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

 Applies To Product(s): HAMMER Version(s): CONNECT Edition, V8i Area: Modeling Original Author: Scott Kampa, Bentley Technical Support Group

# Problem

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

# Solution

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.