How to select the right centrifugal pump

Understanding system head curve is critical to building a well-matched pump and piping system.

By Robert X. Perez, P.E.

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During my career, I have coached numerous new engineers in the selection of centrifugal pumps for process applications. On many occasions, and after years of seeing the costly results of pump theory knowledge gaps, I found that engineers get caught in selection traps that have resulted in less than ideal pump selections. I eventually realized that the root cause of the poor pump selections was a lack of training in centrifugal pump hydraulics basics, and how hydraulic design parameters can dramatically impact a pump’s life cycle costs.

Choosing a centrifugal pump from the countless options available can be daunting, but someone has to make the decision. Many factors, such as the required flow, differential pressure, and suction conditions must be weighed against the capital costs and cost of energy for the pumps considered. To determine the right pump, you must consider the overall cost of ownership, which includes capital cost, operating costs, and maintenance cost. What good is a low cost pump if it is inefficient or is costly to maintain?

My selection methodology focuses mainly on hydraulic design considerations but also touches on mechanical design details, including key hydraulic deign parameters, such as specific speed (Ns) and suction specific speed (Nss) , NSPHa, NPSHr, and NPSH margin ratio. Analyzing these basic hydraulic parameters allow you to quickly determine if a centrifugal pump makes sense for your particular application. If you do decide a centrifugal pump will work for your application, then you need to be able to evaluate the various bids returned by pump manufacturers.

The importance of system head curve

Every pump manufacturer would like to supply the perfect pump for every application they quote. However to do this, the manufacturer requires the future pump user to provide them with an accurate system head curve that describes the capacity and head needed for your operating conditions. In this section, we will define what a system head curve is, why they are important, and how they are generated.

 

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A system-head curve is an analytical or graphical representation of the relationship between the flow and the hydraulic head requirements of a given piping system. Hydraulic head requirements are related to the suction head, the discharge head, and the hydraulic head losses of the piping system. Since hydraulic head requirements and losses are functions of the flowrate, size and length of pipe, as well as the size, number and type of fittings, each system head curve tends to have a unique shape.

Normally, at least one point on the system curve is given to pump manufacturers in order to help them select the pump properly. However, it is highly desirable to graphically superimpose the entire system curve over the head-capacity curve of the pump in order to better understand the interactions between the pump performance curve and the system head curve. By definition, the intersection of the pump performance curve with the system head curve defines the operating point of the pump and piping system. Once the system curve is defined, we can plot various pump curves on top of the system curve and hopefully select one that matches the process needs. Without this system curve, there is not much of a chance of coming up with the right pump.

To create a system curve, we must first plot the desired capacities against the required head over the total anticipated operating range of the pump. The head will be measured in feet or meters and the capacity will be measured in gallons per minute or cubic meters per hour. The general equation for a system curve is given by the following equation:

Hsystem= (Pdownstream - Pupstream) + Elevation + All line losses

Putting this equation in words, we would say that the system head is the sum of (1) the difference between the downstream pressure and the upstream pressure; (2) the elevation difference between the downstream liquid level and the upstream liquid level; and (3) all the line losses. For the remainder of this article we will use the following shorthand version of the equation:

Hsystem = (Pds - Pus) + Hstatic + L

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