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Re: Rigid vs Flexible residential diaphr

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>I think that the reason that pressure vessel codes, design and fabrication 
>are much more stringent than building codes is because of the catastrophic 
>failures that did occur with pressure vessels at one time.
Pretty much right, but there's more to it, namely that the insurance business was taking some serious hits, and they wanted to do something about it. Instead of renting a couple of congressmen (about as common in 1910 as at other times) they sought out the ASME and set up rules for construction. The rules were called 'a dangerous form of socialism' and 'an affront to personal liberty' (history _does_ repeat itself ;->) but they worked. After all the pre-code vessels were retired (or blew up) the accident rate dropped like a rock. We still have about the same number of injuries and deaths as in 1900, but since then the number of vessels in service is up by hundreds of thousands, maybe more. 

>And, when design of pressure vessels is changed by people who say, "We 
>much more than those old f**ts, and can design these vessels with a smaller 
>factor of safety, and less inspection and less ... " then I think that you 
>will see a resurgence of pressure vessel failures.
Truth to tell, now that manufacturing has become passé in the US, there are fewer old farts and more academics on code committees. We're also seeing more effort at stretching the life of existing equipment, _and_ a 12% decrease in design margins. So if Petroski's 20 year cycle works for pressure vessels like it works for aircraft and bridges, I expect you're right.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)