How does pump inertia effect the pump calculations during a transient simulation?
Why don't I see any change in transient results when changing the pump inertia?
The transient solver in HAMMER assumes that for all rotational equipment – pumps and turbines – the “moment of inertia” refers to WR^2, weight moment of inertia. Thus, the inertia units for pumps and turbines are identical. In the SI system, typical units are N-m^2 and kgf-m^2, where kgf is kilogram-force; in US units, lbf-ft^2, where pound-force lbf is the most common choice for the force component. The force units kgf and lbf are often written without the suffix “f” when the meaning is clear.
The use of the pump inertia input in a transient simulation in HAMMER depends on the Transient Pump Type:
With the shut after time delay transient pump type, HAMMER assumes that the pump's applied electrical torque instantly drops to zero at the time specified in the "Time (Delay until shut down)" property. At this point, the impeller will still be spinning and partially keeping the momentum of the water column moving, but will be slowing down based on the Inertia. In general, the higher the inertia, the longer it will take for the pump to shut down.
When using the variable speed/torque option (including pump start), the Operating Rule can either control the speed of a pump or the applied torque, depending on what you select for the Control Variable. This is explained in more detail in these two articles:
Pump Startup occurs too quickly / initial upsurge too severe
Residual flows different for different pump shut down methods.
When the Control Variable is set to Speed, the Operating Rule is directly controlling the pump impeller speed, in which case the pump inertia does not influence the calculations (since you're essentially including its effect in the operating rule pattern).
When the Control Variable is set to Torque, the Operating Rule is directly controlling the electrical torque applied to the pump, and the impeller speed will be based on factors including the inertia that you enter. Speed is a part of the Four-Quadrant characteristic curve that HAMMER uses to simulate the hydraulics of the pump during the transient simulation, and is a function of the Specific Speed that you enter. More on this here: How are the pump flows used in the transient analysis derived in HAMMER?
Note: See Help topics "Pump Inertia" and "Pump and Motor Inertia Calculator" for more information on entering the Inertia value.
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