31. Fire and explosion risk indexes

I. Self-ignition point: the lowest temperature of a substance that causes self-ignition.

II. Concentration limits of ignition: lower and upper ones.

The lowest concentration of flammable gas or vapor at which it can already be ignited is called lower limit of ignition.

The highest concentration of flammable gas or vapor at which it can still be ignited is called upper limit of ignition.

III. Flashpoint: the lowest temperature of a substance that causes flash.

Using flash point flammable liquids are divided into two classes:

• high-inflammable: those with the flash point lower or equal 61 deg C, such as gasoline, spirit, acetone;

• inflammable: those with the flash point over 61 deg C, such as oil, black oil.

IV. Ignition point: the lowest temperature of the substance that causes ignition.

V. Temperature limits of ignition: temperatures corresponding to concentration limits.

VI. Flammability group:

• inflammable (flammable): liable to catch fire;

• hard-inflammable: liable to catch fire only in presence of igniter;

• nonflammable: unable to catch fire.

The indexes applicability for certain substance (table 4) depends on what aggregation state it has: liquid, gaseous, solid or dust.

Table 4

Applicability of fire-explosive indexes

Index	Fire-explosive indexes application for
	gas	liquid	solid	dust
Flammability group	+	+	+	+
Flash point	-	+	-	-
Ignition point	-	+	+	+
Self- ignition point	+	+	+	+
Lower (upper) concentration limits of ignition	+	+	-	-
Temperature limits of ignition	-	+	-	-

32. Fire and explosion risk area categories

In reference to standard «SNIP 24-86» all work areas are divided into 5 categories: «A», «Б», «В», «Г», «Д».

Work area is relevant to some category dependently on characteristics of substances stored or used in the area (table 5).

Table 5

Fire and explosion risk area categories

Area Category	Characteristics of materials and substances stored in the area
1	2
«A» fire and explosion risk area	Inflammable substances with: lower limit of ignition 10% or lower; flashpoint lower or equal 28 deg C; in condition that mentioned substances can make explosive mixtures in volume covering 5 % of the total area volume. Such as: painting workshops, areas storing pressured gas.
Б fire and explosion risk area	Inflammable substances with: lower limit of ignition over 10%; flashpoint over 28 to 61 deg C; in condition that mentioned substances can make explosive mixtures in volume covering 5 % of the total area volume. Such as: areas using ammonia, and emitting flammable dusts.
«B» fire risk area	Inflammable substances with: flashpoint over 61 deg C; solid flammable substances liable only to catch fire but not explosion.
«Г»	Nonflammable substances in hot, molten and red-hot condition; And also substances which are burned as fuel.
«Д»	Areas using nonflammable substances in cold condition.

The importance of accurate defining Category of fire-explosive risk for an area or a building is extremely high because the considered Category significantly defines requirements to building’s construction and floor plan, organization of fire safety and its technical equipment.