copper -> steel conversion factors? Also copper to steel joints?

[AlfJones]

Hello all.

I am looking to build a boiler, but I want to build the thing in steel rather than copper. For our cousins in the US, europe and down in Oz, this will seem perfectly normal, but in the UK, raising the idea of a steel boiler in a group of model engineers gets a worse reaction that if you went into the car park and jumped up and down on their cars.

I'm hoping to stay away from the anger and indignation in this post, and get some basic facts.

Many of the designs I am looking at call for the use of 3mm copper for the main vessel. I also need to take into account a wastage factor of around 10%, so lets say it's 3.5mm. That would suggest a 3.5mm steel construction might be enough, if we assume that steel is as strong as copper, which is should be ( i think )

In my head, the copper will be in an annealed state after heating, so will go soft, where as the worst the steel will do is get harder.

However, a number of sources suggest that if building a boiler out of steel, significantly thicker sheet will be needed to get a like for like level of strength, with some suggesting up to 6mm sheet would be needed. This seems very thick to me, but I am absolutely prepared to accept that my world view is wrong on this, I just can't understand why.

Can anyone help me with this? What sort of conversion factors should I use when converting a copper based boiler to a steel boiler? Do I really need to double the thickness for what is a significantly stronger material?


Secondly, I will need to make some joins between copper and steel. My thoughts are that Silver Solder will be fine, but I wanted to check this to make sure - is there anything I need to be aware of? Special fluxes? Specific solders etc?

 

[steamer]

Hi Alfjones,

Steels here in the states for use with boilers include A36 and SA106 tubing...among others.

It's rather difficult to say for sure, but a low carbon steel suitable for pressure vessal fabrication and joining methods is prefered. Again....there's no "conversion chart" for this, and a boiler designed for one material would need a complete redesign and engineering study for the other material.

Beware of simplifications......nothing is ever simple.

Dave

 

[Jasonb]

Alf where did you get your 10% wastage from?

The likes of John Haining use a factor* of 0.5 to 0.8 in their calcs so thats 20-50% and would account for why the same part of a similar boiler boiler in copper may be 1/8 but 5swg in steel or 3/32" copper will be 6swg in steel

J

* this is a factor for 1/4 or thinner plate, less would be needed on thicker plate as you will get the same amount of wastage whether it be 1/4" or 1" plate so 10% on 1/8" plate is 12thou allowance but 10% on 1" plate is 100thou. The rule of thumb seems to be allow for about 1/8" of wastage through the life of the boiler. hence 1/8" copper needs to be 1/4" steel.

 

[ShedBoy]

http://www.pnc.com.au/~wallison/AALS/AMBSC/AMBSC.htm
Here is th ruling we use in Australia. I have all 3 codes and they make excellent reading even if you are not biulding one. I prefer to build in steel also but there is more rules for steel here also.

Brock

 

[Jasonb]

Do be aware that some of the things in the Australian codes are frowned upon here and visa versa. As An example only yesterday I met a guy from Aus who is building a similar traction engine to mine and we met up as he was over on Business. He had to use a different method of staying the top of the firebox as Girder stays are not allowed there but he was able to make the throatplate as a brazed fabrication where here it would be flanged. He also had to use screwed stays as soldered ones are not allowed if the length is more than 100 times the dia.

Having said that the Aus code does give a lot of formulas so you can work out whats needed and should be used as a basis for your design before running it past whoever will be testing your boiler be it club or professional inspector.

J

 

[GWRdriver]

We need to back up a step or two before your questions can be answered responsibly. First what model are you proposing and more importantly what scale? The smaller the scale the less appropriate and inadvisable steel becomes. Also, in the USA, the only binding regulation is the ASME code for full size power boilers and this can't be met by all but the largest model boilers, but even so no responsible boiler builder would use less than 1/4" plate for a steel boiler and for 7+"ga locos 5/16" steel is common. You can see what limitations this might impose on a small scale boiler. In general, unless the copper components are renewable, flues specifically, IMHO it wouldn't be value-effective to permanently (silver solder) copper into steel due to the difference in expected life span.