How to divide explosion and fire hazard areas?

In chemical enterprises, the material media of many production devices are explosive and fire-hazardous. Explosions and fire accidents caused by them are an important issue threatening the safe production of chemical enterprises. For example, in the synthetic ammonia production system using coal coke as raw material, the explosive gas from raw material to purification is lighter than air; in the synthetic ammonia production system using oil as raw material, the explosive gas from oil gasification to purification is heavier than air; some floors of the main plant of synthesis, compression, and urea are in an explosive environment where the explosive gas is lighter than air. In this environment, sparks from electrical equipment are often one of the main causes of explosion accidents.

As we all know, a mixture of flammable gas or steam and air will be ignited when it encounters sparks, electric arcs or dangerous high temperatures, which will cause combustion or explosion. Petrochemical and chemical companies often have to process and handle flammable liquids or gases. A considerable number of raw materials in the petrochemical industry are flammable, such as oil, natural gas, and hydrogen in common raw materials; most of the alkanes and hydrocarbon compounds in semi-finished products are flammable substances; gasoline, diesel, etc. in finished products are also flammable substances. These flammable substances will inevitably escape or leak from pipelines, reactors, and storage tanks during processing and storage, and form an explosive mixture after mixing with oxygen in the air. If there is an ignition source on site at the time, an explosion will occur. The explosion produces high temperatures and shock waves, causing casualties and huge property losses.

If an oil refinery or one of its devices is identified as an explosion-hazardous place because its raw materials or products contain flammable substances, it is extremely uneconomical and obviously unreasonable. The presence of flammable substances forms a potential explosive environment. The so-called potential means that they do not appear all the time. Some appear frequently, while others appear less frequently. In this case, there are places with high danger and places with low danger. Therefore, it is necessary to "classify" these dangerous places. According to the frequency of the gas environment in the place and the length of time it exists, the degree of danger of the place is classified so that different explosion-proof measures can be adopted according to the type of dangerous area.

The demarcation of explosion-hazardous areas must first locate and determine the release source, divide the explosion-hazardous areas according to the level of the release source, and then adjust the area division based on the ventilation conditions where the release source is located.

In each engineering project, each processing equipment (such as tanks, pumps, pipelines, containers, etc.) containing flammable materials should be considered as a potential release source, such as the discharge port of flammable gas or liquid, sampling point, leaking valve, etc., are all release sources. Equipment containing flammable materials that will not be released into the environment, such as all welded pipes, etc., cannot be considered as a release source.

The division of explosion-hazardous areas is determined according to the frequency and duration of the occurrence of explosive mixtures, and is divided into Zone 0, Zone 1, and Zone 2.

The scope of the fire and explosion hazardous area should be determined based on the duration of the explosive mixture and the frequency of its occurrence, the type, quantity, physical and chemical properties of the hazardous materials, ventilation conditions, production conditions, and the accumulation and diffusion formed by ventilation, the density, quantity, generation speed and release direction and pressure of the hazardous gas or vapor. Inside the building, the scope of the hazardous area should be determined in units of factory buildings. Within the scope of the hazardous area, the appropriate electrical equipment should be selected based on the type and level of the hazardous area, and taking into account the type and use conditions of the electrical equipment.

The demarcation of explosion-hazardous areas, in addition to the provisions of the regulations, is also related to the following factors:

The amount of flammable substances released;

The release rate;

The concentration of the released explosive gas mixture;

The boiling point of the flammable liquid, the initial boiling point of the liquid mixture;

The lower explosion limit;

The flash point;

The ventilation volume, etc.

The increase and decrease of these quantities are directly related to the size of the explosion hazard area. Because the factors defining these quantities are complex, the parameters listed above that affect the size of the range are only based on IEC regulations, which are only a principle regulation so far, and there is no specific size to follow. Since the process, equipment, instrumentation, ventilation layout and other conditions of actual production equipment are different, in the specific design, they must be properly selected in combination with the actual situation to ensure safety. For specific division dimensions, please refer to the relevant specification GB50058-92.

In order to prevent fire and explosion accidents caused by sparks, arcs or dangerous temperatures in electrical equipment and circuits, fire and explosion hazard areas are divided into 3 categories and 8 zones according to the degree of fire and explosion danger and the status of dangerous goods. Appropriate measures are taken according to different categories and zones to prevent the occurrence of electrical fire and explosion accidents.

The partition walls between the factory buildings adjacent to the explosion-hazardous areas should be dense and solid non-combustible solids. The doors on the partition walls should be made of solid non-combustible materials and have sealing measures and automatic closing devices.