I am about to calculate waterhammer for a sewerage rising (pumped/forced) main using the Bentley HAMMER software. The main is currently Hobas (a type of sand-filled resin with fibreglass (fiberglass) reinforcing material). We are going to slip-line this pipe using polyethylene (PE) pressure pipe. The Hobas pipe is DN450 with an ID of 487mm and the PE pipe has an OD of 355mm. We are planning to fill the gap between the two with a cementitious grout.. I presume the PE pipe may float a little in the grout and so I think it is reasonable to assume the grout will be around the entire outer face of the PE pipe, though unevenly.
My problem is how to calculate a realistic celerity for this compound pipe.
It seems to me that the two pipes will be acting together and the celerity would have to be higher than either of them acting alone, particularly since they are bridged with the grout.
The only model I have found, and which seems reasonable to me, has been used for steel pipe having a cement mortar lining. It transforms the thickness of the two materials using the ratio of their elastic moduli and combining their wall thicknesses.
t eq. = t + T(Ecl/Est)
where
t eq = transformed pipe wall thickness (mm)
t = steel pipe wall thickness (mm)
T = cement mortar lining thickness (mm)
Ecl = Young's Modulus for cement mortar (MPa)
Est = Young's Modulus for steel (MPa)
My reference notes that this will give a conservative value and assumes that perfect bonding will NOT occur between the steel and the cement mortar, which would require the wall to be modelled as a T shape having far higher E value. I think it is quite reasonable to assume in my case that the plastic pipes will not bond well, if at all, to the cement grout and so the simple transformation given in the above formula seems appropriate to me. Therefore I propose to use this model.
I would like others to comment please.
Hi FrankS,
Your case is interesting to me and maybe I can share input that may in a way contribute. The concrete compressibility is greater than the steel and the formula you have shown is applicable. In your model is totally different with your formula, I would rather treat the pipe as PE only (for bonding problem) due to the fact that vacuum pressure is involved in this scenario.
Hi AC_Jr.,
I agree that under vacuum conditions, the PE would be acting alone (because there would be no bond between the PE and the cement grout), and my proposed model would be invalid. However, I expect to get the system to a point where vacuum won't occur, and so the pipes and the grout would act together under a positive pressure only. Also, if I treated it as PE alone under this scenario, I would get a far lower peak positive pressure. On the other hand, I can't rely on the strength of the Hobas pipe and the grout. We are sliplining the Hobas because it is failing under fatigue loading, being mitigated at present with VFD. So if I did as you suggested (and designed it for the lower transients attributable to a PE pipe alone), the higher transients actually occurring would I suspect cause the Hobas pipe to fail, and then the PE pipe would truly be acting alone.
I think I will look at both scenarios to check the effects on the system. Thanks for your contribution.
Burst will likely affect the top of pipe, where there may be less cement (compared to bottom) and less soil pressure.
If the cement is uneven, it is as though the pipe bedding was uneven and the flexible inner PE pipe will not be uniformly restrained against movement either longitudinally or around a given cross-section. This may cause stress points and aggravate fatigue impacts for the PE. Consequently:
1) Reducing the pressure cycling to within the working pressure of the PE pipe (alone) to maximise its life. This requires a run with a sequential shut-down, pause and sequential re-starts to full capacity to establish suitable delays.
2) Analyse the transient pressures resulting from a PE-only pipe and also with the stronger hybrid. Set the operating times such that you can achieve 1) in either case.
3) In the event of a power failure, check for short-lived sub-atmospheric pressures along the line and if these can occur the cement lining may de-laminate or get pulverised over time (by analogy with a concrete-encased penstock). If so, consider the impact of overburden and groundwater: the PE lining should be assumed to have to take the whole load since the cement is uncertain.
4) The new PE pipe will eventually age also, so be clear about the expected additional life for the pipeline. In the event of a pipe break (rapid drain), sub-atmospheric pressures could occur perhaps down to full vacuum. If the PE lining collapses, the cement may make it difficult to "re-inflate".
These ideas are offered gratuitously and neither the author nor its employer accept any responsibility for their accuracy or applicability to this design. It is the responsibility of the designer to consider and use or reject information.
Hi Guys,
The pipe in the past is operational having the same material and it was upgraded using PE pipes thus the present situation is in reverse mode. Jean-Luc is right if you can arrest vacuum there will be no problem in using PE pipe a liner. There might be minimal impact when it comes to bursting effect if job is satisfactory done in this case. What will be worried maybe is actually during power failures... that will trigger vacuum. Carefull analysis with correct devices to be installed will greatly help. These can be achieved only if model (celerity) has been measured as built in site with proper testing based on the model created...nice discussions thanks both of you.