Calculating Nominal Torque for a Transient Pump Startup or Variable speed

  Product(s): HAMMER
  Version(s): CONNECT Edition, V8i
  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 multiply against. So, when the pattern has a 1.0, it will use an applied electrical torque of 1.0 times 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 sign in first before using the link). Use the "Pump Power and Torque calculator" section, entering the flow, head, efficiency, fluid weight (keep that as-is 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 run, right click on the pump and choose Pump Curve. Enable the option to display the efficiency curve. Right click in the graph, choose Chart Options, select the "efficiency operating point" from the list and click the Data tab to see the exact value. Note: for a variable speed pump whose initial Relative Speed Factor is less than 1, the nominal efficiency point computed by the transient solver may be slightly different than what you see in the Pump Curve graph, since that is generated based on the initial conditions solver. So, in such a case, it is best to perform an initial transient run (for example with just a few seconds for the duration), then open the Transient Analysis Output Log (from the Report ribbon), scroll to the "NODE DATA" section and note the "Nominal Percent Efficiency".
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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