24"/30" Water Transmission Main 1

[Denob]

Project Description: 29,000 L.F. of 30” OR 24” water transmission main and 8,000 L.F. of 16” water main—all within road right-of-way. The waterline will be 4 to 8 feet behind the edge of asphalt of a 2-lane (one lane each direction) heavily trafficked open-ditch roadway. The depth will generally be 6 to 8 feet deep, except when crossing flood control channels, canals, gas/petroleum pipelines, and a railroad. Possible pipeline materials are PVC, Ductile Iron, and Steel (concrete pipe disqualified due to recent failures in this area). I have several design questions that are probably common for large diameter pipeline projects:

1. I assume PVC (C-905) is the cheapest pipe material for 16” but what about 24” and/or 30”? The latest I was able to come up with was $81 per foot for 30” ductile iron and $83 per foot for 30” PVC (material and shipping only).

2. What would be acceptable wall thicknesses or DR’s? The typical operating pressure will be about 70 psi. I would like to get the cheapest pipe I can without compromising longevity and reliability.

3. Assuming costs are the same for each material, which material is the best? I am leaning toward PVC because of its non-corrosive nature.

4. Concerning vertical changes in elevation, at what depth should I consider having bends instead of deflections? Does it make much difference hydraulically (maybe it’s better to use 22.5 deg. bends)? For example, if I needed to drop the current elevation by 5 feet in order to obtain minimum clearance with a flood control channel, would it be better to have 4x45 deg. bends, 4x22.5 deg. bends, or to have the water line deflect down to meet clearance requirements (let’s assume ½ foot offset per 20 segment—or 200 feet of horizontal distance required to drop 5 feet vertically)? It is my understanding that fittings such as bends are expensive for pipe at this diameter. At the same time, using bends would require less excavation.

5. How often and at what locations should Combination Air Release Valves be located (no surge analysis to be done)? I understand that ARV’s are most effective at peaks, but is this always necessary? What if there’s only a slight deflection in the waterline? It seems excessive to place an expensive ARV at even the most minor peaks. In my preliminary design, I am pairing the ARV’s with a Butterfly Valve at every 3000 ft. Is this adequate spacing for ARV’s? Any general advice on ARV’s if no surge analysis will be done?

These are the only questions I can think of right now. Any help on any of the above questions is greatly appreciated.

 

[BobPE]

Wow, this is a bit beyond the forum to design your project for you here. Approximately 29,000 o4 24 to 30-inch is a substantial engineering undertaking. That is not fair to you nor us. You have some very important questions that as the design engineer, you need to seek sound advice, and not just from the forums here.

My first impression is concerning the surge anaylysis. If none has been done, and noone knows the reasons why a project like this cannot continue without one, we can, and I might add should, not help you. Start with this surge analysis first. It will answer most of your questions, including material selection and appurtenant valve placement.

BobPE

 

[Denob]

Thanks BobPE. I hope other posts are as helpful as yours.

 

[JStephen]

1) Comparative material prices will vary depending on where you are. I would think there would be differences in labor cost, as well.
2) Acceptable wall thicknesses are as required by the design that you will do. AWWA standards include some information for wall thickness of DI pipe due to earth loading, etc., but I'm not sure to what extent they cover PVC. See AWWA Manual M11 for information on steel piping.
3) I was under the impression that PVC was used mainly due to cost concerns. The fittings are still usually DI, so you don't eliminate corrosion concerns necessarily.
4) Consider how you'll restrain the ells in a vertical configuration.

You mention that you won't consider concrete due to recent failures, then also say you don't plan a surge analysis. Could there be a connection there?

That's my two cents worth on the above.

 

[Denob]

Thank you very much, JStephen. Your advice is actually constructive.

1) Yes, there does seem to be variation in material and labor costs depending on the area. I should have thought this through before asking this question.

2) As far as wall thickness/DR, the calculations suggest that a thin wall thickness/high DR would suffice...i'm just a bit uneasy because the min. wall thickness that my city standard specification suggests is much thicker.

3) Does this mean that PVC would still require cathodic protection because of the DI fittings?

4) Does the term 'ell' refer to a 90-deg. bend or any-deg. bend? DI or steel would be welded and PVC would have a harness restraint system, if my memory serves me correctly.

The concrete failures were from other various projects around the city that my company had nothing to do with. I don't know if they did or did not use a surge analysis. My project manager does not want to use a surge analysis because he says that this system will be running at a constant pressure from the water plant non-stop. I questioned him about the lack of a surge analysis but he seems quite adamant.

 

[semo]

1) Not only does the price vary with location, it also varies with the market conditions (ie. steel price/petroleum price). I've seen times when the large DIP was considerably cheaper than the PVC. Your labor costs depend a lot on the pipe weight and installation requirements (wrapping, etc.).

2) This is where the surge analysis comes in. You can design for normal pressures; but, what happens if an open valve/hydrant is suddenly closed and you get a surge that exceeds the pipe rating because you used 80 psi (DR 51) PVC pipe? Not to mention the loadings placed on the pipe as mentioned above.

3) Your corrosivity depends on the water characteristics (is it corrosive to the inside of the pipe and how much?), the type of pipe, and the soil/groundwater characteristics (are they corrosive to the outside of the pipe and how much?). If the soil is corrosive, things can be done to extend the life of the pipe such as exterior coatings and making sure they are not compromised (scratched/missing/etc.), cathodic protection, wrapping the pipe with polyethylene, etc.

4) I don't think there is an exact definition of an Ell. I consider and Ell to be an elbow of any angle. I label them as 90 degree Ell, 45 degree Ell, etc. And with this large a pipe, your restraints could be critical. A few cubic feet of concrete probably won't do it. The soils again have a bearing on how much restraint you need. EBAA has a program available to help determine the restraint needed with their fitting restraints.

Steel might be welded, DI won't, and PVC won't. You will need restraints for DI and PVC.

I agree with Bob that your answer will not be found here. You might get ideas on what to look at; but, a complete analysis of the design needs to be done. Your manager might think a few thousand dollar analysis is trivial; but, just wait until a 24" line costing over $1mil blows a crater in the street that would swallow a few pickup trucks.

 

[plasguy]

I think you will be much further ahead with PVC due to longevityhere are a variety of pressure ratings available as per AWWA C905 going from 80 psi to 165 psi for those sizes. All design information canbe found in AWWA design Manual M23 for PVC pipe. PVC fabricated fittings (similar to steel) are also available for complete corrosion resistance. It is intersting to note that ductile iron has gotten progressively thinner over the last 20 years so any corrosion activity will affect it very quickly. There are also about 3 or 4 PVC manufacturers that make these sizes in NA so bidding will be very competitive. You may wish to contact IPEX for good technical support

http://www.ipexinc.com

 

[dicksewerrat]

I wouldn't design with the working pressure of the system. Use something like twice the working pressure. Sudden valve closings in other parts of the system will affect the new section. It doesn't take much to bring the pressure up to 150 psi. also if the city requires a min. wall, go with that. Then that part of the system will be approved. bends should be restrained and there are ways of doing that. Bends are not that expensive that you would try to deflect joints.

 

[Denob]

Typically, 20' segments of pipe can deflect at the joints at 1 or more degrees depending on the diameter and material, or so the manufacturer's guides say. It seems to me that for minor changes in elevation, deflecting pipe at the joints would be cheaper than 45 degree fittings...or is it?

 

[doshi]

Cheapest!!!
Is it that one time installed cost being the only consideration in economic analysis? Consider life cycle, replacement cost, maintenance and operation costs all factored in to it.
Look at
AWWA C 605: AWWA Standard for underground Installation of Polyvinyl Chloride (PVC) Pressure Pipe and Fittings for Water
AWWA C 900: AWWA Standard for Polyvinyl Chloride (PVC) Pressure Pipe and Fabricated Fittings, 4 inch Through 12 inch for Water Distribution
AWWA C 905: AWWA Standard for Polyvinyl Chloride) (PVC) Pressure Pipe and Fabricated Fittings, 14 inch Through 48 inch for Water Transmission and Distribution
AWWA M 23, Manual of Water Supply Practices, PVC Pipe Design and Installation, latest revision.
AWWA M51 Air-Release, Air/Vacuum, & Combination Air Valves
Unibell Pipe Association Handbook of PVC Pipe: Design and Construction Fourth Edition Dallas, Texas
Uni-Bell PVC Pipe Association: UNI-B-3 Recommended Practice for the Installation of Polyvinyl Chloride (PVC) Pressure Pipe (Nominal Diameters 4-36 Inch)
Look at the following link for DIP and PVC pipe comparison

http://www.dipra.com/publications/

MATERIAL COMPARISONS
A Comparison of Engineering Considerations for Pressure Pipe
Ductile Iron Pipe versus pvc: This brochure compares the structural and performance attributes of Ductile Iron pipe and pvc pipe and provides valid current information to engineers who must determine a basis for selecting pipe materials. In addition to comparing physical test data on the two products, this brochure also compares applicable AWWA design standards for each material.
I work for QA/QC in a Civil Engineering Design Firm and help our Design team with Technical issues.