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505.3 Flammability Limit
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Unless the circumstances stipulated in Section 505.3.1, Section 505.3.2, or Section 505.3.3 exist, in systems conveying flammable vapors, gases, or mists, the concentration shall not exceed 25 percent of the lower flammability limit (LFL). [NFPA 91:4.2.3]
505.3.1 Higher Concentrations
Higher concentrations shall be permitted where the exhaust system is designed and protected in accordance with NFPA 69 using one or more of the following techniques:
- Combustible concentration reduction
- Oxidant concentration reduction
- Deflagration suppression
- Deflagration pressure containment [NFPA 91:4.2.3.1]
Contaminated air shall not be recirculated to occupied areas unless contaminants have been removed. Air contaminated with explosive or flammable vapors, fumes, or dusts; flammable or toxic gases; or radioactive material shall not be recirculated.
505.3.2 Ovens and Furnaces
Higher concentrations shall be permitted for ovens and furnaces designed and protected in accordance with NFPA 86. [NFPA 91:4.2.3.2]
505.3.3 Deflagration
Higher concentrations shall be permitted where deflagration venting is provided in accordance with NFPA 68. [NFPA 91:4.2.3.3]
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