Requirements related to alarm equipment setting
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Gas leak alarm alarm point setting At present, various gas fuels such as natural gas, liquefied petroleum gas, and city gas are widely used in homes and industrial places. While they bring convenience to people's daily lives and industrial production, they are also accompanied by various safety hazards. A small amount of gas leakage will form a low concentration in the air, which will not cause fire or explosion accidents and will not cause much harm. However, if there is a lack of monitoring and the gas leakage is large or slowly accumulates, it will cause a higher concentration of flammable gases in the air. Once it reaches a certain level, there will be a risk of fire and explosion when encountering an open flame. This level is usually called the lower explosion limit (LEL) of the gas. Different flammable gases have different The lower explosion limit value, for example, for methane (natural gas), the lower explosion limit concentration is 5% of the content in the air. That is to say, if the methane content in the air is less than 5%, it will not cause fire and explosion accidents due to open flames and sparks. Once it reaches or exceeds 5%, such an atmosphere will cause a fire and explosion accident when encountering an open flame or cremation. In order to prevent accidents, gas monitoring and alarm systems with reliable quality and good debugging can indeed play an important role in the safety of gas use at home and industrial sites. The adjustment of the gas concentration alarm point is a key indicator of the quality of the combustible gas alarm system and whether it can effectively prevent fire and explosion accidents caused by combustible gas leakage. Usually, the alarm point of a combustible gas alarm is set within the range of 1%-25% of the lower explosion limit of the detected combustible gas. For example, for methane (natural gas), its lower explosion limit is 5%, that is, LEL=5%=50000ppm. The alarm point range of the corresponding natural gas alarm should be between 1%LEL – 25%LEL, that is, 0.05% (500ppm) – 1.25% (12500ppm), such as 3000ppm ； If you want to increase the safety factor, you can also set it to 1000ppm (0.01%). However, blindly pursuing high sensitivity and lowering the alarm set point will lead to a deterioration of the alarm's anti-interference ability. Non-dangerous trace leaks and changes in ambient temperature, humidity or atmosphere will cause false alarms of the alarm, thus reducing the accuracy and reliability of the alarm system. This is also what must be paid attention to when setting the alarm point. Refer to the figure below to see the correct setting range of the corresponding alarm points for different flammable gases.: CH4(LEL=5%=50000ppm) 0% 0 5% 0 50000ppm 1%LEL 25%LEL 100%LEL LPG(LEL=3%=30000ppm) 0% 0 3% 0 30000ppm 1%LEL 25%LEL 100%LEL H2 (LEL=4%=40000ppm) 0% 0 4% 0 40000ppm 1%LEL 25%LEL 100%LEL Different from the above, since artificial gas contains highly toxic carbon monoxide, a small amount of leakage without explosion risk can cause fatal poisoning accidents. Preventing poisoning is often the primary role of artificial gas alarms in monitoring leaks. The setting of alarm points should fully consider the carbon monoxide content of artificial gas, and set the alarm point in a concentration range equivalent to 100PPM-300PPM of carbon monoxide. * * Relevant standards require the setting of alarm points for combustible gas alarms: People's Republic of China * * Standard GB15322 "Combustible Gas Detector" should be the concentration setting value range: Applicable gas types Alarm concentration setting value range Combustible gas low limit 1%LEL~25%LEL High limit 50% Artificial gas (hydrogen) Low limit 125PPM-750PPM High limit 1250PPM Artificial gas (carbon monoxide) Low limit 50PPM-300PPM High limit 500PPM People's Republic of China Urban Construction Industry Standard CJ3057-1996 "Household Gas Leak Alarm" Response concentration setting value range: Alarm concentration technical requirements: Natural gas 0.1%~1% Liquefied petroleum gas 0.1%~0.5% Artificial gas with CO content ≤10% 0.04%~0.5% Artificial gas with CO content ≤10% 0.04%~0.25% Artificial gas with CO content ≤10% 0.025%~0.15%
How to choose a suitable toxic and harmful gas detector: For various production occasions and detection requirements, choosing the appropriate gas detector is something that everyone engaged in safety and health work must pay great attention to. Here we will introduce some specific situations for your reference. 1) Confirm the type and concentration range of gas to be detected: The types of gases encountered in each production department are different. When choosing a gas detector it is important to consider all possible scenarios. If methane and other less toxic alkanes are common, LEL detector is undoubtedly the most appropriate choice. This is not only because the LEL detector has a simple principle and is widely used, but it also has the characteristics of convenient maintenance and calibration. If toxic gases such as carbon monoxide and hydrogen sulfide are present, a specific gas detector must be selected first to ensure the safety of workers. If there are more organic toxic and harmful gases, considering the low concentration that may cause human poisoning, such as aromatic hydrocarbons, halogenated hydrocarbons, ammonia (amines), ethers, alcohols, lipids, etc., you should choose the photoionization detector introduced in the previous chapter, and absolutely do not use the LEL detector, because this may cause casualties. If the gas types cover the above categories of gases, choosing a composite gas detector may achieve twice the result with half the effort. 2) Determine the use occasion: Different industrial environments require different types of gas detectors. A) Fixed gas detector: This is a detector commonly used in industrial equipment and production processes. It can be installed at specific detection points to detect specific gas leaks. Fixed detectors are generally two-piece. The detection head composed of sensors and transmitters is installed as a whole at the detection site. The secondary instrument composed of circuits, power supplies and display alarm devices is installed as a whole in a safe place for easy monitoring. Its detection principle is the same as that mentioned in the previous section, but in terms of process and technology, it is more suitable for the characteristics of continuous and long-term stability required for fixed detection. They should also be selected according to the type and concentration of the gas on site. At the same time, attention should be paid to installing them in the parts where specific gases are most likely to leak. For example, the most effective height for sensor installation should be selected based on the specific gravity of the gas, etc. B) Portable gas detector: Because the portable instrument is easy to operate and small in size, it can be carried to different production areas. The electrochemical detector is powered by alkaline batteries and can be used continuously for 1,000 hours. ； New LEL detectors, PIDs and composite instruments use rechargeable batteries (some have used memoryless nickel-metal hydride or lithium-ion batteries), so that they can generally work continuously for nearly 12 hours. Therefore, such instruments are increasingly used in various factories and health departments. If this type of instrument is used as a safety alarm in an open place, such as an open workshop, you can use a body-worn diffusion gas detector because it can continuously, real-time and accurately display the concentration of toxic and harmful gases on site. Some of these newer instruments also come with vibrating alarm attachments to avoid inaudible audible alarms in noisy environments, and computer chips that record peak values, STEL (15-minute short-term exposure levels) and TWA (8-hour statistical weighted average) - to provide specific guidance for worker health and safety. If you enter a confined space, such as a reaction tank, storage tank or container, sewer or other underground pipeline, underground facility, agricultural closed granary, railway tank car, shipping cargo hold, tunnel and other workplaces, testing must be carried out before personnel enter, and the testing must be conducted outside the confined space. At this time, it is necessary to choose a multi-gas detector with a built-in sampling pump. Because the gas distribution and gas types in different parts (upper, middle, and lower) of the confined space are very different. for example: Generally speaking, the specific gravity of combustible gases is relatively light, and most of them are distributed in the upper part of the confined space. ； Carbon monoxide has the same specific gravity as air and is generally distributed in the middle of a confined space. ； Heavier gases such as hydrogen sulfide exist in the lower part of the confined space (as shown in the figure). At the same time, oxygen concentration is also one of the types that must be detected. In addition, if the possible volatilization and leakage of organic substances in the tank are considered, a detector that can detect organic gases is also needed. Therefore, a complete confined space gas detector should have a built-in pumping function to enable non-contact and segmented detection. ； With multi-gas detection function - to detect dangerous gases distributed in different spaces, including inorganic gases and organic gases ； With oxygen detection function - to prevent hypoxia or oxygen enrichment ； It is a portable instrument that is small in size and does not affect workers' work. Only in this way can the absolute safety of workers entering confined spaces be guaranteed. In addition, after entering a confined space, the gas components must be continuously detected to avoid changes in the concentration of volatile organic compounds or other toxic and harmful gases caused by personnel entry, sudden leaks, temperature changes, etc.