I have been thinking about throat sheets a lot lately. In particular, throat sheets with a square, welded corner joint at the transition to the shell. I have designed and built boilers with this type of throat sheet, though mostly for hobby boilers. I prefer flanged corners, but my bias may be as much about aesthetic as good engineering practice. Cost is another factor. Flanging a throat sheet is hard work. If you are lucky enough to have access to a flanger or a buck to hammer the sheet on, the labor involved can make a finished throat sheet the most expensive plate component on a new locomotive style boiler.

The choices designers make when laying out a new locomotive boiler are dictated primarily by design pressure; material availability; external dimensional requirements; additional mechanical loads on the boiler (i.e. engines on a Shay mounted to the wrapper, etc.); and cost. In my view, cost is the predominant reason for using a square corner throat sheet, but is the result a boiler with an inadequate throat sheet design? I’d like to know what the community thinks about this.

I think many of us don't know how to calculate the different designs - the flanged doesn't really include the radiused area as surface needing staying, and square cornered one would, of course, which may require more or bigger stays or some thickess increases. What's least clear in my mind is how the different types of corners would react under expansion and pressure stress loads. I mentally envision the stresses sort of distributing themselves smoothly around the flanged corner, but loading up in the square one......but I'm a self taught wrench trying to do a better job of understanding and caring for boilers, not a ME.

Maybe less ignorant guys can figure this one out.

dave

Greetings,

On the N&W, they had developed and were testing in service weldment throatsheets. They were entirely of square corner construction, both from sidesheet to front and from front to barrel. To overcome the issue of unsupported area they installed fairly stout gussets up the corner on a 4"ish pitch. I recall what if any bracing was in place for the barrel joint.

Cheers, Jason

We have had this discussion a number of times in our little local circle of misfits! The concern centres around what Dave mentions - the stresses that 'may' develop at the squared corners. This is especially concerning in some new, welded traction engine boilers where squared corner fireboxes are used. How many heating and cooling cycles are in the lives of those designs?
The jury is still out ,one supposes? Few of these boilers will see the working hours/miles of the traditional boilers. They are not likely to see the heavy types of work that past boilers saw, at least on a sustained basis. To date, do we know of any reported failures of the welded corners - probably not?
Has to be a question for the slide rule (oops - ancient technology) types!

The Georgetown Loop 12 is a good example of this. It was built with square corners on the inside sheets, this did lead to failures in the corners from the flexing associated with the sudden thermal cycling. That was an INSIDE corner though, those exposed directly to flame (heat) grow rapidly.....as we all know.
Perhaps it's worth pointing out that the only two sheets the N&W ever put in with square joints were front fluesheets (heavy fillet weled to shell) and throats.

Jason

Jason- I wonder if a drawing for these exists in the N&W archives. I've found drawings in their on-line catalog for several apparently experimental designs (oil firing for an A, a design using ~50 very small transverse arch tubes, etc.). It'd be interesting to see the details.

The drawings I have are in a copy of the "Master Boiler Maker Proceedings", from which year I don't remember other than late 40's. All my books are in Pennsylvania.

Cheers, Jason

This was an unsusal situation.....the side and crown sheets were .375, the rear tube sheet twice that (Jason, correct me if my memory is faulty here.) All welded "modern" boiler with welded in rigid stays. The front of the crown sheet failed across the top of the RTS through stress corrosion cracking - I always wondered just what the very large differences in thickness also might have contributed to the problem.

By the way, not the Shay #12, the little prairie.

dave

Was the Georgetown Loop locomotive repaired? If you can say, what was the remedy for this issue - simply rewelding?
Thank you.

The front few inches of crown sheet was cut out and new metal welded in. This required the top few rows of tubes to be removed for access, then new tubes installed again. At least, that's what was done on my watch.....not sure if Jason had any repairs to make or if Phil did any heavy boiler work before sending it on.

dave

Thanks Dave.
Canadian Pacific Railway had two G-5 Pacifics built in '46 with welded boilers. They were built at Montreal Locomotive Works -
http://news.google.com/newspapers?nid=1 ... 11,4220270
- but I recall reading that the boilers came from Alco. I have never been able to find mechanical accounts of how successful these might have been in service, or what level of maintainence they required. Has anyone seen anything in this regard?
The CPR engines were No.1216 and No.1231.
Thank you.

Well, since the N&W was brought into this conversation, I have some info on the boiler work the N&W performed. I actually have been collecting documents for a article on N&W boiler welding and the square corner does become a topic of discussion.

Briefly, the N&W began welding locomotive fireboxes together at an early date, in the teens. As the art of welding progressed so naturally did the N&W's use of welding. Around 1921 a movement began to eliminate the flanged rear tube sheet on combustion chamber boilers. They went with a 'set-in' rear tube sheet, that is set into the combustion chamber and fillet welded on the fire and water sides of the tube sheet. Apparently this did not work well as there were many variations of attaching the tube sheet to the combustion chamber, such as fillet welds, and fillet welds combined with plug welds. Around 1923 the square corner of the rear tube sheet was re-enforced with various forms of gussets, presumably to stiffen the joint.

Thus far I have not been able to determine how extensive the installations of the set-in tube sheets were or how long all this was in service. The 1218 and 611 both had flanged rear tube sheets. Obviously the N&W thought this was a great idea and spent considerable time on the design, but in the end it was not successful.

I believe there may be some dynamic at work that applies to small boilers, say 24" in shell dia. or less, that does not apply to larger mainline locomotives. This results in some of this square corner construction to survive service. The stress or loading of the smaller boiler is just different than in larger boilers, perhaps by simply overbuilding the miniature boilers, as things do not scale in a linear manner?? Paul, if you re-call some of our committee discussion on this when we were trying to establish diameters at which the new part Pl would apply to. I recall an anecdotal story about a miniature boiler with absolutely no water in the boiler and it did not explode!

Jason, I do not know of square cornered throat sheets- inside or outside. I'd like a citation to the info you saw.

Hope this makes sense to all...

Robert Yuill

Robert, do you suppose the failures came from a combination of joint design and welding technology in its infancy? Were these joints made with gas welding or electric welding? It seems to me the idea of welded boilers was way ahead of the welding technology available at the time. I wonder what those N & W engineers would accomplish if they had access to the welding capabilities of the 1940's, let alone today.

Hobby boilers tend to be so inherently overbuilt. I have seen 1-1/2" scale engines run out of water with a full coal fire in them and just stop moving. My guess is there just isn't enough heat generated by the fire in these small boilers to produce any significant damage. Corrosion seems to be a bigger killer of these small boilers than low water.

The 'set-in' tube sheets were stick welded, what type of pre-heat or controlled cooling/PWHT was used is not known. At the time it was general knowledge that when gas welding, significant compensation had to be made in the placement of the sheets. So they knew that steel can be placed in a strain due to the contraction of welds. Certainly welding technology was in its infancy in the 1920's.

Joint design probably had less of and effect than the low level of technology combined with, dare I say- no quality control?

Robert Yuill

Gary and I replaced the doorsheet and rear fluesheet will with ones of a flanged design, the door hole opening was reduced in size to that of something more appropriate (if you recall it was huge and square), front fluesheet was replaced to accommodate application of a proper tube pattern, all doorsheet/backhead and throatsheet bolts were replaced with larger to accommodate the higher loading resulting from a larger than standard bolt spacing, all bolts in sidesheets were replaced with larger for the same reason and transverse stay rods were installed (as is done on european locomotives which have wagon top boxes with vertical crown stays).
All of this work was done to make the boiler legal under FRA regs as was dictated by the results of form 4 calcs I performed.
We were also in process of superheating the locomotive (already had all material on hand but the unit runs) only lacking about $50,000 worth of work to complete the process.
The only work performed after I left (as a result of Railstar losing the operating rights) was absolutely misguided. Almost three times the cost of finishing the superheat conversion was spent to put yet another set of tube sheets in with a non locomotive type tube pattern. In fact, the pattern in it now is actually referenced in several texts (from the teens and 20's) as an old design that is absolutely not to be used as it shortens tube life due to overheating/burning of the material. It's all a shame, that locomotive could have really performed!
Dave, there is no one employed at that operation capable of doing boiler work.



Sir Yuill!

I do not remember what year these are from..... and I am surprised they are still hiding in my camera but this is the reference.

Cheers, Jason