Small Steel Boilers

[Dave BondParticipant @davebond87219]

With advances in welding and the cost of copper, why is it that steel boilers are only made for larger scales?

So for example something the size of a sweet pea boiler.

Edited By Dave Bond on 18/04/2011 12:48:31

[Dave BondParticipant @davebond87219]

[JasonBModerator @jasonb]

Three reasons

 

1. You need to use much thicker wall tube and plate to allow for wasting (corrosion) so where 3mm copper may do the job you would need 5 or 6mm steel. This increase in thickness reduces the capacity of the boiler and if you are going to keep the same amount of water around the firebox you will end up with a smaller grate area.

 

2. Copper is a better conductor of heat which is an advantage in the smalkler sizes

 

3. You can make a copper boiler yourself but a steel one really needs to be professionally welded

 

Jason

[Dave BondParticipant @davebond87219]

So accepting the above how come you don’t seem to be able to get hold of a small steel boiler. I realize the disadvantages, but the offset against cost I imagine would make them increasingly common?

Even professional steel boiler makers are not interested in making small boilers.

[KWILParticipant @kwil]

It is the same with any business, if there is insufficient demand at a price that makes the production worth while (ie profitable) then there will be no willing suppliers.

 

As to copper (and the necessary silver solder), yes it is getting more expensive, but so are all the other items we use, so is this a hobby you can no longer afford?

[Dave BondParticipant @davebond87219]

” As to copper (and the necessary silver solder), yes it is getting more expensive, but so are all the other items we use, so is this a hobby you can no longer afford? “

 

That is the wrong attitude, the hobby is not just for those that have lots of money, many people have done a lot with very little. As for this topic it was a question as to why it is not more common and the reasoning behind this. All part of knowledge gathering and the exchange if ideas.

 

The hobby is accessible to everyone, clubs have tools and machinery and if you ask around many companies chuck out machines and metal offcuts. My last project a GWR railcar only cost £100 because of the variety of metal offcuts and bits that companies are chucking out. The lathe I am currently using was a cast off from industry and, I have recently offered a colechester lathe free to a good home in the same spirit.

 

For anyone who cannot afford a copper boiler or castings, can build an electric or IC model for a fraction of the price. Or going even further with limited club machinery could build a small stationary engine. Anyone regardless of ability or money can make a start in the hobby.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dave, as far as costs go, you are not taking all of Jasons points into further consideration. OK to build a boiler out of steel may well be cheaper for the material side of things, but consider the point about having to have a larger section of steel to copper and then heating the same amount of water in it. Firstly you will will find the boiler will have to be bigger and that may well put it out of proportion to the model you are building, and secoundly, you will find the amount of energy to raise the water to boiling point will be getting close to all most double that of a copper boiler, because you will have to put energy into the boiler itself as well as the water and a steel boiler having the same capacity of copper with the added thickness that Jason has pointed out will require more energy input.

 

Consider (and I’m exaggerating here a bit) you turning up a a club event with your steam model and you all start firing up your boilers of the same size, but yours is the only steel one, and by the time you have raised your steam every body else has started to pack up to go home. So why would a manufacturer produce boilers that nobody had any use for. You need to take running costs into consideration as well as construction cost to evaluate whether it is cheaper or not.

 

Regards Nick.

 

[JasonBModerator @jasonb]

Oh and the forth reason for not using steel in the smaller sizes or any other for that matter is the life expectancy. Copper if looked after will last a lifetime but steel you may well be replacing tubes in 6-8 years and the barrel in maybe 20-30. So if you said you were steaming for 40years then one copper boiler at todays price will be cheaper than one steel one now, another in 20yrs and two retubes.

 

There is also the cost of boiler treatment which is really needed with a steel boiler.

 

Also take into account the fact that someone will have to produce a new drawing, do the calcs and get the design approved for a steel version of any copper boiler but why go to all that trouble for something that will be less efficient and have a limited lifespan.

 

Its a no brainer really, if the only reason you can come up with is initial cost then that is not what the majority seem to go for so therefore no market for small steel boilers.

 

J

[Chris GunnParticipant @chrisgunn36534]

I would say that maybe the fifth reason against small steel boilers is the difficulty in constructing a 5″ loco boiler in steel, welding the boiler in some of the nooks and crannies, and expanding small tubes in a confined space would be just 2 problems, as well as sourcing the materials. I know someone will have done it, but it is not for the inexperienced.

Chris Gunn

[Hugh GilhespieParticipant @hughgilhespie56163]

I don’t know about the construction issues of copper versus steel for boilers but the heat capacity of the metal in a steel boiler wouldn’t be an issue. The relevant values , in kJ/kg k are 0.49 for steel against 0.39 for copper. This is insignificant compared to the heat capacity for the water in the boiler at about 4.2 in the same units. However, the main heat flow required is that needed to turn the water into steam. The energy of vapourisation for steam at about 7 bar is about 2060 kJ/kg. The differences in the thermal conductivity of steel and copper are more important, copper is about 8 times as conductive as steel so for a given rate of steam production under the same heat input conditions and the same shell thickness, the area of the steel boiler would need to be 8 times as large. If the steel boiler shell also needs to be thicker then an even larger heated area would be needed.

[John OlsenParticipant @johnolsen79199]

Practically, the conductivity of the metal makes little difference in boilers. This is because the biggest problem is getting the heat out of the hot gas into the metal, rather than getting it through the metal. Think of the difference in temperature betwen the flue gas and the metal, and compare with the difference in temperature from one side of the metal to the other. Not easy to measure the latter, but it will be pretty small (Some of the heat actually gets to the metal by direct radiation from the hot fire rather than by conduction from the gas, but the point is still valid.)

 

The heat capacity makes no difference at all when the plant is running in a steady state, and is pretty small compared to the amount of energy involved.

 

Corrosion is the main determinant, it sets a lower limit to the thickness of steel that it is reasonable to use. There has to be enough extra thickness for the corrosion to be found by inspection before it is too serious, and since the rate of corrosion does not depend on the size, the allowance makes the thickness of plate for a small boiler pretty unwieldy.

 

regards

John

[Peter GardnerParticipant @petergardner86280]

Practice in USA is now almost universally to use steel boilers with copper tubes.

For 7 1/4 G locomotives and above, the boiler shell thickness is about the same for copper and for steel due to the greater strength and corrosion allowence factors cancelling out, ie shell thickness of about 3/16 inch in either case. Having recently seen quite a few copper boilers failing at 10 to 15 years, I’m not sure that longevity is neccessarily on the side of copper. In hard water areas, unless a rigorous approach is taken to descaling, overheating of tubes, particularly at the firebox ends, can cause silver soldered joints to fail which are difficult if not impossible to repair. With regular descaling and routine water treatment a steel boiler would last at least as well. The main problem is finding a manufacturer of small scale steel boilers. Google has failed to find anyone still in the business. Does anyone out there know of a CE approved manufacturer?

Peter Gardner

[Richard ParsonsParticipant @richardparsons61721]

John Olsen How right you are! Heat transfer is the biggest problem.

Steel boilers do not have to be welded! They can be silver brazed or hard brazed using the same methods as as for copper. Hard brazed! I hear the Gnomes of Elfin Safety shrieking, brass looses zinc in hot water. Tripe! de-zincification can be prevented by adding 0.3% arsenic to then brass and any way most brazing rods are guaranteed against this problem.

The steel boilers can be protected against rusting by the use of a good engine cylinder paint (these are proof to 500°F – 343°C). Sacrificial anodes are also very effective providing the boiler is kept full of de-aerated water (done by warming the stuff). De-aeration is why ordinary brass is used in house-hold plumbing

Do not try to braze stainless steel as this is a receipt for disaster. Stainless steel covers its self almost instantly with an oxide film so molecular joints (like brazing) do not have any strength to them.

Stainless pressure vessels require very special treatment and highly skilled welders. This is because of chrome leaching out of the matrix.

Do not try to galvanise the boiler or use galvanise steel. The surface plating de-laminates at about 200°C and the deposition into the surface of the steel cause’s serious damage to the steel’s structure.

[michael howarth 1Participant @michaelhowarth1]

Richard, can you give us a bit more on this dezincification business. There are lots of references to it in the books and magazines that I read but no real explanation other than zinc disappears from brass when the brass is heated. Details of the exact conditions under which this occurs or is likely to occur are never given and no one ever seems to have personal knowledge of an instance of brass failure due to dezincification. There seems to be a great deal of mystery attached to this phenomenon and the general attitude seems to be “it happens and that is good enough for you”.

 

I asked a local boiler inspector about this once and he became very defensive when queried about his experience of dezincification but admitted that he had never known of a boiler explosion or whatever attributed to it. He did know of a fatal case however. It happened on a warship in the American civil war…….I think the brass prop shaft gland failed and it sank….confederate I believe.

 

Any information on this subject welcome.

[michael howarth 1Participant @michaelhowarth1]

Further to the above, many model locomotives were fitted with brass boilers,certainly up to about 1950 and some are still running I believe. I also seem to remember one or two more recent designs by Tubal Cain specifying brass. Were these models really so disastrous or dangerous?

[JasonBModerator @jasonb]

Peter there are several steel boiler makes

 

Bell Boilers- Wayne Bell

 

AJB Engineering – Tony Baldwin

 

Gloucester steam

 

Steam Technology also do steel boilers

Richard painting a boiler is unlikely to prevent it wasting from the inside and just heating the water is not ideal, a proper boiler water treatment is the best way to prolong the life of a steel boiler something like Multitreat TS for steel or their TC for copper

 

I’m also fairly sure that a brazed or silver soldered boiler would not pass a boiler test here but I could be proved wrong

 

Jason

[Clive HartlandParticipant @clivehartland94829]

The only de-zincification of brass I ever came across was in Rifle cartridges, this usually occurs over long periods of storage.

When found the whole ‘lot’ are condemned.

The de-zincification causes the cases to seperate when fired and can leave the bell mouth of the case stuck in the breech.

It usually shows visually as a dark discolouration on the outside of the case but often only happens when fired.

I do believe sulphur in the water can also start de-zincification of brass parts.

 

Clive

[John BaguleyParticipant @johnbaguley78655]

My brother and I regularly run a 5″ gauge loco based on an American Forney. This has a steel boiler and it steams like a witch. It takes no longer to ‘brew up’ than any equivalent sized copper boiler. As mentioned before, the conductivity of the boiler material makes little, if any, difference. I’ve also noticed that the boiler pressure is very stable, unlike some copper boilers, possibly due to the greater mass of hot metal in the steel boiler.

 

The loco was bought from a workshop clearance for a song purely because it had a steel boiler. It had stood for at least 20 years, had no paperwork, and so had to be considered possible scrap. We removed the boiler and took it to the club for an inspection and a test which it passed with no problem at all. All the steel is 6mm thick and all the welding looks of a professional standard. The thing weighs a ton! One of our inspectors tests gas tanks for a living and happens to have an ultrasonic tester which showed no sign of thinning of the steel. The only problem was a very slight weep from one of the copper tubes which was cured with Loctite.

 

Funny enough, also in the same workshop clearance was a steel boiler for a Sweet Pea. That looked professionally made and never been used.

 

I think for a 5″ gauge wide firebox loco, steel is a definite alternative to copper. Probably not for a narrow firebox job though where the thickness of the steel will affect the grate area adversely.

 

With regard to dezincification – I’ve found a few cases on fittings when working on older loco boilers. One steam valve snapped clean off with little effort when I tried to remove it from the turret. A similar thing occurred with a brass blowdown valve. I always use bronze or gunmetal for any fitting that screws into a boiler. Brass should be fine for anything outside the boiler.

 

John

[Ian S CParticipant @iansc]

I would have thought that as far as heating is concerned, a steel boiler with copper tubes would be the better boiler, the heating is from the inside, ans he steel (not sure of wording here)will tend to insulate the boiler slowing radiation of heat from the shell.

Amoung the hot air fraternity there are some that think they should use copper and aluminium for the hot bits, where as the way to go is use stainless steel, the heat’s only got a few thou to get to the inside, and the other metals can’t take the heat with out corrosion. Sorry wandering a bit. I use TIG welded stainless at red heat for long periods of time, but thats with out water, I’m still interested in reading of stainless boilers. Ian S C

[JasonBModerator @jasonb]

Its not just the tubes you get the heat from, there is a lot a radiant heat through the sides and crown of the firebox into the surrounding water space, thats why the thicker steel reduces this waterspace or the grate area and copper can be better on smaller boilers.

Your insulating properties of steel would also prevent radiant heat through the sides/crown from the fire and likely cancel out any insulating of teh outer surface.

 

J

[Richard ParsonsParticipant @richardparsons61721]

Steel boilers do not have to be welded, they can be brazed. A properly designed and competently made braze has 80 – 90% of the strength of a weld. Anyone who can make a good silver braze can make a good braze (if they have the heat available). There were even low melting brazing rods (old Curly mentions one in Part2 of Roedean one Sifbronze ). Brazing was often used in motor car body repair shops as it did not tend to change the structure of the mother metals. There is a good PDF on this website on the design of brazed joints here. I have shortened the address because I understand long hyperlinks cause trouble. Dezincification occurs because zinc in the presence of water forms Zinc Hydroxide which is soluble. But that is not the whole story. Zinc first has to form Zinc Oxide which it does in air (this protects the brass alloy. But in water especially hot water Zinc Oxide (which is insoluble) reacts with the water and forms the much more soluble Zinc Hydroxide. Remove the oxygen (or air) and all is well. There is an interesting item on this website http://www.hghouston.com/coppers/brass75.htm on the topic.

The practice of brazing car bits was outlawed by the Gnomes of Elfin Safety partly because of poorly designed joints and because of dezincification.

The gnomes of Elfin Safety learned about dezincification or Condenseritis, which was something which plagued ships of the Royal Navy but not the German ships at the turn of the last centenary. The addition of 0.03% Arsenic which occurred naturally in German copper ores was the cure (I wrote 0.3% in my first posting I was wrong). Your local (unfriendly) gnome only knows the FAQS and not the full story.

JasonB. Painting the boiler yes but you paint the insides of the boiler. I have written a ‘Thought Piece’ on boilers and boiler making but I have never submitted it to Model Engineer because it needs a picture of an ‘241-A’ class Locomotives made to M. Chapelon’s design. There is still one preserved at Verine in Switzerland you will see a picture at http://www.241a65.ch/ .

If you want to find out how to paint the inside of a boiler look have a look at the ‘Rotordip system’ which was used on car bodies. It involves degreasing, your ‘treatment’ –phosphoric acid- etc and paint.

Ian SC In the unpublished ‘Musings on Boilers’ I have written a little about welding stainless. I have a friend over here in Hungary who makes stainless steel welded pressure vessels. His people have to take scrapings of the welds and send them off for analysis. You will find a brief description of the problem at this site http://www.brewingtechniques.com/library/backissues/issue2.6/palmer/html .
I am sorry it is so long

[Dave BondParticipant @davebond87219]

So with all this in mind, is it possible to construct your own steel boiler and gain a boiler certificate without the need to get CE certified etc.

 

Also with regard to copper tubes or a copper firebox in a steel boiler, would the different expansion rates of the materials not cause issues with brazed joints?

[JasonBModerator @jasonb]

It mainly comes down to being a “competant welder” so you either have to submit test joints to the boiler inspector or have the relevant “codes” to prove your competance. This would certainly have to be done through an independant boiler inspector as I doubt any club inspector would test your welding. You also need to have the paper trail for all materials used.

 

Have a look at page 18 of this document.

 

Dave a steel boiler with copper tubes will not have them brazed in they will have been expanded into the tubeplate. but it would be advisable not to bring the booiler upto temp too fast as a powerfull plower may heat the tubes faster than teh surrounding water/steel.

 

You will also note that club inspectors cannot test stainless steel boilers.

 

Also if a steel boiler is brazed rather than welded the builder would have to get the design approved as it would not have been made to an approved design and could well be why we don’t see them

 

J

[Richard ParsonsParticipant @richardparsons61721]

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.

For the boiler barrel I would use the same thickness of mild steel as the original designer used copper, subject to a lower limit of 2.5mm thick. The plates I would cut from a suitable thickness of mild steel plate. In the plans of the 5” gauge Metro the late Martin Evans shows the flanges on his plates as ranging between ³/₈” (10mm appx) and ⁷/₁₆” (11mm appx) wide. This I presume to get a suitable width of silver solder to add strength to the joint. 

With a bit of luck I hope to see steel boiler plans written up soon. Remember steel is has more than twice the tensile strength of copper so if you copy the copper design in steel using the same thickness of metal you will be more than O.K.

rdgs

Dick

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

[JasonBModerator @jasonb]

“With a bit of luck I hope to see steel boiler plans written up soon. Remember steel is has more than twice the tensile strength of copper so if you copy the copper design in steel using the same thickness of metal you will be more than O.K.”

 

That may be OK where you are based but what about allowance for wasteage. No insurance company inspector is likely to pass a drawing with say 3mm steel for the barrel and plates of a typical 5-6″ dia boiler.

 

As an example one of the model ploughing engines can be built with 10swg copper though with it now being available in metric 3mm would be whats used. There is an approved drawing for the same boiler in steel which uses 6mm plate and wall thickness on the barrel.

 

One other factor to bear in mind if using expanded in tubes is that the makers of the expanders give a minimum thickness of tube plate, at copper thicknesses for the tubeplate you run a real risk of shearing straight through the tube or at the very least getting a problematical joint.

 

Jason

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

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