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How to protect areas with a risk of explosion?

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.

Risk assessment

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.

Areas with a risk of explosion of gases or vapors

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.

Areas with combustible dust

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).

Classification of explosives  

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).

Methods of protection in explosive atmospheres

​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:

  • use of electrical equipment designed for explosive atmospheres with regard to the type of volatile substance and the environment,
  • ventilation and removal of dust from surfaces.

Electrical equipment in explosive atmospheres

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

Mechanical protection of equipment

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.

Surface temperature of the device

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.

Categories according to the type of environment and hazardous substances in the environment

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:

Mining environment:

  • M1: in the event of a failure of one safety element, safety is maintained by another element and the equipment remains in operation,
  • M2: the device must be switched off in the event of an explosion.

Above-ground environments:

  • 1: highest level of protection (zones 0 and 20),
  • 2: high level of protection (zones 1 and 21),
  • 3: normal level of protection (zones 2 and 22).

Certification

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:

  • CE mark - "self certifying" procedure to verify that the products are designed to the appropriate standards,
  • 4-digit number of the Notified Body that issued the product certification,
  • equipment category,
  • type of explosive atmosphere (D for dust and G for gas),
  • type of protection according to technical standard,
  • temperature classification or numerical indication of the maximum surface temperature in 

    °

    C

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.

Ventilation and motorization of air dampers

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