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dead-end water mains
8

dead-end water mains

dead-end water mains

(OP)
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

RE: dead-end water mains

2
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..    

RE: dead-end water mains

3
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

RE: dead-end water mains

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.

RE: dead-end water mains

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.

RE: dead-end water mains

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

RE: dead-end water mains

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.

RE: dead-end water mains

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).

RE: dead-end water mains

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').

RE: dead-end water mains

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).

RE: dead-end water mains

(OP)
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.

RE: dead-end water mains

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.

RE: dead-end water mains

(OP)
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.

RE: dead-end water mains

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

RE: dead-end water mains

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..



RE: dead-end water mains

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).   

RE: dead-end water mains

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.

RE: dead-end water mains

danryan,

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

RE: dead-end water mains

(OP)
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.

RE: dead-end water mains

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.

RE: dead-end water mains

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.

RE: dead-end water mains

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.

RE: dead-end water mains

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.

RE: dead-end water mains

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?

RE: dead-end water mains

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.

RE: dead-end water mains

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.

RE: dead-end water mains

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.     

RE: dead-end water mains

(OP)
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.

RE: dead-end water mains

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."

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