The OP indicated that the classification had already been evaluated; presumably by someone qualified to do so. If no qualified person has been involved so far, I suggest getting one. Otherwise, this is essentially a DIY, which this site frowns on.
Originally Posted by martnr
To answer this particular post, there is no set amount of volatiles; however, it's obvious that there must be some limits. API RP 500 makes some suggestions in its Appendix D. For gases and fluids, vapor pressures, process flow rates, process temperatures and potential leaks or other releases (such as relief plates or valves or collection spigots) to the surrounding atmosphere all play part in the evaluation. Add to that means and methods of ventilation and you have a start. Appendix E outlines the evaluation methodology.
NFPA 30 is the typical Standard for evaluating adequate ventilation. Beyond flow rates, evaluating the location for potential "dead spaces" where volatiles could collect is also important.
It should be noted that NFPA 30 is a "Code", intended for enforcement even if it isn't locally adopted. API RP 500 is only a "Recommended Practice" but it is ANSI sanctioned, so both Standards are enforceable by FedOSHA and most State OSHAs (including Alaska) under their various General Duty Clauses. Like NEC Appendices, API RP 500 Appendices aren't technically part of the Standard, but ignoring them without a damn good reason is still an OSHA no-no.
Robert B. Alexander, P.E.
Answers based on 2014 NEC unless otherwise noted.