In 2004, NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids established a “sizing basis” for flammable duct work (as well as Combustible Dust ductwork). This sizing basis comes from the fact that the code prohibits the flammable/explosive concentrations from exceeding 25% of the LEL…
4.2.3
Unless the circumstances stipulated in 4.2.3.1, 4.2.3.2, or 4.2.3.3 exist, the following concentration limits shall apply:
(1) In systems conveying flammable vapors, gases, or mists, the concentration shall not exceed 25 percent of the LFL.
(2) In systems conveying combustible particulate solids, the concentration shall not exceed 25 percent of the MEC.
(3) In systems conveying hybrid mixtures, the combined percentage of the lower flammable limit (LFL) of flammable vapors and the percentage of the minimum explosible concentration (MEC) of combustible dusts shall not exceed 25 percent.
4.5.9*. The system shall be designed and maintained to ensure that the air-gas velocity used meets or exceeds the minimum required to keep the interior surfaces of all piping or ducting free of accumulations under all normal operating modes. [660:9.4.3.1.1]