Metrohm NIRS XDS Process Analyzer – DirectLight/NonContact - Manual (Page 15)

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2.1.2 Purging
COMMON EQUIPMENT IN CLASS I AREAS
As strictly defined by NFPA 496, this method is a start-up process of Class I area pressurizing which
removes flammable vapors from a protected enclosure. This is accomplished by exchanging a known
volume of protective gas, while maintaining a minimum positive enclosure pressure of 0.10 inches of
water.
2.2 Class, Group and Division Ratings
The 1993 edition of NFPA 496 recommends 4 volume exchanges for all enclosures and 10 volume
exchanges for all motors.
NOTE: The word “purging” is commonly used as a term to define the complete process of
pressurizing protected enclosures in Class I areas.
Class Ratings
Classes are used to define the explosive or ignitable substances which are present in the
atmosphere.
Class I Flammable gases or liquid vapors (See examples in Group Ratings)
Class II
Ignitable metal, carbon or organic dusts (See examples in Group
Ratings)
Class III Ignitable fibrous materials
Group Ratings
Groups are used to define substances by rating their explosive or ignitable nature, in relation to
other known substances.
TYPICAL CLASS I SUBSTANCES
Group A
Acetylene (XDS Process Analytics system is not used in Group
A)
Group B Hydrogen or > 30% Hydrogen by Volume
Group C Ethyl Ether & Ethylene
Group D Acetone, Ammonia, Benzene & Gasoline
TYPICAL CLASS II SUBSTANCES
Group E Aluminum, Magnesium & Alloys
Group F Carbon, Coke & Coal
Group G Flour, Grain, Wood, Plastic & Chemicals
Division Ratings
Divisions are used to define the degree of hazard by determining the explosive or ignitable
substance’s expected concentration in the atmosphere.
Division 1 Contains substances under normal conditions
Division 2 Contains substances under abnormal conditions
IMPORTANT NOTES:
Division 1 areas must be surrounded by Division 2 areas. Class II, Group E areas must be rated as
Division 1 areas.