Steam & Excursion > The 4:1 safety factor and reality of in-service forces...

Over the years, serveral aspects of where the 4:1 requirement originated have been kicked about.
That accepted standard has to do with the strength of the materials, and the stronger cross-sections of things like rivets, staybolt, thread strength, etc.

Another factor was the political power of the PRR,  in the early 1900s,  because if it’s size, to influence the lawmakers and the early regulators.
One thing that Pennsy had experience in was the explosions that followed Loco head-on collisions.   PRR people realized that simply building boilers to withstand the vagaries of service stresses ( lower factors of safety) was not good enough in case of collisions.

So, a lot of the structural integrity is incorporated to allow the boiler shell to withstand the increased stresses of a crash....at speed.
So, there are a lot of factors, other than just feel-good, security-blanket, dart-board figures.

Now, let’s address the reduced strength of the firebox sheets, with low water conditions....
Softening of the firebox sheets, primarily the crown sheet, is critical in the threaded holes of the sheets.
As the overheated metal softens, it softens & slips down the threads of the stay bolts....   The early solution was the use of crown bolts with tapered threads ...   The taper is inverted, so that the largest diameter is greater at the bottom of the bolt than at the shaft.Tapered crown bolts are screwed up from the bottom of the crown sheet, often with outer, spherical nuts, at the outer ends.

[ There is a growing practice of using welded-only stays ( instead of threaded stays) in the firebox sheets...The sheets’ staybolt holes are counter-sunk to allow a stronger root-weld at each of the welded stays.  An advantage is that the weldment carries the heat of the fire smoothly between the side-sheets and the stays.]

So, what happens,  in a progressive ( meaning that the water level continues to recede) overheating of the crown,
is that in the area of the overheating, the crown begins to strip/slip down the threads, as the sheet steel gets softer and softer... 
Explosions happen when a large area, is pushed-off the threads under hundreds of tons of pressure on the sheets.
Each crown bolt can support 16-square inches , times 250 to 300 lbs. per square inch.... that’s a A LOT OF pressure.

When the massive collapse occurs an area of 20 or 30 bolts lets go all at once, sending the boiler skyward, propelled like a rocket..
What’s at work is the steel becoming softened by the heat.

Remember that firebox sheets are nominally 3/8” thick —-( calculate how many threads are holdin, at 12TPI.).
The fireside of any thicker firebox sheets cannot be kept cool by the water.
So, any-thicker firebox sheets are subject to surface melting.  3/8” thick is about the thickest sheets that can be used for fireboxes...

If you experiment by heating 3/8” steel sheet, the steel gets softer and softer...
If you heat the steel to a barely-visible, very dark, dull-red, it will be near 1000 deg F.....the dark-red steel is 8-Times weaker than steel that is submerged... A visible dull-red would be 16-times weaker,  & a glowing-red is 32-Times weaker.  
 ( That’s the reason  for a blacksmith’s forge)
Ergo, dropped crown sheets are very easy to occur, if even slightly overheated...!

It was found that with ALL-tapered crown bolts, it aggravated the severity of explosions by having the sheets withstand (contain) the pressure, even as the water level gets lower and lower...then WHAM!!
ALL the subjected bolts’ threads & holes completely strip at once, the explosion happens...

The CP railroad, at sometime in the ‘30’s & ‘40’s adopted a different approach to crown stays-design.
Their design utilizes straight threaded crown stays starting with the FRONT TWO ROWS of bolts, side to side, across the crown sheet.  Next, they  used the tapered bolts for two rows, then, two rows of straight threaded stays, alternating the length of the crown sheet.

So, when CP 1278’s low-water event occurred, the upper front two rows stripped/slipped off of the bolts, offering a controlled release of pressure —— letting the 400 degree water erupt, exploding, into into the firebox, relieving the pressure, but scalding the crew.  The “controlled release”,  prevented a more massive explosion and launching of the boiler.   

If it were up to me, I would have mandated the CP ‘s crown-stay design, to have been adopted ( for new construction) by the now-updated ( revised) boiler-construction and inspection FRA Loco Inspection Rules.

So, there’s a lot to be grasped as to what happens during a low water, dropped crown sheet event.
The earlier-cited NTSB report on the Gettysburg Boiler Explosion, has very good illustrations of the layout of a typical CP- safety
crown sheet —- showing the alternated rows of tapered and straight crown stays...

Makes sense to me...

​W.

Not proofed, yet

 



Edited 7 time(s). Last edit at 10/13/18 07:05 by wcamp1472.