Exhaust ducts flammable vapors

Dedicated exhausts used for flammable vapors should be of ferrous construction. Flammable liquids do not have an adverse effect on materials so there is no reason to use plastic ducts. Flammable vapors could condense and the duct should be sprinklered. This condensation of the material could occur where the duct is subject to change in temperature or the solvent stream is subject to a change in velocity. 

Although hoods are most commonly used to control concentrations of toxic vapors, they can also serve to dilute and exhaust flammable vapors. Although theoretically possible, it is extremely unlikely (even under most worst-case scenarios) that the concentration of flammable vapors will reach the lower explosive limit (LEL) in the exhaust duct. However, somewhere between the source and the exhaust outlet of the hood, the concentration will pass through the upper explosive limit (UEL) and the LEL before being fully diluted at the outlet. Both the hood designer and the user should recognize this hazard and eliminate possible sources of ignition within the hood and its ductwork if there is a potential for explosion. The use of duct sprinklers or other suppression methods in laboratory fume ductwork is not necessary, or desirable, in the vast majority of situations. 

UMC 505.1 requires that ducts which convey explosives or flammable vapors, fumes or dusts (toxics are included in this group) shall extend directly to the exterior of the building, without entering other spaces. We believe it isunacceptableto connect gas cabinet exhaust to a woikstation or any other exhaust ventilation system. 

Exhaust ducts with a cross-sectional diameter of 10 inches or more, which contain flammable vapors, must contain fire protection per the Uniform Mechanical Code Section 609.7 and UFC Article 5104.3.2.4. UMC 609.7 and NFPA 318 2-1.2 cover design criteria for sprinkler systems for such ductwork (see reference below). 

Laboratories are normally classified nonhazardous locations if the quantities of flammable and combustible liquids are within the requirements of NFPA. Normally a vapor collection hood is provided when sampling and measurements are conducted with exposed liquids. The primary concern is the exhaust of vapors and the storage and removal material saturated with liquids. The exhaust hood, ducting and a radius of 1.5 meters (5 ft.) from the exhaust vent should be considered an electrically classified area. 

---