Calculating Nominal Torque for a Transient Pump Startup or Variable speed

  Product(s):  HAMMER
  Version(s): V8i, CONNECT Edition
  Area: Modeling

Problem

When using "Torque" as the "Control Variable" for a pump whose Transient Pump Type is set to "Variable Speed/Torque", what should be entered for the "Torque (Nominal)"? How can this be calculated?

Solution

When using Torque as the Control Variable, the nominal torque value must be entered. This is the value that the multipliers in the pump's Operating Rule pattern will multiple against. So, when the pattern has a 1.0, it will use an applied electrical torque of 1.0 X the nominal torque. So, the nominal torque should normally be equal to the torque when the pump is on, so that a pattern multiplier of 1.0 will reflect the full torque applied.

If you are not sure how to calculate the nominal torque to enter, it can be calculated with the below spreadsheet (be sure to log in first before using the link). Use the "Pump Power and Torque calculator" section, entering the flow, head, efficiency, fluid weight (keep that the same if using the default fluid properties in HAMMER) and rotational speed, and use the computed Shaft Torque as the nominal torque.

Pump flow and head: run a steady state simulation with the pump on and check the head and flow in the correct units
Efficiency: with the above steady state, right click on the pump and choose Pump Curve. Enable the option to display the efficiency curve.
Pump rotational speed: this is entered in the Transient tab of the pump definition

Pump Power and Torque (from the OpenFlows | Hydraulics and Hydrology File Gallery)

Here are the steps of the calculation:

1) First, calculate the Hydraulic Power of the pump based on the initial head and flow
2) Calculate the shaft power based on the hydraulic power and efficiency.
3) Calculate the Shaft Torque based on shaft power and rotational speed

References:

Fluid Transients in Systems, Wylie, 1993, pg. 146
http://www.engineeringtoolbox.com/pumps-power-d_505.html
http://www.engineeringtoolbox.com/pumps-speed-torque-d_1114.html

See Also

Pump efficiency impact on transient results

Pump Startup occurs too quickly / initial upsurge too severe

Calculating initial electrical torque for use with the Load Rejection turbine Operating Case

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