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Vaults that contain tanks of Class I liquids shall be provided with continuous ventilation at a rate of not less than l cfm/ft
2
of floor area [0.00508m
3
/(s x m
2
)], but not less than 150 cfm (4 m
3
/min). Failure of the exhaust airflow shall automatically shut down the dispensing system. The exhaust system shall be designed to provide air movement across all parts of the vault floor. Supply and exhaust ducts shall extend to a point not greater than 12 inches (305 mm) and not less than 3 inches (76 mm) above the floor. The exhaust system shall be installed in accordance with the provisions of NFPA 91. Means shall be provided to automatically detect any flammable vapors and to automatically shut down the dispensing system upon detection of such flammable vapors in the exhaust duct at a concentration of 25 percent of the LFL.