3. Introduction to Ex

The Hazardous Areas (Ref: IEC 60079-10, 14)

Areas in which an explosive atmosphere is present , or may be expected to be present , in quantities such as to require special precautions for the construction , installation and use of equipment. (An Area is a three dimensional region or space)

The Hazardous Contents associated with an area , are quantified through the process of Area Classification

Objectives of Area Classifications

The basic underlying philosophy is SAFETY of life and property by quantifying the location.

A special methodology is used by virtue of which an industrial location ( 3 dimensional space ) is analyzed and classified into Classes & Zones , keeping in view the chemicals/substances involved & the industrial procedures being carried out in that location.

An approximate structure of the procedure is outlined in the following block diagram

Some everyday situations where an explosive atmosphere exists but is usually ignored and the safety is taken for granted.

Petrol – Gasoline Station – Gasoline – Diesel, CNG, PETROL, DIESEL & LPG (GAS in Cylinders ) dispensing stations.

All these locations are classified as Class 1, Zone 1 & Zone 2 locations

The CNG Compressor room and the cascade is Class 1, Zone 1.

The Petrol, diesel & CNG DISPENSERS are all specially built machines with Class 1, Zone 1 certified components.

The Underground storage tank-inside is Most dangerous and is classified as Class 1, ZONE 0.

The air vents and the sumps are Class 1 Zone 1 locations

Hazardous Area near the Dispensing Unit

Gazette of Pakistan

has used the CLASS & DIVISION classification Methodology . The underlying philosophy is same.

Ref: Compressed Natural Gas Production & Marketing Rules 1992 Gazette of Pakistan.

Physical Layout of  Hazardous Area (into Zones)

Ideally, the object of the design engineer is to keep Zone 0 small , surrounded by a comparatively larger Zone 1 area , surrounded by an even larger Zone 2 area.

The Safety level increases from zone 2  to Zone 0

Another view

(Short Note :How to control the extent of the zone)

Reference: IEC 60079-10-1 the basic elements for establishing the types of Hazardous Area Zones are as follows:

1.     Identification of Sources of Release
2. ……..   And  Grades of Release
3. ……….The degree of Dilution
4.  ..The availability of VENTILATION 

(In an industrial process) The flammable gas or vapors are contained within the process equipment; any release from the process equipment will definitely create an explosive atmosphere depending upon the Grade of release.

A simple rule of thumb is: REF: IEC 60079-10-1.  A continuous grade of release normally leads to zone 0, a primary grade to zone 1 and a secondary grade to zone 2.

The extent of the zone depends on the estimated or calculated distance over which an explosive atmosphere exists before it disperses to a concentration in air below its LOWER FLAMMABLE LIMIT. (The explosive Triangles just disappear )

One way to dilute the release is to utilize the process of VENTILATION.

(It is the movement of air and its replacement with fresh air due to the effects of wind, temperature gradients, or artificial means .Example: Fans or extractors)

So Ventilation is one way to ‘control’ the extent of the zone.

These terms : Good Ventilation , Fair Ventilation  & Poor Ventilation are the terms used under the main heading of VENTILATION ->Classification of Hazardous Areas

The effectiveness of Ventilation can be expressed in terms of DEGREES of DILUTION, namely :-

1. High Dilution
2. Medium Dilution
3. Low Dilution

Logically, dilution is the result of the DISPERSION of the explosive mixture , thus the

sequence of events can be expressed in the following block diagram.

Now the next logical requirement is the AVAILABILITY of VENTILATION.

An Example (from IEC 60079-10 ) to show the classification of Hazardous Area & related parameters.

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