Copper boilers and cast bronze domes

[syrtismajor]

Just a simple question that I can't seem to find the answer too:
With cast bronze domes for copper boilers, what is the thinnest the material can be?
To give a bit of background, this is for a 3.5" (3/4 scale) steam loco with a commercial built coal fired copper boiler. I am building the rest of the loco to LBSC's 'words and music' where he described building the steam dome from copper and brass. There is then an after thought about using a casting where it 'just needs to be cleaned up'.
The problem arises in that the casting I bought (a gorgeous lump of bronze from Reeves) is over sized and needs turning to give room for the fixing bolts.
It is 11/4" high and 11/2" in diameter. If I turn it to drawing however, the thickness of the dome wall about the mounting flange will be about 3/32". This seems a little thin to me...
Do I leave it at about 1/8" and change the location of the mounting bolts or continue turning away the bronze?
Many thanks in advance

 

[Dan Rowe]

I am a bit confused here. You state that it is a comercial built boiler. To me that means they built the whole pressure vessel and the dome is simply a decoration.

If that is the case it seams like you can go with the drawings if you feel there is enough mechanical strength for the job.

If the dome is under pressure then I can not answer the question.

Dan

 

[syrtismajor]

Yeah, on second reading my question may be a little hard to understand...

I am a bit confused here. You state that it is a comercial built boiler. To me that means they built the whole pressure vessel and the dome is simply a decoration

The boiler is commercial built but it is just that, a boiler. It has all the bushes but not fittings:

(ignore the toy steam engine in front of it, that was just for a size comparison)

The large bush on the firebox wrapper is the dome bush which has no mounting holes drilled (it often makes me wonder how they sealed it to hydraulically test it ???)

This means that the dome in question will indeed be under pressure during operation and if it is machined to drawing, its wall thickness will be less that 3/32 (2.4mm)

In summary, what is the safest minimum wall thickness that a 1.1/2" diameter bronze dome can be?

 

[steamer]

Impossible to answer without dimensions for the rest of the dome. If it's a commercial boiler, you should be able to get those dimensions from the manufacturer via the serial number on the boiler.

Dave

 

[steam oil]

Usually there is a inner dome and a outer dome. The dome you bought from reeves sounds like the outer dome and is purely for decoration. The dimensions for the inner dome should be on the drawings.

Brian

 

[syrtismajor]

Ok, I return with pictures as they are worth a thousand words ;D

Firstly, the dome in question is not the decorative cover but is the inner dome and will house the regulator and safety valve.

The boiler was manufactured by Cheddar Steam (UK) in 2002 and is just the boiler. The bush was machined and silver soldered into the top of the firebox wrapper by them. It is 2"OD and 1.5"ID leaving a 0.25" Flange:

As you can see it has not had any mounting holes drilled into it.

The two domes are as follows (the part machined inner dome on the left and the cover on the right):

The problem arises in that the drawings give OD of the top of the inner dome as 1.5" while the maximum width of the flange is 2". If I machine the top of the dome to 1.5"OD the wall thickness will be less that 0.094" because the ID of the cast inner dome is 1.3438":

The ID of the dome does taper up to the top so it is only an issue directly above the mounting flange but the question is still the same, is a wall thickness of 0.094" (3/32 or 2.4mm) too thin for a cast bronze dome of 1.5" OD to withstand up to 80Psi?

If I do not machine the dome to the stated OD, then the mounting holes will not be central in the 0.25" flange of the dome bush as they will be pushed out by the slightly wider diameter dome for mounting screw clearance. Would this be an issue if I shouldn't machine the dome to drawing?

Hopefully this makes things a little clearer!
Thanks again

 

[maverick]

One way to neatly sidestep the issue would be to machine a new inner dome from solid,
thereby giving enough material to allow more thickness. As far as calculating the minimum
safe thickness of the cast inner dome, there are several people on this board with the know how.
Sadly, I'm not one of them.

Regards,
Mike

 

[GWRdriver]

Syrtis,
Here is one way to make this determination . . . if this inner dome was made using copper tube as the tubular portion of the body an .094" wall thickness would be comfortably sufficient for 80psi. The tensile strength of copper can be assumed to be roughly 20-25kpsi with a bursting pressure cold of 600psi. For lack of knowledge of exactly what bronze alloy you have we will assume the values for "gunmetal", the most common alloy for model steam work, which from an on-line alloy table has roughly a 40-45kpsi tensile strength. Based upon this I would say the .094 wall thickness will be perfectly adequate.

I knew John Woodruff of Cheddar Models and the fact that this boiler is IMHO incomplete, and could not have been pressure tested, doesn't surprise me. Otherwise the boiler should be well built.
The most critical job, and perhaps you know this already, is drilling the tapping the dome holes without the drill bit digging in and ruining your day. My thinking would be, if it isn't stable and square sitting on the bottom of the firebox, I'd want to make a cradle of some kind (wood will do) to hold the boiler square and solid for drilling and tapping. Also, if I was doing this I would first look to see if I could use studs rather than screws to hold the inner done down. There may not be room, but I like studs because there is far less likelihood of pullouts or stripping or cross-threading those particular holes.