Lap seam boiler constrction, ex-Proposed New-Builds

Bagnall 2221/1927, Lewisham, also has a lap seam boiler and has steamed within the last ten years.

 

Will Warwickshire's boiler also have lap seams, for all-0ut authenticity, or will it be a modern all-welded affair? Presumably nobody is using butt joints these days unless the boiler concerned was originally thus constructed.

 

Manning boilers generally didn't use flanged plates except for the copper firebox. The steel plates that formed the wrapper, throatplate and backplate were joined together by means of a rolled section. It ws a simple form of construction but brought with it the possibility of grooving at the seams.

 

Middleton has had two new boilers made this century and both are riveted with butt straps and conventionally screw stayed. The only welding has been with the firebox where the wrapper is welded to the flanged tubeplate and doorplate..

 

I think there may be some confusion over terminology here. AFAIK all welded boiler barrels employ butt welded joints. The few traditionally constructed barrels made in recent years employ a rivetted butt strap as mentioned above - usually with a double row of rivets each side. Don't forget that for every loco boiler (or barrel) built in recent years there have been many more 'locomotive type' built for steam road vehicles. I recollect a long discussion (some 40 odd yrs ago) with my, then, boiler inspector regarding the lap joint on my traction engine boiler. As a rule this type of joint required more stringent examination and testing than other types as the result of a catastrophic failure of one on a vertical steam crane boiler. A lap seam boiler barrel is not truly round so, when under pressure the barrel tries to assume a round shape flexing the steel along the joint. This causes stress cracking and localised corrosion leading to failure - hence they come under suspicion and few, if any, have been built for many years.
Ray.

 

Welded boilers can also be susceptible to the same problem. Butt welded joints may become 'peaked' and the amount of peaking has to be measured when manufactured. Boilers with peaking are not truly round, just like a lap seam boiler. Depending on the amount of peaking different inspection regimes are required. There were several explosions of welded boiler barrels in the 1980's because of this. Have a look at http://www.safeduk.co.uk/download/A20039_SBG_2___Edition_3.pdf for more info. If you can't be bothered reading all 46 pages, have a glance at Section 2 'Background'. Personally, I think that riveted boilers which have had replacement sections of barrel welded in should be subject to the same inspections but I'm not aware that this is routinely done.

 

There are a number of ex German WWI Feldbahn "Brigadeloks" in the UK these days, several of these probably have their original boilers and these are built with overlap joints.
The overlap joints are not dangerous per see, but it is tricky to tell the difference between a correctly built boiler barrel with overlap and one that is potentially dangerous.
If the ends of the boiler plate are just riveted together the boiler barrel is the first turn of a spiral and thus not circular and will eventually crack.


But if the ends of the plates are pre bent before riveted together, then a circular barrel can be built also with this technique.
This illustration from a Swedish steam locomotive manual from 1902 shows a correct joint, made more easy to understand with my coloured lines.

Note that the interface between the plates is not tangential to the barrel and the rivets in the joint do not point radially.
This can be tested if you make a board template with the correct radius and make a cut out for the rivet joint. The plates on both sides of the joint must follow a continuous circular contour.

If made correctly, these boilers can be safe and last long. And they are accepted for old boilers in Sweden and many other countries.

 

This is a classic example. A Decauville Mallet of the Norwegian 750 mm gauge line Nesttun-Os, south of Bergen.
The locomotive and its boiler was built by Atellier de Tubize in Belgium in 1893 and was probably built in the correct way.
Later the bottom half of the barrel was changed in Norway and in September 1920 the boiler exploded on the loco Bjørnen (The Bear). Both old and new overlap joints were ripped open. The cause is most probably a faulty repair of the boiler, where the boilersmiths have not been aware of the problem or not meticulous enough. The railway ordered new boilers for all their Decauville Mallets.
Luckily no one was seriously hurt in the explosion.

 

Not seen this arrangement before. Whilst accepting your argument abut it remaining circular, the section shown is at the circumferential joint. The frightening thing about that boiler, if that is truly representative, is that it is a single riveted lap joint.

 

The old illustration from 1902 tells the principle, does not say what type, size or pressure the boiler is. Just that the principle was supposed to be well known, no comments about it.
The Norwegian illustration and all the narrow gauge boilers I have seen, like the German Feldbahns, O&K industrials (also a great number also of those in the UK) etc. all have a double zig-zag overlap joint.
As you know the tension in the cylindrical plate of the boiler barrel plate increases proportional to the diameter of the barrel, thus this arrangement is most common in narrow gauge boilers.

Added:
Thus for those of you that come across this type of boiler: make board templates and check out the circularity. In this way we can kill the myth that overlap joints are inherently dangerous.

 

Most new barrels when fitted to Traction engine boilers tend to be either submerged arc welded longitudinal butt seams or riveted butt strapped seams. Traction engine boilers tend to be in the region of 30" diameter. At one time I was involved with a large 1904 Aveling and Porter roller which had it's original barrel which had a riveted Butt strap, I'm told that this would probably be a wrought iron boiler.

 

Sure, you're right. We all agree that butt-strap joint is better. But traditionally traction engine boilers were for a long time a stronghold for lap joints, also when they became much more rare for locomotive boilers.