Dears ,
I am modelling a 2 km pvc line of 1600 mm diameter. the static head is only 4 m & the total head is 18 m. when i model the event of pumps shut down , the flow is only reducer through the pumps & cut off after over than 80 seconds whatever the pump inertia is. I noticed that the flow from the supply tank continues through the pumps even after power failure. is it the real case ? how can prevent it to model what happens in my model ?
Best ,
Hello Ahmed,
Depending on the transient settings for the pump, it may take some time for the flow to stop. You may want to check the time history graph for the results if you have not already.
If that is not the issue, I will need a little more information. What pump valve type are you using for the pump being shut down? If you have a control valve entered, this might account for the flow. Depending on the time to close, the valve may remain open after the pump shut down. This will mean that either forward or reverse flow will be allowed through the pump (following the four-quadrant curve used for the pump). You could try setting the valve type to "Check Valve" and see how that impacts the results.
If that doesn't help, I may need to see a copy of the model. There are two options for sharing your model files on BE Communities. If you would like the files to be visible to other members, compress the files into a zip file and upload them as an attachment using the ‘Advanced Reply editor’ before posting. If your data is confidential, you can follow the instructions in the link below to send it to us via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley.
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If you upload the model to Sharefile, please post here to let me know that it is available.
Regards,
Scott Kampa
Bentley Technical Support
Don't forget that the pump inertia is insignificant compared to the inertia of the liquid in your system.
Imagine a freight train that was 2km long and weighed 4000 tonnes - that's how much water you have in the pipeline. Even if you cut the engine, the train will keep rolling for a while - especially since you have such a flat system (approx. 0.2% uphill slope).
So it sounds like this could be a realistic result that the model is giving you...though I am sure Scott can double check if you send him the model.
Assuming you don't have any significant negative pressures or vapor pockets forming, then it may be ok for the water to keep flowing for a while under its own inertia?
Mal
Answer Verified By: Sushma Choure
Hello all,
Scott, the file is attached.
Mal, I see that the inertia causes the fluid to continue downstream the pump even after power failure. But , would it cause the fluid to be pulled out through the pump ?
Best regards,
Ahmed Elshorbagy
One thing you may want to do is look at the elevations in the model, including around the pumps. The elevations at three of the four pumps is set to -5 meters, the other is set to 0 meters.
In addition to possible issues with the elevation, this also could be related to the relatively high flows being generated. The flow in the initial conditions is based on the Discharge to Atmosphere element you have at the downstream end of the model. You may want to make sure the values for the typical flow and pressure drop are accurate and that the flow into the D2A is correct.
I did a quick test on your model, replacing the D2A with a reservoir with an elevation of 10 meters. With this setup, the pump goes to zero much more quickly, about half the time it took before. The flow through each of the four pumps is about 1140 L/s compared to a flow of 1500 L/s through each pump if the D2A is present in the model.
This goes back to what Mal said in his post. With a large flow through your system, it will take longer for the pump to speed down to zero flow. If the initial conditions flow results through the pumps and at the D2A are accurate (as well as the inertia entered for the pump), the results seem to be as expected. The fact that a lower flow situation yields zero flow through the pumps more quickly also seems to support this.
So, I would start by looking a the initial conditions results to make sure they are correct. With large flows, it will take time for the pump to reach zero flow. If the flows should be smaller, some adjustment to the D2A initial conditions will be needed.
Please let us know if you have any questions.
Scott