Casting Boiler shells?

[pyrobrewer]

I am wondering if anyone has information on comparative strength of drawn coppers sheet / tube versus cast copper.

I recently priced 6 inch copper tube @ $15 per centimetre making a 300mm (12') piece $450!

I understand that the drawn / rolled copper is stronger for a given thickness than cast. I can scrounge scrap copper for free or next to it so using more is not an issue I just carve my wax thicker. I can also include decorative details (or even set with gems ).

I am happy to test several examples to destruction to prove suitability. Failures can be recast!

So my question is how much thicker does a cast shell need to be to equal the strength of say hard drawn copper tube 3mm thick?

The Jeweller in me wants to create a one piece boiler shell with fire tube holes and thickened areas for threading directly negating the need for a lot of Brazing. I would be using investment lost wax casting technique I am familiar with.

 

[Maryak]

IMHO...............DON'T

Sorry to be so blunt but you cannot guarantee the structural integrity of such an item,,,,,,,,,,,,,,,,,,,,,1 bubble/hole you cannot see = 1 blowup you and/or your nearest and dearest may not survive.

Best Regards
Bob

 

[abby]

I have to respectively disagree with you Bob , any pressure vessel must be given a hydraulic test to twice it's working pressure to ensure that it is sound and the likelihood of sudden catastrophic failure is remote.
However pyrobrewers question deserves a sensible answer as many older steam models did in fact use cast metals in their boiler construction.
Drawn copper is the traditional material for live steam boilers , it is easy to obtain in a wide variety of sizes , and the quality and suitability can be certified.
It can be easily worked and joined by methods suitable for home construction.
The downside is the cost but model engineering is not a cheap hobby.
Casting sound copper is not an easy proposition as it is very prone to porosity and cracking , melting requires attention to quality of source material , correct fluxing and de-oxidising.
If these problems can be overcome there is still the problem of obtaining insurance , which will be required if the model is to be used in public , or a test certificate if the model is sold .
If the boiler is purely for your own amusement I for one would be very interested in seeing how you go about the casting , but suggest steel might be a cheaper alternative.
As for your question unfortunately I can't advise on the relative strength of cast versus wrought but it will certainly be strong enough for the job.

 

[pyrobrewer]

IMHO...............DON'T

Sorry to be so blunt but you cannot guarantee the structural integrity of such an item,,,,,,,,,,,,,,,,,,,,,1 bubble/hole you cannot see = 1 blowup you and/or your nearest and dearest may not survive.

Best Regards
Bob

With respect I was not looking for a guarantee. Guarantees are for real engineering like the Titanic and space shuttle. I'm not sure that you can get a guarantee that a drawn copper boiler will not fail despite it meeting code.

Risk assessment is about likelihood and consequences

Consequences in this case, as you rightly point out could be dire indeed

My op concerns reducing the likelihood of a catastrophic failure by hopefully stimulating an informed discourse along sound engineering principles.
Given that I may have porosity in the casting how much over-engeneering do i build in to a casting to equal failure point of drawn copper.

Of course you may be absolutely correct, but i think the question is still valid and so far not answered.

 

[lensman57]

Hi,

I tend to agree with Bob, there is no way of knowing how to engineer a cast boiler shell and rate it against a drawn shell unless you have two identical boilers, one made of the drawn stuff and the other from casting and then test both to destruction under lab conditions and then compare the data. Yes I know that a computer model can give you the same data but who has such resources.
For my model engines I am sticking to air for the same reasons as you have mentioned, the cost of the boilers.

Better to be safe than sorry.

Regards,

A.G

 

[compspecial]

I am sure Richard Trevithick used cast (iron) boiler shells with great success and considered them to be stronger than rivetted wrought iron, I don't know about cast copper though other than the fact that casting are usually more brittle.
Stew.

 

[GWRdriver]

FIRST of all, if you have been priced at $450/LF (for US standard 6" Type L HD copper tube) your are shopping at the wrong place! That is double the price of what it can be bought for. Secondly, unless you value your time at $2.50/hr I would expect that attempting to cast a copper boiler will cost you many times in time, if not money, and frustration the cost of using a reasonably priced drawn tube . . . and it still might be failure.

Back in the 1950s the Little Engines Co (Lomita CA) decided that it would make building a boiler much easier for customers (and thus sell more locomotive casting kits) by producing a fully cast firebox assembly for a few of their engines. These castings weren't ever cheap, but they provided the complete rear end (the outer wrapper and firebox w/staying) ready to be coupled with the barrel tube and front head. They were cast in silicon bronze for at least three of their engines, the American 4-4-0, the CP Huntigdon 4-2-4, and the 1"scale 0-6-0 switcher. At first, when the castings were carefully done by a competent foundry, these fireboxes were OK, but eventually they weren't done with care, or by experienced foundrymen, and the quality and soundness of the castings became erratic, causing more problems and costs for builders than they solved.

On the first boiler I ever built ( 1968 ) I made a mistake. In order to avoid having to buy the copper and form the plates for a 7.5"ga locomotive (6" diam boiler) I decided to make patterns (quick and easy) to have the four plates required (front & back heads, firebox heads) cast in silicone bronze. We had a superb local bronze foundry which did them relatively inexpensively. At some point during the silver soldering of the boiler ALL of the heads cracked form thermal differential which was going to be present in a boiler soldering job no matter what. Resoldering the cracks only worsened the problem and eventually boiler, and the large (excessive) investment in time and material, all just to avoid forming the plates, had to be declared a total loss. All I would have had to do was use maybe 4-6 hours, make the forms, form the heads, and be done with it. I still have the patterns . . . to remind me never to be so penny-wise and pound foolish again.

 

[pyrobrewer]

I am sure Richard Trevithick used cast (iron) boiler shells with great success and considered them to be stronger than rivetted wrought iron, I don't know about cast copper though other than the fact that casting are usually more brittle.
Stew.

This reminds me of a little anecdote of Trevithick and friends slaking their thirst in a Pub with the engine unattended outside (no car-parks then) - It Exploded!!!
Not sure how true this is, but it would make you choke on your pint!

 

[chrsbrbnk]

Might be reasonable to build the boiler pressure vessel out of drawn copper then attach the cast detail facade on. giving you both details and more reliability. Chris

 

[Arjumand]

There is a saying 'There is no substitute for practical experience' and I would think this is very applicable to boiler making! If you do not have the experience in the construction of boilers, do not attempt to build one, all it takes is a small mis-step, flaw, in construction or materials, combined with the unlimited potential of steam, the word 'BOMB' comes to mind!!!! There are numerous makers of boilers for the hobbyist, these people have built boilers of all sorts of requirments for the customer,are licenced and can pass any and all ASME testing for the application, be it railroad,steamboat, traction engine, etc. With the cost of material's this day and age and the 'required equipment' to produce a boiler, it doesnt equate to a 'home built' one. You may find some recorded disaster's about boiler explosions on You-Tube, just my thoughts on the subject!

 

[Ken I]

There might also be legal problems to consider.

Codes vary but here in South Africa if the potential energy exceeds 10000 Joules total energy or 50kPa pressure then the codes apply - no exceptions.

10000 Joules isn't much especially when steam is involved.

(Extract from Bureau of Energy Efficiency)
For a boiler is operating at a pressure of 8 kg/cm², steam saturation temperature is
170°C, and steam enthalpy or total heat of dry saturated steam is given by:
hf +hfg = 171.35 +489.46 = 660.81 kCal/kg.
If the same steam contains 4% moisture, the total heat of steam is given by:
171.35+ 0.96 x 489.46 = 641.23 kCal/kg

For 10000 Joules that’s less than 4cc of water – which says something about the awesome power of steam.

If someone gets injured by an "illegal" boiler you could find yourself in big trouble.

As a certified engineer who has tested and signed off on full scale boilers and pressure vessels I must agree with Maryak

DON'T

Regards,
Ken

 

[aonemarine]

I would be curious to see how a cast firebox works out, and I also understand why they say "dont". I have to think that it is possible to do but would question its long term safety. I dont think I would try it as a pure copper casting. Maybe silicon bronze?
Sitting here scratching my head, I swear I remember seeing a cast marine boiler at one point in time.

 

[Ken I]

It's not like it isn't doable - it is !

But

Can you do the design calculations ?

At the very least you would need to build two and test one to destruction to demonstrate a factor of safety of at least 5 and preferably 10 - and that after several hundred cold and hot cycles (for fatigue).

A FOS of 10+ isn't that hard on small scale.

Even then it still won't be certifiable.

A person who knows how to do it wouldn't ask the question "can I ?" - hence my gut reaction advice is - don't.

Regards,
Ken