The Steamboating Forum

I'm completely brand new to building boilers. I am beginning to put together the Model M boiler from Tiny Power. It is a water tube boiler of the Worthington style.

The only note for preparing a smaller tube (standard 1/2" black iron) to weld into a larger tube (standard 3/4" black iron) occurs in a note for the upflow assembly. Here it says that the hole in the 3/4" pipe should be just 1/32nd smaller than the outside diameter of the 1/2" pipe.

But now I am working on the mud pipe and the water grate tubes. The water grate tubes are extra heavy 1/2" and they are being welded into standard 1 1/4" inch mud pipe. I first drilled out the mud pipe with 1/2" holes in a step-drilling process heading towards the 13/16 inch holes which are the ones that would be 1/32nd smaller than the O.D. of the 1/2" pipes. As soon as I cut one of the 13/16th holes, it seemed that it was an unnecessary reduction of materials and potentially a weaker weld. Is this still the recommended process?

Or should I leave something more like the 1/2" hole in the mud pipe and have the additional material? The I.D. in the extra heavy 1/2" pipe is only .55"

Any and all advice would be greatly appreciated. I will try to upload one picture.

CannonFodder (Robert)

[img][IMG]http://i1243.photobucket.com/albums/gg5 ... brhtjn.jpg[/img][/img]

When I've done this in the past, I've set things up so the smaller pipe actually enters the large one. Are there any cross-sections that would make this clearer?

- Bart

Hi, thanks for the quick response!

Here is a drawing clarifying my previous question.

"A" is as noted in the "M" drawing (although it was for another area of the boiler) - with the hole only 1/32nd smaller than the O.D. of the smaller pipe.

"B" is as I think you mentioned. This seems good. Any limits on fit? How much slop would be allowable in the hole? Do you ever trim the smaller pipe to fit?

"C" was the other thought I had, with the hole only being big enough to somewhat match the I.D. of the smaller pipe. What do you think?



CannonFodder

Okay, I finally got the drawing to upload, sorry.

CannonFodder

Your sketch "B" is the correct method to use. "A" is called a corner weld and is not allowed in boilers per the ASME code.

As to fit, I'd drill the holes out w/ a 27/32 drill; this is plenty close enough (.844 vs .840 nominal OD).

- Bart

Awesome gentlemen, thank you very much!

CannonFodder (Robert)