SCIENTIFIC AND METHODOLOGICAL BASES FOR DESIGNING SPECIAL SHOE FOR HIGH TEMPERATURES

ABSTRACT

The article studies the influence of harmful factors on a person during firefighting, the requirements for indicators of the purpose of personal protective equipment for the firefighter's legs and the application of a scientific and methodological approach to the design of special-purpose footwear for firefighters.

АННОТАЦИЯ

В статье изучены влияния вредных факторов на человека при пожаротушении, требования, предъявляемые к показателям назначения средства индивидуальной защиты ног пожарного, и применение научно-методологический подход к проектированию обуви специального назначения для пожарных.

Keywords: Shoes, lining, thermal insulation, material, leather, waterproof, density, fabric, factor.

Ключевые слова: Обувь, подкладка, термоизоляцион, материал, кожа, водонепроницаемость, плотность, ткань, фактор.

The sustainable pace of economic development of our Republic largely depends on the state of working conditions in the workplace. Especially priority in this direction has recently become the issue of providing workers with overalls, special footwear and other means of individual and collective protection.

The share of research on the creation of multicomponent and functional materials through purposeful changes and regulation of the microstructure is increasing in the world. The creation of a waste-free, resource-saving, environmentally friendly innovative technology of multilayer materials based on natural and synthetic fibrous raw materials is of particular importance. Such compositions are widely used in the production of 50% of modern footwear, fur products, 60% of leather goods, in particular, in the production of thermal insulation lining materials for shoes [1].

A fire is an open thermodynamic system that exchanges substances and energy with the environment. The main phenomena accompanying a fire are the processes of combustion, gas and heat exchange, which change in time and space.

The process of burning combustible substances and materials in a fire is a fast-flowing chemical oxidation reaction and physical phenomena, without which combustion is impossible, accompanied by the release of heat and the glow of hot combustion products with the formation of a laminar or turbulent diffusion flame. Combustion leads to the release of heat products, an increase in the surface temperature and an increase in the rate of oxidation in the zone of its chemical reaction. Under the influence of heat released in the combustion zone, heating, destruction, evaporation and ignition of the following sections of combustible substances and materials occur. This process is accompanied by gas and heat transfer, which are called general phenomena characteristic of any fire, regardless of its size and location. General phenomena can lead to the manifestation of particular ones: explosions, deformation and collapse of technological apparatus and installations, building structures, boiling up or ejection of oil products from tanks, etc. (picture 1).

Figure 1. Mass and heat transfer on fire: a - heat transfer in a fire; b - zones on fire: 1 - combustion zone; 2 - zone of thermal influence; 3 - smoke zone

Often, the combustion temperature of some materials in a fire reaches 1000° C. Practice shows that at a temperature equal to 80-100°C in dry air and at 50-60°C in humid air, a person without special thermal protection can stay only a few minutes. A higher temperature or prolonged stay in this zone leads to burns, heat stroke, loss of consciousness, and even death (Table 1). Therefore, when choosing the degree of protection of a firefighter to eliminate the combustion process, it is necessary to take into account the harmful environmental factors that constitute a potential hazard to human health [1].

Table 1.

Conditions for a person to stay in a fire depending on the heat flux density

The safe time for people to stay in the heat-affected zone is determined by the length of combat work, which excludes dangerous overheating of the human body. The presence of personnel in the danger zone is initially limited to 10-15 minutes, and the period for restoring the thermal state of the body to the initial level is at least 1 hour.

Figure 2. Scheme of the influence of harmful factors on a person during fire fighting

When performing combat operations to extinguish fires and eliminate the consequences of accidents, a firefighter experiences the maximum possible impact of harmful factors. Standard GOST 12.0.003-74 “Hazardous and harmful production factors. Classification” the following groups of relevant factors are established, which are presented in Figure 2 [2-3].

Proper assessment of the risk to which a firefighter is exposed during the performance of basic duties requires the use of special-purpose footwear with an appropriate level of thermal insulation properties.

The materials used for footwear must meet both the generally recognized requirements for footwear (technological, hygienic, aesthetic), and special, due to the specific operating conditions in the field of high temperature (heat resistance, reflective properties, etc.). In addition, from an economic point of view, the material must withstand the specified service life and have a low cost.

The choice of materials for the manufacture of footwear for special purposes is significantly limited by the relevant regulatory and technical documentation. According to GOST 53265-2009 "Personal protective equipment for firefighter's feet", the requirements presented in Table 2 are regulated for special footwear for firefighters.

The requirements for the indications for the appointment of personal protective equipment for the legs of a firefighter SIZNP must correspond to the values \u200b\u200bgiven in table 2

Table 2.

Requirements for the indicators of the purpose of personal protective equipment for the legs of a firefighter

When selecting criteria that determine the requirements for the design of special-purpose footwear that protects against high temperatures, special attention should be paid to the type of footwear that provides protection from the highest possible level of harmful factors. According to [4], the following requirements for the design of shoes for this purpose are regulated:

• safety shoes should be made from 245 to 307 sizes;
• an internal safe gap in the toe of safety shoes with an impact energy of (200 + 5) J must be at least 20 mm;
• the mass of a half-pair of safety shoes size 270 should not exceed 1600 g;
• the height of safety shoes should be no more than 345 mm;
• the design of leather safety shoes should ensure ease of dressing and fixation on the lower leg;
• the color of safety shoes must be black;
• all fittings and parts used (buckles for adjusting the width of the shaft, nails, etc.) must be made of anti-corrosion materials or have an anti-corrosion coating;
• special footwear should not interfere with dressing on alarm for the standard time of all types of combat clothing;
• the depth of the reef of the sole and heel must be at least 1 mm.

At the same time, the priority factor that has a negative impact on the human body under conditions of high thermal load is extreme temperature (> 200°C), which causes increased sweating from the surface of the foot. In this case, special footwear as a means of personal protection is designed to prevent the effects of heat from the outside, on the one hand, and provide an optimal microclimate inside the shoe, on the other.

Table 3.

Requirements for the physical and mechanical properties of leather personal protective equipment for firefighter's legs

The properties of rubber SIZNP should correspond to the values given in table 3.

Rubber PPE should have impact protection in the rear of the foot, shin and ankle (see Table 4, indicator 8).

Table 4.

Properties of firefighter's rubber leg protection

The range of domestic and foreign manufacturers of footwear for this purpose is shown in Figure 3.

Figure 3. Model range of footwear assortment for firefighters

A significant disadvantage of these products in an environment with extremely high temperatures can be considered the lack of the possibility of timely drying.

In order to eliminate this drawback, a shoe model was developed that, due to the design, can provide quick drying in natural or artificial conditions.

The proposed boots are made with high berets 345 mm high in accordance with [4].

Shoes are made by direct casting of raw rubber compounds. Top model of traditional design, i.e. with adjustable lace-up berets. On each beret there are 8 blocks and 3 hooks. Rigid fixation of the shin with shoes will reduce injuries to the feet of firefighters when performing combat operations to extinguish fires and facilitate movement on inclined and wet surfaces (Fig. 4).

Figure 4. Boots for firefighters

Upper material - chrome leather 2.0 - 2.2 mm thick with water-repellent impregnation. Lining - membrane fabric "СROSSTECH" [5].

In boots made of fire-resistant chloroprene rubber produced by CJSC Faraday Companies (Russia), a metal anti-puncture insole, a protective metal toe cap and a fire and oil and petrol resistant chloroprene sole are used to increase the protective properties of footwear. Lining package of materials from laminated GORE-TEX membrane fabric and insulation. Boots are designed for use at ambient temperatures from minus 400C to plus 400C. The mass of a pair of boots 270 (42) size is not more than 3200 g. Boots are equipped with special stockings made of natural or artificial fur, cloth or baize, designed to protect against convective heat and adverse climatic influences.

The main distinguishing characteristics of modern special footwear for firefighters: the presence in the footwear of an impact-resistant metal toe cap that protects the toe of the foot from impacts with a force of 200 J; hard backs, giving shape stability to the heel of the shoe; metal insole protects the foot from cuts and punctures; heat-resistant sole withstands temperatures from - 40 to + 250 0С; side loops for easy and quick donning; as a lining, a membrane breathable material was used, which allows the skin of the foot to "breathe" and eliminates the "greenhouse" effect in summer; removable insoles of a porous structure, evenly distributing the load on the foot, which contributes to less fatigue of the legs [6].

The main properties of the material are water resistance and wear resistance. It owes these properties to the membrane, which is a very thin fluoroplastic film. It has such a structure that its pores are 20,000 times smaller than a drop of water, so they do not allow moisture to enter the boot, and at the same time, 700 times more than a vapor molecule, due to which evaporation is removed from the boot, which allows the foot to "breathe" and stay dry. The presence of membrane fabric in shoes provides good air circulation and microclimate inside the boot. Membrane fabrics are not thermal insulators.

In addition, the Deaf valve model prevents water, small stones and sand from entering the inside of the boot. In the middle part of the figured valve, a leather detail is sewn - an amplifier that protects the leg from mechanical influences.

The figured elongated soft cuff provides a comfortable fit to the lower leg and improves the appearance of the boot. The figured cuff serves for protection of a shin from injuries and convenience at operation.

In the manufacture of footwear for protection against the cold, they tend to use designs with fewer seams to preserve the heat-shielding properties of the product [7].

Thus, the creation of high-quality thermal footwear is based on a set of requirements for various characteristics that regulate the degree of compliance with the conditions of the type of activity (tactical-technical, ergonomic, hygienic and aesthetic requirements). Achieving the required level of the above indicators in most cases is ensured through the use of appropriate raw materials, fittings, etc.

Taking into account the fact that primarily protective properties are applied to heat-insulating shoes, the considered model should be checked for compliance with the quality indicators declared by NBP158-97.

A preliminary analysis of the determination of the heat-shielding efficiency of safety shoes can be carried out on a mathematical model of heat transfer in the toe part of the shoe, which allows you to conduct a numerical experiment with various combinations of model parameters. Thus, it becomes possible to reduce numerous costly physical tests. The geometric model of the study area - the toe of the shoe - is shown in Figure 5 (explanations for the positions of the figure 5 are presented in table 5).

Figure 5. Geometric model of the toe of the firefighter's safety shoes: a - longitudinal section of the shoe; b - a fragment of the longitudinal section of safety shoes in the toe part

Table 5.

Thermophysical properties of materials of the system "human foot - safety shoes - environment"

The processes of heat transfer in shoes are described by the equation of non-stationary heat conduction [5], which in the case under consideration has the form

                                           (1)

We accept that the temperature inside the toe of the shoe is a function of radius r and time t, that is, T = T (r, t); the temperature at the point 0 - T₀ is known (the temperature of the human body, equal to 30 °C).

To obtain a unique solution to this equation, it is necessary to supplement it with initial conditions Т(r,0) and conditions on the boundary

T₀ = Т_внут, Т_н = Т_внеш

In a spherical coordinate system, equation (1) takes the following form

                                           (2)

Applying the finite difference method, we obtain for the i-th grid node:

                                (3)

where i = 1,2,...,n — 1; j = 0,1,2,...,m; t - time step t (∆t = t_j+1 - t_j); ∆r - step by coordinate  r (∆r = r_i+1 — r_i); _i, _i и c_i - values of thermal conductivity, density and heat capacity of a layer of material enclosed between nodes (i-1) and i.

Total calculation time t_common = 360 sek. At step ∆t =30 sek, m =12.

Let us solve equation (3) with respect to

         (4)

где i = 1,2,...,n -1; j = 0,1,2,...,m.

The system of relations (4) makes it possible to find the temperature at all nodes of the grid at the moment of time t₁ = ∆t, t₂ = 2∆t, t_j = j∆t, up to t_com= 360 с.

The calculation made using a mathematical model made it possible to obtain graphs of the dependence of heat transfer processes on time, shown in Figure 6.

Figure 6. Dependence of the temperature distribution on the surface of the layers of the system "human foot - special footwear - environment" on the time of exposure to high thermal load:
 integumentary tissues of the foot; sek, internal details of the top of the shoe; В-В - frame parts of the upper of the shoe, outer parts of the shoe upper

Analyzing the data of the presented graphs, one should keep in mind the following provision of NPB 158-97, that the arithmetic mean of the temperature on the inner surface of the composition of the layers of special footwear should not exceed 50°C for at least 5 minutes. The most informative in this case is the heat transfer graph of the internal parts of the upper of the shoe. The permissible temperature limit is reached after 50 sec. from the moment of exposure to only an external temperature of 200 ° C, which does not meet the requirements of regulatory and technical documentation. In this case, the effect of heat flow in this case was not taken into account.

In order to confirm the results obtained from the results of mathematical modeling, a series of experiments was carried out to obtain numerical values of the temperature of the shoe space. The temperature-sensitive sensor was fixed on the internal parts in the toe before placing the safety shoes in a heating cabinet with a constantly maintained temperature of 200°C. Fixing the results of the measuring device every 10 seconds. made it possible to confirm the numerical indicators of mathematical calculations (Fig. 7).

Figure 7. Graph of the dependence of the temperature on the inner surface of the composition of the layers of special footwear on the external temperature

Thus, the application of a scientific and methodological approach to the design of special-purpose footwear for firefighters made it possible to reveal that the choice of a package of materials is the dominant factor. However, the developed design of the model provides an improvement in hygienic properties by improving the quality of care for the model without losing priority protective indicators.

References:

1. Ўзбекистон Республикаси Президентининг “Чарм-пойабзал ва мўйначилик соҳаларини ривожлантириш ва экспорт салоҳиятини оширишни янада рағбатлантириш чора-тадбирлари тўғрисида”ги ПҚ-3693-сон қарори. Тошкент ш., 2018 йил 3 май.
2. ГОСТ 28507-90 Обувь специальная кожаная для защиты от механических воздействий. Общие технические условия
3. ГОСТ 12.0.003-74 «Опасные и вредные производственные факторы. Классификация».
4. Шарапа Т.П., Автореферат диссертации "Разработка и исследование новых конструкций и технологии спецобуви для защиты от повышенных температур".
5. Maksudova U.M., Rafikov A.S., Mirzaev N.B.,  Theoretical foundations of obtaining lining composite materials/International Jornal of Advanced Research in Science,  Engineering and Technology/ vol. 5, Issue 10, October 2018. P. 6994-6996.
6. U.M.Maksudova, N.B.Mirzaev, D.Z.Pazilova, Sh.Sh.Sheraliev, Analysis of innovative technologies in the manufacture of composite lining materials/ International Jornal of Advanced Research in Science, Engineering and Technology/ vol. 5, Issue 10, October 2018. P. 7013-7015.
7. Rafikov, A., Alimkhanova, S. Mirzaev, N. Multilayer nonwoven lining materials made of wool and cotton for clothing and footwear Journal of Industrial Textiles this link is disabled, 2022, 51(4), pp. 6173S–6194S
8. Спиридонов А.А. Планирование эксперимента при исследовании технологических процессов / -Москва. “Машиностроение”. 1981. С.184.