Classification of Gas and Vapor
Group of Gas
Group II (surface industries) comprises three subdivisions IIA , IIB and IIC defined according to the maximum experimental safe gap ( MESG ) for Exd envelopes or after the minimum ignition energy (EMI ) for intrinsically safe equipment . Most A2S products are designed for group IIC ( the most dangerous ) and can be used almost anywhere. However, some attention to explosion proof products (projectors, cabinets ) for which we generally have 2 separate solutions for groups IIB and IIC .
It should indeed use the ExdIIC solutions in the presence of hydrogen, acetylene, carbon disulphide or ethyl nitrate, and this because of the high cost of such equipment . The ExdIIB solutions are sufficient for all other types of gases or vapors effect.
Equipment using only the method of protection increased safety Exe does not appear Subdivision A , B or C because it can be used irrespective of the gas present.
Classes of temperature
Each gas has a self- ignition temperature of its own . When this temperature is reached, the gas ignites spontaneously, without sparks or arcing . The auto-ignition temperature should be considered as a limit not reached, either in normal operation or not. Standards define six classes of temperature (T1 to T6) corresponding to 6 maximum surface temperatures (450°C at 85°C).
Temperature class T6 is the strictest .
Classification of dust
Each type of dust has been studied in detail to determine their flammability characteristics (INRS Brochures/Guide silos … ) . The characteristic dust is their auto-ignition temperatures differ depending on whether the dust is in clouds or layers. Like many other parameters can change the auto-ignition values (particle size , humidity, température…), a calculation incorporating a safety margin must be made for each case.
- In the case of dust cloud , the maximum surface temperature of the equipment shall not exceed two-thirds of the ignition temperature in degrees Celsius, the dust / air mixture (T1).
- In the case of dust layers less than 5 mm, a safety factor of 75°C must be deduced from the temperature of self-ignition layer, in degrees Celsius , dust concerned (T2).
The choice of material will be done by taking the worst result.
The table below shows the results obtained for the most common dust :
Practical example:
On a manufacturing milk powder , the user wants to install a projector in ATEX zone 21 . The maximum surface temperature to be taken into account for the milk powder is 125 ° C. The PX04 projectors are suitable for zones 21 and 22 (category 2D). However a large portion of this range may be used as the maximum surface temperature is greater than 125 ° C. Only 70W versions (T90°C) and 150W (T115°C) can be installed in this area .
NOTE: ATEX certified products on the ” DUST “, the maximum surface temperature is shown in full in the dust marking on the label.
