Special Concerns For Sizing Pumps

There are several special concerns with sizing pumps. For a general overview of the process of sizing pumps see: Sizing Pipes and Pumps Concurrently.

End of Curve Flow Rate

Centrifugal pumps typically have an end of curve, or runout, flow rate. They cannot operate reliably past this point. This can be entered in Pump Configuration, but this will not actually prevent the model from running the pump with higher flows. Instead, the curve will be extrapolated past this point and a warning will be generated.

If it is important to prevent the sizing results from considering flows in this region as valid, a Design Requirement for maximum flow rate equal to the end of curve value should be placed on the pump.

Partial Service Duration

Without other input, ANS will assume that any pump included in the sizing operates constantly.

If this is not the case, then either an appropriate Service Duration or Multiplier should be defined in Assign Cost Libraries.

Variable Speed Pumps with Partial Control Applicability

ANS can size pumps with a user defined fixed speed other than 100%, or a controlled pump that modifies speed to meet a flow or pressure setpoint. However, pressure-type variable speed controlled pumps allow the user to optionally enforce Control only when the actual pressure is above/below a setpoint. This causes an issue with sizing.

If ANS allowed the pump to change from a fixed 100% speed pump (non-controlling pressure) to a variable speed pump (when the setpoint is violated), it would be changing the characteristics of the system. This has a direct impact on the sizing process, so it cannot be allowed - ANS will always model this type of pump as fixed 100% speed.

Either an Always Controlling pump must be specified, or Design Requirements on pressure must be added to the appropriate pipes.

Unrealistic Sizing Results From Dependent Design Cases

If there are two design cases that have different pump flow rate requirements, the ideal pump curve may be unrealistic.

For example, one operating case may require 500 gal/min, and another may require 750 gal/min. It is desired to select the ideal pump. After running ANS, it is determined that the pump should generate 50 feet of head in the 500 gal/min case and 70 feet in the 750 gal/min case. To meet this requirement, we need a pump curve that increases from 50 feet of head to 70 feet with a large increase in flow. This is an unrealistic demand.

To prevent this, a head rise Design Requirement can be added to maintain the high flow head at 70-90% of the low flow head. This will effectively force the sizing to consider only realistically shaped pump curves.