I have three running pumps in my model and for pump emergency (abnormal) shutdown I did the following setting:
the full stop time for one pump shutdown scenario (speed drops from 2900 to zero) is about 2.85 sec,
for 2 pumps simultaneous shutdown, the stop time is 4.5 second
but the full stop time for the 3 simultaneous pumps shutdown is about 26 seconds
I just confused about the results, since each pump stops based on defined inertia, so the stop time should be the same.
because of this slow stopping in three running pumps scenario, there is no cavitation and negative pressure in the system while for a single running pump there is cavitation and vapor generation. This means that three simultaneous pumps shutdown in better than two simultaneous pumps shutdown and two is better than one, but in reality, it is not correct, more pumps shutdown worse conditions.
I think the stop time should be same for each pump and independent from how many pumps are running simultaneously,
have any idea how can I fix this problem or is this normal?
In the pump shutdown event the pump's torque instantly drops to zero at the time specified in the "Time (Delay until shut down) sec" property. With this, the impeller will still be spinning and keep water column moving forward, 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.
See the article related to this here;
How does pump inertia effect the pump calculations during a transient simulation?
This is what could be happening for the third pump. Check the pump inertia for the third pump. It could be causing the pump speed to not go down as expected. You can view the pump speed in the extended node results. See the article for it below;
How to view extended node transient results for hydropneumatic tanks, pumps, or turbines
Let me know if this helps.
Bentley Technical Support
Based on the screenshots provided, I suspect the issue could be due to the timing of the closure of a check valve. Notice in the first two screenshots for the single pump and two pump shutdown, there is a gradual downward trend in the speed and then it sharply drops to zero - this also happens in the third screenshot of the three pumps shutting down, but does not occur until about 26 seconds. This kind of sudden drop to zero speed typically corresponds to the closure of a check valve. So, it could be that with three pumps shutting down, the hydraulics dictate that reverse flow conditions do not occur until later, so the check valve takes longer to close. If you're seeing a more severe transient response in the one pump and two pump cases, that could be due to the closure of the check valve.
Please check the configuration of the check valve option in the pump properties, pipe properties, or check valve node element. Also look at your user notifications and/or a graph of flow to see when the check valve closure occurs. If you are using a check valve node element with a non-zero closure time, note that reverse velocity can build up, potentially causing a worse transient than would occur with a faster closing check valve (see more on that here). A close examination of the results and profile animation could help determine if this is related.
Also as Yashodhan mentioned, check to make sure that the inertia selected in the transient tab of the pump definitions (for all three pumps, if they use different pump curves) is appropriate.
If you need further help, please provide a copy of the model: Sharing Hydraulic Model Files on the OpenFlows Forum
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.
That screenshots belong to the same pump but for different scenarios.
I have three pups operating parallel with the same curve and specification.
Check valve closure could be the main reason for dropping the pump speed and I will check the notification for check valve first closure time.
but I expected the more negative surge pressure (more vapor generation) for the 3 simultaneous pumps shut down rather than two and one, but the result shows that the one pump shutdown is the worse case, is it correct in reality or this is the software issue?
Jafar Dashtbani said:but I expected the more negative surge pressure (more vapor generation) for the 3 simultaneous pumps shut down rather than two and one, but the result shows that the one pump shutdown is the worse case, is it correct in reality or this is the software issue?
In my experience, users often see surprising results in transient simulations that they may not have expected. This is due to the complex interactions and phenomenon that can occur. In your particular case, shutting down three pumps instead of one or two may change the hydraulics of the network such that the check valve closes at a later time, when the velocity is different, causing a different response to that closure. There could be a complex interaction going on between your pumps, the check valve, the velocity in the pipes, the inertia of multiple pumps vs. the inertia of one pump alone)
So, the short answer to your question is that the results should be accurate as long as your data input and assumptions are accurate (and as long as the user notifications do not indicate any major problems), but without a deep examination of the model, I cannot say specifically why the results are different other than my observations above. I would recommend taking a close look at the interactions occurring in your model using profile animations as mentioned in my previous reply, to get more confidence in exactly how the network is reacting in each case. (with close attention to the check valve closure)