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…