How to identify potential toxic gases before working in confined spaces
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Every year, there are some accidents that occur due to work in confined spaces, resulting in serious casualties. The underlying reason is often the failure to identify potential toxic substances in confined spaces, the lack of testing to determine the concentration of such toxic substances, and entering these confined spaces recklessly. Of course, in the chemical industry, there are specific technical standards to guide work in confined spaces, and as a result, there are fewer accidents related to such work compared to other industries, though they do occur. Sometimes, it can be difficult to identify toxic gases in confined spaces. For example, not long ago, inspection experts asked why hydrogen sulfide testing was not conducted for all work in confined spaces We say that these operations in confined spaces involve production equipment such as storage tanks or towers, whose internal media are well-defined; there is no hydrogen sulfide present, and it’s especially the case with non-toxic media, as none of them involve toxic gases. I’m not sure if this statement is correct? Of course, not all inspectors agree. It was also pointed out that there is an area where excavation work for the expansion structure’s foundation pit and rebar installation are being carried out; shouldn’t hydrogen sulfide testing be conducted there? I would like to ask for everyone’s advice here regarding the identification of toxic gases in confined space operation teams
Let me share my personal understanding. Firstly, regarding occupational hazard factors, it is essential to have appropriate testing methods for those hazards that may exist in confined spaces, and there is nothing wrong with that. Meeting the requirements of GBZ2 is sufficient. There are two possible interpretations regarding the expert’s preference for hydrogen sulfide: 1. The expert may come from a petrochemical company or have conducted many inspections in this field; since most reactions involving hydrocarbons take place in the presence of hydrogen, whenever sulfides are generated at any stage, it is very likely that a certain amount of hydrogen sulfide will be produced as well ； 2. In municipal and rural areas, poisoning incidents occur frequently during the construction of sewer systems, manholes, and fertilizer fermentation facilities; the toxic gases involved are mainly hydrogen sulfide and carbon monoxide, and experts may be considering this factor. If it can be confirmed that there is absolutely no possibility of the presence of hydrogen sulfide or similar substances in the soil environment near the confined space in the facility, then it can be argued that hydrogen sulfide need not be considered in the hazard assessment; however, if there are sewage facilities such as sewage tanks in the vicinity, it is recommended that this factor be included in the hazard assessment.
Thank you for your enthusiastic explanation! The doubts raised during the expert examination process should be given due consideration. Indeed, the goal is to identify potential hazards as clearly as possible when working in different confined spaces, in order to avoid mistakes. For general production equipment, it is relatively easy to locate; the main consideration is the connection to the medium or related equipment. As for environmental pits, pools, and the like, things get a bit more complicated; is the area that needs to be monitored larger? Should hydrogen sulfide, carbon monoxide, and methane be the main considerations?
That’s what my previous reply meant – the identification of occupational hazard factors in the anti-toxic and anti-explosion areas of the factory facility is a process that has been thoroughly developed and refined through proper design. On the contrary, there are often weaknesses in the process of environmental assessment; for example, when constructing or excavating in areas designated for industrial use, if sewage storage tanks or pipelines are located in close proximity, it is necessary to give more careful consideration to ensuring proper sealing to prevent leaks. This is especially true in areas within the industrial complex or where the groundwater level is high, as it’s impossible to be certain about what lies beneath the ground.
Before working in confined spaces, the identification of potential toxic gases is primarily carried out using oxygen detectors; operations are not permitted if the oxygen level is not within the required range.
The space is small; insufficient oxygen supply can lead to suffocation, while too much oxygen can act as an accelerant inside the container. It is essential to know the appropriate circumstances and to implement stronger protective measures to ensure safety
1. First, it is necessary to identify the substances that need to be tested in confined spaces. If there is no risk of hydrogen sulfide throughout the plant, testing for its concentration may merely serve as a precautionary measure. It is essential to analyze and test all the toxic and harmful substances present in such confined spaces； 2. Sampling: In my opinion, sampling is a very crucial factor, but many organizations overlook this aspect, believing that as long as samples are taken, the requirements are met. This mistake often leads to more serious accidents. Sampling must be carried out by process engineers, operators, and laboratory technicians together, at the locations where toxic and harmful gases might be present. The sampling device should be capable of taking samples from deep areas and from various directions; it’s absolutely unacceptable to simply take samples carelessly at the entrance of a hole or pit and call it a day… 3. Portable monitoring instruments are highly advisable, as they allow for real-time detection of the gas components in the working environment, issuing alerts immediately in case of any abnormalities. That’s why I always bring portable detectors when entering confined spaces – it’s a safe, effective, and reliable solution.
I fully agree with the issues reflected in Article 2! In chemical plants, when poisoning or asphyxiation accidents occur during work in confined spaces, inadequate testing or perfunctory inspections are the main contributing factors
A hazardous atmosphere refers to an environment in occupational activities that may cause death, loss of consciousness, loss of the ability to escape or save oneself, injury, or acute poisoning, including one or more of the following conditions: concentrations of flammable gases, vapors, and aerosols exceeding 10% of the lower explosive limit (LEL)； The concentration of explosive dust in the air reaches or exceeds the lower explosion limit ； The oxygen content in the air is below 18% or above 22% ； The concentration of harmful substances in the air exceeds the occupational exposure limits ； Any other environmental conditions in which the concentration of harmful substances exceeds levels that pose an immediate threat to life or health (IDLHs).
Before entering a confined space to work, the person in charge of the task must check for any harmful gases present inside. This can be done using instruments or by testing with small animals; however, it is prohibited to use lit matches or fuses to investigate the space, in order to prevent explosions. Where there is a possibility of oxygen deficiency, excessive oxygen levels, toxic or harmful gases, flammable or explosive gases, dust, etc., gas testing should be conducted in advance, with the testing time and results recorded. If the work is interrupted, gas testing must be performed again before resuming. Gas monitoring should be conducted when the gas environment may change during entry into confined spaces. For gas monitoring, continuous monitoring should be preferred; if intermittent monitoring is used, the interval should not exceed 2 hours. Continuous monitoring instruments should be installed near the working area, and in a location where supervisors and entrants can see or hear them. Only personnel with the appropriate qualifications may take samples and conduct tests. The detection instruments used to monitor gases in confined spaces must be within their calibration validity period. Before and after each use, check whether the testing instrument is in normal working condition. The sampling should be representative, with special attention paid to the areas where the personnel may work. The sample should include samples from the top, middle, and bottom of the space. The order of testing should be oxygen content, flammable and explosive gases, and toxic and harmful gases. Any gas purging should be stopped during sampling to reduce the risks for the sample collectors and ensure that the sampled gas corresponds to the actual environment. When inspectors are unable to conduct sufficient sampling outside the confined space, they need to enter the space to carry out initial sampling, and must obtain permission to enter the confined space in accordance with special control procedures. The oxygen concentration inside and outside the confined space, as shown by the test results, should be identical. If there is a discrepancy, the cause of the deviation should be identified before granting access to the space, and the oxygen concentration should be maintained between 19.5% and 23.5% (taking into account the errors of the measuring instruments). Whether there is welding, hammering, etc. or not, the concentration of flammable and explosive gas or liquid vapors in confined spaces must meet the following criteria: when the lower explosion limit of the combustible gas is greater than 4%, a detection value of less than 0.5% is considered acceptable ； When the lower explosion limit of the combustible gas is less than 4%, analytical test results of less than 0.2% are considered acceptable. The concentration of toxic and harmful substances in confined spaces must not exceed the limits specified as the \"maximum allowable concentration of toxic substances in workshop air\" in **(or the local area). If any item fails the inspection, entry is prohibited or work must be stopped immediately. If a confined space contains flammable and explosive gases, liquids, or solids, as well as toxic and harmful substances, it must be cleaned or purified. Methods of cleaning include emptying the space, sweeping (such as flushing, steaming, washing, and rinsing), neutralizing, and replacing the hazardous substances.
Gas detection is the most effective method.