Electric spark temperature. Open fire, hot combustion products and surfaces heated by them

In production conditions, ignition sources can be very diverse both in the nature of their occurrence and in their parameters.
Among the possible sources of ignition, we highlight open fire and hot combustion products; thermal manifestation of mechanical energy; thermal, manifestation of electrical energy; thermal manifestation of chemical reactions.

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); In this case, a large amount of heat is released and the combustion process, as a rule, is 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 repair 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 is installed during the work.
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 the operation of furnaces and internal combustion engines is achieved by using spark arresters and spark arresters. 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 temperature of sparks from 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 conveyor belts and drive belts slip, when fibrous materials are wound 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.
Systematic lubrication of bearings (especially plain bearings) is of great importance. 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, arrange for forced cooling of the bearing with 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 fibers getting 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 chemical fiber factories, etc.) reduce the gaps between shaft journals and bearings, bushings, casings, shields and other anti-winding devices are used to protect the 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 are formed as a result of 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. To avoid explosions of air compressors, in addition to monitoring the temperature and air pressure, optimal lubricating oil supply standards are established and strictly maintained, and the discharge air ducts and receivers are systematically cleaned of flammable deposits.
Thermal manifestation of electrical energy. The thermal effect of electric current can manifest itself 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 combust in air are 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.

Electrical sparks are quite common causes of fires. They can ignite not only gases, liquids, dust, but also some solids. In electrical engineering, sparks are often used as an ignition source. The mechanism of ignition of flammable substances by an electric spark is more complex than ignition by a heated body. When a spark is formed in a gas volume between the electrodes, molecules are excited and ionized, which affects the nature of chemical reactions. At the same time, an intense increase in temperature occurs in the volume of the shield. In this regard, two theories of the mechanism of ignition by electric sparks were put forward: ionic and thermal. At present, this issue has not yet been sufficiently studied. Research shows that both electrical and thermal factors are involved in the mechanism of ignition by electric sparks. At the same time, in some conditions, electrical ones predominate, in others, thermal ones. Considering that the research results and conclusions from the point of view of the ionic theory do not contradict the thermal theory, when explaining the mechanism of ignition from electric sparks, the thermal theory is usually followed.
Spark discharge. An electric spark occurs if the electric field in a gas reaches a certain specific value Ek (critical field strength or breakdown strength), which depends on the type of gas and its state.
Reflection of a sound pulse of an electric spark from a flat wall. The photograph was obtained using the dark field method.| Passage of a sound pulse through a cylindrical wall with holes. The photograph was taken using the dark field method. An electric spark produces an extremely short flash; the speed of light is immeasurably greater than the speed of sound, the magnitude of which we will discuss below.
Electric sparks that can appear when there is a short circuit in electrical wiring, during electric welding work, when electrical equipment sparks, or during discharges of static electricity. The size of metal droplets reaches 5 mm during electric welding and 3 mm during a short circuit of electrical wiring. The temperature of metal drops during electric welding is close to the melting point, and metal drops formed during a short circuit of electrical wiring are higher than the melting point, for example for aluminum it reaches 2500 C. The temperature of the drop at the end of its flight from the source of formation to the surface of the combustible substance is taken in calculations to be 800 WITH.
An electrical spark is the most common thermal ignition pulse. A spark occurs at the moment of closing or opening an electrical circuit and has a temperature significantly higher than the ignition temperature of many flammable substances.
An electric spark between the electrodes is produced as a result of pulsed discharges of capacitor C created by an electrical oscillating circuit. If there is liquid (kerosene or oil) between tool 1 and part 2 at the moment of discharge, then the processing efficiency increases due to the fact that metal particles torn from the anode part do not settle on the tool.
An electric spark can be born without any conductors or networks at all.
Transient flame propagation characteristics during spark ignition (Olsen et al. / - hydrogen (successful ignition. 2 - propane (successful ignition. 3 - propane (ignition failure). The electric spark is of two types, namely, high and low voltage. A high-voltage spark created by some high-voltage generator breaks through a spark gap of a pre-fixed size. A low-voltage spark jumps at the point of break in the electrical circuit when self-induction occurs when the current is interrupted.
Electrical sparks are sources of small energy, but, as experience shows, they can often become sources of ignition. Under normal operating conditions, most electrical appliances do not produce sparks, but certain devices typically produce sparks.
An electric spark has the appearance of a brightly glowing thin channel connecting the electrodes: the channel can be complexly curved and branched. An avalanche of electrons moves in the spark channel, causing a sharp increase in temperature and pressure, as well as a characteristic crackling sound. In a spark voltmeter, ball electrodes are brought together and the distance at which a spark jumps between the balls is measured. Lightning is a giant electrical spark.
Schematic diagram of an AC activated arc generator.| Schematic diagram of a condensed spark generator.
An electric spark is a discharge created by a large potential difference between electrodes. The electrode substance enters the spark analytical gap as a result of explosive emissions-torches from the electrodes. A spark discharge at a high current density and high temperature of the electrodes can turn into a high-voltage arc discharge.
Spark discharge. An electric spark occurs if the electric field in a gas reaches a certain certain value Ec (critical field strength or breakdown strength), which depends on the type of gas and its state.
An electric spark breaks down NHs into their constituent elements. Upon contact with catalytically active substances, its partial decomposition occurs even with relatively little heating. Ammonia does not burn in air under normal conditions; however, there are mixtures of ammonia and air that will ignite when ignited. It also burns if it is introduced into a gas flame burning in air.
An electric spark decomposes the gas into its component elements. Upon contact with catalytically active substances, its partial decomposition occurs even with relatively little heating. Ammonia does not burn in air under normal conditions; however, there are mixtures of ammonia and air that will ignite when ignited. It also burns if it is introduced into a gas flame burning in air.
An electric spark allows you to successfully perform all kinds of operations - cutting metals, making holes in them of any shape and size, grinding, coating, changing the surface structure... It is especially beneficial to process parts of a very complex configuration made of metal-ceramic hard alloys, carbide compositions, magnetic materials, high-strength heat-resistant steels and alloys and other difficult-to-process materials.
The electric spark that occurs between the contacts when the circuit breaks is extinguished not only by accelerating the break; This is also facilitated by the gases emitted by the fiber from which the gaskets 6 are made, specially laid in the same plane with the movable contact.
Schematic diagram of the ignition system.| Battery ignition system diagram. An electric spark is produced by applying a high voltage current pulse to the spark plug electrodes. The breaker ensures the opening of contacts in accordance with the sequence of cycles, and the distributor 4 provides high voltage pulses in accordance with the operating order of the cylinders.
Installation for ultrasonic cleaning of glass parts with evacuation of the working chamber. An electric spark removes a thin layer of glass from the surface being treated. When blown through this arc, an inert gas (argon) is partially ionized and contaminant molecules are destroyed by ion bombardment.
Electrical sparks in some cases can lead to explosions and fires. Therefore, it is recommended that those parts of installations or machines on which there is an accumulation of electrostatic charges are specially connected to the ground with metal wire, thereby allowing electrical charges free passage from the machine to the ground.
An electric spark consists of quickly decaying atoms of air or other insulator and is therefore a good conductor for a very short time. The short duration of the spark discharge has long made it very difficult to study, and only relatively recently has it been possible to establish the most important laws to which it obeys.
Spark discharge. An electric spark occurs if the electric field in a gas reaches a certain value Ek (critical field strength, or breakdown strength), which depends on the type of gas and its state.

An ordinary electric spark, jumping through a generator device, gave birth, as the scientist expected, to a similar spark in another device, isolated and several meters away from the first. Thus, for the first time, what was predicted was discovered. Maxwell, a free electromagnetic field capable of transmitting signals without any wires.
Soon an electric spark ignites the alcohol, phosphorus and, finally, gunpowder. The experience passes into the hands of magicians, becomes the highlight of circus programs, everywhere arousing burning interest in the mysterious agent - electricity.
Flame temperatures of various gas mixtures. A high-voltage electrical spark is an electrical discharge in air at normal pressure under the influence of high voltage.
An electric spark is also called the form of passage of electric current through a gas during a high-frequency discharge of a capacitor through a short discharge gap and a circuit containing self-induction. In this case, during a significant fraction of the half-cycle of the high-frequency current, the discharge is an alternating-mode arc discharge.
By passing electric sparks through atmospheric air, Cavendish found that nitrogen was oxidized by atmospheric oxygen into nitric oxide, which could be converted into nitric acid. Accordingly, Timiryazev decides, by burning air nitrogen, it is possible to obtain nitrate salts, which can easily replace Chilean saltpeter in the fields and increase the yield of turf crops.
By passing electric sparks through atmospheric air, Cavendish found that nitrogen was oxidized by atmospheric oxygen into nitric oxide, which could be converted into nitric acid. Consequently, Timiryazev decides, by burning air nitrogen, it is possible to obtain nitrate salts, which can easily replace Chilean saltpeter in the fields and increase the yield of turf crops.
High-frequency currents are excited by electrical sparks in the wires. They spread along wires and emit electromagnetic waves into the surrounding space, interfering with radio reception. This interference enters the receiver in various ways: 1) through the receiver antenna, 2) through the wires of the lighting network, if the receiver is networked, 3) by induction from lighting or any other wires through which interfering waves propagate.
The effect of an electric spark on flammable mixtures is very complex.
Obtaining an electric spark of the required intensity during battery ignition is not limited to the minimum number of revolutions, but when igniting from a magneto without an accelerator clutch, it is ensured at approximately 100 rpm.
Ignition by an electric spark, compared to other methods, requires minimal energy, since a small volume of gas in the path of the spark is heated by it to a high temperature in an extremely short time. The minimum spark energy required to ignite an explosive mixture at its optimal concentration is determined experimentally. It is reduced to normal atmospheric conditions - a pressure of 100 kPa and a temperature of 20 C. Typically, the minimum energy required to ignite dust-air explosive mixtures is one or two orders of magnitude higher than the energy required to ignite gas and steam-air explosive mixtures.
Ignition switch. During a breakdown, an electric spark evaporates a thin layer of metal deposited on the paper, and near the breakdown site, the paper is cleared of metal, and the breakdown hole is filled with oil, which restores the functionality of the capacitor.
Electric sparks are the most dangerous: almost always their duration and energy are sufficient to ignite flammable mixtures.

Finally, an electric spark is used to measure large potential differences using a ball gap, the electrodes of which are two metal balls with a polished surface. The balls are moved apart and a measured potential is applied to them. Then the balls are brought closer together until a spark jumps between them. Knowing the diameter of the balls, the distance between them, pressure, temperature and air humidity, find the potential difference between the balls using special tables.
Under the influence of an electric spark, it decomposes with increasing volume. Methyl chloride is a strong reactive organic compound; Most reactions with methyl chloride involve replacing halogen atoms with various radicals.
When electric sparks are passed through liquid air, nitrous anhydride is formed as a blue powder.
To avoid an electric spark, it is necessary to connect the disconnected parts of the gas pipeline with a jumper and install grounding.
Change in concentration limits of ignition depending on spark power. An increase in the power of electric sparks leads to an expansion of the area of ignition (explosion) of gas mixtures. However, here too there is a limit when further changes in the ignition limits do not occur. Sparks of such power are usually called saturated. Their use in devices for determining concentration and temperature limits of ignition, flash point and other values gives results that are no different from ignition by heated bodies and flames.
When an electric spark is passed through a mixture of sulfur fluoride and hydrogen, H2S and HF are formed. Mixtures of S2F2 with sulfur dioxide form thionyl fluoride (SOF2) under the same conditions, and mixtures with oxygen form a mixture of thionyl fluoride and sulfur dioxide.
When electric sparks are passed through air in a closed vessel above water, a greater decrease in the volume of gas occurs than when phosphorus is burned in it.
The amount of electric spark energy required to initiate the explosive decomposition of acetylene strongly depends on pressure, increasing as it decreases. According to the data of S. M. Kogarko and Ivanov35, the explosive decomposition of acetylene is possible even at an absolute pressure of 0 65 from, if the spark energy is 1200 J. Under atmospheric pressure, the energy of the initiating spark is 250 J.
In the absence of an electric spark or flammable impurities such as grease, reactions usually occur noticeably only at high temperatures. Ethforan C2Fe reacts slowly with dilute fluorine at 300 , while k-heptphoran reacts violently when the mixture is ignited by an electric spark.
When electric sparks are passed through oxygen or air, a characteristic odor appears, the cause of which is the formation of a new substance - ozone. Ozone can be obtained from completely pure ear oxygen; it follows that it consists only of oxygen and represents its allotropic modification.
The energy of such an electric spark may be sufficient to ignite a flammable or explosive mixture. A spark discharge at a voltage of 3000 V can ignite almost all steam and gas-air mixtures, and at 5000 V it can ignite most combustible dusts and fibers. Thus, electrostatic charges arising in industrial conditions can serve as an ignition source, capable of causing a fire or explosion in the presence of flammable mixtures.
The energy of such an electric spark may be large enough to ignite a flammable or explosive mixture.
When electric sparks are passed through oxygen, ozone is formed - a gas that contains only one element - oxygen; Ozone has a density 1 to 5 times greater than oxygen.
When an electric spark passes through the air gap between two electrodes, a shock wave occurs. When this wave acts on the surface of the calibration block or directly on the PAE, an elastic pulse with a duration of the order of several microseconds is excited in the latter.

Question 1: Classification of ignition sources;

IGNITION SOURCE - the energy source that initiates combustion. Must have sufficient energy, temperature and duration of exposure.

As noted earlier, combustion can occur when gas is exposed to various ignition sources. According to the nature of origin, ignition sources can be classified:

open fire, hot combustion products and surfaces heated by them;
thermal manifestations of mechanical energy;
thermal manifestations of electrical energy;
thermal manifestations of chemical reactions (from this group open fire and combustion products are separated into a separate group).

Open fire, hot combustion products and surfaces heated by them

For production purposes, fire, combustion furnaces, reactors, and torches for burning vapors and gases are widely used. When carrying out repair work, the flames of burners and blowtorches are often used, torches are used to warm frozen pipes, and fires are used to warm the soil when burning waste. The temperature of the flame, as well as the amount of heat that is released, is sufficient to ignite almost all flammable substances.

Open flame. The fire hazard of a flame is determined by the temperature of the torch and the time of its influence on flammable substances. For example, ignition is possible from such “low-calorie” ignitions as a smoldering cigarette butt or cigarette butt, or a lit match (Table 1).

Sources of open fire - torches - are often used to heat a frozen product, for illumination when inspecting equipment in the dark, for example, when measuring the level of liquids, when making a fire in the territory of objects with the presence of flammable liquids and gases.

Highly heated combustion products are gaseous combustion products that are obtained from the combustion of solid, liquid and gaseous substances and can reach temperatures of 800-1200oC. A fire hazard is posed by the release of highly heated products through leaks in the masonry of fireboxes and smoke ducts.

Industrial ignition sources are also sparks that occur during the operation of furnaces and engines. They are solid hot particles of fuel or scale in a gas stream, which are obtained as a result of incomplete combustion or mechanical removal of flammable substances and corrosion products. The temperature of such a solid particle is quite high, but the reserve of thermal energy (W) is small due to the small mass of the spark. A spark can only ignite substances that are sufficiently prepared for combustion (gas-steam-air mixtures, settled dust, fibrous materials).

Fireboxes “spark” due to design flaws; due to the use of a type of fuel for which the firebox is not designed; due to increased blowing; due to incomplete combustion of fuel; due to insufficient atomization of liquid fuel, as well as due to non-compliance with cleaning periods for stoves.

Sparks and carbon deposits during internal combustion engine operation are formed due to improper regulation of the fuel supply system and electric ignition; when fuel is contaminated with lubricating oils and mineral impurities; during prolonged operation of the engine with overloads; in case of violation of the deadlines for cleaning the exhaust system from carbon deposits.

The fire hazard of sparks from boiler houses, chimneys of steam and diesel locomotives, as well as other machines, and fires is largely determined by their size and temperature. It has been established that a spark d = 2 mm is fire hazardous if it has a temperature of » 1000°C; d=3 mm - 800°C; d = 5 mm - 600°C.

Hazardous thermal manifestations of mechanical energy

In production conditions, a fire-hazardous increase in body temperature as a result of the conversion of mechanical energy into thermal energy is observed:

upon 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;
when compressing gases and pressing plastics.

The degree of heating of bodies and the possibility of the appearance of an ignition source depends on the conditions for the transition of mechanical energy into thermal energy.

Sparks that are produced by impacts of solid bodies.

The size of impact and friction sparks, which are a piece of metal or stone heated to the point of glowing, usually does not exceed 0.5 mm. The spark temperature of unalloyed low-angle steels can reach the melting point of the metal (about 1550°C).

In industrial conditions, acetylene, ethylene, hydrogen, carbon monoxide, carbon disulfide, methane-air mixture and other substances ignite from the impact of sparks.

The more oxygen in the mixture, the more intense the spark burns, the higher the flammability of the mixture. The spark that flies does not directly ignite the dust-air mixture, but, if it hits settled dust or fibrous materials, it will cause the appearance of smoldering centers. Thus, in flour mills, weaving and cotton spinning enterprises, about 50% of all fires arise from sparks that are generated by impacts of solid bodies.

Sparks that are produced when aluminum bodies hit an oxidized steel surface lead to a chemical reaction with the release of a significant amount of heat.

Sparks generated when metal or stones hit cars.

In machines with mixers, crushers, mixers and others, sparks may form if pieces of metal or stones get in with the products being processed. Sparks are also formed when the moving mechanisms of machines strike their stationary parts. In practice, it often happens that the rotor of a centrifugal fan collides with the walls of the casing or the needle and knife drums of fiber separating and scattering machines, which rotate quickly and hit stationary steel gratings. In such cases, sparking is observed. It is also possible if the clearances are adjusted incorrectly, with deformation and vibration of the shafts, wear of the bearings, distortions, or insufficient fastening of the cutting tool on the shafts. 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.

Ignition of a flammable medium due to overheating due to friction.

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 parts that rub, the heat that is released is promptly removed by a special cooling system, and is also dissipated into the environment. 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, ignition of a flammable medium or materials occurs from overheating of machine bearings, tightly tightened oil seals, drums and conveyor belts, pulleys and drive belts, fibrous materials when they are wound on the shafts of machines and devices that rotate.

In this regard, the most fire-hazardous are the sliding bearings of heavily loaded and high-speed shafts. Poor quality of lubrication of working surfaces, their contamination, misalignment of shafts, overloading of machines and excessive tightening of bearings - all this can cause overload. Very often the bearing housing becomes contaminated with flammable dust deposits. This also creates conditions for them to overheat.

At facilities where fibrous materials are used or processed, they ignite when wound on rotating units (spinning mills, flax mills, operation of combines). Fibrous materials and straw products are wound onto the shafts near the bearings. Winding is accompanied by gradual compaction of the mass, and then its strong heating during friction, charring and ignition.

Heat release when gases are compressed.

A significant amount of heat is released when gases are compressed as a result of intermolecular motion. Failure or absence of the compressor cooling system can lead to their destruction in an explosion.

Dangerous thermal manifestations of chemical reactions

In the production and storage of chemicals, a large number of chemical compounds are encountered, the contact of which with air or water, as well as mutual contact with each other, can cause a fire.

1) Chemical reactions that occur with the release of a significant amount of heat have a potential risk of fire or explosion, since there is a possible uncontrolled heating process of reacting, newly formed or nearby flammable substances.

2) Substances that are spontaneously flammable and spontaneously combust on contact with air.

3) Often, due to the conditions of the technological process, substances located in the apparatus can be heated to a temperature exceeding their spontaneous combustion temperature. Thus, the products of gas pyrolysis when producing ethylene from petroleum products have a self-ignition temperature in the range of 530 - 550°C, and leave the pyrolysis furnaces at a temperature of 850°C. Fuel oil with a self-ignition temperature of 380 – 420°C is heated to 500°C in thermal cracking units; butane and butylene, which have a self-ignition temperature of 420°C and 439°C, respectively, when producing butadiene heats up to 550 - 650°C, etc. When these substances escape outside, they spontaneously ignite.

4) Sometimes substances in technological processes have a very low auto-ignition temperature:

Triethylaluminum - Al (C2H5)3 (-68°C);

Diethylaluminum chloride - Al (C2H5)2Сl (-60°С);

Triisobutylaluminum (-40°C);

Hydrogen fluoride, liquid and white phosphorus - below room temperature.

5) Many substances are capable of spontaneous combustion when in contact with air. Spontaneous combustion begins at ambient temperature or after some preliminary heating. Such substances include vegetable oils and fats, iron sulfur compounds, some types of soot, powdered substances (aluminum, zinc, titanium, magnesium, etc.), hay, grain in silos, etc.

Contact of self-igniting chemicals with air usually occurs when containers are damaged, liquid spills, substances are packaged, during drying, open storage of solid crushed and fibrous materials, when pumping liquids from tanks, when there are self-igniting deposits inside the tanks.

Substances that ignite when interacting with water.

At industrial facilities there is a significant amount of substances that are flammable when interacting with water. The heat released during this process can cause ignition of flammable substances formed or adjacent to the reaction zone. Substances that ignite or cause combustion upon contact with water include alkali metals, calcium carbide, alkali metal carbides, sodium sulfide, etc. Many of these substances, when interacting with water, form flammable gases that ignite from the heat of reaction:

2K +2H2O=KOH+H2+Q.

When a small amount (3...5 g) of potassium and sodium interacts with water, the temperature rises above 600...650°C. If they interact in large quantities, explosions occur with the splashing of molten metal. When dispersed, alkali metals ignite in moist air.

Some substances, such as quicklime, are non-flammable, but the heat of their reaction with water can heat nearby combustible materials to the point of spontaneous combustion. Thus, when water comes into contact with quicklime, the temperature in the reaction zone can reach 600°C:

Ca + H2O = Ca(BOH)2 + Q.

There are known cases of fires in poultry houses where hay was used as bedding. Fires occurred after treating poultry buildings with quicklime.

Contact with water of organoaluminum compounds is dangerous, since their interaction with water occurs with an explosion. Intensification of a fire or explosion that has started may occur when attempting to extinguish such substances with water or foam.

Ignition of chemical substances upon mutual contact occurs due to the action of oxidizing agents on organic substances. Chlorine, bromine, fluorine, nitrogen oxides, nitric acid, oxygen and many other substances act as oxidizing agents.

Oxidizing agents, when interacting with organic substances, will cause them to ignite. Some mixtures of oxidizers and flammable substances can ignite when exposed to sulfuric or nitric acid or a small amount of moisture.

The reaction between the oxidizer and the flammable substance is facilitated by the grinding of the substances, its elevated initial temperature, as well as the presence of initiators of the chemical process. In some cases, the reactions are explosive.

Substances that ignite or explode when heated or mechanically affected.

Some chemicals are unstable in nature and can degrade over time under the influence of temperature, friction, shock and other factors. These are, as a rule, endothermic compounds, and the process of their decomposition is associated with the release of a large or less amount of heat. These include nitrates, peroxides, hydroperoxides, carbides of some metals, acetylenides, acetylene, etc.

Violations of technological regulations, use or storage of such substances, or the influence of a heat source on them can lead to their explosive decomposition.

Acetylene is prone to explosive decomposition under the influence of elevated temperature and pressure.

Thermal manifestations of electrical energy

If the electrical equipment does not comply with the nature of the technological environment, as well as in the event of non-compliance with the operating rules of this electrical equipment, a fire and explosion hazard may arise in production. Fire and explosion hazards arise in production processes during short circuits, breakdowns of the insulation layer, excessive overheating of electric motors, damage to certain sections of electrical networks, spark discharges of static and atmospheric electricity, etc.

Discharges of atmospheric electricity include:

Direct lightning strikes. The danger of a direct lightning strike lies in the contact of the GE with the lightning channel, the temperature in which reaches 2000 ° C with an action time of about 100 μs. All flammable mixtures ignite from a direct lightning strike.
Secondary manifestations of lightning. The danger of secondary lightning manifestations lies in spark discharges that arise as a result of the inductive and electromagnetic influence of atmospheric electricity on production equipment, pipelines and building structures. The spark discharge energy exceeds 250 mJ and is sufficient to ignite flammable substances from Wmin = 0.25 J.
High potential skid. High potential is carried into a building through metal communications not only when they are directly struck by lightning, but also when the communications are located in close proximity to the lightning rod. If the safe distances between the lightning rod and communications are not observed, the energy of possible spark discharges reaches values of 100 J or more. That is, it is sufficient to ignite almost all flammable substances.

Electric sparks(arcs):

Thermal effect of short-circuit currents. As a result of a short circuit, a thermal effect occurs on the conductor, which heats up to high temperatures and may be a flammable medium.

Electric sparks (metal drops). Electric sparks are formed during a short circuit in electrical wiring, electric welding, and when the electrodes of general-purpose incandescent electric lamps melt.

The size of metal droplets during short circuit of electrical wiring and melting of the filament of electric lamps reaches 3 mm, and during electric welding 5 mm. The arc temperature during electric welding reaches 4000 °C, so the arc will be a source of ignition for all flammable substances.

Electric incandescent lamps. The fire hazard of lamps is due to the possibility of contact between the flammable lamp and the bulb of an incandescent electric lamp, heated above the auto-ignition temperature of the luminaire. The heating temperature of a light bulb bulb depends on its power, size and location in space.

Sparks of static electricity. Static electricity discharges can be formed during the transportation of liquids, gases and dust, during impacts, grinding, spraying and similar processes of mechanical influence on materials and substances that are dielectrics.

Conclusion: To ensure the safety of technological processes in which contact of flammable substances with ignition sources is possible, it is necessary to know exactly their nature to avoid impact on the environment.

Question 2: Preventive measures to exclude the impact of ignition sources on the flammable environment.;

Fire-fighting measures that exclude contact of a flammable medium (FME) with an open flame and hot combustion products.

To ensure fire and explosion safety of technological processes, processes of processing, storage and transportation of substances and materials, it is necessary to develop and implement engineering and technical measures that prevent the formation or introduction of an ignition source into the gas system.

As noted earlier, not every heated body can be a source of ignition, but only those heated bodies that are capable of heating a certain volume of the combustible mixture to a certain temperature when the rate of heat release is equal to or exceeds the rate of heat removal from the reaction zone. In this case, the power and duration of the thermal influence of the source must be such that the critical conditions necessary for the formation of a flame front are maintained for a certain time. Therefore, knowing these conditions (conditions for the formation of IZ), it is possible to create such conditions for conducting technological processes that would exclude the possibility of the formation of ignition sources. In cases where safety conditions are not met, engineering and technical solutions are introduced that make it possible to exclude contact of the hydraulic system with ignition sources.

The main engineering solution that prevents contact of a flammable medium with an open flame, hot combustion products, as well as highly heated surfaces is to isolate them from possible contact both during normal operation of the equipment and in case of accidents.

When designing technological processes with the presence of “fire” devices (tubular furnaces, reactors, torches), it is necessary to provide for the insulation of these installations from the possible collision of flammable vapors and gases with them. This is achieved:

placement of installations in enclosed spaces, separated from other devices;
placement in open areas between “firing” apparatus and fire-hazardous installations of protective barriers. For example, placing closed structures that act as barriers.
compliance with fireproof regulated gaps between devices;
the use of steam curtains in cases where it is impossible to ensure a fire-safe distance;
ensuring the safe design of flare burners with continuous combustion devices, the diagram of which is shown in Fig. 1.

Figure 1 - Flare for burning gases: 1 - water vapor supply line; 2 - ignition line of the next burner; 3 - gas supply line to the next burner; 4 - burner; 5 - torch barrel; 6 - fire arrester; 7 - separator; 8 - line through which gas is supplied for combustion.

Ignition of the gas mixture in the next burner is carried out using the so-called flame that runs (the previously prepared combustible mixture is ignited by an electric igniter and the flame, moving upward, ignites the burner gas). To reduce the formation of smoke and sparks, water vapor is supplied to the torch burner.

with the exception of the formation of “low-calorie” IZ (at facilities, smoking is allowed only in specially equipped areas).
using hot water or water steam to warm frozen areas of process equipment instead of torches (equipping open parking lots with hot air supply systems) or induction heaters.
cleaning pipelines and ventilation systems from flammable deposits using a fireproof agent (steaming and mechanical cleaning). In exceptional cases, it is allowed to burn waste after dismantling pipelines in specially designated areas and permanent hot work sites.
monitoring the condition of the masonry of smoke channels during the operation of fireboxes and internal combustion engines, to prevent leaks and burnouts of exhaust pipes.
protection of highly heated surfaces of technological equipment (returbent chambers) by thermal insulation with protective covers. The maximum permissible surface temperature should not exceed 80% of the auto-ignition temperature of flammable substances that are used in production.
preventing dangerous sparks from furnaces and engines. In practice, this area of protection is achieved by preventing the formation of sparks and using special devices to catch and extinguish them. To prevent the formation of sparks, the following is provided: automatic maintenance of the optimal temperature of the combustible mixture supplied for combustion; automatic regulation of the optimal ratio between fuel and air in the combustible mixture; prevention of prolonged operation of furnaces and engines in forced mode, with overload; use of those types of fuel for which the firebox and engine are designed; systematic cleaning of the internal surfaces of fireboxes, smoke ducts from soot and engine exhaust manifolds from carbon-oil deposits, etc.

To catch and extinguish sparks that are formed during the operation of furnaces and engines, spark arresters and spark arresters are used, the operation of which is based on the use of gravity (sediment chambers), inertial (chambers with partitions, meshes, nozzles), centrifugal forces (cyclone and turbine-vortex chambers ).

The most widely used in practice are spark arresters of gravitational, inertial and centrifugal types. They are used, for example, in smoke ducts of flue gas dryers, exhaust systems of cars and tractors.

To ensure deep purification of flue gases from sparks, in practice, not one, but several different types of spark arresters and spark arresters are often used, which are connected to each other in series. Multi-stage spark collection and extinguishing has proven itself reliably, for example, in technological processes for drying crushed combustible materials, where flue gases mixed with air are used as a coolant.

Fire safety measures that eliminate dangerous thermal manifestations of mechanical energy

Preventing the formation of ignition sources from dangerous thermal effects of mechanical energy is an urgent task at fire and explosion hazardous facilities, as well as at facilities where dust and fibers are used or processed.

To prevent the formation of sparks during impacts, as well as the release of heat during friction, the following organizational and technical solutions are used:

Use of non-sparking tools. In places where explosive mixtures of vapors or gases may form, it is necessary to use explosion-proof tools. Instruments made of bronze, phosphor bronze, brass, beryllium, etc. are considered intrinsically safe.

Example: 1. Spark-proof railway braking shoes. tanks.2. Brass tool for opening calcium carbide drums in acetylene stations.

The use of magnetic, gravitational or inertial catchers. Thus, to clean raw cotton from stones before entering it into machines, gravitational or inertial stone catchers are installed. Metal impurities in bulk and fibrous materials are also captured by magnetic separators. Such devices are widely used in flour and cereal production, as well as in feed mills.

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.

Preventing impacts of moving machine mechanisms on their stationary parts. The main fire prevention measures aimed at preventing the formation of impact and friction sparks come down to careful regulation and balancing of shafts, proper selection of bearings, checking the size of the gaps between moving and stationary parts of machines, their reliable fastening, which excludes the possibility of longitudinal movements; preventing machine overload.

Installation of non-sparking floors in fire and explosion hazardous areas. Increased requirements for spark safety are put forward for industrial premises with the presence of acetylene, ethylene, carbon monoxide, carbon disulfide, etc., the floors and platforms of which are made of a material that does not generate sparks, or are lined with rubber mats, walkways, etc.

Preventing combustion of substances in areas of intense heat generation due to friction. For this purpose, to prevent overheating of the bearings, the sliding bearings are replaced with rolling bearings (where such a possibility exists). In other cases, automatic control of their heating temperature is carried out. Visual temperature control is carried out by applying heat-sensitive paints, which change their color when the bearing housing is heated.

Prevention of bearing overheating is also achieved by: equipping automatic cooling systems using oils or water as a coolant; timely and high-quality maintenance (systematic lubrication, prevention of over-tightening, elimination of distortions, cleaning the surface from contamination).

To avoid overheating and fires of conveyor belts and drive belts, work with overload must not be allowed; you should monitor the degree of tension of the tape, belt, and their condition. Avoid blocking the elevator shoes with products, distorting the belts and rubbing them against the casings. When using powerful, high-performance conveyors and elevators, devices and devices can be used that automatically signal when working with overload and stop the movement of the belt when the elevator shoe is blocked.

To prevent fibrous materials from winding on the rotating shafts of machines, it is necessary to protect them from direct collision with the processed materials by using bushings, cylindrical and conical casings, conductors, guide bars, anti-winding shields, etc. In addition, a minimum gap is established between the shaft journals and the bearings; systematic monitoring of the shafts where there may be windings is carried out, timely cleaning them from fibers, protecting them with special anti-winding sharp knives that cut the fiber that is being wound. For example, scooping machines at flax mills have such protection.

Prevention of overheating of compressors when compressing gases.

Prevention of compressor overheating is ensured by dividing the gas compression process into several stages; arrangement of gas cooling systems at each compression stage; installing a safety valve on the discharge line downstream of the compressor; automatic control and regulation of the temperature of the compressed gas by changing the flow rate of the coolant supplied to the refrigerators; automatic blocking system, which ensures that the compressor is turned off in the event of an increase in gas pressure or temperature in the discharge lines; cleaning the heat exchange surface of refrigerators and internal surfaces of pipelines from carbon and oil deposits.

Preventing the formation of ignition sources during thermal manifestations of chemical reactions

To prevent the ignition of flammable substances as a result of chemical interaction upon contact with an oxidizing agent, water, it is necessary to know, firstly, the reasons that can lead to such interaction, and secondly, the chemistry of the processes of self-ignition and spontaneous combustion. Knowledge of the causes and conditions for the formation of dangerous thermal manifestations of chemical reactions allows us to develop effective fire-fighting measures that exclude their occurrence. Therefore, the main fire-fighting measures that prevent dangerous thermal manifestations of chemical reactions are:

Reliable tightness of the devices, which excludes contact of substances heated above the auto-ignition temperature, as well as substances with a low auto-ignition temperature, with air;

Prevention of spontaneous combustion of substances by reducing the rate of chemical reactions and biological processes, as well as eliminating conditions for heat accumulation;

Reducing the rate of chemical reactions and biological processes is carried out by various methods: limiting humidity during storage of substances and materials; reducing the storage temperature of substances and materials (for example, grain, animal feed) by artificial cooling; storage of substances in an environment with low oxygen content; reducing the specific surface area of contact of self-igniting substances with air (briquetting, granulating powdered substances); use of antioxidants and preservatives (storage of mixed feed); eliminating contact with air and chemically active substances (peroxide compounds, acids, alkalis, etc.) by separately storing self-igniting substances in sealed containers.

Knowing the geometric dimensions of the stack and the initial temperature of the substance, it is possible to determine the safe period for their storage.

Elimination of heat accumulation conditions is carried out in the following way:

limiting the size of stacks, caravans or heaps of stored substances;
active air ventilation (hay and other fibrous plant materials);
periodic mixing of substances during long-term storage;
reducing the intensity of formation of flammable deposits in process equipment using trapping devices;
periodic cleaning of process equipment from self-igniting combustible deposits;

prevention of ignition of substances when interacting with water or air moisture. For this purpose, they are protected from contact with water and moist air by storing substances of this group in isolation from other flammable substances and materials; maintaining an excess amount of water (for example, in devices for producing acetylene from calcium carbide).

Prevention of ignition of substances upon contact with each other. Fires from the ignition of substances upon contact with each other are prevented by separate storage, as well as by eliminating the causes of their emergency release from apparatus and pipelines.

Elimination of ignition of substances as a result of self-decomposition during heating or mechanical impact. Prevention of ignition of substances prone to explosive decomposition is ensured by protection from heating to critical temperatures, mechanical influences (impact, friction, pressure, etc.).

Prevention of the occurrence of ignition sources from thermal manifestations of electrical energy

Prevention of dangerous thermal manifestations of electrical energy is ensured by:

correct choice of the level and type of explosion protection of electric motors and control devices, other electrical and auxiliary equipment in accordance with the fire or explosion hazard class of the zone, category and group of the explosive mixture;
periodic testing of the insulation resistance of electrical networks and electrical machines in accordance with the preventive maintenance schedule;
protection of electrical equipment from short circuit currents (short circuit) (use of high-speed fuses or circuit breakers);
prevention of technological overload of machines and devices;
prevention of high transient resistances through systematic review and repair of the contact part of electrical equipment;
eliminating static electricity discharges by grounding technological equipment, increasing air humidity or using antistatic impurities in the most likely places where charges are generated, ionizing the environment in devices and limiting the speed of movement of liquids that are electrified;
protection of buildings, structures, free-standing devices from direct lightning strikes with lightning rods and protection from its secondary effects.

Conclusion on the issue:

Fire prevention measures in enterprises should not be neglected. Since any savings on fire protection will be disproportionately small in comparison with losses from a fire that occurs for this reason.

Lesson conclusion:

Eliminating the impact of the ignition source on substances and materials is one of the main measures to prevent the occurrence of a fire. At those facilities where it is not possible to eliminate the fire load, special attention is paid to eliminating the ignition source.

A spark discharge occurs in cases where the electric field strength reaches a breakdown value for a given gas. The value depends on the gas pressure; for air at atmospheric pressure it is about . As pressure increases, it increases. According to Paschen's experimental law, the ratio of breakdown field strength to pressure is approximately constant:

A spark discharge is accompanied by the formation of a brightly glowing, tortuous, branched channel through which a short-term pulse of high current passes. An example would be lightning; its length can be up to 10 km, the channel diameter is up to 40 cm, the current strength can reach 100,000 amperes or more, the pulse duration is about .

Each lightning consists of several (up to 50) pulses following the same channel; their total duration (together with the intervals between pulses) can reach several seconds. The temperature of the gas in the spark channel can be up to 10,000 K. Rapid strong heating of the gas leads to a sharp increase in pressure and the appearance of shock and sound waves. Therefore, a spark discharge is accompanied by sound phenomena - from a faint crackling sound from a low-power spark to the rumble of thunder accompanying lightning.

The occurrence of a spark is preceded by the formation of a highly ionized channel in the gas, called a streamer. This channel is obtained by blocking individual electron avalanches that occur along the path of the spark. The founder of each avalanche is an electron formed by photoionization. The streamer development diagram is shown in Fig. 87.1. Let the field strength be such that an electron ejected from the cathode due to some process acquires energy sufficient for ionization at the mean free path.

Therefore, electrons multiply - an avalanche occurs (the positive ions formed in this case do not play a significant role due to their much lower mobility; they only determine the space charge, causing potential redistribution). Short-wave radiation emitted by an atom from which one of the internal electrons has been removed during ionization (this radiation is shown in the diagram by wavy lines) causes photoionization of molecules, and the resulting electrons generate more and more avalanches. After the avalanches overlap, a well-conducting channel is formed - a streamer, through which a powerful flow of electrons rushes from the cathode to the anode - breakdown occurs.

If the electrodes have a shape in which the field in the interelectrode space is approximately uniform (for example, they are balls of a sufficiently large diameter), then breakdown occurs at a very specific voltage, the value of which depends on the distance between the balls. This is the basis of the spark voltmeter, which is used to measure high voltage. During measurements, the greatest distance at which a spark occurs is determined. Then multiply by to obtain the value of the measured voltage.

If one of the electrodes (or both) has a very large curvature (for example, a thin wire or a tip serves as the electrode), then at not too high a voltage a so-called corona discharge occurs. As the voltage increases, this discharge turns into a spark or arc.

During a corona discharge, ionization and excitation of molecules do not occur in the entire interelectrode space, but only near the electrode with a small radius of curvature, where the field strength reaches values equal to or exceeding . In this part of the discharge the gas glows. The glow has the appearance of a corona surrounding the electrode, which gives rise to the name of this type of discharge. The corona discharge from the tip has the appearance of a luminous brush, and therefore it is sometimes called a brush discharge. Depending on the sign of the corona electrode, they speak of positive or negative corona. Between the corona layer and the non-corona electrode there is an outer corona region. The breakdown mode exists only within the corona layer. Therefore, we can say that the corona discharge is an incomplete breakdown of the gas gap.

In the case of a negative corona, the phenomena at the cathode are similar to those at the cathode of a glow discharge. Positive ions accelerated by the field knock out electrons from the cathode, which cause ionization and excitation of molecules in the corona layer. In the outer region of the corona, the field is not sufficient to provide electrons with the energy necessary to ionize or excite molecules.

Therefore, electrons that penetrate into this region drift under the influence of zero to the anode. Some electrons are captured by molecules, resulting in the formation of negative ions. Thus, the current in the external region is determined only by negative carriers - electrons and negative ions. In this region, the discharge is not self-sustaining.

In the positive corona, electron avalanches originate at the outer boundary of the corona and rush towards the corona electrode - the anode. The appearance of electrons that generate avalanches is due to photoionization caused by radiation from the corona layer. The current carriers in the outer region of the corona are positive ions, which drift under the influence of the field to the cathode.

If both electrodes have a large curvature (two corona electrodes), processes characteristic of a corona electrode of a given sign occur near each of them. Both corona layers are separated by an outer region in which counter flows of positive and negative current carriers move. Such a corona is called bipolar.

The independent gas discharge mentioned in § 82 when considering meters is a corona discharge.

The thickness of the corona layer and the strength of the discharge current increase with increasing voltage. At low voltage the size of the corona is small and its glow is imperceptible. Such a microscopic corona appears near the tip from which the electric wind flows (see § 24).

The crown, which appears under the influence of atmospheric electricity on the tops of ship masts, trees, etc., was in ancient times called St. Elmo's fire.

In high-voltage applications, particularly high-voltage transmission lines, corona discharge leads to harmful current leakage. Therefore, measures must be taken to prevent it. For this purpose, for example, the wires of high-voltage lines are taken with a fairly large diameter, the larger the higher the line voltage.

Corona discharge has found useful application in technology in electric precipitators. The gas to be purified moves in a pipe along the axis of which a negative corona electrode is located. Negative ions, present in large quantities in the outer region of the corona, settle on gas-polluting particles or droplets and are carried along with them to the outer non-corona electrode. Having reached this electrode, the particles are neutralized and deposited on it. Subsequently, when the pipe is struck, the sediment formed by the trapped particles falls into the collection tank.

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 the electric field of the 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 glowing 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. The electric field strength in the spark decreases from several tens of kilovolts per centimeter (kV/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