When I run my pump station failure scenario with no protection (Pump shutdown of all 4 pumps after 5 seconds, control valve closes over 300 seconds) at a time step of 0.1s the model results in negative transient pressures (min of -9.71 psi at Pump suction). When I run the same scenario and only change the time step to 0.01 seconds, the model results in positive pressures (min of 3.79 psi at Pump suction). I am not sure which results to believe...Please Help! Thanks!
Time Step 0.1s
Time Step of 0.01s
I concur with Scott's answer. I'll also add that wave propagation and reflection tends to be chaotic in nature in some situations, especially on the suction side of a pump, at dead ends, in closed off places with zero flow (no friction to help dampen the transient energy) and in systems that experience vapor pocket formation and collapse (such pockets can form and collapse rapidly).
In the case of a pump suction side, the waves travel so fast and reflect back and forth so rapidly that a small change like the one in question can have a relatively large impact, since it impacts the timing of the wave reflection and interaction. I would trust the results with the smaller timestep more, especially if there are other fast changes or vapor/ air pockets occurring in this system. A timestep of 0.1 seconds is large for a transient simulation and 0.01 seconds is more typical.
If you're not doing so already, I would highly recommend animating profiles in the Transient Results Viewer to understand what happens during the transient event. You may need to limit the number of report points to ensure that the profile is showing enough detail and not missing out on things that are happening between timesteps.
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Answer Verified By: Sushma Choure