Welded Boilers

There's comment on the 82045 topic about welded boilers which I would appreciate further discussion on:

I would suggest that you refer to the rules and regulations with regard to boilers.[/quote:socvgfy4]

I'm quite intrigued as to what these rules and regulations are? Weld repairs to boilers have been around for an awful long time on locomotive boilers and are very much the norm on industrial ones. OK, it's not simply a question of cutting out a piece of plate and bunging a new piece in but it isn't rocket science and Insurance Co's (Boiler Inspectors) are well versed in what is acceptable and what is not. I prefer a screw-stayed and lap riveted boiler with a copper box because it's the traditional way but there are an awful lot of advantages to having all steel boilers with welded stays and welded seams and, provided that the boiler has been designed this way, they are a lot less trouble to maintain and repair.

 

on the continent isnt welded boilers the normal way they prefer to build new boilers, i can see the advantages of welded steel fireboxes after all one of the best steaming boilers that you can get uses a steel firebox and welded construction and isnt steel easier to work than copper and more cost effective when you think of what copper plate to the correct spec costs

 

In Poland just about everything is welded steel...with weld on top of weld covered in weld lines and very rough cut edges for the harder working ones.

 

I think OVSB said something like, "While there are welders, my boilers will last forever"

 

Sounds about right for a Polish boiler! Greek ones aren't any better from those I've seen.

 

If you look carefully at many fireboxes, the outer wrapper often has welded seams somewhere. Many overhauled boilers have lower firebox sections replaced with... Welded seams. I seem to remember that for the "Bloomer" project, a new/revised British Standard was developed to cover a welded steel locomotive boiler. A steel inner firebox has the advantage of the elimination electrolytic corrosion and reduced differential expansion that can lead to stay failure.

With respect to repairs, it is possible to build up surfaces inside the firebox that are burned or pitted. due to the higher strength and elastic limit of steel, there is less risk of tube leakage where tubes are exapnded into the front tube plate. The downside to a steel inner firebox is the lower conductivity of steel compared with copper. There is also a greater risk of corrosion due to acidic ash should the grate be left covered in ash for a period of time.

With respect to stays, can they be plain and just welded at each end or must they be threaded?

 

Welded stays are certainly allowable and obviously the cheapest option. There are various thoughts on these, though. For example, do you make the hole a clearance fit or do you make it a tight fit? Do you do a full 'J' prep on it with full penetration or do you simply have the stay protruding an appropriate amount and provide a simple fillet weld? I've heard arguments for all these variations.
There are similar arguments with boiler tubes. UK practise is usually to expand the tube, seal weld and lightly re-expand, with or without beading over first at the firebox end and simply expand at the smokebox end. I believe German practise is to do a penetration weld with no expansion at both ends.

 

From an Eisenbahn Kurier video I have about Meiningen Works the method of fitting tubes is to weld them at the firebox tubeplate and expand at the smokebox end.

 

Stay holes on welded stays should be a clearance fit with no weld prep. Otherwise the assembly is too rigid and the stay life is much reduced, with increased stress grooving in the plate around the stay area. Unfortunately in the UK there is a reluctance to move to welded stays in steel fireboxes, and the experience and knowledge base hasn't developed sufficiently. So there is a mistaken belief in tight stay holes and full weld preps by some people. However the rest of the world learnt that lesson decades ago. I have a copy of an old German manual by Henschel which explains all that in detail. Similarly in the USA the method of applying welded stays was established long before they finished with steam.

Concerning boiler tubes. They should always be expanded in, especially at the firebox end before welding. The reason is that otherwise the heat conduction between the end of the tube and the tubeplate is insufficient, and the tube end burns away in a ring between the weld and the region where the water in the boiler cools the rest of the tube. The narrow gap between those areas and the tubeplate acts as an insulator, and so at times when the boiler is exposed to high firebox gas temperatures, that part of the tube heats up excessively. In serious cases the tube can eventually break apart completely. I'm very surprised to hear that Meiningen has been seen to weld tubes in without expanding, though I have heard of a preserved engine elsewhere suffering such a problem. That's a serious error which frankly needs urgent correction.

 

A gentleman called Alan Haigh of Morley, Leeds published a couple of books which might prove interesting. One looked at the LNER diagram 100/100a boiler showing how it could be developed in accordance with modern standards. Since Mr Haigh designed boilers for a living his views, coming from the industrial boiler side of the fence, are worth a glance or two. Be warned though, he has little time for the traditional methods of locomotive boiler construction.

On staying the the Tross double-conoidal sunken weld stay is considered to be the best form of welded stay. It was adopted as standard on the German F.R.

 

BS2790 requires that the tubes be expanded before welding. However, Beel Boilers made a boiler for me in which they didn't expand the tubes. When I queried this with them, their MD, who was on the BS2790 committee, claimed that the BS was wrong(!) and it was continental (German) practise to not expand them. To cut the story short, I discovered the real reason they weren't expanding them was that they had found the tubes (Italian) were cracking when they did so. Needless to say, they had to change all the tubes and do it properly. All 528 of them!

 

The boiler design Alan suggests is essentially to BS2790 with a full J prep for the stays with no protrusion on the firebox (fire) side to avoid burning. He doesn't make comment on the clearance of the stay hole and I haven't got BS2790 to hand to look it up.

 

So, if welded construction practice proven around the world could be writtin into a British Standard, then togehter with modern CAD and CNC plate cutting processes, could the cost of producing replacement boilers be considerably reduced?

 

The simple answer to that is no.

When tubes are expanded, the material is subjected to very high pressures by the rollers. The mechanism is not stretching of the tube material but compressing it between the roller and the internal surface of the hole. Which results in the external diameter of the tube wall becoming larger than the hole - but constrained by the hole. That's why they become water tight. Simple expansion of the tube due to heating could not make the tube fit the hole, let alone provide the necessary thermal conductivity. Also one should remember that when at rest the tubeplate is at much the same sort of temperature anyway. The holes don't get smaller, the plate as a whole becomes larger. So in fact the holes grow as well.

 

Thanks for taking the time to enlighten us on the finer points of weld preps for stays. What I find highly amusing is that if you talk about replacing a traditionally built boiler with a welded steel one it's all part of the restoration and provokes little comment. But to change the colour on the outside 0.5mm generates pages and pages of highly moralistic feelings.

The easiest part to change is the paint, the most complex is the boiler. What does that tell you about our priorities?

 

I'm sure the debate over the merits of all-welded v rivet and steel v copper fire boxed boilers will continue. Whatever the favoured construction method the main consideration must be for the heritage movement to continue, and perhaps to improve, its very good safety record. Adherance to the HRA's guidelines is a useful step in that process. http://ukhrail.uel.ac.uk/hra/guidelines ... atment.pdf
http://ukhrail.uel.ac.uk/hra/guidelines ... oilers.pdf
http://ukhrail.uel.ac.uk/hra/guidelines ... tPlugs.pdf
http://ukhrail.uel.ac.uk/hra/guidelines ... ePlugs.pdf

 

These guidelines are generally based around BR policies and there are some contentious issues in them. They haven't turned out to be as good as they should have been.