Water Main Sizing

[nichole3]

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.

 

[cvg]

that doesn't even make sense

 

[jgailla]

Somebody's pulling your leg.

 

[bimr]

I agree that somebody is pulling your leg.

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.

 

[jgailla]

bimr,
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.

 

[cvg]

6 inch can sometimes provide fireflow for short distances assuming you have adequate pressure. However, in general for longer distances and lower pressures, 8 inch is usually required. Most street grids are too widely spaced to accomodate a 6 inch water distribution system. I have never seen it used in a modern system. I have seen miles of smaller (2 - 4 inch) older system pipes replaced with 8 inch.

 

[youngcivil101]

This is the part where you go to the city/county specifications and see for your self if this infact is correct. Never have other people make assumptions for you.
Besides....they add up to 14'' and I don't think that pipe is manufactured in 14''. It depends.

 

[Zambo]

youngcivil101,

what adds up to 14?

Nichole3 has 2+6=8 which is at least mathematically correct. Although the engineering is in doubt.

 

[youngcivil101]

Zambo

Oops, my readings horrible without my glasses. Adding the two does give you 8''.

 

[Ron]

Well, in pipe work, 2+6 does not equal 8!. A 2" dia. pipe + a 6" dia. pipe added together only have about 60 percent of the capacity of an 8" pipe.

 

[youngcivil101]

Well then 2+6 doesn't equal 60% either.

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....

 

[nichole3]

I was meeting with the DPW Assistant Superintendant of the Town in which my project is located. Basically, our utility plan showed an 8" main in the main driveway of the proposed apartment complex with a 6" fire protection and a 4" domestic service at each building. The assistant superintendant and his boss told me that a 10" main would be required because 6" and 4" equal 10". I was also told by them that this was the standard for water main sizing in the Town and I could find it in AWWA.

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.

 

[cvg]

if you have 2 buildings does that mean (6 + 4) x 2 = 20" main?

 

[Ron]

Their sizing logic makes no sense, whatsoever!
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.

 

[jgailla]

Do you have a hydrant flow test? This is required by code in many areas in order to permit the sprinkler system.
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.

 

[youngcivil101]

jgailla-

Couldn't of said it any better myself! Sometimes you have to revert back to hydraulics and your pressure head equations!

 

[msquared48]

Sounds like someone at the city is being very conservative, simplifying the review problems for their non-engineer plan reviewers.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rconner]

Perhaps AWWA Manual M31, Distribution System Requirements for Fire Protection would be a good reference for your purposes.