Classification studies
Responsibility for ensuring that classification is carried out rests with the person in charge or, if such a person has not yet been appointed, with the person responsible for the operation.
Classification shall be carried out by a technician or a group of technicians who are thoroughly familiar with the operation and with the functional safety of the equipment in question and the properties of the explosive substances, and who are able to comprehensively assess the explosion risks present. Classification studies should be carried out at an early stage in the planning of a facility and when changes are made to the operation.
In the case of laboratories, the person in charge may, following a risk assessment, decide to combine the classification with, or replace it by, specific conditions or instructions.
It is important that facilities are designed so that hazardous areas are few and limited in extent. In addition, efforts should be made to ensure that hazardous areas mainly consist of the zone with the lowest risk class (E2 instead of E1).
To assess different design alternatives with regard to the consequences for classification and equipment, ongoing consultation should take place during the work with experts in the classification field and with technicians.
A classification plan shall be prepared through a systematic analysis based on the factors stated below.
To facilitate the preparation of a classification plan, certain typical examples are available. In individual cases, however, local conditions must always be taken into account.
The extent of each hazardous area is assessed with regard to the properties of the substance handled, the hazard source and the surroundings. The extent of the zones is determined individually, depending on the different hazard sources present.
The extent of the hazardous area is mainly determined by the following factors:
– the nature of the operation, work process
– type of equipment
– type of explosive substance
– physical state and particle size
– quantity of explosive substance
– air movement, ventilation
– room and environmental conditions.
The extent of the hazardous area is usually affected as follows:
– increases with increasing quantity of explosive substance
– increases with decreasing particle size
– decreases with improved ventilation
– increases or decreases with local conditions. An obstacle may, for example, impede ventilation. A wall may limit dispersion.
When carrying out classification, consideration shall be given to the need for such operational measures that result in closed systems being opened for, for example, filter replacement, maintenance work, emptying or cleaning. When classifying a hazardous area, future changes should be taken into account that may mean the hazardous area is assigned a stricter classification. If the production equipment is designed in such a way that the explosive substance may be released, which is the case for most mechanical installations (apparatus), it is necessary to determine the type of source of risk. The source of risk may be primary or secondary. This is determined based on the definition in the article Sources of risk and hazardous areas. Examples of a primary source of risk include all open handling of explosive substances where dust, liquid or vapour may be emitted to the surroundings to a significant extent and thereby cause damage if the substance ignites. A primary source of risk generally results in a zone E1 area. Examples of a secondary source of risk are places where, during normal operation, explosive substances are not dispersed to the surroundings but where, due to operational disturbances, etc., which do not need to be of an accident or catastrophic nature, releases may occur. Such places may be flange joints, mechanical seals, non-tight-fitting covers, etc. Once the source of risk has been determined, it is necessary to determine the magnitude of the release and to consider other factors that may affect the zones and their extent.
Measures in hazardous areas
When changes are made to handling, the classification plan must be checked and, if necessary, revised in light of the changes made. Changes in work routines, etc., may also affect the classification. When handling explosive goods, spillage, dust, etc., may occur. Cleaning routines shall be in place. By collecting spillage, dust, etc., in a suitable container, the hazardous area is minimised. Dust should, where possible, be collected where it is generated, for example during mechanical processing, and disposed of, for example by local exhaust ventilation to a separator. Equipment shall not be unnecessarily exposed to deposits of dust, sublimate, etc., from explosive substances. Only equipment necessary for the operation shall be installed within a hazardous area. The above measures shall be taken into account already during the design of new facilities.
Classification plan
Classification shall be documented in a classification plan. The classification plan shall state:
– the division of the hazardous area into zones and their extent both horizontally and vertically
– for zone E1, the temperature class. If this is ET 2, the ignition temperature shall also be stated.
A classification plan may be prepared in several ways, for example in tabular form or as a drawing similar to the typical examples in Section 6. These examples have deliberately been designed differently to show different possibilities. The marking of zones shall be clear and unambiguous.
One way to mark zones on a drawing is to place boxes in the corners (of rooms or zone boundary lines) indicating E1, E2 or E3, see, for example, Figure 1.
The basis used to determine each zone and its extent, for example the quantity handled and type of explosive goods, the design of the ventilation system and the ventilation flow rate, shall be documented and retained.
SEK Swedish Electrotechnical Standard
