To answer the questions on the roof sheet, there are several things to consider.

As far as the K-4 is concerned, we feel pretty confidant we figured out what the Pennsy did. When the earlier Belpare boilers were built, they were designed to have the crown/roof sheet bolts thread through the roof sheet and stick out enough to be able to thread a nut on the ends as well. When you use nuts that bear against the sheet you can use a thinner plate than you can without them. This is because you have not only the threads through the sheet as an attachment, but you also have a nut holding the sheet as well. This type of construction allows for a higher “constant” to be used in the flat stayed surface formulas in the ASME Locomotive Codes. The higher the constant, the thinner the minimum required thickness.

The Pennsy at some point (1940 +/-) eliminated the nuts and went with the bolts threaded into the sheets only method, presumably to save on costs. Everything we’ve seen so far indicates that they kept the same bolt pitches and didn’t make the sheet thicker to make-up for the lower constant. How they got away with it regulatory wise I don’t know, but in actual practice it worked. We went back as far as the 1920s era Locomotive Codes and couldn’t make it work. This particular calculation changed very little up to and including the last Loco Code of 1952.

The K-4 and most other Pennsy engines before the 1940s were built with 3/8” thick roof sheets. With the 5.0625” transverse bolt pitch, and 3/8” thickness, this gives a working pressure of only 175 psi for a 205 psi boiler. This isn’t even considering normal wastage which has made it even thinner. When you use the constant for nutted construction, you get a working pressure of 210 psi. This is one of the “design flaws” that Mike Tillger talked about.

This is where you get to the debate of “it worked before so why mess with it”. The problem is that you have to be able to hang your hat on numbers that support using a given part. If you can’t do that, then it’s all over and you replace the part.

I hope this helps,

R L Musser

Would longer bolts with nuts on the rebuilt K-4 solve the problem of the "thin" sheet?

Jim,

Every one of our engines had the bent stud problem and I know of many other outfits that had them as well. In fact, just yesterday I calculated one for a customer that was too thin. That being said, there are engines out there with sufficient thickness to where bending is not an issue. The problem is not that they will blow off or they can’t withstand the pressure, it’s the bending of the lid and studs from tightening that is the problem. The concern is with the repeated bend cycles weakening the materials.

So, it you put a nut on an extended staybolt rather than peening it tight, how do you avoid it leaking through the threads and between the nut and the sheet?

Isn't there special bucking/caulking tool that is used for this? Been a long time since I studied this aspect of boiler construction. Perhaps Rick or someone else can help clarify if I am correct.

I have my own questions regarding the subject. I spent some time writing a message earlier only to have the server connection lost, and thus the message went "poof." Rick, do you recall the factor of safety based on the roof sheet and staybolt combination as is at MAWP? Although it's obviously not 4, it would have to be well above 3, I'd think. Also, I agree that to correct the issue requires changing all of the bolts so installing a new, thicker roof sheet is sort of a no-brainer as the bolt work is a major part of the job. What about going ahead and changing the crown sheet as well? At least then you'll get another lifetime out the job based on the better care and less use that most steam sees nowadays.

Speaking of that, why not just derate the boiler to get the engine(s) out to generate at least some revenue and goodwill, then take care of the big fix at the next 1472 inspection? It's not as if any of the three under restoration are going really need that additional bit of horsepower for most any use they would likely see.

Just a few thoughts for discussion...

Best,
Mike

Rick had to take off, but like the White House Press Secretary, I’d like to “clarify” what he meant to say. The roof sheet on #1361 calculated out to a MAWP of approximately 155 PSI. For a boiler meant to hold 205 PSI that amounts to a F of S of 3.0, only 75% of what it should be.