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Detection Technology Of Toxic And Harmful Gases in Confined Space

2021-6-30      View:

OSHA defined the procedure for entering the confined space in osha29cfr1910.146 in April 1993. It defines the confined space as follows:

  • The space is large enough for workers to enter the work

  • Workers' access is restricted

  • The structure design is not suitable for workers to work continuously in them, while the confined space should also have the following characteristics:

  • Containing, or may contain, dangerous gases (atmosphere) present

  • Contains substances that may cause erosion

  • Have a structure that can cause the entry personnel to fall into or suffocate

  • Have recognized health hazards

Examples of confined spaces are everywhere in our daily work: tanks and containers, sewers and other underground pipelines, underground facilities, grain silos, railway tankers, ship tanks, tunnels, closed transport channels, etc.

What are the dangers in the confined space? In fact, in addition to mechanical accidents such as falling and bumping, the greatest danger to workers in confined spaces is the toxic and harmful gas. The research reports published by NIOSH and OSHA show that many fatal accidents in confined space are related to the gas composition in the space. These dangerous components may exist before workers enter the confined space or may be formed by their activities in the meantime. It can be said that the lack of detection of dangerous gas before and during the work of workers is an important reason for most accidents in confined spaces.

The dangerous gases in confined space can be divided into three categories: oxygen level (insufficient or excessive), combustible gas, and toxic gas. In many cases, carbon monoxide and hydrogen sulfide are considered to be the main causes of human death. But the reality may be more complex, and some toxic gases are not common, but the hazards they cause may be more lethal, especially for workers with long-term physical damage and life threat. For example, workers may not encounter the toxic and harmful gases (oxygen, combustible gas, carbon monoxide, hydrogen sulfide) mentioned above when cleaning the benzene tank, but the residual benzene vapor in the tank can be enough to cause the workers' short-term or long-term toxic reaction (the immediate lethal amount of benzene is 500ppm, that is, 0.05% volume ratio!).

Oxygen deficiency and excess

Oxygen deficiency in confined space is a frequent situation. There are many reasons for oxygen deficiency, such as microbial behavior (such as decomposition of rats mentioned at the beginning), replacement (other gases such as nitrogen, dry ice, and other entering space to reduce air volume), combustion, oxidation (such as rust), absorption and adsorption (such as wet activated carbon). Working behavior is also an important reason. The use of solvents, coatings, cleaners, or heating can affect the oxygen content. Two concentration points should be considered in the detection of oxygen concentration in confined space: too low oxygen concentration will cause asphyxia; Oxygen that exceeds a constant may cause unimaginable acceleration or improvement of combustion or other chemical reactions. The concentration of oxygen in normal air is about 20.9%, and other components (over 78%) are mainly nitrogen, and also include a small amount of water vapor, carbon dioxide, and other trace gases.

The effect of hypoxia and oxygen enrichment on the human body

Oxygen concentration (% volume) symptom (under atmospheric pressure)

>23.5% oxygen-rich, with strong explosion risk

20.9% oxygen concentration is normal

19.5% oxygen minimum allowable concentration

15-19% reduces efficiency and can cause head, lung, and circulatory system problems

10-12% shortness of breath, loss of judgment, purple lips

8-10% of the patients suffering from mental loss, fainting, unconscious, pale face, purple lips, nausea, and vomiting

6-8% 8 minutes, 100% fatal, 6 minutes 50% fatal, 4-5 minutes can be cured after treatment

Convulsions, respiratory arrest, death in 4-6% 40 seconds

The above data may vary depending on the health and physical condition of the individual

Combustible gas and steam

Combustible gases and vapors encountered in confined spaces may come from several aspects. Leakage between underground pipelines (between cable and urban gas pipelines), residual inside the container, bacterial decomposition, work products, etc. Common combustible gases include methane, natural gas, hydrogen, volatile organic compounds, etc.

We will encounter a variety of combustible gases and vapors, both when we enter the confined space and after entering the work. When their concentration is enough, many substances' vapor and gas become combustible dangerous gas. If they encounter a fire source and provide certain energy, they will burn or explode. Sources of fire in confined spaces may include work activities that generate heat, fire tools, light sources, power tools, electronic instruments, and even static electricity.

There are four conditions for combustion or explosion: the gas must contain the proper amount of oxygen, a proper amount of gas (exceeding the explosion limit of combustible gas), a fire source, and enough molecular energy to maintain the fire chain reaction. These four conditions are generally referred to as "five quadrilateral". If any of these four is not or is insufficient, combustion cannot occur, which is why oxygen concentration must be detected when measuring explosive gas.

Toxic gases and vapors

There may be many toxic gases in the confined space. It can be existing in a confined space or produced during work. Sometimes, some substances will appear in the short-term sudden volatilization of some substances, so the types of toxic gases in confined space may be various. The toxic substances in the air are generally classified according to their physiological effects on workers exposed to the site. Toxic substances can be used for two time periods of symptoms: acute or chronic. Therefore, it is necessary to establish the exposure level of various substances in the workplace to provide the basis for safety protection. The working hours of workers are related to the concentration of toxic gases, and exceeding the exposure level of these regulations will pose a great threat to the personal safety of workers.

The following are the exposure limits set out in some regulations and regulations:

The weighted mean value (TWA) of 8 hours is the allowable concentration of toxic gas in the normal working environment.

The short-term exposure level (STEL) of weighted average was statistically analyzed for 15 minutes, and the concentration of toxic gas allowed by workers working in short term was analyzed.

The concentration of toxic gas causes the workers to die for a short time.

TWA, STEL, idhl values of common toxic gases listed by OSHA


Ammonia (NH3) 25.0ppm35.0ppm 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

No 25.0ppm - 100 ppm

SO2 2.0ppm 5.0ppm 100 ppm

VOC 10.0ppm 25.0ppm with substance

The relationship between the concentration of hydrogen sulfide gas and human health is given below:

Symptoms and effects of concentration (PPM)


10 allowable exposure concentration

8 hours

50-100 mild eye and respiratory discomfort for 1 hour

100-500 significant eye respiratory discomfort for 1 hour

500-700 unconscious death 1 / 2 - 1 hour

>1000 unconscious, dead

In a few minutes

How to select the appropriate toxic and harmful gas detector:

gas detector

In conclusion, to ensure the safety of workers, before entering any confined space, the gas composition in the gas shall be tested, and the detection shall be conducted in the following order under non-contact conditions, and sufficient oxygen concentration shall be ensured first; No combustible gas; The concentration of toxic gas is lower than the exposure limit of OSHA. Toxic gases in confined spaces include hydrogen sulfide (H2S) and carbon monoxide (CO), and other toxic gases, such as volatile organic compounds and vapors (VOCs).

For different production occasions and testing requirements, it is necessary for every person engaged in safety and health to choose the appropriate gas detector.

Because the gas types encountered by each production department are different. All possible situations should be taken into account when selecting the gas detector. The types of dangerous gases that may be encountered when workers enter the confined space shall be correctly estimated, taking into account various situations. For example, in urban utilities, oxygen deficiency and gas detection are 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 situations, such as petrochemical and chemical industries, the existence of volatile organic compounds (such as aromatic and halogenated hydrocarbons) may be considered in the production process and working process. Even if the concentration is very low, there may be no explosion risk, but it will cause acute and chronic poisoning of workers, These substances need to be tested at this time, such as using the Rae photoionization detector (PID), etc., and never use the gas detector to cope with it, because this may cause injury and death of personnel.

Considering the actual work, gas detection in the confined space must be carried out before the personnel enters the confined space, and the non-contact detection shall be conducted outside the confined space. At this point, a multi-gas detector with a built-in sampling pump must be selected. Because the distribution of gas and the gas types of different parts (upper, middle, lower) in the confined space are very different. For example the proportion of combustible gas, in general, is relatively light, and most of them are distributed in the upper part of the confined space; Carbon monoxide and air are the same proportion, generally distributed in the middle of the confined space; Heavy gases such as hydrogen sulfide and VOC exist in the lower part of the confined space. At this time, it is necessary to continuously detect all classes of the space through the suction pump outside.

A complete gas detector in confined space should be a portable multi-gas (composite) detector with built-in pumping function - so that it can easily detect various dangerous gases distributed in different spaces, including oxygen, inorganic gas and organic gas in non-contact and partial position; At the same time, it should also have real-time data acquisition and display, and instantaneous sound and light alarm; Small size, does not affect the characteristics of workers, only in this way can ensure the absolute safety of the staff entering the confined space. It is important to note that when selecting such a detector, it is best to select an instrument with the function of switching sensors' > sensors separately to prevent damage from affecting other sensors due to one sensor damage. Meanwhile, to avoid the blockage of the suction pump due to water inflow, the intelligent pump with stop pump alarm should be selected to be safe. After non-contact testing and confirming that the space is safe to enter, the inspectors can issue access permission to allow workers to enter the confined space for work. However, gas detection cannot be stopped. The workers entering the room and the outside watchers must also carry out continuous detection of the gas in the space. To avoid the change of toxic and harmful gas concentration due to leakage, gas release and temperature change, the process shall continue until the workers leave the confined space.

In short, the detection of toxic and harmful gases is a powerful measure to ensure the safety of workers entering the confined space. We should choose the appropriate gas detector according to the specific use environment and the required functions. At present, the detector available for our choice includes fixed / portable, diffusion/pump suction, single gas / multi-gas, inorganic gas / organic gas and other diverse combinations. Only by selecting the appropriate gas detection instrument can we really achieve half the effort and prevent the accident. It is recommended to use the limited space gas detection box and the chemical accident comprehensive detection box