Model Boilers, use of Brass and Copper

[johnofwessex]

Miniature locomotive boilers are almost always copper, however 'toy' for wanto f a better word ones eg Mamod & Wilesco are brass, despite issues with Zinc Pest.

 

Any reason for using brass rather than copper? ot VV?

 

 

[Simond]

Brass boilers are probably made from drawn tube, with soft soldered stamped end plates, using external firing and low pressure.  So low-stressed assembly, made to a price, by relatively unskilled labour.  And directly painted.

 

A model engineering boiler is typically a complex form, working at higher pressure, hence higher temperature, so hard soldered, made by skilled craftspersons, and usually clad over insulation (though probably not in 0 gauge).  And much more expensive, and expected to last.

[47137]

Pretty much exactly 42 years ago I completed a simple traction engine with a reciprocating cylinder. This was the practical part of A level metalwork (EWTP), I was allowed to do this but not the theory lessons because they clashed with physics!

 

The boiler is a length of copper tube. The ends I cut from an offcut of the same tube, flattened out and flanged over some kind of former (forgotten what - probably a round bit of wood) to make two caps which fitted snugly into the tube. This all silver soldered together and then pressure tested with water i.e. a hydraulic test. I misread the pressure gauge and took mine to 150 psi, should have been 50 psi but nothing went amiss. Imagine being allowed to do this sort of thing without supervision in a school nowadays! There is a boss for the safety valve and I recall this had a bolt inserted in place of the safety valve for the pressure test. Outside is a larger tube made from tinplate to take the paint.

 

- Richard.

Edited June 26, 2022 by 47137

[rogerzilla]

The Mamod boilers are notorious for dezincification, depending on what water has been used and whether the boiler has been emptied after use, or just topped up.  I reboilered a SE3 (the big 2-cylinder one) earlier this year because the end caps were like a colander.

 

They are exempt from testing because they run at a mere 1 bar and the stored energy is only about 0.4 bar-litres.  The worst that can happen is probably a jet of scalding water.

Edited September 14, 2022 by rogerzilla

[Michael Hodgson]

On 26/06/2022 at 18:35, 47137 said:

 

This all silver soldered together and then pressure tested with water i.e. a hydraulic test. I misread the pressure gauge and took mine to 150 psi, should have been 50 psi but nothing went amiss. Imagine being allowed to do this sort of thing without supervision in a school nowadays!

 

- Richard.

If it failed during a hydraulic test surely all that would have happened is you would get wet?  And if it withstands a higher pressure than intended that's agood thing isn't it?

[didcot]

Yes, that's why a hydraulic test is used. Water isn't compressable and a leak will result a sudden drop in pressure. More importantly there is no stored energy or hot water to hurt you.

Water boils at a higher temperature as the pressure increases. From memory water becomes super heated around 225psi. A boiler failure means a sudden escape of steam, which now at atmospheric pressure will mean a sudden increase in expansion and therefore damage etc. Well something along those lines. Never underestimate the power of a steam vessel. 

[Yarravalleymodeller]

On 25/06/2022 at 17:02, johnofwessex said:

Miniature locomotive boilers are almost always copper, however 'toy' for wanto f a better word ones eg Mamod & Wilesco are brass, despite issues with Zinc Pest.

 

Any reason for using brass rather than copper? ot VV?

 

 

Brass doesn't suffer from zinc pest, certainly not in the sense that say diecasting alloys do. 

 

You can end up with a loss of zinc from the alloy either superficially on the surface(when brass goes red but polishes back to yellow) or systemically but this is not the same process as "zinc pest". 

 

You can actually see this on brass components of full size engines sometimes if exposed to too much heat the brass takes on a red appearance as the zinc is lost and you are left with copper. 

 

Worst case, and this is a joke and you shouldn't mess with boilers, if it happens to your brass boiler well chemically speaking congratulations you now own a copper boiler... what a turn up for the books. 

[Shed Driver]

Full size boiler fittings are not brass they are bronze  Copper/Tin

[Yarravalleymodeller]

2 hours ago, Shed Driver said:

Full size boiler fittings are not brass they are bronze  Copper/Tin

Indeed, but you can see this dezincification in brass fittings, any fancy bits exposed to heat. 

 

You can also experience a similar tin loss in bronze. 

 

Point being you are not going to encounter "zinc pest" that is an entirely different metallurgical phenomena limited to only some alloys with zinc as component. 

[RedgateModels]

On 26/06/2022 at 18:35, 47137 said:

Pretty much exactly 42 years ago I completed a simple traction engine with a reciprocating cylinder. This was the practical part of A level metalwork (EWTP), I was allowed to do this but not the theory lessons because they clashed with physics!

 

The boiler is a length of copper tube. The ends I cut from an offcut of the same tube, flattened out and flanged over some kind of former (forgotten what - probably a round bit of wood) to make two caps which fitted snugly into the tube. This all silver soldered together and then pressure tested with water i.e. a hydraulic test. I misread the pressure gauge and took mine to 150 psi, should have been 50 psi but nothing went amiss. Imagine being allowed to do this sort of thing without supervision in a school nowadays! There is a boss for the safety valve and I recall this had a bolt inserted in place of the safety valve for the pressure test. Outside is a larger tube made from tinplate to take the paint.

 

- Richard.

 Pretty much my O level metalwork project too. Ours were tested on live steam, with us crouched behind the breeze block wall around the forge area for "safety" 😮. I'll grab a snap this evening if I remember .....

[didcot]

We run demin water systems at work and brass fittings are banned from use. It has to be stainless steel. Zinc free solders are also required too.

[ikcdab]

Just about on topic.  The latest blondihacks video has an interesting discussion about boiler materials.

 

https://youtu.be/yiAWsopdr0A

 

 Ian