Jim Elsey helps you avoid common centrifugal pump mistakes
by Jim Elsey
June 18, 2018

To preclude the ingestion of air, do not operate the pump when the fluid level is below the critical submergence. The vortexing phenomena is a direct function of the fluid velocity. You can preclude vortexing by the use of baffles and/or larger pipe diameters such as bell flanged inlets. There are numerous reference charts on submergence to use when looking at the suction side design. The best one would be from the Hydraulic Institute. A conservative rule of thumb is to have one foot of submergence per foot of fluid velocity.

Pumps cannot efficiently move fluids mixed with air if the percentage is greater than 4 or 5 percent. Most pumps start to lose performance around 2 to 3 percent air entrainment. Almost all pump designs will cease to perform at around 14 percent entrainment. Exceptions can be disc pumps, self-primers and some vortex or recessed impeller type pumps.

My pump bearing feels hot. This is a common comment, but it is subjective, not objective. It is difficult for the typical person to hold their hand on a bearing housing that is over 120 F.

It is perfectly normal for a bearing to be operating at 160 to 180 F. Use a thermometer or infrared device to measure the temperature and deal
in facts.

Viscosity is the kryptonite of centrifugal pumps. Most centrifugal pumps become too inefficient or exceed their horsepower (hp) limits in a viscosity range between 400 and 700 centipoise that depends on pump size. Always check with the manufacturer when pumping viscous fluids for corrected curves and power limits for the frame, bearings and shaft.

Horsepower requirements progressing along the pump curve change for different impeller geometries. Low and medium specific speed pumps require more hp the farther out on the curve you operate, which is fairly intuitive reasoning. For high specific speed pumps (axial flow), the highest hp required will be at the lower flows. This is also why it is common to start up these types of pumps with the discharge valve open so as to not overload the driver.

There is a simple way to think of specific speed. Specific speed (Ns) is a tool used by designers to look at the performance and geometry of a hypothetical impeller. Don’t want to get all caught up in the math involved? A low specific speed impeller will have the flow enter parallel to the shaft centerline and leave the impeller at 90 degrees to the centerline. A medium specific speed impeller will enter parallel to the shaft and exit the impeller at 45 degrees to the centerline.

A high specific speed impeller will operate with the flow entering parallel to the shaft centerline and leave parallel to the centerline.

Bon appetit!

To read other articles in the 'Common Pumping Mistakes' column, go here.

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