Boiler silver soldering and use of Silfos

I have acquired a boiler which is at least 10 years old and the firebox silver soldering may have been done with silfos - 15. The phosphorus content of this alloy makes flux unnecessary but the phosphorus is attacked by the sulphur from the coal combustion and eventually the joints will go spongy and porous. Data sheet warnings for this brazing alloy warn against the use in a sulphurous environment. The boiler has not been steamed and while it passed a hydraulic test when it was built it now has a number of small leaks on the inside ends of the firebox stays which are threaded gunmetal.

I am looking for advice on how to proceed as I need to seal the joints in the best case or , in the worst case , I can prove that silfos was actually used, then the boiler may have to be scrapped.

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Thank you for your suggestion I have already had discussions and sought advice from Johnson Matthey, Cup Alloys and my Club Boiler inspectors. No clear consensus on best course of action yet, hence throwing the problem to a wider audience in the hope that I can pick up the best advice. The problem is the original boiler maker is now in his eighties and he is not 100% sure what he used. He still has a stock of Silfos from a project in the Nuclear industry where fluxless brazing of large copper conductors was required. He thinks he may have used the silfos in the firebox but not on the outside joints. Not sure why he did it this way around. If I could be sure that the firebox stays in the firebox ( which are threaded gunmetal ) were not soldered using silfos I would be happy. I still have some small leaks but they can be sealed hopefully.

If it comes to scrapping the boiler, maybe think of using gas firing rather than coal? Just a thought. In the longer term I think there are major advantages in using propane in any case. I am seriously looking at doing so on the two locos that I am building.

Andrew.

Hi Andrew,

Check out my profile and if you feel it's relevant then consider the content of the personal message.

I can honestly say that in my 50+ years in the brazing business, I have not come across a situation where anyone has felt the need to rectify a copper/copper joint, previously made with Silfos type materials, with either silver solder or soft solder.

There is a very simple and quick way to answering the question.

Clearly you cannot consider a phosphorus bearing alloy for repairing your leaks.

Regards

Keith

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Wow!

Sorry br, I can't be more friendly, but thank you for your comment.

Keith

Edited By Keith Hale on 06/01/2021 09:28:11

Few thoughts from an outsider - feel free to disagree!

• Andrew says his boiler has never been steamed, therefore the leaks have nothing to do with mixing Silfos and Sulphurous Coal. I suggest Silfos is a red-herring at this point.
• Not all coal contains Sulphur, and solid fuels like Charcoal and Smokeless Briquettes are sulphur free. Why not dodge the problem, if it exists at all, by only running the engine on Sulphur free fuels? Easier now than it was. Back in the day domestic coal in the UK was random stuff - the amount of sulphur and other contaminants depended on where it was mined. As what ended up in the firebox was unknown, it paid to avoid Silfos. Still true today now our coal is imported from Russia, the USA and Australia - coal is a natural product and exactly what's in it varies wildly. So don't burn unknown coal when Silfos is suspected and alternatives are available. Most of us are clever enough to not fill petrol cars with diesel, and selecting a suitable fuel for a sulphur sensitive engine isn't that challenging, is it?
• London Smog was caused by large scale burning of dirty coal. The authorities solved the problem with smokeless zones, in which burning mucky coal is forbidden. Most of us go for the convenience of gas, oil or electric, but low-sulphur smokeless solid fuels are readily available, including Anthracite, Coke, and barbecue charcoal etc.
• Two main problems not burning real coal; finding an alternative solid fuel that produces enough heat in a firebox designed for coal, and - most serious - losing the authentic smell of a real coal-fired steam engine! (Slightly toxic of course, but they do smell nice.)

Dave

Edited By SillyOldDuffer on 06/01/2021 10:43:42

Thank you all for your responses and suggestions. I am concerned about the possible use of Silfos in the firebox because of the possible failure under working conditions i.e. sulphur attack on the phosphorus fluxed alloy. The good news that the stays are gunmetal and were threaded into the inner firebox sheets. I only have leaks on the side sheets in the firebox. Mechanically the silfos joints will be strong enough because of the threaded stays although some might leak in due course. Perhaps hard water scale will keep those leaks at bay?

My original post was to invite suggestions for repairing the leaky stays. As the Moderator has suggested the Silfos is to a certain extent a red herring because for the purposes of repairs, 455 silver solder will be OK according to Johnson Matthey. Talking to CuP Alloys they felt that if the internal firebox stay ends had been soldered with Easy Flo No. 2 there might be a risk in using 455 for repairs because of the higher melting point and the risk of melting nearby stays that are pressure tight at the moment. The leaks are fairly minor ( slow bubbling with 40psi air pressure ). I have Oxy/Propane equipment and am looking into getting a cylinder of Propylene which has a higher calorific value than Propane but safer than Acetylene.

Propane as a source of heat is an interesting idea but I have no experience of burners, fuelling techniques, etc. I am happier sitting on a tender of coal rather than a gas cylinder. Also has been pointed out it doesn't smell right!!

I have not messaged Keith Hale directly because I feel we can all benefit from the experience of his years of experience in these matters if he cares to reply in the forum. I would though be interested in seeing what Keith suggests I use to seal these leaks and out of interest is there anyway that the actual joint alloy could be sampled? I don't know anyone with access to a mass Spectrometer!

A jeweller who buys old gold

Hi All,

Add to the list of lifes certainties;

Flat roofs leak and

Leaks in a brazed joint are caused by operator malpractice. (No exceptions!) Refer to BS EN 14324 Guidance on the application of brazed joints.

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Mass spectrometers in a jewellers or pawnbrokers? They use hallmarks and trust/gamble on what an experienced eye tells them

Keith

Not in the Birmingham Jewellery Quarter Keith. Too much Asian unmarked gold.