Dust as a hazardous substance
Roughly 80% of all dust that accrues during industrial manufacturing is combustible and can, if a potentially explosive dust-air mixture is present, lead to an explosion. Widely encountered combustible dusts include e.g. sawdust, grain dust, cellulose, baking flour and fish meal and materials such as laundry detergents, toner, vitamin C or metallic dust as well. Even a seemingly harmless material such as lactose can explode under the right conditions.
ATEX must be observed wherever such dusts are created as waste products during processing or are generated as an end product in relevant quantities. Only those items of equipment that have protection ratings for the prevention of sparks or high temperatures may be operated in hazardous areas. The implementation of these protection classes is defined in the EU standards.
A special risk obtains in various industrial branches:
Potentially explosive dust-air mixtures can be produced during the fragmentizing of materials in crushers, which can cause severe damage. For this reason crushing technology is particularly affected by the new ATEX.
There is also a risk of explosion in bucket conveyor plants when dust is transported. Whether sawdust, flour, coal or metallic dust is the load - all sensors set up in the hazard zone, perhaps for monitoring off-track running or for counting the buckets, must carry the new ATEX approval.
ATEX must be adhered to in wood processing operations as well: dust deposition at wood cutting saws, for which a speed of 20 m/s must be ensured, may only be safety monitored with Ex-approved sensors.
Vacuum conveying facilities in grain elevators in the food processing industry or other sectors are required to have ATEX-compliant equipment.
Agitators and mixers in the food processing industry, for the manufacture of pharmaceutical products or in the chemical industry and pelleting presses laden with dust are also affected.
Zone classification in the Ex dust area
Facilities in which combustible dusts can be generated are divided into three zones. These zones establish the differences in the periods of occurrence of a cloud of combustible dust in the air during normal operations.
Zone 20 EX atmosphere present frequently or constantly or long-term
Zone 21 EX atmosphere occasionally present
Zone 22 EX atmosphere present very seldom and then only for a short period.
The operator is responsible for zoning for which he can receive support from institutions such as the TÜV or the factory inspectorate.
ATEX 100a prescribes a uniform identification of the equipment approved for use in potentially explosive areas. For dust Ex this consists of:
the EX mark
the equipment group: II (all areas except mining)
the equipment category: 1D = use in zones 20, 21, 22
2D = use in zones 21, 22
3D = use in zone 22
the maximum surface temperature: Txxx°C
the name of the examination authority
the number of the examination certificate
Example:Ex II 1D T 100°C
designates a piece of equipment belonging to equipment group II, which can be used for dust in zones 20, 21 and 22 and which even in the case of a breakdown does not become hotter than 100°C.
Maximum surface temperature
This is the maximum temperature that can be reached on the surface of a sensor if it is operated under installation conditions within the specifications prescribed in the data sheet. This value is not exceeded even if the sensor is defective. The maximum surface temperature is a criterion for the application of a sensor, since various maximum values may not be exceeded according to the type of dust. A distinction must be made between a dust cloud and accumulated dust.
The term dust cloud describes dust particles that along with air form a potentially explosive mixture. Each dust cloud has a specific ignition temperature - that of sulfur amounts to 240°C, for example. A piece of equipment that is to be used in this environment may reach a maximum surface temperature of 2/3 of the ignition temperature. In the case of sulfur dust this therefore yields a maximum approved surface temperature of 160°C.
Dust layers can ignite on hot surfaces. This can result in a dust explosion if the ignited dust is stirred up. Each type ,of dust has its specific glow temperature, which depends on the thickness of the layer. For a 5-mm thick layer of sulfur this temperature is 250°C. If a sensor is supposed to be used in an area in which deposits can occur, the surface temperature may reach a maximum of the glow temperature minus 75K. For sulfur this would mean 175°C would be permitted.
Selection of the sensor
The selection of a sensor that is suitable for an EX area that is exposed to dust cannot be made until after the zoning allocation carried out by the operator. In addition, the type of dust and the ambient temperature must be taken into consideration. Both the area of deployment and the maximum surface temperature of the sensor are considered as criteria to the same extent. If dust layers can also occur in addition to dust clouds in an application, the lower of the two relevant limiting temperatures are used as the maximum value for the selection of the sensor. In the case of sulfur the maximum approved surface temperature of the sensor comes to 160°C.
Approval for safety applications
Sensors for personal security must have a qualifification approval according to EN 954-1 and must be labeled accordingly. Sensors that are not labeled must not be used for applications of this kind.