Buried Thermoplastic Pipe Celerity-Effect of Embedment 2

[stanier]

The celerity of a thermoplastic material is lower than for rigid materials. However in a buried pipeline it is considered that the soil support will provide some increase in modulus of the pipe/soil combined structure.

In modelling such pipelines for surge most profesional software packages allow the modelling of a circular tunnel. This assumes that the material doesnt deform when such a pressure wave travels along the pipe and thus the celerity is similar to that of a rigid material.

The modulus of soils is however low compared even to thermoplastic materials. So one may argue it has no bearing. On the other hand the response of viscoelastic materials such as soils is strain rate dependent and may actually have higher moduli than found in the labratory for rapid waves.

Do Forum memebers know of any research texts or books that cover the topic of the effects of soil embedment on celerity of low modulus materials in pipelines?

 

[rconner]

Good question. While I am admittedly not an expert on this subject, I am aware that some research and testing perhaps at least in part as you seek was done even long ago by some of the plastic pipe industry (I think back in the 1960's-1970's) and mentioned in an article, "Designing PVC Pipe for Water-Distribution Systems". This article was written by Mr. Robert Hucks (who I believe was then manager of a Johns-Manville research lab) and published in the July 1972 AWWA Journal. I noticed this article contained the statement, "Community-water-distribution systems require pipe that will perform without failure under the high flow rate and water hammer pressures created during fire-fighting activities." Under the paragraph, "Magnitude of Water Hammer Surges", the author said, "Initial work at the author's laboratory indicated that the pressure wave (water hammer) generated by rapid valve closure was greater in magnitude than that predicted by the Logan-Kerr equation (though how much greater was not revealed in this article). It appears the author’s company then “supported” perhaps a couple different research programs at Utah State University (USU) examining this subject. The first USU work, that apparently did not include any burial, reportedly concluded that the traditional equations, I guess at least when pipe actual modulus was considered, “accurately predict” surge, but they recommended further, buried condition work. Mr. Hucks then said per the subsequently performed work, with a rather shallow depth of “well-compacted” backfill (18 inches of cover?), “Preliminary results indicate the wave velocities are 7 per cent higher under buried conditions than for the previous study.” I believe he also mentioned that they felt axial restraint might be even more of a factor than radial in increasing pvc wave velocity/surge. [While not disclosed in the Hucks AWWA paper that included a graph of pressure vs. velocity changes up to only 8 fps, I read another report discussing the 1972 USU water hammer research on buried plastic pipe that interestingly reported in one run of valve closure at 10 fps flow velocity, one 6” PC160 DR26 pvc pipe was sort of inexplicably burst/split from one end to the other!, though an explanation for this failure was not disclosed in that report.]
Mr. Hucks also disclosed in the AWWA report the results of some “Field Studies of Operating Systems”, wherein researchers attempted to capture surges in three different (I assume actual) systems with pvc pipe. He reported based on “short-term measurements” the surge was 60 psi (20 psi above the steady state pressure) in the first system with “negligible elevation changes” and only some very small diameter pipe. In the second with up to 6” diameter pipe, reportedly supplied by three wells through “large hydropneumatic storage tanks”, he reported there was a 142 psi surge (102 psi over operating) with “rapid hydrant opening and closing”. In the third, with also up to 6” pipe, he reported a maximum of 162 psi surge (50 psi over a 112 psi operating pressure). In the third, he reported they subsequently took some “long-term measurements”, apparently revealing “pressure excursions” he said due to “high demand” to “225 psi approximately twelve times during the two-month period”. Mr. Hucks reported that the situation in the third case was considered serious enough to “advise the operators that PVC pipe failures could be expected within the next ten years without taking corrective steps.” Mr. Hucks also reported (but apparently not considered in the water-hammer research), “Since PVC pipe moves axially as well as circumferentially in response to pressure surges (“Poisson’s ratio effect”), pipe may undergo scratching of the outside when buried and in contact with sharp stones in the backfill. This area requires further study.”
I should probably also mention that the research referred to by Hucks involved “ASTM” specification pvc pipes, much of which is less stiff then contemporary AWWA specification pvc pipes.

In addition to stiff soil backfill, I think also all types of pipes are sometimes encased or partially encased in areas with concrete and/or flowable fill or grout encasement, and/or passing into and out of concrete etc. structures, and near inevitably plastic pipes are also connected at some point to stiffer pipes (such as steel or ductile iron piping), an effect that probably should also be considered by competent engineers in surge analyses. In addition to stiff soil backfill, I will note also all types of pipes are sometimes encased or partially encased in concrete and/or flowable fill or grout encasement, and/or passing into and out of concrete etc. structures, and near inevitably plastic pipes are also connected at some point to stiffer pipes (such as steel or ductile iron piping), an effect that probably should also be considered by competent engineers in surge analyses.

Interestingly, while the current 2002 AWWA Manual M23 manual for pvc pipe does include the statement, “The pipeline designer should be aware that the geometry and boundary conditions of many systems are complicated and require the use of refined techniques similar to those developed by Streeter and Wylie”, it appears to still advocate after all this that surge be determined by “elastic wave theory of surge analysis” (with a modulus of 400,000 psi assumed for the pipe, etc.) I believe some other authorities are advocating more conservative surge provisions for pipe selection. Perhaps there has been more recent water hammer research that covers more issues than that of Mr. Hucks – I will wait along with you to see. Sorry for the long post, but perhaps this reference and information will be helpful to you.

 

[plasguy]

Hello, ys thus topic has been studied extensively by the PVC pipe industry and educational institutions such as Utah State Univ.

Your best best is to buy a copy of the PVC Pipe Handbook from the Uni-Bell PVC Pipe Asociation in Dallas, TX. It can be ordered thru

http://www.uni-bell.org.

You will find after reviewing the methodology that the combination fo flexibel pie pipe plus compacted soil can far outperform rigid pipe alone in similar soil conditions.

Good luck. Plasguy

 

[Bbird]

From the practical view point the celerity of a fully rigid pipe in a rock tunnel is about 1300m/s and the normal wave speed for a thermal plastic pipe would be between 400 to 700m/s (GRP, PVC etc). The soil would have little impact unless the circumference is fully restrained to the point the pipe radial expansion is prevented. Although I haven't investigated the theoretical celerity equation recently I don't believe it is sensitive to the support condition once the low elastic modulus of the pipe material and the wall thickness have been cast in stone.

The high celerity generate a high peak surge pressure and the low celerity corresponding a low peak pressure. The latter takes longer to die down.

However no system is ideal steel pipe and rock tunnel alike in practice show a modest drop from the theoretical celerity. Also a tiny percentage of air in the pipeline, say due to aeration, can significantly alter the celerity too.

I haven't had a need to look at the type of soil support when analysing a water hammer problem in a PVC pipe. If a time history pressure reading is captured the peak to peak duration plus the distance of the pipeline will tell us the combined celerity of the system. I doubt in practice one can measure celerity to +-15% accuracy and hence the soil supporting condition, which invariably has to be some kind of a backfill for a thermalplastic pipe, could have a significant effect on the surge behaviour.

 

[stanier]

Bbird,

Thanks to the thoughts. You are thinking along the same lines as I. I would like to think that soemone has researched this and there is some data out there. Perhaps the plastic pipe industry has and the results dont suit their purpose. If the celerity went up becuase of a good embedment it takes away one of their selling points. The pressure rise may require the use of a higher class plax\stic and then it wouldnt compete with DICL!

In reality if a plastic pipe is well compacted, in say pea gravel, you would expect the celerity to be similar to a lined pipe. After all the rapid diametral strain,as the wave passes, must impose a reaction from the soil. If the soil is compacted and consolidated where can it go? Would the response be similar to a grouted pipeline in a tunnel?

 

[Bbird]

stanier,

The pea gravel has an elastic modulus and is easily deformable. Unless the pipe is concrete surrounded the celerity isn't going to change much.

The standard waterhammer analysis uses the method of characteristic at specified intervals. To avoid interpolation between grid points everybody cooks the celerity and changes its value to force the characteric lines passing through the grid intersection. Up to 10% change in the celerity can hardly change the surge behaviour.

Both theory and practical investigations confirm the celerity can be nearly halved just by 0.1% of air content.

Thus there is very little practical benefit to bottom out small difference from the hardness of the soil on the thermalplastic pipe's wave velocity.

 

[stanier]

Bbird,

Pea gravel does not deform when a wheel load is applied to a trench? How is it much different in modulus to a pressure grouted system that you may model as a circular tunnel? Has anyone done the testing? Where are the numbers?

I appreciate your point about air in the system but most utilities want air valves as they have been convinced air in theior systems will affect the hydraulics.

Hard soil will have limit cone penetration that infers high modulus. I really need to get onto someone who has done the testing rather than theorising on this matter because we end up dealing in opinions not engineering facts.

 

[BRIS]

ARD Thorley in “Fluid Transients in Pipeline Systems”

notes for a steel lined tunnel a convenient expression for wave speed is:


C= (roe (1/k + 2D/(GD+2Ee))^0.5

e = thickness of the liner
E= young’s modulus
D = diameter
G = modulus of elasticity of the rock
K = bulk modulus of the liquid

And he goes on to note the equation may also be used to obtain a reasonable approximation for the wave speed in buried pipes where it is believed the backfill material around the pipe provides additional support. Note that this is not always the case especially with the more elastic pipes such as those made from PVC and similar plastics.

Note - for plastic pipe the affect of temperature and rate of strain on the material properties are probably far more important factors in determining c than the affects of the backfill are .

 

[stanier]

Hi BRIS,

I have ARD Thorleys 2nd Edition and will look that up.

As for temperature and rate of strain this would be accounted for in the equation in the value of E modulus. generally I use instantaneous value of E at an estimated wall temperature. This follows work by Lars Eric Jansen of Borealis fame.

Now all I have to do is modify the restraint coefficient to come up with a modified celerity.