings and balance drums. Often overlooked, there are significant losses caused by turbulent flow created by vortices (eddy currents) that recirculate in front of an impeller and disturb the laminar flow required for the pump to operate as designed. Visualize that the angle of the incoming fluid flow should match the attack angle (leading edge) of the rotating impeller vane for efficient operation. These “upsetting vortices” are created by excessive impeller or ring clearances and are exacerbated by operating the pump away from BEP.

Just as you would not drive your car at 70 miles per hour in first gear or 55 miles per hour with the brake pedal fully depressed, you should not operate the pump with the discharge valve 50 percent shut or with the bypass or control valve fully ported back to suction, but this is happening every day in plants around the world, and the wasted energy can never be recovered.

Many pump operators do not have an accurate idea of where their pump is operating on the curve, and even more operators do not have a good grasp of their system curve dynamics or even the concept of a system curve. Simply stated, the pump will operate on its performance curve where the system curve dictates, that is where the two curves intersect. If you are not monitoring the differential pressure across the pump then you have limited knowledge of where the pump is operating on its curve, therefore you can’t know if the pump is being operated at its BEP.

A few methods exist to improve the efficiency of the pump itself and typically involve addressing the surface finish of the casing and impeller. As always, there are pros and cons to the methods.
Friction losses in a pump are directly proportional to the relative roughness (smoothness) of the casing, impeller and other waterway surfaces. One method is to apply superficial coatings that will yield varying results depending on the size of the pump and how well the surface was prepared and the coating applied. Note that the coating will also change clearances with the unintended consequence of reducing the expected benefit, especially on a smaller pump. The bigger the pump, the more benefit will be realized. Another method is to produce a highly polished finish (typically just on the impeller) to reduce fluid friction, which will offer good results but can also cause erosion and corrosion depending on the fluid.

More of a maintenance step than a design parameter are the pump clearances, which have a significant effect on the pump efficiency. Pump efficiency will decrease exponentially with increased clearances once the small range of initial design clearance is exceeded.

Last, but not least, investment castings or similar improved casting processes will yield smooth surfaces and also allow a few more feet of head development, which results in more efficient pumps by a few points. There are other methods, but most will yield results that do not offer sufficient payback for the expense incurred or are unique to a certain pump
or system.

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