During a recent design review, we encountered some questionable "normal breathing" designs. I have always used the simplest of tests to ensure the tank has ADEQUATE venting during filling and emptying. Although OSHA allows the designer to use API 2000 (1968) or "other accepted standards," the simplest test is that the VENTING capability MUST be as large as the filling or withdrawal connections, but in no case smaller than 1.25".

However, unfortunately, designers fail to read ALL of .106(b)(2)(iv), which goes on to state: (emphasis by me)

If any tank or pressure vessel has:

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