If a pump’s speed will be controlled (i.e., ramped up or down, started or shut down during the simulation period) you need to enter the pump’s rotational inertia. Inertia is the product of the rotating weight with the square of its radius of gyration. Pumps with more rotating mass have more inertia and take longer to stop spinning after power fails or the pump is shut off. The trend has been towards lighter pumps with less inertia.

Pumps with higher inertias can help to control transients because they continue to move water through the pump for a longer time as they slowly decelerate. You can sometimes add a flywheel to increase the total inertia and reduce the rate at which flow slows down after a power failure or emergency shut down: this is more effective for short systems than for long systems.

The value of inertia you enter in Bentley HAMMER CONNECT must be the sum of all components of the particular pump which continue to rotate and are directly connected to the impeller, as follows:

• 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.

While this may seem like a long list, it is often enough to enter only the pump and motor inertia and neglect the other factors. For design purposes, this tends to yield conservative results, because the simulated pump will stop more rapidly than the real pump would. Surge-protection designed to control the somewhat larger simulated transients should be adequate.

If the motor and pump inertia are not available, they can be estimated separately and then summed (if they remain coupled after a power failure) using an empirical relation developed by Thorley:

If uncertainty in this parameter is a concern, several simulations should be run to assess the sensitivity of the results to changes in inertia.