Testing of underground ductile iron pipe 3

[chief600]

a project is calling for 30" ductile iron class 52 pipe with push joints to be installed under a roadway with both ends left open for pressure testing after the roadway is back filled. The drawings call for quick flanges (sometimes referred to as EZ flanges or break flanges) to be installed on the ends of the pipe along with steel blind flanges to enable the pressure testing of the piping.

My question is if this quick flange and blind flange combination is a proper method of capping the ends of the pipe? The test pressure of 200 psi would produce very large forces on the ends of pipe of around 135,000 lbf.

 

[MJCronin]

I agree that you will be developing massive forces on a blind flange at this pressure and diameter. This places huge loads on the end device.... In industrial system testing (ASMEB31.3)this is accomplished by an end flange and a "testing blind". The blind is loaded in bending and converts test pressure into bolt loads

I suggest that you talk with the vendors of the piping and the "push-on" devices about testing of thier systems. I am sure that they have encountered this problem before.

As I recall, the "push-on" devices rely on developing tensile forces onto a lip moulded into the pipe wall, or into a device that relies on friction and compresses the outside pipe diameter.

Please tell us more about your final resoultion, what brand of piping and connector you use and the propper way to test in this situation...

Best Regards

-MJC

 

[cvg]

I believe you would want to restrain the entire length of pipe under the roadway (unless this is a very wide road). I would assume that the restrained length would be more than just one joint long. Megalug might be a good option.

http://www.ebaa.com/index.php?page=1100

 

[bimr]

I agree with cvg's post, you need to restrain almost the entire length of pipe under the roadway. It may be possible to brace the pipe ends during testing so that the middle does not push apart. Normally 2-3 pipe lengths would need restraints, not just the pipe ends.

 

[rconner]

While I agree with all posters that thrust forces can be substantial, and with latter posters as well that restrained pipe can be helpful in/simplify hydrostatic testing of some push-on joined pipelines at some locations (and restrained pipe also is sometimes specified for crossings).
However, restrained joint pipes in general are NOT really necessary, at least just for hydrostatic testing, of many even quite large diameter and high pressure unrestrained piping sections. In a case in point I am aware of a project several years ago, that I suspect involved in toto a half million feet or so of 1200mm ductile iron piping. Except for rather small footages, all piping was just standard unrestrained joint pipe. It is my understanding every stick of that pipe was successfully subjected to a 30-32 bar (435-465 psi) field hydrostatic test. By and large, this was accomplished by quite substantial and temporary external buttressing behind end closures for each test section, up to the task. I believe the bulkhead thrust on 1200 mm DIP at 30 bars pressure is about 834 kips. In the particular case I happened to see on that job, the Contractor employed a system of interlocking precast concrete blocks he simply made a form for and poured (with lifting loops cast in). Imagine a very large sort of lego block construction, large enough by gravity and friction to effectively sustain the load (by the way, this firm happened to be Egyptian). I think locations of future valve vault construction may have sometimes been chosen to put down and take up such temporary buttresses. This was incidentally a quite “sustainable” construction, as they simply loaded up the blocks and trucked them from testing location to location (who knows, they may still be using same!) Virtually the same thing can be accomplished with a transverse big steel I-beam or two(many Contractors have such leftover beams on their yards), and a good many railroad ties stacked vertically against a soil wall/cross-trench behind same (I have seen up to 48” unrestrained lines tested up to 175 psi in this manner.) Can throw in a big hydraulic cylinder to keep things “tight”, even if blocking slightly undersized. When the testing is over, load everything up and maybe no one is out really anything, other than labor to construct, compared to the cost of restrained joint pipes?
As to closures used, while there are a great many sorts of closures available for ductile iron piping I guess I have nothing against the field adaptable flanges, but of course in and of themselves they basically just provide a platform to fix a blind flange to a pipe plain end. In this and all Engineering work, Newton’s third law must nevertheless be respected by Engineers, particularly when large forces are involved (maybe the next joint buried down the line a few feet may be pulled out, and should be inspected?) I believe some of these devices bite into the outside metal of the plain end for restraint, that could be a concern if the permanent connection to the line might eventually result in another rubber gasket trying to seal down on the same axial area (I guess I’ve never heard of a leak from such, but maybe something to think about?)

 

[cvg]

pre-cast or cast in place blocks, steel struts etc can be used, however bear in mind that for this size pipe, the bearing area of a thrust block will be about 100 sq ft more or less. the bearing area must be kept below grade and rely on the strength properties of the soil. the passive pressure of the soil will not exert any force on the blind flange, instead it relies on the movement of the pipe slightly before it feels any resistance at all. nearby structures or underground utilities may be affected by thrust blocking as well. the cost of excavation, design, placement and removal of the block may well exceed the cost to just install joint restraints. The choice of joint restraint vs thrust blocking for hydro testing is usually left up to the contractor. For permanent installations, I know some agencies that no longer accept permanent concrete thrust blocks. Large chunks of concrete left buried in the public right of way becomes a liability in the future.

 

[chief600]

Thanks for all the answers. This particular project calls for the continuation of the piping at a later date. So concrete thrust blocks, while a logical solution, would have to be removed for the continuation of the piping and therefore not practical. I beieve a 1" steel place, 15 feet by 8 feet placed vertically against virgin soil, with concrete traffic barriers placed between the end of the pipe and the steel plate and wegded tightly with wood would work.

Using mechanical joint fittings with retainer glands is another option but more costly. My concern would be that if the ends of the pipe are not braced the joints may still seperate.

 

[cvg]

megalugs are designed to completely restrain the pipe. believe me, they will not separate. no bracing would be needed. but again, why isn't this left up to the contractor to decide which option is preferred? your steel plate option sounds a bit more iffy to me than megalugs

 

[bimr]

Agree with CVG. While the cost for megalugs may be a little higher, this is offset by the substantial reduction of risk in relying on unknown soil properties. In the end, it is normally the Contractor's responsibility.

 

[dicksewerrat]

What is so difficult about putting the half clamps on the pipe and stringing the threaded rod? If this is a 100 foot crossing, and you are excavting and placing the pipe properly, there will be very little additional time.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[rconner]

The OP stated the project has 30" "push-on joint" ductile iron pipe (given the question asked, I guess I assumed this was now plain unrestrained pipe). If it is not practical or cost effective to apply external buttressing to external caps or plugs, another option is to use mechanical joint caps with Megalugs on the ends and simply replace the plain rubber standard push-on pipe gaskets (likely now Fastite or Tyton?) with gaskets to fit the same push-on socket pipes except with remarkable restraint capabilities (e.g. see video at

http://www.american-usa.com/products/fastite-joint-pipe-with-fast-grip-gasket/resources

)