B31.1 Stub-Ins

[TBP]

I'm involved with a retro-fit project that calls for 8" drips to be stubbed-in off the bottom of a 10" steam main. It's currently being determined if the main is Sch 40 or 80. The branch pipe will be a properly bevelled "fish-mouth" with a full-penetration weld connecting it to the main.

I've got my B31.1 out, and have looked at Nayyar's "Piping Handbook", but I'd like to hear from someone that has actually run the calculations for this kind of joint before. Anything special that should be considered?

 

Hello TPB,

Anything special that should be considered?

Well, yes, you will have to do the calculations shown in B31.1, paragraph 104.3.1.

The idea here is that if there is not enough "surplus" material surrounding the hole that you made to make the unreinforced fabricated branch connection (nee "stub-in&quot, you will have to provide a reinforcing pad (with a vent hole in it) to satify the requirements of B31.1. If you want to see some examples of how to do these calculations, look at Appendix H of B31.3 (unfortunately there is no such Appendix in B31.1) as it presents several "worked out" examples in a step-by-step format - really very informative.

If you have much internal pressure ("steam main" seems to imply that you do) you should be prepared to provide additional reinforcement. If I were doing this retrofit, I would use sweep-o-lets (of the thickness appropriate for the pipe schedules)or at least weld-o-lets to make the branch connections (these welding fittings have "integral" reinforcing). I would also take some UT thickness measurements around the area to make sure how much of the original wall thickness is remaining after years(?) of corrosion. I would also perform MT or PT examination of the finished welds to make sure there are no cracks - also do a good VT to make sure there is no weld undercut.

Keep in mind that B16.9 welding tees are the first choice, then sweep-o-lets, then weld-o-lets, then fabricated REINFORCED branch connections, then fabricated UNREINFORCED branch connections. This is the reverse order in which we would expect to see fatigue failures.

I am sure some other of our colleagues can add some wisdom to this discussion.

Best regards, John.

 

[TBP]

This project is slated to happen at a plant where a friend of mine works. He's in maintenance, and feels he may get left holding the bag. (I can relate to that...) This arrangement has be designed by an outside consultant, and (of course) gone out for bids. The winning contractor was WAY under the other quotes. (Anyone seeing a familiar pattern here?) My friend has asked me to take a quick look at this before the work starts. There are no pads, saddles, tees, or any kind of o'let identified. Just a plain-Jane fishmouth stub-in. Safety valves lift at 170 PSIG, operating pressure is around 125 PSIG.

This application very clearly falls under B31.1. I don't have (or have access to) a copy of B31.3. This is a goofy job anyway, and I have no idea why it's even being undertaken. The 4" drips that are being replaced have been in there for decades. There have been no problems, or water hammer events. If this was my plant, I would be spending this money on problems that actually exist, and they do have a couple of those. I'd go with TEEs. But it's neither my plant, nor my job. However, it is my friend, and I'd like to be able to give him some accurate advice, backed up with a short calculation, or two.

Thanks for your input John, and I welcome anyone else who thinks they can help out.

 

Hi TBP (got it right this time),

Perhaps you can relax a little. If we assume A53, Gr B, schedule 40 seamless pipe at 170 psig, and further assume that there is not a significant loss of (original) wall due to corrosion, the contractor would really have to bungle the job to cause a problem. However, the welding should be given an appropriate degree of NDE to assure the mechanical integrity of the repair.

Of course this just addresses internal pressure design. The expansion/contraction (secondary) stresses still have the potential for making fatigue be the limiting failure mechanism. The fact that the original design is being replaced "in kind" is a positive sign but there is no substitute for involving a professional engineer in such repairs.

Best regards, John.

 

[TBP]

Nothing personal directed at you or anyone else in particular, but I have NOT generally been well served by the local professional engineering community in matters like this. Projects like the one I've described are not considered particularly high profile, around here anyway. They tend very much to be delegated to rookies just out of school, or middle aged duds. "Give it to old Charlie. Surely he can't screw THIS one up." Wanna bet? As far as the low bid contractor who's doing this job goes, his company name might as well be "McBungle Mechanical". They're plumbers & sheet metal workers, who'll hire a welder. (I have nothing against plumbers & tin knockers, but these guys wouldn't know what a pound of steam was if they had it in their pants pocket.)

There are a huge number of plants run totally by the MBAs and accounting people. They want "low bid" on everything from engineering, to material, to contractors. And they're getting just exactly what they're paying for. There's a lot of scary stuff being installed out there.

Sorry for the rant. Thanks for considering my question, John. I appreciate your input.

 

[MJCronin]

To all,

I cannot add to anything said here, except I agree completely with John Breen and again enjoy the comments of TBP.....

I believe that "McBungle Mechanical" has now gone international and that the MBAs and accountants want to issue an IPO this summer......

The issue of "responsibility avoidance" has been brought to a new absurd height (depth ?) here in Pittsburgh Pennsylvania.

Realizing the risks of any kind of commercisl work, local Mechanical Contractors are now advertising for mechanical PEs as "field engineers" who must be willing to PE Stamp any and all "field changes" brougt to them....

Strange, they don't seem to be getting many applicants for these jobs....

MJC