The assessment as to the extent of the Zone is a requirement of DSEAR, the methodolgy for assessment clearly defined in EN 60079-10-1 and should be done by a suitable Competent person. It is not within the scope of this site to provide a detailed guide to zoning as the standard does that quite well, but merely to point out some points which are sometimes misunderstood or overlooked..
The zone definitions are reasonably well known, taking gas as an example.
Gas Zones: ( Defined in EN 60079-10-1)
Zone 0:Area in which an explosive gas atmosphere is present continuously or for long periods or frequently
Zone 1: Area in which an explosive gas atmosphere is likely to occur in normal operation occasionally
Zone 2: Area in which an explosive gas atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only
Deciding on Zones (normally called Zoning) is often done a very simplistic level, on the estimated potential presence of the gas, sources, leaks etc. simply on a time basis.
The following time periods are regularly quoted despite the fact that they are not defined in any standard:
Zone 0: Explosive atmosphere for more than 1000h/yr
Zone 1: Explosive atmosphere for more than 10, but less than 1000 h/yr
Zone 2: Explosive atmosphere for less than 10h/yr
The question is largely ignored as to what these periods refer to i.e. the explosive atmosphere being potentially present.
Some assessors have been known to just use the typical examples of zone extents as shown in the standards rather than conducting proper evaluation. This may be acceptable if all the details are the same in reality as the example, but that is rarely the case.
In order to assess the risk the hazards need to be defined, for gas:1
|Nature of the gas||Nature of the source (release)|
|What gas(es) are present?||Probability of occurence (high, medium or low)|
|Is this heavier or lighter than air?||Potential Duration of release|
|What is the Lower Flammable Limit (LFL)?||Potential Volume of release|
|What is the gas group?|
Evaluating the zone limits is not just a time factor calculation but involves the nature of the source i.e. physical parameters of the Gas/Vapour.
The method and calculations to evaluate the zones are given in EN 60079-10. Of course assumptions can be made, but these should be detailed and perhaps justified so the risk can be assessed.
The sketch of a liquid storage tank is a typical example of zoning as given in the standard, I have not included example dimensions as they can be misleading often used in place of actual assesment.
It is commonly used example showing Zone 0 in a fuel storage silo, Zone 1 directly around the vent pipe, Zone 2 outside the silo where probability of reaching LFL is lower and then Zone 1 in the sump where fuel can collect. There are typical examples in EN 60079-10-1. However, the extent of the zone will change for a different type of fuel e.g. Diesel, which does not give off a lot of vapours compared with petrol storage.
Note that as the extent of the zone is evaluated as 'distance to dilution below the LFL' and a time probability factor if there is a zone 1 then there is usually a zone 2 (in an open space). Of course for low level source time/pressure/volume, Zone 1 extent may be insignificant.
A simple example of a potential source of hazard is leakage from flange.
The probability and scale of a flange failure needs to be assessed, then the consequences. The distance to the point where the leakage is below the Lower Flammable Limit (LFL) would need to be calculated based on ventilation, pressure, nature of the liquid or gas and the time factor considered to give the extent of the Zones.
In the event that a probable leak/ failure does not give sufficient gas to meet the LFL at the normal process operating temperature then there is no zone, although it may be prudent to declare a small Zone 2 if considered critical for flammability protection2.