I have researched this subject on these forums and found some good reference ANSI FCI 70-2 (old B16.104) and API 598 forum. I'm still confused though.
I have a vendor who quoted me 'bubble tight shutoff' for several full bore ball valves. No mention even of FCI 70-2 (Classes I thru VI for control valves - some ball valve companies do) or even API 598 on his literature. I looked through all the specs and can't find a definition of the Bubble Tight Shutoff term. (I've 'googled' it to death.)
Can anybody relate the term to a 'spec/standard/test' at all ?

Bubble tight means different things to different people, if your buying a valve from these guys specify the test you want to conform to there are several classifications that are used for valve tests as you have mentioned. Go back and have them confirm they conform to your requirements. That said my experience is bubble tight is synonomous with class V test level (but it wouldnt stand up in a court).

The term "bubble tight seal" is very old.  My understanding is that it originated from a test where a valve was installed in an air system with an open end pointing up.  Some soap was poured into the open end and air was applied to the other side.  The valve passed if no bubbles formed in the soap.  I'm not sure if this is true or not, but it was the only explanation I ever heard that was at least plausible.

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