Gene Vogel
EASA Pump & Vibration Specialist
Service centers are often called on to provide replacement pumps or pump motors or to advise on pump retrofit and re-application projects. A good understanding of the parameters that govern pump performance is essential to help customers with these opportunities. The information here relates to rotodynamic pumps (centrifugal and axial flow impellers) and not to positive displacement pumps.
Unlike motors, pumps are rated by head and flow, not by power. There’s no such thing as a 50 hp pump or a 100 kW pump. The pump can operate over a range of heads and flows, and the power required is determined by those and by the efficiency of the pump at the particular head-flow operating point. It is helpful to know that “head” correlates to a measure of pressure. For water it is a simple conversion: 2.31 ft. head = 1 psi (1 meter = 9.8 kPa). There is a simple formula that describes the relationship between head, flow, pump efficiency and pump power:
The formula is very helpful for making quick estimates of the power required for a pumping application with known head and flow values. But you can only get accurate power values from the pump curve. All rotodynamic pumps have a pump curve prepared by the pump manufacturer. EASA provides several technical resources related to reading pump curves; those details are beyond the scope of this article. What is important here is that the power requirements for a pump vary with flow rate. So knowing the range of flow rates for the pump is essential to sizing a motor to the pump.
A pump may operate across a fairly wide range of flow rates, known as the Allowable Operating Range. Ideally, the pump should be designed to operate as close as possible to the Best Efficiency Point (BEP) and within the Preferred Operating Range. Pump efficiency will drop dramatically as flow rates move away from the BEP, and turbulent flow will reduce the reliability of the pump. Of course, the head that a pump can deliver must also match the application. If the maximum pump head is below the system demand, the pump will not produce flow (bad!). If the maximum pump head is much greater than the system demand (more than double), the operating point will not be near the BEP, and both efficiency and pump reliability will suffer. These basic concepts are all evident on the pump curve.
Cavitation Process
Another important concern when selecting a pump for a specific application is the possibility that cavitation may occur. If the pump is to operate across a range of flow rates (rather than always operating near a single flow rate), cavitation will be more likely at the higher flow rates. Pumps have Net Positive Suction Head Required (NPSHR) ratings, which allow the likelihood of cavitation to be evaluated. The NPSHR values for any flow rate are provided on the pump curve. Generally, lower-speed pumps are less susceptible to cavitation than higher-speed pumps. If the application has low suction head demands, a lower operating speed will be an advantage. At lower operating speeds, a larger pump impeller diameter will be required, and thus a physically larger and more expensive pump may be needed.
So the process of sizing a pump and motor starts with properly sizing the pump, using the range of head and flow requirement of the application. Once the pump is properly sized and selected, the motor is sized and selected to match the pump requirements. For most pumps, the power requirement varies with flow rates. Power requirements may increase with increased flow, or they may decrease with increased flow. The pump curve will provide that information. Obviously, the motor must have adequate power to meet the pump demand at the application flow rate with the highest power requirement. That’s the minimum power requirement for the motor. But it is likely the pump will have an Allowable Operating Range wider than the application demands.
Power Considerations
So if application demands were to change at some future time, the pump might be expected to operate at a point where the power requirements were greater than that minimum power requirement. It is wise to consider the maximum power the pump could require under any operating conditions. This value is provided on the pump curve as the No Overload Power (NOL) rating. In some cases, the difference between the minimum power requirement for the application and the NOL rating may be absorbed by the motor service factor. In other instances sizing for NOL power may require a higher power motor.
Conclusion
Sizing a pump and a pump motor for an application is more than a trivial endeavor. The application head and flow requirements must be known. The pump power formula provided above, with the “k” to match the selected units, will provide a good estimate of the size of the machine. Pump vendors have pump selection charts which are generalized versions of the pump curve that will help with pump selections, and those charts and related reference data will provide NOL power ratings (see Figure 1).
The person responsible for the final selection of a pump and motor should know how to read pump curves and motor performance data, and those resources must be available to them. Searching the EASA website (easa.com) will yield a plethora of technical information to help with the process.