I know very little about boilers but may I draw this Australian/New Zealand specification to your attention

In a previous life in the 70's I worked in the nuclear power industry and can say that stainless boilers, albeit of a very different type, were the norm in an environment where steam temperatures and pressures are very high, failure would be catastrophic, a long life is essential and repairs extremely difficult. Mind you the boiler shells were a good inch thick and encased in reinforced concrete several feet thick!

Ian.

Stainless steel can be silver soldered the same as copper. For a small test boiler which doesn't need certification is there any reason stainless cannot be used?

(Naturally pressure testing will need to be done)

The problem with stainless steels is that you need to be sure exactly which one you have, not all of them would be suitable for boilers.

Without being an expert, I would think that 316 would be most suitable but I would want to see a test certificate than corresponded with printed markings on the sheet.

316 can be silver soldered successfully using Tenacity 5 flux or equivalent and preferably solder with a 55% silver content.

Brian

The problem in the UK is the need to persuade a Club that the boiler is safe. Clubs have to insure against accidents to members, visitors and the passing public. Insurance Companies don't check boilers themselves: instead the job is delegated to the club. To do meet this responsibility, clubs appoint a knowledgable member as Inspector, and he has to assure himself all is well.

Easy job when a certificated commercial boiler turns up. Not too difficult when the Inspector knows and trusts the builder, and the builder competently followed a proven design using conventional materials and techniques. Tricky when someone turns up out of the blue with a mysterious second-hand engine, internal condition unknown, obviously suspicious repairs and no documentation.

Extra difficult I think when an Inspector is faced with a new to him boiler built with unfamiliar materials and techniques. Silver-soldered joints and copper boilers are well understood by the model engineering community and build errors implementing proven copper boiler designs are relatively straightforward to detect and fix. Conversely, stainless-steel boilers are likely outside the Inspector's personal experience, require proof that the work was done by coded welders, and that they used the correct materials. (Not unknown scrap!) And on top of that special equipment is needed to detect faulty welds.

Well-made Stainless steel boilers are technically superior to Copper boilers. Unfortunately, it's much harder to prove stainless steel boilers really are 'well-made'. Thus, in the UK at least, Copper boilers are the preferred option.

The Insurer would take a dim view of an Club found accepting boilers they were unqualified to assess. They might refuse liability for a claim, ramp up the premiums, or refuse to insure the club in future.

Should be possible to have a home-made boiler professionally assured and certificated. However, I doubt it's easy! They too would want to understand the strengths and weaknesses of the design, exactly what materials were used and what qualifications the welders had. Then you pay for the welds to be inspected with X-rays or whatever, and all the usual pressure tests. Professional rates, not cheap!

Or run the engine on a private track and accept liability yourself. This is OK unless you maim a neighbours child and have to pay for life-changing injuries…

Dave

304L is probably the preferred. Your replies all refer to large, high pressure boilers which need certification, of which I am well aware of. I am talking about a 30psi boiler for the test running of stationary engines. Does anyone have any experience of this type of boiler in stainless? And why refer only to TIG welding? Is it really any different to MIG or MMA?

Duplex stainless is preferred but if that is unavailable SS316L is fine for our little boilers (class 1 and below, but need to check regional requirements). All 4 of the stainless boilers I’ve designed and built have been very successful. If you use a backing plate then back purging is not necessary so a little planning will pay off. The design for stainless is slightly different to steel and copper; the design needs to take into account the corrosion cracking issues especially with austenitic stainless. I personally wouldn’t use MIG; some of the tests I did showed the normal MIG gas caused the welds to become sensitized (stark difference to TIG). MMA inclusions are a big problem in SS. The Wahya series had a high level FEA pic showing the calculations to deal with material issues. I’ve submitted an article some time back on stainless boiler manufacture and some of the things to look out for, not sure if it is in the pipeline for print. The part on the Ballaarat boiler will have a little more info…

The designation of sch40 or sch40s depends on which ANSI standard is being applied. ANSI B36.10 does not use the s after 40 but ANSI B36.19 does. since at 2" there is no dimensional difference it does not matter if you put the s in or not. #

The issues of stress corrosion and stress cracking possibly apply more to a model boiler than to industrial applications. An industrial application may only have a few pressure or heat cycles a year, a model sized boiler may have more in one day than an industrial application. The smaller boiler may also locally heat up and cool down faster and since stainless has lower thermal conductivity than copper and less ductility this cycling could be more damaging.

Martin C

Corrosion cracking should not be confused with low cycle fatigue. Corrosion cracking is an accumulative failure type spurred on by the surface tensile stresses, temperature and initiation element concentrations. I’m afraid I don’t agree that model boilers are more susceptible. If anything they are less so due to the limited use. Low cycle fatigue due to thermal cycling is another matter and will only accelerate corrosion cracking if the design does not cater for it. With respect to ductility, this is again a design issue and any design needs to take this into account.

The oldest stainless boiler in current use at our club is 27years (SS316L, and that’s a class 2 boiler). Stainless is much cheaper than copper, and in my humble opinion easier for the younger generation to fabricate who would likely have had more experience with TIG than brazing.

Mark: SH10 will be fine for a 30Psi boiler!

But welding of any type, irrespective of the material being welded, must be carried out by a coded welder according to the current UK code.

So while I agree a coded welder of the younger generation might agree with your statement, the majority of builders in clubs have either never been coded or are retired and so are no longer so.

Jason you too quick!