How to Select and Size the Right Centrifugal Pumps

How to Size and Select the Correct Centrifugal Pump

At one time or another we have all heard the same question: How do I know that the pump I just selected is the right one for the application? Ever since the advent of the Massachusetts Pump, first developed in Boston in 1818, that question has been uttered by a number of people. Actually two different questions are being raised: how do I know that I have selected the right pump configuration for my needs and how do I know that the pump size I have selected is proper for the application?

Performance difficulties in pumps can often be traced back to either an improper pump selection at the start, or system related issues. Lacking some of the sophisticated controls available today, the common  centrifugal pump is a simple device that will operate where the system design allows it to operate. That point of operation may or may not coincide with the original calculation.

To end up with trouble-free pump performance, it is critical that several factors be considered when selecting the configuration and size of the pump necessary for the application. Factors that include location, available space, maintenance requirements, reliability, and finally the hydraulic requirements, are all factors in the selection process.

For example, the location of the pump in a closet or ceiling area will dictate that a close-coupled pump be considered over a frame-mounted pump. If a mechanical room is the location, then a frame mounted pump may be suitable, but the space may be cramped. Consideration for the maintenance foot print versus the pump foot print should be given as the space required to perform maintenance on the pump may not be sufficient to allow for ease of service.

Finally, the hydraulic requirements will dictate the model of pump you select. A split case pump may seem more appropriate over an end suction pump for moving 2500 gallons of chilled water in a primary distribution loop. Taken together, these varying factors allow you to quickly whittle down to a possible model configuration.

Other times you will discover that the particular application could handle almost any of the various configurations available. What then? Determine which pump operates at the best location on the pump curve taking into consideration efficiency, point of operation on the curve, horsepower and net positive suction head (NPSH) requirements.

That leads us to the second half of our question- how do I select the proper pump size for my application?  To select the proper pump size you need to review the individual performance curves for the pump configurations you are evaluating.

The performance of every centrifugal pump is reflected graphically on a characteristic curve. A typical characteristic curve identifies the capacity capability as reflected in gallons per minute (GPM) against total dynamic head (TDH) in feet, pump efficiency (Ep), brake horsepower (BHP) and required NPSH. The curve in Figure 1(on page 1) reflects the typical head/capacity curve for a pump displaying the differential head produced in feet, versus flow, in GPM.

Figure 2 reflects the addition of the horsepower, pump efficiency and NPSH curves to provide the proper means for evaluating the pump for our needs.

The ideal spot for a pump to operate at is known as the Best Efficiency Point, or BEP. This is the point where capacity and head combine to provide the maximum efficiency performance from the pump. As displayed on a typical computer software selection program, that location in Figure 2 is 1025 GPM at 72 TDH. At this point the pump is at its maximum efficiency of 84%. The identical pump curve is displayed in the traditional curve format displayed in Figure 3.

Ideally, when you are evaluating several different pumps for use you want to select a pump that will meet the hydraulics and come as close as possible to the BEP. Inevitably, compromise is required when making your selection and you must pick between two or three sizes that fall on either side of the BEP. For example, let us presume that you need to meet 1250 GPM at 52 TDH. The pump in Figure 3 will meet this requirement, but will also be operating out toward the end of its curve. With an end of curve operation you need to be concerned that you will have sufficient NPSH available for the pump to properly operate. Note that the amount of NPSH required increases further out on the pump curve where the flow is the highest. If the amount of NPSH available is a limitation, you may want to consider a pump one size larger that will lower your NPSH requirement.

End of curve operation resulting from the use of an undersized pump also limits the leeway that you will have available if you need to make adjustments for the actual flow and head variations in the system when the pump is up and running. In addition, fluid velocities through the suction and discharge nozzles of the pump can be extremely high resulting in increased pump and system noise as well as higher system pressure drops.

Selecting a pump that has an operating point far to the left of BEP can also be undesirable. Operation in this region imposes significantly higher thrust loads on the pump bearings and the mechanical seal faces resulting in diminished life or failure. In addition to these concerns, the pump is significantly oversized, resulting in lower efficiency and higher operating and capital cost.

So what pump should you select? Ideally, select a pump with the flow and head requirements that are slightly to the left of BEP. This provides the user with the capability to make adjustments if necessary to flow and head conditions for the system to operate properly, while providing a high level of efficient operation.