Material Type-Water Main and Force Main 3

[jgailla]

I am looking for different opinions on the material and thickness (pressure class) used for water mains and sewer force mains. There is usually a local utility standard that I have to meet, but in this case the local utility has no established standards.

In the absence of any local standard or special considerations, assuming you are in the US, what type of material and pressure class would you use and why for a force main and water main?

We have a 2 1/2 mile long force main with a gravity discharge. There are no other connections to the force main, although there may be connections in the future. Maximum working pressure is less than 50 psi. Size is 12" diameter.
The water main is a grid pattern on a 400 acre mixed use development. Maximum working pressure is 75 psi. Sizes range from 8" to 16" diameters with some short 6" stubs for fire hydrants. There are fire hydrants on the lines, but no fire department connections, as these would be on the individual parcels.
The topography is generally flat. Freezing temperatures are not a consideration.

I would appreciate any opinions on materials and pressure class for this situation. In the absence of local standards I used the design standards of a nearby utility whose manual and specifications appear to be the most thought out. I would also appreciate any thoughts about whether there is an industry standard. I have worked with about 5 local standards, and they all have slight differences in materials and pressure class for water and force mains.

 

[bimr]

The reviewing authority has the ultimate determination on piping materials. That would probably be the State where your project is located.

Here is a typical specification:

All material including pipe, fittings, valves, and fire hydrants shall conform to the latest standards by the AWWA and ANSI/NSF, where such standards exist, and shall be acceptable to the reviewing authority.

In the absence of such standards, materials meeting applicable product standards and acceptable to the reviewing authority may be selected.

 

[cvg]

DIP, PVC, PE, VCP, CMLC (steel) and PCCP (concrete steel cylinder) are all used and can all be specified to work for your situation. But you have not given nearly enough information to help make a decision.

soil type? ground water level? amount of fill over pipes?
water source - reservoir or pump station? surge pressures?
12" force main? sounds large for a 400 ac development...
what type of pipe is commonly installed in your area? What are the cost differences in materials and installation in your area. What about O&M? who will end up maintaining the system and do they have any say in the design?

 

[jgailla]

bimr and cvg. Thanks for the responses.
bmir, the specs are calling out the appropriate AWWA/NSF standard.
cvg, Soil type is SP, SP-SM, nothing of note in the geotech report for contaminants or poor soil conditions.
Ground water level is 3'-6' below grade.
Pipes have 3' of cover except at conflicts, where there's up to 6'. Anything over 6' deep is in a casing.
Water source is an elevated tank connected to a well and also connected to an existing system with several wells and elevated tanks. I do not expect surge pressures to be an issue due to the layout of the system.
12" force main is pretty large for a 400 ac development, but the design peak flow is 1200 gpm due to some conservative design criteria by the owner.
Pipe commonly installed in the area is PVC or ductile iron. PE pipe is usually not allowed in the area except for directional drills. VCP is not allowed. I am not familiar with CMLC or PCCP.
Ductile iron is significantly more expensive than PVC and is only used when mandated by a public utility. PVC is almost exclusively used.
The current property owner will own and maintain the system. We have gotten no input from the owner or their representatives, even though we have tried to get input.

My main reason for asking the question is to try to get a survey of typical uses. An internal reviewer said that we were not using industry standard. We are using DR25 PVC water main and DR18 PVC force main.

There are five utilities that I have worked with, all in Florida and Georgia. Minimum standards are:
Utility 1: DR25 PVC water main and DR18 PVC force main.
Utility 2: DR18 PVC water main and DR25 PVC force main.
Utility 3: DR18 PVC water main and DR14 PVC force main.
Utility 4: Class 250 DIP water main and DR25 PVC force main.
Utility 5: Class 150 DIP water main and Class 150 DIP force main. (this one is variable depending on your political status)

 

[cvg]

sounds like pvc or dip are both defacto standards in your area. I see no reason why you couldn't use either one. PVC may reduce pumping cost slightly and give higher flow rates as well as being cheaper to install. Check the O&M cost for PVC vs DIP - will it really last 100 years like DIP (as claimed by DIPRA...) Why not spec both PVC and DIP as alternates and take the lowest bid?

 

[RWF7437]

"The current property owner will own and maintain the system. We have gotten no input from the owner or their representatives, even though we have tried to get input."

What State are you in ?
Can the current property owner legally operate a public water system in your State ?
What experience does the owner have in this area?
How will Operation and maintenance be paid for ?
If O&M "fails", who will take over ?

Add these questions to your list of things to ask the owner, please.

 

[jgailla]

RWF, Thanks for the response. The property owner is the federal government, so there are no issues with O&M funding. It is extremely difficult to get information from the public works department.
DIP and PVC are the only materials I've seen used in the area. I'm comfortable with what we specified for the water main (DR25 PVC for a 75 psi system), but the internal reviewer's comments made me want to get some more opinions.
I'm in these fora quite a bit and I respect the opinions of those who have posted on this subject. Thanks again cvg, bimr, and RWF.

 

[dicksewerrat]

Look at what the gov't has asked for in some of their bids recently and the Army Corps of Engineers spec. Maybe show them some specs from local bids.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[civilperson]

For the 75 psi working pressure, use 150 psi components.

 

[jgailla]

civilperson,
Why would one use 150 psi pipe as opposed to 100 psi in a 75 psi system? It is my understanding that a factor of safety is already included in the pressure class.
It's the same way with a dedicated fire line tested to 200 psi. If the test is to 200 psi, the working pressure is no more than 150 psi, and therefore DR18 (PC 150) should be appropriate.
Please let me know what I am missing.

 

[dicksewerrat]

Water hammer

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

Also, as a corrosion allowance.

 

[jgailla]

dicksewerrat and bimr, thanks.
Shouldn't the water hammer be calculated or estimated and the pipe designed for that, rather than an arbitrary step up one pressure class? In a gridded system fed by elevated tanks and using gate valves, where does the water hammer occur? I thought you had to have an instantaneous shutoff of a pump or a valve shutoff like a plug valve closing quickly.

If using PVC, is a corrosion allowance taken into consideration? I thought corrosion would apply to metal pipe.

 

[bimr]

If using PVC, you have to consider the long term strength of the pipe. The strength of the thermoplastic pipe will degrade over time.

See Uni-Bell for further information on that.

Note that the strength of PVC pipe is also determined at 70 Degree F. As you move away from that temperature, the pipe strength also increases or decreases with temperature.

 

[rconner]

All of the piping materials discussed on this thread are not new and have been available for decades, and most large utilities and probably also experienced regulator/funding agencies have some experience with all of them. I have noticed that some flavor for the overall preferences of utilities that deal with these pipes (that certainly may not necessarily be arbitrary) can perhaps be gleaned e.g. by looking at data in some USEPA and “Ten States Standards” documents e.g.

http://www.epa.gov/OGWDW//disinfection/tcr/pdfs/issuepaper_tcr_ds-inventory.pdf,

http://www.epa.gov/owm/mtb/force_main_sewers.pdf

, and

http://10statesstandards.com/wastewaterstandards.html#49

etc.

While they are often operated at ostensibly some lower pressures than many water mains (as appears to be the case of the present inquiry), for many decades it has been known (at least by some authorities) that there are some arguably unique issues to be considered particularly with lesser strength plastic materials and also with some long, larger diameter force mains in general of all materials.

If you were to check the archives of this or other lists (using the ”Advanced Search” feature), or do a good web search, you would quickly find that these issues of general pipe selection and force mains have been continually debated (as well as good and bad results when utilities took someone’s advice!) All pipes have arguable advantages and disadvantages, some more than others, and there are many issues that are not necessarily covered in minimum consensus standards etc. Despite the fact that all materials now have AWWA standards and manuals etc., they are not really “equal” in any respect other than they all generally have at least one hole in both ends. Non-obvious problems or failures (again you can find many of these also with web searches) are one thing that has perhaps shaped some utility preferences. In the 21st century, in my opinion no one incidentally should have to accept based on perception or assumptions/calculations alone that e.g. a particular pipe with a given pressure rating will have less pumping cost/energy consumption or better flow than another of equal or greater pressure rating that has a larger internal diameter. Despite hyped marketing claims of smoothness advantages, these claims should probably be substantiated with actual, long-term field head loss experience/tests of comparative pipes in the given service. At least when modern pipes (with contemporary linings etc.) of the last several decades are considered, and for whatever it is worth, what field testing I have seen documented appears to indicate that to the contrary water in general flows more liberally through the bigger hole. Finally, as is the case with many other things in life and while short-term economic pressures are no doubt great, the least cost/lesser strength materials may not necessarily in all cases be the best choice, and. I noticed at e.g. .

http://www.portlandtribune.com/news/story.php?story_id=120605143613485800

and

http://northwest.construction.com/news/oregon/archive/0806.asp

these lessons are apparently still being learned into this 21st century.

 

[RWF7437]

But.......

"Forensics studies by a city expert show the fittings in some parts of the line are overtorqued, causing the leaks, Ryan says. A consultant is advising the entire line be replaced."