Locomotive boiler question

[crueby]

One of those odd thoughts rolled through the brain this morning...  On locomotive boiler smokeboxes, the front usually has a huge thick door, very heavy metal, lots of clamps.


Why?  The smokebox is not holding pressure, just needs a door to seal in the hot air going past on its way up the stack. Seems like they way overengineered the doors. What am I missing?

[cnr6400]

For good draft in a boiler, there ideally needs to be a negative pressure / partial vacuum in the smokebox (S-B) to draw the fire through the tubes. A decent air seal is needed at the S-B door. If one central clamp were used the S-B front and door would need to be REALLY heavy to ensure a seal even with wide temperature variations potentially causing distortion of S-B front and door and gaps in the sealing. North American S-B's used relative light pressed steel plate for box fronts, and used multiple clamps to make a sealed joint at many points around the door. Short span between clamps means distortion is controlled locally and no big gaps can develop. Cast S-B doors and pressed steel S-B doors were both used. 

[Jo]

Yes agreed the Smokebox door is holding back a pressure differential, just look at the blast going up the chimney 

Jo

[crueby]

Cool, thanks!

[ChuckKey]

Yes agreed the Smokebox door is holding back a pressure differential, just look at the blast going up the chimney 

Jo

I take it that's Guinness.

[10KPete]

Yes agreed the Smokebox door is holding back a pressure differential, just look at the blast going up the chimney 

Jo

Which is an important part of the blast nozzle design, maximizing the blast while keeping the smoke box pressures under control.

Excellent subject!

 

Pete

[Jasonb]

On locomotive boiler smokeboxes, the front usually has a huge thick door, very heavy metal, lots of clamps.

Not being a loco man can I ask what is considered "very heavy metal" and are "lots of clamps" used on American locos as ones here would tend to only have a single dart and locking lever in the middle. Same with traction engines even the largest heavy workhorses like ploughing engines only have the central lock and the door would have been pressed from 3/8 to 1/2" plate, smokebox itself rolled from 3/8" plate.

[Zephyrin]

The advantage of this kind of door is that it eliminates the horizontal bar across the smoke box for the central locking dart, making easier access to the interior when carrying out frequent maintenance work.
https://upload.wikimedia.org/wikipedia/commons/b/b1/Porte_de_bo%C3%AEte_%C3%A0_fum%C3%A9e_de_type_unifi%C3%A9.JPG

And this frequency of maintenance work greatly depends on the design of the inside of the smoke box.

Many US locos had a self cleaning device using part of the exhaust to sweep the bottom of the smoke box, where the cinders and unburnt residues tend to accumulate, to expel them through the chimney.

[cnr6400]

"Not being a loco man can I ask what is considered "very heavy metal" and are "lots of clamps" used on American locos as ones here would tend to only have a single dart and locking lever in the middle. Same with traction engines even the largest heavy workhorses like ploughing engines only have the central lock and the door would have been pressed from 3/8 to 1/2" plate, smokebox itself rolled from 3/8" plate."

1/2" thick plate was used on many engines in US and Canada for smokebox front plate. Pressed doors were also often 1/2". Cast doors varied from 1/2" to 1" thick depending on size and where the thickness varied in the door. Smokebox diameters varied, on small engines they might be 62", large engines they could be 84" dia. On the UP Big Boy they were 109". Some pics of smokeboxes on Canadian engines are attached. No 9 is a small 0-6-0 switching engine and the big one is partly stripped CN 6200, a 4-8-4 Northern. Many engines had the front plate attached by dozens of studs and nuts to a heavy ring inside the front end diameter of the smokebox. For major service like a boiler re-tube the whole front plate and door would be removed for access. For regular service and periodic clean-out the central door would be opened after loosening the multiple clamps and swinging them out of the way. Just FYI.

[Kim]

Fascinating thread!  Thanks for starting it Chris, and thanks for all the contributions.  I'm learning a lot here
Kim

[crueby]

Fascinating thread!  Thanks for starting it Chris, and thanks for all the contributions.  I'm learning a lot here
Kim

I'm learning a lot too - had not considered the lower pressure that the draw would create, and how the door has to hold up with that large a surface area. Any leaks there, and it would reduce the draft back at the firebox. Thanks everyone!

[cnr6400]