New C900 Pressure Classification 2

[coloeng]

Recently AWWA changed the pressure classification of C900 PVC pipe from 100 psi(DR-25), 150 psi(DR-18)& 200 psi(DR-14) to 165 psi, 235 psi & 305 psi. They have reduced the factor of safety form 2.5 to 2.0 and eliminated the built-in surge allowance. I would just like some of your thoughts on this.

 

[rconner]

I see that you have not gotten any responses to your inquiry this week. While perhaps my “thoughts” are arguably unimportant, I will nevertheless provide some history that I believe may well be relevant to your question. Per ASTM standards books the original version of ASTM D2241 (Standard Specification for POLYVINYL CHLORIDE (PVC) PLASTIC PIPE (SDR-PR) was published in 1964. It does not appear that this ASTM standard for at least a great many years? had any specific mention of water hammer or surge at all, but instead it did make a statement in an appendix e.g. to the effect, “The pipe is rated for use with water at 23 degrees C.(73 degrees F.) at the maximum internal pressures shown in Table X1.1. Lower pressure ratings than those calculated in accordance with 4.5 may be recommended.” Per that early standard “SDR”or “R” etc. referred (as does later/current “DR”) basically to the ratio of the pipe diameter to the wall thickness. The sort of pressure rating philosophy of that early ASTM standard appeared (e.g. with assumed, common unplasticized pvc compound strength per both standards) to be virtually the same as the now brand new C900, in that SDR26 pipe carried a 160 psi “pressure rating” (the new promotion is that only very slightly thicker DR25 has a 165 psi rating), SDR 17 pipe carried a 250 psi rating (the new promotion is very slightly thinner DR18 pipe for 235 psi rating), and 13.5 pipe carried a 315 psi rating (the new promotion is for very slightly thinner DR14 pipe for 305 psi rating).
A few years after the first ASTM standard, a gentleman named Robert Hucks, who I believe was then manager of a Johns-Manville research lab, published an article in the July 1972 AWWA Journal entitled, "Designing PVC Pipe for Water-Distribution Systems" . I noticed that 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, actual working systems with pvc pipe. He reported based on “short-term measurements” (with at least the monitoring technology of the day) 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.”

On or before 1974 the USA plastic pipe industry lobbied strongly with AWWA et al to accept the first version of an AWWA standard for 4”-12” pvc pipes, that eventually issued as AWWA C900 in 1975. As the experience that had been amassed (worldwide) with ASTM style pvc pipes over many years to that date was some checkered (“chequered”, to you international folks), the original at least some more conservatively designed (then thicker) 1975 C900 pipe proposed, with inclusion also of at least some small level of surge (of course shortly after the research per Hucks and others in working pvc systems) as well as a reported 2.5 safety factor on the total pressure (working plus that defined surge) in thickness calculation, was instrumental in the successful balloting of this standard. It is also some interesting that a safety factor vs total pressure including surge and importance of consideration for surge was even highlighted in what appeared to be a promotional article I read sourced from the 1977 Uni-Bell “Handbook of PVC Pipe” entitled, “Water Hammer – Surge Pressure with Vengence” (their spelling).

In 1994 (of course 30 years after the first edition ASTM standard) and after nearly 20 years of the first design C900 pipes, I am aware that the American Water Works Association Research Foundation sponsored a “data gathering questionnaire” that was reportedly sent to many utilities. The reported results of this questionnaire revealed that approximately 80% of the aggregate utilities’ installed pvc pipe was then “C900” and 20% was per ASTM D2241. While received results of this questionnaire revealed some positive perceptions and satisfaction with pvc pipe, it noted ovc pipe was “second in overall performance compared to other pipe materials” (I believe first was ductile iron pipe), and it also concluded that,

“An analysis of reported data shows that problems associated with pvc water pipe manufactured to the ASTM D2241 Standard is about twice as high as for pipe manufactured to the AWWA C900 standard.”

Beyond that I would only note that at least for many years leading up to the present day I have seen most notable authorities state that if total pressure conditions including surge were at all beyond the ASTM D2241 ratings for a particular SDR pipe, that thickness pipe should not be used, or greater thickness or other pipe etc. selected for the application (while who knows what if any accurate surge analyses are done I suspect most pipes bought and installed per the ASTM specification and subsequently surveyed in 1994 were basically chosen as best they could figure out with that “design philosophy”).

Of course also, while some strokes of the pen or keyboard have apparently now “eliminated the built-in surge allowance” from the new C900 (so that it appears basically just like the old ASTM “design”), it is my opinion this does not eliminate the risks and realities (concerning all aspects/complexities of working pipeline systems and as published per Mr. Hucks et al) concerning all potential causes and magnitude of water hammer, and maybe even some larger surge than some anticipate from simplistic wave spped analyses in installed pvc pipes!

There is some more information in the archives of these and other forums (e.g.

http://www.eng-tips.com/viewthread.cfm?qid=125930)

Everyone have a good weekend.


“Those who cannot remember the past are condemned to repeat it.” (Mr. George Santayana, before 1905)

 

[coloeng]

Thanks rconner, that was some interesting information.