dead-end water mains 8

[garrettanthony]

Hi,

Does anyone know of an industry/federal-level reference for the maximum length of dead-end water mains - mains on the order of 8" to 12" in diameter? Thank you, Garrett

 

[semo]

I along with our regulating authorities consider a dead end main to be one that is not looped. If there isn't a user at the end of the line, why is it being installed "and" placed on the system in the first place. I don't know many clients that just spend money to install waterlines to nowhere.

The main issue is water quality. You will have to perform the hydraulics to size the line for the existing or future flows. The latter is what can cause quality problems. If the line is sized for a large future flow; but, a low flow will exist for a time, the water can become stagnant.

As rconner stated in the last response, the water will become stagnant in an unlooped line if it is not flushed from the line (by users or a flushing hydrant) in a timely fashion. Stagnation can cause low chlorine residual, bacteria, odors, colors, etc. depending on the type of water being placed into system.

Looping the line will allow it to transfer water from one part of the system to another and potentially keep it fresher. Running two lines side by side from adjacent locations and tying them together at the end will not improve the quality as the water is from the same location and will not move through the pipe except when the users on that line draw water.

There is no scientific calculations to determine the length that I know of; but, often I will determine how much water the pipe will hold and based upon the existing flow see how many days storage are in the line. Then depending on the estimated time that the line will be in low flow condition, I will advise the client that they will have quality problems. They may decide to institute a flushing program for that line (even if its based upon complaints) or they can decide to install a smaller line now and upsize it later when the high flow conditions arise.

 

[danryan]

There are lots of reasons for "true" dead ends, even if they are temporary. For example, a phased construction of a subdivision where a road is being extended. If the waterline assignment is in the roadway, which is very common, the line will get installed at that point. There may be future sections which will add looping to the line but for a definite period you could be faced with a long single feed with minimal use.

The single biggest case of dead ends is still fire hydrant leads. When you flush one that has been sitting for a while the first water that comes out is pretty nasty.

I think common sense is probably the best solution when you face changing demand conditions due to new development. If looping is not feasible economically then a flushing program is appropriate.

 

[cvg]

Flushing is appropriate and absolutely necessary in these cases. However, most cities hesitate to accept these partially constructed loops into a city system because the city is then required to expend the time and money to flush the mains. The developer generally can't be relied upon to do the flushing. And any customers connected to the temporary dead end main are at risk of health problems from the stagnant water. When those customers get sick, they call and complain to the City, not to the developer who constructed the line. A better solution is to install the loop and to reimburse the developer later when the next housing phase is constructed.

 

[rconner]

semo,

Thanks for your excellent comments. As to your statement, " Running two lines side by side from adjacent locations and tying them together at the end will not improve the quality as the water is from the same location and will not move through the pipe except when the users on that line draw water...", I of course in offering my hypothetical suggestion of two new parallel mains (perhaps even in the same trench) did not know exactly what "tie in" sources were available for the new parallel mains. If the tie in is to the same trunk main and say only a few feet apart on same, I suspect you are correct that there could be little flow through the new narrow loop except as varied demand around same dictates. However, if there happened to be say a couple mains in a major street ROW (say across the street from each other), with say even a slight distribution pressure difference, where one end of the narrow loop could be connected to each, would there not be a flow created through the new narrow loop, even when NO customers are drawing water from same?

 

[semo]

You are correct rconner. I wasn't thinking of two separate lines with different pressures being connected. My thoughts were of connecting onto the same line. Your scenario would provide a constant flow of water which would tend to keep the water fresher. Sometimes I have a tendency to only think in one direction.

 

[biglil]

This sounds like a thrust problem. Usually the project engineer is required to calculate the length of pipe if using restrained joints, ie ductile iron. Follow the method prescribed in AWWA M## (the Ductile Iron one, I think it's also discussed in AWWA
stee pipe manual...M11...?...M41..??). Often times the owner (if a publilc works agency) will tell you what system pressure to use.

 

[rconner]

I'm not sure exactly what thrust restraint/pressure problem you are talking about, but I suspect once a new loop is connected to two feeder mains with slightly different pressure, I suspect the pressure from the highest pressure main source, and then in effect all might be reduced slightly (particularly if there is any new main draw/demand).
In most cases it would probably be reasonably safe to use the specified utility test pressure for the new main as the thrust restraint design pressure in thrust length caluclations per AWWA Manual M41. If one line is installed to a "dead end", the thrust at the dead end or for that matter against the backside of cut in tee(s) or closed valve is of course "full bulkhead" thrust or PxA (design pressure in psi x cross-sectional area of the pipe in in2). If a narrow loop were installed instead to a return bend at its end, I believe the thrust at the return bend is in effect twice that of a dead-end (or 2PA). At any thrust foci the magnitude of total thrust must of course be withstood by concrete reaction blocking and/or a determined length of restrained joint piping.

 

[garrettanthony]

Returning to the original question, I stumbled across this in the 2003 edition of NFPA 1141:

"9.2.7 Dead-end mains shall not exceed 183 m (600 ft) in length for main sizes less than 250 mm (10 in.) in diameter."

This is an industry standard as regards fighting fires. Any other industry/federal-level standards would be appreciated.

 

[rconner]

Garrett,
It appears this thread has found perhaps the most notable or detailed guidance on this basic subject to focus on fire protection (that was of course not specifically mentioned in the intitial inquiry). In this specific regard, I believe that there is now also a relatively new (first edition 1989, but now in its third printing) AWWA manual "M31", "Distribution System Requirements for Fire Protection". It appears this roughly 60 page manual highly stresses the importance of "dual" (providing both water supply and fire protection) system "reliability", and I noticed the first thing it mentioned when it talked about "out of service conditions" was "loops". While not an issue in your case but perhaps of general interest of those reading this thread, I noticed it also states (on the very first page of Chapter 1 "Fire Flow Requirements") , "The most significant aspects are: installing and maintaining fire hydrants, providing adequate storage capacity, and meeting requirements for minimum pipe sizes (e.g. 6-in. [150-mm] pipes in loops and 8-in. [200-mm] dead ends) in neighborhood distribution mains when much smaller pipes would suffice for delivery of potable water only."