Selecting a sensor for hazardous area use

01 November 2007

European legislation, derived from the ATEX directives, control how equipment is used in hazardous areas. Paul Gay describes the directives have affected the sensor market

Hazardous areas in European oil, gas, chemical and process industries are defined and operated according to legislation written by each EU member country and drawn up with strict reference to the EU’s ATEX directives. Each country has interpreted the directive and produced its own set of rules to enforce the ATEX requirement.

There are two directives. Article 100a – known as the product directive – lays down how equipment for use in
potentially explosive atmospheres should be designed. It refers to explosion protection concept and therefore defines how products, such as process industry sensors, should be designed. EU directive 137, published in 1999 is a different kettle of fish.

The user directive, as article 137 has become known, deals with the safety of workers employed in the hazardous area environment. It states who is responsible for safety and outlines procedures required to ensure safe operation. It goes on to define three zones of hazardous area where the various degrees of explosion protection should be employed. The harshest zone has been nominated zero. In Zone 0 potentially explosive mixtures of gases, vapours or dust will always be present, such as the inside of a vessel or pipe transporting process fluids. Zone 1 is where potentially explosive mixtures are likely to occur during process operation. This could be close to a flange or filling point or perhaps near an overflow or
venting access to a storage vessel. Zone 2 is the least hazardous where an explosive mixture is unlikely to be present and then only for a short period time.

The hazardous area implies that flammable material is present. Oxygen, in the air surrounding us, is required to create a fire or explosion. The fire triangle is completed with an ignition source – a spark, a flame or a surface with elevated temperature. Equipment for use in zone 0 area must be intrinsically safe so there is no possibility of an ignition source. The application of the explosion proof concepts, described by the product directive, is laid down in the user directive.

To perform safely, sensors used in hazardous areas must be designed specifically for the environments in
which they operate. Sensors designated intrinsically safe have insufficient energy to cause ignition of the rated hazard. IS devices normally use a safety barrier designed to limit the amount of current, and hence energy, that can pass in the circuit under any fault condition. A passive barrier establishes protection mechanisms that prevent over-voltage and limit current.

Another technique is to contain any explosion within the device. So a nonincendive sensor can be contained in an explosion-proof housing, so that if an incident occurs, a containment vessel made to withstand the force of the expected reaction will not allow the force or any fire to propagate.

A third method involves systems, which purge-and-pressurise with inert gases and prevent sensors and other devices from reacting in a hazardous area. There are several variations, but essentially these systems introduce nonflammable gas such as nitrogen or carbon dioxide throughout the conduit, components, and equipment to eliminate the possibility for flammable material to enter. An adaptation would be a sealed system that allows no room for flammable materials.

Paul Gay is the editor of our sister magazine HazardEx the Journal and its international edition Hazardous Area International. Further details on ATEX can be found at the Hazardex website www. or look at the guidelines issued by the Health and Safety Executive at:

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