INFLUENCE OF WELDING PROCESSES FOR UNDERGROUND PIPELINE REPAIR ON WELDER SAFETY

The object of research is the welder safety during the repair and construction of underground pipelines. It is established that manual electric welding is characterized by significant influence on the human body, since the distance from the place of welding to the welder is 20–30 cm, the temperature in the pillar of the welding arc reaches 6500 K. It is contributes to the allocation in the surrounding space of a significant amount of metal vapor and fine particles (welding aerosol and toxic gases). Theoretical and experimental investigations of the initial part of the welding torch (at a height of 0.4–0.5 m) using manual, semi-automatic welding in the environment of protective gases and welding under a layer of flux have been carried out. Dependencies for determining axial temperatures and velocities are proposed. It is substantiated that the spreading of aerosols during welding of pipelines in the trench is determined by the physical processes of spreading the welding torch. The hygienic characteristics of the electrodes of the ANO type in the welding processes are investigated. It is established that the total amount of aerosol in the process of welding using ANO-4 electrodes with basic coating reaches 31 g/kg and toxic substances in aerosol up to 9 g/kg. The amount of gases in front of the welder shield during the welding of electrodes by ANO-8 reaches 20 mg/m 3 . Empirical equations for determination of aerosols concentration and gaseous concentration of harmful substances are obtained. In addition, an equation for determining the index of workplace pollution of welders, which made it possible to predict individual parameters of working conditions, is obtained. The mechanism of propagation of aerosols in the trench requires a special consideration, which can be carried out by the method of mathematical modeling, because such physical and chemical processes, as longitudinal and transverse dispersion, molecular diffusion play a role. The analysis of empirical formulas which can be recommended for determination of parameters in the initial section of a welding torch using flash welding of a pipeline in a trench is done.


Introduction
The main technological process in which a large amount of harmful emissions (nitric oxide, carbon monoxide, manganese oxide, fluoride compounds, ozone, dust and the heat associated with them) is released is welding.
Hygienic studies of welding processes include experimental and industrial studies. The experiment gives general data on the quantitative and qualitative composition of dust and gases, electromagnetic radiation, sound pressure level and the like. Industrial studies reveal the dynamics of these harmful factors during the day, week, season.
Welding of underground pipelines during their repair is carried out in a half-open space, and since the distance from the place of welding to the welder is only 20-30 cm, there is a need to study the effect of welding processes when repairing underground pipelines on the safety of the welder.
The subject of the study is to increase the level of hygiene and safety during electrical welding during the repair of underground pipelines.
The theoretical and methodological basis of research is scientific works [1][2][3] in the direction of the effective-ness of managerial decisions on labor protection in the welding industry.
The main tasks of the work are the assessment of sanitary and hygienic working conditions, as well as studies of the allocation and spread of aerosols during the welding process.
In this regard, it is relevant to conduct research to reduce the risk of occupational diseases working by increasing the efficiency of welding production.
Thus, the object of research is the welder safety in the repair and construction of underground pipelines.
The aim of research is studying the effect of harmful substances on the human body when welding in half-open space (underground pipelines).

Methods of research
Experimental studies were conducted under production conditions. Welding work took place in the repair of in-depth gas pipelines and was carried out in trenches. In this case, welding processes occur with a rapid change in the temperature of the metal to be welded or cut. In a wide temperature range, various physical and chemical ISSN 2664-9969 processes occur. All applied heat sources are characterized by high thermal power, contributes to the formation of a welding torch (or cutting jet).
For example, in electric arc welding, the temperature in the column of the welding arc reaches 6500 K, and in the areas of the electrodes through which the welding current passes, it is close to the boiling point of the metal and reaches 2500-2600 K. This contributes to the release of a significant amount of metal vapor into the surrounding space, which condenses, forming fine dust (welding spray). A number of toxic gases are also emitted [1,2].
The study of the laws of the welding torch is also the subject of work [4,5]. The mechanism of aerosol propagation in trenches requires special consideration, which can be carried out by mathematical modeling, since such physicochemical processes as longitudinal and transverse dispersion, molecular diffusion play a role here.

Research results and discussion
The burning of the welding arc is accompanied by a spray of drops of metal and slag from the weld pool, which is in the nature of microexplosions.
Convective heat, which is given by the arc and heated part to the surrounding air, causes the appearance of a rising contaminated stream -the welding torch ( Fig. 1). Like any other convective flow, the welding torch consists of a booster (initial) and main sections. In the accelerating section with a height of 0.3-0.4 m, air leaks to the arc, heats up and rises, its velocity increases significantly. It is also clearly seen that the accelerating section of the welding torch has a smaller diameter than the main one, the convective flow is compressed by air, and leaks from the sides.
The main section of the welding torch is characterized by a decrease in axial velocities and temperatures as the torch rises and mixes it with the surrounding air. The experimental dependences for υ m and ∆t m in the case of manual welding with electrodes with a diameter of 3 and 5 mm are also shown in Fig. 2.
Processing the obtained data, taking into account the laws of thermal jets and the magnitude of convective heat Q C , allowed the author of [4,5] to conclude that the axial velocity (in m/s) and excess temperature ∆t m (in °C) in the main section of the welding torch are quite well described by theoretical dependences: where γ -distance along the vertical axis to a given point of the torch, m a b The distribution of axial relative concentrations and excess temperatures in the flare is subject to one dependence. Studies have also confirmed that with increasing arc power, the absolute values of the welding torch parameters increase. According to the indicated dependences, it is possible to calculate the values of axial velocities in the welding torch and the value of axial excess temperatures at a height = 0.4 m from the surface of parts and more.
As a result of theoretical and experimental studies of the initial section of the welding torch (at a height of 0.4-0.5 m) during manual, semi-automatic and automatic welding in a shielding gas medium and welding under a flux layer, the following dependences are proposed to determine the maximum axial temperatures and velocities [4]: -for manual and semi-automatic welding, K: -for automatic welding, K: where γ -height from the surface of the welded part to a given point, m; I D -welding current strength, A, when welding under a flux layer k p = 6.5 and k a = 28.
where T ∞ -ambient temperature, which can be taken equal to 293 K. After substituting the digital values ∆t m from formula (3), the formula for determining the axial velocity (in m/s) in ISSN 2664-9969 the initial section of the welding torch (for manual welding), proposed in [6,7], will look like: . . m/s. Fig. 3 shows the data of experimental studies of the axial velocities of air in a welding torch during welding in CO 2 with currents of 200, 1200, and 3000 A [6]. Air velocity measurements were made by a special thermoelectric anemometer, the sensor of which had a thin platinum thread, and were not sensitive to the flux of radiant heat. Experimental points are the results of averaging multiple measurements of maximum velocities in each mode. As can be seen from the graphs of Fig. 3, the initial (accelerating) section in the welding torch has a height of 0.4-0.5 m, the maximum air velocities are noted at a sample height of 0.3 m from the surface of the welded parts, that is, where the cross section of the torch is narrowed. The most important characteristic of the welding torch is the magnitude of convective heat Q C , which determines the main parameters of convective flow. Various researchers in their works [7][8][9] recommend taking values of Q C that differ significantly from each other. It is found that for fast-moving welding arcs when welding with a consumable electrode with a diameter of 3 mm Q C = 8-9 %, and for thick electrodes Q C = 10-11 % of the total thermal power of the arc Q eq . When welding in CO 2 , the amount of convective heat in the welding torch increases and amounts to 12-14 % Q eq . A significant part of the heat enters the welding torch not only from the surface of the parts, but also directly from the welding arc itself, and, with an increase in its power, the value Q C increases.
In the arc gap, intense heat is removed from the welding arc by forced convection due to the action of the cathode jet moving from the electrode to the workpiece at a velocity of about 100 m/s. Some heat enters the welding torch due to condensation of the metal vapor. A very important factor is the dissociation in the welding zone of a part of СО 2 into С and О 2 and further molization, which occurs in the welding torch with the release of heat.
All these processes with an increase in power consumption and an increase in temperature in the column of the welding arc are intensified, while the amount of convective heat provided is increased [10-12].
A number of empirical dependencies proposed by some researchers deserve attention. Studies [8] allows to obtain the following empirical formulas that can be recommended for determining the parameters in the initial section of the welding torch during butt welding: -for convective flow velocity, m/s: where N -power spent in welding, kVA; γ -distance from the welding point along the axis of the upward flow, m; -for the volume moved by a stream of air, m 3 /s: 5 3 .
; . γ (9) In dependences (8) and (9) obtained by the author of [6], taking into account the well-known laws of compact convective flows, the power N consumed during welding is introduced instead Q C . The calculations of the torch parameters are done at values from 0.1 to 0.5 m. For example, at a power of 10 kVA, the axial air velocity at a height of 0.5 m is 0.68 m/s. The obtained value is quite close to the values υ m , although the technological process differs significantly from welding with a consumable electrode.
Thus, it is substantiated in the work that the distribution of aerosols when welding pipelines in a trench is mainly determined by the physical processes of propagation of the welding torch. Based on this, the nature of the change in the concentration of welding aerosols in trenches with a pipeline is simulated.

Conclusions
It is shown that the distribution of aerosols during welding of underground pipelines in a trench is mainly determined by the physical processes of propagation of the welding torch. As a result of comprehensive studies, empirical dependencies are obtained to determine the concentration of gaseous harmful substances and to determine the pollution index of welders' jobs, which will make it ISSN 2664-9969 possible to predict individual parameters of the working conditions of welders.