Centrifugal pumps are ubiquitous at process plants but many users don't understand how to select and install them. Some strict rules apply and the devil is in the details. So, this article presents some pointers I've gleaned over the years.
Pump selection should start with the system curve. Plants that supply vendors with a system curve have addressed 80% of the pump selection process. Unfortunately, plant-supplied system curves are rare and this leads to excessive costs.
Developing the curve first requires establishing the piping lay out, allowable system pressures and flow rates. Once isometrics are drawn, you can calculate and graph the system curve. The pressure/flow rate plot should represent the changes that flow will experience over the life of the process. This plot is essential for the pump vendor to make a valid recommendation.
The only other plot needed is the range of Net Positive Suction Head Available (NPSHA) the pump can experience. Determining the total NPSHA requires calculating suction-side static, velocity and friction heads.
You should compare these plots to the pump curve the vendor supplies. The intersection of the system curve and the pump curve represents the ideal design point. Comparing the NPSHA to the Net Positive Suction Head Required (NPSHR) will indicate if the pump is a viable candidate.
When selecting between two particular pumps, try to choose the one with the greatest distance between its maximum and minimum wheel (impeller) size shown on the pump curve. Specifying a pump with a wheel diameter nears its minimum means you can't shave the impeller down to adapt to process changes. Installing the largest possible wheel means you can't debottleneck the process later by increasing the wheel diameter, and will need to purchase a new pump.
Comparing pump curves requires engineering judgment and experience. Sometimes, though, the choice is clear. For instance, Figure 1 shows curves for two pumps — one, an A-Frame, with a recommended wheel of 9.25 inches; the other, an S-Frame, with a 6.13-inch wheel. Both pumps will operate at the same flow rates when compared to the system curve. However, the larger A-Frame pump has an impeller only 0.25 inch larger than the minimum size for that pump. In contrast, the impeller for the S-Frame is about halfway between the minimum and maximum sizes. Clearly, the S-Frame Pump provides more flexibility to accommodate changes.
A smaller wheel has a higher NPSHR and lower required power draw, but is less efficient. The larger the wheel is, the more efficient the pump if you can operate near or at the Best Efficiency Point (BEP), indicated by the triangular areas on Figure 1. While you shouldn't ignore efficiency in pump selection, BEP operation, shaft length (L) and diameter (D), needed horsepower, NPSHR and process adaptability typically dominate design criteria.
Filtration is an example of a pump running across the curve during required operation. The pump starts at a high flow rate and low head. As the cake builds, discharge head increases. So, it's impossible to design a centrifugal pump to operate at the BEP in this application. This can result in vibration on both sides of the BEP and significant shaft deflection problems.