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:
- more than one (1) fill or withdrawal connection, AND
- SIMULTANEOUS filling or withdrawal can be made
the vent size shall be based on the MAXIMUM anticipated simultaneous flow.
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1910.106(b)(2)(iv)(a) Atmospheric storage tanks shall be adequately vented to prevent the development of vacuum or pressure sufficient to distort the roof of a cone roof tank or exceeding the design pressure in the case of other atmospheric tanks, as a result of filling or emptying, and atmospheric temperature changes. 1910.106(b)(2)(iv)(b) Normal vents shall be sized either in accordance with:
1910.106(b)(2)(iv)(b)(1) The American Petroleum Institute Standard 2000 (1968), Venting Atmospheric and Low-Pressure Storage Tanks, which is incorporated by reference as specified in § 1910.6; or
1910.106(b)(2)(iv)(b)(2) other accepted standard; or
1910.106(b)(2)(iv)(b)(3) shall be at least as large as the filling or withdrawal connection, whichever is larger but in no case less than 1¼ inch nominal inside diameter.
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1910.106(b)(2)(iv)(d) If any tank or pressure vessel has more than one fill or withdrawal connection and simultaneous filling or withdrawal can be made, the vent size shall be based on the maximum anticipated simultaneous flow.
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This essentially means that if we have three (3) fill lines that total 7 inches, and it is possible that we could be filling the tank SIMULTANEOUSly through all three (3) fill lines, then we would need 7″ venting size. The same would apply to discharge lines: three (3) discharge lines that total 7 inches, AND it is possible that we could be emptying the tank SIMULTANEOUS through all three (3) discharge lines, then we would need 7″ of vacuum venting size.
Maybe API 2000 allows for some fudging of these numbers, but this is basic physics and is an EXCELLENT litmus test during a conceptual design review.