Hammer Column Separation Sewage Rising Main

Matija Oliver said:

is there a chance that in reality it would not actually come to the column slam event as the column on the right hand side of the air valve would not have anything to bounce back against (it would just continue to discharge at the discharge manhole while the cavity expands-air valve intakes air)? Am I wrong in thinking this?

I have read this many times and I think I may understand what you're asking, but I'm still a bit fuzzy. Are you expecting that the downstream water column will continue to move away from the air pocket, draining out part of the downstream pipe? The air and vapor pocket modeling in HAMMER is an estimate based on the assumptions and limitations documented in this article from our Wiki: Assumptions and limitations of tracking air or vapor pockets in HAMMER

My guess is that if you animate the profile path in the Transient Results Viewer, after the air pocket forms, the downsurge wave continues past the air valve, reflects off the downstream endpoint, and then travels back toward the air valve as a higher pressure wave. I see that the downstream boundary condition seems to be at an elevation that is roughly equal to or maybe slightly higher than the air valve location. By the end of the simulation (you may need to extend it if needed), do you find that some air is let back in once conditions settle down to the final steady state?

It may help to look at the Time History tab of the Transient results viewer - graph flow as well as air volume on either side of the air valve, which may give some further insight.

If this does not help, please elaborate on your question, provide further screenshots (or video) of the animated profile path and graphs, or provide a copy of the model with any steps needed: Sharing Hydraulic Model Files on the OpenFlows Forum

Hi Jesse,

Thank you for the quick reply.

Are you expecting that the downstream water column will continue to move away from the air pocket, draining out part of the downstream pipe?

I was thinking along those lines, yes. I thought that the downstream side would continue to drain while the air valve intakes air and that the wave reflection would not be so significant as we are discharging to atmosphere. Every time the downstream side wave reaches the discharge point I would assume that it is further mitigated.  Furthermore, I assume that there would be no enough momentum to "return" downstream column up to the air valve elevation again- I tried to sketch this below. I have attached the video recording of the analysis and the model, and we can observe that there is almost no upsurge until the column slam event happens. I would say that the pipeline almost settled down to the final steady state just before the column slam. I hope I managed to clarify further what I meant.

I would be very grateful if you could have a look and share your opinion. I absolutely accept that my logic might be completely wrong because of my lack of experience.

Monread RM.wtgpkg.zip

Transient result Viewer

Thank you and Kind Regards,

Matija

Thanks for providing the model file. Here is my interpretation of the transient simulation from the graphs and profile animation in the Transient Results Viewer:

1) When the pump shuts down the downsurge wave front reaches the air valve location, causing subatmospheric pressures, so the air valve opens and starts letting air in.
2) At around 60 seconds, things have settle down to the point where the model tries to start reaching the new steady state/equilibrium point. Since the downstream reservoir is at a slightly higher elevation than the air valve (87.57 m vs. 87.10 m), the downstream water columns starts to flow in reverse back toward the air valve (you can see this in a flow time history for S705:AV-1). The upstream reservoir is at a lower elevation but the water column on the upstream side of the air valve in question does not move in reverse toward the pump (only slight movement due to elasticity) because the pump has the check valve option enabled.
3) From the movement of water from the reservoir (87.57 m) to the air valve (87.10 m), the air pocket is eventually expelled completely and an upsurge results from the water columns colliding. The air valve's outflow orifice diameter is set fairly large (50 mm) which influences the rate that the air is expelled. Using a smaller diameter can help slow this down, reduce velocity and therefore the upsurge. (refer to the Joukowsky equation)
4) The upsurge waves reflect back off the endpoints, meet again, cause a low pressure "downsurge" wave, which introduces air at the high point again.
5) Conditions settle again, air is released, an upsurge occurs, and this repeats several times (conditions do not fully settle down until near the end of the simulation or even beyond)

Thank you for the detailed description and for reviewing the model.

I agree on majority of your points. However, I am wondering now if I should have modelled the downstream "reservoir" somehow different. This is the discharge location of the wastewater rising main (a gravity manhole). Maybe a discharge to atmosphere would be more suitable as there should be only reverse flow in the pipe and not flow from the "reservoir" as this is just a discharge manhole. However, even with a discharge to atmosphere we can observe a column slam, but not so severe. It just seems counterintuitive to me that that column of water is still present, I would think that a lot of the volume would discharge through the gravity system. Considering that there is approximately 2.2m3 of air volume intake in the pipeline(approx 14m of the pipe would be filled with air), and as you noted majority of this volume will go into the downstream side of the pipe. Do you think that anyway the 0.47m difference would be enough to return the column back to the air valve elevation and cause the collision?

Regards the existing air valve, I wanted to model it as a worse case scenario, that’s why I selected 50mm discharge orifice. The site team could not locate it and the local authority does not know anything about it.

However, by trying to avoid an air vessel proposal, I think we will try to propose replacement of that existing air valve with an air valve with the surge protection discharge orifice. When I replace the existing air valve with an air valve with surge protection (C80-SP) something strange happens. Even if I extend simulation time to over 4h, the air never gets expelled as per screengrab below. This would be more in line with my thinking that the downstream water column would not have enough momentum to expel that air and it would just stay in the pipe. What are your thoughts on this?

Thank you so much for all your help with this. It is very appreciated.

Matija Oliver said:

It just seems counterintuitive to me that that column of water is still present, I would think that a lot of the volume would discharge through the gravity system. Considering that there is approximately 2.2m3 of air volume intake in the pipeline(approx 14m of the pipe would be filled with air), and as you noted majority of this volume will go into the downstream side of the pipe. Do you think that anyway the 0.47m difference would be enough to return the column back to the air valve elevation and cause the collision?

Part of the problem is with the assumptions and limitations in HAMMER's tracking of air and vapor pockets, as described in this article previously shared. Since HAMMER does not track the air/liquid interface, the boundary water level (HGL) remains the same at both ends, which is why the water columns still travels back toward the air valve due to the 0.47 m difference in elevation between the reservoir (or D2A elevation) and the air valve elevation. Meaning, when air is introduced into the air valve, by default HAMMER does not track the air pocket traveling down the pipe and causing the water surface elevation to drop as it drains down the pipe, and when air is introduced into the D2A, it does not track that air going down the pipe and the resulting drop in HGL from the water surface elevation going down. So, the water column continues to move toward the air valve, eventually expelling the full air pocket and resulting in an upsurge from the velocity of that column.

In reality if the reservoir location is an open end, some of that downstream end may drain down but only enough to expel the air pocket out of the air valve in question. Depending on the timing of the waves and water column elasticity, the 0.47 m drop in water surface elevation may or may not be enough to expel the full air pocket.

The 2.2 m^3 volume of air that HAMMER reports is an estimate based on the aforementioned assumptions. In order to get a more accurate calculation of the volume of air introduced into the air valve, you would need to account for the air/liquid interface tracking along the pipe. This can be accomplish to some extent by using the Extended CAV calculation option. Note that this option only enables the tracking of air in the adjacent two pipes and only if the adjacent nodes are at lower elevations than the air valve. So, you may need to combine a few pipes (perhaps using Skelebrator, or manually) if the air/liquid interface needs to extend beyond that (the slope of the pipes seems fairly continuous for some distance so you might be OK doing this)

With this, you should be able to achieve a more accurate simulation of the volume of air that enters the air valve, and get a better sense of whether the drop in water level at the downstream end (D2A) would cause enough volume change to push out that entire air pocket. However, it still isn't an exact simulation because the air pocket at the D2A site does not work with the Extended CAV option as explained here. Additionally, I believe you said there is already uncertainty with the orifice size which will also skew the results. So, you may need to use a sensitivity analysis with the size.

Some engineering judgement may need to be employed here with these computer model assumptions in mind. Perhaps something conservative. Maybe you can consider that the results are telling you that there might be a problem here with air pocket slam in some conditions and air valve orifice sizes and that the pipeline may be at risk. You could then instead focus on mitigation strategies.

Matija Oliver said:

When I replace the existing air valve with an air valve with surge protection (C80-SP) something strange happens. Even if I extend simulation time to over 4h, the air never gets expelled as per screengrab below. This would be more in line with my thinking that the downstream water column would not have enough momentum to expel that air and it would just stay in the pipe. What are your thoughts on this?

In order to troubleshoot this, I would need more information about how you configured this "C80-SP". What model element did you use? (A surge valve? What settings?) How exactly does this device operate? Can you provide a new copy of the model with steps to reproduce?

Transient simulations will be very sensitive to the way that elements are configured, so it is important that the configuration is correct to avoid skewing the results or instability (which appears to be occurring in this case). If you think the setup is OK here, I would try a smaller calculation timestep, review other user notifications and ensure that you're using the latest version of HAMMER (10.03.02.75).