Calculation of the average daily concentration of dust in the air. Methods for determining dust concentration in the air of a working area

Name the types of artificial grounding conductors.

Extension and contour + horizontal and vertical (conditional)

20. How can you reduce the resistance of the ground electrode?

The total grounding resistance depends, as mentioned above, on the resistance of the soil layers adjacent to the ground electrode. Therefore, it is possible to reduce the grounding resistance by lowering the soil resistivity only in a small area around the ground electrode.

An artificial reduction in soil resistivity is achieved either chemically using electrolytes, or by laying grounding conductors in pits with bulk coal, coke, or clay.

Dustiness

1, What is called dust?

Dust refers to crushed particles solid, capable of being suspended in the air for some time.

2. What is the hygienic hazard of dust?
Dust is a hygienic hazard, as it negatively affects the human body. Under the influence of dust, diseases such as pneumoconiosis, eczema, dermatitis, conjunctivitis, etc. can occur. The finer the dust, the more dangerous it is for humans. The most dangerous particles for humans are considered to be particles ranging in size from 0.2 to 7 microns, which, when entering the lungs during breathing, are retained in them and, accumulating, can cause illness.

There are three ways dust can enter the human body: through the respiratory system, gastrointestinal tract and skin.

3, what is maximum permissible concentration harmful substance?

Maximum permissible concentration (MPC) is a sanitary and hygienic standard approved by law. MPC is understood as such a concentration of chemical elements and their compounds in the environment, which, with everyday influence over a long period of time on the human body, does not cause pathological changes or diseases established modern methods research at any time in the life of the present and subsequent generations.

The essence of the gravimetric method for determining dust concentration.

The essence of the method is that a certain volume of dust-laden air is passed through a highly efficient filter and the mass concentration of dust is calculated by increasing the mass and volume of filtered air:

5. How is dust count concentration measured?

Its essence lies in the preliminary separation of dust from the air and its deposition on glass slides, followed by counting the number of particles using a microscope. By dividing the calculated number of particles by the volume of air from which they are deposited, we obtain the countable concentration of dust (particles/l):

6. How is the volume of air sucked through the filter measured using the gravimetric method of measuring dust concentration?

V0 – volume of filtered air reduced to normal conditions (temperature 0 °C and barometric pressure B0 = 760 mm Hg), m3.

where P0, P – barometric pressure, Pa, respectively, under normal and operating conditions (P0 = 101325 Pa, P = B×133.322 Pa); T – air temperature at the place of dust collection, °C; V – volume of air passed through the filter at temperature T and pressure B, m3,

Where w– volumetric speed of air suction through the filter, l/min;
t– duration of sampling, min.

7. What sanitary and technical measures can reduce dust concentrations in workplaces to the MPC level?

7.4. To reduce dust levels and create acceptable microclimate parameters in vehicle cabins, it is necessary to seal doors and windows and use installations for cleaning, heating or cooling air.

7.5. The use of machines with internal combustion engines without effective means of neutralization and purification of exhaust gases is not allowed. Neutralizers and cleaning agents must ensure the content of harmful substances in the air working area at levels not exceeding the maximum permissible concentration. The use of leaded gasoline is prohibited.

7.6. The traffic schedule of vehicles should not allow their accumulation with running engines on work sites, ledges, or sections of the road. The minimum distance between heavy-duty dump trucks (10 tons and above) must be at least 30 m. When organizing loading operations, preference should be given to a loop pattern for vehicle access to the loading site.

7.7. The rock mass loaded into the body of a dump truck, wagon or onto a conveyor belt must be irrigated during the warm season. The irrigation torch must cover the loading area.

7.8. To improve air exchange in the sections, guides and protective aerodynamic devices should be provided to regulate natural air flows.

7.9. During long-term inversions and calms, in the event of accumulation of harmful gases at workplaces in stagnant zones of open-pit mines more than 100 m deep, artificial ventilation must be provided using special devices.

7.10. When designing, manufacturing or importing mining, transport and other machines, one should take into account their possible use in various climatic-geographical regions and mountain-geological zones of the country (presence of: polar day and night, permafrost, specific rocks, strong winds, calms, temperature inversions, wide temperature range outside air from + 40 °C to - 60 °C, prolonged fogs), as well as the content of toxic substances in exhaust gases, which must comply with domestic standards.

There are many industry documents describing indoor dust conditions. These are SNIPs, GOSTs, and they consider it from their own professional points of view. But nowhere in them are there numbers limiting the dust content in domestic and office premises. This is primarily due to the fact that in the decoration of premises of these categories the most different materials. Namely, from the applied finishing materials, materials used in the equipment of premises and the design of premises (ventilation and air conditioning). And having established dust standards for household and office premises designers risk not meeting them.

In 2004, the broadest document defining standards for dust content in the air was put into effect. This is the "Interstate standard GOST ISO 14644 -1-2002, Clean rooms and associated controlled environments, Part 1, Classification of air purity."

This is such a long and uncomplicated name. For us, in in this case interesting table. 1. from section 3.

Previously, there was GOST R 50776-95, which is characterized by normalization of the content of microorganisms (see Table 1, column highlighted in pink), and the values ​​of the amount of dust are not rounded.

Considering that we need guidelines for dust concentration, the data from these two GOSTs are summarized in one table.

Table 1, cleanliness classes for airborne particles for clean rooms and clean areas

ISO Class N (N - classification number)	Maximum permissible concentration of particles, particles/m 3, with sizes equal to or greater following values, µm						MK
	0,1	0,2	0,3	0,5	1,0	5,0
ISO Class 1	10	2	nd	nd	nd	nd	nd
ISO Class 2	100	24	10	4	nd	nd	nd
ISO Class 3	1000	237	102	35	8	nd	nd
ISO Class 4	10000	2370	1020	352	83	nd	nd
ISO Class 5	100000	23700	10200	3520	832	29	5
ISO Class 6	1000000	237000	102000	35200	8320	293	50
ISO Class 7	NK	NK	NK	352000	83200	2930	100
ISO Class 8	NK	NK	NK	3520000	832000	29300	500
ISO Class 9	NK	NK	NK	35200000	8320000	293000	NK
Due to the uncertainties involved in particle counting, concentration values ​​of no more than three significant figures should be used for classification.

nc - countable concentration of particles of a given size for a given class is not controlled,

nd - particles of a given and bigger size there shouldn't be any in the air

MK - maximum permissible number of microorganisms, pcs/m 3

I have not yet found data related to the category of air cleanliness in domestic and office premises. Although I came across standards for clean rooms in medical institutions.

And knowing about the strict regulation of dust content in the air of clean industrial premises having a category, we can conclude that classes (categories) 7, 8, 9 are closest to office (7, 8) and household (9) premises.

Conclusion

Although GOST defines the category “for clean rooms and clean areas,” we are interested in ISO class 9, as (in my opinion) the closest to domestic premises, and ISO classes 7 and 8 for office premises equipped with air conditioning and air filtration, respectively.

The figures given can only be used as guidelines when making estimates for electronic and air filters. computer technology and its operating regulations.

For accurate calculations, you should use the values ​​of dust levels specified in the passports of the premises where the equipment is located.

For your information The amount of dust in the atmospheric air can vary greatly. In areas with continuous green areas, above lakes and rivers, the amount of dust in the air is less than 1 mg/m3, in industrial cities - 3-10 mg/m3, in cities with poorly equipped streets - up to 20 mg/m3. Particle sizes range from 0.02 to 100 microns. Sanitary standards of the USSR-(SN 245-71) limit the average daily maximum permissible concentration of non-toxic dust in the atmospheric air of populated areas 0.15 mg/m3, however, in reality the dust concentration is often higher, so it is better to proceed from experimental data on the degree of air pollution in a particular area. The concentration of suspended substances in the atmospheric air of Novosibirsk exceeds the Maximum Permissible Concentrations. If the maximum permissible concentration is 0.15 mg/m³, then in 2004 it was 0.26 mg/m³, in 2005 – 0.21 mg/m³, and in 2006 – 0.24 mg/m³. In the center of the capital of Estonia, Tallinn, a fine dust concentration of up to 0.07 mg/m 3 was recorded. In England, the air in cities where residential areas with fireplace heating are combined with large industrial enterprises is characterized by a dust content of up to 0.5 mg/m3, In the USA, the concentration of dust in the air reached 1.044 mg/m3, In Germany, the highest concentration of dust was observed in the cities of the Ruhr - up to 0.7 mg/m 3. The main danger to the human body is precisely particles ranging in size from tenths of a micrometer to 10 and especially up to 5 microns. The structure of dust in domestic premises and offices differs from atmospheric dust and dust in industrial premises and significantly depends on their decoration and the equipment and furniture placed in the room.

Prepared by A. Sorokin,

Production premises

Goal of the work: determination of dust concentration in the air by weight method and sanitary assessment of dust in the production environment.

Basic concepts and definitions

Dust called a dispersed system consisting of tiny solid particles suspended in a gas environment (aerosol) or settled (aerogel).

Dust is divided into atmospheric and industrial. Sources of industrial dust formation are technological processes and production equipment associated with grinding (crushing, grinding, cutting) and surface processing of materials (grinding, polishing, napping, etc.), transportation, movement and packaging of crushed materials, etc. Atmospheric dust includes industrial (pollution atmospheric air emissions from industrial enterprises) and natural, occurring during weathering of rocks, volcanic eruptions, fires, wind erosion of arable land, dust of cosmic and biological origin (pollen, spores, microorganisms). TO industrial enterprises emitting dust particles into the atmosphere include enterprises of ferrous metallurgy, heat power engineering, chemical, oil refining, industrial building materials and etc.

Hygienic standards GN 2.2.5.686–98 “Maximum permissible concentrations (MPC) of harmful substances in the air of the working area” and GOST 12.1.005–88 “SSBT. General sanitary and hygienic requirements for the air of the working area" established maximum permissible concentrations for more than 800 various substances(in mg/m3). MPC of harmful substances in the air of a working area is considered to be such a concentration that, during daily work for 8 hours or another duration, but not more than 41 hours per week, during the entire working period, cannot cause diseases or health problems detected by modern research methods in in the process of work or in the long term of the life of the present and subsequent generations. In adj. Table 1 shows the maximum permissible concentrations of substances in the air of the working area.

Dust is classified according to the following criteria: the type of substance from which the particles are composed, the degree of dispersion (grinding), the degree of harmful effects on the human body, explosion and fire hazard.

Based on their origin, dust is divided into three main subgroups:

1. Organic:

Natural (plant origin - wood, cotton, and animal origin - bone, wool);

Artificial (dust from plastics, rubber, resins, dyes and other synthetic substances).

2. Inorganic:

Metal (steel, copper, lead);

Mineral (sand, lime, cement).

3. Mixed.

Based on dispersion, dust is divided into three groups:

1) visible (particle sizes greater than 10 microns);

2) microscopic (0.25-10 microns);

3) ultramicroscopic (less than 0.25 microns).

The danger of dust increases as the size of the dust particles decreases, since such dust remains as an aerosol in the air longer and penetrates deeper into the pulmonary canals.

The harmful effects of dust on the human body depends on the degree of dust in the air, characterized by concentration (mg/m3), and various properties dust: chemical composition, solubility, dispersity, particle shape and adsorption capacity. Based on their effect on the body, dust is divided into toxic and non-toxic.

Dust enters the human body in three ways: through the respiratory system, gastrointestinal tract and skin.

Depending on the composition, dust can have the following effects on the body:

1. Fibrogenic effect - connective tissue grows in the lungs, disrupting the normal structure and functions of the organ (quartz, rock).

2. Irritating effect on the upper respiratory tract, mucous membrane of the eyes, skin (calcareous, fiberglass).

3. Toxic effect - toxic dusts, dissolving in the biological media of the body, cause poisoning (lead, arsenic).

4. Allergic effect (wool, synthetic).

5. Biological action (microorganisms, spores).

6. Carcinogenic effect (soot, asbestos).

7. Ionizing effect (uranium, radium dust).

Dust particles ranging in size from 0.1 to 10 microns penetrate deeply into the lungs. Smaller ones are exhaled back, and larger ones settle on the mucous membranes of the nasal cavity, pharynx, trachea and are expelled with mucus when coughing and sneezing. Some of the dust is retained in the nose and nasopharynx and, together with saliva and mucus, enters the digestive organs. When inhaled, smaller, non-settled dust particles penetrate into the deep respiratory tract, right up to the lung tissue. Particles not exceeding 7 microns are retained in the lungs. When dust enters the respiratory tract, it can cause occupational diseases - pneumoconiosis (restriction of the respiratory surface of the lungs and changes in the entire human body), chronic bronchitis, diseases of the upper respiratory tract. The chemical composition of dust determines the nature of certain occupational diseases. For example, when inhaling coal dust, a type of pneumoconiosis occurs - anthracosis, aluminum altinosis, free silicon dioxide SiO 2 - silicosis, etc.

When dust gets on the skin, it penetrates the sebaceous and sweat glands and disrupts the body's thermoregulation system. Non-toxic dust has an irritating effect on the skin, eyes, ears, gums (roughness, peeling, acne, asbestos warts, eczema, dermatitis, conjunctivitis, etc.).

The solubility of dust depends on its composition and specific surface area (m 2 /kg), since the chemical activity of dust in relation to the body depends on total area surfaces. Sugar, flour and other types of dust, quickly dissolving in the body, are eliminated without causing much harm. Dust insoluble in the body (plant, organic, etc.) lingers in the airways for a long time, leading in some cases to the development of pathological abnormalities.

The shape of dust particles affects the stability of the aerosol in the air and its behavior in the body. Spherical particles fall out of the air faster and penetrate lung tissue more easily. The most dangerous dust particles are those with a jagged, prickly surface, as they can cause injuries to the eyes, lung tissue and skin.

The adsorption properties of dust depend on the dispersion and total surface area. Dust can be a carrier of microbes, fungi, and mites.

Dusts can also acquire electric charge due to the adsorption of ions from the air and as a result of friction of particles in the dust flow, which increases their harmful effects. Non-metallic dust is charged positively, and metallic dust is negatively charged. Oppositely charged particles are attracted to each other and settle out of the air. With the same charge, dust particles, repelling one another, can hover in the air for a long time. Charged particles stay in the lungs longer than neutral ones, thereby increasing the danger to the body.

A negative property of many types of dust is their ability to ignite and explode. Depending on the value of the lower flammability limit, dusts are divided into explosive and fire hazardous. Explosive dusts include dusts with a lower flammability limit of up to 65 g/m 3 (sulfur, sugar, flour), and flammable dusts include dusts with a lower flammability limit above 65 g/m 3 (tobacco, wood, etc.).

To protect against dust in production, a complex of sanitary-hygienic, technical, organizational and medical-biological measures is used. By effective means protection are: the introduction of comprehensive mechanization and automation of production operations with automatic or remote control and control, sealing of equipment, instruments and communications, placement of dangerous components and devices outside work areas, replacement of dry methods of processing dust-producing materials with wet ones, use of local suction from equipment and equipment, automatic blocking of starting devices of technological and sanitary-hygienic equipment, hydrodust removal. These products are general methods of protecting workers and equipment from dust. Respirators, gas masks, pneumatic helmets, pneumatic masks, impenetrable dust-proof clothing, safety glasses, etc. are used as personal protective equipment against dust. Time protection, ultraviolet irradiation in fotariums, alkaline inhalations, medical examinations, personal hygiene, and the use of special nutrition also play an important role.

The air of the working area (a space up to 2 m high above the floor or platform where permanent and temporary residences of workers are located) is cleaned in the following ways: when breaking materials dry, catchers of dust suspended in the air are installed, pneumatic transportation of the resulting product is used, and suction is provided ( aspiration) dust from under shelters in places of its formation. The vacuum created during aspiration in the shelter connected to the air duct exhaust ventilation, does not allow contaminated air to enter the air of the working area. Exhausts from equipment and apparatus are performed interlocked with the starting device of the main equipment. Before being released into the atmosphere or work area, dusty air is pre-cleaned.

An important indicator operation of dust removal equipment is the degree of air purification:

Where m 1 And m 2– dust content in the air before and after cleaning, respectively, mg/m3; V 1 And V 2– air volume before and after cleaning, respectively, m3.

Air purification from dust can be coarse (coarse dust is retained - particle sizes greater than 100 microns), medium (dust with a particle size of less than 100 microns is retained, and its final content should not be more than 100 mg/m3) and fine (fine dust is retained (up to 10 microns) with the final content in the air of supply and recirculation systems up to 1 mg/m 3). Dust removal equipment is divided into dust collectors And filters. Dust collectors include dust sedimentation chambers, single and battery cyclones, inertial and rotary dust collectors. Depending on the principle of operation, filters are classified into electric, ultrasonic, oil, fabric, bag, etc. (see Fig. 2.1–2.3).

A	B

Rice. 2.1. Dust collection chambers:

A– simple; b– labyrinthine

Rice. 2.2. Cyclone diagram:

1 – inlet pipe; 2 – bottom of the conical part; 3 – centrifugal pipe

Rice. 2.3. Electric ( A) and ultrasonic ( b) filters:

1 – insulator; 2 – filter wall; 3 – corona electrode; 4 – grounding;

5 – ultrasound generator; 6 – cyclone

To determine the air quality in the workplace, there are control methods that are divided into two groups: the first - with the separation of the dispersed phase from the aerosol (weight and counting methods), the second - without the separation of the dispersed phase from the aerosol (photoelectric, electrometric, radiation and optical methods) . The most commonly used methods are weight and counting methods. Typically, in the practice of inspection control, preference is given to the weight method.

Weight method

The weight method is the most hygienically sound method for assessing dust content in the air of a working area. It is the basis current system occupational safety standards (OSS) as standard. The essence of the method is that a certain volume of dusty air is passed through a highly efficient filter and the mass concentration of dust is calculated by increasing the mass and volume of filtered air:

Where With– mass concentration of dust, mg/m3; G n– mass of dust deposited on the filter, mg; V 0– volume of filtered air reduced to normal conditions (temperature 0 o C and barometric pressure B 0= 760 mm Hg. Art.), m 3.

, (2.2)

Where P0, P– barometric pressure, Pa, under normal and operating conditions, respectively ( P0= 101325 Pa, P= B×133.322 Pa); T– air temperature at the place of dust collection, o C; V– volume of air passed through the filter at temperature T and pressure IN, m 3 ,

Where w– volumetric speed of air suction through the filter, l/min;
t– duration of sampling, min.

Counting method

A number of industries place increased demands on the cleanliness of the air environment, for example, for the manufacture of radio-electronic equipment, film and photographic materials, medical supplies and so on. Act here departmental norms to air quality, which establish maximum permissible dust concentrations in counting indicators, expressed in the number of particles per liter or per cm 3. Air dust control in this case is carried out using the counting method. Its essence lies in the preliminary separation of dust from the air and its deposition on glass slides, followed by counting the number of particles using a microscope. By dividing the calculated number of particles by the volume of air from which they are deposited, we obtain the countable concentration of dust (particles/l):

,

Where K p– the number of fields of view (grid cells) in 1 cm 2 of the microscope eyepiece; n avg– the average number of dust particles in one field of view, determined based on counting in five different cells; F– area of ​​the base of the container from which dust particles are deposited, cm 2 ; V, h– volume and height of this container, respectively, cm 3 and cm.

To determine the countable concentration of dust, conimeters are used, consisting of a humidifying tube, a piston pump, a receiving chamber and a glass slide, in-line VDK ultramicroscopes, photopulse devices, etc. The most common is an automatic particle counter of the AZ-2M type, which allows simultaneously with measuring the countable concentration to determine the dispersed composition dust.

Related information.

Federal Agency for Maritime and River Transport

Federal state budget educational institution

Higher vocational education

"STATE MARINE UNIVERSITY NAMED AFTER ADMIRAL F.F. USHAKOV"

Department of Life Safety

Practical work № 3

on the topic of:

“Determination of the class of working conditions by factor

“ASSESSMENT OF THE HARMFUL IMPACT OF DUSTS”

Cadet group 1922

Somkhishvili Irma

Checked by: senior teacher

Pisarenko G.P.

Option 22

I. PURPOSE OF THE WORK

Explore general properties industrial dust and requirements sanitary standards; familiarization with the structure and operation of the aspirator; determine the dust content in the air by weight method and give a sanitary assessment of dust content.

II. GENERAL INFORMATION ABOUT INDUSTRIAL DUST

Industrial dust refers to solid particles suspended in the air, i.e. These are dispersed systems, namely aerosols, where the dispersed phase is particles ranging in size from 10 -2 to 100 microns, and the dispersed medium is air.

The formation of industrial dust occurs during reloading and transportation of bulk cargo, mechanical grinding of solids.

Industrial dust can also include soot resulting from incomplete combustion fuel in marine diesel engines and steam generators.

Industrial dust can be quantitatively characterized by the average particle size, size distribution curve, specific surface area, i.e. the ratio of the total surface of dust particles to their mass or volume. The most important characteristic is the concentration of dust in the air.

Dust enters the human body through the respiratory system, gastrointestinal tract, eyes and skin. For humans, the greatest danger is posed by dust particles smaller than 10 microns, as can be seen from the data given in Table 1

Table 1

A particular danger to the human body is dust consisting of particles of a toxic substance, or dust with sorbed toxic substances on the surface. For example, toxic dust includes coal sand, calcium carbide, lime, lead, etc. A special feature is the presence of adsorbed carcinogenic substances on the surface of the particles, namely 3,4-benzpyrene - this is a condensed aromatic hydrocarbon with carcinogenic properties, i.e. May cause cancer when applied to the skin or when applied under the skin of animals.

The harmful effect of dust on the human body is determined by its content in the air of working premises, that is, the dust concentration, which can usually vary from 10 -8 to 10 5 mg/m 3 . Increased dust concentrations cause intense harmful effect on the human body.

Based on the degree of impact on the human body, harmful substances (including aerosols) are divided into 4 hazard classes:

1st – extremely dangerous substances;

2nd – highly hazardous substances;

3rd – moderately hazardous substances;

4th – low-hazard substances.

The hazard class of harmful substances is established depending on standards and indicators.

A harmful substance is assigned to a hazard class based on the indicator whose value corresponds to the highest hazard class. It is also necessary to keep in mind that some industrial dusts are explosive.

One of the dangerous dusts for the human body is maritime transport is grain dust, which consists of organic components

(bacteria, spores, etc.) and inorganic (particles of sand, clay, soil). The silicon dioxide content in grain dust reaches 10%.

Prolonged contact with grain dust can lead to the development of pneumoconiosis. Short-term exposure to the mucous membrane of the eyes and upper respiratory tract causes irritation and the development of inflammatory processes. At mechanical impact Blistering rashes appear on the skin (“grain scabies”), possibly also a bacteriological lesion with severe headache, chills, palpitations, dizziness and nausea (“grain fever”).

To prevent harmful effects of industrial dusts

A set of measures is applied to the human body:

Maximum permissible concentrations (MAC) of various dusts in the air of the working area are being developed and established;

Ventilation units and aspiration systems are designed and installed;

Developed and applied individual means protection;

III. BASIC OPERATIONS AND CALCULATIONS FOR ANALYSIS OF DUST CONTAINMENT IN WORKING PREMISES

a) Dust study protocol

b) Assessment of dust content in the workplace/room

1. To quantify a dusty work area, it is necessary to know the mass of dust per unit volume. Dust concentration can be determined various methods, the simplest and most reliable is the weight one. The essence of the method is to weigh a special filter before and after drawing a known volume of dust-laden air through it.

where: C – dust concentration in the air, mg/m3;

P 1 – filter mass before dust collection, mg;

P 2 – filter mass after dust collection, mg;

V 0 – volume of air at the sample site, o C.

V o =

where: V is the volume of air drawn through the filter under experimental conditions (at t (o C) and pressure B (hPa);

Dear readers, in this article we will talk about how the category of a room with dust is determined.

Despite the fact that the mathematical apparatus of SP 12.13130.2009, which is intended to determine the category fire danger rooms with dust is quite simple, but determining a number of parameters causes certain difficulties.

Let's look at everything in order. To begin with, it should be noted that rooms with dust can be classified as category B for explosion and fire hazard or explosion and fire hazard.

Before proceeding to the calculation of whether a room belongs to one of categories B for fire hazard, it is necessary to justify by calculation whether the room where the formation of an air suspension is possible belongs to category B for fire and explosion hazard.

The main calculation formulas are contained in section A.3 of Appendix A of SP 12.13130.2009.

In accordance with formula A.17 of the set of rules, the estimated mass of dust suspended in the room as a result of an emergency situation should be taken as the minimum of two values:

— the sum of the masses of swirling dust and dust released from the apparatus as a result of the accident;

— a mass of dust contained in a dust-air cloud, capable of burning when an ignition source appears.

It should be noted here that not all dust is capable of burning, i.e. the coefficient of participation of combustible dust in the explosion is ≤0.5, which is confirmed by formula A.16 of the set of rules.

The coefficient of participation of suspended dust in combustion depends on the fractional composition of the dust, namely a parameter called the critical particle size.

For most organic dusts (wood dust, plastics, flour, etc.), the critical size value is about 200-250 microns.

Dust consisting of larger particles will not participate in combustion, except when it is burned in special hearths (furnaces). When the category of a room with dust is determined, as a rule we are dealing with either completely fine dust, the particle size of which is less than critical (for example, powdered sugar), or with dust, which includes particles of various sizes, both larger and smaller than the critical one. Such dust includes wood dust, grain dust, etc.

The fractional composition of dust is determined experimentally by sifting through a system of special sieves called “fractionator”. It is hardly possible to find such data, although for a number of industrial dusts (powders), data on the fractional composition can be requested from the manufacturer.

In the absence of data, it is assumed that all dust particles have a size less than critical, i.e. capable of spreading fire. The mass of dust that can come out of the device as a result of an emergency is determined by the characteristics of the technological process.

The mass of swirling dust is that part of the deposited dust that can become suspended as a result of an emergency.

In the absence of experimental data, it is assumed that 90% of the mass of deposited (accumulated) dust can become an air suspension. Dust, which is released in small quantities in the production area during normal operation, settles on the enclosing structures (walls, floor, ceiling), on the surface of the equipment (cases of technological devices, transport lines, etc.), on the floor under the equipment.

At the designed production facility, the frequency of dust collections is determined: routine and general. According to SP 12, it is accepted that all the dust that settles in hard-to-reach places for cleaning accumulates there during the period between general dust collections. Dust that settles in places accessible for cleaning accumulates there during the period between current dust collections. Estimation of the proportion of dust settling on a particular surface (accessible or difficult to access) is possible only experimentally or by modeling methods.

Assessing the dust collection efficiency of designed production facilities, as a rule, is also impossible, therefore it is conventionally accepted that all dust released from the equipment into the room settles inside the room.

The amount of dust settling on different surface areas located in the room also varies. Dust, which is released normally, floats in the air and, due to gravity, gradually settles on various surfaces.

At the same time, it is expected that the largest amount of dust settles on more low levels premises, provided that the source of dust (equipment) is also located on the lower level. It is obvious that horizontal surfaces can accumulate dust in almost unlimited quantities; a limited amount of dust settles on vertical surfaces, depending on the type of surface.

For, the amount of dust that settles on the walls is as follows: painted metal partitions– 7-10 g/m2, brick walls– 40 g/m2, concrete walls– 30 g/m2. Most likely, the data presented can be used for other industries.

Now let's turn to the formula for calculating the amount of dust depending on the volume of the dust-air cloud. It should be noted that there are no analytical expressions by which the volume of a dust-air cloud can be calculated in the domestic literature.

It has not yet been possible to find such data in foreign fire-technical literature, probably because such an approach is not used in the USA and Europe (meaning the calculation of categories). Therefore, in practice, the volume of the dust cloud has to be estimated in some way.

For example, we can conditionally take as the characteristic shape of a cloud a cone with a height from the floor to the dust source and a base with a radius several times greater than this height. Although, I’m not sure how true this assumption is, since there are no experimental data available.

In addition to the critical size, the stoichiometric dust concentration is also a determining parameter.

Stoichiometric dust concentration is the concentration of dust at which its complete combustion occurs, taking into account the amount of oxygen contained in a unit volume of air.

The stoichiometric dust concentration can be determined by calculation only for substances and materials for which the chemical composition is known. These include the majority polymer materials(polyethylene, polypropylene, polystyrene, etc.), various medications, powders of metals and alloys.

For other materials, for example for plant materials (wood and grain dust, tea, etc.) and food materials (flour, powdered milk, cocoa, etc.), the stoichiometric concentration must be determined either experimentally, or the chemical composition of the corresponding material from which the dust consists is sought.

Determining the stoichiometric concentration comes down to solving the following sequential problems:

1. The chemical composition of the dust is determined.

2. Recorded chemical equation reactions of complete combustion of dust.

3. The mass of oxygen required for complete combustion of 1 kg of dust is determined.

4. The mass of oxygen contained in 1 m 3 of air is determined, taking into account the design temperature.

5. The mass of dust that can completely burn in the mass of oxygen contained in 1 m 3 of air is determined. The resulting value is the stoichiometric concentration of dust in the dust-air cloud.

Determining the category of a room with dust does not take into account such an indicator of fire danger as the lower concentration limit of flame propagation (LCFL). As a rule, the concentration of dust in a dust-air cloud at emergency situations exceeds the NLPR.

And finally, a couple of very interesting videos about dust explosions in industries. Even without knowledge of English, everything is shown clearly and interestingly. I recommend watching!

I look forward to seeing you again on fire safety!