How to model a downfall pipeline?

The solution to a situation like this can depend on many factors and will likely require some engineering judgment. Use an animation of the profile path in the Transient results viewer to help you visualize the transient response and decide what surge control strategy might work best.

You can also take advantage of scenarios and alternatives to try various approaches and compare their effectiveness. You may find that a larger tank is necessary or one with a higher or lower initial gas pressure. Some guidance on hydropneumatic tank modeling can be found here:

communities.bentley.com/.../modeling-reference-_2D00_-hydropneumatic-tanks.aspx

In some cases depending on the topology for example, you might find that a hydropneumatic tank alone may not be a good fit. In these cases you can try combining it with other measures such as the addition of air valves at key points or increasing the pump inertia to simulate a flywheel.

Further general guidance can be found in the Help topic called "Developing a Surge-Control Strategy".

Jesse Dringoli

Thank you very much for your reply. However I have already tried everything that I remember, and I have no solution. Even a 20000 l tank is not a reliable solution, because due to the slope of the profile sooner or later it runs out of water. Increase the inertia of the pump does not work too. So I've been thinking if some change on the piping could help. It is not possible to use some type of valve in the end of the piping, in order to limit the pressure or push up the hydraulic grade? I already tried a PSV, but it does not solve my problem either. I put some air valves in the piping, which help a bit but is not enough (picture).

I hope you can help me, because I'm running out of ideas.

Thank you

Ricardo Silva

It's hard to tell without seeing the model, but from your profile picture I think I see what you mean - the ground profile is slightly downhill in the direction of flow. So, as long as the upstream pump remains off, the system would eventually partially drain by gravity (at least the section on the left side of the profile that is above the elevation of the downstream tank/reservoir). Without a pump to add energy (to "push up" the HGL), there's not much you can do to prevent the system from draining . Perhaps a small amount of draining is OK and you're more interested in the sub-atmospheric pressure seen on the right side of the profile? Since the right side of the profile appears to be above the boundary condition at the far right, you may be able to help prevent subatmospheric pressure during a transient event. For example if you can't slow the momentum change of the pump using a flywheel, you could try adding a secondary tank at say, the high point at around station 5000.

Judging by the volume shown in the upper part of the screenshot, it looks like you may have some air valves that introduce air at some points, and possibly cause severe surges when the air is expelled (if the outflow orifice isn't sized carefully) which could be subsequently causing severe downsurges. You can animate the profile path to see if that is happening.

Again, it's ultimately up to you as the engineer to decide what needs to be done to mitigate the transient response to the modeled situation. Bentley can assist in explaining any assumptions behind the program and how to use tools to help you make these decisions, and other engineers may share their experiences on the forum.

If you suspect something is not entered correctly, if the model is experiencing an "initial surge" (which can happen due to inconsistency between the initial conditions results and the first transient timestep) or if you have a question or doubt related to data entry or modeling tools, it would be best to submit a copy of the model. There are two options for sharing your model files on 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.

communities.bentley.com/.../7079.be-communities-secure-file-upload

Hi Ricardo,

If I understand correctly, you are pumping wastewater over a hill, and you are worried about transients when the pumps stop.

You latest screenshot tells me that the maximum hydraulic grade line envelope (the red line) barely exceeds the steady state hydraulic grade line. This is good - it means you aren't likely to get pipes breaking due to over-pressurization.

The blue line tells me that you have some negative pressures along the pipeline, but they are not severe enough to cause vaporization. A few more air/vacuum breaker valves in strategic locations should reduce the negative pressures further.

So what are you main concerns with the model you have now?

Regards,

Mal

Thank you so much for your answer.

I know it is up to me to make the decision, but sometimes I run out of ideas how to solve some problems. The idea for another HT in point 5000m looks good, I'll start by trying it and ajust the air valve too. If this fails, I will need to consider an intermediate reservoir at a lower point and pumping water from there.

I did not know it was possible to send the files. Thank you for that explanation as well.

Best regards
Ricardo Silva