Discussion on the problems of the hottest combusti

2022-08-20
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Discussion on the problems of combustible gas detection instruments

combustible gases such as methane (CH4), carbon monoxide (CO), hydrogen (H2), ethanol, etc. are released from the coal wall or leaked from the container. When they reach a certain concentration in the air mixture, they may cause combustion, even explosion, casualties and property losses when they encounter the fire source. In the production process, in order to avoid such fire and explosion accidents, stable and reliable combustible gas detection instruments are set near the coal wall that can produce methane (CH4) and other combustible gases, or in the production workshops of relevant chemical products. It can detect the concentration of combustible gas in time, give an alarm, and start the related equipment to avoid accidents

1 working principle of combustible gas detection instrument

combustible gas detection instrument is mainly composed of sensor, primary instrument and secondary instrument. The sensor is mainly used to detect the concentration of combustible gas in the mixture of air and combustible gas. It can accurately convert the combustible gas concentration into the corresponding signal and transmit it to the primary instrument. The primary instrument can pass the received voltage (current) signal through shaping and amplification to the secondary instrument and related equipment

more than 800 chemical patents have been granted. The sensors in the combustible gas concentration detection instrument have different forms due to different sensing elements, such as semiconductor gas sensing type, optical interference type, thermal conductivity type, catalytic combustion type, etc. Among them, sensors made of catalytic combustion elements are widely used in instruments for detecting combustible gases in most countries in the world, including China. The working principle of this sensor is shown in Figure 1

Figure 1 working principle of combustible gas detection sensor

the catalytic combustion principle is based on the flameless combustion generated when the combustible gas contacts with the catalytic element, and the ratio of its calorific value to the lowest explosive limit concentration of the combustible gas is basically constant. The resistance of the catalytic element of the bridge is changed by heating. In Figure 1, R1 and R2 on the bridge arm are standard resistors, R3 and R4 are catalytic treated detection elements (called black elements) and passivated compensation elements (called white elements), that is, R3 and R4 are commonly known as black-and-white elements. When there is no combustible gas in the ambient gas, the bridge circuit is balanced and the output is zero; When there is combustible gas, flameless combustion occurs on the surface of the detection element, the temperature rises, the resistance value changes, and the bridge circuit loses balance. At this time, a voltage (current) signal proportional to the concentration of combustible gas is output. When the concentration of combustible gas in the measured gas exceeds the set alarm value, the primary instrument drives the secondary instrument to give an alarm, and starts the associated equipment. For example, when the methane sensor of the working face detects that the methane concentration exceeds the limit, the secondary instrument will be driven immediately to give an alarm sound, and the ventilator will be turned on and the power supply of the working face will be stopped at the same time. In the production site of surface chemical products, when the concentration of combustible gas leaked out of the container exceeds the limit, the detector will send an alarm signal, and at the same time, start the ventilation equipment to dilute the concentration of combustible gas, and timely remind the staff to check the leakage and take corresponding treatment measures

2 characteristics and calibration of combustible gas sensor

2.1 characteristics of combustible gas sensor

(1) characteristics of sensor. Generally speaking, such sensors have good linear stability when the concentration of combustible gas is 0~100%; It is not easy to be disturbed by background gas (such as CO2); It is easy to realize the compensation of ambient temperature; Short response time and high accuracy. The methane sensor mentioned above obviously has these characteristics

(2) the sensor element of the sensor should have the ability to resist impact and "poisoning". It is required that the combustible gas sensor has this special ability, which is based on the working environment. For example, when the container leaks combustible gas, some of it leaks out slowly. If the spouting leakage is near the sensor, then the concentration of the sensing element in contact with the combustible gas will greatly exceed the minimum explosion limit concentration, that is, the sensor is impacted by the high concentration of combustible gas. For high-quality sensors, it can better restore the sensitivity within its range after impact, but some sensor elements have a sharp reduction in catalytic performance after impact, resulting in the loss of reaction sensitivity. In addition, some sensing elements are exposed to compounds containing hydrogen sulfide or halogens in the mixture of detected combustible gases, resulting in the loss of catalytic activity and sensitivity attenuation of the sensing element. This phenomenon is called "poisoning" of the sensing element. Therefore, we must choose anti poisoning components as sensors

2.2 calibration of combustible gas detection sensor

in order to ensure that the detection instrument can detect the concentration of combustible gas sensitively and accurately, the instrument must be calibrated and checked regularly. Therefore, we should pay attention to the following aspects

(1) calibration cycle and qualification of calibration personnel. The national metrological verification regulation stipulates that "the calibration cycle is one year", "the instrument (device) often vibrates abnormally or has doubts about the indicated value, and is sent for inspection at any time after replacing the main components". All instrument manufacturers have pointed out the calibration cycle of their instruments in the product instructions, but they have different calibration cycles for the same type of instruments. According to the provisions of relevant data and my experience, the calibration cycle for combustible gas sensors used in underground coal mines should be calibrated every three months, The combustible gas detection sensor in the ground chemical product production site should be calibrated every 3 months, while the combustible gas detection sensor in the ground chemical product production site should be calibrated every 6 months. The qualification of calibration personnel must be the personnel who have received special training from relevant units and passed the examination of the local measurement department, and then issued a certificate of qualification. Only then can they work with certificates to calibrate and verify the sensors regularly

(2) calibration method. Generally, instrument manufacturers have detailed instructions on the calibration and verification of their products. The general method is: the standard gas flows out of the steel cylinder, passes through the pressure reducing valve, and then provides the standard gas shown in to the sensor for calibration through a special calibration cover. In order to better control the flow of standard gas, some calibration personnel add a rotameter behind the pressure reducing valve to quantitatively supply gas to the sensor, which can avoid the waste of standard gas due to inaccurate manual control. It should be noted that before calibrating the sensor underground, portable instruments must be used to test whether the concentration of combustible gas in the ambient gas is within the safety range specified in the regulations, otherwise calibration cannot be carried out to avoid safety accidents

3 arrangement of combustible gas detection sensor

the sensor must be placed in a place where combustible gas is easy to leak or release and diffuse, so that the sensor element can easily contact the measured gas. However, how to arrange the sensors, and how many sensors are arranged? The premise is that both safety and economy should be considered, but safety must be given priority. Main principles: ① regional layout and key parts 4 The minimum reading value of torque: 0.01nm 0.1nm. For the mining working face and mining area roadway with combined layout, the number of sensors can be set according to one or more ratios arranged at about 10m in the closed and semi closed workshops for the production of chemical products with combustible gas on the ground. The so-called rearrangement refers to the upper, lower and upper corners of the mining face with combustible gas release, and the entrance of the return air roadway; Beside the process equipment in the production workshop of the chemical plant, near the sampling port, filling port, storage tank drainage port and other release sources on the machine pump pipeline. ② The principle of keeping the gas in close contact with the sensor. The installation position can be determined according to the specific gravity of the measured gas. If the specific gravity of the measured gas is lighter than that of air, it should be placed above. If methane is easy to accumulate in the upper corner of the mining face, a methane sensor must be installed. ③ For flowing air, the sensor must be placed on the downwind side. ④ A certain number of sensors should be placed in the analysis room, control room and power transformation and distribution room in the fire and explosion-proof zone of the production workshop. ⑤ Sensors that must be installed are required by the operating instructions of the detection system manufacturer

take the compressor room of the ground chemical plant as an example to illustrate the method of arranging sensors on site. When the number of equipment is relatively concentrated, it should be controlled regionally to avoid being arranged close to the sealing end of the pump or the water outlet of the valve, so as to avoid frequent alarm. First, arrange points near the column heads of buildings at both ends of a row of pumps, and the points between the two ends are different in combination with liquefied petroleum gas or class a oil materials. The former is dense and the latter is sparse. It is arranged near the column head in combination with the column spacing as far as possible, and a sensor is placed every other column. A row of columns on one side of the motor can be equipped with a sensor every two columns, which forms a triangular relationship with the pump side. As shown in Figure 2

Figure 2 Schematic diagram of sensor distribution in compressor room of chemical plant

in addition, combustible gas detection instruments should be regularly maintained and managed. To ensure that its good three exhibitions will promote the industrialization process rate (utilization rate, intact rate and accuracy rate) of new material technology in Ningbo and even China. To this end, we must formulate a strict maintenance management system and implementation rules, and implement maintenance management personnel. Be sure to calibrate the sensor regularly. Clean the dust cover, conduct normal patrol inspection for each shift, and carefully record the shift handover. In short, the maintenance and management should be strengthened to ensure that the flammability testing instruments work timely, accurately, sensitively and reliably

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