Tapping Boiler shell directly

[Berkman]

Are there any issues with drilling and tapping a boiler shell directly without a welded bushing to add a water inlet? Probably 5/16 mtp or 1/8 npt.

[SteveR]

Depends on the shell thickness and shell curvature and operating pressure and shell material and ??. 1/8 NPT has 27 threads/inch and I would want at least 6 full threads myself for a 1/8 NPT fitting into steel at 100psi (just a guess) Small radius shell would have a thread engagement different from a flat plate, such that you might need even more threads. Alternative to contiguous welding, you could tack weld and then braze/silver solder a bushing to get the extra threads and simplify.

Good luck,
SteveR

[Berkman]

10 inch pipe, 10 3/4 OD, sch 40.

[Chris Hollands]

FYI - this is from the Australian model steel boiler code -

Tapped holes for bolted or studded connections shall have sufficient depth to allow a minimum length of threaded engagement equal to the major thread diameter - this would be for smaller taper threads as well I'm guessing .

[Pontiacguy1]

If it is 10" schedule 40, that means it has a wall thickness of almost 3/8 of an inch (0.365" I believe), so that is plenty enough depth to drill and tap directly into the shell for certain sizes. 1/8" NTP and 1/4" NTP will be fine to tap at that thickness, you won't have any problems with leaks or anything unless you mess up the threads somehow. Anything larger might require a bushing of some type. I've never had a problem with any of the threads that I tapped into that. Using 1/4" wall thickness, I usually weld in a bushing. I've also seen people 'build up' the surface by about 0.100" by welding, and then drill/tap a 1/8" NTP in the middle of that.

[Berkman]

Thanks for the replies! That's what I thought but wanted to be safe. Don't have a TIG so didn't feel comfortable doing the bushing welding.

[Bill Shields]

And what do you think we all did before TIG was available?

We stick welded everything

Your pipe is more than thick enough to stick weld a bushing in place.

Having spent most of my life around industrial boilers, I have an aversion to threading directly, although I know it is a common practice in the live steam hobby world.

[SteveR]

Some quick work in Fusion - using 28 TPI straight thread.
Your case:

6 inch copper "L" type

[cbrew]

sch 40 pipe, i would have zero issue up to 1/8 npt. plenty of meat for proper thread engagement

[SteveR]

One more caveat for this - if you're tapping pipe thread, don't tap as deep as you normally would. You want to make sure it snugs up good and tight.

SteveR