Water Main Sizing
Water Main Sizing
(OP)
I am working with a Town on a multi-family residential development. In reviewing the water distribution system, the Town said their standard for sizing the main is to add the size of the domestic and fire protection sevices at the building, resulting in the minimum water main size required. Example; 2" domestic + 6" fire = 8" main in the street. The Town also said that the sizing method could be found in the AWWA Standards.
Does anyone know if this sizing requirement exists in the AWWA Standards and in which book I would be able to find it?
Thanks for your help.
Does anyone know if this sizing requirement exists in the AWWA Standards and in which book I would be able to find it?
Thanks for your help.





RE: Water Main Sizing
RE: Water Main Sizing
RE: Water Main Sizing
The minimum water main size for a modern municipal water distribution system is 8-Inch. It is sized to be capable of transferring fire flows.
RE: Water Main Sizing
6" could be acceptable if it meets fire flow requirements. This would be used in a tightly gridded residential system, perhaps, although I've never used 6" lines for anything but hydrant stubs or building risers.
8" is the minimum for ungridded runs.
I think both of the above are in NFPA 1141, but I'm not sure.
RE: Water Main Sizing
RE: Water Main Sizing
Besides....they add up to 14'' and I don't think that pipe is manufactured in 14''. It depends.
RE: Water Main Sizing
what adds up to 14?
Nichole3 has 2+6=8 which is at least mathematically correct. Although the engineering is in doubt.
RE: Water Main Sizing
Oops, my readings horrible without my glasses. Adding the two does give you 8''. :)
RE: Water Main Sizing
RE: Water Main Sizing
Nicole, why the America Water Works Association? If that will give you guidance than more power to you. I would simply refer to either a county or city public works department for thier input. Stick to the specs....
RE: Water Main Sizing
This was something I never encountered before. I asked a coulpe Fire Protection Engineers and they haven't heard of that standard either. I figured I would post the question to make sure I wasn't missing something.
Thanks everyone for your responses.
RE: Water Main Sizing
RE: Water Main Sizing
Even when you have fire service requirements, the size of the main does not increase drastically.
Most municipalities subscribe to the requirements of the AWWA....it's a good reference...stick with it.
RE: Water Main Sizing
Once you have the hydrant flow test, you'll need to get the requirements for the fire flow.
In this case, I would guess that the governing flow would be 1500 gpm from the most hydraulically distant hydrant with a residual pressure of 20 psi at the street.
You would then run a spreadsheet (simple Hazen-Williams pressure drop for straight runs) or WaterCAD model (for gridded or complicated runs) to size the line so that the pressure drop at 1500 gpm does not cause a residual pressure below 20 psi.
The method you've described from the town is ludicrous. Even in the case that you couldn't get any data and had to guess, the areas of the pipes should be added, not the diameters. The flow is proportional to the area, and the area is proportional to the diameter (over 2) squared. Simple math will tell you that you can't add the diameters together.
RE: Water Main Sizing
Couldn't of said it any better myself! Sometimes you have to revert back to hydraulics and your pressure head equations!
RE: Water Main Sizing
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Water Main Sizing