Inerting Extinguishing System
Inciting systems prevent the formation of explosive air-vapor mixtures. The inert gas takes up space, displacing the oxygen, thus preventing accumulation of sufficient oxygen to support combustion. Nitrogen, and occasionally carbon dioxide, are usually used as inciting agents for such applications. Inert gases such as argon have special application for materials such as zirconium that burn in nitrogen. The limiting oxygen concentration (LOC) to support combustion can vary depending on the inert gas used, and flammability tests maybe needed with complex mixtures to experimentally measure the lower limits of flammability based on the volumetric concentration of the combustible gas.
Inerting systems are commonly employed to pad or blanket the space above flammable liquid storage tanks or other vessels including mixing tanks, ball mills, centrifuges, and other equipment. Inerting is also used to reduce the concentration of flammable vapor in a vessel prior to cleaning or maintenance. In this case, purging the enclosure with the inert material reduces the concentration of the flammable material below its lower flammable limit without the vapor space passing through the flammable range by eliminating the oxygen. Once the flammable vapors have been removed, the inert gas must be replaced with oxygen and confined space entry procedures followed before the vessel can be entered. Inerting systems for combustible gas releases by vent stacks have been employed to control accidental ignitions. NFPA 69, “Explosion Prevention Systems” provides guidance on the design and calculation methods used in these systems and those designed to prevent accumulation of flammable vapors.
Water as an inert diluent has been used for many years in steam smothering systems in process plants. Care is needed since the steam can condense out and the space become flammable again. Also, steam discharge can cause static chaige accumulation. Atmospheres such as methane and air that would otherwise be explosive can be made inert by the addition of fine water fogs with water droplet size of approximately 1 micron. However, since highly atomized water spray systems typically produce 100- to 200-micron droplets which are far too large, water spray inerting is not as practical as one might assume.