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mechanical joints vs flanged

mechanical joints vs flanged

mechanical joints vs flanged

I'm working on a water distribution system for a subdivision.  I typically work on roadway alignment stuff, so this is a bit out of my range.  When do I use mechanical joins vs flanged joints at T's, hydrants, etc?  Any input would be greatly appreciated.

RE: mechanical joints vs flanged

generally, flanged is used above ground.  Mechanical is usually used where thrust restraint is needed.  The more common type is bell and spigot joints which are the cheapest and easiest to install.  These are only suitable for long runs of straight pipe where thrust restraint is not necessary.

RE: mechanical joints vs flanged

Flanged Joints are rigid, but are also restrained.  They do not allow for any deflection which may come about due to minor misalignments and/or settlement which are common in underground installations. They are only used in aboveground installations.

Mechanical Joints are flexible, but do not provide any restraint by themselves (The follower gland just presses the gasket so that it seals the joint). They are able to compensate for the minor misalignments and/or settlement in underground installations. There are retainer glands available that do provide restraint for mechanical joints. These are similar to the standard follower gland, but have set-screws, or a variation there of, that dig into the barrel of the pipe to provide the restraint.

Compared to push-on joints (the other common joint used in underground installations) the mechanical joint is labor intensive and is only used at fittings, at least in my area. The only advantage over push-on joints is that in pipe sizes commonly used in distribution systems (say 16” and smaller) mechanical joints have a slightly larger allowable deflection.

The water supplier should have standard details that would cover almost all common situations (tees, valves, bends, hydrants etc.).

For what it's worth, the general practice in my area is for pipe to have push-on joints and fittings to have mechanical joints w/ retainer glands. Thrust blocks would be used at all fittings. Hydrant leads would be the same as above, but short hydrant leads (say under 20 ft long) would also have the auxiliary valve rodded back (ductile lugs and all-thread) to the tee and the hydrant rodded back to the auxiliary valve.

RE: mechanical joints vs flanged


Flanges ARE used underground (e.g. for joining valves to tees). However, as you point out, flanges don't allow deflection. Consequently, the length of flanged assemblies should be kept to a minimum so that differential settlement doesn't overstress the piping system.


RE: mechanical joints vs flanged


You are correct for a tapping sleeve and valve. In my area a flanged joint would not be used between a standard tee and valve. In my area, it would be MJ tee, spool piece, MJ valve.

RE: mechanical joints vs flanged


Interesting. Where are you located? I'm in Central California, and have worked in Southern California. We routinely bury flanged assemblies consisting of standard tees, valves, and reducers. I have even seen city standard details that require a flanged hydrant bury, though most are MJ.


RE: mechanical joints vs flanged

Petro pipelines use buried flanges in many locations.  We put a flange protecting stainless steel band with grease fittings around it and fill the gap.

   Going the Big Inch! worm

RE: mechanical joints vs flanged

The use of the flanged joint goes back more than 300 years.  I believe the use of what became basically the now standardized mechanical joint design only goes back 85 or so years, to U.S. Patent No.1,365,530 of Mr. W. D. Moore of ACIPCO.  Both joints (with some improved gasket means now available) are good joints and still used effectively for many applications today.  However, in addition to what has already been said, their bolted assemblies are relatively labor-intensive and their performance rather labor-reliant compared to some contemporary and essentially boltless push-on joining structures that have been introduced into the marketplace in the decades since.  I believe there will perhaps  eventually also be more future preference for contemporary joints that do not require such bolting, based on ergonomics and trench safety etc.
As a result of its geometry and rigid bolting nature, the flanged joint in theory requires perfect alignment of support along the axis of the lines to avoid placing bending loads on the pipe, fittings, and flanged fabrications etc. in construction (and perfection in particularly underground construction can be difficult to obtain in some locales in even this 21st century!)  I think this reason, perhaps also along with realities of differential settlement, vibrations, seismic movements, some injudicious choices of bolting material relative to corrosion etc. in some areas is why some AWWA manuals and standards contain the cautionary statement, “The use of flanged joints underground is generally not recommended because of the rigidity of the joint.”
As one poster has replied however flanged joints are still used internationally, and even in a few areas of the USA off the branches of underground tees.            

RE: mechanical joints vs flanged


I’m located in southeastern Pennsylvania.

I have used flanged fittings underground, but their use has been limited to very specific instances. When bringing a line up into or down out of a pump house, my early designs had the underground vertical-to-horizontal bend rodded up to the aboveground flanged bend, but this configuration looked “clumsy” coming through the floor. Later designs used a flanged bend underground for the vertical to horizontal transition with a relatively short FL x PE through the wall to a coupling just outside of the building. Nowadays, I would just use a restrained MJ bend.

How do you integrate the flanged assemblies with the remainder of the work on a regular basis? Do you use a short FL x PE spool piece from the last fitting?

To bring this around to acmish’s original post, the mores of the particular fields of engineering and the standard practice in various geographical areas differ, sometimes greatly. The most relevant information for your particular application will most likely be standard details from the entity that will be accepting the completed construction. Their details (or the details of a nearby water provider if they do not have standard details) should reflect what standard practice in your area is.

RE: mechanical joints vs flanged

to integrate with the rest, I have used fl x pe as suggested, with a butt strap fully welded to the mainline.  This was for CML&C steel pipe - for a 24" outlet from 48" main.  

RE: mechanical joints vs flanged


I've been very busy, so I'm just now getting back to this thread. I have used a variety of means to connect flanged assemblies to the world around them. Sometimes it's per the client's preference, sometimes it's mine.

Considering a flanged tee or cross with valves attached, I have used FLxMJ valves, FLxFL valves with flanged coupling adapters, FLxFL valves with FLxPE spools, duct tape smile

Another regional difference is that ductile iron pipe is not used as often in California compared to locations that are closer to the manufacturers. We commonly use PVC for small diameter / low pressure (<<150 psi) applications and fabricated steel for larger diameters and higher pressures.


RE: mechanical joints vs flanged


First of all, be careful if you're moving out of your field of design expertise; maybe you should refer the work to another designer with the needed skills. If you're designing a subdivision you'll need to conform to the standards of the entity having jurisdiction over the approval of your design. Depending on the location of the town/city/district and their level of sophistication and experience, you might find very thorough guidance or maybe not so thorough. Anyway, the in-place standards is a good place to start. If the jurisdiction doesn't have good guidance, you should consider obtaining standrads from another area town/city/district that is experienced and has good details and specifications.

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