Explosion Testing for dust, gases & vapours

Limiting oxygen concentration (LOC)

Maximum oxygen concentration to prevent ignition

The limiting oxygen concentration (LOC) is the highest concentration of oxygen that prevents a combustible dust forming an explosive atmosphere. The test is mandatory if inerting is to be used as a method of dust explosion prevention under ATEX or DSEAR. Testing is carried out according to BS EN 14034-4:2004 (Determination of the limiting oxygen concentration LOC of dust clouds). The test is carried out in the 20 litre sphere apparatus. Five grams of combustible dust is placed in the dust container and the explosion chamber is filled with a known concentration of oxygen in nitrogen. An automatic test sequence is initiated to pressurise the dust container to 20 bar and activate the ignition source 60 msec after the dust has been dispersed. Once ignition is achieved at a known oxygen concentration, the level of oxygen is reduced until a point is reached at which no ignition occurs. At an oxygen level 1 % above the minimum found, the quantity of dust injected is adjusted to find the most explosive concentration in a reduced oxygen atmosphere (which is generally at lower dust concentrations than at normal atmospheric conditions). Subsequent tests are carried out with this mass of dust, and the oxygen concentration is gradually reduced until no ignition is found in three consecutive tests. An ignition is deemed to have occurred if the maximum explosion pressure is at or above 0.5 bar. Test work is normally undertaken with nitrogen as the inerting gas, but tests may also be carried out using carbon dioxide or argon at additional cost. In practical applications, it is usual to include a safety margin when applying the test results to plant design. The safety margin should reflect the size of the plant and its control system for rectifying oxygen increases, plus the level of oxygen monitoring installed, the sensor accuracy and response time. The safety margin is unlikely to be less than 2%. For example, if dust explosion testing determines the LOC to be 8%, the maximum permitted oxygen concentration (MPOC) allowed on plant might be set to 6%.

LOC test results table

Material

LOC

Coal dust

14% in N 2

Maize starch

9% in N 2

Aluminium dust

5% in N 2

Sewage sludge

11% in N 2

Explosion Testing

Limiting oxygen

concentration (LOC)

Maximum oxygen

concentration to prevent

ignition

The limiting oxygen concentration (LOC) is the highest concentration of oxygen that prevents a combustible dust forming an explosive atmosphere. The test is mandatory if inerting is to be used as a method of dust explosion prevention under ATEX or DSEAR. Testing is carried out according to BS EN 14034- 4:2004 (Determination of the limiting oxygen concentration LOC of dust clouds). The test is carried out in the 20 litre sphere apparatus. Five grams of combustible dust is placed in the dust container and the explosion chamber is filled with a known concentration of oxygen in nitrogen. An automatic test sequence is initiated to pressurise the dust container to 20 bar and activate the ignition source 60 msec after the dust has been dispersed. Once ignition is achieved at a known oxygen concentration, the level of oxygen is reduced until a point is reached at which no ignition occurs. At an oxygen level 1 % above the minimum found, the quantity of dust injected is adjusted to find the most explosive concentration in a reduced oxygen atmosphere (which is generally at lower dust concentrations than at normal atmospheric conditions). Subsequent tests are carried out with this mass of dust, and the oxygen concentration is gradually reduced until no ignition is found in three consecutive tests. An ignition is deemed to have occurred if the maximum explosion pressure is at or above 0.5 bar. Test work is normally undertaken with nitrogen as the inerting gas, but tests may also be carried out using carbon dioxide or argon at additional cost. In practical applications, it is usual to include a safety margin when applying the test results to plant design. The safety margin should reflect the size of the plant and its control system for rectifying oxygen increases, plus the level of oxygen monitoring installed, the sensor accuracy and response time. The safety margin is unlikely to be less than 2%. For example, if dust explosion testing determines the LOC to be 8%, the maximum permitted oxygen concentration (MPOC) allowed on plant might be set to 6%.