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Boiler tube subject to ext pressure
QUESTION:Does anyone on this group have information for sizing the wall thickness
required for a copper model steam boiler firetube 2 1/2 in in dia.
subject ot 120 PSI of external pressure. The copper tube web site
doesn't mention external pressure. The tube will be in the annealed
state after soldering.
ANSWER: Weeeeell, like the answer to most engineering problems, "that depends".
Apart from the material properties and the wall thickness, tubes
subjected to external pressures may also experience a mode of failure
called catastrophic collapse (the tube, in effect, gets squashed flat).
The onset of this is quite difficult to ascertain because quite a small
out-of-roundness condition will have a large effect on the tube's
resistance to catastrophic collapse.
The way this is usually dealt with is as follows:
1) make the tube wall very thick, say 10 to 20 times thicker than the
theoretical analysis would indicate.
2) attach circumferencial reinforcing diaphragms (disks) along the
length of the tube on the outside, or inside.
3) build a test setup and test a piece of considered tube to
destruction, noting the collapsing pressure (pressure gauge will take a
sudden drop at collapse) and take point 1) above into consideration.
I am sure the website would be interested in hearing the views of aircraft
and submarine designer's views on this subject.
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