Hi
I'm looking for any guidance / experience in calculating wavespeed and considering transient performance in a section of asbestos cement pipe pipe which will be re-lined using a flexible liner system. The liner product is semi-structural and inserted in a folded form and then inflated, and is made up of various plastic/fabric layers. There will be some small annulus remaining between the liner and host pipe (i.e. not a close-fit liner system) which will not be grouted but is expected to fill with water over time.
The system will be subject to some transient behaviour, the most significant likely to be occasional low/vacuum pressures if the upstream pump stops suddenly/loses power. In normal operation the pump starting and stopping will be at a controlled rate which will minimize transients.
For positive transient pressures, is it reasonable to assume the pressure will be transferred to the host pipe and result in a similar (much higher) wavespeed to the host pipe? at least as a worst case (i.e. resulting in the highest wave speed)
For negative transient pressures, the above approach would seem to remain valid (at least as a worst case) providing pressure remains above the full-vacuum / vapour pressure and the internal water column does not collapse. At full vacuum / vapour pressure however, the liner pipe might collapse/deform, which is unlikely to damage it, but this might also cause (or require) the water in the annulus to vaporize. Is it possible in this case the section of pipe may behave more like some kind of pressure vessel and have a significant damping effect on the transient? As a worst case would this result in wavespeed of the host pipe remaining applicable/valid? or is this just too unpredictable / un-knowable for meaningful analysis?
Any thoughts / experience welcome
Below are a couple previous posts I found on similar questions, although these were just dealing with wavespeed for composite pipes with a PE liner, which is full structural liner pipe and would be able to resist collapsing
thanks,
Jack
communities.bentley.com/.../26659.aspx
https://communities.bentley.com/products/hydraulics___hydrology/f/haestad-hydraulics-and-hydrology-forum/54711/hammer-wave-speed-calculator/184877#184877
Hello,
This could be a case for modeling a compound pipe (with lining) in HAMMER. In such cases it is better to do a sensitivity analysis with a range of celerity values to determine the changes in transient results. See this article which discusses how you can model this: Transient implications of soil, pipe lining, material and thickness
Jack Foster said:here will be some small annulus remaining between the liner and host pipe (i.e. not a close-fit liner system) which will not be grouted but is expected to fill with water over time.
Why water needs to fill this annulus? Is there no way of grouting to bond this liner to the host pipe? Is there a special purpose this water is providing. I am guessing if there is gap filled with water, it would be slightly compressible allowing for withstanding higher positive pressures. Is this the intended purpose?
Jack Foster said:Is it possible in this case the section of pipe may behave more like some kind of pressure vessel and have a significant damping effect on the transient?
This could be possible with a "water in bladder" type hydropneumatic tank (pressure vessel) where water is filled between the bladder and the hydropneumatic tank. The pre-charge is pressure in this case is applied to the area between the tank wall and the bladder which is going to be filled with water, as is the case with the lining in your query. See the section titled "Bladder" in this article: Modeling Reference - Hydropneumatic Tanks
You can see this article to set the preset pressure based on initial volume in the pipe section / unit length (full capacity volume). In this case it would be a pipe modeled as a pressure vessel.
Let me know if this helps.
Regards,
Yashodhan Joshi