Specific classification standards for dust explosion-proof areas
According to the frequency and duration of explosive dust environment, explosive dust environment is divided into three areas. When selecting dust explosion-proof motor, the applicable motor can be determined according to the characteristics of the place corresponding to the explosion-proof grade.
Zone 20: places where combustible dust clouds in the air exist continuously or for a long time or frequently for a short time in an explosive environment.
Examples of places where zone 20 may occur:
1. Internal places of dust containers;
2. Storage tanks, silos, etc., cyclone dust collectors and filters;
3. Dust conveying system except some parts of belt and chain conveyor;
4. Agitator, crusher, dryer, loading equipment, etc.
Zone 21: during normal operation, it is likely to exist in explosive environment in the form of combustible dust cloud in the air.
In most cases, the scope of zone 21 can be specified by the evaluation of emission sources related to the environment in which explosive dust / air mixtures are formed.
Examples of places where zone 21 may occur:
1. When explosive dust / air mixture appears inside the dust container, frequently move or open the external place of the dust container closest to the access door for operation;
2. When no measures are taken to prevent the formation of explosive dust / air mixture, it is located outside the dust container closest to the loading and unloading points, feeding belts, sampling points, truck unloading stations, belt unloading points, etc;
3. If the dust accumulates and the dust layer may be disturbed due to process operation to form explosive dust / air mixture, the place outside the dust container;
4. Places inside dust containers where explosive dust clouds may appear (but they are neither continuous, long-term, nor frequent), such as the interior of simple bins with long self cleaning intervals (if only occasionally loading and / or discharging) and the accumulation side of filters.
Zone 22: places that are unlikely to exist in explosive environment in the form of combustible dust clouds in the air during normal operation. If they exist, they are only temporary.
Examples of places where zone 22 may occur:
1. The exhaust port from the ventilation hole of the dust bag filter may escape explosive dust / air mixture in case of failure. Places near the equipment that are rarely opened, or places near the equipment that are prone to leakage due to dust ejection higher than the ambient pressure according to experience; Nearby places where pneumatic equipment and flexible connections may be damaged.
2. A storage bag containing many powdered products. During operation, the storage bag may fail, causing dust diffusion.
3. When measures are taken to prevent the formation of explosive dust / air mixture, the places generally divided into zone 21 can be reduced to zone 22. Such measures include exhaust ventilation. Measures shall be taken near (dust bag) loading and discharging points, feeding belts, sampling points, truck unloading stations, belt unloading points and other places.
4. Places where the controllable (cleaned) dust layer formed may be disturbed and produce explosive dust / air mixture. Only before the formation of hazardous dust / air mixture, the dust layer is removed by cleaning, it is a non hazardous place.
Zone 21 is the highest dust explosion-proof grade of dust explosion-proof motor, which can be used instead of zone 22, and zone 20 is prohibited from entering electrical appliances.
The difference between B and A in the dust explosion-proof sign: B indicates that the explosion-proof joint surface of the motor is a straight joint surface, and a indicates that the explosion-proof joint surface of the motor is a plane joint surface, which can be used mutually.
These two kinds of dust explosion-proof signs generally have no difference in price, but the method of selecting explosion-proof structure is different. Both A21 and B21 have the grade requirements for explosion-proof motors in dust environment zone 21.