Crap. 3

5.1.2.4. Sparks of static electricity
Spark energy ( W i), J, which can arise under the influence of voltage between the plate and any grounded object, is calculated from the energy stored by the capacitor from the formula
(85)
Where WITH- capacitor capacity, F;
U- voltage, V.
The potential difference between a charged body and the ground is measured with electrometers under real production conditions.

If W and³0.4 W m.e.z ( W m.e.z ¾ minimum ignition energy of the medium), then a spark of static electricity is considered as an ignition source.
The real danger is posed by “contact” electrification of people working with moving dielectric materials. When a person comes into contact with a grounded object, sparks with an energy of 2.5 to 7.5 mJ occur. The dependence of the energy of an electrical discharge from the human body and the potential of static electricity charges is shown in Fig. 4.
5.1.3. Mechanical (friction) sparks (sparks from impact and friction)
The size of impact and friction sparks, which are a piece of metal or stone heated to the point of glow, usually does not exceed 0.5 mm, and their temperature is within the melting point of the metal. The temperature of sparks formed during the collision of metals capable of entering into chemical interaction with each other with the release of a significant amount of heat can exceed the melting point and therefore it is determined experimentally or by calculation.
The amount of heat given off by a spark when cooling from the initial temperature t n to the self-ignition temperature of the flammable medium t sv is calculated using formula (84), and the cooling time t is calculated as follows.
The temperature ratio (Qp) is calculated using the formula
(86)
Where t c - air temperature, °C.
Heat transfer coefficient ( a), W×m-2×K-1, calculated by the formula
(87)
Where w and - spark flight speed, m×s-1.
Spark speed ( w i), formed upon impact of a freely falling body, is calculated using the formula
(88)
and when hitting a rotating body according to the formula
(89)
Where n- rotation speed, s-1;
R- radius of the rotating body, m.
The flight speed of sparks generated when working with a percussion tool is taken equal to 16 m×s-1, and from those struck when walking in shoes lined with metal heels or nails, 12 m×s-1.
The Biot criterion is calculated using the formula
(90)
Where d and - spark diameter, m;
li is the thermal conductivity coefficient of the spark metal at the self-ignition temperature of the combustible substance ( t sv), W m -1×K-1.
According to the values ​​of the relative excess temperature qп and the criterion IN i is determined from the graph (Fig. 5) by the Fourier criterion.

Crap. 5

The cooling time of a metal particle (t), s, is calculated using the formula
(91)
Where F 0 - Fourier criterion;
WITH and is the heat capacity of the spark metal at the self-ignition temperature of the combustible substance, J×kg-1×K-1;
ri is the density of the spark metal at the self-ignition temperature of the combustible substance, kg×m-3.
If there are experimental data on the igniting ability of friction sparks, a conclusion about their danger for the analyzed flammable environment can be made without performing calculations.
5.1.4. Open flames and sparks from engines (furnaces)
The fire hazard of a flame is determined by the intensity of the thermal effect (density heat flow), area of ​​impact, orientation (relative position), frequency and time of its impact on flammable substances. The heat flux density of diffusion flames (match, candle, gas burner) is 18-40 kW×m-2, and pre-mixed ( blowtorches, gas-burners) 60-140 kW×m-2 In table. 6 shows the temperature and time characteristics of some flames and low-calorie heat sources.
Table 6

Spark discharge

Spark discharge(electric spark) - a non-stationary form of electrical discharge occurring in gases. Such a discharge usually occurs at pressures on the order of atmospheric pressure and is accompanied by a characteristic sound effect - the “crackling” of a spark. The temperature in the main channel of the spark discharge can reach 10,000. In nature, spark discharges often occur in the form of lightning. The distance “pierced” by a spark in the air depends on the voltage and is considered equal to 10 kV per 1 centimeter.

Conditions

A spark discharge usually occurs when the power of the energy source is insufficient to support a steady-state arc discharge or glow discharge. In this case, simultaneously with a sharp increase in the discharge current, the voltage across the discharge gap for a very short time (from several microseconds to several hundred microseconds) drops below the extinction voltage of the spark discharge, which leads to the termination of the discharge. Then the potential difference between the electrodes increases again, reaches the ignition voltage, and the process repeats. In other cases, when the power of the energy source is sufficiently large, the whole set of phenomena characteristic of this discharge is also observed, but they are only a transient process leading to the establishment of a discharge of another type - most often an arc one. If the current source is not capable of maintaining a self-sustained electrical discharge for a long time, then a form of self-sustained discharge called a spark discharge is observed.

Nature

A spark discharge is a bunch of bright, quickly disappearing or replacing each other thread-like, often highly branched stripes - spark channels. These channels are filled with plasma, which in a powerful spark discharge includes not only ions of the source gas, but also ions of the electrode substance, which intensively evaporates under the action of the discharge. The mechanism for the formation of spark channels (and, consequently, the occurrence of a spark discharge) is explained by the streamer theory of electrical breakdown of gases. According to this theory, from electron avalanches arising in electric field discharge gap, under certain conditions, streamers are formed - dimly glowing thin branched channels that contain ionized gas atoms and free electrons split off from them. Among them we can highlight the so-called. leader - a weakly luminous discharge that “paves” the path for the main discharge. Moving from one electrode to another, it closes the discharge gap and connects the electrodes with a continuous conductive channel. Then the main discharge passes in the opposite direction along the laid path, accompanied by a sharp increase in the current strength and the amount of energy released in them. Each channel rapidly expands, resulting in a shock wave at its boundaries. The combination of shock waves from the expanding spark channels generates a sound perceived as the “crack” of a spark (in the case of lightning, thunder).

The ignition voltage of a spark discharge is usually quite high. Tension electric field in a spark decreases from several tens of kilovolts per centimeter (sq/cm) at the moment of breakdown to ~100 volts per centimeter (v/cm) after a few microseconds. The maximum current in a powerful spark discharge can reach values ​​of the order of several hundred thousand amperes.

A special type of spark discharge - sliding spark discharge, which occurs along the interface between a gas and a solid dielectric placed between the electrodes, provided that the field strength exceeds the breakdown strength of air. Areas of a sliding spark discharge, in which charges of one sign predominate, induce charges of a different sign on the surface of the dielectric, as a result of which spark channels spread along the surface of the dielectric, forming the so-called Lichtenberg figures. Processes similar to those occurring during a spark discharge are also characteristic of a brush discharge, which is a transition stage between corona and spark.

The behavior of a spark discharge can be seen very well in slow-motion footage of discharges (Fimp. = 500 Hz, U = 400 kV) obtained from a Tesla transformer. The average current and pulse duration are not sufficient to ignite an arc, but are quite suitable for the formation of a bright spark channel.

Notes

Sources

• A. A. Vorobyov, High voltage technology. - Moscow-Leningrad, GosEnergoIzdat, 1945.
• Physical Encyclopedia, vol. 2 - M.: Great Russian Encyclopedia p. 218.
• Raiser Yu. P. Physics of gas discharge. - 2nd ed. - M.: Nauka, 1992. - 536 p. - ISBN 5-02014615-3

see also

Wikimedia Foundation. 2010.

See what “Spark discharge” is in other dictionaries:

- (spark), unsteady electrical a discharge that occurs when, immediately after a breakdown of the discharge gap, the voltage across it drops for a very short time (from several fractions of a microsecond to hundreds of microseconds) below the voltage value... ... Physical encyclopedia

spark discharge- An electrical pulse discharge in the form of a luminous thread, occurring at high gas pressure and characterized by high intensity of spectral lines of ionized atoms or molecules. [GOST 13820 77] spark discharge Full discharge in... ... Technical Translator's Guide

- (electric spark) a non-stationary electric discharge in a gas that occurs in an electric field at a gas pressure of up to several atmospheres. It is distinguished by its sinuous, branched shape and rapid development (approx. 10 7 s). Temperature in the main channel... Big Encyclopedic Dictionary

Kibirkštinis išlydis statusas T sritis fizika atitikmenys: engl. spark discharge vok. Funkenentladung, f; Funkentladung, f rus. spark discharge, m pranc. décharge par étincelles, f … Fizikos terminų žodynas

Spark, one of the forms of electrical discharge in gases; usually occurs at pressures on the order of atmospheric pressure and is accompanied by a characteristic sound effect: the “crackle” of a spark. IN natural conditions I.r. most often observed in the form of lightning... ... Great Soviet Encyclopedia

An electric spark is a non-stationary electric discharge in a gas that occurs in an electric current. field at gas pressure up to several. hundreds of kPa. It is distinguished by a tortuous branched shape and rapid development (approx. 10 7 s), accompanied by a characteristic sound... ... Big Encyclopedic Polytechnic Dictionary

- (electric spark), non-stationary electric. discharge in a gas that occurs in an electrical field at gas pressure up to several. atm. It is distinguished by its sinuous, branched shape and rapid development (approx. 10 7s). Tempo pa in ch. channel I. r. reaches 10,000 K... Natural science. encyclopedic Dictionary

Calculation of fire (explosion) source parameters

At this stage, it is necessary to evaluate the ability of ignition sources to initiate flammable substances.

Four ignition sources are used in the calculation:

a) secondary action of lightning;

b) short circuit sparks;

c) electric welding sparks;

d) bulb of an incandescent lamp.

e) burning insulation of an electrical cable (wire)

Secondary lightning impact

The danger of secondary lightning exposure lies in spark discharges resulting from the inductive and electromagnetic effects of atmospheric electricity on production equipment, pipelines and building structures. The spark discharge energy exceeds 250 mJ and is sufficient to ignite flammable substances with a minimum ignition energy of up to 0.25 J.

The secondary effect of a lightning strike is dangerous for the gas that has filled the entire volume of the room.

Thermal effect of short-acting currents

It is clear that during a short circuit, when the protection device fails, the resulting sparks can ignite the flammable liquid and explode the gas (this possibility is assessed below). When the protection is triggered, the short circuit current continues a short time and is only capable of igniting PVC wiring.

The temperature of the conductor t about C, heated by the short circuit current, is calculated by the formula

where t n is the initial temperature of the conductor, o C;

I short circuit - short circuit current, A;

R - resistance (active) of the conductor, Ohm;

short circuit - short circuit duration, s;

Cpr - heat capacity of the wire material, J * kg -1 * K -1 ;

m pr - weight of the wire, kg.

In order for the wiring to ignite, it is necessary that the temperature tpr be greater than the ignition temperature of the polyvinyl chloride wiring trec. =330 o C.

We take the initial temperature of the conductor equal to the ambient temperature of 20 o C. Above in Chapter 1.2.2, the active resistance of the conductor (Ra = 1.734 Ohm) and the short circuit current (I short circuit = 131.07 A) were calculated. Heat capacity of copper C pr = 400 J*kg -1 *K -1. The mass of a wire is the product of density and volume, and volume is the product of length L and cross-sectional area of ​​the conductor S

m pr =*S*L (18)

Using the reference book, we find the value = 8.96*10 3 kg/m 3 . In formula (18) we substitute the value of the cross-sectional area of ​​the second wire from the table. 11, the shortest, that is, L=2 m and S=1*10 -6 m. The mass of the wire is

m pr =8.96*10 3 *10 -6 *2=1.792*10 -2

With the duration of the short circuit. =30 ms, according to table 11, the conductor will heat up to the temperature

This temperature is not enough to ignite PVC wiring. And if the protection is turned off, then it will be necessary to calculate the probability of the PVC wiring catching fire.

Spark short circuit

During a short circuit, sparks appear, which have an initial temperature of 2100 o C and are capable of igniting the flammable liquid and exploding the gas.

The initial temperature of the copper drop is 2100 o C. The height at which the short circuit occurs is 1 m, and the distance to the flammable liquid puddle is 4 m. The diameter of the drop is dk = 2.7 mm or dk = 2.7 * 10 -3.

The amount of heat that a drop of metal is capable of giving up to a flammable medium when cooling to its ignition temperature is calculated as follows: the average flight speed of a drop of metal during free fall w avg, m/s, is calculated by the formula

where g is the acceleration of gravity, 9.81 m/s 2 ;

H - fall height, 1 m.

We find that the average flight speed of a drop in free fall is

The duration of a drop falling can be calculated using the formula

Then the volume of the droplet Vк is calculated using the formula

Drop mass mk, kg:

where is the density of the metal in the molten state, kg*m -3.

The density of copper in the molten state (according to the teacher) is 8.6 * 10 3 kg/m 3, and the mass of the drop according to the formula (22)

m k =8.6*10 3 *10.3138*10 -9 =8.867*10 -5

Flight time of a metal drop in a molten (liquid) state p, s:

where C p is the specific heat capacity of the melt of the droplet material, for copper C p =513 J*kg -1 *K -1 ;

S to - drop surface area, m 2, S to =0.785d to 2 =5.722*10 -6;

T n, T pl - the temperature of the drop at the beginning of the flight and the melting temperature of the metal, respectively, T n = 2373 K, T pl = 1083 K;

T o - ambient air temperature, T o =293 K;

Heat transfer coefficient, W*m -2 *K -1.

The heat transfer coefficient is calculated in the following sequence:

1) first calculate the Reynolds number

where v=1.51*10 -5 1/(m 2 *s) is the coefficient of kinematic viscosity of air at a temperature of 293 K,

where =2.2*10 -2 W*m -1 *K -1 - thermal conductivity coefficient of air,

1*10 2 W*m -2 *K -1 .

Having calculated the heat transfer coefficient, we find the flight time of a metal drop in a molten (liquid) state using formula (23)

Because< р, то конечную температуру капли определяют по формуле

The self-ignition temperature of propane is 466 o C, and the temperature of the drop (spark) by the time it approaches the pool of flammable liquid is 2373 K or 2100 o C. At this temperature, isoprene will ignite and burn steadily, and propane will explode even when a short circuit spark occurs. The flash point of isoprene is -48 0 C.

Open fire and hot combustion products. Fires and explosions often arise from constantly operating or suddenly appearing sources of open fire and products accompanying the combustion process - sparks, hot gases.
An open fire can ignite almost all flammable substances, since the temperature during flaming combustion is very high (from 700 to 1500 ° C); it stands out a large number of heat and the combustion process is usually prolonged. Sources of fire can be varied - technological heating furnaces, fire reactors, regenerators with burning of organic substances from non-flammable catalysts, furnaces and installations for incineration and waste disposal, flare devices for burning side and associated gases, smoking, the use of torches for heating pipes, etc. e. The main fire protection measure against stationary sources of open fire is their isolation from flammable vapors and gases in the event of accidents and damage. Therefore, it is better to place fire-powered apparatus in open areas with a certain fire separation from adjacent apparatus or to isolate them by placing them separately in enclosed spaces.
External tubular fire furnaces are equipped with a device that allows, in case of accidents, to create a steam curtain around them, and in the presence of adjacent devices with liquefied gases (for example, gas fractionation units), the furnaces are separated from them by a blank wall 2-3 m high and a perforated pipe is laid on top of it to create steam veils. To safely ignite furnaces, electric igniters or special gas igniters are used. Quite often, fires and explosions occur during fire (for example, welding) repair work due to the unpreparedness of the equipment (as discussed above) and the sites where they are located. Fire renovation work, except
the presence of an open flame, accompanied by scattering
from the sides and the fall of hot metal particles onto the underlying areas, where they can ignite flammable materials. Therefore, in addition to the appropriate preparation of the devices to be repaired, the surrounding area is also prepared. All flammable materials and dust are removed within a radius of 10 m, combustible structures are protected with screens, and measures are taken to prevent sparks from entering the underlying floors. The vast majority of hot work is carried out using specially equipped stationary sites or workshops.
For hot work, in each individual case, a special permit from the administration and a sanction from the fire department are obtained.

If necessary, additional security measures are developed. Hot work sites are inspected by fire department specialists before and after completion of work. If necessary, a fire station with appropriate fire equipment.
For smoking on the territory of the enterprise and in workshops, special rooms are equipped or appropriate areas are allocated; To warm frozen pipes, use hot water, steam or induction heaters.
Sparks are hot solid particles of incompletely burned fuel. The temperature of such sparks is most often in the range of 700-900 ° C. When released into the air, the spark burns relatively slowly, since carbon dioxide and other combustion products are partially adsorbed on its surface.
Reducing the fire hazard from sparks is achieved by eliminating the causes of spark formation, and, if necessary, by trapping or extinguishing sparks.
Catching and extinguishing sparks during operation of furnaces and engines internal combustion achieved by using spark arresters and spark arrestors. The designs of spark arresters are very diverse. Devices for catching and extinguishing sparks are based on the use of gravity (precipitation chambers), inertial force (chambers with partitions, nozzles, meshes, louvered devices), centrifugal force (cyclones).

catchers, turbine-vortex), forces of electric attraction (electric precipitators), cooling of combustion products with water (water curtains, capture by the surface of water), cooling and dilution of gases with water vapor, etc. In some cases, they are installed

/ - firebox; 2 - settling chamber; 3 - cyclonic spark arrester; 4 - afterburning nozzle
several spark extinguishing systems in series, as shown in Fig. 3.7.
Thermal manifestation of mechanical energy. The transformation of mechanical energy into heat, which is dangerous in terms of fire, occurs during impacts of solid bodies with the formation of sparks, friction of bodies during mutual movement relative to each other, adiabatic compression of gases, etc.
Impact and friction sparks are formed when there is a sufficiently strong impact or intense abrasion of metals and other solids. The high temperature of friction sparks is determined not only by the quality of the metal, but also by its oxidation by atmospheric oxygen. The spark temperature of unalloyed low-carbon steels sometimes exceeds

1500° C. The change in the temperature of impact and friction sparks depending on the material of the colliding bodies and the applied force is shown in the graph in Fig. 3.8. Despite the high temperature, impact and friction sparks have a small heat reserve due to the insignificance of their mass. Numerous experiments have established that

Rice. 3.8. Dependence of the temperature of impact and friction sparks on the pressure of colliding bodies

The most sensitive to impact and friction sparks are acetylene, ethylene, carbon disulfide, carbon monoxide, and hydrogen. Substances that have a long induction period and require a significant amount of heat for ignition (methane, natural gas, ammonia, aerosols, etc.) are not ignited by impact and friction sparks.
Sparks falling on settled dust and fibrous materials create smoldering areas that can cause a fire or explosion. Sparks produced when aluminum objects strike the oxidized surface of steel parts have great ignition potential. Prevention of explosions and fires from sparks, impact and friction is achieved by using non-sparking tools for everyday use and during emergency work in explosive workshops; magician
thread separators and stone catchers on the lines for supplying raw materials to impact machines, mills, etc.; making machine parts that can collide with each other from non-sparking metals or by strictly adjusting the size of the gap between them.
Tools made of phosphor bronze, copper, aluminum alloys AKM-5-2 and D-16, alloy steels containing 6-8% silicon and 2-5% titanium, etc. are considered non-sparking. It is not recommended to use copper-plated tools. In all cases, where possible, impact operations should be replaced by non-impact ones*. When using steel impact tools in explosive environments, the work area is heavily ventilated, and the impacting surfaces of the tool are lubricated with grease.
The heating of bodies from friction during mutual movement depends on the condition of the surfaces of the rubbing bodies, the quality of their lubrication, the pressure of the bodies on each other and the conditions for heat removal to the environment.
Under normal conditions and proper operation of the rubbing pairs, the excess heat generated is promptly removed to the environment, ensuring that the temperature is maintained at a given level, i.e., if Qtp = QnoT, then /work = Const. Violation of this equality will lead to an increase in the temperature of the rubbing bodies. For this reason, dangerous overheating occurs in the bearings of machines and devices, when slipping conveyor belts and drive belts, when winding fibrous materials on rotating shafts, during mechanical processing of solid flammable substances, etc.
To reduce the possibility of overheating, rolling bearings are used instead of plain bearings for high-speed and heavily loaded shafts.
Great importance has systematic lubrication of bearings (especially plain bearings). For normal bearing lubrication, use the type of oil that is accepted taking into account the load and shaft speed. If natural cooling is not enough to remove excess heat, forced cooling of the bearing is arranged running water or circulating oil, provide temperature control

the ratio of the bearings and the liquid used to cool them. The condition of the bearings is systematically monitored, cleaned of dust and dirt, and overload, vibration, distortion and heating above the established temperatures are not allowed.
Avoid overloading the conveyors, pinching the belt, loosening the tension of the belt or tape. Devices are used that automatically signal when working with overload. Instead of flat-belt drives, V-belt drives are used, which practically eliminate slipping.
From the entry of fibers into the gaps between the rotating and stationary parts of the machine, the gradual compaction of the fibrous mass and its friction against the walls of the machine (in textile factories, flax and hemp-jute factories, in drying shops of factories chemical fibers etc.) reduce the gaps between shaft journals and bearings, use bushings, casings, shields and other anti-winding devices to protect shafts from contact with fibrous materials. In some cases, anti-winding knives, etc. are installed.
Heating of flammable gases and air during their compression in compressors. The increase in gas temperature during adiabatic compression is determined by the equation

where Tll1 Tk is the gas temperature before and after compression, °K; Pm Pk - initial and final pressures, kg/cm2\ k - adiabatic index, for air? = 1.41.
The gas temperature in the compressor cylinders at a normal compression ratio does not exceed 140-160 ° C. Since the final gas temperature during compression depends on the degree of compression, as well as on the initial gas temperature, in order to avoid excessive overheating when compressed to high pressures, the gas is compressed gradually in multistage compressors and cooled after each compression stage in interstage refrigerators. To avoid damage to the compressor, monitor the temperature and pressure of the gas.
An increase in temperature during air compression often leads to compressor explosions. Explosive concentrations result from the evaporation and decomposition of lubricating oil under elevated temperature conditions. Sources of ignition are sources of spontaneous combustion of oil decomposition products deposited in the discharge air duct and receiver. It has been established that for every IO0C increase in temperature in the compressor cylinders, oxidation processes are accelerated by 2-3 times. Naturally, explosions, as a rule, occur not in compressor cylinders, but in discharge air ducts and are accompanied by the combustion of oil condensate and oil decomposition products accumulating on the inner surface of the air ducts. In order to avoid explosions of air compressors, in addition to monitoring the temperature and air pressure, install and strictly maintain optimal norms supply of lubricating oil, systematically clean the discharge air ducts and receivers from flammable deposits.
Thermal manifestation of electrical energy. Thermal effect electric current may appear in the form of electrical sparks and arcs during a short circuit; excessive overheating of motors, machines, contacts and individual sections of electrical networks during overloads and transient resistances; overheating as a result of the manifestation of eddy currents of induction and self-induction; during spark discharges of static electricity and atmospheric electricity discharges.
When assessing the possibility of fires from electrical equipment, it is necessary to take into account the presence, condition and compliance of existing protection from environmental influences, short circuits, overloads, transient resistances, discharges of static and atmospheric electricity.
Thermal manifestation of chemical reactions. Chemical reactions that occur with the release of a significant amount of heat pose the potential for a fire or explosion, since in this case the reacting or nearby flammable substances can be heated to the temperature of their spontaneous ignition.
Chemical substances are divided into the following groups based on the danger of thermal manifestations of exothermic reactions (more about this is discussed in Chapter I).
A. Substances that ignite upon contact with air, i.e., having a self-ignition temperature below the ambient temperature (for example, organoaluminum compounds) or heated above their self-ignition temperature.
b. Substances that spontaneously ignite in air - vegetable oils and animal fats, coal and charcoal, iron sulfur compounds, soot, powdered aluminum, zinc, titanium, magnesium, peat, waste nitroglyphthalic varnishes, etc.
Spontaneous combustion of substances is prevented by reducing the oxidation surface, improving the conditions for heat removal to the environment, reducing the initial temperature of the environment, using inhibitors of spontaneous combustion processes, isolating substances from contact with air (storage and processing under the protection of non-flammable gases, protecting the surface of crushed substances with a film of fat, etc. .).
V. Substances that are flammable when interacting with water are alkali metals (Na, K, Li), calcium carbide, quicklime, powder and shavings of magnesium, titanium, organoaluminum compounds (triethylaluminum, triisobutyl aluminum, diethyl aluminum chloride, etc.). Many of this group of substances, when interacting with water, form flammable gases (hydrogen, acetylene), which can ignite during the reaction, and some of them (for example, organoaluminum compounds) explode upon contact with water. Naturally, such substances are stored and used, protected from contact with industrial, atmospheric and soil water.
d. Substances that ignite upon contact with each other are mainly oxidizing agents that can, under certain conditions, ignite flammable substances. The reactions of interaction of oxidizers with flammable substances are facilitated by the grinding of substances, elevated temperature and the presence of process initiators. In some cases, the reactions are explosive. Oxidizing agents must not be stored together with flammable substances; any contact between them must not be allowed, unless this is due to the nature of the technological process.

e. Substances capable of decomposing with ignition or explosion when heated, impact, compression, etc. influences. These include explosives, nitrate, peroxides, hydroperoxides, acetylene, porophor ChKhZ-57 (azodinitrilisobutyric acid), etc. Such substances during storage and use protect against dangerous temperatures and dangerous mechanical influences.
Chemical substances of the groups listed above cannot be stored together, or together with other flammable substances and materials.

In industrial conditions, a fire-hazardous increase in body temperature as a result of the conversion of mechanical energy into thermal energy is observed during impacts of solid bodies (with or without the formation of sparks); with surface friction of bodies during their mutual movement; during mechanical processing of hard materials with cutting tools, as well as during compression of gases and pressing of plastics. The degree of heating of bodies and the possibility of the appearance of ignition sources depends on the conditions for the transition of mechanical energy into thermal energy.

Fig-5-9. Turbine-vortex spark arrester: / - housing; 2 - fixed turbine; 3 - trajectory of movement of solid particles

Rice. 5.10. Dependence of the temperature of a steel spark on the force and the colliding material (according to MIHM data): 1 - with an abrasive disc; 2 - with a metal disk. Linear impact speed 5.2 m/s

Sparks generated by impacts of solid bodies. When some solid bodies collide strongly enough, sparks are generated (impact and friction sparks). The spark in this case is a particle of metal or stone heated to the point of glow. The sizes of impact and friction sparks depend on the properties of the materials and the energy characteristics of the impact, but usually do not exceed 0.1...0.5 mm. The spark temperature, in addition, depends on the interaction process (chemical and thermal) of the metal particle with the environment. Thus, when metals are impacted and abraded in an environment that does not contain oxygen or other oxidizer, no visible sparks are formed. Additional heating of metal impact sparks during flight in the environment usually occurs as a result of their oxidation by atmospheric oxygen. The spark temperature of unalloyed low-carbon steel can reach the melting point of the metal (about 1550 ° C). It will increase with increasing carbon content in the steel, and decrease with increasing alloying additives. The dependence of the spark temperature on the material of the colliding bodies and the applied specific load is shown in Fig. 5.10. According to the graphs, the spark temperature increases linearly with increasing load, and sparks formed when steel hits corundum have a higher temperature than when steel hits steel.

In industrial conditions, acetylene, ethylene, hydrogen, carbon monoxide, and carbon disulfide ignite from impact sparks. Impact sparks (under certain conditions) can ignite methane-air mixtures. The ignition ability of impact sparks is proportional to the oxygen content in the mixture, which these sparks can ignite. This is understandable: the more oxygen in the mixture, the more intense the spark burns, the higher the flammability of the mixture.

The igniting ability of impact sparks is established experimentally - depending on the impact energy.

A flying spark does not directly ignite dust-air mixtures, but when it hits settled dust or fibrous materials, it causes the appearance of smoldering centers. This apparently explains the large number of flashes and fires caused by mechanical sparks in machines where there are fibrous materials or deposits of fine combustible dust. Thus, in the grinding shops of mills and cereal factories, in the sorting, loosening and carbon monoxide shops of textile factories, as well as in cotton gin plants, more than 50% of all fires and fires occur from sparks generated by impacts of solid bodies.

Sparks are formed when aluminum bodies hit an oxidized steel surface. In this case, a chemical interaction occurs between the heated aluminum particle and iron oxides, releasing a significant amount of heat:

2A1 + Fe 2 O 3 = A1 2 O 3 + 2Fe + Q.

Due to the heat of this reaction, the heat content and temperature of the spark increase.

Sparks generated when working with impact tools (hammers, chisels, crowbars, etc.) often cause fire and explosion hazards. There are known cases of outbreaks and explosions in pumping and compressor stations, as well as in production premises when tools are dropped, or keys are struck when tightening nuts. Therefore, when performing work in places where the formation of an explosive mixture of vapors or gases with air is possible, you should not use impact tools made of spark-producing materials. Instruments made of bronze, phosphor bronze, brass, beryllium, aluminum alloy AKM-5-2, duralumin with limited (up to 1.2... 1.8%) content, magnesium.. (alloy D-16 and etc.) and even tools made of high-alloy steels. The use of copper-plated tools does not achieve the goal, because the soft layer of copper quickly wears out. When using steel tools, you should protect them from falling and, if possible, replace impact operations with non-impact ones (for example, replace metal cutting with a chisel with sawing, etc.), and use mobile ventilation units to disperse flammable vapors or gases at work sites.

Sparks generated when metal or stones hit cars. In devices with stirrers for dissolving or chemical treatment solid substances in solvents (for example, celluloid mass in alcohol, cellulose acetate in acetone, rubber in gasoline, nitrocellulose in an alcohol-ether mixture, etc.), in centrifugal impact machines for grinding, loosening and mixing solid flammable substances (hammer and impact-disc mills, feed crushers, cotton ginning and scattering machines, etc.), into mixing devices for mixing and preparing powder compositions, into devices centrifugal action to move gases and vapors (fans, gas blowers, centrifugal compressors), pieces of metal or stones can get in with the processed products, resulting in the formation of sparks. Therefore, processed products should be sifted, winnowed, washed, or used magnetic, gravitational or inertial catchers.

Rice. 5.11. Stone catcher: / - pneumatic pipeline; 2 - bunker; 3 - inclined surfaces; 4 - unloading hatch

Fibrous materials are especially difficult to clean because solids become entangled in the fibers. Thus, to clean raw cotton from stones before entering it into machines, gravitational or inertial stone catchers are installed (Fig. 5.11).

Metal impurities in bulk and fibrous materials are also captured by magnetic traps (separators). In Fig. Figure 5.12 shows a magnetic catcher, most widely used in flour and cereal production, as well as in feed mills. In Fig. Figure 5.13 shows a cross-section of an electromagnetic separator with a rotating drum.

It should be noted that the efficiency of the traps depends on their location, speed of movement, uniformity and thickness of the product layer, and the nature of the impurities. They are installed, as a rule, at the beginning of the production line, in front of impact machines. Separators usually protect machines from mechanical damage. Their installation is also dictated by sanitary and hygienic requirements.

Rice. 5.12. Magnetic separator with permanent magnets: / - housing; 2 - permanent magnets; 3 - bulk material

Rice. 5.13. Electromagnetic separator with a rotating drum: / - housing; 2 - stationary electromagnet; 3 - product flow; 4 - adjusting screw; 5 - rotating drum made of non-woven

magnetic material; 6 - pipe for the purified product; 7 - pipe for captured impurities

If there is a danger of solid non-magnetic impurities getting into the machine, they carry out, firstly, careful sorting of raw materials, and secondly, the internal surface of the machines, against which these impurities can hit, is lined with soft metal, rubber or plastic.

Sparks generated when moving machine mechanisms hit their stationary parts. In practice, it often happens that the rotor of a centrifugal fan comes into contact with the walls of the casing or the rapidly rotating serrated and knife drums of fiber separating and scattering machines hit stationary steel gratings. In such cases, sparking is observed. It is also possible if the gaps are adjusted incorrectly, with deformation and vibration of the shafts, wear of the bearings, distortions, or insufficient fastening on the shafts cutting tool etc. In such cases, not only sparking is possible, but also breakdown of individual parts of the machines. A breakdown of a machine component, in turn, can cause the formation of sparks, since metal particles enter the product.

The main fire prevention measures aimed at preventing the formation of impact and friction sparks come down to careful adjustment and balancing of shafts, correct selection of bearings, checking the size of the gaps between the rotating and stationary parts of the machines, their reliable fastening, eliminating the possibility of longitudinal movements; preventing machine overload.

Before putting into operation, a machine in which rotating parts may collide with stationary parts must be checked (while stationary and then at idle) for the absence of distortions and vibrations, the strength of fastening of the rotating parts, and the presence of the necessary clearances. During operation, if any extraneous noise, shocks or shocks appear, you must stop the machine to troubleshoot the problem.

Increased requirements for intrinsic safety are imposed on production premises with the presence of acetylene, ethylene, carbon monoxide, carbon disulfide vapor, nitro compounds and similar flammable or unstable substances, the floors and areas of which are made of a material that does not generate sparks, or are lined with rubber mats, paths, etc. The floor of the premises where it is processed nitrocellulose is also kept hydrated. Carts and trolleys must have soft metal or rubber rims on their wheels.

Any movement of bodies in contact with each other requires the expenditure of energy to overcome the work of friction forces. This energy is mainly converted into heat. In normal condition and proper operation of the rubbing bodies, the generated heat Q t p is promptly removed by a special cooling system Q cool, and is also dissipated into the environment Q OkP:

Q tr = Q cool + Q env.

Violation of this equality, that is, an increase in heat generation or a decrease in heat removal and heat loss, leads to an increase in the temperature of the rubbing bodies. For this reason, fires of a flammable medium or materials occur from overheating of machine bearings, heavily tightened oil seals, drums and conveyor belts, pulleys and drive belts, fibrous materials when they are wound on rotating tool shafts and mechanically processed solid combustible materials.

Rice. 5.14. Sliding bearing diagram: / - shaft spike; 2 - bearing shell; 3 - bed

Fire caused by overheating of machine bearings And devices. The most fire hazards are the sliding bearings of heavily loaded and high-speed shafts. Poor quality of lubrication of working surfaces, their contamination, shaft misalignments, machine overload and excessive tightening of bearings - all this can cause overheating of bearings. Very often the bearing housing becomes contaminated with deposits of combustible dust (wood, flour, cotton). This also creates conditions for their overheating. The approximate value of the temperature of the sliding bearing (see Fig. 5.14) can be determined by calculation. The temperature of the bearing surface when its operating mode is violated changes over time. For a period of time dx we can write the following equation heat balance:

d Q t р = dQ heat+ dQ oxl+ dQ 0 Kp , (5.7)

Where dQ T p- the amount of heat generated during the operation of the bearing;

dQ heat - the amount of heat used to heat the bearing; dQoxl - amount of heat removed compulsory system cooling; d Q 0 K p - heat loss from the bearing surface to the environment.

The amount of heat released during friction of surfaces is determined by the formula

Q tr = f tr Nl,

Where f tr - friction coefficient; N- load; / - relative movement of surfaces.

Then, in relation to the bearing (for rotational motion), the work of friction forces is determined by the expression

dQ t p = f Tp Nd III /2πndτ = πf TR Nd III ndτ,(5.8)

Where P- shaft rotation speed (1/s); d- diameter of the shaft tenon. Assuming the friction coefficient to be a constant value and denoting the product of constant values A, will have:

dQ Tp = adτ.(5.9)

The amount of heat expended to heat the bearing dQ heating when the temperature rises by dT, will be equal to:

dQ narp = mcdT,(5.10)

Where T- mass of heated bearing parts; With- average specific heat capacity of the bearing material.

The amount of heat dQ 0 XJI , discharged by the forced cooling system can be taken equal to zero, which corresponds to the most dangerous operating mode of the bearing.

The amount of heat dQoup, lost by the bearing surface into the environment will be equal to:

dQ env = α( T P- T B)Fdτ,(5.11)

where α is the heat transfer coefficient between the bearing surface and the environment; T p And T in- temperature of the bearing surface and air; F- heat exchange surface (bearing surface washed by ambient air).

Substituting the found values dQ Tp , dQ narv And dQ 0 Kp into equation (5.7), we obtain the equation

adτ = mcdT+a(T n -T B)Fdτ,(5.12)

the solution of which under the initial conditions of the accident (T P = T V) gives:

Coefficient a is determined from the conditions of heat transfer from the surface of the cylinder to the environment with free air convection.

The resulting equation (5.13) makes it possible to determine the temperature of the bearing at any time during the emergency mode of its operation or to determine the duration of the emergency mode, during which the temperature of the bearing surface reaches a dangerous value.

The maximum bearing temperature (at τ = ∞) can be determined by the formula

To avoid a fire and explosion hazard, in this case, instead of sliding bearings, rolling bearings are used, they are systematically lubricated, and the temperature is controlled.

In complex machines (turbines, centrifuges, compressors), bearing temperature is controlled using instrumentation and control systems.

Visual control of the bearing temperature is carried out by applying heat-sensitive paints that change color when heated to the bearing housings. Overheating of bearings can be prevented by forced lubrication systems, the design of which must ensure control of the presence of oil, replacement of used oil with fresh oil (with specified performance characteristics), and quick and easy removal of oil leaks from machine parts.

An example is the modernization of the lubrication system for bearings of drying cylinders and felt rollers of paper and cardboard machines at a pulp and paper mill in the Arkhangelsk region. As a result of this modernization, fires and fires in the relevant systems have practically ceased.

Initially, droppers were provided to visually monitor the flow of oil into the bearings. They were placed under the covers of cars, in the area high temperatures, which practically excluded the possibility of systematic control. According to the proposal of the on-site fire department and the fire-technical commission of the enterprise, the droppers were replaced with rotameters located outside the machine. This made it possible to visually monitor the flow of oil, reduce the number of detachable connections in the oil system, thereby reducing oil leaks on the frames and bearing assemblies.

In addition, according to the original design, the oil in the bearings was replaced only during scheduled preventative repairs or scheduled maintenance. It was difficult to control the presence of lubricant during operation of the machine. The serviceability of the bearings was checked by ear. During the reconstruction of the machines, it was installed centralized system lubricant: from a container (10 m3) installed in separate room, a gear pump began to supply filtered oil into pressure pipelines and through branches - to rotameters, from rotameters - to bearings. Having passed through the bearing, the oil entered the sump and filter, where it was cleaned of mechanical impurities, cooled and again entered the working tank. The pressure, temperature and oil level in the tank were controlled automatically. When the oil pumps stopped and the pressure in the pressure line dropped, a sound and light alarm was triggered and the backup pumps were turned on.

To clean cars from oil leaks and dust settling on them, it turned out effective application 2% solution of technical detergent TMS-31 (at 50...70 ° C). A stationary system for washing units and mechanisms is installed along the entire length of the machine. The introduction of a cleaning system made it possible to wash away oil stains and dust every shift without stopping the machine. In addition, 10 tons of kerosene were removed from production, and working conditions for workers were significantly improved.

Overheating and ignition of conveyor belts and drive belts occur mainly as a result of prolonged slipping of the belt or tape relative to the pulley. This slippage, called slippage, occurs due to a mismatch between transmitted force and tension of the belt branches (tape). When slipping, all the energy is spent on friction between the belt and the pulley, resulting in the release of a significant amount of heat. Most often, slipping of conveyor belts, elevator belts and belt drives occurs due to overload or low belt tension. In elevators, the cause of slipping is most often the blockage of the shoe, that is, a condition when the elevator bucket cannot pass through the thickness of the transported substance. Overloading and slipping can result from belt pinching, distortions, etc.

The maximum temperature of the drum or pulley during prolonged slipping of the belt or belt can be determined by formula (5.14).

To avoid overheating and fires of conveyor belts and drive belts, work with overload must not be allowed; the degree of tension of the belt, belt, and their condition should be monitored. Avoid blocking the elevator shoes with products, distorting the belts and rubbing them against casings and other nearby objects. In some cases (when using powerful high-performance conveyors and elevators), devices and devices are used that automatically signal the operation of the transmission with overload and stop the movement of the belt when the elevator shoe is blocked.

Sometimes, to reduce slipping, the transmission belt is sprinkled with rosin, but this gives only a short-term effect. Treating the belt with rosin promotes the formation of static electricity charges, which poses a certain fire hazard. In this case, it is better to use a V-belt drive.

Fire of fibrous materials when they are wound onto shafts observed in spinning mills, flax mills, and also in combines when harvesting grain crops. Fibrous materials and straw products are wound onto the shafts near the bearings. Winding is accompanied by gradual compaction of the mass, and then strong heating it when rubbed against the walls of the car, charred and, finally, ignited. Sometimes fires occur as a result of fibrous materials being wrapped around the shafts of conveyors moving waste and finished products. In spinning mills, fires often occur as a result of a broken cord or braid that drives the spinning machine spindles.

The winding of fibrous materials onto the rotating shafts of machines is facilitated by the presence of an increased gap between the shaft and the bearing (getting into this gap, the fiber becomes wedged, pinched, and the process of winding it onto the shaft begins with increasingly stronger compaction of the layers), the presence of bare sections of the shaft with which the fibrous materials come into contact, as well as the use of wet and contaminated raw materials.

To prevent the winding of fibrous materials on the rotating shafts of machines, it is necessary to protect the shafts from direct contact with the processed fibrous materials by using bushings (Fig. 5.15), cylindrical and conical casings, conductors, guide bars, anti-winding shields, etc. In addition, it is necessary to install minimum clearances between the shaft journals and bearings, preventing their increase; conduct systematic monitoring of the shafts where there may be winding, promptly clearing them of fibers, protect them with special anti-winding sharp knives, cutting the wound fiber. Such protection is provided, for example, by scutching machines at flax mills.

Rice. 5.15. Shaft protection against winding of fibrous materials: A- loosely fitted straight bushing; b- fixed conical bushing; 1 - bearing; 2 - shaft; 3 - protective sleeve

Thermal manifestation of mechanical energy under production conditions is observed during the operation of presses and compressor units. The fire hazard of these mechanisms is discussed in Chapters 10 and 11 of this textbook.

§ 5.4. Thermal manifestation of chemical reactions -