Many industries have to deal with a risk of explosions. These are mainly the chemical and petrochemical industries, but also mining, pharmaceutical, food and waste management industries. There are binding standards and regulations for such environments, with legislation varying by region and country.
To properly secure an area against explosion, you must first determine the degree of danger and the type of explosive atmosphere. Based on this information, you can then implement various explosion protection methods.
These are spaces where an explosive and flammable gas-air mixture may be formed or is still present. These are mainly the chemical and petrochemical industries, pumping stations and deep mines. Depending on the degree of danger, Zone 0 (highest or permanent risk), Zone 1 (risk of explosion is likely to arise under normal conditions) and Zone 2 (low risk under normal conditions, may happen in the event of an unexpected leakage of a flammable substance) are distinguished.
These are atmospheres where a cloud of flammable dust may form or be permanently present. These are mainly mills and sawmills, but also various other areas of the food industry (processing of cocoa, flour, starch, etc.), the chemical industry and pharmacy. The degree of risk is indicated similarly to areas with a risk of explosion of gases and vapors - 20 (highest or permanent risk), 21 and 22 (low risk).
An explosion may occur due to a spark or contact of the volatile substance with a hot surface. However, different substances have different properties and therefore a degree of danger. Accordingly, they are divided into groups I, II and III and further denoted by the letters A, B and C.
Group I is reserved for mining locations, Group II for other explosive gases and vapors, Group III for explosive dust. The letter C indicates the most volatile substances, the letter A the least volatile.
In the USA and Canada, mining and other gases and vapors are marked with the number I, dusts with the number II. The degree of explosion hazard is indicated by the letters, but in reverse order: A (highest risk) - G (lowest risk).
It is necessary to prevent the explosion by all available means, ie it is necessary to minimize the concentration of the volatile substance and prevent its contact with the source of the explosion (spark or hot surface, but also flame, sunlight, friction, etc.). The basic methods of explosion protection therefore include:
It can be concluded from the above information that it is necessary to use only electrical equipment certified for these specific environments. Conventional electrical equipment generates a number of tiny sparks that you do not normally notice, but can cause an explosion. Device temperature can also be dangerous.
Indication of equipment in explosive atmospheres. Source: Wikimedia.org
The design of electrical equipment in explosive atmospheres must prevent contact of flammable substances with sparks or hot surfaces. Therefore, electrical equipment for explosive environments must have so-called intrinsically safe circuits or special closures for electrical circuits - for example, oil, sand, pressure, the equipment can also be filled with a special material. The danger of spreading the explosion is solved by the so-called solid closure.
Each device has a certain surface temperature. In explosive atmospheres, the maximum surface temperature must not exceed 2/3 of the flash point temperature of the explosive. Therefore, if there is a substance in the environment that is at risk of ignition at 60 °C, only equipment with a maximum surface temperature of 40 °C may be used. Surface temperature of the device is indicated by the temperature class (T1 - T6).
ATTENTION: The ignition temperature of a substance is the temperature at which the self-ignition of a substance occurs without an external source of ignition - preventing contact with fire or spark is therefore not sufficient.
Only suitable electrical equipment corresponding to given or even higher risk may be used in potentially explosive atmospheres. This means that if, for example, the device is suitable for environments with the presence of Group IIB substances, it can also be used in environments with Group IIA gas, but not IIC. If the equipment has only a number (I, II, III), it can be used for all hazard categories - A, B and C. However, the numbers I, II, III are not interchangeable, so for example equipment for substances of category IIC is not suitable for environment at risk IA or IIIB.
Devices are then divided into categories:
The EU directive of equipment and protective systems for use in explosive atmospheres is called ATEX. Devices that comply with this directive are marked with the Ex sign in the hexagon.
The label on the device also contains:
To export equipment outside the EU, you must comply with the legislation in the importing country. NEC 500 is the most commonly used certification in the US and Canada.
It is important to maintain the lowest possible concentration of explosive gases and dusts to prevent an explosion. Dust must be regularly cleaned and vacuumed, even from joints, but especially from hot surfaces. Hazardous gases and vapors must be ventilated by extractors and fans.
Fans for explosive environments with an appropriate certification are used for the removal of hazardous gases, including fire dampers, which prevent the spread of a possible fire. Fans can be built into the wall or ventilation ducts.
An important part of fans and air dampers for explosive atmospheres is, of course, a motor, which must have protection according to the type of environment and a robust metal cover. Only electric motors with an fail-safe function may be used in an explosive atmosphere so that the fans do not stop in the event of an explosion and power failure.
A fan for explosive atmospheres. Source: Ventilatory.cz