RRR_BethBr At the risk of re-resurrecting an old thread - I think you're both correct. According to toot-toot, crown bar construction was an earlier/different method of boiler construction than what your cross-section depicts, which is the typical stay bolt method. By definition, the boiler you posted wouldn't need crown bars, because it was designed not to have them. According to my research, crown bars would have been found either a) spanning top sheet b) underneath the roof sheet, or c) both places, and attached/stayed to each other by metal supports I've seen called "sling stays". In either case, the crown bars would still be bolted to whatever structure they were supporting (the roof or top sheets). Think of the roof bracing of a cathedral, and you'll have some idea of how a crown-bar firebox would look (though not exactly, of course).
At the risk of re-resurrecting an old thread - I think you're both correct. According to toot-toot, crown bar construction was an earlier/different method of boiler construction than what your cross-section depicts, which is the typical stay bolt method. By definition, the boiler you posted wouldn't need crown bars, because it was designed not to have them.
According to my research, crown bars would have been found either a) spanning top sheet b) underneath the roof sheet, or c) both places, and attached/stayed to each other by metal supports I've seen called "sling stays". In either case, the crown bars would still be bolted to whatever structure they were supporting (the roof or top sheets). Think of the roof bracing of a cathedral, and you'll have some idea of how a crown-bar firebox would look (though not exactly, of course).
I had the impression that crown bars were designed so that the stay bolts could pass through both the crown sheet and roof sheet at approximately 90 degrees. That would be something automatically taken care of with a Belpaire--hence no need for crown bars. However, the below source takes a different view:
The 1916 "Locomotive Dictionary" defines "crown bar":
"A beam extending across the water space above the crown sheet of wagon top boilers to support the stay bolts holding up the crown sheet where the opening for the steam dome prevents staying the crown sheet to the roof sheet. They are usually supported at the ends by castings resting on the top edges of the side sheets of the firebox and by sling stays from the roof sheet or dome shell. Thimbles on the crown sheet stay bolts maintain the proper spacing between the crown sheet and the crown bars. Not in general use now."
Note that toot toot mentions crown bars as being on Belpaire fireboxes.
I am not finding any photos of Belpaire fireboxes with a steam dome above the firebox. In particular, on the PRR.
I did find a cross-section drawing of a firebox (anthracite burning, it looks like) with a crown bar and sling stays:
I would think the left side would represent a section looking backwards--hence the big round hole for inserting coal. There are three pipe-looking things above the crown sheet, two of which present at an angle. I think these go from the roof sheet back to the rear sheet for the purpose of staying that large expanse of sheet metal above the height of the crown sheet.
And the right side is looking forward, showing the boiler tubes. Note also the small round "circle" up towards the top of the steam space that represents the pipe containing the throttle linkage.
It looks to me that wagon-top boilers would have to have the steam dome opened to clean the top of the crown sheet, because the crown bars were indeed in the way. On a wagon-top boiler. Of which it does not appear there were any Belpaires.
Ed
Here's another drawing showing a boiler with a crown bar:
Note that it is not a locomotive, but a portable boiler.
And, changing the subject, a French 4-4-6 cab forward: