Classes
Classes apply to the Division Classification, as well as the Zone Classification.
Class I: A location where there is a danger of explosion due to the
presence of a flammable gas or vapor.
Class II: A location where there is a danger of explosion due to the presence of a flammable dust.
Class III: A location where there is a danger of explosion or flash fire
due to the presence of flammable fibers
or flyings.
Division Classification
Division 1: A location where an
explosive mixture of gas, vapor, dust, fibers or flyings and air may exist under normal operating conditions.
Division 2: A location where an
explosive mixture of gas, vapor, dust, fibers or flyings may exist under abnormal conditions, such as the accidental rupture of a storage vessel, or container, or the failure of a ventilation system.
Division Gas Groups
The following are representative gas groupings.
|
|
Group A–Atmospheres containing
Acetylene
Group B–Atmospheres containing Hydrogen
Group C–Atmospheres containing Ethyl, Ether, Ethylene
Group D–Atmospheres containing Acetone, Gasoline, Propane
Zone Classification
Zone 0: A location in which an explosive gas-air mixture is continually present or present for long periods of time.
Zone 1: A location in which an explosive gas-air mixture is likely to occur in normal operation.
Zone 2: A location in which a gas-air mixture is not likely to occur and if it does occur it will be for short periods of time.
Classifying a location to Zone/Division is a complex problem that is usually accomplished in the design stage by the process engineers and/or chemical engineers. Each room, section or area shall be
considered individually in determining its classification.
The main factors to be considered are:
|
|
• the probability of the presence of
gas;
• the quantity and the duration of
the gas;• the amount of
ventilation;
• the nature of the gas–lighter or
heavier than air
• the consequence of an
explosion.
Zone Gas Groups
The following are representative gas groupings.
Group 1: Methane–Relates to underground applications only, example: coal mines.
Group A–Propane
Group B–Ethylene
Group C–Hydrogen-Acetylene
Gases, vapors or mists present in the atmosphere are classified by the energy required to ignite the various gas/air
mixtures.
As can be seen under Division Gas Groups and Zone Gas Groups, the alpha designator is different for the Division and Zone. The Zone indicator is in rough correlation to that of the European Norm, whereas the Division classification relates only to the U.S. Division system. |
|
