The term ATEX is derived from the French denomination "Atmosphères Explosibles" (explosive atmospheres): The ATEX Standard comprises the Equipment Directive 2014/34/EU as well as the Workplace Directive 1999/92/EC. In a previous article about the ATEX Directive we put more emphasis on general issues and theory. In this article we use the example of a flexible claw coupling to explain in more detail how this standard is implemented in practice.
European Directive 2014/34/EU
The ATEX directive classifies equipment into groups and categories, and defines the requirements to be met for the assessment of an assembly or a component intended for use in a potentially explosive atmosphere. In extreme cases, an EU type examination certificate is mandatory which involves specific quality assurance measures during production and particular tests to be carried out for the manufactured products.
In individual cases, it may also be sufficient to confirm the compliance of the product with the European directive by means of an EU Declaration of Conformity. Each European country is responsible for the enforcement and implementation of this directive through its own regulations. This is done on the basis of so-called harmonised standards and technical regulations.
What is ATEX-relevant in regards to a flexible claw coupling?
According to the directive, a claw coupling is not classified as equipment, but as a component. Consequently, the required assessment efforts are less extensive. What remains, however, is to consistently assess possible risks, i.e. the assessment of ignition hazards with the aim to determine the so-called “explosion level”.
To do so, you must closely review all country-specific standards and regulations to verify whether the product – in this case the flexible claw coupling – conforms with or deviates from the requirements of this directive.
Evaluation of the Ignition Hazards
Let us now look at the procedure for an ignition hazard assessment for a claw coupling within the frame of the ATEX directive. One of the ignition sources is described in more detail below:
The fundamental aim of the assessment is to define whether an ignitable spark is or can be caused by the intended operation of the claw coupling. A spark may, for instance, be caused by electrostatic discharge of a coupling component that has been electrostatically charged.
Electrostatic charge occurs due to surface friction, whereby the charge potential depends on the material and its conductivity. The higher the electrical resistivity is – i.e. the lower the electrical conductivity – the more prone it is to electrostatic charge.
Resistivity of the Material
Each country-specific standard defines the resistivity as of which a material is classified as insulating, and thus, as non-conductive. The torque-transmitting elastic intermediate ring of a flexible claw coupling, for example, is made of a non-conductive material, and can therefore be charged when surfaces slide upon each other.
Though it is not intended that the surfaces within a coupling rub against each other, this cannot be excluded if the permissible alignment values are exceeded in operation. This is a foreseeable event which represents a potential ignition hazard and, for this reason, must be reviewed thoroughly in terms of the compliance with the EC Directive.
Electrostatic Properties of the Elastic Intermediate Ring
First, the electrical chargeability of an elastic intermediate ring must be measured and counter-checked in comparison to the limit value specified by a technical standard (e.g. IEC/TS 60079-32-1:2013). If this value is exceeded, the elastic intermediate ring is capable of being charged and can release electrical charges in the form of sparks.
However, it has not been defined yet, whether the sparks dispose of sufficient energy to ignite gas or dust. This energy potential depends on the size of the free surface area of the non-conductive material which faces the ignition mixture.
Only when the projected area of the chargeable surface exceeds the size determined by one of the harmonised standards (EN ISO 80079-36), laboratory tests must be conducted to determine the electrostatic charge transfer, i.e. the minimum ignition charge. The minimum ignition charge describes the ignition sensitivity of a substance as against the discharge of static electricity, and the assessment of explosive hazards can be conducted according to the ATEX Directive.
To assess the electrostatic ignition hazards of a claw coupling within the frame of the ATEX Directive, the following analysis steps have to be performed:
- Determination of the size of the projected surface area of the electrostatically chargeable elastic intermediate ring
- Only if the limit size is exceeded, the surface resistivity of the elastic intermediate ring must be measured to classify the material as insulating or non-insulating.
- Only if the elastic intermediate ring is insulating, a charge measurement must be performed and the charge transfer be determined.
- The final step is the explosion risk assessment for the explosion groups defined in the Directive under consideration of the charge transfer.
In practice, this can mean that the evaluation of the electrostatic charge is not necessary for a coupling up to a certain size, and that the Directive has been satisfied with the calculation of the projected surface area.
The above example demonstrates that the EU type examination certificate also involves many analyses and tests that have to be conducted to fulfill the requirements relating to the assessment of electrostatic ignition hazards in accordance with the Directive 2014/34/EU.