Structural Vs. Pipe Dummy Supports

[DGrayPPD]

Good afternoon all,

Please see the attached file for a reference.

We've recently had a string of projects where more and more client's are starting to request the use of these structural beam pipe supports rather than traditional pipe base ells/dummy leg supports. Their reasoning seems to be that the pipe dummy legs are failing inspections due to the 1/4" weep hole becoming plugged over time and trapping moisture inside. This of course is leading to internal corrosion of the support itself. The structural beam supports are their way of fixing that issue.

I'm curious if others are noticing a trend in this direction as well for pipe supports, or if these have just been a couple oddball client requests. As far back as I can remember, pipe dummy legs/base ells have always been pretty common practice in the piping world.

I'm wondering, if the 1/4" weep hole being plugged is the issue, couldn't a slightly larger hole be drilled, say 3/8" or 1/2", to help aid in that problem? Or could it be more of an issue of the inspections not being carried out routinely enough to prevent the weep holes from becoming clogged?

Thoughts? Oh, and all the projects have been B31.3 as well.

"Thinking is the hardest work there is, which is probably the reason why so few engage in it."

-Henry Ford

 

[DekDee]

DGrayPPD,
Where does the requirement for weep holes (classified as vent holes in B31.3) in support legs come from ?
I can find no reference in B31.3.
They are required in reinforcing pads and saddles but why would one be required in the pipe dummy leg ?
Regards,
Shane

 

[DGrayPPD]

DekDee,

To provide a visual indicator of any internal corrosion underneath the dummy weld itself. You'd be able to see the leakage through the weep hole. And you're right, I don't believe there is anything in the code that specifically requires the weep hole. It has just been another one of those good practices I've followed over the years.

Regards.

"Thinking is the hardest work there is, which is probably the reason why so few engage in it."

-Henry Ford

 

[DSB123]

Seen plenty of Structural "I" sections being used as anchors at piping bends where you have been able to kick through the web due to excessive corrosion. The benefit of Structural sections is you can see the corrosion whereas for "pipe" dummies you cannot detect the internal corrosion and the only time it is evident is when the pipe has corroded through. (i.e. "Open" sections are better than "Closed" sections)

 

[DekDee]

Maybe a stupid question but how do you get internal corrosion in an airtight cylinder ?
If you drill a weephole you have automatically made the dummy leg susceptible to internal corrosion.

 

[DGrayPPD]

I've always understood them to also vent any trapped gasses during welding, same as a re-pad.

"Thinking is the hardest work there is, which is probably the reason why so few engage in it."

-Henry Ford

 

[DSB123]

DekDee,
If you do not have a weep hole then how would you know if the bend over which the dummy is welded has eroded/corroded and was leaking? Also the weep hole is needed in an "airtight" cylinder to let the welding gasses out during fabrication.

 

[DekDee]

There is no "welding gas" trapped inside.
The vent hole required in B31.3 is to allow venting during welding and PWHT.
It is to vent heated air not any welding gas.
I have welded hundreds without a "weep/vent" hole and all you get is a slight "pop" of the molten metal at the closure of the weld when the heated air is finally trapped.
If it was to have PWHT I would probably agree that a "weep/vent" hole would be required - (how many dummy legs / supports have PWHT ?).
The point I am trying to make is they are not a code requirement but a designers preference and as the OP has noted having them can actually create additional problems
Regards,
Shane

 

[saplanti]

The H section that you mentioned is commonly used in the piping support industry. However if the temperature difference between pipe and integral support becomes large, H section support as an integral to the pipe starts introducing secondary thermal stresses to the connection and pipe wall. These stresses are large. In this kind of applications the pipe dummy (which has flexible section at the connection) is much better choice to reduce/eliminate these stresses.

Additionally, your client might have pretty good idea about their plant, they might already have this kind of support, they are very happy about it, and the temperature difference does not create problem. But the welding, sometimes, becomes nightmare in the structural section applications due to different material on each side, and length of the weld.