With the rapid development of the petrochemical industry, the application range of flammable, explosive, toxic and harmful gases has also been expanded and increased. In the course of production, transportation and use of these chemical dangerous goods, in the event of a leak or other accident, it will cause serious accidents such as poisoning, death, fire and explosion, and seriously endanger people’s lives and property.
In recent years, sudden accidents of chemical hazards, with the acceleration of production rhythms and the increasing frequency of production activities, have greatly increased the chance of accidents. The occurrence of a large number of toxic and harmful gases or the highly volatile toxic and hazardous chemicals that are suddenly leaked and spread causes large-scale pollution, and sudden incidents that endanger people have occurred. Therefore, strengthening the capability of rapid emergency detection of sudden chemical accidents is an important part of whether the firefighting force can succeed in the disposal of chemical accidents.
The types of sudden chemical accidents that the fire brigade responded to included: First, chemical unit leaks, fires, and explosions; second, chemical accidents; and third, chemical terrorist incidents.
Once a chemical dangerous goods gas leakage accident occurs, corresponding measures must be taken as soon as possible to dispose of it, and the accidental loss should be minimized. The timely and reliable detection of certain gas contents in the air, obtaining first-hand accurate data, adopting effective and correct disposal methods, and reducing the leakage and causing the accident to expand are the necessary conditions for avoiding major property and casualties.
With the development of science and technology, various kinds of gas detectors came into being, which brought convenience and safety to the handling of sudden chemical events. Therefore, it is very important to fully understand, select, and correctly apply these gas detectors.
1 Kind of gas detector
Gas detectors are also called gas detectors. Mainly used in chemical production such as gas, liquefied petroleum gas, natural gas units and oil and gas fields, mine mining detection and alarm. The fire emergency department uses this type of hand-held mobile detection instrument as a basis for disposal, evacuation, and decontamination in emergency events to ensure that it can correctly and quickly dispose of and ensure the safety of firefighters and people.
A wide variety of gas detectors can be divided into the following seven categories based on the sensor principle and technical characteristics:
( 1 ) Semiconductor type ( surface control type, resistance control type, non-resistance type )
This detector sensor mainly uses semiconductor gas-sensitive materials. Semiconductor gas sensors have the advantages of high sensitivity and fast response. It is currently one of the largest and most widely used sensors in the world. According to the detection method of gas sensitive features, it can be divided into two types: surface control type, resistive type and non-resistive type.
The change in surface resistance depends on the exchange of electrons between the adsorbed gas and the semiconductor material on the surface. A gas with high electronic compatibility, such as oxygen and NO2 in the air, receives electrons from a semiconductor material and adsorbs negative charges. As a result , the conductive electrons in the surface space charge region of the N -type semiconductor material are reduced and the surface conductance is reduced. This puts the device in a high-impedance state. Once the device is in contact with the gas to be measured, it reacts with the adsorbed oxygen, releasing several electrons bound by oxygen, increasing the surface conductance of the sensitive membrane and reducing the resistance of the device. These devices are currently commercialized with gas sensors such as SnO2 and ZnO .
Resistive semiconductor gas sensors work by detecting changes in gas sensors with gas content. Most of the metal oxides as sensitive materials are composed of non-stoichiometric components. When the flammable gas is contacted, the resistance of the sensitive body is changed by changing its internal structural composition (lattice defects). With the research and development of new materials, the characteristics and application of resistive semiconductor gas sensors have been greatly improved. For example, the concentration of NH3 detected is in the range of 5ppm-50ppmppm , and the detection of CO gas sensor up to 200 ppm is very sensitive.
Non-resistive semiconductor gas sensors operate on the principle that the current or voltage of the gas sensor changes with the gas content. There are MOS diode and junction diode types, and field effect tube gas sensors. The diode gas sensor is made by using some gases to be adsorbed at the interface between the metal and the semiconductor and affecting the band gap of the semiconductor or the work function of the metal, so that the diode rectification characteristics change. Diodes such as Pd/Ti and Pd/ZnO can be used for H2 detection. The detection gases are mostly combustible gases such as hydrogen and silane. The CO2 concentration can be measured in the range of 0.05%~2% . The advantage is good selectivity, CO , CH4 , H2 and other interference, humidity does not interfere.
( 2 ) Solid Electrolyte Type
The solid electrolyte gas sensor uses a solid electrolyte gas sensitive material as a gas sensor. The principle is that the gas generates ions through these gas-sensitive materials, thus forming electromotive force, measuring the electromotive force value, thereby achieving the purpose of measuring the gas concentration. The sensor has high conductivity, good sensitivity and selectivity, and has been widely used in petrochemical, environmental protection, mining and other fields. Second only to metal oxide semiconductor gas sensors. Such as YS T-Au- WO3 gas sensitive material measurement H2S , H + 4Ca CO3 measurement NH3 and so on.
( 3 ) Contact Burning
Can be divided into two kinds of direct contact combustion and catalytic contact combustion. The working principle is: when the gas sensitive material is energized, the flammable gas is oxidized and burned or oxidized and burned under the action of the catalyst (flameless combustion), the temperature is increased, the resistance of the component is increased, the balance of the bridge is broken, and the resistance thereof The value changes, generates a slight voltage difference signal, and is converted by D , which is measured and converted into a gas concentration value. The lower explosion limit is generally used as the full scale. This sensor can only measure combustible gases and is insensitive to non-flammable gases. For example, a Pt wire coated with a sensor made of an active catalyst has broad-spectrum characteristics, that is, it can detect various combustible gases. The contact combustion type gas sensor is very stable at ambient temperature and can detect most of the flammable gases at the lower explosion limit. At present, it is widely used for the monitoring and alarm of flammable gases in petrochemical plants, mine tunnels, etc.
( 4 ) Electrochemical type ( potential electrolysis type, Gafanni battery type, ion electrode type )
Electrochemical gas sensors include constant-potential electrolysis, Gavarni cell, and ion electrode types. Fixed potential electrolysis gas sensor, also known as control potential electrolysis gas sensor. It is composed of a working electrode, an auxiliary electrode and a reference electrode, and a gas-permeable separator made of PTFE. Between the working electrode and the auxiliary electrode, the reference electrode is filled with an electrolyte, the potential of the sensor working electrode is controlled by a potentiostat so that the potential of the reference electrode is kept constant, and the gas molecules to be measured reach the surface of the sensitive electrode through the gas permeable membrane. Under the catalysis of the porous noble metal, an electrochemical reaction (oxidation reaction) takes place; at the same time, oxygen is reduced at the auxiliary electrode. The current generated by this redox reaction is controlled by the diffusion process. The diffusion process is related to the concentration of the gas to be measured. As long as the diffusion current generated on the sensitive electrode is measured, the concentration of the gas to be measured can be determined. After a certain voltage is applied between the sensing electrode and the auxiliary electrode, an oxidation-reduction reaction occurs between the gas and the working electrode in the electrolyte, and the equilibrium potential of the electrode changes, and the value of the change is proportional to the gas concentration. The gas is electrolyzed. If the applied voltage is changed, the redox reaction proceeds selectively. Therefore, the gas can be detected at regular intervals. Electrochemical sensors include: carbon monoxide, hydrogen sulfide, sulfur dioxide, nitric oxide, nitrogen dioxide, ammonia, chlorine, hydrogen cyanide, ethylene oxide, hydrogen chloride, and the like. Oxygen detectors are also a type of electrochemical sensor. Due to the relatively low cost of electrochemical type and catalytic combustion type measuring heads, the response to leakage is rapid and continuous detection, so the benefits are higher. The different components contained in different electrochemical gas sensors determine that it reacts with the corresponding toxic gas.
The Gafanni battery type gas sensor consists of a separator, a lead electrode (positive), an electrolyte, and a platinum electrode (negative) to form a Gafanni battery. O to the oxygen sensor, for example, when the measured gas diffuses to reach the negative electrode surface by a PTFE diaphragm, a reduction reaction can occur. It is reduced to OH- ion at the platinum cathode , the lead at the anode is oxidized to lead hydroxide, and the current is generated in the solution. The current flowing through the external circuit at this time is proportional to the speed of oxygen through the PTFE membrane. The partial pressure of oxygen at the cathode is almost zero, and the oxygen transmission rate is proportional to the external oxygen partial pressure. When the oxygen passes through the electrolyte, an oxidation-reduction reaction takes place between the anode and the cathode, the anode metal is ionized, electrons are released, and an electric current is generated. The size of the current is proportional to the amount of oxygen.
The ion electrode type gas sensor is composed of an electrolyte, a fixed reference electrode, and a pH electrode. Through the gas permeable membrane, the gas to be measured is equilibrated with the outside and chemical equilibrium is achieved in the electrolyte. The measured gas uses CO2 as an example. According to the law of mass action, the HCO3 concentration must be proportional to the H+ concentration and the CO2 partial pressure within the set range . The CO2 concentration can be known from the pH value .
The appropriate combination of electrolyte and electrode can detect a variety of gases. Sensors such as NH3 , SO2 , NO2 ( pH electrode) HCN ( Ag electrode), and halogen (halide electrode) have been put into practical use.
( 5 ) Infrared
The principle of the infrared sensor is to generate infrared light through an infrared generator, pass through a gas chamber filled with sample gas, and then be received by a dedicated receiver of various gases. According to the use of different elements of a particular wavelength absorption principle of manufacturing.
Infrared sensors are the most accurate gas technology in common applications. They have good sensitivity and extremely low false alarm rates. No spare parts are consumed, so post-maintenance costs are much lower than other technologies; there is a significant time lag between gas leaks and detection, but the price is expensive.
Due to the limitation of infrared wavelengths, it is only applicable to hydrocarbons with carbon-hydrogen bonds, and cannot detect CS 2 , H2 , CO , NH3, etc., and hydrocarbons such as acetylene and benzene cannot be detected. It is therefore more suitable for long-chain hydrocarbons than catalytic combustion sensors. Although its initial investment is expensive, the overall price is lower than catalytic combustion technology.
( 6 ) Photoionization
Photoionization sensors use ultraviolet light to ionize gas molecules and are used to detect volatile organic compounds.
The main detection features of PID gas analyzer are:
I detection response is fast, PID response is generally a few seconds, suitable for rapid emergency detection needs. The PID is continuously measured and can be continuously measured in real-time on site.
II has a wide linear detection range and can detect more than 40% of volatile organic compounds (VOCs) and NH , HS , AsH3 , PH3 , and CI . , Br2 and NO and other inorganic toxic gases. The harmful substances in many accidents are VOCs , so PIDs with high sensitivity and wide linear range for VOC detection can have important applications in emergency accident detection.
III small size, light weight, easy to carry to any need to detect the environment. The built-in powerful suction pump can draw the gas to be tested where the inconvenience arrives (the sampling line can be tens of meters long ) ;
The IV sensor is not poisoned: Unlike most other sensors, the PID is not damaged ( poisoned ) by high concentrations of the test substance ; its recovery time is the same as its response time is only a few seconds, and the real-time concentration can be measured at any time ;
V But PID cannot detect non-volatile organics such as N2 in air .
( 7 ) Chemical tape
Chemical tape technology uses chemically immersed tape to detect toxic gases. This method is very much like the litmus test paper will change color when it encounters a certain gas; the tape machine measures through the photocell, analyzes the color change of the tape, and converts it into a gas concentration value.
The advantage of chemical tape technology is that the tape machine provides physical evidence of gas leakage as a result of the color change reaction. They are also affected by interfering gases, but they are less specific than the electrochemical, solid-state metal oxides, and are therefore more specific. In addition, chemical tape technology can detect more types of gases than electrochemical ones.
What is worth mentioning is that gas detection methods such as gas detection tube method and chemical sheet detection method are similar to the chemical tape detection method without using a paper tape machine. These two gas detection methods are not only more economic in terms of price and storage cost, but also easier, quicker, and more reliable in emergency rescue of chemical accidents, which is far more than gas detectors using sensors .
The gas detection tube method is a commonly used quantitative field quick and easy detection method. The gas detection tube is a small glass tube filled with both ends of the color indicating powder, and a concentration scale is marked on the wall of the tube, indicating that the powder is a porous solid fine particle that adsorbs a chemical reagent. Each chemical reagent usually has color development characteristics for only one compound or a group of compounds. When sampling, the sampling flow rate and sampling volume shall be used for sampling. When the inhalation air passes through the detection tube, the toxic gas to be measured in the air will rapidly react with the indicator powder in the tube to show the color. By observing the scale position indicated by the length of the indicator's discolored column, the concentration of the analyte can be read out to obtain a semi-quantitative test result. In the case of known toxic gas chemical name categories, a corresponding type of detection tube is used to detect the concentration range of the toxic gas in the air, or a plurality of detection tubes are used for the test.
The chemical tablet detection method is a quick and easy detection method on site. Cured by a special chemical formula on the transparent sheet to cure, when a certain chemical gas comes into contact with it to produce a chemical reaction, it will show different colors. Each chemical test piece has a quantitative reaction value. When the concentration of chemicals in the atmosphere does not reach its set value, no reaction will occur and the color will change. Chemical test strips have reached more than ten varieties, and have a good detection sensitivity and accuracy, the reaction is very sensitive and fast. The phosgene reaction concentration was 1 ppm , hydrogen sulfide was 5 ppm , and the reaction time was several tens of seconds to several seconds. It is suitable for the detection of unexplained gas leaks in the handling of emergency incidents and the rapid identification of toxic gases. For on-site alerting, decontamination, and other work, the degree of trust is greater than other various sensor-type gas detectors. Chemical test strips can be worn on the hands or legs without batteries or preheating; 10 to 20 chemicals can be tested simultaneously. Chemical test strips are inexpensive and can be stored for one to two years.
2Application of gas detectors in the disposal of gas leakage accidents
Firefighting forces respond to sudden chemical accidents. How do you choose and apply dozens of gas detectors? This article discusses the selection and use of gas detectors that will involve application, maintenance, and storage. The main role of gas detectors is to conduct monitoring and obtain relevant data for the commander to make correct judgments in the event of a chemical accident, so that further measures can be taken to save or reduce the loss of life and property. It can be said that there is no effective monitoring of the occurrence of chemical accidents. Without relevant data, the various disposal methods are blind and even dangerous and fatal!
In the event of an emergency chemical accident, how to choose the right gas detector?
According to its detection function, gas detectors are divided into: flammable and explosive (also known as combustible gas detectors , explosion detectors), toxic and harmful gas detectors (also known as toxic gas detectors, and poisoning meters) and military toxicants. This article focuses on the first two.
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Japan Ricoh Combustible Gas Leakage Detector
According to its use methods are divided into: portable (pocket, handheld, wear), semi-fixed (mobile, vehicle-mounted) and fixed.
According to its use time limit is divided into: one-time, time-limited and long-term.
According to its scope of use is divided into: emergency detection, production testing, health testing, military detection.
Divided by its detection methods: test paper, test tube, sensor, sampling and analysis
According to its detection function is divided into: single (single gas), compound (multi-gas), analytical
Divided by its working function: diffusion, pump suction, take
Sample analysis
According to its gas characteristics are divided into: inorganic gas type, organic gas type, combined type
According to the above classification, it can be seen that what type of detectors and methods should be used in the rescue of emergency chemical accidents will have a great bearing on the success or failure of emergency chemical accident rescue operations.
( 1 ) Defining the gas species to be tested
Before arriving at the scene of an accident, the type of chemical accident must first be determined. Such as flammable and explosive chemicals or toxic and hazardous chemicals? Still unknown? Then we must further grasp the physical and chemical properties of the chemical. The flammable and explosive chemicals mainly possess many physical and chemical characteristics such as the lower explosion limit, ignition point, and specific gravity. The toxic and hazardous chemicals mainly have many physical and chemical properties such as hazardous concentrations and corrosiveness; while some chemicals are flammable and easy to use. Explosions are poisonous and hazardous chemicals that must be considered in both aspects. However, in the detection of physical and chemical properties of common gas detectors, only the lower explosion limit and concentration data are available. However, other data on the physicochemical properties of the chemicals may be helpful in the process of use. For example, the specific gravity is very important for the position of the combustible gas detector. Corrosion protection is very important for personal protection . Therefore, it is also a very important step to grasp the physical and chemical properties of this type of chemicals at the accident site.
( 2 ) To know the gas detector concentration measurement range
Each type of gas detector has its own fixed detection range. Only when the measurement is completed within its measurement range can the instrument be accurately measured. Common gas detectors are targeted for concentration detection range, resolution, allowable concentration, and maximum exposure concentration. The toxic gas detector concentration measurement range is too narrow to be suitable for use during emergency rescue, which will limit the commander to make more accurate judgments and deployment; and the low concentration measurement range is also easy to use when working at higher concentrations for a long time. Damage to the detector may even cause permanent damage to the detector. LEL gas detectors, if used directly in excess of 100% LEL , may completely burn out the sensor. Therefore, choosing a gas detector with a wide range of measurement will help the emergency rescue and disposal work.
( 3 ) To determine the location of use
The occasions and environmental locations for emergency rescue of chemical accidents can be described as varied and live differently from normal normal chemical industrial environments. Some of the accidents occurred in large chemical and chemical plants, but they sometimes entered tank containers; some accidents occurred on the highway, and others required rescue in the sewer ditches. The emergency rescue of chemical accidents at these different locations is different for the selection and application of gas detectors. In large industrial chemical plant accidents, multiple sets of gas detectors are required to divide hazardous areas, alert areas, and safety areas. The detectors in the danger zone need to be pumped, highly sensitive and have a wide range of detectors; while those in a confined space that require composite gas detection must know both the oxygen content and the presence of several gases.
In general, combustible gases have relatively small relative densities and they are mostly distributed in the upper part of the confined space. The relative density of carbon monoxide is close to that of air, which is generally distributed in the middle of the confined space; and the heavier gas such as hydrogen sulfide is present in the lower part of the confined space. Finding gas leaks and large leak detection parties
The choice of method and instrument is different.( 4 ) To understand the gas detector warm-up time
Preheating can take several minutes for most gas detectors. Therefore, the correct method of use is to open the gas detector when the alarm is activated, or to open the gas detector when reaching the on-site safety zone; rather than opening the gas detector when entering the danger zone. At the same time pay attention to the low amount of battery, it will also affect the warm-up time.
The above analysis of gas detectors is a relevant issue when used on site, and maintenance and storage are also very important after use. The following questions should be given the same attention. It is related to whether or not the instrument can be put into normal condition when it is used again, and it is crucial to exert its function.
I Note zeroing and calibration of gas detectors
Gas detectors, like other detection instruments, are measured using relatively comparable methods. Therefore, gas detectors need to perform zeroing and calibration of the instrument frequently, which is an essential task for ensuring the accuracy of the detector measurement. Gas detectors need to be zeroed before use, so that the value obtained during formal survey work is standard. Most modern gas detectors are automatically zeroed, and some gas detectors still require manual operation. Electrochemical sensing detectors have zero drift over a long period of time and can be corrected by periodic calibration. When a gas detector replaces a sensor probe, it needs to be calibrated with a standard gas, and it is not always possible to use it with a new sensor probe.
II Note Gas Sensor Sensor Lifetime
All types of gas sensors have a certain useful life. The useful life of the sensor is determined by many factors, including heat, humidity, dust, and cumulative gas exposure. In portable instruments, the life span of LEL sensors is generally about two to three years; the life of photoionization detectors is four years; Zuo Shi uses electrochemical specific gas sensors, generally one to two years; oxygen sensors. The life expectancy is about one year. Therefore, it is necessary to detect the sensor at any time and use it within the effective period of the sensor as soon as possible. If it fails, replace it in time.
Short exposure to high concentrations of gas will affect the performance of the sensor in the near future, and even affect the life of the sensor.
Misunderstanding of 3 gas detector applications
Gas detectors are often ignored in applications, or can be referred to as human error. Failure to correctly understand and correct these mistakes or neglected problems may lead to loss of life and property.
( 1 ) The use of catalytic combustion sensors must pay attention to whether the oxygen concentration is normal enough. Catalytic combustion sensors require at least 10% oxygen to react properly to show the correct concentration. If the concentration is greater than 23% , the result is wrong; if the concentration is too low, even if the pure gas reaches 100% LEL , the reading may be 0% LEL . Therefore, using a gas detector to enter a confined space must know the oxygen concentration, or use an infrared technology sensor.
( 2 ) Gas detectors for common catalytic combustion sensors, also known as combustible gas detectors, or pyrometers. Most manufacturers use methane, a specific gas, for calibration before leaving the factory; at the same time, they are also indicated on the operating instructions or on the back of the instrument to remind the user that the gas detector is calibrated with methane. Because the catalytic combustion sensor is not a single choice for combustible gases. Therefore, when there is a propane leak, using a gas detector calibrated with methane as the specified gas, the result will be erroneous; the measured lower explosion limit will vary by 30% to 50% . Such a large difference is a very dangerous and fatal data for workers in hazardous areas.
( 3 ) Catalytic combustion sensors are not suitable for the measurement of long-chain alkanes, such as gasoline, diesel, aromatics, etc., which can have high flash point. This is because the measurement bridge of the catalytic combustion sensor will produce more heat in the same volume, larger molecules of alkanes will not easily pass through the fire protection grille, and the reaction will be insensitive; while smaller molecules of alkanes such as methane, etc., will be easily fired through sintering. Grid, so the measurement results are more accurate.
( 4 ) Specification of China's petrochemical sector: The primary alarm high limit setting value for combustible gas detectors is less than or equal to 25% LEL , and the secondary alarm high limit setting value is less than or equal to 50% LEL ; When the gas concentration exceeds % LL, it must exit. At this time, the concentration does not reach the 5% LEL alarm limit. When the city gas leaks, the smell of leaking gas is smelled. At this time, the concentration is 20% LEL . Otherwise it is not the fault of the gas company that the meter is inaccurate.
( 5 ) Many people mistakenly use a lighter to test the reaction directly at the detector. Catalytic combustion sensor poisoning often occurs directly at high concentrations. When the flammable gas concentration is high, more heat will be generated on the measuring bridge. Oxygen deficiency at this time, combustion is not complete, the formation of carbon black deposited on the surface of the knot may cause damage to the sensor. Catalytic combustion sensor poisoning is often caused by the detection of organic silicides. For example, lubricating oils, cleaning agents, etc. decompose the catalyst in a high-temperature environment and form solid substances on the catalyst surface, resulting in reduced sensitivity. When a higher concentration of organic silicon compounds is invaded, the catalytic combustion sensor may be immediately damaged.
( 6 ) If a catalytic combustion type gas detector is calibrated with methane, it is a very dangerous mistake to detect benzene, which is a dangerous toxic gas and a combustible gas chemical. The lower explosion limit of benzene is 1.2% , and its correction factor on the LEL detector is 2.51 . That is, the concentration of benzene shown on an LEL detector calibrated with methane (lower explosion limit of 5% ) is only the actual concentration. 40%. From the perspective of poisoning, it is also not allowed to use catalytic combustion gas detectors to detect such chemicals as benzene. The lethal amount of benzene is 500ppm , and as long as the catalytic combustion sensor shows 1% LEL , its benzene concentration reaches 1300ppm , which is far beyond the safety range.
4 Conclusion
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