Under atmospheric conditions, gases, vapors, mist, or dust combustible substances form a mixture with air, and when ignited, combustion will be transmitted to the entire unburned mixture.
In chemical plants, there will be explosive substances such as combustible gas or combustible dust, so how to classify and group explosive substances, and how to classify and classify explosion hazard places?
1. Classification and grouping of explosive substances
1.1 Classification and grouping of explosive gases
Explosive gases should be classified according to their maximum test safety clearance (MESG) or minimum ignition current ratio (MICR) and should comply with Table 1.
Explosive gases should be grouped by ignition temperature and should comply with Table 2
1.2 Classification and grouping of combustible dusts
a. Combustible dust can be divided into two types: conductive dust and non-conductive dust according to its conductive characteristics.
b. Combustible dusts are grouped according to their minimum ignition temperature, and the specific grouping method is the same as that of the ignition temperature of explosive gases.
The ignition temperature of combustible dust is divided into the minimum ignition temperature of the dust and air mixture (i.e., the minimum ignition temperature of the dust cloud, Tcl) and the minimum ignition temperature of the dust layer. The minimum ignition temperature of the dust layer is usually divided into the minimum ignition temperature (T5mm) with a dust layer thickness of 5mm and the minimum ignition temperature (T12.5mm) with a dust layer thickness of 12.5mm.
2 Classification, zoning and zoning of explosion hazard sites
2.1 Classification of Explosion Hazard Locations
According to the state of explosive substances, explosion hazardous places are divided into two categories: gas explosion hazardous places and dust explosion hazardous places.
2.2 Zoning and zoning of explosion hazard sites
According to the frequency and duration of explosive atmospheres, hazardous places are divided into three regional levels: Zone 0, Zone 1 and Zone 2.
Places where explosive atmospheres are continuously present or exist for a long time. It is generally found in Class A substance storage tanks and reactors.
Places where explosive atmospheres may occur during normal operation. It is generally found in areas where combustible substances leak such as feeding, packaging, and sampling in Class A workshops
During normal operation, explosive atmospheres are not possible, and if they occur, they occur occasionally and only for a short period of time.
The regional division of explosive gas atmosphere should be carried out in accordance with GB3836.14-2000 and GB50058-1992 according to the actual production situation.
2.3 Zoning of combustible dust environments
According to the frequency and duration of the combustible dust/air mixture and the thickness of the dust layer, the combustible dust environment can be divided into three regional levels: zone 20, zone 21 and zone 22.
Locations and inside containers where combustible dust occurs continuously or frequently in quantities sufficient to form a mixture of combustible dust and air and/or may form an uncontrollable and extremely thick dust layer during normal operation.
During normal operation, there may be locations where the amount of dust is sufficient to form a combustible dust and air mixture, but is not classified as Zone 20. This area includes places directly adjacent to the point where dust is charged or discharged, where dust layers occur and where combustible dust and air mixtures that may occur under normal operating conditions can be generated.
Under abnormal conditions, combustible dust clouds appear occasionally and only exist for a short period of time, or where combustible dust occasionally accumulates or may exist in a dust layer and produce a combustible dust-air mixture. If it is not guaranteed that the accumulation of combustible dust or dust layer will be excluded, it shall be divided into 21 zones.
3. Legend and schematic diagram of explosion-proof partition
Part of this article is excerpted from: AQ 3009-2007 Safety Code for Electrical Explosion Protection in Hazardous Locations