Hazard and detection of toxic and harmful gases in confined spaces
On a clear day, the workers opened the water meter cover and looked up the table. The danger occurred. What would you estimate? Yes, suffocation and death! Because after the event, the oxygen concentration at the bottom of the well is only 5%! Far below the safety limit. However, this watch has been conducting frequent sampling tests in the past and has never detected insufficient oxygen. what is the reason? A careful study found that the abnormal situation was caused by the powdery corpse of a mouse found at the bottom of the well, that is, the decomposition of this small organism consumes a large amount of oxygen, resulting in such a high degree of hypoxia.
This is a dangerous example of a typical confined space, and we enter into a confined space every day: telecommunications workers enter the cable trench, cleaners get down the sewer, maintenance workers drill into the tanker, and so on. Therefore, the danger of entering a confined space must also be taken seriously. So, which environments are confined spaces?
The Occupational Safety and Health Association (OSHA) specifies the procedure for entering a confined space in OSHA29CFR 1910.146 of April 1993. The confined space defined by it is:
§ The space is large enough for workers to enter the work. § Worker access is restricted. § Structural design is not suitable for workers to work continuously. Confined spaces should also have the following characteristics:
§ Contains, or may contain, hazardous gases (atmosphere). § § A recognized health hazard contains substances that may cause erosion. § Structures that can trap or suffocate people.
Examples of confined spaces exist in our daily work: various types of tanks and containers, sewers and other underground pipes, underground facilities, grain silos, railway tankers, ship cabins, tunnels, closed transport corridors, and more.
What are the dangers in a confined space? In fact, in addition to mechanical accidents such as falling and bumping, the biggest danger to the workers in confined spaces is the toxic and harmful gases. Research reports published by the National Institute for Occupational Safety and Health (NIOSH) and the National Institute for Occupational Safety and Health (OSHA) have shown that many deadly confined space accidents are related to the gas composition of the space in which they are located. These dangerous components may exist either before the workers enter the confined space or because of their activities in between. It can be said that the lack of detection of dangerous gases before and during the entry of workers into confined spaces is an important cause of most confined space accidents.
Dangerous gases in confined spaces can be broadly classified into three broad categories: oxygen levels (insufficient or excessive), flammable gases, and toxic gases . In many cases, carbon monoxide and hydrogen sulfide are considered to be the cause of death. But the reality may be more complicated. Some toxic gases are not common, but the damage they cause may be more deadly, especially for long-term physical damage and life threats to workers. For example, when cleaning benzene tanks, workers may not encounter the toxic and harmful gases ( oxygen, combustible gas, carbon monoxide, hydrogen sulfide ) mentioned above , but the residual benzene vapor in the tank is enough to cause workers to suffer short-term or long-term poisoning. The reaction ( immediate lethal dose of benzene is 500 ppm, that is, 0.05% by volume!).
Insufficient oxygen and insufficient oxygen in excess confined spaces are common situations. There are many reasons for insufficient oxygen, such as microbial behavior (such as the decomposition of rats mentioned at the beginning), replacement (other gases such as nitrogen, dry ice, etc. into the space to reduce the amount of air), combustion, oxidation (such as rust) and absorption and adsorption ( Such as wet activated carbon). Work behavior is also an important reason. The use of solvents, paints, detergents or heating can affect the oxygen content. Oxygen concentration detection in confined spaces takes into account two concentration points: too low a concentration of oxygen can cause asphyxiation; and excess of a constant oxygen may cause an unimaginable acceleration or increase in combustion or other chemical reactions. The concentration of oxygen in normal air is about 20.9%, and the other components (more than 78%) are mainly nitrogen, but also a small amount of water vapor, carbon dioxide and other trace gases .
Effects of hypoxia and oxygen enrichment on the human body Oxygen concentration (% by volume) Symptoms (at atmospheric pressure)
>23.5% Oxygen-rich, there is a strong explosion hazard
20.9% normal oxygen concentration
19.5% minimum allowable concentration of oxygen
15-19% reduces work efficiency and can cause head, lung and circulatory problems
10-12% shortness of breath, loss of judgment, purple lips
8-10% loss of intelligence, fainting, unconscious, pale, purple lips, nausea and vomiting
6-8% 8 minutes, 100% fatal, 6 minutes 50% fatal, 4-5 minutes can be cured after treatment
4-6% twitching within 40 seconds, respiratory arrest, death data may be due to individual health and physical fitness. Combustible gases and vapors encountered in confined spaces may come from several aspects. Leakage between underground pipes (between cable ducts and city gas pipes), residual inside containers, bacterial decomposition, work products, etc. Common combustible gases include: methane, natural gas, hydrogen, volatile organic compounds, and the like.
Whether in the confined space or after entering the work, we encounter a variety of flammable gases and vapors. When their concentration is sufficient, many substances' vapors and gases become flammable dangerous gases. If they encounter a fire source and provide a certain amount of energy, they will burn or explode. Sources of ignition in confined spaces may include: work activities that generate heat, fire tools, light sources, power tools, electronics, and even static electricity.
Combustion or explosion must meet four conditions: the gas must contain the right amount of oxygen, the right amount of gas (beyond the lower explosive limit of the combustible gas), the source of ignition, and sufficient molecular energy to maintain the fire chain reaction. These four conditions are generally referred to as "fire quadrilaterals." If any of these four is not or insufficient, combustion cannot occur, which is why the oxygen concentration must be detected when measuring explosive gases.
There may be many toxic gases in the toxic gas and vapor confined space, which may exist in the confined space and may be generated during the work. Sometimes, the short-term sudden evaporation of certain substances may also have some inexplicable substances, so the types of toxic gases in confined spaces may be various. Toxic substances in the air are generally classified according to their physiological effects on workers exposed to the site. Toxic substances can cause symptoms in people for two periods: acute or chronic. Therefore, it is necessary to establish the exposure level of various substances in the workplace to provide a basis for safety protection. There is a certain relationship between the working hours of workers and the concentration of toxic gases. Exceeding these prescribed exposure levels poses a great threat to the personal safety of workers.
The following are the exposure limits specified by some regulations and regulations:
The 8-hour time statistical weighted average (TWA) is the concentration of toxic gases allowed in the normal working environment.
The 15-minute time counts the short-term exposure level (STEL) of the weighted average, the concentration of toxic gases allowed by the worker's short-term work.
Immediate lethal dose (IDHL), the concentration of toxic gases that cause workers to die for a short time.
OSHA listed TWA, STEL, IDHL value of common toxic gases TWA STEL IDLH
Ammonia (NH3) 25.0ppm 35.0ppm 500 ppm
Carbon monoxide (CO) 25.0ppm -- 1500 ppm
Chlorine (Cl2) 0.5ppm 1.0ppm 30 ppm
Hydrogen cyanide (HCN) -- 4.7ppm 50 ppm
Hydrogen Sulfide (H2S) 10.0ppm 15.0 ppm 300 ppm
Nitric Oxide (NO) 25.0ppm -- 100 ppm
Sulfur dioxide (SO2) 2.0ppm 5.0ppm 100 ppm
Volatile Organic Compounds (VOC) 10.0ppm 25.0ppm Change with Substance The following gives the relationship between the concentration of hydrogen sulfide gas and human health:
Concentration (ppm) Symptom and effect time
10 Allowable exposure concentration
8 hours
50-100 mild eye and respiratory discomfort for 1 hour
100-500 significant eye respiratory discomfort 1 hour
500-700 Unconscious to death 1/2 - 1 hour
>1000 Unconscious, Death How to choose the right toxic and harmful gas detector within a few minutes:
In summary, in order to ensure the safety of workers, the gas components should be tested before entering any confined space, and in the non-contact case, the following sequence should be tested to ensure that there is sufficient oxygen concentration; There is no flammable gas; the concentration of toxic gases is below the exposure limit of OSHA. Toxic gases present in confined spaces include: hydrogen sulfide (H2S) and carbon monoxide (CO), as well as other toxic gases such as volatile organic compound gases and vapors (VOC).
For all kinds of different production occasions and testing requirements, the choice of a suitable gas detector is a must for every person engaged in safety and health work.
Because the type of gas encountered in each production department is different. All possible situations should be considered when selecting a gas detector. Correctly estimate the types of hazardous gases that may be encountered when workers enter confined spaces, taking into account various situations. For example, in urban utilities, anoxic and flammable gas detection is essential. Considering the possible presence of carbon monoxide and hydrogen sulfide, at least four toxic and harmful gases need to be detected; if it is in industrial applications, such as petrochemicals, The chemical industry also needs to consider the existence of volatile organic compounds (such as aromatics, halogenated hydrocarbons, etc.) that may be encountered during production and work. These substances may not explode even at low concentrations. But it will cause acute and chronic poisoning of workers. At this time, it is necessary to test these substances, such as using RAE's photoionization detector (PID) , and never use a combustible gas detector, as this may It can cause injury and death to people.
In view of the actual work, gas detection in a confined space must be carried out before personnel enter the confined space, and non-contact detection is performed outside the confined space. In this case, you must select a multi-gas detector with a built-in sampling pump. Because the gas distribution and gas type of different parts (upper, middle, and lower) in the confined space are very different. For example, in general, the proportion of combustible gases is relatively light, and most of them are distributed in the upper part of the confined space; while the specific gravity of carbon monoxide and air is similar, generally distributed in the middle of the confined space; like hydrogen sulfide and volatile organic compounds (VOC) The heavier gas is present in the lower part of the confined space. At this time, it is necessary to continuously detect the various layers in the space by the suction pump.
A complete confined space gas detector should be a portable multi-gas (composite) detector with built-in pumping function - so that it is easy to detect different types of dangerous gases in different spatial distributions in non-contact and sub-sections. It includes oxygen, inorganic gases and organic gases; it should also have real-time data acquisition and display, instant sound and light alarms; small size, does not affect the work of workers, etc., in order to ensure the absolute safety of workers entering the confined space. It should be noted that when selecting such detectors, it is best to select instruments with individual sensor functions to individually switch to prevent other sensor usage from being affected by one sensor damage. At the same time, in order to avoid the occurrence of blockage of the getter pump due to water ingress, it is safer to select an intelligent pump design with a pump stop alarm. After conducting non-contact testing and confirming that space safety can be entered, the inspector can issue an entry permit to allow the worker to enter the confined space to work. However, the gas detection work cannot be stopped, and the workers and outside waiters who enter it must Continuous detection of gas in the space. To avoid injury caused by changes in the concentration of toxic and harmful gases due to leakage, toxic gas release, temperature changes, etc., this process continues until the worker leaves the confined space.
In short, the detection of toxic and harmful gases is a powerful measure to ensure workers' safety in confined spaces. We need to choose the right gas detector according to the specific environment and the function required. Currently, the detectors available for us include fixed/portable, diffused/pumped, single/multi-gas, inorganic/organic gases, and many more. Only when a suitable gas detection instrument is selected can you do more with less, and prevent it from happening. It is recommended to use a limited space gas detection box and a chemical accident comprehensive detection box.
This is a dangerous example of a typical confined space, and we enter into a confined space every day: telecommunications workers enter the cable trench, cleaners get down the sewer, maintenance workers drill into the tanker, and so on. Therefore, the danger of entering a confined space must also be taken seriously. So, which environments are confined spaces?
The Occupational Safety and Health Association (OSHA) specifies the procedure for entering a confined space in OSHA29CFR 1910.146 of April 1993. The confined space defined by it is:
§ The space is large enough for workers to enter the work. § Worker access is restricted. § Structural design is not suitable for workers to work continuously. Confined spaces should also have the following characteristics:
§ Contains, or may contain, hazardous gases (atmosphere). § § A recognized health hazard contains substances that may cause erosion. § Structures that can trap or suffocate people.
Examples of confined spaces exist in our daily work: various types of tanks and containers, sewers and other underground pipes, underground facilities, grain silos, railway tankers, ship cabins, tunnels, closed transport corridors, and more.
What are the dangers in a confined space? In fact, in addition to mechanical accidents such as falling and bumping, the biggest danger to the workers in confined spaces is the toxic and harmful gases. Research reports published by the National Institute for Occupational Safety and Health (NIOSH) and the National Institute for Occupational Safety and Health (OSHA) have shown that many deadly confined space accidents are related to the gas composition of the space in which they are located. These dangerous components may exist either before the workers enter the confined space or because of their activities in between. It can be said that the lack of detection of dangerous gases before and during the entry of workers into confined spaces is an important cause of most confined space accidents.
Dangerous gases in confined spaces can be broadly classified into three broad categories: oxygen levels (insufficient or excessive), flammable gases, and toxic gases . In many cases, carbon monoxide and hydrogen sulfide are considered to be the cause of death. But the reality may be more complicated. Some toxic gases are not common, but the damage they cause may be more deadly, especially for long-term physical damage and life threats to workers. For example, when cleaning benzene tanks, workers may not encounter the toxic and harmful gases ( oxygen, combustible gas, carbon monoxide, hydrogen sulfide ) mentioned above , but the residual benzene vapor in the tank is enough to cause workers to suffer short-term or long-term poisoning. The reaction ( immediate lethal dose of benzene is 500 ppm, that is, 0.05% by volume!).
Insufficient oxygen and insufficient oxygen in excess confined spaces are common situations. There are many reasons for insufficient oxygen, such as microbial behavior (such as the decomposition of rats mentioned at the beginning), replacement (other gases such as nitrogen, dry ice, etc. into the space to reduce the amount of air), combustion, oxidation (such as rust) and absorption and adsorption ( Such as wet activated carbon). Work behavior is also an important reason. The use of solvents, paints, detergents or heating can affect the oxygen content. Oxygen concentration detection in confined spaces takes into account two concentration points: too low a concentration of oxygen can cause asphyxiation; and excess of a constant oxygen may cause an unimaginable acceleration or increase in combustion or other chemical reactions. The concentration of oxygen in normal air is about 20.9%, and the other components (more than 78%) are mainly nitrogen, but also a small amount of water vapor, carbon dioxide and other trace gases .
Effects of hypoxia and oxygen enrichment on the human body Oxygen concentration (% by volume) Symptoms (at atmospheric pressure)
>23.5% Oxygen-rich, there is a strong explosion hazard
20.9% normal oxygen concentration
19.5% minimum allowable concentration of oxygen
15-19% reduces work efficiency and can cause head, lung and circulatory problems
10-12% shortness of breath, loss of judgment, purple lips
8-10% loss of intelligence, fainting, unconscious, pale, purple lips, nausea and vomiting
6-8% 8 minutes, 100% fatal, 6 minutes 50% fatal, 4-5 minutes can be cured after treatment
4-6% twitching within 40 seconds, respiratory arrest, death data may be due to individual health and physical fitness. Combustible gases and vapors encountered in confined spaces may come from several aspects. Leakage between underground pipes (between cable ducts and city gas pipes), residual inside containers, bacterial decomposition, work products, etc. Common combustible gases include: methane, natural gas, hydrogen, volatile organic compounds, and the like.
Whether in the confined space or after entering the work, we encounter a variety of flammable gases and vapors. When their concentration is sufficient, many substances' vapors and gases become flammable dangerous gases. If they encounter a fire source and provide a certain amount of energy, they will burn or explode. Sources of ignition in confined spaces may include: work activities that generate heat, fire tools, light sources, power tools, electronics, and even static electricity.
Combustion or explosion must meet four conditions: the gas must contain the right amount of oxygen, the right amount of gas (beyond the lower explosive limit of the combustible gas), the source of ignition, and sufficient molecular energy to maintain the fire chain reaction. These four conditions are generally referred to as "fire quadrilaterals." If any of these four is not or insufficient, combustion cannot occur, which is why the oxygen concentration must be detected when measuring explosive gases.
There may be many toxic gases in the toxic gas and vapor confined space, which may exist in the confined space and may be generated during the work. Sometimes, the short-term sudden evaporation of certain substances may also have some inexplicable substances, so the types of toxic gases in confined spaces may be various. Toxic substances in the air are generally classified according to their physiological effects on workers exposed to the site. Toxic substances can cause symptoms in people for two periods: acute or chronic. Therefore, it is necessary to establish the exposure level of various substances in the workplace to provide a basis for safety protection. There is a certain relationship between the working hours of workers and the concentration of toxic gases. Exceeding these prescribed exposure levels poses a great threat to the personal safety of workers.
The following are the exposure limits specified by some regulations and regulations:
The 8-hour time statistical weighted average (TWA) is the concentration of toxic gases allowed in the normal working environment.
The 15-minute time counts the short-term exposure level (STEL) of the weighted average, the concentration of toxic gases allowed by the worker's short-term work.
Immediate lethal dose (IDHL), the concentration of toxic gases that cause workers to die for a short time.
OSHA listed TWA, STEL, IDHL value of common toxic gases TWA STEL IDLH
Ammonia (NH3) 25.0ppm 35.0ppm 500 ppm
Carbon monoxide (CO) 25.0ppm -- 1500 ppm
Chlorine (Cl2) 0.5ppm 1.0ppm 30 ppm
Hydrogen cyanide (HCN) -- 4.7ppm 50 ppm
Hydrogen Sulfide (H2S) 10.0ppm 15.0 ppm 300 ppm
Nitric Oxide (NO) 25.0ppm -- 100 ppm
Sulfur dioxide (SO2) 2.0ppm 5.0ppm 100 ppm
Volatile Organic Compounds (VOC) 10.0ppm 25.0ppm Change with Substance The following gives the relationship between the concentration of hydrogen sulfide gas and human health:
Concentration (ppm) Symptom and effect time
10 Allowable exposure concentration
8 hours
50-100 mild eye and respiratory discomfort for 1 hour
100-500 significant eye respiratory discomfort 1 hour
500-700 Unconscious to death 1/2 - 1 hour
>1000 Unconscious, Death How to choose the right toxic and harmful gas detector within a few minutes:
In summary, in order to ensure the safety of workers, the gas components should be tested before entering any confined space, and in the non-contact case, the following sequence should be tested to ensure that there is sufficient oxygen concentration; There is no flammable gas; the concentration of toxic gases is below the exposure limit of OSHA. Toxic gases present in confined spaces include: hydrogen sulfide (H2S) and carbon monoxide (CO), as well as other toxic gases such as volatile organic compound gases and vapors (VOC).
For all kinds of different production occasions and testing requirements, the choice of a suitable gas detector is a must for every person engaged in safety and health work.
Because the type of gas encountered in each production department is different. All possible situations should be considered when selecting a gas detector. Correctly estimate the types of hazardous gases that may be encountered when workers enter confined spaces, taking into account various situations. For example, in urban utilities, anoxic and flammable gas detection is essential. Considering the possible presence of carbon monoxide and hydrogen sulfide, at least four toxic and harmful gases need to be detected; if it is in industrial applications, such as petrochemicals, The chemical industry also needs to consider the existence of volatile organic compounds (such as aromatics, halogenated hydrocarbons, etc.) that may be encountered during production and work. These substances may not explode even at low concentrations. But it will cause acute and chronic poisoning of workers. At this time, it is necessary to test these substances, such as using RAE's photoionization detector (PID) , and never use a combustible gas detector, as this may It can cause injury and death to people.
In view of the actual work, gas detection in a confined space must be carried out before personnel enter the confined space, and non-contact detection is performed outside the confined space. In this case, you must select a multi-gas detector with a built-in sampling pump. Because the gas distribution and gas type of different parts (upper, middle, and lower) in the confined space are very different. For example, in general, the proportion of combustible gases is relatively light, and most of them are distributed in the upper part of the confined space; while the specific gravity of carbon monoxide and air is similar, generally distributed in the middle of the confined space; like hydrogen sulfide and volatile organic compounds (VOC) The heavier gas is present in the lower part of the confined space. At this time, it is necessary to continuously detect the various layers in the space by the suction pump.
A complete confined space gas detector should be a portable multi-gas (composite) detector with built-in pumping function - so that it is easy to detect different types of dangerous gases in different spatial distributions in non-contact and sub-sections. It includes oxygen, inorganic gases and organic gases; it should also have real-time data acquisition and display, instant sound and light alarms; small size, does not affect the work of workers, etc., in order to ensure the absolute safety of workers entering the confined space. It should be noted that when selecting such detectors, it is best to select instruments with individual sensor functions to individually switch to prevent other sensor usage from being affected by one sensor damage. At the same time, in order to avoid the occurrence of blockage of the getter pump due to water ingress, it is safer to select an intelligent pump design with a pump stop alarm. After conducting non-contact testing and confirming that space safety can be entered, the inspector can issue an entry permit to allow the worker to enter the confined space to work. However, the gas detection work cannot be stopped, and the workers and outside waiters who enter it must Continuous detection of gas in the space. To avoid injury caused by changes in the concentration of toxic and harmful gases due to leakage, toxic gas release, temperature changes, etc., this process continues until the worker leaves the confined space.
In short, the detection of toxic and harmful gases is a powerful measure to ensure workers' safety in confined spaces. We need to choose the right gas detector according to the specific environment and the function required. Currently, the detectors available for us include fixed/portable, diffused/pumped, single/multi-gas, inorganic/organic gases, and many more. Only when a suitable gas detection instrument is selected can you do more with less, and prevent it from happening. It is recommended to use a limited space gas detection box and a chemical accident comprehensive detection box.
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