Dealing with boiler expansion?

[unclebobkt]

  ... .

Don't forget boilers move a lot that's one reason why welded boilers tend to suffer from cracking far more than rivited ones, and the reason why major examinations take place only 10 yearly on riveted boilers but every 5 on welded ones.

 

      Beatty 139.,

 

  Thank you for your reply.

  Now - I have two queries -

>.1.- Is this movement of boilers caused by the thermal expansion & cooling or by mechanical  movements caused by imperfections in the track, tolerances & workmanship in the loco's. construction and suchlike? 

>.2.- I realize that this query might be daft, but I'll accept that!,  instead of being rivetted or welded why cannot boilers, and especially parallel ones,  be made in one piece and thus with no seam? 

     

[lefrog97]

Beatty 139.,

 

Thank you for your reply.

Now - I have two queries -

>.1.- Is this movement of boilers caused by the thermal expansion & cooling or by mechanical movements caused by imperfections in the track, tolerances & workmanship in the loco's. construction and suchlike?

>.2.- I realize that this query might be daft, but I'll accept that!, instead of being rivetted or welded why cannot boilers, and especially parallel ones, be made in one piece and thus with no seam?

Not really my place to comment; but I would imagine that for number2; it would pose a major problem if boiler pressure rose too high; buckling and possibly even bursting could be all too fatal an occurrence! Other than the safety aspect; I'd imagine it all comes down to how easy it is to make them

[Grovenor]

 

Not really my place to comment; but I would imagine that for number2; it would pose a major problem if boiler pressure rose too high; buckling and possibly even bursting could be all too fatal an occurrence! Other than the safety aspect; I'd imagine it all comes down to how easy it is to make them

Any boiler will burst if pressure rises too high! That is why safety valves are so important.The weakest point in the boiler is normally the joints in the plates, I would therefore expect a seamless tube to be stronger if it was possible to manufacture it economically.

Keith

[lefrog97]

Any boiler will burst if pressure rises too high! That is why safety valves are so important.The weakest point in the boiler is normally the joints in the plates, I would therefore expect a seamless tube to be stronger if it was possible to manufacture it economically.

Keith

But the seamless tube Boiler wouldn't allow for any thermal expansion; in neither the longitudinal or latitudal directions; unlike a riveted boiler. Welded boilers would be the same; they don't allow for as much thermal expansion as a good old fashioned riveted boiler. It would be possibly more economical to manufacture a single piece boiler, but then again it may be just as economical if not more to stick with the traditional methods of boiler manufacture. In fact; I think (not sure though) that some original locomotives like Trevithick's engine were single piece cylinders; which then evolved into riveted plate boilers

[Edwin_m]

I would have thought that even if the boiler tube was seamless it would need some kind of seams at each end, which are likely to be the weak points in any case because of the extra stress at the corner. 

[Grovenor]

 

But the seamless tube Boiler wouldn't allow for any thermal expansion; in neither the longitudinal or latitudal directions; unlike a riveted boiler. Welded boilers would be the same; they don't allow for as much thermal expansion as a good old fashioned riveted boiler.

Sorry, that's just rubbish, the thermal expansion occurs because the steel gets hot from the heat of the fire and the steam and hence expands according to its coefficient of expansion, it makes no difference to this whether it is seamless, rivetted or welded.

Regards

Keith

PS i don't think Trevithick had any means of making seamless or welded boilers, the illustrations show plenty of rivets. I'm not sure that current technology would allow economic manufacture of seamless tubes the size needed for a loco boiler, hence the use of welding.

[jjb1970]

Boiler tubes and drums have to satisfy two requirements that impose very different challenges. They have to be strong enough to withstand the forces acting on them from the steam or water pressure (high powered boilers are generally water tube) and also thermal stresses. One tends to favour a heavier, thick tube or drum, the other favours a thin tube or drum. Where you find the best compromise tends to be determined by the operating regime of the boiler (is it one that is being run up and shut down regularly or sits at a steady load) and how much you want to pay.

[Arthur]

There is no way a seamless boiler of locomotive size and type can be made, they are not cylindrical at the firebox end, being essentially flat sided and so necessitating some sort of fabrication.

 

There are two ways of making seamless 'vessels'.

 

Seamless tubes are made by piercing a hot billet, drawing it over a mandrel and then rolling it. This method would not scale up to boiler size.

 

Large cylindrical pressure vessels are forged, and there is no more costly manner in which to make a steel product. You can forge a cylindrical vessel and then forge the ends closed. Vessels to withstand exceptionally high pressures, power station boilers for example, have been, and are still, made this way, but at very high cost. They are essentially cylindrical tubes with closed ends. Over the years improved welding and non destructive examination methods have enabled welded vessels to increasingly meet these specs., and even with forged vessels, the fittings are still welded on.

 

Traditional heavy forging of a hollow vessel;

 

 

Just not economic, nor neccessary, for locomotive boilers operating at a couple of hundred rather than a couple of thousand psi.

[Siberian Snooper]

The Yarrow type marine boilers that I have seen or worked on, are forged, the internal shape is cylindrical but the external is thicker in the tube hole area's and all have riveted end caps.

[MarkC]

As an aside, boiler expansion, or rather a lack of it, was one of the things that contributed to the boiler explosion on board the 'Norway' (ex 'France') that tragically led to loss of life, & ultimately to the early scrapping of the ship. Basically, there were more thermal cycles being applied to the boilers than were envisaged when the ship was built for the trans-Atlantic trade; the ship was now being used for 'island hopping' cruises round the Caribbean, with frequent start ups & shut downs of the power plant. This had led to some on board welding procedures to repair occasional cracks that weren't as per Class Rules. This was then compounded by poor water treatment procedures, and also the sliding feet which allowed the boiler to expand & contract easily were not being greased adequately, hence there were extra stresses induced. Then, one day, the inevitable happened...

 

http://www.ssmaritime.com/norway-NTSB-report.htm

 

http://www.ntsb.gov/investigations/AccidentReports/Reports/MAB0703.pdf

 

A classic example of lots of small things coming together & resulting in an accident...

[Beatty 139]

A round seamless tube would be the perfect boiler but, then you start to add the bits that make it work such as holes for take offs, fire boxes etc and even then as it is not a sphere, the ultimate pressure vessel shape, we end up with ends, joints etc. etc.

 

Where the junctions of shapes join is where you get flexing largely due to differential expansion and forces working in opposition to each other steel will expand by aprox 0.0012mm for every degree Celsius of temperature change.

 

The issue is with welded steel boilers is largely down the temperature fluctuations that take place in a locomotive type boiler, one of the harshest that steam rasing boilers come across. At each junction the boiler will flex and these repeated flexing results in stress cracking. With a riveted boiler however well it's made small amounts of movement take place at each joint releving some of this stress.

 

A welded vessel will ring like a bell because it's one solid part once fabricated, a riveted one will sound dull, as it's a collection of parts.

[avonside1563]

If you want a good account of how locomotive boilers developed and some of the failures that can occur I would suggest obtaining a copy of 'Locomotive Boiler Explosions' by Christian H Hewison, Published by David & Charles in 1983. This is an excellent and informed read for those who have an interest in steam and preservation.

[jjb1970]

I must admit I always shudder when I think of the sort of water quality railway boilers had to tolerate. There are really two things that determine boiler life, managing thermal stresses and controlling water quality. When I think of the way feed water and condensate are molly coddled in power stations and the efforts to control chemistry, using demineralising plants etc(much more so even than in marine boilers) and then look at what poor old steam locomotives had to put up with I can't help giving a shudder. Admittedly the boilers are very different in being much lower pressure and fire tube rather than water tube but still......

[PatB]

A round seamless tube would be the perfect boiler but, then you start to add the bits that make it work such as holes for take offs, fire boxes etc and even then as it is not a sphere, the ultimate pressure vessel shape, we end up with ends, joints etc. etc.

 

Where the junctions of shapes join is where you get flexing largely due to differential expansion and forces working in opposition to each other steel will expand by aprox 0.0012mm for every degree Celsius of temperature change.

 

The issue is with welded steel boilers is largely down the temperature fluctuations that take place in a locomotive type boiler, one of the harshest that steam rasing boilers come across. At each junction the boiler will flex and these repeated flexing results in stress cracking. With a riveted boiler however well it's made small amounts of movement take place at each joint releving some of this stress.

 

A welded vessel will ring like a bell because it's one solid part once fabricated, a riveted one will sound dull, as it's a collection of parts.

 

Riveted structures also have the advantage over welded that any crack which does start will eventually reach either a rivet hole or the edge of a plate and will stop. In a welded structure the crack can easily either cross a welded seam or turn and run along it. Perhaps not so much an issue with pressure vessels, as I'd assume that a crack would be either detected or would cause catastrophic failure by that point, but it was certainly an issue with early welded construction in ships. J E Gordon's very readable books Structures and The New Science of Strong Materials contain excellent and easy to understand explanations of these and many other phenomena.

[unclebobkt]

  ... .

The issue is with welded steel boilers is largely down the temperature fluctuations that take place in a locomotive type boiler, one of the harshest that steam rasing boilers come across. At each junction the boiler will flex and these repeated flexing results in stress cracking. With a riveted boiler however well it's made small amounts of movement take place at each joint releving some of this stress.

  ... .

 

  A day or two ago I looked-at a video presentation of a boiler-explosion at a Canadian university.

  The boiler was used for students' instruction - thus there were many start-ups and close-downs: thermal cycling.

  BP. was 150.PSI., the boiler was a standard fire-tube and was of welded construction - viewed from the chimney's end the welded seam was at abour 2 o'clock.

  The inspectors reckoned that the initial failure occured along the line of the weld;  apparently the material used in welding was NOT compatible to the steel used in the boiler-shell,  corrosion occured because of electrolytic action and then ... !

     

[jjb1970]

There are some good reasons why good coded welders with the right tickets to weld high pressure systems are so well paid. Some of the more esoteric steels used in very high pressure systems can be a real pig to weld and need suitable filler metal as well as some very tightly defined pre- and post weld heat treatment then there is the non destructive examination. Nowadays most people try to avoid radiography as there are some safety complications associated with control of the sources and exposure. If you are good enough to get the ticket, run the welds quickly and pass the NDE you can make a lot of money doing pressure vessel welds.

[LMS2968]

As I recall locomotive boilers are actually the strongest part of the machine, and provide support for the frames rather than the other way around.  in the Harrow and Wealdstone crash of 1952 the city of Glasgow boiler was re-used, I doubt if much else survived.

 

JF

 

 

A lot more than that, I think!

 

http://lmsforum.forumup.co.uk/viewtopic.php?t=64&mforum=lmsforum

http://lmsforum.forumup.co.uk/viewtopic.php?t=73&mforum=lmsforum

http://lmsforum.forumup.co.uk/viewtopic.php?t=74&mforum=lmsforum

http://lmsforum.forumup.co.uk/viewtopic.php?t=78&mforum=lmsforum

[unclebobkt]

       Bestest & good thanks to all who have replied to this topic - your answers greatly appreciated, as is the general diffusion of knowledge.

  'Give instruction to a wise man ... .', (Prov. IX:9.),.

     

[Arthur]

 

A tube, i.e. a boiler, made out of steel plate is indeed going to be a very strong structural member, stronger in most (probably all) ways to two heavy plates riveted together with spacers, the frames.

[avonside1563]

A tube, i.e. a boiler, made out of steel plate is indeed going to be a very strong structural member, stronger in most (probably all) ways to two heavy plates riveted together with spacers, the frames.

Indeed it is Arthur, you only have to look at a Ploughing Engine with the weight of the winding drum and associated gearing, and the forces involved when ploughing, all bolted onto the underside of the boiler. Some of the larger engines weigh in at over 25 tons.