Welding protection against fire, explosion and arc radiation
Welding protection harmful gases and welding fumes
Harmful gas
In the process of CO2 gas shielded welding, under the action of the high temperature of the arc and the strong ultraviolet rays, the harmful gases formed are mainly carbon monoxide, carbon dioxide, nitrogen oxides and ozone.
Carbon monoxide（CO）
In the process of CO2 gas shielded welding, there are two main sources of CO: one is generated by the decomposition of CO2 under the action of a high-temperature arc; the other is formed by the reaction of CO2 with molten metal elements, as shown in Figure Source of CO .
CO is a toxic gas, which enters the body from the respiratory tract, is absorbed into the blood through the alveoli, and combines with hemoglobin to form carboxyhemoglobin, which hinders the oxygen-carrying capacity of the blood, and makes the human tissue hypoxic to form carbon monoxide poisoning. The maximum allowable concentration of CO is 30 mg/m3, which is lighter than air and exists above the welding site.
Carbon dioxide (CO2)
Carbon dioxide is the protective gas for CO2 gas shielded welding. After inhaling excessive CO2 gas, the victim’s eyes and respiratory system may be stimulated. In severe cases, there may be cognitive disturbances, difficulty breathing, pulmonary edema, and death from asphyxiation. At present, the maximum allowable concentration of CO2 has not yet been stipulated. Some countries stipulate that it is 9000 mg/m3. CO2 is heavier than air and often accumulates.
Below the welding site.
Nitrogen oxides（NO\NO2）
Due to the high temperature of welding, the nitrogen and oxygen molecules in the air are dissociated and recombined to form ammonia oxides, mainly nitrogen dioxide (NO2). Severe, whole body paralysis, after high concentration of nitrogen dioxide inhaled into the alveoli, the reaction is accelerated due to the high humidity in the alveoli. About 80% of nitrogen dioxide entering the human body is trapped in the alveoli and gradually reacts with water to generate nitric acid or nitrous acid. The maximum allowable concentration of nitrogen dioxide is 5 mg/m3, which is heavier than air and often accumulates below the welding site.
Ozone （O3）
Oxygen in the air, under the excitation of short-wave ultraviolet rays of CO2 gas shielded welding, photochemically produces ozone (03). Ozone is a light blue toxic gas. When the concentration exceeds the allowable value, it often causes cough, fatigue, Dizziness, chest tightness, body aches, and severe bronchitis. The maximum allowable concentration of ozone (O3) is 0.3mg/m3, which is heavier than air and often accumulates Exists below the welding site.
Welding fumes
Formation of welding fumes
In the process of CO2 gas shielded welding, welding fume is the general name of fume and dust, the diameter of which is less than 0.1 um is called smoke, and the diameter between 0.1 and 10 μm is called dust. The main components of welding fume are very complex. When welding ferrous metals, the main components of fume are Fe, Si, Mn and other metals and their compounds.
Welding fume mainly comes from metal evaporation in the welding process, followed by metal oxides formed by the oxidation reaction of liquid metal and oxygen in the arc area of CO2 gas shielded welding under the high temperature of the welding arc, which diffuses to the job site to form a mixture. soot.
Hazards of welding fumes
Welders are exposed to welding fumes for a long time. If the protection is not good and too much fumes are inhaled, it will cause nausea, headache, pneumonia, bronchitis, and serious diseases such as welder’s pneumoconiosis, metal heat and manganese poisoning. See Table The amount of welding fumes and main toxic substances in CO2 gas shielded welding for the amount of CO2 gas shielded welding fumes and main toxic substances.
Welder's pneumoconiosis
Welders inhale high concentrations of welding fumes for a long time, causing systemic diseases mainly caused by diffuse progressive fibrosis in the lungs. The onset of pneumoconiosis in welders is generally relatively slow, and symptoms of pneumoconiosis in welders usually appear 10 or even 10-20 years after exposure to welding fumes. The main symptoms are cough, shortness of breath, chest tightness and chest pain. Some welders’ pneumoconiosis patients show symptoms such as general weakness, loss of appetite, weight loss and neurasthenia.
Manganese poisoning
When the welder performs CO2 gas shielded welding, the respiratory tract is the main way to absorb manganese. The manganese vapor in the welding process is quickly oxidized into MnO and Mn304 fumes in the air, and the smoke and dust exceeding the standard concentration of manganese and its compounds are inhaled for a long time. It can cause manganese poisoning.
The diameter of manganese fume is very small and the dispersion is very large, so it can spread rapidly. Therefore, it is not easy to form high-concentration manganese fume in open air or well-ventilated welding sites. When welders weld in containers and pipes for a long time, if the protection If the measures are not good, the welder may have manganese poisoning.
Manganese fume mainly acts on the peripheral nervous system and central nervous system of welders, and can cause serious organic lesions. The onset of manganese poisoning is slow, and the incubation period is generally more than 2 years. Chronic poisoning is the characteristic of manganese poisoning. The early symptoms are fatigue, dizziness, headache, drowsiness, memory loss and autonomic nerve dysfunction. , Squatting is somewhat difficult, etc. The concentration of manganese in the air at the welding site is specified as 0.2 mg/m³ by the relevant hygiene standards.
Welder metal heat
In the process of CO2 gas shielded welding, a large number of 0.05 ~ 0.5 μm iron oxide manganese oxide particles and other oxides enter the human body’s peripheral bronchioles and alveoli through the welder’s respiratory tract, thereby causing the welder’s metal thermal reaction disease. The main symptoms of welder’s metal fever are fever after work, followed by chills, physical fatigue, metallic taste in the mouth, nausea, throat itching, difficulty breathing, chest pain, loss of appetite, etc.
Welding protection Fire and explosion-proof
Possibility of explosion at welding site
An explosion is a phenomenon in which a substance releases a large amount of gas and energy in the form of mechanical work in an instant.
There are several situations in which explosions may occur during welding:
Explosion of combustible gas
Combustible gases widely used in industry, such as acetylene (C2H2), natural gas (CH4), etc., are evenly mixed with oxygen or air to a certain limit, and they will explode when encountering a fire source. This limit is called the explosion limit and is usually expressed as the volume percentage of the flammable gas in the mixture. For example, the explosion limit of acetylene mixed with air is 2.2%~81%; the explosion limit of acetylene mixed with oxygen is 2.8%~93%; the explosion limit of propane and butane mixed with air is 2.1%~9.5% and 1.55%~8.4% respectively .
Explosion of flammable liquid or flammable liquid vapour
When there is a flammable liquid in or near the welding site, the vapor of the flammable liquid or the flammable liquid reaches a certain concentration, and it will explode when it encounters electric welding sparks (for example, gasoline vapor mixed with air, the explosion limit is only 0.7% ~ 6.0%).
Explosion of combustible dust
Combustible dust (such as magnesium, aluminum, cellulose dust, etc.), suspended in the air, reaches a certain concentration range, and it will explode when encountering a fire source (such as electric welding sparks).
Welding direct use of flammable gas explosion
For example, when using an acetylene generator, an explosion will occur during feeding, refueling (too much phosphorus in calcium carbide or sparks caused by collision), and tempering caused by improper operation.
Explosion of airtight containers
Welding of closed or pressurized containers can also explode if proper measures are not taken.
Fire and explosion-proof measures
- It is forbidden to put inflammable and explosive materials in the welding site. Fire-fighting equipment (such as fire extinguishers) should be provided in the site to ensure sufficient lighting and good ventilation.
- Storage vessels or pipelines for oil or other flammable and explosive materials, and oxygen cylinders should not be stored within 10m of the welding site.
- When welding pressurized containers, airtight containers, various oil drums and pipelines, and workpieces stained with flammable substances, they must be inspected in advance, and toxic, harmful, flammable, and explosive substances must be removed by flushing, and the containers and pipelines must be removed. After the pressure is removed, the closed state of the container is eliminated, and then the welding is carried out.
- When welding airtight hollow workpieces, vent holes must be left. When welding pipes, both ends must not be blocked.
- When welding in workshops, places with flammable and explosive substances, or near gas pipes and acetylene pipes (bottles), the consent of the fire department must be obtained. Take strict measures during operation to prevent sparks from splashing and causing fire.
- Welders are not allowed to perform welding operations on wooden boards and wooden brick floors.
- Welders are not allowed to weld with bare handles or grounding wires, and are also not allowed to connect the secondary circuit wires indiscriminately.
- When gas welding and gas cutting, qualified calcium carbide, acetylene generator and flashback preventer should be used, pressure gauge (acetylene, oxygen) should be checked regularly, and qualified rubber hose should be used.
- When leaving the welding site, the gas source and power supply should be turned off, and the fire should be extinguished.
Welding protection Arc radiation protection
Welding arc radiation sources mainly include ultraviolet, infrared and visible light. They are produced by the heating of objects and belong to the thermal spectrum.
UV
Appropriate amount of ultraviolet rays is beneficial to human health, but excessive irradiation of strong ultraviolet rays generated by welding arcs has certain harm to human health. UV damage to the human body due to actinic action, it mainly causes damage to the skin and eyes.
The effect of uv on the skin
Ultraviolet rays of different wavelengths are absorbed by different depths of the skin. When the skin is affected by strong ultraviolet rays, it can cause dermatitis, such as diffuse erythema, sometimes small blisters, exudates and edema, burning sensation and itching. The skin’s response to UV light varies depending on its wavelength. When ultraviolet rays with longer wavelengths act on the skin, erythema usually appears after an incubation period of 6 to 8 hours, which lasts for 24 to 30 hours, and then slowly disappears, forming a long-term non-fading pigmentation. When the wavelength is shorter, the erythema appears and disappears faster, but the headache is heavier, and little pigmentation remains. When the effect is strong, it is accompanied by systemic symptoms: headache, dizziness, easy fatigue, nervous excitement, fever, insomnia, etc. Systemic symptoms are due to the collapse of the body’s cells under the action of ultraviolet rays, resulting in humoral spread, and the direct effects of ultraviolet rays on the central nervous system.
UV-induced electrooptic ophthalmia
Acute keratoconjunctivitis caused by excessive exposure to ultraviolet light is called electro-optic ophthalmia. This is a special occupational eye disease in the direct operation and auxiliary workers of open arc welding. Ultraviolet rays with shorter wavelengths, especially those below 320 nm, can damage the conjunctiva and cornea, and sometimes even the iris and retina.
The main reasons for the occurrence of electro-optic ophthalmia are: several welding machines work at the same time, and when the distance is too close, they are easily exposed to the radiation of the adjacent arc light during the operation; Open the mask too early; the auxiliary worker is exposed to arc light due to the uncoordinated coordination when assisting welding, and the welder has not prepared for protection (such as wearing goggles, deviating head, closing eyes, etc.) ; Insufficient lighting in the work place, the welding seam cannot be seen clearly, so that the first ignition and then the wearing of a mask and other passers-by are subject to sudden strong exposure, etc.
The degree of damage to the eyes when exposed to ultraviolet rays is proportional to the time of exposure, inversely proportional to the distance from the source of exposure, and related to the projection angle of the light. When the light is irradiated at right angles to the cornea, the effect is the largest, and the larger the deflection angle, the smaller the effect.
Exposure to strong ultraviolet rays for a short period of time can cause the disease. The incubation period is generally 0.5 to 24 hours, and most patients develop symptoms 4 to 12 hours after exposure. First, there is a high degree of shyness in the eyes, a tearing foreign body sensation, tingling, redness and spasm of the eyelids, and often headache and blurred vision. Generally, after treatment and care, it will recover well after a few months and will not cause permanent damage.
UV damage to fibers
Ultraviolet radiation of welding arc has strong destructive ability to fibers, especially cotton fabrics. As a result of actinic action, cotton work clothes can be oxidized and deteriorated and broken, and the colored printing and dyeing materials are significantly faded. This is one of the reasons why the cotton work clothes of open arc welding welders are not durable, especially when argon arc welding, plasma arc welding and other operations. more obvious.
Infrared
The harm of infrared rays to the human body is mainly caused by the thermal effect of tissues. Infrared rays with longer wavelengths can be absorbed by the skin surface, making people feel hot; short-wavelength infrared rays can be absorbed by tissues, heating blood and deep tissues, resulting in burns. During the welding process, the eyes are exposed to intense infrared radiation, and immediately feel intense burns and burning pains, giving off flash illusions. Long-term exposure may cause infrared cataracts, vision loss, and even blindness in severe cases. It can also cause retinal burns.
The infrared intensity of argon arc welding is 1.5 to 2 times that of manual arc welding; and plasma arc welding is greater than that of argon arc welding.
Visible light
The luminosity of the visible light of the welding arc is about 10,000 times greater than the luminosity that the naked eye can normally bear. When irradiated, the eyes are sore, blurred and can’t see clearly, and the long-term effect will cause vision loss, and the labor force can be lost in a short period of time. Often referred to as electric welding “dazzling”, gas welding flames also emit this light.
To sum up, the welding arc is an extremely strong radiation energy. After it is directly radiated or reflected to the unprotected parts of the human body, it will produce radiation and chemical pathological effects. The effects of radiation on unprotected visual organs are shown in Table Effects of arc light on visual organs.
