Small Steel Boilers

[Gordon WParticipant @gordonw]

I know nothing about model boiler regs. For full size boilers the plate sizes are quite easy to calculate stress wise. Gets complicated with different codes from around the world, eg allowance for corrosion, on inside and outside. I guess the same allowances will have to be made on little ones, percentage wise this must get tricky.

[John Allan Watson BrownParticipant @johnallanwatsonbrown]

Has anybody seen the article in engineering in miniature, July 2011 issue page 20 by David Beale? Steel boiler made from certified fittings. It is a rather large scale boiler, of an approved design from the Steam Boat Association (SBA). I have used the steel butt weld fittings on ships pipe work but for a boiler, as the threads have stated a qualified welder is required to comply with the inspector.

[Speedy Builder5Participant @speedybuilder5]

Stainless steel – what is the truth in this material ? Some web sites say don’t use it as you will get boiler failure at the steam / water interface caused by intercrystaline corrosion. Yet an advertiser in ME will sell SS boilers with copper tubes. Surely these must be a combination of SS welding and silver solder. What do our experts say ?

[JasonBModerator @jasonb]

Robert The advert is a bit misleading I think as it says

 

“Copper & Steel Tig welded boilers” but just says “stainless steel” against a vestle. The one on teh left of the words Stainless is a mild steel boiler with expanded copper tubes.

 

If you look at the Steam Technology (Steam tec make the boilers for Maxitrak)website under products they say the do “water containers” in Stainless not boilers

 

http://www.steam-technology.co.uk/

 

Steam Tec are actually a commercial boiler/pressure vestle makers and thats just the name they use for their model products, main site is

 

http://www.welding-innovations.co.uk/

 

J

 

PS Having said that Stainless boilers are common on mainland Europe

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

[Alf JonesParticipant @alfjones33318]

Hello.

 

My apologies for re-awakening a dormant thread, but I have read this thread, and others, and I am still none the wiser.

 

As I understand it, most of mainland Europe, and the US as well, use steel for boilers. I also believe that the vast majority of pressure vessels made at 1:1 scale, including boilers far in excess of any specification we have for at model scale, are also steel or stainless.

 

So I’m not sure what is special about model steam in the UK where we insist on Copper?

 

Some arguments have been put forward in this thread that I’m not sure are correct, such as time to raise steam – back of the envelope calculations suggest ….. to me at least…. that the time difference will be minimal.

Some line up better, such as wastage allowances, but I don’t see why that can’t be built in – the process is well known and calculations are available.If a design calls for 3mm copper, I wouldn’t object to using 4mm or 5mm steel instead. It’ll likely work out cheaper in materials.

 

If I can fit the boiler into scale, and it’s safe, I don’t really understand the problem.With Stainless, I don’t believe that the issue is as significant as is being made out. Stainless is used, and has been used for years, by the flash steam world at vastly higher temps ( 700 degrees + ) and pressures ( 2000psi + ) for seasons on end with no sign of any issues despite regular testing.

 

The welding can be trickier, but the world HAS moved on from the times of LBSC, no matter if sometimes I wish it hadn’t. Welding technology has moved on hugely, and there are a lot more competent Stainless welders these days, I think partly because the systems that have make high quality stainless welds easier to make. Parts that would have previously been made from formed sheet can now be CNC’d out of solid. In all, there are a lot more options available.

 

For myself personally, producing a steel boiler would be a lot cheaper, and likely as easy, if not easier, than producing a copper boiler, especially at large sizes. Stainless would add to the time and cost, but again from a purely personal point of view, I believe I can still produce/have produced, a stainless boiler at the same cost as copper which will be many times more resilient in every way to a copper boiler.

 

There is a separate case of the Insurance companies, but I believe that if the welds are signed off and the pressure tests passed, Insurance will be given.

 

So I get to the point where, if I was undertaking building a largish boiler which would work quite hard, and I had to choose between a copper/silver solder boiler and a stainless boiler, or even just an alloy steel boiler, then I personally would feel far more comfortable with a steel boiler.

 

Given that, and given that most of the rest of the world uses steel, I *STILL* don’t understand the resistance to it? It almost feels like us UK model engineers are looking for reasons, any reasons, not to make a move to it.

 

Please don’t take this thread as a rant, it’s not. It’s just me being confused, and looking to understand the situation better.

 

 

 

 

 

 

 

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

[DustyParticipant @dusty]

The bottom line is that the vast majority of model engineers are not competent to weld a steel boiler. Meaning that they are not coded welders, with the exeption of one or two club boiler inspectors are neither capable or willing to undertake the inspection of a welded steel boiler. Insurance companies would not insure boilers that were not produced by coded welders and almost certainly not those inspected by club boiler inspectors who did not have the relevant qualifications.

You are going to pay for both the welding and the inspection by proffesionaly qualified people. These impinge on the cost and make a home made copper boiler not such an expensive item in relation to a welded steel boiler. If we are talking about proffesionaly made copper boilers that is a different scenario.

[JasonBModerator @jasonb]

“but I don’t see why that can’t be built in – the process is well known and calculations are available.If a design calls for 3mm copper, I wouldn’t object to using 4mm or 5mm steel instead. It’ll likely work out cheaper in materials.”

 

3mm copper would more than liekly need 6mm steel for a start.

 

As I’ve said earlier your average 3 1/2g loco or 2″ traction engine with an 3mm copper boiler has say 10mm gap between the outer wrapper and the firebox. If you the change to steel you will either end up with an impractical 4mm water space or to keep the water spave the same reduce the grate area by 6mm all round so on a grate area on a copper boiler of 4″ x4″ = 16 sq inch you would end up with agrate in steel of 3.5″x3.5″ = 12.25″ which is quite a loss of area.

 

On larger models like 71/4g or 3″ plus traction engines then steel becomes a better bet but this thread was about SMALL boilers.

 

As Dusty says its relatively easy to get a copper boiler tested at club level, some will also do steel but if they don’t you could be looking a couple of hundread quid a year which will soon eat into any saving on materials. Stainless would definately have to be done by a professional inspector in the UK.

 

J

[Chris GunnParticipant @chrisgunn36534]

Did anyone visit the Midlands ME Exhibition and see the all steel boiler for a steam launch, made without a weld or braze anywhere, it was all bolted construction ? Of course I know it is easier to do this in this type of boiler when one does not have the constraint of trying to fit the boiler into a locomotive chassis. I have just built a welded vertical steel boiler with copper tubes for my Donkey engine, which was very easy to build, mainly because of the layout. I still say that it would be hard to build a small, 3.5 or 5″ gauge traditional locomotive boiler in steel, and weld it, and as cost was one of the major factors raised in the original post, would all the new techniques be available to the amateur?

Chris Gunn

[Richard ParsonsParticipant @richardparsons61721]

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’

[Alf JonesParticipant @alfjones33318]

Posted by JasonB on 31/10/2011 16:48:54:

“but I don’t see why that can’t be built in – the process is well known and calculations are available.If a design calls for 3mm copper, I wouldn’t object to using 4mm or 5mm steel instead. It’ll likely work out cheaper in materials.”

 

[quote] 

3mm copper would more than liekly need 6mm steel for a start. [/quote]

 

This is the kind of information I was looking for. Please could you provide more information about this calculation? I had assumed that a steel would require less material than copper, before a wastage allowance was built in, assuming the use of mild steel. If it was a mild steel, I believe that the assumption is 7% per year ( compunding) as long as the boiler is kept full at all times. It looks like you have a different figure – please could you point me in the direction of your source so I can read up on it?

 

[quote]

As I’ve said earlier your average 3 1/2g loco or 2″ traction engine with an 3mm copper boiler has say 10mm gap between the outer wrapper and the firebox. If you the change to steel you will either end up with an impractical 4mm water space or to keep the water spave the same reduce the grate area by 6mm all round so on a grate area on a copper boiler of 4″ x4″ = 16 sq inch you would end up with agrate in steel of 3.5″x3.5″ = 12.25″ which is quite a loss of area

 

I wonder if this is an argument based on scale appearance rather than boiler construction? My thinking is along the lines of a free-lance design, or maybe semi-freelance, so for me this wouldn’t be a major issue I think.

 

As Dusty says its relatively easy to get a copper boiler tested at club level, some will also do steel but if they don’t you could be looking a couple of hundread quid a year which will soon eat into any saving on materials. Stainless would definately have to be done by a professional inspector in the UK.

 

J

 

Right, now we’re getting somewhere. If thats the main issue then for me thats an easy thing to sort out. Remembering that I posed my question relevant to me, then this seems to solve it.

I do have access to a chap who is fully certified for steel, stainless and also aluminium welding ( I’m assuming thats a completely no go zone?? ) for use with pressure vessels. He also does work with such dark arts as stir welding ( which I understand ) and electron beam welding ( which I really don’t ) Via him, stainless steel falls from the proverbial heavens ( or more accurately the most astonishingly large and wasteful “scrap” bin you have ever seen. 6 foot rods in the scrap bin…..! ) and his firm will also provide test certification for not a very significant fee.

 

Thats why, for me, a steel boiler is so appealing – the cost is so low.

 

I understand this obviously isnt the case for everyone – I’m very lucky to be in this position – but I may as well take advantage of it if I can – that money could be far better spent at Tracy Tools or Chronos.

 

 

 

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

[Hugh GilhespieParticipant @hughgilhespie56163]

Hi Alf,
My information is a bit out of date but and my memory is also suspect but as far as I recall, the ASME codes for steel steam piping require about a 10% thickness increase to allow for corrosion in service. There is also another, smaller allowance for toleraance on wall thickness.
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, that is it is fairly impervious and also has similar thermal expansion properties to the base metal so it doesn’t tend to spall off with thermal cycling. However, with excess oxygen in the feed water, very localised corrosion cells can be created that lead to deep pinholes being formed and eventual failure.
With ‘proper’ boilers, the feed water is always treated to remove dissolved oxygen. A device called a deaerator is used to scrub the dissolved oxygen from the water with low pressure steam and almost always a chemical oxygen scavenger is added, used to be hydrazine but less toxic chemicals are more common now. An important part of the commissioning procedure for high pressure boilers is the magnetite formation boil-out. Basically the boiler is fired with treated water, usually to give about 150 psi and the pressure maintained for 24 hours to get a good film of magnetite on the metal surface.
Unless you intend to operate the boiler over 500 C, then low carbon steel, A106 or similar, will be just fine. If you do want to go higher – unlikely but who knows, the grade known as P22. 0.5% Chromium, 2.25% Molybdenum is good to 560 C – from memory.
I’m not sure how relevant any of this is to model boilers as the corrosion processes are very slow and I think it rather unlikely that any model boiler would be operated for the 1000’s of hours that would be needed to cause serious corrosion. Probably a much greater risk would be from general rusting when the boiler was sitting idle but that can be dealt with very easily. Either store it very dry or keep it full with water with a couple of percent of washing soda dissolved, provided you keep the pH above about 10 you won’t get any rusting.
Regards, Hugh

[SpringbokParticipant @springbok]

Wayne Bell (reluctantly) made 2 of 7.5 scale boilers for me but told me that he would not make anything smaller.

If you do go down the road of steel be prepared to get an engine hoist or a double hernia. Seriosly studying the boilers I could not envisage anything smaller being made Wayne’s welding is superb and I am sure that there are few specialist in this field who could emulate him.. Every bit of steel had to be documented and sources named.

The paperwork alone came to about 30 pages per boiler.

I would concur with what is said above go copper it is you best if not only option.

 

Bob

[JasonBModerator @jasonb]

I’ll dig the formula out tonight, and don’t forget the fire will also waste the metal so its not just from the wet side you need to allow wastage.

 

As an back to back comparison John Haining in his countryman’s SteamManual gives drawings for the Durham & N. Yorks traction engine in both steel & copper

 

wrapper 0.212″ steel, 0.125″ copper

firebox 0.192 steel, 0.093 copper

 

Now as its hard to get imperial plate (.212″ = 5SWG) the nearest metric size up is 6.0mm

 

The other thing as well as wastage that can affect the tubeplates is if you are using expanded in tubes particularly copper then you can’t go too thin on the plate as you won’t get the seal, the makers of the expanders give min thickness depending on tube size.

 

The grate/water space is still relevant weather scale or freelance as you just end up with a larger boiler or less grate for a given width & length.

 

If you can get it made, tested and CE marked at good price then go for it. Though you will likely have to submit your design surance co for them to have your calcs checked before they will insure as its not an established design.

[Richard ParsonsParticipant @richardparsons61721]

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

[Sub MandrelParticipant @submandrel]

Well done for mentioning LBSC Dick,

 

In teh Roedean articles he said ‘steel is fine in full size where 1/16 of rust wouldn’t matter but on this boiler you’d have a hole right through’. we have to accept that steel rusts at the same rate regardless of boiler diameter. It’s linear vs suare vs cube, or as LBSC put it many times ‘you can’t scale nature’.

 

Using exotic steels that need even more advanced joining techniques deafeats the cost argument.

 

The Original poster mentioned ‘Tich’ as an example. I made a small vertical boiler no smaller than a tich (but with simpler tubes). the cost of the materials and silver solder was a fraction of the cost of a full set of Tich castings, so for the original questions about SMALL boilers the cost argument doesn’t hold up.

 

That said, I am sure there is amiddle ground, probably a large 3 1/2″ boiler or a 5″ one, where a welded briggs type boiler (as per the aussie code) is probably cheaper and easier to construct. But you do need to be a better welder than me.

 

Welding and silver soldering are both arts, but I think its easier to get a safe, successful silver solder, even if it looks a bit messy, as you just need to get the joint full of solder. With welding you don’t know the penetration and quality of the join without cutting it – hence the need for a qualified welder with a proven technique. Also apoor solder joint that still passes a pressure test won’t degrade significantly in use. A marginal weld that allows a little water penetration could degrade very quickly in use.

 

Neil

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dick, magnetite is a black mineral of iron oxide, Fe3O4 often occurring with titanium or magnesium, and an important ore of iron. A magnetically polarised piece of this mineral is called a lodestone. Well that’s the way my dictionary describes it.

 

Many years ago the company I used to work for, used to add a portion of it into the mix when making resin coated foundry sand. It looked a bit like black sand, but was quite heavy for its volume, I believe it used to be mined in Russia. It would react near a magnet in just the same way as iron fillings do, but more dense, but the magnetic flux was clearly visable. Never knew why it was added to the sand, but before they used that, they used mill scale, and after magnetite was either too pricey or unavailable, they used some other stuff of a similar nature but I can’t remember what that was.

 

I don’t think it is the same as the oxide you get on hot rolled steel.

 

Regards Nick.

[Kerrin GalvinParticipant @kerringalvin72662]

Hi All,
It has been mentioned, the Austrailian Model Engineers have 3 boiler codes, small boilers (gauge 1ish size), copper boilers upto approx 8″, steel boilers a bit bigger. Plus another which must be nearly out of draft for stainless steel. These are available for purchase from the AMBSC web site.
For those that might like to see a 7 1/4 stainless steel boiler, oh & the constuction of a Darjeeling B class check out. http://www.rsmenz.com look under news & events. My understanding is that this boiler is the first to be built to the draft code.

[Hugh GilhespieParticipant @hughgilhespie56163]

Hi,

Once again a content warning! My brain is slowly but surely leaking away all the information it used to have, and as all this is based on stuff I did 20 years ago it cannot be considered reliable!

Next a couple of corrections, senior moments in my earlier post. I got my chromium and molybdenum the wrong way round, the correct figures for ASTM A335 Grade P22 is 2.25% chromium and 1 % molybdenum.

Finally, for some fairly gentle information on magnetite films in boilers, this is a good link.

http://www.angelfire.com/mt/chemicalcleaning/magnetite.html

Regards, Hugh

[Neil WyattModerator @neilwyatt]

I am sure there have been reports on this forum of brass boiler fittings failing due to de-zincification.

Neil

[John FieldingParticipant @johnfielding34086]

I think some folks are missing an important point regarding the differences between steel and copper for boilers. The need to make steel boiler shells thicker is not so much to do with the ultimate tensile strength but more to do with potential pitting and corrosion. Normally water treatment plants are required to ensure the water is of an acceptable standard and often chemicals are added to adjust the local water supply to that which will give the lowest corrosion.

The other point is the tensile strength of steel versus copper. Although for a given thickness of boiler shell wall thickness steel is better than copper the elastic limits are quite different. Steel has a lower ability to "swell" under pressure, which is good as it can withstand higher pressure than copper of the same wall thickness. But copper has a greater elastic limit and can balloon safely under high temperatures and to some extent return to its original form after an over pressure event. Steel on the other hand resists swelling better but once it reaches it ultimate limit tends to rupture.

Now there is also a huge difference between mild steel tube bought for structural and other uses and genuine steam pipe and boiler shell duty. Most steel pipes are rolled and seam welded, which is a potential weak point in the circumference when internal pressure is applied. Genuine boiler tubes and casings are seamless drawn and hence much more expensive. There are nowadays not many steel manufacturers who can supply genuine steam pipes and boiler shell material. I am only aware of one in Europe, in Germany, who can supply genuine steam boiler tubes etc. There may be more but I haven't come across them. I know for a fact that the locos down in the depot locally import their boiler tubes from Germany and they are ten times the cost of locally made welded seam tube.

Welding of pressure vessels is a highly skilled job and in this country (South Africa) may only be undertaken by government certified coded welders, who are few and far between with the necessary ticket. I have two friends who are coded welders, one has the top certification possible the other is one step below so he cannot tackle all the jobs and has to pass on them.

Steel when subjected to repetitive heating and cooling work hardens and stress fractures can appear, it needs to be a very special grade to withstand this arduous duty. Vickers Steel in the old days was the only British steel maker who could make the required plate and tubes. Riveted boiler construction obviates the need for welding but in some cases it cannot be avoided when repairs have to be made.

Lastly, as others have mentioned, copper will outlast steel in boiler applications by a large amount, so if you want to make it and forget it then copper is the way to go. Annual boiler tests will have in most cases to be done but a well constructed copper boiler should have no difficulty in lasting 30 years or more as long if it has been handled correctly and not allowed to run low on water.

My 2c worth!

[Author]

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