In production conditions, the fire hazardous increase in temperature of bodies as a result of the transformation of mechanical energy into thermal is observed when solid bodies (with or without spark formation); with surface friction of bodies during their mutual movement; With mechanical processing of solid materials, cutting tools, as well as when compressing gases and pressing plastics. The degree of heating of the bodies and the possibility of appearance of the sources of ignition depends on the conditions for the transition of mechanical energy into thermal.

Fig-5-9. Turbinno-vortex sparking: / - housing; 2 - fixed turbine; 3 - The trajectory of the movement of solid particles

Fig. 5.10. The dependence of the temperature of the steel spark from the effort and the disadvantaged material (according to the MIMM): 1 - with an abrasive disk; 2 - with a metal disk. Linear spread rate of 5.2 m / s

Sparks formed during solid blows. With sufficiently "strong collision of some solid bodies, sparks are carved (sparks and friction). The spark in this case is a particle or stone chopped to the glow. The size of the sparks of the blow and friction depend on the properties of the materials and the energy characteristics of the blow, but usually do not exceed 0.1 ... 0.5 mm. Spark temperature, moreover, depends on the process of interaction (chemical and thermal) metal particle with the environment. So, when the metals hit and abrasion in a medium that does not contain oxygen or other oxidizing agent, visible sparks are not formed. An additional heating of the metal sparks of the blow during flight in the environment is usually due to the oxidation of their air oxygen. The temperature of the sparks of unalloyed small carbon steel can reach the melting point of the metal (about 1550 ° C). It will increase with an increase in carbon content steel, decrease with an increase in alloying additives. The dependence of the spark temperature on the material of the compound bodies and the accompanying specific load is shown in Fig. 5.10. According to the schedules, the spark temperature increases linearly with an increase in the load, and the sparks formed when the corundum has taken on the corundum, which are used, have a higher temperature.

In production conditions, acetylene, ethylene, hydrogen, carbon monoxide, carbon is ignited from the spark of the strike. Sparks of impact (under certain conditions) are able to ignore methane-air mixtures. The igniting ability of the sparks of the strike is proportional to the oxygen content in the mixture that these sparks can set fire to. This is understandable: the greater in the oxygen mixture, the more intense the spark is burning, the higher the combustibility of the mixture.

The flammable ability of the spark of the strike is established experimentally - depending on the energy of the impact.

The flying spark does not directly ignore the dusty mixtures, but, hitting the axial dust or fibrous materials, causes the appearance of foci. This, apparently, explains a large number of outbreaks and lighting from mechanical sparks in machines, where there are fibrous materials or deposits of small fuel dust. Thus, in grinding mills of mills and miner roads, in the sorting and tearful and carbon monoxis of textile factories, as well as in cotton-cleaning factories, more than 50% of all lighting and fires occur from sparks veins during solid blows.

Sparks are formed when they blow aluminum bodies about a steel oxidized surface. In this case, there is chemical interaction between the heated aluminum particle and iron oxides with the release of a significant amount of heat:

2A1 + Fe 2 O 3 \u003d A1 2 O 3 + 2FE + Q.

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

Sparks formed during operation by shock tools (hammers, chisels, lows, etc.), often cause fire-hazardous situations. There are cases of outbreaks and explosions in pumping and compressor stations, as well as in the industrial premises when the tool falls, shutting the keys at the time of tightening nuts. Therefore, in the production of work in places where the formation of an explosive mixture of vapors or gas with air is possible, you should not use shock tools from spark-forming materials. Intrinsically safe consider tools made of bronze, phosphorous bronze, brass, beryllium, aluminum AKM-5-2 alloy, durally with limited (up to 1.2 ... 1.8%) content, magnesium .. (alloy D-16 and Dr.) And even tools from high-alloy steels .. The use of a copper tool does not reach the target, for the soft layer of copper is quickly engaged. When using steel tools, they should be protected from falling and, if possible, replace the impact operations) unstressed (for example, the cutting of the metal to replace sawing, etc.), and for the dispersion of combustible vapors or gases at the work of work, we use mobile ventilation units.

Sparks formed when inserting metal or stones.In apparatuses with stirrers for dissolving or chemical treatment of solids in solvents (for example, celluloid mass in alcohol, acetylcellulose in acetone, rubber in gasoline, nitrobolotka in alcohol ethereal mixture, etc.), in the shock-and-centrifugal machine for grinding , breaking and mixing solid combustible substances (hammer and shock-disk mills, feed crushers, cotton-cleaning and trunk machines, etc.), in mixing machines for mixing and compiling powder compositions, centrifugal apparatuses for moving gases and vapor (Fans, fans, centrifugal compressors) can get together with cultivated products of metal or stones, resulting in sparks. Therefore, the processed products should be sifted, drained, rinse or use magnetic, gravitational or inertial catchers.

Fig. 5.11. StoneLog: / - pneumatic pipeline; 2 - bunker; 3 - inclined surfaces; 4 - Unloading Luc

It is especially difficult to clean fibrous materials, since solid impurities are confused in the fibers. So, for cleaning raw cotton from stones before entering it in the car, gravitational or inertial stones are installed (Fig. 5.11).

Metal impurities in bulk and fibrous materials are also captured by magnetic cattors (separators). In fig. 5.12 depicts a magnetic trap, the most widely used in flour and cheerful production, as well as in the feed plants. In fig. 5.13 shows a section of an electromagnetic separator with a rotating drum.

It should be noted that the efficiency of the intakes depends on their location, speed of movement, uniformity and thickness of the product layer, the nature of impurities. Install them, as a rule, at the beginning of the technological line, before machines of the shock action. Separators typically protect machines and mechanical damage. Their installation is also dictated by sanitary and hygienic requirements.

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

Fig. 5.13. Electromagnetic separator with a rotating drum: / - housing; 2 -The-line electromagnet; 3 - product flow; 4 - adjusting screw; 5 - Rotating drum from not

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

If there is a risk of singing in the car of solid non-magnetic impurities, they are carried out, first, thorough sorting of raw materials, secondly, the inner surface of the machines, which these impurities can hit, fucked with soft metal, rubber or plastic.

Sparks formed when blowing moving machines of machines about their fixed parts. In practice, it is often that the rotor of the centrifugal fan comes into contact with the walls of the casing or quick-repairing sawn and knife drums of the wolves but separated and trepal machines hit the fixed steel lattices. In such cases, sparking is observed. It is also possible with improper adjustment of the gaps, during deformation and vibration of shafts, wear bearing, skews, insufficient mounting on the shafts of the cutting tool, etc. In such cases, not only sparking, but also breakage of individual parts of the machines is possible. The vehicle breakdown, in turn, may be the cause of the formation of sparks, since metal particles fall into the product.

The main fire and prophylactic measures aimed at preventing the formation of sparks of impact and friction is reduced to careful adjustment and balancing of shafts, the correct selection of bearings, check the values \u200b\u200bof the gaps between rotating and fixed parts of the machines, their reliable fastening, eliminating the possibility of longitudinal displacements; Preventing machine overloading.

Before starting to work, the machine in which the collision of rotating parts on fixed, should be checked (in a fixed state, and then idling) on \u200b\u200bthe absence of distortion and vibrations, strength of the rotating parts, the presence of the necessary gaps. In the process of working when the appearance of extraneous noise, shocks and concussions, you must stop the machine to troubleshoot.

Incidentally safety requirements are presented to production premises with acetylene, ethylene, carbon monoxide, carbon vapors, nitro compounds, and those like them flammable or unstable substances, floors and platforms in which they are made of material that does not form a spark, or linse with rubber mats, tracks and T . P. Paul of premises where the nitrobolotka is processed, besides, supported in a moistened state. Trolleys and trolleys should have rims from soft metal or rubber on wheels.

Any movement of contact with each other bodies requires energy costs to overcome the work of friction forces. This energy is mainly turning into warmth. In the normal state and the correct operation of the rubbing bodies, the heat released Q T p in a timely manner is allocated to the special cooling system Q okl, and also dissipates into the environment q OKP:

Q. Tr \u003d Q ox + Q OKR.

Violation of this equality, that is, the increase in heat dissipation or decrease in the heat sink and heat loss, leads to an increase in the temperature of the rubbing tel. For this reason, the combustion of the combustible medium or materials from overheating of bearings of machines, highly tightened seals, drums and conveyor belts, pulleys and drive belts, fibrous materials when winding them on rotating tool shafts and mechanically processed solid combustible materials are wrapped.

Fig. 5.14. Sliding bearing diagram: / - shaft spike; 2 - bearing shell; 3 - Stanina

Safety from overheating machine bearingsand devices.The most fires are hazardous sliding bearings of highly loaded high-robust shafts. Poor quality lubrication of working surfaces, their pollution, shafts, overloading the machine and excessive tightening of bearings - all this may be the cause of overheating of bearings. Very often, the bearing housing is polluted by the deposits of combustible dust (wood, flour, cotton). It also creates conditions for their overheating exemplary magnitude of the sliding bearing temperature (see Fig. 5.14) can be determined by calculation. The temperature of the bearing surface when you violate the mode of its operation varies in time. For a segment of time dXyou can write the following thermal balance equation:

d.Q t p \u003d dQ.narch +. dQ.oXL +. dQ. 0 k p. , (5.7)

where dQ T P.- the amount of heat released when the bearing operation;

dQ.narr - the amount of heat going on the heating of the bearing; dQOXL -the amount of heat applied by the forced cooling system; d.Q 0 K P - heat loss with the surface of the bearing in the environment.

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

Q. Tr \u003d. f. Tr. NL,

where f. Tr - friction coefficient; N.- load; / - Relative movement of surfaces.

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

dQ. T p \u003d. f TP ND III / 2πNDτ \u003d πF Tr. ND III NDτ,(5.8)

where p- the rotational speed of the shaft (1 / s); d.- The diameter of the shaft spike. Assuming the friction coefficient of the value of constant and designating the product of permanent values but,will have:

dQ TP \u003d adτ.(5.9)

The amount of heat spent on the heating of the bearing dQ.nerch with raising temperature on dT,it will be:

dQ NARP \u003d MCDT,(5.10)

where t.- mass of heated bearing parts; from- The average specific heat capacity of the bearing material.

The amount of heat dQ 0 xji,an allocated forced cooling system can be taken equal to zero, which corresponds to the most dangerous mode of the bearing.

The amount of heat dQOP,the bearing surface to the environment will be equal to:

dQ.oKR \u003d α ( T.p- T b) fdτ,(5.11)

where α is the heat transfer coefficient of the bearing surface and the medium; T P.and T B.- the temperature of the surface of the bearing and air; F.- The heat exchange surface (the surface of the bearing is washed by the ambient air).

Substituting the values \u200b\u200bfound dQ TP, DQ Narvand dQ 0 kp.to equation. (5.7), we get the equation

adτ \u003d mcdt + a (t n -t b) fdτ,(5.12)

solution of which under the initial conditions of the accident (T n \u003d t c)gives:

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

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

The maximum bearing temperature (at τ \u003d ∞) can be determined by the formula

To avoid a fire-hazardous situation, in this case, instead of the sliding bearings, rolling bearings are used, they are systematically lubricated, control the temperature.

In complex machines (turbines, centrifuge, compressors), the temperature control of bearings is carried out with the help of kipi.

Visual control of the bearing temperature is carried out by applying heat-sensitive paints, changing their color when heated, on the bearing housings. Prevent the bearing overheating allows the system of forced lubrication, the device of which should ensure control of the availability of oil, replacing the exhaust oil with fresh (with given performance), fast and easy removal of oils with parts of the machine.

As an example, it is possible to modernize the lubrication system of the bearing of drying cylinders and weapons of paper and cardboard machines on the pulp and paper mill in the Arkhangelsk region. As a result of this modernization, fires and sunbathing in the relevant systems almost stopped.

Initially, droppers were provided for visual control over the flow of oil into bearings. They were placed under the cables of machines, in the high temperature zone, which practically eliminated the possibility of systematic control. According to the proposal of the Fire Firework and the Fire and Technical Commission, the dropper enterprise was replaced by rotamers made beyond the vehicles. This allowed visually controlling the flow of oil, reduce the number of detachable compounds in the oil system, thereby reducing the oil leaks on the beds and nodes of bearings.

In addition, according to the initial project, the oil in bearings was replaced only with planned warning repairs or planned maintenance. Control the presence of lubrication during the operation of the machine was difficult. The properness of bearings was checked "for rumor". During the reconstruction of the machines, a centralized lubricant system was mounted: from a container (10 m 3) installed in a separate room, the filtered pump was filtered into pressure pipes and through branches to rotames, from rotameters to bearings. Having passed through the bearing, the oil fell into the sump and the filter, where it was cleaned from mechanical impurities, was cooled and re-operated into the working capacity. Pressure, temperature and oil level in the tank were controlled automatically. When stopping the oil pumps and the pressure drop in the pressure line, sound and light alarms were triggered, backup pumps were included.

For cleaning machines from oils and deposited on them, the dust was effective to use a 2% solution of technical detergent TMS-31 (at 50 ... 70 ° C). Over the entire length of the car, a stationary system for washing of aggregates and mechanisms is arranged. The introduction of the cleaning system made it possible for each other, without stopping the machines, flush oils and dust. In addition, 10 tons of kerosene seized from production, the working conditions of working ones are significantly improved.

Overheating and ignition of conveyor belts and drive belts Coming mainly as a result of long-term slippage of the belt or tape relative to pulley. Such a slippage, called bounce, arises due to the inconsistency between the transmitted force and the tension of the branches of the belt (ribbons). When the buxation, all the energy is spent on the friction of the belt of the pulley, as a result of which a significant amount of heat is distinguished. The most frequently bucking of the conveyor belt, the ribbons of elevators and belt gear occurs due to overload or weak belt tension. The elevators the cause of the bucks is most often a shoe challenge, that is, such a state when the Elevator bucket cannot pass through the thickness of the transported substance. To overload and bouncer can cause ribbon, dusting, etc.

The maximum temperature of the drum or pulley with a long-term slip of the ribbon or belt can be determined by formula (5.14).

To avoid overheating and sunbrokes of conveyor belts and drive belts, it is impossible to work with overload; The degree of tension of the tape, the belt should be monitored, their condition cannot be allowed to break the shoes of the elevators products, bias of ribbons and friction them about casing and other nearby objects. In some cases (when using powerful high-performance conveyors and elevators), devices and devices are used, automatically signaling the operation of transmission with overload and stopping the movement of the ribbon during the slaughter of the elevator shoe.

Sometimes to reduce the bucking, the transmission belt sprinkled with rosin, but it only gives a short-term effect. Treatment of the same belt Riniphol contributes to the formation of static electricity charges, which represents a certain fire danger. It is better to use clinorem in this case.

Safety of fibrous materials when winding them on the shafts It is observed in spinning factories, flax supplies, as well as in combines when cleaning grain crops. Fibrous materials and straw products are wound on the shafts near bearings. Washing is accompanied by a gradual mass seal, and then with strong heating of it with friction on the wall of the machine, char harness and, finally, ignition. Sometimes, the light occurs as a result of winding fibrous materials on the trees of conveyors moving waste and finished products. In spinning factories, sunbathing often arise as a result of a cord cliff or braid, with the help of which spinning verses are rotated.

The winding of the fibrous materials on the rotating shafts of the machines contributes to the presence of an increased gap between the shaft of the eyebission (falling into this clearance, the fiber is encouraged, it starts to wind up it on the shaft with an increasingly strong seal of the layers), the presence of centered sections of the shaft, with which fibrous materials come into contact, as well as the use of wet and polluted raw materials.

To prevent the winding of fibrous materials on rotating machines of machines, it is necessary to protect the shafts against direct contact with the processed fibrous materials by using bushings (Fig. 5.15), cylindrical and conical hooks, conductor guides, guide slats, anti-winding shields, etc. In addition, it should be installed minimal gaps between shaft and bearings, not allowing them to increase; Maintain systematic observation of shafts where it may be wound, cleaning them from the fibers in a timely manner, to protect them with special anti-winding sharp knives that cut the wound fiber. Such protection have, for example, trunk machines on flax supplies.

Fig. 5.15. Protection of the shaft from the winding of fibrous materials: but- freely planted straight sleeve; b.- fixed conical sleeve; 1 - Bearing; 2 - shaft; 3 - Protective sleeve

The thermal manifestation of mechanical energy in production conditions is observed during the operation of presses and compressor installations. The fire danger of these mechanisms is considered in chapters 10 and 11 of this textbook.

§ 5.4. Thermal manifestation of chemical reactions -

Spark discharge

Spark discharge (Electric spark) - non-stationary form of electrical discharge occurring in gases. This discharge usually occurs at the pressure of the order of atmospheric and is accompanied by a characteristic sound effect - "cod" sparks. The temperature in the main channel of the spark discharge can reach 10,000. In nature, spark discharges often occur as a lightning. The distance, the "punitive" spark in the air depends on the voltage and is considered to be equal to 10 sq. 1 centimeter.

Conditions

Spark discharge usually occurs if the power of the energy source is insufficient to maintain a stationary arc discharge or a glow discharge. In this case, simultaneously with a sharp increase in the discharge current, the voltage on the discharge gap for a very short time (from several microseconds up to several hundred microseconds) drops below the voltage of the spark discharge extension, which leads to discharge. Then the potential difference between the electrodes is growing again, reaches the ignition voltage and the process is repeated. In other cases, when the power of the energy source is large enough, the entire set of phenomena characteristic of this discharge is also observed, but they are only a transition process leading to the establishment of a discharge of another type - most often arc. If the current source is not able to support an independent electrical discharge for a long time, then there is a form of an independent discharge, called a spark discharge.

Nature

Spark discharge is a bunch of bright, quickly disappearing or replacing each other's filamentous, often strongly branched stripes - spark channels. These channels are filled with plasma, which in a powerful spark discharge includes not only the ions of the source gas, but also the ions of the substance of the electrodes, intensively evaporated under the action of the discharge. The mechanism for the formation of spark channels (and, consequently, the occurrence of the spark discharge) is explained by the streamer theory of electrical breakdown of gases. According to this theory, from electronic avalanches arising in the electric field of the discharge gap, under certain conditions, streamers are formed - dimly glowing fine branched channels, which contain ionized gas atoms and free electrons cleaved from them. Among them can be allocated. The leader is a weakly glowing discharge, "Putting" path for the main discharge. It, moving from one electrode to another, overlaps the discharge gap and connects the electrodes with a continuous conductive channel. Then, in the opposite direction, the main discharge is held in the laughed path, accompanied by a sharp increase in current force and the amount of energy released in them. Each channel is expanding rapidly, as a result of which a shock wave occurs at its borders. A combination of shock waves from expanding spark channels generates the sound perceived as a "crackle" sparks (in case of lightning - thunder).

The ignition voltage of the spark discharge is usually large enough. The electric field strength in the spark drops from several tens of kilovolts per centimeter (kV / cm) at the time of breakdown to ~ 100 volts per centimeter (V / cm) after several microseconds. The maximum current in a powerful spark discharge can reach the values \u200b\u200bof the order of several hundred thousand amps.

Special view of spark discharge - sliding spark discharge, arising along the surface of the gas and solid dielectric partition, placed between the electrodes, provided that the field strength of the punching strength of the air is exceeded. The area of \u200b\u200bthe sliding spark discharge, in which the charges of any single sign prevail, are induced on the surface of the dielectric charges of another sign, as a result of which spark channels are steel along the surface of the dielectric, while forming the so-called figures of Lichtenberg. The processes close to what is happening in the spark discharge is also characterized by a wretched discharge, which is the transitional stage between the corona and spark.

The behavior of the spark discharge can be very good to see on slow motion shooting of discharges (FIMP. \u003d 500 Hz, U \u003d 400 kV) obtained from the Tesla transformer. The average current and pulse duration is insufficient for the ignition of the arc, but for the formation of a bright spark channel is quite suitable.

Notes

Sources

• A. A. Vorobiev, high voltage technique. - Moscow-Leningrad, Gosnergoisdat, 1945.
• Physical encyclopedia, T.2 - M.: Large Russian Encyclopedia p.218.
• Raizer Yu. P. Gas discharge physics. - 2nd ed. - M.: Science, 1992. - 536 p. - ISBN 5-02014615-3.

see also

Wikimedia Foundation. 2010.

Watch what is a "spark discharge" in other dictionaries:

- (spark), unsteady electric. The discharge occurring in the case when immediately after the breakdown of the discharge interval, the voltage on it drops over a very short time (from several. MKS fraction to hundreds of ISS) below the voltage ... ... Physical encyclopedia

spark discharge - Electric pulsed discharge in the form of a luminous thread occurring at high gas pressure and characterized by a large intensity of spectral lines of ionized atoms or molecules. [GOST 13820 77] Spark discharge full discharge in ... ... Technical translator directory

- (spark electric) nonstationary electric discharge in a gas that occurs in an electric field at a gas pressure to several atmospheres. It has a winding branched form and rapid development (approx. 10 7 s). Temperature in the main channel ... Big Encyclopedic Dictionary

KIBIRKŠTINIS IŠLYDIS STATUSAS T SRITIS FIZIKA ATITIKMENYS: ANGL. Spark Discharge Vok. FunkenentLadung, F; FunkenLadung, F Rus. Spark discharge, M pranc. Décharge Par Étincelles, F Fizikos Terminų žodynas

Spark, one of the forms of electrical discharge in gases; It usually occurs at the pressure of the order of atmospheric and is accompanied by the characteristic sound effect of the "cod" sparks. In natural conditions I. r. Most often observed in the form of lightning ... ... Great Soviet Encyclopedia

The spark is electrical, non-stationary electric discharge in the gas occurring in the electric. Field with gas pressure to several. hundred kpa. It has a winding branching form and rapid development (approx. 10 7 (s), accompanied by a characteristic sound ... ... Big Encyclopedic Polytechnic Dictionary

- (electrical spark), non-stationary electric. The discharge in the gas occurring in the electric. Field with gas pressure to several. atm. Different with a winding branched form and rapid development (approx. 10 7c). Paced PA in ch. Channel I. r. reaches 10,000 to ... Natural science. encyclopedic Dictionary

Page 5 of 14

Blows of solid bodies with the formation of sparks.

With a certain strength of some solid bodies, sparks that call the sparks of the blow or friction can be formed about each other.

Sparks are heated to high temperatures (hot) particles of metal or stone (depending on which solid bodies are involved in collision) with a size of 0.1 to 0.5 mm and more.

The spark temperature of the strike from ordinary structural steels reaches the melting point of metal - 1550 ° C.

Despite the high temperature of the sparks, its flammable ability is relatively low, since due to small sizes (mass), the supply of thermal energy sparks is very small. Sparks are capable of ignite the vapor-high mixtures having a small induction period, a small minimum ignition energy. Acetylene, hydrogen, ethylene, carbon monoxide and serougerium are of the greatest danger in this regard.

The flammable ability of the spark, located at rest, above the flying, as the fixed spark is slower than cooled, it gives heat to the same volume of the combustible medium and, therefore, can heat it up to a higher temperature. Therefore, sparks that are alone are able to ignore even solids in crushed form (fiber, dust).

Sparks in the conditions of production are formed when working with an impact tool (wrench, hammers, chisels, etc.), when masting metal impurities and stones in machines with rotating mechanisms (devices with stirrers, fans, fans, etc.) , as well as with the blows of mobile mechanisms of the machine on fixed (hammer mills, fans, devices with folding lids, hatches, etc.).

Activities to prevent the dangerous manifestation of sparks from impact and friction:

1. Application in explosive areas (premises) to use an intrinsically safe tool.
2. By blowing clean air the places of production of repair and other works.
3. Exclusion from the machines of metal impurities and stones (magnetic cattles and stone chiefs).
4. To prevent the sparks from the blows of movable mechanisms of machines about fixed:
   1. careful adjustment and shaft balancing;
   2. verification of gaps between these mechanisms;
   3. preventing machine overload.
5. Apply intrinsically safe fans for transporting steam and gas-air mixtures, dust and solid combustible materials.
6. In the premises of obtaining and storing acetylene, ethylene, etc. Floors to perform from an insincerible material or stipulated by their rubber mats.

Surface friction tel.

Moving relative to each other in contact with bodies requires energy costs for overcoming friction forces. This energy is almost entirely turning into warmth, which, in turn, depends on the type of friction, the properties of the rubbing surfaces (their nature, the degree of contamination, roughness), from pressure, surface size and initial temperature. Under normal conditions, the heat released in a timely manner is allocated, and the normal temperature mode is ensured. However, under certain conditions, the temperature of rubbing surfaces can increase to dangerous values \u200b\u200bunder which they can become a source of ignition.

The reasons for the growth of the temperature of the rubbing bodies in the general case is an increase in the amount of heat or a decrease in the heat sink. For these reasons, hazardous overheating of bearings, transport tapes and drive belts, fibrous combustible materials occur in the technological processes of industries, while winding them into rotating shafts, as well as solid combustible materials during their machining.

Activities for the prevention of dangerous manifestation of surface friction Tel:

1. Replacing the bearings of sliding on rolling bearings.
2. Control over lubrication, bearing temperature.
3. Control over the degree of tension of the conveyor belts, belts, not allowing the operation of machines with overload.
4. Replacing flat transfers to clinorem.
5. To prevent the winding of fibrous materials on rotating shafts, use:
   1. the use of free-suiced sleeves, housings, etc. To protect the open sections of the shafts from contact with fibrous material;
   2. overload prevention;
   3. the device of special knives for cutting the winding fibrous materials;
   4. installation of minimum gaps between the shaft and the bearing.
6. With mechanical processing of combustible materials, it is necessary:
   1. comply with cutting mode
   2. tool tool in a timely manner
   3. use local cutting of cutting location (emulsion, oil, water, etc.).

Calculation of the parameters of fire sources (explosion)

At this stage, it is necessary to assess the possibility of ignition sources to initiate combustible substances.

The calculation takes four ignition sources:

a) the secondary effect of lightning;

b) sparks of short circuit;

c) sparks of electric welding;

d) incandescent bulbs flask.

e) burning electrical insulation (wires)

Secondary exposure to lightning

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

The secondary action of the lightning strike is dangerous for the gas, which filled the entire volume of the room.

Thermal effect of short-acting currents

It is clear that with a short circuit, when the defense device refuses, the sparks that appear are capable of ignite the housing and blowing up the gas (this feature is estimated below). When the protection is triggered, the short circuit current lasts a short time and can only ignite the polyvinyl chloride wiring.

The temperature of the conductor T Prie about with the heated short circuit current is calculated by the formula

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

I k.z. - short circuit current, and;

R is the resistance (active) conductor, OM;

k.Z. - short circuit duration, with;

With pr - heat capacity of the material of the wire, j * kg -1 * to -1;

m Pr - mass of wires, kg.

In order for the wiring to ignites it is necessary that the temperature T PR was larger than the temperature of the inflammation of the polyvinyl chloride wiring T Best.Pr. \u003d 330 o C.

The initial temperature of the conductor is taking equal to the ambient temperature of 20 o C. Above in chapter 1.2.2, the active resistance of the conductor (Ra \u003d 1.734 ohms) and a short circuit current (I KZ \u003d 131.07 A) were calculated. Heat capacity with PR \u003d 400 J * kg -1 * to -1. The mass of the wire is a product of density on the volume, and the volume - the product of the length L on the cross section of the conductor s

m \u003d * s * L (18)

According to the directories, we find the value \u003d 8.96 * 10 3 kg / m 3. In formula (18) we substitute the value of the sections of the second wire, from the table. 11, the shortest, that is, L \u003d 2 m and S \u003d 1 * 10 -6 m. The mass of the wire is equal to

m Pr \u003d 8.96 * 10 3 * 10 -6 * 2 \u003d 1,792 * 10 -2

With the duration of a short circuit K.Z. \u003d 30 ms, Table 11, the conductor will heat up to temperature

This temperature is not enough to ignite the polyvinyl chloride wiring. And if the protection turns off, it will be necessary to calculate the likelihood of a polyvinyl chloride wiring.

Sparks of short circuit

With a short circuit, sparks occur, which have an initial temperature of 2100 ° C and are able to ignite the housing and blowing the gas.

The initial temperature of the copper drop of 2100 o p. The height at which a short circuit occurs, 1 m, and the distance to the puddles of the LVZH 4 m. Drop diameter D K \u003d 2.7 mm or D K \u003d 2.7 * 10 -3.

The amount of heat that is capable of metal can give a combustible medium when cooled to the temperature of its ignition is calculated as follows: The average speed of the metal drop flight with a free fall of W cf, m / s, calculated by the formula

where G is the acceleration of free fall, 9.81 m / s 2;

H is the height of the fall, 1 m.

We get that the average speed of the drop of drops with a free fall

Duration of falling drops can be calculated by the formula

Then they calculate the volume of the VK by the formula

Mass drop M k, kg:

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

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

m K \u003d 8.6 * 10 3 * 10,3138 * 10 -9 \u003d 8,867 * 10 -5

Time of metal drops in molten (liquid) state P, C.:

where with p is the specific heat of the melt of the material of the drop, for copper with p \u003d 513 j * kg -1 * to -1;

S to - surface area of \u200b\u200ba drop, m 2, s k \u003d 0,785d to 2 \u003d 5,722 * 10 -6;

T n, T pl - the temperature of the drop at the beginning of the flight and the melting point of the metal, respectively, T n \u003d 2373 K, T pl \u003d 1083 K;

T o - ambient temperature, T o \u003d 293 K;

The heat transfer coefficient, W * M -2 * to -1.

The heat transfer coefficient is calculated by the following sequence:

1) First calculate the number of Reynolds

where V \u003d 1.51 * 10 -5 1 / (m 2 * c) is the coefficient of kinematic viscosity of air at a temperature of 293 K,

where \u003d 2.2 * 10 -2 W * M -1 * K -1 is the thermal conductivity coefficient of air,

1 * 10 2 W * M -2 * to -1.

Having calculated the heat transfer coefficient to find the time of the metal drop flight in the molten (liquid) state by formula (23)

As< р, то конечную температуру капли определяют по формуле

The temperature of the self-ignition temperature is 466 o C, and the temperature of the drop (sparks) by the time it is puddled to the puddle of the hip 2373 to or 2100 o C. At this temperature, the isoprene will marry and will be stable to burn, and propane will explode in the event of a short circuit spark. Flash temperature isoprene -48 0 S.