Stainless steel boilers

Not exactly a beginners thread but as I have no information I thought it apt.

In my professional life we manufacture production line machines, mostly from stainless steel where foods are concerned. So this makes me luck at having an endless supply of Stainless steel. By chance I was looking at the Maxitrack website and found that they offer stainless steel boilers. This comes as news to me and was wondering if there are any plans available for a bench test boiler or any boilers for that matter?

We have a few welders who are very capable of welding pipes (tig– purged) for our machines, as is required for hygiene, so I am sure one of them will be able to weld me a boiler.

How would a club boiler inspector feel about one of these?

Your thoughts and experiences please gents.

Brian

Club inspectors can't inspect stainless as its not covered by the club testing code, see item 3.5

I feel this is one of the reasoins that Stainless is not used much in this counrty, go over to Europe and stainless is quite common, that may be the best place to look for designs.

J

Edit, I remember this came upo before, if you look closely at the wording on Maxitrak it says copper and MS boilers but Stainless vestles not boilers, or something like that. Although Marco who does their boilers is quite capable of stainless work as part of their main fabrication company Welding Innovations Ltd.

"In addition to the list below, we can build any boiler to your requirements, Water Container's in Stainless or other materials.  Also, we can build boilers in other materials such as carbon steel."

Edited By JasonB on 01/04/2012 09:03:06

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

Come and live in FRANCE. There has never been an instance of a stainless steel model boiler failing and they are quite popular. However I have to say that the testing regulations are VERY different to the UK - that said I havn't heard of a model boiler accident be it copper, mild or stainless steel.

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.

To add to what Ian said: stainless steel is also used extensively in medical autoclaves - here the temperatures and pressures required aren't anything like as high as in a power station but large changes in temperature and high temperature gradients are very frequent. Stainless steel seems to cope with this fine - biggest clinical autoclave I've seen had a 4 cubic meter capacity with a steam supply from an oil fired steam plant and it ran for decades.

I suspect that in the past stainless steel was avoided because of limitations in welding technology in smaller workshops - modern TIG technology has changed things a lot.

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)

Obviously the Welders referred to are Coded..And so would know all the pitfalls re carbon precipitation which is less of an issue with low temp liquids contained within, but with steam and pressure, any carbon precipitation results in very rapid pinhole development and subsequent rusting, which would make a pressure vessel interesting after a little use..

Joe

Edited By Joseph Noci 1 on 27/01/2022 11:10:19

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

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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

One of the things that came out of the nuclear industry's use of stainless steel for hot pressurised fluids was that austenitic stainless steels, such as the 300 series, could suffer from intergranular corrosion if there was any chloride in the fluid. As a result they had a requirement of (IIRC) less than 30ppm in any fluid that was used, even when hydrostatic pressure testing before being put into use. This could cause issues for places with hard water that may have dissolved chloride salts, including common salt NaCl, causing internal structural damage that could only be detected over time with x-ray or ultrasonic NDT. The biggest issue for the pipes made from 321 or 316 stainless steels where I worked was in the highly stressed and thin corrugated bellows of flexible pipework. The pressure testing water could gather in the bellows and deposit chlorides in them that was hard to remove. Since the town water was very hard and also contained a lot of iron we used a water treatment system to remove almost all the impurities with RO for pressure testing purposes. We later found the NDT department were using untreated hot water on the pipes as part of their colour contrast crack detection. We had to write a new process specification for them to ensure they also followed the no-chloride rules.

Martin C

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?

Welding stainless without backing gas on both sides of the weld to keep oxygen away would result in a weld that was highly contaminated with oxides. As mentioned in a post by Brian Dickinson in 2012 you need to purge the inside with an inert gas, usually argon, to make satisfactory welds in stainless steels. MIG welds also tend to produce unacceptable sharp changes in section that act as stress raisers and can also cover up all sorts of poor preparation and internal shrinkage cracking. A TIG weld in stainless made by a good welder is a thing of beauty and looks like it has been made by a machine.

For pressure vessels you need a full penetration weld to avoid stress raisers. With TIG you can create a weld pool and hold it in position until it has had a chance to go all the way through with closed butt welds before moving around the joint, you can't do this with MIG. With open butt welds you have to do something similar but form a bridging weld pool that is then continued around the joint . For anything over 1mm wall thickness we used to form a weld preparation by bevelling the edges so that we could get this full penetration weld then had to have multiple passes and a capping pass. The time to produce this weld preparation and subsequent weld in something like 2" NB sch 40 pipe was about 1 hour. We would not have wasted time on it if we could have just MIG welded them and produced a satisfactory job.

Martin C

Edited By Martin Connelly on 27/01/2022 13:52:21

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…

Martin, do you mean SCH40S? And nearly 4mm wall thickness is overkill for a 30psi boiler surely. I am thinking SCH10S.

I appreciate the answers but other than Luker everyone is refering to industrial aplications. I am talking about something that produces less pressure than your car tyres.

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

I am reading my way through 80 years of ME, and there are odd comments about not using stainless. From memory it is to do with corrosion, as pointed out chloride, but also I think from sulpher. Industrial boilers don't tend to use coal firing so the sulpher problem never arises.

Bit like the comments about silphos decades ago to an absolute no no now.

The fail safe designs using copper and silver solder really do seem to be a very cheap and easy solution.

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!

This was a boiler project I started the '60s made from 316 ss for a 5" Firefly. Backhead, throat-plate and boiler barrel were TIG welded by a coded welder and all the rest was silver soldered with easyflo and Tenacity 5 flux. Removing the flux once over heated was a real pain. The project was abandoned as there was no way it would be accepted back in 1968 as it was in Stainless Steel.

Bob