In oil refining, petrochemical, chemical and other industries, there are inevitably various flammable and explosive gases (or vapors). Once these gases leak and accumulate in the surrounding environment, they will become a serious hidden danger that may lead to burning and explosion accidents. In order to prevent problems before they happen, a reliable combustible gas detection and alarm device is used to closely monitor the concentration of combustible gas in the process equipment or storage and transportation facility environment to ensure safe production.

The following mainly talks about the experience of the selection and installation of combustible gas detection alarm in the rectification design.

1 Selection of flammable gas detection alarm

In the selection of instruments, it is based on the principle of combining advancement and applicability. Note that the product features and functions should meet the requirements of stable, reliable and safe production, and the performance and price ratio should be reasonable. At the same time, the combustible gas detection alarm must be certified by the national designated agency and authorized inspection unit for measuring instrument manufacturing certification, explosion-proof performance certification and fire protection certification. The combustible gas detection alarm is composed of two parts: detector and alarm. The specific selection is as follows.

According to the different measured medium, select detectors with different measurement principles. As a sensitive element for combustible gas or vapor, the detection technology of detectors has developed rapidly. The forms are divided into electrochemical (constant potential electrolysis, diaphragm electrode method), optical (infrared absorption method, optical interference method), and electrical method (heat conduction method, Catalytic combustion method, semiconductor method).

The catalytic combustion detector has obvious advantages such as fast response, good linearity, good repeatability of the instrument display, small interference, high precision, broad spectrum and good stability. Therefore, in the design, an intrinsically safe catalytic combustion type combustible gas detection and alarm device is preferred (the catalytic combustion method is detonating for hydrogen, and the dedicated catalytic combustion type hydrogen detector or toxic gas detection method should be used for the detection of hydrogen: Electrochemical or semiconductor detection elements).

The specific conditions of the manufacturer also have different requirements for the performance indicators of the alarm. Some production enterprises mainly use on-site instruments for production process detection, such as on-site temperature display, pressure display, liquid level display, etc. The requirements for flammable gas detection and alarming only need an alarm with indication and alarm functions. The installation method of the alarm can be wall-mounted. However, some manufacturing companies require higher levels of automation, and the production process detection is based on centralized control. That is, temperature, pressure, flow, liquid level, etc. are all displayed and controlled in a centralized manner. Important parameters require historical data recording and historical data query. The requirements for combustible gas detection alarms should not only have indication and alarm functions, but also have switch output or 4-20 mA DC output or RS-232/RS-485 interface (using RS-232, RS-485 For communication, signal conversion can be avoided, and the authenticity can be improved), and the installation method of the alarm is mostly panel-mounted. The block diagram of the alarm system is shown in Figure 1.

can independently complete the flammable gas detection, alarm setting, indication and alarm functions. The difference is that plan b) has the functions of plan a), and can also input the detection signal into the DCS for recording, control, filing and online query. When the DCS is used for recording, the signal should be introduced from the display alarm, and the detector signal should not be directly connected to the DCS.

1.2 Setting of alarm value

When combustible gas (or steam) is mixed with air, an air-combustible mixed gas will be formed. According to the concentration of combustible gas (or steam) in the mixed gas, the mixed gas can be divided into 3 areas-poor volume area, explosion District, abundance area. The lower limit of the explosion zone is called the lowest explosion limit, written as LEL; the upper limit is called the highest explosion limit, written as UEL. The lowest explosion limit and highest explosion limit of each gas are determined, and the volume fraction of explosive gas can be obtained by consulting the combustible gas and toxic gas characteristics table in the literature [1].

For a single alarm (first level alarm) instrument, the set value is less than or equal to 25% LEL. For dual-alarm (second-level alarm) meters, set the first alarm point at 20%LEL. , The second alarm point is set at 40%LEL. Of course, the user can make appropriate adjustments to the alarm points according to the needs of the scene. But it cannot exceed 50% LEL.

1.3 Selection example

In the engineering design of a certain liquefied petroleum gas station, the scheme shown in Figure la) was selected according to the actual situation of the production site. The instrument selects the JB-QT—SNE4100B series combustible gas detection and alarm device (wall-mounted) produced by Shenzhen Nanyou Nuoan Electronics Co., Ltd. The product adopts a number of new technologies such as micro-processing chip control, EEPROM data storage and digital display, and adopts multiple processing methods such as software compensation and data filtering in data processing, which not only improves the detection accuracy of the alarm device, but also improves the resistance of the alarm device. Interference ability, effectively preventing false alarms. In terms of instrument function, in addition to the conventional sound and light alarm function, it also has self-check, reset, alarm record query, output contact control, automatic switching of main and standby power supplies, and can detect short-circuit and open-circuit faults of the detector.

The system is easy to compose, and the investment is low. The alarm adopts the commonly used 220 VAC power supply, which is easy to put into operation and ensure safe production.

2 Installation of flammable gas detection alarm

2.1 Detector detection network setting

It is very important to determine the installation position of the detector. In some cases, it may directly affect the timely alarm. For this reason, when setting up a flammable gas detection network, the following issues should be paid attention to.

The effective coverage horizontal plane radius of the combustible gas detector should be 7.5 m indoors and 15 m outdoors. Within the effective coverage area, a detector can be set. "In addition, the setting of the detector should also consider the release source located on the upwind side (or downwind side) of the local site’s minimum frequency wind direction. Detectors are installed separately in the pump room, compressor room, and bottling room valve group that may have leaking gas. The bottling room is generally a semi-closed factory building, and one can be installed about 10 m away.

2.2 Determination of the installation height of the detector

The installation height of the detector depends on the density of the gas or vapor to be measured. For detecting combustible gas detectors that are heavier than air (under standard conditions, gas density greater than 0.97 kg/m3 is considered heavier than air, and less than 0.97 kg/m3 is considered lighter than air). The installation height should be away from the floor. (Or floor) 0.3-0.6 m, the detector for measuring liquefied petroleum gas, the installation height is 0.3 m from the floor. If it is too low, it is easy to cause damage to the detector due to rain or splashing. If it is too high, it will exceed the height at which combustible gas heavier than air is easy to accumulate. The detector for detecting combustible gas lighter than air should be installed at a height of 0.5～2 m higher than the release source.

2.3 Installation precautions

1) The installation position of the detector should be convenient for calibration and maintenance. A certain amount of free space should be maintained around it. It is advisable to leave a clearance of not less than 0.3m. 2) The detector should be installed in a place where there is no impact, no vibration, and no strong electromagnetic field interference. 3) The display alarm of the detection alarm device should be installed in the control room or operation room or duty room in the safe area. 4) The electrical wiring of the combustible gas detection alarm: The installation and wiring of the combustible gas detection alarm shall be carried out in accordance with the requirements specified by the manufacturer, and shall comply with the relevant regulations for the installation and wiring of explosion-proof instruments. The laying of wires and cables is carried out in accordance with the principles of orderliness, beauty and convenient operation. When laying in the ground, cables, wires, and protective pipes are laid, and the depth of the ground is greater than or equal to 700 mm