Hazardous Areas and Zonal Classification

Introduction

A HAZARDOUS area / environment may be potentially prone to:

• An accident
• The creation of a dangerous situation
• Being beset with dangerous situations
• Flash points
• Being explosive
• Being inflammable or flammable
• Being radioactive etc

Hazards can be defined as the following:

Urban Structure Fires: Perhaps the most common human-caused hazard (often a disaster) is fire in large occupied buildings. Causes can be accidental or deliberate, but unless structures have been built to safe fire standards, and sound
emergency procedures are used, heavy loss of life can result.

BLEVE: An entire community was involved at Mississauga, Ontario, Canada when 250,000 had to be evacuated to avert disaster following a train accident which triggered a series of BLEVE (Boiling Liquid Expanding Vapour Explosions).

Other Explosions: Great loss of life occurred in Halifax, Nova Scotia, Canada in 1917 when a ship carrying explosives collided with another. Australia’s most disastrous explosion was in the Mt Kembla mine, Wollongong, in 1902, when 95
miners died. One of the worst non-mining explosions occurred in 1974 at the Mt St Candice Convent in Hobart, when seven died in a boiler explosion

Basis of area classification

The first step in Area Classification is to list and identify the areas in the plant where there is the possibility of a conducive atmosphere for explosion or fire to occur. Based on the knowledge so acquired, the design, selection and operation of the equipment has to be influenced in such a way that the risk of fire or explosion
taking place is minimized. It is useful to understand what a Non-hazardous (safe) Area is. “An area classified as non-hazardous has a small probability of a flammable mixture being present. It is also called a “safe area” and includes most control rooms.”

Area classification cannot be set rigidly into any standard. Each installation will be different in some respect and therefore each site must be examined on its individual merits. There are three situations that can occur in an operating plant with reference to hazardous areas:

• A situation where an explosive atmosphere is present always or for long periods because of operational requirement, i.e. continuous.
• A situation where explosive atmosphere occurs frequently, or if infrequently may persist for a considerable time, i.e. primary.
• A situation in which explosive atmosphere occurs rarely and normally results from failure of equipment or procedures, i.e. secondary.

The above criteria are used in classifying the areas under ‘Source of Release’ methodology of classification. Some typical examples of sources of release are:
• Open surface of liquid
• Virtually instantaneous evaporation of a liquid (for example from a jet or spray)
• Leakage of a gas mixture
• principles governing the sources of release
• Sources giving a continuous grade of release
• Sources giving a primary grade of release
• Sources giving a secondary grade of release

Zonal Classification

 Gases, vapour and mists

Areas where there is the likelihood of the presence of explosive gas-air mixtures are referred to as zones. Zones are classified as shown in the table below. The higher the number in this ‘Zonal classification’ the smaller is the risk of an
explosion. This is as per IEC 79:

Dusts

In respect of dust, the situation had been much more fluid. In recent times effort has been made to address this by classifying the Zones in a way which is similar to that adopted for gas and vapour.

In carrying out an area classification, it is necessary to:

• Identify those parts of the plant where flammable dust can exist including, where appropriate, the interior of process equipment
• Assess the likelihood of occurrence of a flammable atmosphere thereby establishing the appropriate zonal classification
• Delineate the boundaries of the zones taking into account the effect of likely air movement
• Take into account, when assessing the area classification of a plant, the influence of the classification of adjacent plants

Also read 

Difference between IEC 61508 & IEC 61511 SLC Version

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