Steam & Excursion > Belpair firebox question

I remember reading and being told that in order to fix the belpair firebox that is on the Pennsy engines that the sheets would need to be replaced and have a smaller pitch for the stays and that a thicker roof sheet won't work as the pitch will still be wrong. Why won't a thicker sheet (which is stronger) work? To me it would make more sense to only replace the roof sheet instead of roof and crown and adjust the pitch?

Per the link below, the maximum pressure for a stayed surface is P = C*(T^2)/(P^2) where C is a constant, T is the thickness of the sheet in 16ths of an inch and P is the pitch.

For a boiler where the sheet is 3/8" thick and has a pitch of 5" and the bolts are threaded into the sheets, the mawp is 161 psi. However if the sheet were increased were increased to 1/2", then the mawp is 307 psi.

https://books.google.com/books?id=-uc_AQAAMAAJ&pg=PA851&lpg=PA851&dq=boiler+calculations+for+stayed+surface&source=bl&ots=yGloJMtRr3&sig=MPOUyii26Y03-K_rNMeKH1DXH40&hl=en&sa=X&ei=lM88VbrOAuqwsATGmYGACQ&ved=0CCwQ6AEwAQ#v=onepage&q=boiler%20calculations%20for%20stayed%20surface&f=false

 

Everything must be kept at the same thickness.  If not the expansion difference will cause  the area around staybolts to deteriorate and if left unnoticed could lead to disastrous results..

Not exactly true Tom, look at the rebuild of Alaska 557, new thicker sheets were installed in her. Went from 3/8" to 7/16" in both the crown and side sheets under the guidance of Robert Franzen

Posted from iPhone

I have also seen boiler prints that call for different thickness of sheets for the firebox and boiler courses. A thicker sheet will expand more than a thinner, but to what degree? I was taught that the general rule of thumb for steel expanding is 0.0008" expansion for 10" per 10 degrees. So from 0F to 400F a 1/2" thick plate of steel expand 0.004" more than a 3/8" thick plate of steel.
tomstp Wrote:
-------------------------------------------------------
> Everything must be kept at the same thickness. 
> If not the expansion difference will cause  the
> area around staybolts to deteriorate and if left
> unnoticed could lead to disastrous results..

> I have also seen boiler prints that call for
> different thickness of sheets for the firebox and
> boiler courses.

He probably wasn't saying firebox sheets
were as thick as the boiler shell.

But the prints I have seen have called for different thickness for the firebox sheets, depending on their location. Difference in expansion to me doesn't make sense when talking about the thickness. The length of a 10' long piece of 3/8" plate should pretty much expand the same about as a 10' long piece of 1/2" plate, right?

I was talking about the firebox  side sheets and crown sheet need to be the same thickness

tomstp Wrote:
-------------------------------------------------------
> I was talking about the firebox  side sheets and
> crown sheet need to be the same thickness

Even that doesn't seem to make sense. Steel expands linearly in all directions when a plate is evenly heated. The linear or X-Y expansion of the plate is independent of the thickness of the plate; it is only controlled by the material coefficient of expansion, the temperature of the object, and the size of the plate.

For instance a 3/8" steel plate heated from 60F to 400F and having 5" staybolt spacing will expand about 20 mils between each staybolt. A 3/4" plate will have exactly the same expansion over the same temperature range. However, the thickness of the 3/4 plate will expand twice as much as the 3/8 plate: 3.1 mils vs about 1.5 mils. Of course, the staybolt in tht staybolt hole is also made of steel, and has approximately the same temperature change, so will not see any change in length relative to the plate thickness.
 

I am no boiler expert at all, but I would think that there is more in this than just expansion and contraction of the roof sheet or the crown sheet.  I think that the staybolt spacing, and the number and placement  of both flexible and non flexible staybolts would have a lot to do with things.  Thicker steel is not always the answer and the combination of thicker steel, and improper types and placement of staybolts can create situations of extreme danger that may not even be noticed by either engine or shop crews, as Tom and I both know regarding one of our most favorite engines.  

In the other thread, it was said that the problem was that the washers and nuts were removed from the crown stays at some point and that is what caused the design to have a safety factor less than 4. It was not said that the stays were too stressed. So if the stays are not under too much stress, why not replace the roof sheet with a thicker sheet? With a roof sheet, one is not worried about heat transfer.

Better read the other thread again. The stays do have more than the limit allowed by FRA.

Posted from iPhone