	Product(s):	HAMMER
	Version(s):	08.11.XX.XX and higher
	Area:	Calculations

Problem

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?

Background

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.

Solution

The use of the pump inertia input in a transient simulation in HAMMER depends on the Transient Pump Type:

Shut After Time Delay

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.

Variable speed/torque (and pump startup)

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.

Does HAMMER account for water in the pump for inertia calculations?

HAMMER will take into account the fluid's momentum along the pipeline. The rotating fluid in a pump will contribute to the pump's overall inertia, but HAMMER does not make any allowance for that.

If you wish, you can include the inertia contribution (weight - WR^2) of any water entrained in the pump. However, a lower inertia from including only the pump and motor in the calculation generally results in the worst-case scenario in transient designs, so underestimating the inertia by omitting the contribution from any entrained water would often be okay.

Aside from this, the value of inertia you enter should account for the sum of all components (weight - WR^2) of the particular pump which continue to rotate and are directly connected to the impeller, as follows (taken from the Help topic "Pump Inertia"):

Motor inertia—typically available from motor manufacturers directly, since this parameter is used to design the motor. The pump vendor can also provide this information.
Pump impeller inertia—typically available from the pump manufacturers’ sales or engineering group, since inertia is used to design the pump.
Shaft inertia—the shaft’s inertia is sometimes provided as a combined figure with the impeller. If not, it can either be calculated directly or ignored. Entering a lower figure for the total inertia yields conservative results because flow in the model changes faster than in the real system; therefore, transients will likely be overestimated.
Flywheel inertia—some pumps are equipped with a flywheel to add inertia and slow the rate of change of their rotational speed (and the corresponding change in fluid flow) when power is added or removed suddenly.
Transmission inertia—some pumps are equipped with a transmission, which allows operators to control the amount of torque transmitted from the motor to the pump impeller. Depending on the type of transmission, it may have a significant inertia from the friction plates and the mechanism used to connect or separate them.