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

 

[rconner]

I'm not sure I've ever seen any sort of specific guidance from authorities on maximum length of specific sizes "dead end" mains; however, I believe I have seen statements from ASCE/AWWA/Ten States Standards etc. indicating dead ends should in general be avoided or minimized where possible e.g. "by making appropriate tie-ins whenever practical", and of course that appropriately sized flushing structures etc. must be provided.
While perhaps the preeminent reson for this sort of precautionary guidance is concern for water quality, I believe ASCE additionally explained in the publication, "Pressure Pipeline Design for Water and Wastewater" that dead end conditions can also result in "unexpected high pressures" (I guess as a result of a sort of superposition of pressure waves in the area of the dead-end). Perhaps these concerns would be heightened in some situations with increasing length of the dead-end, and the answer to any specific design situation might lie in what acceptable level of water quality and provisions for surge can be expected/assured for the situation..

 

[RWF7437]

In addition to all of the things rconner has said, there are two other reasons water mains are "looped" rather than dead ended.

The first is reliability. It is desirable to feed water to any location in the system from at least two directions. That way, during an emergency or routine shut down of one line, service can be provided from the other direction.

Second when water is fed from two dircetions to something like a fire hydrant velocities and friction losses will be lower. This saves energy and cost.

The real answer to your original question may be found by analyzing the system for the maximum required flow and residual pressure at the end of the dead end line. Usually this is the required fire flow which on most public systems will be on the order of 1000 to 500 gpm and the minimum pressure is usually required, by health standards, to be 20 psig.

good luck

 

[danryan]

The City of Austin has recently set maximum lengths based on 100 gallons of capacity. They claim (but I can't find anything in AWWA standards to verify) that this is AWWA set for fire hydrant leads and fire suppression systems. So for a 6" pipe, 68 feet is the max, for an 8" 38 feet.

 

[RWF7437]

The City of Austin "standard" sounds exactly upside down. It would seem, for the reasons stated in my previous post, that the larger the line size the longer the dead end may be allowed. Keeping the water in the line from becoming "stagnant" seems a secondary consideration compared to reliabilty and economics. At least to some of us. Never seen such a "standard" in any AWWA publication.

 

[CESSNA1]

GARRETANTHONY:

I have always seen very short ones usually just after the last lateral. There is also a cement thrust block used to prevent the end from coming off due to water pressure.

Remember Mulholloand
When he built the water line ftom the California desert to supply Los Angeles in the 1920's there was a large section (30-40 miles?) that collapsed due to a vacuum created in the main. The flowing water hit a dead end and reflected back creating a negative pressure in the pipe.

Regards
Dave

 

[danryan]

RWF7437, can you explain your comment a little further in terms of reliability? How is a longer single-feed more reliable? The requirement is based strictly on volume, and as you surmised it is based on these lines not becoming stagnant.

I don't think reliability enters the equation here - the line has to be sized appropriately regardless of other considerations. The City has experienced enough complaints regarding water taste and appearance that they decided to establish a maximum volume in a dead end lead.

In terms of looping there is no question it is the best way to go if possible. The main reasons it is not feasible are typically economic; i.e. it is cheaper to run a single larger line than a looped smaller line if the length is 1/2 for the larger line.

 

[rconner]

danryan,

Is there any chance they were referring to some sort of fire fighting capacity (to leave some level of actual or theoretical residual pressure in the mainline) and maybe "left a zero" off that??

If you had pretty low pressure to begin with in some areas, I can see you might not want much length of say 6" pipe (say to provide 1,000 gpm?) in a firefighting dead-end (not to mention worries of surge with at least with some piping materials).

 

[danryan]

rconner, no. Fire flow is entirely out of that determination. I can't think of a case where modeling would not be used to prove fire flow availability. Modeling would drive the selection of pipe size for the lead. And the standard hydrant lead in this area is a 6" pipe, but your typical lead length on a street will not exceed 60' anyway (unless you have a really wide ROW, say over 100').

 

[rconner]

danryan,

I guess I see at least now what you/they are talking about -- a 6" nominal inside diameter pipe, 68 feet limit long as if it were e.g. a storage tank would have a volume of about 100 gal., and an 8" 38 feet long about the same (while I guess this could conceivably have something to do with turnover/water quality, I must profess I don't know exactly where the 100 gallons of capacity comes from to set these lengths).

 

[garrettanthony]

Thank you EVERYONE for the responses to the question. The case I have in mind concerns a campus with difficult terrain crowded with structures.

I understand the water-supply principles behind the avoiding of dead-end mains. What puzzles is the lack of a scientific and quantifiable basis for just how long a dead-end main can be, according to the various contingencies.

We all know that dead-ends are not desirable, and should be avoided, but at what point are they _unacceptable_ to the civil engineer? And why?

If the client asks, "Can we make that dead-end main 50 feet longer?", and I say "No", he asks "Why?".

Apparently the only answer to the question is in terms of policy and rules-of-thumb, not a reasoned industry standard supported by calculations.

Thank you again for the responses. If further information presents itself, it would be much appreciated.

 

[RWF7437]

As I tried to say in my previous post; you can calculate the maximum length of a dead end main if you know the size of the main, the flow desired or required at the end of the main and the lowest residual pressure allowed at the beginning of the main where it tees into another main.

For example if you have a 200' long dead end main, a normal static pressure of 65 psi where the main tees into another main and you know the flow required (usually the fire flow) is 500 gpm and the lowest permitted residual pressure is 20 psi you can calculate the head loss in that main for 500 gpm. If that head loss is less than 45 psi (65-20) then you know the main can deliver at least 500 gpm. If the head loss is more then the main can only deliver something less than 500 gpm and probably needs to be up-sized. Do the math and you'll get a "feel" for how it all works.

 

[garrettanthony]

Thank you. This addresses the residual pressure, yet there are other design factors of water quality, vacuum conditions, that may be more of a limiting factor than residual fire flow pressures.

 

[RWF7437]

What you say is true, but I've never seen it in 45 years in the water business.

 

[TerraSouth]

I have a subdivision that I'm designing and the only way to get water to it is with dead end main (!)

hmmm. About a Mile of main at that..

 

[rconner]

I'm not sure I've ever known this done for this purpose, but would two small parallel pipes (e.g. 6") laid close to each other and maybe even in the same trench, with a "U" or return bend at the far end (deep in the subdivision), and then the side legs of this feeding the buildings they face, sort of look like a dead-end in macro view but perform hydraulically and water quality-wise as a "loop". Perhaps this would also minimize water waste, or need to flush to maintain water quality in the future? I realize this would add a few bucks/foot cost to the initial project over e.g. one same size or a little larger main (if that was needed for fire protection in the dead-end).

 

[danryan]

Well it's not really a dead end if you are using water from the end of it. A dead end in my view is a section of line which has no turnover and will need to be flushed to maintain any kind of residual for disinfection. So a single feed to a subdivision doesn't fall into the same category as a piece of line to a fire hydrant or into a building's sprinkler system.

The main issue with a single feed is going to be head loss (for fire flow) and maintenance (e.g. how are you going to fix a break without interrupting service for an extended period of time). But single feeds are done all the time.

 

[TerraSouth]

danryan,

Thanks for the clarification.. I feel better now! 8^)

 

[garrettanthony]

I think that a single 8"-diameter, 2000-foot-long, water pipe serving only a small building located at the end of the pipe would qualify as a "dead-end main".

Though water drawn from the end of the main is constant, the demand is small in proportion to the volume of water in the main. This raises water quality issues, even if there is sufficient fire-fighting pressure.

Does anyone have a definition for "dead-end main"? There doesn't seem to be agreement on the exact meaning of the term.

 

[rconner]

While I guess one can argue semantics/definitions (and perhaps there may be bases for doing so when confronted with codes or specifications with terms not well-defined), it appears technical issues that at least may need to be addressed per various authorities in any water distribution or combined system and particularly maybe "single feeds" or dead-ends (as mentioned in this thread) include at least:

1. Water quality.
2. Any (re-bounding etc.) surge effects (e.g. from a blanked/buttressed pipe end or closed valve).
3. Adequate supply and fire flow (and tolerable effects of same).
4. Reliability (with first proper use of available isolation valving, how many customers/what level of fire protection etc. will be affected by a problem or maybe even cataclysmic event? anywhere in the system, and for how long?)

I actually remember hearing of an instance many years ago where one un-looped line of nearly two miles of 12" piping was reportedly at least initially supplying water to only one house trailer at the far end. This was a "single feed" I guess in more ways than one, main but from a practical point of view I think it was pretty much a "dead end" that required flushing. I believe water at every point in an open loop is basically/nearly always moving, one way or another and with some velocity, whereas there would generally be less dependable movement at least near the far end of a single feed or dead end main. A word (with perhaps at least unattractive connotations?) that might be attributed to water not moving is of course "stagnant". It appears also a break near the supply end of a “single feed” main could conceivably shut that entire main down (as well as water supply/fire protection to everything it serves) until it is fixed.