Working with Variable Speed Pumps (VSP) in a Transient Simulation

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



Various problems with pump results in a transient simulation are seen when the relative speed factor in the initial conditions is less than 1.0 (see initial relative speed factor, and calculated relative speed factor).


The most common issue that will occur with this is an initial surge. This will occur when the user also sets up an operating rule with a starting multiplier that is the same as the relative speed factor. The operating rule is strictly a multiplier and is not related to the relative speed factor value itself. For instance, if you have an initial relative speed factor of 0.8 and create an operating rule with a pattern that has in an initial multiplier of 0.8 as well, this can cause an initial surge in the model result. Basically, this results in a pump that starts with a relative speed factor of 0.8 at the initial time step and drops immediately to 80% of 0.8 (0.8 * 0.8) in the next time step. This disparity can cause an initial surge to occur in the model since a change is occurring in the first transient timestep.

To resolve this, set the initial multiplier to 1.0 (separate field above the operating rule pattern), and adjust the rest of the operating rule pattern accordingly.

Another factor to consider in this situation is the rotational speed entered in the "transient" tab of the pump definition. In a case where the pump's initial relative speed factor is less 1.0, HAMMER will use an adjusted initial rotational speed (in rpm) based on the rotational speed in the Transient tab of the pump definition multiplied by the initial relative speed factor. So, you'll need to make sure that the rotational speed entered in the pump definition is still set as the full speed (corresponding to 1.0/100% relative speed factor).

To confirm the actual calculated pump rotational speeds during the transient simulation, add a number in the "Report period" field of the pump, then look at the pump speed graph in the Extended Node Data section of the Transient Results Viewer. 

It is also important to be aware of the impact of pump inertia as explained in this article. Regardless of the initial relative speed factor, during the transient simulation HAMMER will use the inertia value that you enter in the transient tab of the pump definition, as the inertia of the rotating parts of the pump.

Pump Flow at end of transient simulation does not match a steady state check

Another problem that can occur is with the pump flow and head results seen at the end of a transient simulation compared to a steady state check with the relative speed factor adjusted to match the rotational speed at the end of the transient simulation. This can happen if the final speed in the transient simulation is not the same as the initial conditions speed. In this case, the differences can be due to how the pump definition (head vs. flow curve) is scaled (for different speeds) during the initial conditions compared to how the four quadrant pump curve (based on the specific speed in combination with the pair of initial head and flow) is scaled during the transient simulation. If the final speed in the transient simulation is not the same as the initial conditions speed, the differences in the scaling due to the selected Specific Speed can be noticeable.

To illustrate, change the Specific Speed in the Transient tab of the pump definition, re-compute and you will see that the final flow of each pump will be distributed differently. This article can be used to estimate the Specific Speed based on the head and flow in the initial conditions. Note that because of the way that the four quadrant pump curve is constructed based upon the initial head and flow, HAMMER assumes that the initial conditions operating point is at or near the best efficiency point. If you had initially chosen a specific speed based on different assumptions, adjusting the specific speed based on the initial conditions operating point may yield results closer to what you expect.

Another possible solution is to start the initial conditions with pumps at the same speed that the transient simulation will end with, then set up the transient operating rule to slowly transition to the actual desired starting speed, then have it shut down and start back up, settling on the speed that was used in the initial conditions.

See Also

Initial surge or transient event after adding a surge tank to a model

Pump startup occurs too quickly / initial upsurge too severe