Steam pipe keeps breaking

Carl_21

I am on the building committee of a 16-story, 1920's-era coop in Chicago.  We have a true 2-pipe steam heating system that we have meticulously maintained and upgraded over the years as new technologies have became available to increase efficiency & reduce costs. 



Our system supplies steam via down-feed risers, 15 in total.  At the beginning of the previous heating season, we had a pipe break in one of these risers at the 2nd floor, i.e., near the end of the supply run.  We replaced the 80-year-old pipe and thought nothing more of it.  However by the end of the season, we lost vacuum on that run again and, failing to find the source of the leak with our usual techniques (looking for water, blowing pressurized air into the line and listening to the walls with a stethoscope, etc.), had to shut it down once again.  Last week we hired a truck with a more powerful air compressor to find the leak, which we did:  the replaced section of pipe had broken again -- breaking the new nipple in the center (i.e., not at the fittings). 



Our first thought is to replace it this time with an expansion loop of 90-degree elbows, assuming the breaking is being caused by thermal expansion. 



What do you all think?  Should we use some other form of expansion fitting?  Might we have another issue that is causing this?  It seems strange that, after 80 years, this section of piping appears so vulnerable to expansion -- thus our wondering if it might be something else. 



(We do not have water hammer.  Aside from this pipe breaking, our steam is apparently being properly and evenly distributed throughout the system.)

Jamie Hall

How very odd

Without looking at the break, preferably with a microscope, it's a little hard to say.  However, it sounds to me like a fatigue failure -- either low cycle, related to the system temperature changing, or high cycle, which might be from a new source of vibration on that particular riser.  Has anything changed in the way the pipe is supported and restrained?  Or is there a new source of vibration?



Either way, the best bet is a grouping of elbows and nipples so that the pipe can both expand and contract linearly and, perhaps more important, so that it can flex (bend) at that location and so the ends can move at least a little bit laterally.  That should take the stress off it.  One hopes...

nicholas bonham-carter

breaking pipe

this seems an intriguing problem! steam systems areusually not subjecting the pipes to a lot of high frequency "on-off" "heat-cold" cycles, so it seems unlikely that your pipes would suffer repeated thermal shock. what is your pressure in ounces?

maybe if you would tell us about the upgrades which have been installed we could come closer to the solution.as far as efficiency goes, good maintainance, and low pressure, are the most effective. there have been some improvements in control technology.  water-hammer has been known to fracture pipes.--nbc

Jamie Hall

Funny thing

about fatigue failure is how little stress is actually required -- but almost always in bending.  I can't honestly say I've ever heard of a fatigue failure in direct tension or direct compression or shear, but in bending they're common enough.  Love to look at one of those broken pipe ends!

ttekushan_3

16 storey downfeed riser

Interesting. It may be helpful to go to the top of that riser and determine if it appears to have slipped downward at all. I'm not sure how that downfeed riser is suspended and at what intervals. If a clamp allowed the pipe to slip or if a clamp has broken, it seems likely that the expansion downward is allowing excessive stress on the section of pipe in question. Also possible is an expansion joint assembly that has stuck or collapsed so there is no longer relief of expansion. In other words, there is some means of "absorbing" the full expansion of that riser and to prevent its expansion from multiplying floor to floor. If there weren't, you'd have pipes inexplicably breaking. Oh wait. You do. That's your clue.



It seems to me that something has changed to cause the breakage of a tried and true steam line --and its replacement. Not too much there to fail other than supports and expansion arrangements and/or prefab expansion joints.



My concern with putting in various elbows, etc. is that you could cause condensate drainage problems, and if you don't have the elbows at full pipe size and arranged in a swing arm configuration, you may find yourself breaking the new arrangement at a fitting instead. Not much improvement! If at all practical, find the source of the lost expansion capacity.



-Terry

Jamie Hall

Support

Terry's comments on support are excellent -- that's what I was trying to get at in my somewhat feeble way when I was enquiring about what might have changed on that riser.  It seems clear that something must have changed.  Either there was a restraint which isn't there any more (for instance, a hanger at the top which has failed or shifted) or there is a new restraint of some kind -- a frozen expansion joint would be one, if there are any, or some change so that both the top and the bottom are now restrained, or something of the sort.



If you can figure out what that might be, he's quite right -- much better to restore the initial condition.  He's also right in thinking that a collection of elbows and nipples would have to installed rather carefully, to avoid creating a place for water to be trapped while at the same time allowing the two long sections of pipe to move relative to each other.  There are a number of expansion joints available -- in a bewildering array of types! (look at [url=http://www.metraflex.com/index.php]http://www.metraflex.com/index.php for instance) -- but in my humble opinion for low pressure systems one can do just as well with an intelligent application of common bits... remember in supporting pipe which can expand, that one end must be restrained in all three dimensions, while the other end must be free to move longitudinally (along the length of the pipe), but not laterally, and the restraint must not put a bending moment on the pipe.

Unknown

Weight of Pipe in Column

As I visualize this we have a 16 stories worth of pipe standing on end. At approximately 12 feet a story that equals 192 feet of pipe.  The diameter of the pipe wasn't mentioned though I'm thinking that the pipe for a building this size is probably 4 inch or more. If you take the weight figure of 10.79 lb/'ft. for 4 inch Schedule 40 you get a weight of 2071.68 lbs as the weight of the whole pipe. If the pipe size is 6 inch -schedule 40 @ 18.97 lbs/ft. you get a total weight of 3661.21 lbs. This is just the weight of pipe standing on end. No internal pressure , thermal expansion etc. This is quite a bit of weight /force especially if it gets slightly out of column.

 I think it's very significant that the pipe keeps blowing at the second floor level where it takes the most weight.  It seems to me that the problem is with supporting the weight evenly through out the length of the pipe and it sounds as if the original support may be either non existent or gone.

As this is quite a sizable project I think I'd get a mechanical engineer involved if nothing else to cover you from a liability standpoint. Just my 2 cents.

- Rod