Advice requested: Installing a PVC water supply line above ground?

[Mliu]

At an industrial complex in the San Francisco Bay Area, we have a buried water supply pipeline. This pipeline serves several low-consumption applications: an equipment pressure washer, some irrigation, etc. But it also supplies the home of a caretaker's residence which is located on-site.

Recently, we've been having a lot of problems with breaks and leaks in this pipeline. The pipeline was built in stages by various previous owners/operators over many years. As a result, it is a hodgepodge of PVC and galvanized of various diameters (3/4" up to 1-1/2"), but not of reducing diameter over its length (e.g., there is a section of 3/4" pipe at the supply end, and larger pipe downstream). It is not a straight run and there are sections of the line where we do not know exactly where the pipe is buried. Based on the projected path in one section, it appears to have been built over by a 150' wide concrete slab. Perhaps the biggest problem is that it's not buried deep enough: it's depth ranges from less than 18" to no more than 2", and there is one 60' section where the pipe (PVC) is literally laying on the surface of the ground. To further complicate matters, the ground is not stable (it's "bay mud" -- similar in properties to land fill) and the ground water table is very high. Lastly, this pipeline runs along a gravel access road that is traveled by heavy trucks. Currently, there is a small leak that we have isolated to an 800' section of this pipeline, but we have not yet located the break.

We are discussing options for replacing this problematic pipeline. One idea is to mount it along the back of the steel industrial buildings that run the length of the access road. The access road is a straight line, and the backs of the buildings are mostly straight and aligned with only occasional offsets in depth, so it would be fairly simple to mount a pipeline to the backs of the buildings using unistrut and clamps. This would be considerably less-expensive than properly trenching and back-filling a buried pipeline, and it would make it infinitely easier for us to locate and repair any problems in the future (although, hopefully, that should be very rare with the new installation). If we do this, we would probably opt for 1-1/4" or 1-1/2" schedule 80 PVC.

Questions:

1. Any objections or major problems with this plan?

2. We are in a very temperate climate: the average lows are in the 40's to 50's, and the average highs are in the 70's to 80's. But historical metrological data for the past 117 years shows the temperature can get as low as 20 deg F and as high as 110 deg F. So we probably should be concerned about the possibility of freezing. We could use pre-formed fiberglass insulation sleeves. But insulation on it's own will not prevent freezing, only delay it. Heat tape would be impractical for such a long run. Would 1-1/4" sch80 PVC, combined with insulation, combined with normal usage from the residence be enough to prevent freezing? (Of course, if the house were to be vacant during a rare freezing spell, we would have to shut-down and drain the line.) How about the installation of a pressure relief valve (to vent excess water pressure in the event of a freeze)? Any other ideas on this topic?

3. Any other issues, concerns, or recommendations?

 

[hj]

It is a "low pressure" application so you do not need schedule 80 pipe, but ANY exposed PVC pipe has to be shielded from UV exposure, usually by applying a coat of paint to the pipe.

 

[Sponsor]

 

[Jimbo]

As mentioned, must be UV protected, but also if the water stands still in the pipe, and the temp rises well above 100º the pressure rating of the pipe drops below 70 psi. If there would be consistent flow in the pipe, this would not be an issue. Using some foam insulation and a foil tape wrap, might help you on two fronts.

 

[Mliu]

It is a "low pressure" application so you do not need schedule 80 pipe, but ANY exposed PVC pipe has to be shielded from UV exposure, usually by applying a coat of paint to the pipe.

I realize schedule 80 is not required, but we wanted to add a "factor of safety" to the installation.

I had also considered the UV issue, but since I figured the pipe would probably have to be insulated, then the insulation would solve that problem as well. If it's determined that insulation is not required, then we would apply an exterior-rated latex paint.

 

[Mliu]

if the water stands still in the pipe, and the temp rises well above 100º the pressure rating of the pipe drops below 70 psi.

That's a good point, as direct sunlight can raise the temperature of the pipe (with stagnant water) well-above the ambient air temperature. To be clear about the temp derating, I checked the specs from a PVC pipe manufacturer: "PSI is based on water at 73-deg F. Derate 50% at 110-deg F and 78% at 140-deg F."

So the ratings would be:

1-1/2" sch40 @ 73° F = 330 psi
1-1/2" sch40 @ 110° F = 165 psi
1-1/2" sch40 @ 140° F = 73 psi

1-1/4" sch40 @ 73° F = 370 psi
1-1/4" sch40 @ 110° F = 185 psi
1-1/4" sch40 @ 140° F = 81 psi

1-1/2" sch80 @ 73° F = 470 psi
1-1/2" sch80 @ 110° F = 235 psi
1-1/2" sch80 @ 140° F = 103 psi

1-1/4" sch80 @ 73° F = 520 psi
1-1/4" sch80 @ 110° F = 260 psi
1-1/4" sch80 @ 140° F = 114 psi

This would be another good reason to go with 1-1/4" schedule 80, again giving us a larger safety margin. The current static water pressure in that pipeline is 130 psi, so we will add a pressure regulator to reduce the pressure below 80 psi.

If there would be consistent flow in the pipe, this would not be an issue. Using some foam insulation and a foil tape wrap, might help you on two fronts.

My concern with standard foam insulation is that it is not rated for exterior use. The foil tape would help, but wrapping 1,000' of pipe with tape would probably not be practical or cost-effective. There are exterior-rated foam and fiberglass pipe insulation sleeves and I'm researching their cost and performance.

 

[Ballvalve]

I used "Yelllowmine" pipe for a hydro project. This pipe is high impact resistant and UV impervious. Mining outfits lay it on the surface for various purposes, and the joints are an interesting combination of o ring and plastic rope retainer for moving it about.

You can glue it also, and is more like a schd 120 rated. Amazing stuff, I have it in a creek bed, and the supports are more problematic than the pipe.

Standard pipe with insulation and the insulation coated with a heavy elastomeric white paint, would likely be a less costly solution. "Snowroof" would be a good coating.

http://www.intpipe.com/yelomine_pipe.aspx

 

[Hackney plumbing]

Expansion and contraction. Nasty

 

[Mliu]

I had already considered thermal expansion/contraction and planned to handle that with fabricated expansion joints and/or expansion loops. Certainly a consideration, but not a problem for our application.

 

[Mliu]

I have researched various outdoor pipeline insulation methods. There is pre-formed fiberglass, but that would require an additional PVC "cut & curl" jacket. There is also pre-fabricated outdoor-rated foam insulation that has a PVC outer jacket already bonded to the foam. Both of these are rather expensive in terms of both materials (>$3.15/ft for fiberglass & "cut & curl" PVC; >$7.00/ft for foam w/ pvc jacket) and installation labor (there is seaming, gluing, and sealing involved for both). Given a choice between the two, the pre-fab foam would be a much better solution: no chance of water saturating the foam as it could with fiberglass, and an easier and cleaner installation.

However, I have come across and interesting insulation concept based on Uponor's Ecoflex pre-insulated pipe. I would use PEX for the pipeline, wrap it in closed-cell foam insulation, and then sleeve it inside 3" HDPE corrugated drain pipe (non-perforated) as the outer jacket. The jacket would eliminate UV concerns, add some insulation properties, and would protect the foam insulation as well as the PEX pipe. At a nominal cost of $1.20 per foot for both the corrugated pipe and the foam , it is also a much more economical solution.

I could use this concept for PVC as well, but PEX offers significant advantages over the PVC that justify it's higher material cost: less installation labor, far fewer joints, flexibility, greater resistance to freezing damage.

 

[Jadnashua]

By the time you wrap that distance of pipe, might it come out a wash if you rented a trenching machine, got it below the local frost line (probably not far), or the minimum code distance for your area? Then, it wouldn't be susceptable to someone backing a truck into it, sitting on it, freezing, or whatever?

 

[Hackney plumbing]

I would use HDPE pipe and bury it.

http://www.jmeagle.com/pdfs/2008 Brochures/PE WaterandSewer_web.pdf

 

[Tom Sawyer]

Me too for a number of reasons. No UV shielding required, no insulation required, easy to install, it's fairly inexpensive.

 

[Mliu]

Trenching 600 feet will cost a minimum of $7,200, plus the cost of the piping & fittings. We would also have to concern ourselves with avoiding damage to existing buried drainage pipes, electrical, etc.

Regarding the sleeved PEX, we would not have to wrap it per se: the foam insulation comes in 6' sticks that we would slide over the PEX, then we would slide the insulated PEX into the corrugated outer sleeve. I would probably want to run a ring of tape around the joints between each stick of foam. I'm not suggesting the task is trivial, but it would not be very labor intensive, nor very time-consuming. The most labor-intensive part of the job would be installing all the unistrut on the back of the buildings to support the pipe. I've done a preliminary cost estimate and parts, labor, and tooling should cost less than $6,000.

Btw, the idea would be to run the pipe up high (~10') on the back of the buildings so it would not be subject to incidental physical damage.

A final decision has not yet been made on this project. We are just exploring all the options and trying to determine which will be most cost-effective and least problematic.