Common Pumping Mistakes
by Jim Elsey
May 29, 2017

Most experienced pump people will, as an accepted best practice, size the motor driver to be non-overloading for the selected impeller diameter curve. Many industries and end users will specify that the motor be sized to be non-overloading for the maximum size impeller in that pump (6.0 inches in our example), and in many cases there will be a service factor that is added to that max impeller quantity or at least to the selected impeller requirement.

In American Petroleum Industry (API) applications and specifications there will be a service factor added to the motor size once the base size is selected (not to be confused with the actual motor service factor from the manufacturer). Sometimes there are cases that require a size that is obviously too large, and documented exceptions are made to reduce the motor by one size. For motors 25 HP and under, the factor is 1.25. For motors 30 to 75 HP, the factor is 1.15. For motors 100 HP and larger, the factor is 1.1.

Another factor to take into consideration is the specific speed (NS) of the pump impeller. At the lower and mid-range specific speeds a pump will require more BHP as the flow is increased. BHP starts at a low number and increases as the flow increases. For high NS pumps (axial flow in the ranges above NS of 6,000) the BHP is high at the low flows and actually reduces as the flow increases.

This could be a topic for another column, and it is not my intent to explain further here. The point is that the motor size needs to be carefully selected and is the main reason why these high NS pumps are started with the discharge valve open in lieu of closed. Otherwise the motor would trip on overload, unless it was oversized.

Internal Combustion Drivers

A few comments about internal combustion drivers: Simply stated, engines do not handle and react to torque load requirements similarly to most induction motors, so they must be oversized for the service. Otherwise, the pump will not run at the correct speed or at all. Additionally, electric motors run very efficiently near full load while air-cooled engines have a tendency to overheat as they approach full load. For details and specifics contact the engine manufacturer.

As a general rule, know that the engine driver should be at least 30 to 40 percent larger than an induction motor for the same service.

When all applicable factors are considered, an engine can sometimes be twice the size of a motor to drive a pump at the same flow and head. Further considerations should be altitude (ASL or distance above sea level) and ambient temperature, both of which directly affect the engine performance. When selecting engines as pump drivers, the only rating that should be seriously considered is the “Net Continuous Horsepower Available at the Output Shaft” rating.

For more Common Pumping Mistakes columns, click here.

15 Tips for Correctly Sizing the Pump Driver

1. Before you start, define the acceptance criteria for what will become the motor sizing specification.

2. Calculate all anticipated hydraulic loads based on the BHP formula (see Equation 1) and/or use the pump selection software. Ask yourself and others if the conditions will remain constant or change.

3. In the initial base calculations allow for appropriate changes in specific gravity, viscosity, fluid temperature, percent of suspended solids, addition of system components, system fouling and corrosion. Selection software can account for some of these parameters, but not all.

4. Design for the maximum load in most every case. There will be exceptions.

5. Select a motor size based on the rated hydraulic conditions (aka duty point) but also considering the maximum and future loads based on the manufacturer’s pump curve.

6. Select an electric motor that does not use the service factor region. I understand that some people will select a motor that will be forced to operate in the service factor region. It is rarely a good idea.

7. Make the customer (end user) a part of the decision process.

8. In most cases using induction motors, 99 percent of the time it will make the best sense to use a NEMA Design B motor.

9. In the case of electric motors do not oversize the driver, since motors run best in the upper 90 percent range of their ranges.

10. In the case of internal combustion engines, oversize by 40 to 200 percent unless lower speed and reduced performance are acceptable.

11. A centrifugal pump will operate where the system curve dictates. How comfortable are you with the accuracy of the system curve calculations? If replacing a pump on an existing system with good empirical data then you can be quite comfortable with the expected pump operating point. If the system is a complicated new design you may want to give yourself a few options.

12. Altitude will negatively affect ratings on both engine and motor drivers.

13. Duty cycle is another important factor in the selection process. Ask these questions and select based on the answers. What percentage of the time will the pump operate and how many starts in an hour? Is the torque changing gradually or suddenly?

14. If there are gear, belt or chain drives with sheaves, the driver will need to have a higher rating due to the losses associated with the drive arrangement. This higher rating requirement can also be true on vertical hollow shaft motors.

15. Specific speed (NS = geometry of the impeller) will affect the required horsepower differently at different points on the pump curve. The higher the NS the more BHP will be required at low flows.

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