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Dangerous 2" Scale BB1 Boiler

Why we test boilers!

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Nigel Bennett19/07/2019 14:30:56
435 forum posts
19 photos

Some years ago I bought a part-built 2" scale Fowler BB1 ploughing engine to John Haining's design from the estate of a deceased model engineer. He'd bought it from somebody else. The workmanship wasn't brilliant, but the boiler had been commercially built in about 1975 and came with a certificate, saying it had withstood 175psi without leakage or distortion. The boiler itself looked lovely, so I was quite happy with the purchase. It was always going to be a long-term project, so I was in no hurry to get on with it. (Ignore the bloody great hole in it in the picture below!)

After about twenty years, I thought I'd get the boiler certificated. So before taking it to the club, I pumped it up. Only I couldn't pump it up. As soon as the water level rose far enough, it poured out of the firebox from around all the tubes, none of which were actually soldered to the firebox tubeplate. So for the builder to claim it had hadn't leaked was an out-and-out lie. But by now it was far too late to complain - even if he was still alive to complain to.
I had a conversation with our chief boiler tester, and he suggested that I expand the tubes and then run round them with Comsol. So I did that, and pumped it up again.

There were a lot of further leaks from stays (screwed and silver-soldered) so I applied more Comsol. A couple of days ago I pumped it up again, and it withstood 150psi with only some very slight weeps from the odd stay. One or two responded to a bit of gentle caulking with a hammer and punch. Then a small leak manifested itself in what was apparently a plain section of boiler barrel, just in front of the firebox. Curious... I lightly tapped the area with a hammer and the leak became a torrent. I filed the area and a brass colour became exposed. The barrel had been brazed to the firebox, and then smoothed over before copper-plating the boiler to disguise it.
That was it. The boiler was scrap. I got out a hole saw and drilled out the area of leakage. When I got deep enough, a semicircular section of barrel became dislodged, leaving its mating half still riveted to an internal piston ring joint. I couldn't finish drilling through the piston ring because there was no fastening of the piston ring to the boiler other than where it was riveted in places to the firebox and it was pushing away from the hole saw. All that had been holding the barrel to the firebox was a manky brazed butt joint, which had locally come apart with the stresses of hole-sawing.

That piston ring was only serving as a location for brazing the two halves together!


These are the two sections of barrel/firebox wrapper removed by hole-sawing - and the braze had given way during the operation.


The whole thing had been a potential bomb. The scary thing is that if the joint had been only a few percent better than it had been, it wouldn't have leaked and I would have been blissfully ignorant of the potential. And it's a ploughing engine - it's not just pressure stresses on the boiler, but driving it over rough ground imparts who knows what additional loads on the joint?
Further investigation showed that the barrel was only 2mm thick - it should have been at least 3/32" (13SWG) and if I had been making the boiler it would have been at least 3mm thick. The backhead had been brazed in place too - and the section through it shows that the penetration of the braze wasn't all that good. And what kind of brazing rod was it anyway? How long would it have taken to de-zincify?


There were other concerns as well, such as very thin fire tubes - they'd have worn through very quickly - and the internal pad under the cylinders was not in the right place (or too small) so the front ring of cylinder bolts were only tapped into 2mm of copper.

I'm not prepared to disclose the name of the maker. Technically I can't prove anything because there is no marking on the boiler or certificate. Suffice it to say that he was later bankrupted. And I hope he had a really miserable time of it.

At least there's no chance of it being used - it's well cut now and I shall weigh it in for scrap.

Moral: Take care when dealing with older boilers built by unknown "professionals", and look at them very carefully before parting with your money. If a boiler has been copper-plated, walk away from it. It's hiding something. And always, always test a boiler you wish to use.

Nigel Graham 220/07/2019 23:49:35
1898 forum posts
26 photos

Or when dealing with drawings from unknown professionals....

I once helped prepare for testing a newly-built boiler whose wide firebox shape suggested an LNER locomotive, in 3.5" or 5"g. The metal-working including the silver-soldering was immaculate, but inverting it revealed the inner firebox looked more MacDonalds than Gresley, so badly crushed-in was the crown.

Closer inspection revealed only about 8 or so stays, and only in the back-head and throat-plate.

The builder was a retired coppersmith on his first foray into model-engineering, so he had decided to start with the boiler as the simplest part for him, whilst admitting not knowing much about locomotives.

He assured us (club members) he'd tested it hydraulically but only to working pressure. We believed him, but realised he'd not have known to look for more than just dripping water.

Fortunately he had the drawing with him. It was a low-quality photocopy significantly lacking any designer's or publisher's name or other source details. The poor chap had followed it accurately, not knowing it was so flawed. We had explain to him why the design was dangerously poor, and break it to him that the boiler would not be repairable.

We wanted to write to ME with a warning; but for some reason he would not reveal where or how he had acquired the drawings. They might have been made privately by some past model-engineer for his own project, perhaps from railway-literature drawings, but who had a weak knowledge of design principles.

We never saw him again.

FMES21/07/2019 07:47:39
607 forum posts
2 photos

A newly constructed silver soldered boiler usually has the strength integrity of a piece of chocolate until it has been fully work hardened, normally by carrying out the hydraulic test in stages and slowly building up to the required pressure.

Pumping a new boiler up to working pressure in one hit will almost certainly cause deformation in all sorts of places.


Dave Halford21/07/2019 11:29:25
1886 forum posts
22 photos

Makes you wonder if boiler and certificate got married sometime since 1975, I've seen boilers that bad on ebay, they can be a handy source of cheap castings or gears when the builder realises they do not have the boiler skills required.

Former Member21/07/2019 11:41:05
1329 forum posts

[This posting has been removed]

Kiwi Bloke21/07/2019 11:55:37
625 forum posts
1 photos

May I suggest that the above extract from the boiler regs is published regularly in ME? Whilst I would expect boiler inspectors to know this, how many builders would know to first pressurise their newly-built boiler this way? Not me, certainly - although I'm not into steam.

old mart21/07/2019 18:57:44
3510 forum posts
216 photos

My only experience of live steam, was as a teenager, getting hold of a Mamod oscillating, very basic, stationary engine. The first thing I had to do was replace the rubber seal in the filler/safety valve, the blow off seal, that is. There was no way of checking the actual pressure the valve would work at with my carefully cut 1/8" long section of bicycle tyre inflator hose. I bent up a length of copper tube and soldered it to the exhaust port and through a hole drilled in the dummy chimney.

I'm pretty sure the boiler was made of brass, I thought that de-zincification only occurred in a salt water environment, which is why bronze is used.

Neil Wyatt21/07/2019 19:26:43
18894 forum posts
734 photos
80 articles
Posted by Kiwi Bloke on 21/07/2019 11:55:37:

May I suggest that the above extract from the boiler regs is published regularly in ME? Whilst I would expect boiler inspectors to know this, how many builders would know to first pressurise their newly-built boiler this way? Not me, certainly - although I'm not into steam.

The problem with publishing extracts is that there's an awful lot more boiler builders should be aware of; if they read a brief warning someone will say 'well I did 'X' so it should have been OK!'

The effect of tales like this should be to persuade people to seek DETAILED guidance on construction and testing.


Edited By Neil Wyatt on 21/07/2019 19:27:32

Former Member21/07/2019 20:40:23
1329 forum posts

[This posting has been removed]

old mart22/07/2019 14:24:50
3510 forum posts
216 photos

The work hardening aspect is interesting. Copper work hardens by stretching, so how much smaller do you have to make the boiler for it to be the correct size after testing?

FMES22/07/2019 15:09:52
607 forum posts
2 photos
Posted by old mart on 22/07/2019 14:24:50:

The work hardening aspect is interesting. Copper work hardens by stretching, so how much smaller do you have to make the boiler for it to be the correct size after testing?

Never had a boiler fail to fit its intended purpose yet, then again if you build and test the boiler first, everything is machined to fit.

And you are only talikng a few 'thous' of growth if you test properly.

There is always a difference in sound after testing too - before the metal will sound dull if you gently tap it, afterwards it will 'ring'


Keith Hale26/07/2019 07:42:12
329 forum posts
1 photos


The strength of a brazed joint in a boiler is not going to be influenced much by testing upto 150 or 200 psi.

Check out the strength of annealed copper. It is way above the operating stresses of a boiler.

The model engineer has a far greater influence. In order to achieve high integrity joints he has to ensure that all parameters are met to create a sound joint. And that means creating the right conditions for capillary flow.

Is the joint gap and length correct?

Is the joint design appropriate for the operating conditions?

Is the right flux being used to meet the melting range of the filler metal, to remove all the oxides present and the time taken to melt it?

Is the joint being heated correctly to promote capillary flow and ensure penetration of the filler metal into the joint?

Is the appropriate grade of copper being used and not a cheap equivalent?

The responsibility for the quality of a brazed joint lies solely behind the torch, not in front of it!

Isn't the technique of gradually increasing stresses during testing simply one of prudence? Does anyone, when testing any new machine,start by winding it up to full bore immediately? If anything goes wrong the ramifications are often less severe. Increasing the loads gradually usually leads to increased confidence.

That is certainly the case with a boiler. It has nothing to do with increasing the strength of a brazed joint. But, I repeat, if it makes you feel good......



PS The source of my information is a metallurgical degree and 50 years spent trying to ensure that every brazed joint is a sound one.

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