COMBUSTION OF SOLIDS AND MATERIALS
When extinguishing fires, most often one has to deal with the combustion of solid combustible substances and materials (TGM). Therefore, knowledge of the mechanisms of the occurrence and development of combustion of THM is important when studying the discipline "Theory of combustion and explosion".
Most THMs belong to class of organic substances(see fig. 5.1), consisting mainly of carbon, hydrogen, oxygen and nitrogen. Chlorine, fluorine, silicon and other chemical elements can be part of many organic substances, and most of the THM elements are flammable.
A significantly smaller amount of THM belongs to a class of inorganic substances,many of which are also fire and explosion hazardous. Fire hazards are well known, for example magnesium, sodium, which are prone to spontaneous combustion on contact with water. In addition, extinguishing metal fires is associated with significant difficulties, in particular, due to the unsuitability for these purposes of most fire extinguishing agents.
It should be borne in mind that when grinding THM, their fire and explosion hazard increases sharply, for example, wood, grain, coal in a state of dust become explosive. Wood dust in the fibreboard workshop starts to explode already at a concentration of 13-25 g / m; wheat flour in mills - at a concentration of 28 g / m 3, coal dust in mines - at 100 g / m 3. Metals, when ground to powder, ignite spontaneously in air. Other examples can be cited.
The composition of THM influences the features of their combustion (see Table 5.1). So, cellulosicmaterials, in addition to carbon and hydrogen, contain oxygen (up to 40-46%), which participates in combustion in the same way as atmospheric oxygen. Therefore, cellulosic materials require a much smaller volume of combustion air than materials that do not contain oxygen (plastics).
Figure: 5.1. Classification of solid combustible substances and materials
This also explains the relatively low heat of combustion of cellulosic materials and their tendency to smolder. Among them, fibrous(cotton wool, linen, cotton), cavities and pores of which are also filled with air, which contributes to their combustion. In this regard, they are extremely prone to smoldering, the method of extinguishing with insulation is ineffective for them, moreover, in real conditions they practically do not extinguish. The combustion of such substances proceeds without the formation of soot.
A characteristic property of other cellulosic materials is their ability to decompose when heated to form combustible vapors, gases and carbon residues. So, during the decomposition of 1 kg of wood, 800 g of combustible gaseous decomposition products and 200 g of charcoal are formed, during the decomposition of 1 kg of peat - 700 g of volatile compounds, and cotton - 850 g. In addition to the nature of fuel, the amount and composition of emitted volatile substances depends on the temperature and the heating mode of this substance.
Table 5.1.
The composition of some cellulosic materials
The flammability of substances and a variety of materials is of prime importance to determine the likelihood of a flame occurring. This characteristic determines the category of fire hazard of structures, premises, industries; allows you to choose the right means for eliminating foci.
The flammability group of all material components of the object determines the success of fighting a fire and minimizes the likelihood of casualties.
Features of various substances
It is known that substances can be in various states of aggregation, which are important to take into account when determining the flammability group. GOST provides for a classification based on quantitative indicators.
If the substance can burn, before for fire safety, the most optimal flammability group is G1 than G3 or G4.
Flammability is of great importance for finishing, heat-insulating, building materials. On its basis, the class of fire hazard is determined. So, gypsum plasterboard sheets have a flammability group G1, stone wool - NG (not burning), and to insulate expanded polystyrene belongs to the flammability group G4, and the use of plaster helps to reduce its fire hazard.
Gaseous substances
Determining the flammability class of gases and liquids, the standards introduce such a concept as the concentration limit. By definition, it is the limiting concentration of a gas in a mixture with an oxidizing agent (air, for example) at which a flame can spread from the point of ignition to any distance.
If such a boundary value does not exist, and the gas cannot spontaneously ignite, then it is called non-combustible.
Liquid
Liquids are called flammable if there is a temperature at which they can ignite. If the liquid stops burning in the absence of an external source of heating, then it is called slow-burning. Non-flammable liquids do not ignite at all in an air atmosphere under normal conditions.
Some liquids (acetone, ether) can flash at 28 ℃ and below. They are classified as especially dangerous. Flammable liquids at 61 ... 66 ℃ and above are classified as flammable (kerosene, white spirit). The tests are carried out in an open and closed crucible.
Solid
In the field of construction, the most relevant is the definition of the flammability group of solid materials. It is preferable to use substances of the flammability group G1 or NG, as the most resistant to ignition.
Classification
The intensity of the combustion process and the conditions for its course determine the likelihood of an intensification of a fire, an explosion. The outcome of an incident depends on the combination of properties of the feedstock.
General division
According to the national standard of fire and explosion hazard, substances and various materials from them are divided into the following groups:
- absolutely non-flammable;
- difficult to burn;
- flammable.
They cannot burn in air, which does not exclude interaction with oxidants, with each other, and water. Consequently, some members of the group, under certain conditions, present a fire hazard.
Compounds that are difficult to burn are those that burn when ignited in air. As soon as the source of the fire is eliminated, the combustion stops.
Combustible substances under certain conditions ignite themselves or in the presence of a fire source, continue to burn intensely.
The flammability classification of construction materials and products is considered in a separate updated standard. National building codes take into account the categories of all types of products used in work.
According to this classification, non-combustible building materials (NG) are divided into two groups depending on the test mode and the values \u200b\u200bof the indicators obtained in this case.
Group 1 includes products, in the study of which the temperature inside the furnace increases by no more than 50 ℃. The decrease in the sample mass does not exceed 50%. The flame does not burn at all, and the heat released does not exceed 2.0 MJ / kg.
Group 2 NG includes materials with the same indicators of temperature increase inside the furnace and weight loss. The difference is that the flame burns for up to 20 seconds, the heat of combustion should not exceed 3.0 MJ / kg.
Flammability classes
Combustible materials are examined according to similar criteria, divided into 4 groups or classes, which are designated by the letter G and the number next to it. For classification, the values \u200b\u200bof the following indicators are taken into account:
- temperature of gases emitted with smoke;
- reduction in size;
- the amount of weight reduction;
- flame retention time without a combustion source.
G1 includes a group of materials with a smoke temperature not exceeding 135 ℃. Loss of length is 65%, weight loss - 20%. The flame itself does not burn. Such construction products are called self-extinguishing.
G2 includes a group of materials with a smoke temperature not exceeding 235 ℃. Loss of length is 85%, weight loss is 50%. Self-burning lasts no more than 30 seconds.
G3 includes materials whose smoke temperature does not exceed 450 ℃. Loss of length is more than 85%, weight loss is up to half. The flame itself burns for no more than 300 seconds.
The flammability group G4 includes materials in which the smoke temperature exceeds 450 ° C. Loss of length exceeds 85%, weight loss - more than 50%. Self-burning lasts more than 300 seconds.
It is permissible to use the following prefixes in the name of each flammability group in ascending order of the digital index:
- weak;
- moderately;
- fine;
- highly flammable materials.
The given indicators of flammability, along with some other characteristics, must be taken into account when developing project documentation, drawing up estimates.
The ability to form smoke, the toxicity of combustion products, the rate of possible spread of fire, the likelihood of rapid ignition are also of great importance.
Class confirmation
Samples of materials are tested in laboratories and in open areas according to standard methods separately for non-combustible and combustible building materials.
If the product consists of several layers, the standard provides for checking the flammability of each layer.
Determinations of flammability are carried out on special equipment. If it turns out that one of the components has a high flammability, then this status will be assigned to the product as a whole.
The setup for carrying out experimental determinations should be located in a room with room temperature, normal humidity, and no drafts. Bright sunlight or artificial light in the laboratory should not interfere with the reading of the displays.
Before starting the study of the sample, the device is checked, calibrated, and heated. Then the sample is fixed in the holder of the inner cavity of the oven and the recorders are immediately turned on.
The main thing is that no more than 5 seconds have passed since the sample was placed. The determination is continued until the temperature balance is reached, at which within 10 minutes the changes do not exceed 2 ° C.
At the end of the procedure, the sample together with the holder is taken out of the oven, cooled in a desiccator, weighed and measured, reckoning them to the flammability group NG, G1, and so on.
Flammability test method
All building materials, including finishing, facing, paint and varnish types of coatings, regardless of homogeneity or multi-layer, are tested for combustibility by a single method.
Pre-prepare 12 units of identical samples with a thickness equal to the real values \u200b\u200bduring operation. If the structure is layered, samples are taken from each surface.
Then the samples are kept at room temperature and normal ambient humidity for at least 72 hours, periodically weighed. Holding should be stopped when a constant mass is reached.
The plant has a standard design and consists of a combustion chamber, air supply and evacuated gases.
The samples are placed in turn in the chamber, measurements are taken, the weight loss, temperature and amount of evolved gaseous products, combustion time without a flame source are recorded.
By analyzing all the obtained indicators, the level of flammability of the material, its belonging to a certain group, is determined.
Application in construction
When erecting buildings, several different types of building materials are used: structural, insulating, roofing, finishing with different purposes and loads. All products must be in stock and presented to potential buyers with certificates.
You should familiarize yourself with the parameters characterizing safety in advance, be firmly aware of what each abbreviation and numbers mean. The law requires the use of materials of the flammability group G1 or NG for the frames of building ceilings.
Non-combustible substances and materials
"... 2) hardly combustible - substances and materials capable of burning in air when exposed to an ignition source, but incapable of burning independently after its removal; ..."
A source:
Federal Law of 22.07.2008 N 123-FZ (as amended on 10.07.2012) "on fire safety requirements"
Official terminology... Academic.ru. 2012.
See what is "Refractory substances and materials" in other dictionaries:
Non-combustible (hardly combustible) substances and materials - - substances and materials that can burn in air when exposed to an ignition source, but are not able to burn independently after its removal. [GOST 12.1.044 89] Term heading: Building materials Encyclopedia headings: Abrasive ... ...
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flammability - flammability - the ability of a substance, material, product to self-combustion. According to G., substances, materials, products, structures are divided into: 1) combustible - capable of self-combustion after removing the ignition source; ... ... Encyclopedia "Aviation"
Non-combustible substances and materials
"... 1) non-combustible - substances and materials that are incapable of burning in the air. Non-combustible substances can be fire-and-explosive (for example, substances that emit flammable products when interacting with water, air oxygen or with each other); ..."
A source:
Federal Law of 22.07.2008 N 123-FZ (as amended on 10.07.2012) "on fire safety requirements"
"... - non-combustible material - a material that, when heated to 750` C, does not burn and does not emit flammable gases in an amount sufficient for their spontaneous combustion; ... "
A source:
Ministry of Transport of the Russian Federation of 12.02.2004 N 12 "On the rules of fire safety when carrying out hot work on ships located at the berths of seaports and ship repair enterprises"
Official terminology... Academic.ru. 2012.
See what "Non-combustible substances and materials" is in other dictionaries:
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Hazardous substances - substances with a potential hazard to humans. According to the potential danger, cause a fire, intensify the dangerous factors of fire, poison the environment (air, water, soil, flora, fauna, etc.), affect humans ... ... Russian encyclopedia of labor protection
Flame retardant materials - materials, the reduced combustibility of which is achieved by special treatment (fire protection). Methods of fire protection include: applying a layer of non-combustible or low-combustible substances to the surface of materials; introduction to the composition ... ... Great Soviet Encyclopedia
Fire safety - This article should be wikified. Please, arrange it according to the rules of article formatting ... Wikipedia
Fire safety - Fire safety is a state of protection of an individual, property, society and the state from fires. "Fire safety" is an illiterate phrase that is used to mean "fire safety". Contents 1 ... ... Wikipedia
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COMBUSTION - Exothermic reaction proceeding in conditions of its progressive self-acceleration. In terms of flammability, substances and materials are divided into three groups: non-combustible (non-combustible) substances and materials incapable of burning in air. Non-flammable substances ... ... Comprehensive security and anti-terrorist protection of buildings and structures
flammability Encyclopedia "Aviation"
flammability - flammability - the ability of a substance, material, product to self-combustion. According to G., substances, materials, products, structures are divided into: 1) combustible - capable of self-combustion after removing the ignition source; ... ... Encyclopedia "Aviation"
According to their ability to burn (flammability), fire hazardous substances are divided into flammable, hardly flammable and non-flammable.
Combustible flammability substances and materials are divided into three groups:
1) highly flammable;
2) substances of "medium flammability";
3) hardly flammable.
Flammable- flammable substances of increased fire hazard, which, when stored outdoors or indoors, are capable of igniting without preheating upon short-term (up to 30 s) exposure to a low-energy ignition source (from a match flame, spark, cigarette, etc.)
1. Non-flammableNot capable of burning in air. Asbestos fabric, asbestos glass fabric, foam asbestos, metals used in construction, building materials: sand, clay, gravel, cement and products from them (brick, concrete), etc.
2. Hardly flammableCapable of igniting in air from an ignition source, but not
capable of burning after removing it. Consisting of combustible and non-combustible materials: glass fiber laminate SK-9A, glass fiber laminate FN-F, felt, foam concretes with polystyrene filler, three chlorine ethylene С2НCl3, weak aqueous solutions of alcohols, etc.
3. CombustibleCapable of spontaneous combustion, as well as ignite and burn independently after removing the ignition source.
Solid:
organic:wood, coal, peat, rubber, cotton, cardboard, rubber, stearic acid, etc .; inorganic:metals (potassium, sodium, lithium, aluminum, etc., and their compounds);
non-metallic:(sulfur, phosphorus, silicon, etc. and their compounds), including dust (organic - coal, wood, sugar, flour, etc.; inorganic - iron, aluminum, silicon, sulfuric, etc.)
Liquid:
oil and petroleum products, alcohols, acids, paraffins, hydrocarbons1, etc., including synthetic materials that melt when heated.
Gaseous:
hydrogen, hydrocarbons, ammonia, etc., as well as vapors of flammable liquids.
Highly flammablesolids (materials): celluloid, polystyrene, wood shavings, peat plates (ignited by the flame of a match, spirit lamp, gas burner).
Medium flammability: wood, coal, paper in bundles, fabric in rolls (requires a high-energy ignition source that can warm up to the ignition temperature).
Flammable: urea (carbamide) CH4ON2, getinax grade B (pressed paper treated with synthetic resin of the resole type), wood after fire retardant treatment, PVC board.
Non-flammable substances. The assignment of many substances and materials to the non-combustible group is largely arbitrary, since we are talking about combustion in atmospheric air (O2 content ~ 21% vol.). However, these substances can interact upon contact with ozone (O3), fluorine F2, liquid oxygen.
In addition, there are several groups of non-flammable substances that are fire hazardous:
1.oxidants: KMnO4, Cl2, HNO3, O2liquid., Na2O2, H2O2, etc .;
2. Substances that emit combustible products when interacting with water and with each other (calcium carbide CaC2, Na, quicklime, CaO, H2SO4 + + Me H2 (burning);
3. thermally unstable substances (ammonium carbonate), (NH4) 2СО3 Н2СО3 + 2NH3;
4. substances capable of explosive transformation without the participation of acid
kind of air (heating or shock), 4BaN6 Ba + Ba3N2 + 11N2 + 297 kJ.
Hardly flammable substances. Such substances are heterogeneous in terms of fire hazard and are divided into three groups:
First group: combustion is only possible with a powerful ignition source, for example in a fire.
Second group: capable of emitting flammable vapors and gases when heated, which have a certain area of \u200b\u200bignition.
Third group: some explosives - ammonium nitrate NH4NO3, NH4NO3 \u003d НNO3 + NH3 - 170 kJ (when heated).
The released NH3 is capable of burning (but if the source is removed, the process stops). Otherwise, the process proceeds if ammonium nitrate is exposed to a heat pulse or a powerful detonator:
NH4NO3 N2 + 2H2О + 0.5О2 + 126 kJ.
1. Non-flammable - incapable of burning in an air of normal composition. A sufficient criterion for referring to this group is the inability of the material to burn at an ambient temperature of 900 ° C, this group includes natural and artificial organic materials and metals used in construction.
2. Hard-combustible - capable of burning under the influence of an ignition source, but not capable of self-burning after its removal. These include: wood subjected to an effective fire-retardant treatment by coating or impregnation (beshefit); felts impregnated with clay mortar, some polymers and other materials.
3. Combustible - capable of burning independently after removing the ignition source. In terms of flammability, flammable substances are divided into non-flammable and hardly flammable.
Flammable - these are flammable substances (materials, mixtures) that can ignite from short-term exposure to the flame of a match or spark. Knee wire and similar low energy ignition sources. These include almost all flammable gases (for example, hydrogen, methane, carbon monoxide, etc.), flammable liquids (GF) with a flash point of no more than 61 ° C in a closed crucible or 66 ° C in an open crucible (for example, acetone , gasoline, benzene, toluene, ethyl alcohol, kerosene, turpentine, etc.), as well as all solid substances (materials) that ignite from the flame of a match or burner, the premiere spreads over the surface of a horizontally located test sample (for example, dry wood shavings , polystyrene, etc.).
Flammable - these are flammable substances (materials, mixtures) that can ignite only under the influence of a powerful ignition source (for example, PVC conveyor belt, urea foam for sealing the surface of a rock mass in underground mines, flexible electric cables with PVC insulation, ventilation pipes made of vinyl leather and etc.).