Many large scale processes that use Cat 1, 2, and 3 flammable liquids may also rely on a Nitrogen purge system to ensure the space within the process vessel is BELOW the Limiting Oxidizer Concentration (LOC) – sometimes called the Minimum Oxygen Concentration (MOC). This safety system is based on the simple fact that if we maintain the level of oxidizer, usually the oxygen in the air but there are other oxidizers that may be present, BELOW the flammable’s LOC/MOC then if we were to have a static discharge in the flammable layer of vapor in the space we would NOT have an ignition because there is not enough oxygen available to support combustion. But these LOC/MOCs are not an exact science and because of this, we need to establish some “safety factors” in how we design our safety systems if we want to rely on some type of purging. And of course, my go-to source for my Recognized and Generally Accepted Good Engineering Practice (RAGAGEP) is NFPA.
NFPA 69, Standard on Explosion Prevention Systems encourages us to use ASTM E2079-07 – Standard Test Methods for Limiting Oxygen (Oxidant) Concentration in Gases and Vapors when we are establishing our LOC for our process flammable atmospheres. But, if we wish to use the LOC’s provided in NFPA 69 Table C.1(a), the standard requires us to use one of two Safety Factors, depending on if the stated LOC is above or below 10% Oxygen.