Quantative “atex” risk assessment: An alternative method for (parts of) rotating equipment

Methods for assessing the risk of mechanical rotating equipment in an explosive atmosphere, both in normal use and during the occurrence of failures so far have been qualitatively supported. The extension of procedural protective measures is based on subjective judgement. If the assessment is carried out by a manufacturer, then the procedural protective measures are assigned as “equipment under control”. When technical protection measures (instrumental safety provisions) are necessary, the equipment in most cases will be assigned as category 2. The equipment brought to the market nowadays is therefore, when designed for hazardous areas, most often suitable for zone 1. These assessments often don't take into account the possibility for use exclusively in zone 2. Even at the slightest chance of a malfunction, a protective measure is defined, which is not always necessary for the safe operation in zone 2, but on the other hand has influence on the availability. Because of the desire to have an objective risk assessment method, a search was started for finding available risk assessment methodologies for situations where additional procedural and/or technical protective measures should be chosen. An analogy is found in the methodology for determining integrity levels of instrumental protective devices as described in IEC 61508. The malfunctioning of ball bearings has been chosen as an example to prove consistency of the method with the general accepted practice.