7.5" Diameter Copper Boiler

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sunworksco

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I'm planning to build a few 7.5" diameter boilers in copper and considering using finger joints.
Anyone have any advice about this method?


3831-b.jpg
 

Jasonb

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What does your local boiler code allow?

Here we would most likely use a butt joint with external strap, I think the Australian code would also want the same.

Jason
 

sunworksco

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There are no codes for this size boiler in California.
I will be running around 100psi.
 

Dan Rowe

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giovanni said:
There are no codes for this size boiler in California.
The fact that this size boiler is not subject to annual inspection does not mean that there are no construction codes that have to be followed.

Directly from:
http://www.dir.ca.gov/Title8/771.html

§771. Boilers Not Subject to Annual Inspection.

--------------------------------------------------------------------------------

(a) The following boilers are not subject to annual inspection and do not require a permit to operate providing they comply with all of the provisions of subsection (b):

(1) Low-pressure boilers.

(2) Miniature boilers.

(3) High-temperature water boilers.

(4) Boilers, including forced circulation boilers, in which none of the following are exceeded:

(A) One hundred square feet (100 sq. ft.) of heating surface.

(B) Steam drum does not exceed 16 inches inside diameter.

(C) Maximum allowable working pressure does not exceed 100 psi.

(D) Water capacity does not exceed 35 gallons when filled to normal operating level.

(E) The BTU input to the burners does not exceed 400,000 BTU/hr.

(b) Boilers exempt from annual inspection in subsection (a) shall comply with all of the following:

(1) All other provisions of these Orders including construction and installation.

(2) Automatically controlled fired boilers shall be fitted with all the applicable controls required for low-pressure boilers in 763(d).

(3) All automatic controls shall be maintained in operating condition.

(c) Nothing in sections (a) and (b) above shall prohibit any qualified safety engineer employed by the Division from requiring any boiler to be prepared for inspection when in his opinion such inspection is necessary to determine the safety of the boiler.


The definition of a miniature boiler in the State of California Boiler Code is here:
http://www.dir.ca.gov/Title8/753.html

Miniature Boiler: A boiler which does not exceed any of the following limits:
16 inches inside diameter of shell.
5 cubic feet gross volume, exclusive of casing and insulation. (This volume includes the total volume of the steam and water containing parts of the boiler plus the volume of the combustion space and gas passages up to the point of attachment of the smokestack or chimney breeching.)
20 square feet water heating surface.
100 psi maximum allowable working pressures.



771 Subsection (b) states that you have to comply with the construction orders.

As this is a copper boiler I would suggest that a copy of Kozo Hiraoka article "The Safety of Cooper Boilers" be obtained as it uses the ASME code for pressure vessels applied to copper construction.

Dan
 

steamin

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I totally agree Dan. Even though there are NO codes in place does not mean anything goes. Sound design based on past experience and established construction techniques should/must be followed. First and fore most, common sense must prevail in any size of boiler construction.

Personally, a tongue and groove copper shell with silver solder for a "pressure" vessel scares the mesh out of me. But, I am from the old school and like to use tried and proven methods that have worked for many decades. I do not like to experiment with new ideas when it comes to boiler making unless I have a bomb shelter to hid in.

I do not mean to be blunt, but beating around the bush is not one of my strong suits.
 

tel

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Not a new idea at all tho' - the finger jointed barrel has been around as long as model boilers have been and many of the older plans called for them. The system is, or at least was (as I've been out of the loop for a while) allowed under the AMBSC code.
 

Dan Rowe

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Tel,
Indeed not a new idea.

The AMBSC Part 1 (2007) section 4.2.2 on joint design states that "joggled lap joints" are not allowed except to add an extension on a split seamless tube for firebox sides.

Section 4.2.4 is the rules for silver soldered but strap lap joints.

Dan
 

Jasonb

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That would sound about right Dan as I have only seen that and a dovetailed version used to extend the wrapper down, not as a barrel joint, thats why I said overhere an external butt joint is prefered.

Jason
 

sunworksco

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Do you have a photo of the external butt-joint?
A finger-jointed silver soldered shell should be twice as strong as a straight-line soldered seam-joint?
If the finger-joint stands up to a 200psi pressure test, will this be acceptable?
 

Jasonb

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Here is one prior to soldering, holes are for rivits. Boiler is 7 1/8 OD 4mm thick

 

tel

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Dan Rowe said:
Tel,
Indeed not a new idea.

The AMBSC Part 1 (2007) section 4.2.2 on joint design states that "joggled lap joints" are not allowed except to add an extension on a split seamless tube for firebox sides.

Section 4.2.4 is the rules for silver soldered but strap lap joints.

Dan
A joggled lap joint is a different animal again tho' - it is where the sheet is joggled and lays over itself.

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tel

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The finger, or castellated joint is made thusly



castjt-063b.jpg
 

tel

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A couple of snips from my copy of the code.



butt strapjoint.jpg


boiler barrel joint.jpg
 

sunworksco

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Thanks for all of the images and comments.
I'm considering having the boiler shells laser-cut in a dove-tail joint,
making the finger-joints very precise for stronger soldered seams.
This should create a certain amount of anti-shear.
The boiler height is only 4.5".
Any suggestions on how to roll the copper sheet?
Plenty of CNC aluminum tank shell rolling machines in the neighborhood but haven't found a roller for small shells.
 

sunworksco

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Could I possibly hybridize the boiler construction by using a steel shell with copper plate bulkheads and copper tubes all silver-soldered together ?
Could I use seamless heavy wall 316L stainless steel shell if I am not welding it but using compression seals and drilling and threading it for fittings ? How is it that kitchen pressure cooker pots can be used for steam ?
 

Jasonb

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We can't use stainless for boilers in the UK so can't say if it would work for you. Several of the European countries use stainless though. You can get problems with galvanic couples and crevice corrosion in threads etc. You also need to ensure you use the correct grades of stainless.

J
 

sunworksco

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Will steel pipe have a crevice corrossion problem?
I think that a steel shell with copper bulkheads would be much less expensive than a copper shell but can the steel pipe be silver soldered to copper bulkheads without any downsides?
 

GWRdriver

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giovanni said:
Will steel pipe have a crevice corrosion problem?
Giovanni,
To answer your question, no not as you refer to, but with all the hundreds of alloys floating about these days it's become important (IMHO), especially when talking boilers, to be specific about material. MILD steel, hot-rolled seamless tubing, of the type used for boilers (one such in the USA is SA-106/B) does not have problems other than rust and electrolytic corrosion both of which are manageable with proper boiler care and chemicals. I suspect the problem you refer to is "stress corrosion cracking" which occurs in stainless steels and this is an electrolytic reaction due to the presence of chloride ions in water. Stainless steels are also subject to weld area cracking which IIRC can be minimized by using specific weld alloys in a specific SS alloy after which the welded assemblies are normalized. This process is generally outside the capabilities and pocketbooks of the average live steamer. Stress corrosion cracking is pervasive and silent, you cannot see it happening; it is akin to dezincification in brass which also can't be seen. This is the main reason why, as Pat refers to above, SS it is a no-no in some countries. There are a few stainless model boilers operating in the US and I believe a few in the Toronto area. One of the US boilers is quite large and was fully engineered and built by Severn-Lamb Ltd in the UK, and so far only one has reported SCC problems but the science predicts otherwise and we err on the side of safety.
I think that a steel shell with copper bulkheads would be much less expensive than a copper shell but can the steel pipe be silver soldered to copper bulkheads without any downsides?
It would be cheaper, and I don't know of such a mixture of metals specifically, but I wouldn't be at all surprised to learn that it had been done successfully. The question I would have would be, when all-steel boilers with rolled-in or silver soldered in copper flues is probably the most widely used material combination for larger model boilers, why bother with copper heads?
Any suggestions on how to roll the copper sheet?
It rolls best when it has been annealed, and rolling from flat will probably call for several anneaings during the process. The length of the copper sheet in the flat will need to be computed from the barrel circumference along the neutral axis of the plate you are using. IIRC the nuetral axis of copper is approx 3/4 of the thickness of the sheet, towards the outside of the roll. You'll need to check my memory on this one but be advised, the neutral axis does not lie in the center of copper sheet.

As to the finger joints, ideally these should be dovetailed joints rather than finger joints so as to put the metal surrounding the joints into compression rather than relying solely on the shear strength of the solder, which admittedly is great, to hold the joint together. I have done it both ways with no problems although the dovetail joint is far more difficult to do and maintain a very small joint clearance and fit. I try whenever possible to use seamless copper tube, but a while back I decided to go with a butt & strap joint on rolled barrels because that is easier to do, less costly when I'm doing work for others, and is equal to finger joints in strength although it too relies primarily on the shear strength of the solder.
 

sunworksco

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Thank you, GWRdiver!
That is brilliant what you wrote!
I will copy your comment into a word doc and will build the mild steel boiler with rolled copper tubes.
Is the boiler tig welded?
What size copper tubes should I use and roller for a 7.5 inch diameter x 4.5 inch height boiler ?
 
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