Water transmission main through residential and power-tips?

[UtahWater]

I am specifying a large (for me...20" ID) transmission line through what will in the foreseeable future be the back yard of residential lots. My gut impression is that fused pipe, vs bell and spigot, will be less prone to leaks that could upset the future homeowners. Does anybody have experience to bolster or refute that perception?
The line will also run through an electrical power corridor, which the power company's license agreement states may be subject to line pullers weighing up to 65 tons. With 5' of cover in a Type IV backfilled trench, it doesn't seem that there would be a pipe that could really handle that kind of loading...so my gut feel is to go with an HDPE, thinking that it is less likely to be brittle than a fusible PVC, assuming we install fusible, based on the residential alignment mentioned above. Any experiences out there that would lead to some warnings or pitfalls or recommendations? Thank you...some of you are amazing in your breadth and depth of knowledge and presence on this site!

 

[bimr]

Not sure that anyone would provide comments without the operating conditions.

Since you know that you have to design for 65 ton line pullers, why would you not use granular trench backfill?

Bell & spigot piping is generally preferred because of the flexible joints that can accomodate some ground movement with breaking or leaking.

 

[cvg]

fused or fully restrained pipe will generally be more "leak free" than unrestrained bell and spigot pipe. However, I would not limit your analysis to just HDPE and PVC. You could also consider welded steel or restrained ductile iron. All could be designed to handle your situation and could be competitive alternatives.

While 65 tons sounds like a lot, gross vehicle weight is only part of the equation. For example, an AASHTO HS20 truck has a 16 kip axle load. If you have two HS20 trucks passing, you have a point live load of 32 kips plus a 1.2 or higher impact factor which puts you at nearly 40 kips. Probably higher than your wire puller. You really need to look at the axle spacing, tire size and pressure etc. to determine the actual live load. Most pipes can handle minimum cover of 3 feet in an unpaved road, so 5 feet is not bad.

 

[UtahWater]

Thank you bimr and cvg.

 

[rconner]

All the basic types of pipe that have been mentioned thus far on this thread have been around for a long time. Of course, specifications for or selections of piping materials should be based on far more than “perception” and I think you are therefore prudent in seeking out experience as well as comparing all manner of engineering and maintenance etc. attributes. These matters have been discussed often on these forums (and discussions can be searched best with a few key words and the”Advanced Search” feature). There can obviously be a whole lot involved with prudent pipe material selection, including the two specific aspects/perceptions you’ve mentioned in your initial inquiry. Beyond that you might be interested in a hypothetical comparison of some current actual field pipeline hydrostatic testing requirements for say ductile iron and hdpe transmission pipelines, as follows:

Let’s say an Owner basically needs a transmission pipeline with a “20-inch” diameter flow area that is 12,000 feet long, and with a maximum working pressure (let’s say at the lowest point of the pipeline) of 170 psi (and with water etc. temperatures that will never exceed 80 degrees F. (thermal is of course an important consideration with plastic pipes) Let’s say that the Owner further likes to see a field hydrostatic test of say 250 psi (of course that is roughly 1-1/2 times the stated working pressure) after complete installation (involving pipe manufacture, shipping, handling, and finally complete burial/impacts etc.) for such a pipeline.

These working and testing conditions would appear to basically require, per Table 9 of AWWA C906 standard, a 24” DR 9-ish hdpe pipe that rates 193-200 psi working pressure etc. Per Table 6 of AWWA C906. It would further appear per Table 6 of that same standard that 24” DIOD (ductile iron O.D.) DR9 hdpe pipe would have a maximum possible flow diameter of 25.80” O.D. – 2(2.867) = 20.07” (OK for the hydraulic conditions stated). Per the proprietary field hydrostatic testing requirements for hdpe (I accessed today at

http://www.cpchem.com/hb/getdocanon.asp?doc=708&lib=CPC-Portal

, it further appears that “Test Phase – Alternate 2” guidelines would perhaps be applicable, and per Table 2…”Make-Up Water Allowance” of this site they say 8.9 gallons per every 100 feet length of pipeline is allowable to be pumped back in after a 2 hour test. Of course this would in turn appear to mean (12,000 feet/100 feet)x8.9 gallons = 1,068 gallons pumped into the buried polyethylene line after the two hour test to get the pressure back up means the test is OK.

On the other hand lets say minimum Pressure Class 200 20” ductile iron pipe also rates some higher than a working pressure of 170 psi and can be field tested up to at least 300 psi (when all components in a system can withstand that level of test). Per Table No. 17-4 of the site at

http://www.acipco.com/adip/products/Sect17.pdf

the inside diameter of this 20” cementlined ductile iron pipe is 20.75” (of course OK and also larger than the above 24” even DIOD hdpe pipe). Per Table 5A of AWWA C600 standard for testing of ductile iron pipelines, a Contractor etc. is allowed a make-up water allowance of 2.24 gallons/1,000 feet length of pipeline per hour. Consequently, for a 2 hour test of the whole DI pipeline, no more than 2 hr x2.24 gal/hr (12,000 feet/1,000 feet) or 53.8 gallons of make-up water is allowable in field testing of this ductile iron pipeline.

I guess it is somehow conceivable (with all the weird things that can happen in pipeline profiles and durations of field testing etc.) that both pipelines are OK, but with allowable make-up water pumped back in to a buried hdpe pipeline of 1,068 gallons vs 53.8 gallons for DIP (nearly 20 times more for the hdpe), it is certainly not obvious that the hdpe is proven better, nor necessarily for that matter more actually proven “leak-tight” at joints, defects or damages, in this installation. I would recommend of course that you run your own numbers, and for your actual design/installation conditions.

As far as “brittle”(ness) is concerned, I guess it is possible that polyethylene is less brittle than pvc. However, what causes pipes to break can be a lot more complicated than that, and gets into detailed fracture mechanics and other fields. In this regard, however, ductile metals in general have far greater fracture toughness, and can also tolerate far greater “critical crack” or flaw/damage “lengths” etc., than the common plastics.

All have a good weekend.

 

[UtahWater]

Thanks, rconner, for the in-depth analysis. Y'all are the mentors I never had! I'll check it out.