Bottom Support and Tube Slenderness Question

[vobs]

I was studying the B&W book and I wonder, how does one justify a "bottom supported" design and allow increasingly larger and larger slenderness ratios in the tubes supporting the steam drum. I realize this is a pretty general question, but I'm looking for some basic info here. Mind you, I have a civil / structural background, so I would say you limit your slenderness ratio to less than 200. Assuming fixed-fixed end condtions for 2" tubes, that would give you a max length of approx. 16 feet. Is there a differnt design approach I should consider? Thanks.

 

[chicopee]

I dont believe that the steam drum is supported by the tubes. I think that the steam drum is suspended or supported from a steel frame work protected by refractory or other insulating material. Which design are you looking at?

 

[vobs]

I don't have a specific design in mind, but I believe you are thinking of a top supported design. Right now I'm interested in bottom supported design. Isn't that where all pressure parts are supported off the lower drum which in turn is supported by a base, saddles or other mechanism?
In the latest B&W book "STEAM", there is Fig. 23 which shows a small indus. boiler. My understanding is that the steam drum is supported by the boiler bank which in turn is supported by the mud drum. Is this correct?

 

[chicopee]

The reason for tubes not to support the steam drum is that in the event circulation is interrupted in the heat absorbing tubes or you had low water condition in the steam drum, then the would get red hot and soften at which point the steam drum would collapse due to its weight with or without water.

 

[rjoaks]

Bottom supported steam drums are supported by larger and heavier downcommer tubes. In some casses these downcommers are not exposed to the products of combustion or are not in the high heat release zone of the furnace.

rjoaks

 

[vobs]

in cases w/o downcomers, what is there a practical upper limit to the distance between drums. What are the criteria for determining that?