A spark discharge occurs when 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 . Increases with increasing pressure. According to Paschen's experimental law, the ratio of the 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 current pulse of high strength passes. An example is lightning; its length is up to 10 km, the channel diameter is up to 40 cm, the current strength can reach 100,000 or more amperes, 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 weak crackling with a low-power spark to thunder that accompanies lightning.
The appearance of a spark is preceded by the formation of a highly ionized channel in the gas, called a streamer. This channel is obtained by overlapping individual electron avalanches that occur in the path of the spark. The ancestor of each avalanche is an electron formed by photoionization. The scheme of streamer development is shown in fig. 87.1. Let the field strength be such that an electron escaping from the cathode due to some process acquires energy sufficient for ionization over the mean free path.
Therefore, the multiplication of electrons occurs - 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, which causes a redistribution of the potential). The short-wavelength radiation emitted by an atom, in which one of the internal electrons was torn out during ionization (this radiation is shown in the diagram by wavy lines), causes the photoionization of molecules, and the formed electrons generate more and more new avalanches. After the avalanches overlap, a well-conducting channel is formed - a streamer, along which a powerful flow of electrons rushes from the cathode to the anode - a breakdown occurs.
If the electrodes have a shape in which the field in the interelectrode space is approximately uniform (for example, it is balls of a sufficiently large diameter), then breakdown occurs at a well-defined voltage, the value of which depends on the distance between the balls. The spark voltmeter is based on this, with which high voltage is measured. When measuring, the largest distance at which a spark occurs is determined. Multiplying then by get 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 point serves as an electrode), then at a not too high voltage, a so-called corona discharge occurs. With increasing voltage, 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 greater than . In this part of the discharge, the gas glows. The glow has the appearance of a corona surrounding the electrode, which is the reason for the name of this type of discharge. The corona discharge from the tip looks like a luminous brush, which is why it is sometimes called a brush discharge. Depending on the sign of the corona electrode, one speaks of positive or negative corona. Between the corona layer and the non-corona electrode is the outer region of the corona. The breakdown regime 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 glow discharge cathode. 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 insufficient to provide the electrons with the energy needed to ionize or excite the molecules.
Therefore, the electrons that have penetrated into this region drift under the action of zero to the anode. Some of the electrons are captured by molecules, resulting in the formation of negative ions. Thus, the current in the outer region is determined only by negative carriers - electrons and negative ions. In this region, the discharge has a non-self-sustaining character.
In the positive corona, electron avalanches originate at the outer boundary of the corona and rush to the corona electrode - the anode. The appearance of electrons that generate avalanches is due to photoionization caused by the radiation of the corona layer. The current carriers in the outer region of the corona are positive ions, which drift under the action of the field towards the cathode.
If both electrodes have a large curvature (two corona electrodes), processes inherent in the corona electrode of this sign proceed 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 a low voltage, the size of the corona is small and its glow is imperceptible. Such a microscopic corona arises near the point from which the electric wind flows (see § 24).
The crown, which appears under the action of atmospheric electricity on the tops of ship masts, trees, etc., was in the old days called the fires of St. Elmo.
In high voltage applications, in particular in high voltage transmission lines, corona leads to harmful current leakage. Therefore, measures must be taken to prevent it. For this purpose, for example, the wires of high-voltage lines take a sufficiently large diameter, the larger, the higher the line voltage.
Useful application in technology corona discharge found in electrostatic precipitators. The gas to be purified moves in a pipe along the axis of which a negative corona electrode is located. Negative ions, which are present in large quantities in the outer region of the corona, are deposited on particles or droplets polluting the gas and are carried along with them to the external non-corona electrode. Upon reaching this electrode, the particles are neutralized and settle on it. Subsequently, when hitting the pipe, the sediment formed by the trapped particles crumbles into the collection.
Depending on the gas pressure, the configuration of the electrodes and the parameters of the external circuit, there are four types of self-sustained discharges:
- glow discharge;
- spark discharge;
- arc discharge;
- corona rank.
1. glow discharge occurs at low pressures. It can be observed in a glass tube with flat metal electrodes soldered at the ends (Fig. 8.5). Near the cathode is a thin luminous layer called cathode luminous film 2.
Between the cathode and the film is aston dark space 1. To the right of the luminous film is placed a weakly luminous layer, called cathode dark space 3. This layer passes into a luminous area, which is called smoldering glow 4, a dark gap borders the smoldering space - faraday dark space 5. All the listed layers form cathode part glow discharge. The rest of the tube is filled with glowing gas. This part is called positive pillar 6.
As the pressure decreases, the cathode part of the discharge and the Faraday dark space increase, and the positive column shortens.
The measurements showed that almost all potential drops occur in the first three sections of the discharge (Aston dark space, cathode luminous film, and cathode dark spot). This part of the voltage applied to the tube is called cathodic potential drop.
In the smoldering glow, the potential does not change - here the field strength is zero. Finally, in the Faraday dark space and the positive column, the potential slowly increases.
This potential distribution is caused by the formation of a positive space charge in the cathode dark space, due to the increased concentration of positive ions.
Positive ions, accelerated by the cathodic potential drop, bombard the cathode and knock electrons out of it. In the astonian dark space, these electrons, which flew without collision into the region of the cathode dark space, have a high energy, as a result of which they more often ionize molecules than excite them. Those. the intensity of the glow of the gas decreases, but many electrons and positive ions are formed. The ions formed at the beginning have a very low speed, and therefore a positive space charge is created in the cathode dark space, which leads to a redistribution of the potential along the tube and to the appearance of a cathodic potential drop.
The electrons that have arisen in the cathode dark space penetrate into the glowing region, which is characterized by a high concentration of electrons and positive ions with a clenar space charge close to zero (plasma). Therefore, the field strength here is very small. In the region of the smoldering glow, an intense recombination process takes place, accompanied by the emission of the energy released in this process. Thus, the smoldering glow is basically the glow of recombination.
Electrons and ions penetrate from the smoldering glow into the Faraday dark space due to diffusion. The probability of recombination is greatly reduced here, since the concentration of charged particles is low. Therefore, there is a field in Faraday dark space. The electrons dragged along by this field accumulate energy, and often eventually the conditions necessary for the existence of a plasma arise. The positive column is a gas-discharge plasma. It acts as a conductor connecting the anode to the cathode parts of the discharge. The glow of the positive column is caused mainly by the transitions of excited molecules to the ground state.
2. spark discharge occurs in a gas usually at pressures of the order of atmospheric pressure. It is characterized by a discontinuous shape. In appearance, the spark discharge is a beam of bright zigzag branching thin strips that instantly penetrate the discharge gap, quickly extinguish and constantly replace each other (Fig. 8.6). These stripes are called spark channels.
 |
T gas = 10,000 K ~ 40 cm I= 100 kA t= 10 –4 s l~ 10 km |
After the discharge gap is “pierced” by the spark channel, its resistance becomes small, a short-term current pulse of high strength passes through the channel, during which only a small voltage falls on the discharge gap. If the power of the source is not very high, then after this current pulse the discharge stops. The voltage between the electrodes begins to rise to the previous value, and the gas breakdown is repeated with the formation of a new spark channel.
Under natural conditions, a spark discharge is observed in the form of lightning. Figure 8.7 shows an example of a spark discharge - lightning, with a duration of 0.2 ÷ 0.3 with a current strength of 10 4 - 10 5 A, a length of 20 km (Fig. 8.7).
3. arc discharge . If, after receiving a spark discharge from a powerful source, the distance between the electrodes is gradually reduced, then the discharge from intermittent becomes continuous, a new form of gas discharge occurs, called arc discharge(Fig. 8.8).
 |  |
|
| ~ 10 3 A | ||
| Rice. 8.8 | ||
In this case, the current increases sharply, reaching tens and hundreds of amperes, and the voltage across the discharge gap drops to several tens of volts. According to V.F. Litkevich (1872 - 1951), the arc discharge is maintained mainly due to thermionic emission from the cathode surface. In practice, this is welding, powerful arc furnaces.
4. corona discharge (Fig. 8.9). arises in a strong inhomogeneous electric field at relatively high gas pressures (of the order of atmospheric). Such a field can be obtained between two electrodes, the surface of one of which has a large curvature (thin wire, tip).
 |
 |
The presence of a second electrode is optional, but the nearest, surrounding grounded metal objects can play its role. When the electric field near an electrode with a large curvature reaches approximately 3∙10 6 V / m, a glow appears around it, which has the form of a shell or crown, from which the name of the charge originated.
4.9. Based on the collected data, the safety factor is calculated K s in the following sequence.
4.9.1. Calculate the average time of existence of a fire and explosion hazardous event (t0) (average time spent in failure) according to the formula
(68)
where t j- lifetime i-th fire and explosion hazardous event, min;
m- total number of events (products);
j- sequence number of the event (product).
4.9.2. A point estimate of the variance ( D 0) the average time of existence of a fire and explosion hazardous event is calculated by the formula 
(69)
4.9.3. The standard deviation () of a point estimate of the average lifetime of an event - t0 is calculated by the formula 
(70)
4.9.4. From Table. 5 choose coefficient value t b depending on the number of degrees of freedom ( m-1) with a confidence level b=0.95.
Table 5
|
m-1 |
1 |
2 |
3 to 5 |
6 to 10 |
11 to 20 |
20 |
|
t b |
12,71 |
4,30 |
3,18 |
2,45 |
2,20 |
2,09 |
4.9.5. Safety factor ( K b) (the coefficient taking into account the deviation of the value of the parameter t0, calculated by formula (68), from its true value) is calculated from the formula
(71)
4.9.6. When only one event occurs during the year, the safety factor is assumed to be equal to one.
5. Determination of fire-hazardous parameters of thermal sources of element failure rate
5.1. Fire hazard parameters of heat sources
5.1.1. Discharge of atmospheric electricity
5.l.l.l. Direct lightning strike
The danger of a direct lightning strike lies in the contact of a combustible medium with a lightning channel, the temperature in which reaches 30,000 ° C at a current strength of 200,000 A and an action time of about 100 μs. All flammable media ignite from a direct lightning strike.
5.1.1.2. Secondary impact of lightning
The danger of the secondary impact of lightning lies in spark discharges resulting from the inductive and electromagnetic effects of atmospheric electricity on production equipment, pipelines and building structures. The spark discharge energy exceeds 250 mJ and is sufficient to ignite combustible substances with a minimum ignition energy of up to 0.25 J.
5.1.1.3. High potential skid
The high potential is brought into the 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. Subject to safe distances between lightning rods and communications, the energy of possible spark discharges reaches values of 100 J or more, that is, it is sufficient to ignite all combustible substances.
5.1.2. Electric spark (arc)
5.1.2.1. Thermal effect of short circuit currents
Conductor temperature ( t pr), °С, heated by a short circuit current, is calculated by the formula
(72)where t n is the initial temperature of the conductor, °C;
I short circuit - short circuit current, A;
R- conductor resistance, Ohm;
tk.z - short circuit time, s;
FROM pr - heat capacity of the conductor, J×kg-1×K-1;
m pr - mass of the conductor, kg.
The flammability of the cable and the conductor with insulation depends on the value of the multiplicity of the short-circuit current I k.z, i.e. from the value of the ratio I short circuit to the continuous current of the cable or wire. If this multiplicity is greater than 2.5, but less than 18 for the cable and 21 for the wire, then PVC insulation ignites.
5.1.2.2. Electric sparks (drops of metal)
Electric sparks (drops of metal) are formed during a short circuit in electrical wiring, electric welding, and during the melting of the electrodes of general-purpose incandescent electric lamps. The size of metal droplets in this case reaches 3 mm (for ceiling welding - 4 mm). During short circuit and electric welding, particles fly out in all directions, and their speed does not exceed 10 and 4 m s-1, respectively. The droplet temperature depends on the type of metal and is equal to the melting point. The temperature of aluminum droplets during a short circuit reaches 2500 °C, the temperature of welding particles and nickel particles of incandescent lamps reaches 2100 °C. The droplet size when cutting metal reaches 15-26 mm, the speed is 1 m s-1, the temperature is 1500 °C. The temperature of the arc during welding and cutting reaches 4000 ° C, so the arc is the source of ignition of all combustible substances.
The zone of particle expansion during a short circuit depends on the height of the wire, the initial velocity of the particles, the angle of departure, and is of a probabilistic nature. With a wire height of 10 m, the probability of particles falling at a distance of 9 m is 0.06; 7m-0.45 and 5m-0.92; at a height of 3 m, the probability of particles falling at a distance of 8 m is 0.01; 0.24, 4 m - 0.66 and 3 m - 0.99.
The amount of heat that a drop of metal is able to give off to a combustible medium when it cools to its self-ignition temperature is calculated in the following way.
The average flight speed of a metal drop in free fall (wк), m×s-1, is calculated by the formula
(73)
where g=9.8l m×s-1 - free fall acceleration;
H- fall height, m
Metal drop volume ( V k), m3, is calculated by the formula
(74)where d k - droplet diameter, m.
Drop mass ( m k), kg, calculated by the formula
(75)
where r is the density of the metal, kg×m-3.
Depending on the duration of the flight of a drop, three of its states are possible: liquid, crystallization, solid.
The flight time of a drop in the molten (liquid) state (tp), s, is calculated by the formula
(76)where C p - specific heat of the metal melt, J×k-1K-1;
m k is the mass of the drop, kg;
S k=0.785 - drop surface area, m2;
T n, T pl is the temperature of the drop at the beginning of the flight and the melting point of the metal, respectively, K;
T 0 - ambient (air) temperature, K;
a- heat transfer coefficient, W, m-2 K-1.
The heat transfer coefficient is determined in the following sequence:
a) calculate the Reynolds number according to the formula
(77)
where d k - drop diameter m;
v= 15.1×10-6 - coefficient of kinematic viscosity of air at a temperature of 20°С, m-2×s-1.
b) calculate the Nusselt criterion according to the formula
(78)
c) calculate the heat transfer coefficient according to the formula
, (79)
where lВ=22×10-3 - coefficient of thermal conductivity of air, W×m-1× -К-1.
If t £ tp, then the final temperature of the drop is determined by the formula
(80)The flight time of a drop, during which it crystallizes, is determined by the formula
(81)where FROM cr - specific heat of metal crystallization, J×kg-1.
If tr
If t>(tр+tcr), then the final temperature of the drop in the solid state is determined by the formula
(83)
where FROM k is the specific heat capacity of the metal, J kg -1×K-1.
The amount of heat ( W), J, given by a drop of metal to a solid or liquid combustible material on which it fell, is calculated by the formula
(84)
where T sv - self-ignition temperature of combustible material, K;
To- coefficient equal to the ratio of the heat given to the combustible substance to the energy stored in the drop.
If it is not possible to determine the coefficient To, then accept To=1.
A more rigorous determination of the final droplet temperature can be carried out taking into account the dependence of the heat transfer coefficient on temperature.
5.1.2.3. General purpose electric incandescent lamps
The fire hazard of lamps is due to the possibility of contact of a combustible medium with the bulb of an electric incandescent lamp heated above the self-ignition temperature of a combustible medium. The heating temperature of the bulb of an electric bulb depends on the power of the lamp, its size and location in space. The dependence of the maximum temperature on the bulb of a horizontally located lamp on its power and time is shown in Fig. 3.
Crap. 3
5.1.2.4. Sparks of static electricity
spark energy ( W i), J, which can arise under the action of voltage between the plate and any grounded object, is calculated from the energy stored by the capacitor from the formula
(85)
where FROM- capacitance of the capacitor, F;
U- voltage, V.
The potential difference between a charged body and the earth is measured by electrometers in real production conditions. 
If a W U³0.4 W m.e.z ( W m.e.z ¾ minimum ignition energy of the medium), then a spark of static electricity is considered as an ignition source.
The real danger is the "contact" electrification of people working with moving dielectric materials. When a person comes into contact with a grounded object, sparks with an energy of 2.5 to 7.5 mJ are generated. The dependence of the energy of an electric discharge from the human body and the potential of static electricity charges is shown in Fig. four.
5.1.3. Mechanical (frictional) sparks (sparks from impact and friction)
The size of impact and friction sparks, which are a piece of metal or stone heated to a glow, usually does not exceed 0.5 mm, and their temperature is within the melting point of the metal. The temperature of sparks formed during the collision of metals capable of entering into chemical interaction with each other with the release of a significant amount of heat can exceed the melting temperature and therefore it is determined experimentally or by calculation.
The amount of heat given off by the spark when cooled from the initial temperature t n up to the temperature of self-ignition of a combustible medium t sv is calculated using formula (84), and the cooling time t is as follows.
The temperature ratio (Qp) is calculated by the formula 
(86)
where t c - air temperature, °C.
Heat transfer coefficient ( a), W × m-2 × K-1, is calculated by the formula
(87)
where w and - spark flight speed, m×s-1.
spark speed ( w i), formed upon impact of a freely falling body, is calculated by the formula
(88)
and upon impact with a rotating body according to the formula
(89)
where n- rotation frequency, s-1;
R- radius of the rotating body, m.
The speed of flight of sparks formed when working with a percussion instrument is taken equal to 16 m s
The Biot criterion is calculated by the formula
(90)
where d u is the spark diameter, m;
li is the coefficient of thermal conductivity of the spark metal at the self-ignition temperature of the combustible substance ( t sv), W m -1 × K-1.
According to the values of the relative excess temperature qp and the criterion AT i determine according to the graph (Fig. 5) the Fourier criterion. 
Crap. 5
The cooling time of a metal particle (t), s, is calculated by the formula 
(91)
where F 0 - Fourier criterion;
FROM and - heat capacity of the spark metal at the self-ignition temperature of the combustible substance, J×kg-1×K-1;
ri is the density of the spark metal at the self-ignition temperature of the combustible substance, kg×m-3.
In the presence of experimental data on the igniting ability of friction sparks, a conclusion about their danger to the analyzed combustible medium can be made without calculations.
5.1.4. Open flames and sparks from engines (furnaces)
The fire hazard of a flame is due to the intensity of the thermal effect (heat flux density), the area of influence, orientation (mutual position), frequency and time of its effect on combustible substances. The heat flux density of diffusion flames (matches, candles, gas burners) is 18-40 kW×m-2, and pre-mixed (blowtorches, gas burners) 60-140 kW×m-2. 6 shows the temperature and time characteristics of some flames and low-calorie heat sources.
Table 6
| Name of the burning substance (product) or fire hazardous operation |
Flame temperature (smoldering or heating), °C |
Burning time (smoldering), min |
| Flammable and combustible liquids |
880 |
¾ |
| Timber and sawn timber |
1000 |
- |
| Natural and liquefied gases |
1200 |
- |
| Gas metal welding |
3150 |
- |
| Gas cutting of metal |
1350 |
- |
| Smoldering cigarette |
320-410 |
2-2,5 |
| Smoldering cigarette |
420¾460 |
26-30 |
| burning match |
600¾640 |
0,33 |
An open flame is dangerous not only in direct contact with a combustible medium, but also when it is irradiated. Irradiation intensity ( g p), W × m-2, is calculated by the formula
(92)where 5.7 is the emissivity of a black body, W × m-2 × K-4;
epr - reduced emissivity of the system
(93)ef - the degree of blackness of the torch (when burning wood is 0.7, oil products 0.85);
ev - the degree of emissivity of the irradiated substance is taken from the reference literature;
T f - temperature of the flame, K,
T sv is the temperature of the combustible substance, K;
j1f is the irradiance coefficient between the radiating and irradiated surfaces.
The critical values of the irradiation intensity depending on the irradiation time for some substances are given in Table. 7.
The fire hazard of sparks from chimneys, boiler rooms, pipes of locomotives and diesel locomotives, as well as other machines, fires, is largely determined by their size and temperature. It has been established that a spark with a diameter of 2 mm is flammable if it has a temperature of about 1000 ° C, a spark with a diameter of 3 mm is 800 ° C, and a spark with a diameter of 5 mm is 600 ° C.
The heat content and the time for the spark to cool down to a safe temperature are calculated using formulas (76 and 91). In this case, the spark diameter is assumed to be 3 mm, and the spark flight speed (wi), m×s-1, is calculated by the formula
(94)where ww - wind speed, m×s-1;
H- pipe height, m.
Table 7
| Material |
Minimum irradiation intensity, W × m-2, with irradiation duration, min |
||
|
|
3 |
5 |
15 |
| Wood (pine with a moisture content of 12%) |
18800 |
16900 |
13900 |
| Chipboard with a density of 417 kg×m-3 |
13900 |
11900 |
8300 |
| Peat briquette |
31500 |
24400 |
13200 |
| Peat lump |
16600 |
14350 |
9800 |
| cotton fiber |
11000 |
9700 |
7500 |
| Laminate |
21600 |
19100 |
15400 |
| fiberglass |
19400 |
18600 |
17400 |
| glassine |
22000 |
19750 |
17400 |
| Rubber |
22600 |
19200 |
14800 |
| Coal |
¾ |
35000 |
35000 |
Calculation of parameters of fire (explosion) sources
At this stage, it is necessary to assess the ability of ignition sources to initiate combustible substances.
Four sources of ignition are taken into account:
a) secondary action of lightning;
b) short circuit sparks;
c) electric welding sparks;
d) bulb of an incandescent lamp.
e) burning insulation of the electric cable (wire)
Secondary impact of lightning
The danger of the secondary impact of lightning lies in spark discharges resulting from the inductive and electromagnetic effects of atmospheric electricity on production equipment, pipelines and building structures. The spark discharge energy exceeds 250 mJ and is sufficient to ignite combustible substances with a minimum ignition energy of up to 0.25 J.
The secondary action of a lightning strike is dangerous for the gas that has filled the entire volume of the room.
Thermal action of short currents
It is clear that in the event of a short circuit, when the protection apparatus fails, the sparks that appear are capable of igniting the flammable liquid and exploding the gas (this possibility is assessed below). When the protection is activated, the short-circuit current lasts a short time and is only capable of igniting the PVC wiring.
The temperature of the conductor t about C, heated by a short circuit current, is calculated by the formula
where t n is the initial temperature of the conductor, o C;
I k.z. - short circuit current, A;
R - resistance (active) of the conductor, Ohm;
k.z. - short circuit duration, s;
C pr - heat capacity of the wire material, J * kg -1 * K -1;
m pr - weight of the wire, kg.
In order for the wiring to ignite, it is necessary that the temperature t pr be greater than the ignition temperature of the PVC wiring t resp. \u003d 330 about C.
The initial temperature of the conductor is assumed to be equal to the ambient temperature of 20 ° C. Above in chapter 1.2.2, the active resistance of the conductor (Ra \u003d 1.734 Ohm) and the short-circuit current (I short circuit \u003d 131.07 A) were calculated. The heat capacity of copper C pr \u003d 400 J * kg -1 * K -1. The mass of the wire is the product of density and volume, and the volume is the product of length L and the cross-sectional area of the conductor S
m pr \u003d * S * L (18)
According to the reference book, we find the value \u003d 8.96 * 10 3 kg / m 3. In formula (18) we substitute the value of the cross-sectional area of the second wire, from Table. 11, the shortest, that is, L \u003d 2 m and S \u003d 1 * 10 -6 m. The mass of the wire is
m pr \u003d 8.96 * 10 3 * 10 -6 * 2 \u003d 1.792 * 10 -2
With the duration of a short circuit short circuit. \u003d 30 ms, according to Table 11, the conductor will heat up to a temperature
This temperature is not enough to ignite PVC wiring. And if the protection turns off, then it will be necessary to calculate the probability of fire of the PVC wiring.
Sparks short circuit
In the event of a short circuit, sparks arise that have an initial temperature of 2100 ° C and are capable of igniting the flammable liquid and exploding the gas.
The initial temperature of the copper drop is 2100 o C. The height at which the short circuit occurs is 1 m, and the distance to the flammable liquid puddle is 4 m. The drop diameter is d to =2.7 mm or d to =2.7*10 -3.
The amount of heat that a metal drop is able to give off to a combustible medium when it cools down to its ignition temperature is calculated as follows: the average flight speed of a metal drop during free fall w cf, m/s, is calculated by the formula
where g is the free fall acceleration, 9.81 m/s 2 ;
H - fall height, 1 m.
We get that the average speed of the drop during free fall
The duration of the fall of a drop can be calculated by the formula
Then the volume of the drop Vk is calculated by the formula
Drop mass m k, kg:
where is the density of the metal in the molten state, kg * m -3.
The density of copper in the molten state (according to the teacher) is 8.6 * 10 3 kg / m 3, and the mass of the drop according to the formula (22)
m k \u003d 8.6 * 10 3 * 10.3138 * 10 -9 \u003d 8.867 * 10 -5
Flight time of a metal drop in a molten (liquid) state p, s:
where C p is the specific heat capacity of the drop material melt, for copper C p = 513 J * kg -1 * K -1;
S k - surface area of the drop, m 2 , S k =0.785d k 2 =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 =2373 K, T pl =1083 K;
T o - ambient temperature, T o =293 K;
Heat transfer coefficient, W * m -2 * K -1.
The heat transfer coefficient is calculated in the following sequence:
1) first calculate the Reynolds number
where v \u003d 1.51 * 10 -5 1 / (m 2 * s) - coefficient of kinematic viscosity of air at a temperature of 293 K,
where \u003d 2.2 * 10 -2 W * m -1 * K -1 - coefficient of thermal conductivity of air,
1*10 2 W*m -2 *K -1 .
Having calculated the heat transfer coefficient, we find the flight time of a metal drop in a molten (liquid) state according to the formula (23)
Because< р, то конечную температуру капли определяют по формуле
The self-ignition temperature of propane is 466 ° C, and the temperature of the drop (spark) by the time it approaches the flammable liquid pool is 2373 K or 2100 ° C. At this temperature, isoprene will ignite and burn steadily, and propane will explode even if a short circuit spark occurs. The flash point of isoprene is -48 0 С.
Question 1: Classification of ignition sources;IGNITION SOURCE - a source of energy that initiates ignition. Must have sufficient energy, temperature and duration of exposure.
As noted earlier, combustion can occur when a variety of ignition sources influence the HS. According to the nature of origin, ignition sources can be classified:
open fire, hot products of combustion and surfaces heated by them;
thermal manifestations of mechanical energy;
thermal manifestations of electrical energy;
thermal manifestations of chemical reactions (open fire and combustion products are separated from this group into an independent group).
Open fire, hot products of combustion and surfaces heated by them
For production purposes, fire, fire furnaces, reactors, 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, 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, are sufficient to ignite almost all combustible substances.
Open flame. The fire hazard of a flame is determined by the temperature of the torch and the time of its influence on combustible substances. For example, ignition is possible from such “low-calorie” IS as a smoldering butt of a cigarette or cigarette, a lit match (Table 1).
Open flame sources - torches - are often used to heat a frozen product, to illuminate when inspecting apparatus in the dark, for example, when measuring the level of liquids, when making a fire on the territory of objects with the presence of flammable and combustible liquids.
Highly heated combustion products - gaseous combustion products that are obtained during the combustion of solid, liquid and gaseous substances and can reach temperatures of 800-1200 ° C. The fire hazard is the exit of highly heated products through leaks in the masonry of furnaces and smoke channels.
Industrial sources of ignition are also sparks that occur during the operation of furnaces and engines. They are solid incandescent particles of fuel or scale in a gas stream, which are obtained as a result of incomplete combustion or mechanical removal of combustible substances and corrosion products. The temperature of such a solid particle is quite high, but the thermal energy (W) is small due to the small mass of the spark. A spark is capable of igniting only substances that are sufficiently prepared for combustion (gas-vapor-air mixtures, settled dust, fibrous materials).
Fireboxes “sparkle” due to design flaws; due to the use of a type of fuel for which the furnace is not designed; due to increased blast; due to incomplete combustion of fuel; due to insufficient atomization of liquid fuel, as well as due to non-observance of the deadlines for cleaning furnaces.
Sparks and soot during the operation of the internal combustion engine are formed when the fuel supply system, electric ignition are improperly regulated; when fuel is contaminated with lubricating oils and mineral impurities; during prolonged operation of the engine with overloads; in case of violation of the terms of cleaning the exhaust system from carbon deposits.
The fire hazard of sparks from boiler rooms, pipes of steam locomotives and diesel locomotives, as well as other machines, a fire is largely determined by their size and temperature. It has been established that a spark d = 2 mm is flammable if it has t » 1000°С; d=3 mm - 800°C; d = 5 mm - 600°C.
Dangerous thermal manifestations of mechanical energy
Under production conditions, a fire hazardous increase in body temperature as a result of the conversion of mechanical energy into thermal energy is observed:
at impacts of solid bodies (with or without the formation of sparks);
with surface friction of bodies during their mutual movement;
when machining hard materials with a cutting tool;
when compressing gases and pressing plastics.
The degree of heating of bodies and the possibility of the appearance of an ignition source in this case depends on the conditions for the transition of mechanical energy into thermal energy.
Sparks, which are obtained from the impact of solid bodies.
The size of impact and friction sparks, which are a piece of metal or stone heated to a glow, usually does not exceed 0.5 mm. The spark temperature of unalloyed low-carbon steels can reach the melting point of the metal (about 1550°C).
Under production 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 combustibility of the mixture. The spark that flies does not directly ignite the dust-air mixture, but, falling on the settled dust or on fibrous materials, it will cause the appearance of smoldering foci. So in flour-grinding, weaving and cotton-spinning enterprises, about 50% of all fires arise from sparks that are cut out when solid bodies strike.
Sparks, which are obtained when aluminum bodies strike an oxidized steel surface, lead to a chemical attack with the release of a significant amount of heat.
Sparks generated when metal or stones hit machines.
In devices with mixers, crushers, mixers and others, if pieces of metal or stones get along with the processed products, sparks can form. Sparks are also formed when the moving mechanisms of machines hit their fixed 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 the ginning and scutching machines, which rotate rapidly and hit the fixed steel gratings. In such cases, sparking is observed. It is also possible with improper adjustment of the gaps, with deformation and vibration of the shafts, wear of the bearings, distortions, insufficient fastening of the cutting tool on the shafts. In such cases, not only sparking is possible, but also the breakdown of individual parts of the machines. Breakage of the machine assembly, in turn, can be the cause of the formation of sparks, as metal particles enter the product.
Ignition of a combustible medium from overheating during 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 mostly converted into heat. In the normal state and proper operation of the parts that rub, the heat that is released in a timely manner is removed by a special cooling system, and is also dissipated in the environment. An increase in heat release or a decrease in heat removal and heat loss leads to an increase in the temperature of rubbing bodies. For this reason, combustible media or materials ignite due to overheating of machine bearings, tightly tightened seals, drums and conveyor belts, pulleys and drive belts, fibrous materials when they are wound on the shafts of machines and apparatus that rotate.
In this regard, the most fire hazardous are the plain bearings of heavily loaded and high-speed shafts. Poor lubrication of working surfaces, their contamination, misaligned shafts, overloading of machines and excessive tightening of bearings can all cause overloading. Very often, the bearing housing becomes contaminated with combustible dust deposits. This also creates the conditions for their overheating.
At facilities where fibrous materials are used or processed, they catch fire when wound on rotating units (spinning mills, flax mills, combine harvesters). Fibrous materials and straw products are wound onto shafts near the bearings. Winding is accompanied by a gradual compaction of the mass, and then its strong heating during friction, charring and ignition.
Release of heat during compression of gases.
A significant amount of heat is released during the compression of gases as a result of intermolecular motion. A malfunction or lack of a compressor cooling system can lead to their destruction in the event of an explosion.
Dangerous thermal manifestations of chemical reactions
In the conditions of production and storage of chemicals, a large number of such 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 proceed with the release of a significant amount of heat have a potential fire or explosion hazard, since an uncontrolled heating process of reacting, newly formed or nearby combustible substances is possible.
2) Substances that spontaneously ignite and ignite spontaneously on contact with air.
3) Often, according to the conditions of the technological process, the substances in the apparatus can be heated to a temperature exceeding the temperature of their spontaneous combustion. Thus, the products of gas pyrolysis during the production of ethylene from petroleum products have a self-ignition temperature in the range of 530 - 550 ° C, and exit the pyrolysis furnaces at a temperature of 850 ° C. Fuel oil with a self-ignition temperature of 380 - 420 ° C is heated up to 500 ° C at thermal cracking units; Butane and butylene, which have a self-ignition temperature of 420°C and 439°C, respectively, heat up to 550 - 650°C when receiving butadiene, etc. When these substances go outside, they self-ignite.
4) Sometimes substances in technological processes have a very low auto-ignition temperature:
Triethylaluminum - Al (C2H5) 3 (-68 ° C);
Diethylaluminum chloride - Al (C2H5) 2Cl (-60°C);
Triisobutylaluminum (-40°C);
Hydrogen fluoride, liquid and white phosphorus - below room temperature.
5) Many substances in contact with air are capable of spontaneous combustion. Spontaneous combustion begins at ambient temperature or after some preliminary heating. Such substances include vegetable oils and fats, iron sulphides, some types of soot, powdered substances (aluminum, zinc, titanium, magnesium, etc.), hay, grain in silos, etc.
The contact of self-igniting chemicals with air usually occurs when containers are damaged, liquid spills, packaging of substances, during drying, open storage of solid crushed, as well as fibrous materials, when pumping liquids from tanks, when there are self-igniting deposits inside the tanks.
Substances that ignite on contact with water.
At industrial facilities, there is a significant amount of substances that ignite when interacting with water. The heat released in this case can cause ignition of combustible substances formed or adjacent to the reaction zone. Substances that ignite or cause combustion when in contact with water include alkali metals, calcium carbide, alkali metal carbides, sodium sulfide, etc. Many of these substances, when interacting with water, form combustible 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 numbers, explosions occur with a splash of molten metal. In the dispersed state, alkali metals ignite in moist air.
Some substances, such as quicklime, are non-combustible, but their heat of reaction with water can heat combustible materials that are nearby to the point of autoignition. So, when water comes into contact with quicklime, the temperature in the reaction zone can reach 600 ° C:
Ca + H2O \u003d Ca (BOH) 2 + Q.
There are known cases of fires in poultry houses, where hay was used as bedding. Fires occurred after the treatment of poultry premises 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 begun can occur when trying to extinguish such substances with water or foam.
The ignition of chemical substances during mutual contact occurs under 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 oxidizing agents and combustible substances are capable of igniting when exposed to sulfuric or nitric acid or a small amount of moisture.
The reaction of the interaction of an oxidizer with a combustible substance is facilitated by the fineness of substances, its increased initial temperature, as well as the presence of chemical process initiators. In some cases, the reactions are in the nature of an explosion.
Substances that ignite or explode when heated or mechanically acted upon.
Some chemicals are unstable in nature, capable of decomposing over time under the influence of temperature, friction, impact 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 small amount of heat. These include saltpeters, peroxides, hydroperoxides, carbides of certain metals, acetylenides, acetylene, etc.
Violations of the technological regulations, the use or storage of such substances, the influence of a heat source on them can lead to their explosive decomposition.
Acetylene has a tendency 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 case of non-compliance with the rules for the operation of this electrical equipment, a fire and explosion hazardous situation may occur in production. Fire and explosion hazardous situations arise in technological processes of production during short circuit, in case of breakdowns of the insulation layer, in case of excessive overheating of electric motors, in case of damage to certain sections of electrical networks, in case of spark discharges of static and atmospheric electricity, etc.
The types of atmospheric electricity include:
Direct lightning strikes. The danger of a direct lightning strike is in the contact of the HS with the lightning channel, the temperature in which reaches 2000 ° C with an action time of about 100 μs. All combustible mixtures ignite from a direct lightning strike.
Secondary manifestations of lightning. The danger of a secondary manifestation of lightning is spark discharges that occur as a result of the inductive and electromagnetic influence of atmospheric electricity on industrial equipment, pipelines and building structures. The spark discharge energy exceeds 250 mJ and is sufficient to ignite combustible substances from Wmin = 0.25 J.
High potential skid. The high potential is brought into the 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 and more. That is, it is sufficient to ignite almost all combustible substances.
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 can be from a combustible medium.
Electrical sparks (drops of metal). Electrical sparks are produced during short circuits in electrical wiring, electric welding, and during the melting of the electrodes of general-purpose incandescent lamps.
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 temperature of the arc during electric welding reaches 4000 ° C, so the arc will be a source of ignition for all combustible substances.
Electric incandescent lamps. The fire hazard of lamps is due to the possibility of contact of the HS with the bulb of an electric incandescent lamp heated above the self-ignition temperature of the HS. The heating temperature of the bulb of an electric light bulb depends on its power, size and location in space.
Sparks of static electricity. Discharges of static electricity can be generated 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 combustible substances with ignition sources is possible, it is necessary to know exactly their nature in order to exclude the impact on the environment.
Question 2: Preventive measures that exclude the impact of ignition sources on a combustible environment .;
Fire-fighting measures that exclude the contact of a combustible medium (HS) with an open flame and hot combustion products.
To ensure the 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 HS.
As noted earlier, not every heated body can be a source of ignition, but only those heated bodies that are able to heat a certain volume of combustible mixture to a certain temperature, when the heat release rate equals or exceeds the heat removal rate 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 the flame front are maintained for a certain time. Therefore, knowing these conditions (conditions for the formation of IS), 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 the contact of the HS with ignition sources.
The main engineering and technical solution that excludes the contact of a combustible 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 devices of “fire” action (tube furnaces, reactors, torches), it is necessary to provide for the isolation of these installations from a possible collision of combustible vapors and gases with them. This is achieved:
placement of installations in enclosed spaces, isolated from other devices;
placement on open areas between the "firing" devices and fire hazardous installations of protective barriers. For example, the placement of closed structures that act as a barrier.
compliance with fireproof regulated gaps between devices;
the use of steam curtains in cases where it is impossible to provide a fireproof distance;
ensuring the safe design of flare burners with continuous combustion devices, the diagram of which is shown in fig. one.
Figure 1 - Torch for burning gases: 1 - steam supply line; 2 - ignition line of the next burner; 3 - gas supply line to the next burner; 4 - burner; 5 - torch barrel; 6 - flame 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 pre-prepared combustible mixture is ignited by an electric igniter and the flame, moving upwards, ignites the burner gas). To reduce the formation of smoke and sparks, water vapor is supplied to the flare burner.
with the exception of the formation of “low-calorie” IZ (smoking is allowed at the facilities only in specially equipped places).
using hot water or steam to warm up frozen sections of process equipment instead of torches (equipment of open car parks with hot air supply systems) or induction heaters.
cleaning pipelines and ventilation systems from combustible deposits with a fireproof agent (steaming and mechanical cleaning). In exceptional cases, it is allowed to burn waste after the dismantling of pipelines in specially designated areas and permanent sites for hot work.
control over the condition of the laying of smoke channels during the operation of furnaces and internal combustion engines, to prevent leaks and burnouts in the exhaust pipes.
protection of highly heated surfaces of process equipment (chambers of returbents) with thermal insulation with protective casings. The maximum allowable surface temperature should not exceed 80% of the auto-ignition temperature of combustible substances that are used in production.
warning of the dangerous manifestation of sparks from furnaces and engines. In practice, this direction of protection is achieved by preventing the formation of sparks and using special devices for trapping and extinguishing them. To prevent the formation of sparks, the following are provided: automatic maintenance of the optimum temperature of the combustible mixture supplied for combustion; automatic regulation of the optimal ratio between fuel and air in the combustible mixture; prevention of continuous operation of furnaces and engines in forced mode, with overload; the use of those types of fuel for which the furnace and engine are designed; systematic cleaning of the internal surfaces of furnaces, smoke channels from soot and exhaust manifolds of engines from carbon-oil deposits, etc.
To trap 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 (sedimentary chambers), inertial (chambers with partitions, grids, nozzles), centrifugal forces (cyclone and turbine-vortex chambers). ).
The most widespread in practice are spark arresters of gravitational, inertial and centrifugal types. They are equipped, for example, with smoke channels of flue gas dryers, exhaust systems for cars and tractors.
To ensure deep cleaning 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 trapping and extinguishing have proven themselves reliably, for example, in technological processes for drying crushed combustible materials, where flue gases mixed with air are used as a heat carrier.
Fire-fighting measures that exclude dangerous thermal manifestations of mechanical energy
Preventing the formation of ignition sources from the dangerous thermal effects of mechanical energy is an urgent task at explosive and fire hazardous objects, as well as at objects 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 tool. In places of possible formation of explosive mixtures of vapors or gases, it is necessary to use an explosion-proof tool. Tools made of bronze, phosphor bronze, brass, beryllium, etc. are considered spark-proof.
Example: 1. Intrinsically safe railroad brake shoes. tanks.2. Brass tool for opening calcium carbide drums in acetylene stations.
Application of magnetic, gravitational or inertial traps. So, to clean raw cotton from stones before it enters the machines, gravitational or inertial stone traps 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 entering the machine, firstly, a thorough sorting of raw materials is carried out, and secondly, the inner surface of the machines, on which these impurities can hit, is lined with soft metal, rubber or plastic.
Prevention of impacts of moving mechanisms of machines on their fixed parts. The main fire prevention measures aimed at preventing the formation of impact and friction sparks are reduced to careful regulation and balancing of the shafts, the correct selection of bearings, checking the size of the gaps between the moving and stationary parts of the machines, their reliable fastening, which eliminates the possibility of longitudinal movements; prevent overloading machines.
Execution in explosion-hazardous rooms of floors that do not spark. Increased requirements for intrinsic 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 form sparks, or are lined with rubber mats, paths, etc.
Prevention of ignition of substances in places of intense heat release during friction. To this end, to prevent overheating of the bearings, plain bearings are replaced with rolling bearings (where such a possibility exists). In other cases, automatic control of the temperature of their heating is carried out. Visual temperature control is carried out by applying heat-sensitive paints that 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, surface cleaning from contamination).
In order to avoid overheating and fires of conveyor belts and drive belts, work with overload must not be allowed; it is necessary to control the degree of tension of the tape, belt, their condition. Blockages of elevator shoes with products, distortions of belts and their friction against casings should not be allowed. When using powerful high-performance conveyors and elevators, devices and devices can be used that automatically signal overload operation and stop the movement of the belt when the elevator shoe collapses.
To prevent the winding of fibrous materials on the rotating shafts of machines, it is necessary to protect them from direct collision with the materials being processed by using bushings, cylindrical and conical casings, conductors, guide bars, anti-winding shields, etc. In addition, the minimum clearance between the shaft pins and bearings is set; there is a systematic monitoring of the shafts, where there may be windings, their timely cleaning of fibers, their protection with special anti-winding sharp knives that cut the fiber that is being wound. Such protection is provided, for example, by scutching machines at flax mills.
Prevention of overheating of compressors when compressing gases.
Compressor overheating is prevented by dividing the gas compression process into several stages; arrangement of gas cooling systems at each compression stage; installation of a safety valve on the discharge line behind 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; an automatic blocking system that 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 the internal surfaces of pipelines from carbon-oil deposits.
Preventing the formation of ignition sources during thermal manifestations of chemical reactions
To prevent the ignition of combustible 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 an 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 prevention measures that exclude their occurrence. Therefore, the main fire prevention measures that prevent dangerous thermal manifestations of chemical reactions are:
Reliable tightness of devices, which excludes the contact of substances heated above the self-ignition temperature, as well as substances with a low spontaneous ignition temperature, with air;
Prevention of spontaneous combustion of substances by reducing the rate of chemical reactions and biological processes, as well as eliminating the conditions for heat accumulation;
Reducing the rate of chemical reactions and biological processes is carried out by a variety of methods: limiting humidity during storage of substances and materials; lowering the temperature of storage of substances and materials (for example, grain, animal feed) by artificial cooling; storage of substances in an environment with low oxygen content; reduction of the specific contact surface of self-igniting substances with air (briquetting, granulation of powdered substances); the use of antioxidants and preservatives (storage of animal feed); elimination of contact with air and chemically active substances (peroxide compounds, acids, alkalis, etc.) by separate storage of 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 of their storage.
The elimination of heat accumulation conditions is carried out in the following way:
limiting the size of stacks, caravans or heaps of the stored substance;
active ventilation of air (hay and other fibrous plant materials);
periodic mixing of substances during their long-term storage;
reducing the intensity of the formation of combustible deposits in process equipment with the help of trapping devices;
periodic cleaning of process equipment from self-igniting combustible deposits;
Prevention of ignition of substances in contact with each other. Fires from the ignition of substances in contact with each other are prevented by separate storage, as well as by eliminating the causes of their emergency exit from apparatuses and pipelines.
Elimination of ignition of substances as a result of self-decomposition upon heating or mechanical action. Prevention of ignition of substances prone to explosive decomposition is provided by protection from heating to critical temperatures, mechanical influences (shocks, friction, pressure, etc.).
Prevention of ignition sources from thermal manifestations of electrical energy
Prevention of dangerous thermal manifestations of electrical energy is ensured by:
the 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 class of fire or explosion hazard of the zone, category and group of explosive mixture;
periodic testing of the insulation resistance of electrical networks and electrical machines in accordance with the schedule of preventive maintenance;
protection of electrical equipment against short-circuit currents (SC) (use of high-speed fuses or circuit breakers);
prevention of technological overload of machines and devices;
prevention of large transient resistances through a systematic review and repair of the contact part of electrical equipment;
exclusion of static electricity discharges by grounding technological equipment, increasing air humidity or using antistatic impurities in the most likely places for generating charges, 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 by lightning rods and protection from its secondary effects.
Fire prevention measures in enterprises should not be neglected. Since any savings on fire protection will be disproportionately small in comparison with the losses from a fire that arose 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 the exclusion of the ignition source.
