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.
- If the flammable’s LOC/MOC is GREATER than 10%, we MUST subtract at least 1.5% from that value given to us in NFPA 69.
- If the flammable’s LOC/MOC is LESS THAN than 10%, we MUST multiply the value given by 0.85
7.2.3.1.2 For gases and vapors, if the LOC values according to ASTM E2079 are not available, then the LOC values obtained in flammability tubes shall be used after adjustment by subtracting 1.5 percent by volume oxidant for LOC values of 10 percent or greater or by multiplying by a factor of 0.85 for LOC values less than 10 percent, as indicated in the adjusted columns in Table C.1(a).
I should also point out that the LOC/MOCs are also DEPENDENT on which interting gas is used in the safety system. Which again requires a “safety margin” in the design of the safety system.
7.7.2.3 A safety margin shall be maintained between the LOC and the normal working concentration in the system.
7.7.2.4 The safety margin shall take into account all of the following factors:
(1) Fluctuations occurring in the system
(2) Sensitivity and reliability of monitoring and control equipment
(3) Probability and consequences of an explosion
7.7.2.5 One of the following requirements shall be met where the oxygen concentration is continuously monitored and controlled with safety interlocks:
(1) Where the LOC is greater than or equal to 5 percent, a safety margin of at least 2 volume percent below the worst credible case LOC shall be maintained.
(2) Where the LOC is less than 5 percent, the equipment shall be operated at no more than 60 percent of the LOC.
7.7.2.6 Risk Assessment.
A documented risk assessment acceptable to the AHJ shall be permitted to be conducted to determine the safety margin to be maintained between the worst credible case LOC and safety interlocks addressed in 7.7.2.5.
7.7.2.7 The requirement of 7.7.2.5 shall not apply to partial oxidation processes.
7.7.2.8 Where the oxygen concentration is NOT continuously monitored and controlled with safety interlocks, one of the following requirements shall be met:
(1) Where the LOC is GREATER THAN OR EQUAL TO 7.5 percent, a safety margin of at least 4.5 volume percent below the worst credible case LOC shall be maintained.
(2) Where the LOC is LESS THAN 7.5 percent, the oxygen concentration shall be designed to operate at no more than 40 percent of the LOC.