Basic concepts and definitions

Industrial enterprises are often characterized by high explosion and fire, since they are distinguished by the complexity of the production units, a significant amount of flammable and combustible liquids, liquefied flammable gas, solid combustible materials, a large number of containers and vehicles in which there are flammable under pressure, an extensive network of pipelines with balancing equipment, large equipment installations.

Fire – this is a special uncontrolled burning hearth, causing material damage.

Burning – a chemical oxidation reaction is accompanied by the release of large amounts of heat and glow. For the occurrence of combustion requires a combustible, an oxidant (generally oxygen from the air) and the ignition source. Furthermore, it is necessary that the fuel substance was heated to a certain temperature and was in a certain proportion with an oxidant, and ignition source would have certain energy. Oxidants are chlorine and fluorine, nitrogen oxides and other substances.

According to GOST 12.1.004-91 Occupational Safety Standards System «Fire safety. General requirements» fire safety – a state of the object, which prevents the possibility of fire, and in the case of its occurrence prevents impact on people hazardous factors of fire and protects property. Considering this definition preventive measures and fire protection system are developed. Regulatory risk of fire shall be equal to not more than 10^-6 per year at selected hot item of the object. The same probability of exposure to fire hazards, based on the individual person (risk) is adopted in the design of fire protection systems.

Hazardous factors of fire are high air temperature and objects, open flames and sparks, toxic products of combustion and the smoke, the reduced oxygen concentration, explosion, damage and the destruction of buildings and structures.

Substances capable of independently burning after removal of the ignition source are called combustible. Substances, which in the air that do not burn are called non-combustible. Nonflammable substances will ignite under the influence of the source of ignition, but stop burning after removing it.

Fire and explosion hazard substances and materials are determined by the parameters (properties) that characterize the limiting conditions for the occurrence of the combustion process. If combustible material is a gas, its main indicators are:

1) The concentration limits of flame propagation (CL) or the limits of ignition;

2) The rate of flame spread U_n;

3) The minimum explosive oxygen content (MEOC);

4) Auto-ignition temperature T_ai;

5) Pressure explosion P_max;

6) The speed of its rise dP/dt;

7) The minimum ignition energy (MIE).

Also apply as indicators: the lower flammability limits (LFL) and the upper concentration limit of flame spread (UFL). In assessing the flammability of liquids indicators listed above are supplemented by the following: flashpoint T_f; ignition temperature T_ig; temperature limits flame spread (TS); lower limit (LL) and the upper limit (UL) – a liquid temperature at which the vapor pressure creates a concentration of the liquid corresponding to flammability limits.

Fire danger of solids and materials characterized by their tendency to self-ignition and fire.

The following types of burning:

a) Flash (inflammation) – quick combustion of the fuel mixture without the formation of high pressure gas;

b) The fire – the emergence of combustion from the ignition source;

c) Ignition – fire, accompanied by the appearance of the flame;

d) Spontaneous combustion – the burning that occurs in the absence of an external source of ignition;

d) Self-ignition – spontaneous combustion, accompanied by the appearance of the flame;

e) Explosion – extremely rapid combustion, in which there is an allocation of energy and the formation of compressed gas capable of producing mechanical damage.

Flash point is the lowest temperature of the flammable material, in which over its surface vapors and gases are formed capable of producing flash in the air from the source of ignition, but the rate of formation of vapors and gases is insufficient for stable combustion. Flashpoint values ​​used for classification of liquids on the degree of fire risk, in determining the production category explosive, explosion and fire risk in accordance with the requirements of SNIP; classes of explosive and fire-hazardous zones in accordance with the requirements of the Rules for Electrical Installation (PUE), as well as the development of measures to ensure fire safety and explosion protection in accordance with GOST 12.1.004-91 «Occupational safety standards system. Fire safety. General requirements» and GOST 12.1.010-76 «Occupational safety standards system. Explosion safety. General requirements».

By flash point flammable substances are divided into two classes:

1) Flammable liquids. These include a liquid having a flash point not exceeding 61 °C (or 66 °C in a crucible covered) – gasoline, ethanol, acetone, etc.

2) Combustible liquids. Liquid having a flash point above 61 ° C are a class of combustible liquids (oil, fuel oil, formalin etc.).

Burning point is the least temperature of combustible substance at which it emits combustible vapors and gases with such speed that at a present of a source of ignition there is a steady combustion.

Self-ignition point is called the lowest temperature of substance at which it lights up in the course of heating without immediate contact with fire.

Auto ignition is possible only at particular ratios of combustible substance and oxidizers. There are concepts: lower and top concentration limits of inflaming. The interval between them is called the range or area of inflaming. Distinguish also temperature limits of inflaming.

Processes of self-ignition depending on an internal impulse happen:

1. Chemical. Chemical self-ignition occurs from exposure to the substance of oxygen, air, water, or by the interaction of substances (spontaneous combustion of oiled rags, protective clothing, wool and even metal shavings).

2. Microbiological. Microbiological spontaneous ignition occurs at a suitable humidity and temperature in vegetable products (a fungus).

3. Heat. Thermal self-ignition occurs as a result of prolonged exposure to small heat source, wherein the substance decomposed and adsorbed as a result of self-heating, oxidation processes (sawdust, hardboard, and parquet at 100 °C).

There are other indicators to assess fire hazard substances determined by standard methods.

Fire and explosive hazardous substances and materials – similar characteristics, explain largely the same performance. The difference between these characteristics is the flame propagation speed, which is substantially higher for the blasting process than during a fire. Knowledge of flame propagation is necessary for the assessment of possible explosive load of explosive buildings and  construction, and also for the calculation and projection of the safety (easily thrown off) designs intended for overpressure dumping.