Small Steel Boilers

I think the reason for the lack of steel designs is simple. It is ‘Tradition’.

As K.N Harris wrote in his book “Model Boilers & Boilermaking” about steel boilers the making he talks about the difficulty of flanging plates etc in mild steel, which he likened to a blacksmithing job (which in 3mm steel, it can well be). I think here that he was imagining that mild steel boilers would be made in the same way as copper boilers. That is by forming or flanging the boiler plates from sheet.

Edited By Richard Parsons on 24/08/2011 17:40:51

Edited By JasonB on 24/08/2011 18:24:49

Edited By JasonB on 25/08/2011 19:35:54

Edited By Alf Jones on 31/10/2011 10:43:26

Many are missing the point which is that a suitable mild (non stainless) steel is some 2 ½ times the tensile strength of copper. It seems to me that the use of copper is entrenched with a 10,000 reasons (mostly spurious) as to why it should must be used. These arguments are of the type used against the use of the Jet engine in the early 30s. One of these was that without ‘prop wash’ an aircraft would never get its tail off the ground. I often look up at airliners flying over head and wonder how they get their tails off the ground.

Everyone is worried about ‘Wastage’. That is rust! I hope someone here will help me on this one. As I understand rusting occurs in by atmospheric oxygen attacking the Iron in steel changing it into one of the oxides of iron. These oxides dissolve in water to form hydroxides, which in turn assist further rusting.

To stop this from happening the easiest way is to exclude the atmospheric oxygen from contact with the steel. In one of my earlier posts in this thread I suggested using paint, in particular engine paint. This stuff I use on my little motor bicycle cylinder and exhaust lasts about 3 years. I ride the thing about 3 to 4 times a week for up to 1 hour per journey. These paints are guaranteed up to 500°F (343°C) and at 80 Psi the temperature of wet steam would be 156ºC. This is about half of the guaranteed maximum for the paint. I would NOT repeat Not use plating it can penetrates up to 0.635mm or more into the surface layers of steel and seems to delaminate at about 200°C. I also have seen silicone materials being used by silver smiths making vacuum castings with an gas air flame being player straight on the silicone without any obvious effect.

The alternative is to paint (or as a supplement to paint) is a good water treatment JasonB has a link to such a product.

Everyone is on about welding. But as with a copper boiler a hard braze can be used and it has the 80% of the strength of the mother metal and it will reveal leaks in the same way as silver solder and is easier to see.

M. Chapelon showed that the heating surface contained in the fire tubes contributed little to the boiler's capacity for raising steam. His master work 241.A.1’ which it is said could produce in excess of 4,000 Horsepower continuously. The French engine used around 20% less coal and around 40% less water than a comparable British engine. Perhaps someone should look again at our boiler design especially if he wants to win the ILMEC.

Dave if you want ‘Musings on Boilers’ I will send it to you but without the illustration of M. Chapelon. master work No ‘241-A65’

Edited By Alf Jones on 31/10/2011 22:15:00

Hugh. Thank you for your remarks. In which you write “As far as boiler corrosion is concerned, the risk is not so much from general surface corrosion but from deep pits. The pitting is likely when the feed water contains dissolved oxygen - the actual metal surface in a boiler that operates at over about 140 C has a magnetite oxide film that is highly protective and behaves in a similar way to the protective oxide films on aluminium and stainless steels”

This has set me thinking and I have 4 questions, which are:-

1. 1. 1. When a magnetite oxide film is formed is it stable at temperatures below 140°C.

2. T2. Those boiler plans I have looked at these boilers generally work at 80 to 100 PSI. (162ºC to 170 ºC). Would this be enough to form the magnetite oxide film?

3. I3. Is this the same film you find on black iron (hot rolled mild steel)?

4. I4. Is the formation of ‘pinholes due to the surface finish or because of ‘non-homologous’ (impurities). I have noticed that highly polished surfaces tend to be more resistant to rusting.

I think you saw what I saw about the protection of mild steel and its use. I feel that most locomotives run for only a few hours per year. As you point out if the boiler were filled with pre-boiled water a thing which an old friend of mine used to do _until he was ‘ousted’ from his club. He wanted to drive his locomotive but only ‘Senior Members’ were allowed to do this. If the boiler is dryed after use it should not rust

May be I have thrown a rock into the pond but it is about time that the wisdom of Old LBSC and others was re examined.

Best regards

Dick Parsons

Edited By Richard Parsons on 01/11/2011 12:34:27