Hazardous Area Classification

INTRODUCTION:

Hazardous area is the area in which an explosive gas atmosphere is present, or likely to be present, in quantities such as to require special precautions for the construction, installation and use of electrical apparatus.

Hazardous area classification is a method of  analysing and classifying the environment where explosive gas mixture may occur to allow the proper selection of instrument to be installed in that environment.

HAZARDOUS AREA CLASSIFICATION

Hazardous areas are classified primarily based on the following concepts:

•         Based on type of substance

The type of substance determines the energy required to ignite the substance (e.g. acetylene requires less energy to ignite than propane). The type of substance also indicates how it will disperse in case of leakage. (e.g. then gas may affect larger area than dust in case of a leakage).

•         Based on frequency and duration for which hazardous condition is present.

HAZARDOUS AREA IDENTIFICATION (EXTENT OF AREA)

Hazardous area identification is done to determine the extent of an area which would be classified as hazardous.

The area surrounding the location identified as HAZARDOUS is extended to such a distance as to where the flammable substance becomes so diluted with air that ignition is no longer possible. This distance is related to the nature and quantity of the gas, degree of ventilation, etc.

The hazardous area classification and identification is done with the understanding that:

• Over specification could result in considerable expenditure in providing suitable instruments and electrical apparatus.
• Under-specification may lead to safety hazard

The hazardous area classification and identification is done by a team of experienced engineers after careful consideration to following aspects:

• The properties of materials handled in the manufacturing
• Possible sources of leakage depending on types of equipment used
• Earlier experience in running similar plants.

Generally, we receive hazardous area classification and identification from process licensor.

STANDARDS FOR HAZARDOUS AREA CLASSIFICATION:

The hazardous area classification and identification is done based on statutory standards. Different regions and countries follow different standards. The standard to be used for hazardous area classification shall be frozen before activities for designing of a plant are started.

Sr No.	Standard reference	Applicable in region/countries	Remarks
1	IS5572	India	Similar to IEC 79-10
2	IEC 79-10	Europe	Only for gases. Standard does not cover dust and fibre.
3	NFPA70 Art 500 of NEC	North America, Canada

TABLE FOR HAZARDOUS AREA CLASSIFICATION:

	Classification based on energy required for ignition		Classification based on probability
State of substance	IEC 79-10	NFPA70	NFPA70	IEC 79-10
Gas	Group II	Class I	DIV1: Hazardous gas can be present in normal functioning DIV2: Hazardous gas can be present in case of abnormal functioning	Zone 0: Continuously present for long periods Zone 1: Intermittent as part of normal operation Zone 2: Rarely, in ABNORMAL condition, for short period(Note 2)
Acetylene	Group IIC	Group A
Hydrogen	Group IIC	Group B
Ethylene	Group IIB	Group C
Propane	Group IIA	Group D
Dust,Flammable (Note 4)	NA	Class II
Non conductive (wheat)	NA	Group G
Conductive but non-metallic (coal)	NA	Group F
Conductive and metallic(aluminium)	NA	Group E
Fibre (particle size much greater than dust)	NA	Class III	DIV1: Flammable fibres are HANDLED
			DIV2: Flammable fibres are STORED (Note1)
Remarks	Group I: coal mines where danger is from methane gas or coal dust.

Note 1:

These divisions for fibres are not defined in NFPA70 but followed in the USA based on ISA standard.

Note 2:

The words “continuously” or rarely do not have any specific meaning. The Italian standard CEI 64-2 gives following definition for divisions(which are equivalent to zones in IEC 79-10)

DIVISION	TIME OF PRESENCE WITHIN A YEAR
0	More than 100 hrs
1	Between 1 hour to 100 hours
2	Between 0.01 hour to 1 hour

Note 3:

The gases are classified based on the minimum energy levels required to ignite the mixture of gas and air. Following table gives minimum ignition energy levels based on which gas grouping is done.

Typical gas hazard	Europe	North America	Min. ignition energy (micro joules)
Acetylene	Group IIC	Class I, Group A	20
Hydrogen	Group IIC	Class I, Group B	20
Ethylene	Group IIB	Class I, Group C	60
Propane	Group IIA	Class I, Group D	180

           

           

Note 4:

Non-conductive means resistivity more than 10⁸ ohm-cm.

Conductive but non-metallic means resistivity between 100 and 10⁸ ohm-cm

Metallic means resistivity less than 100 ohm-cm.

GUIDELINES FOR CLASSIFICATION OF ZONES:

ZONE 0 AREAS:

Some of the examples of Zone 0 areas are listed below:

• Vapour space inside close tanks
• Space in the immediate vicinity of a tank vent containing hydrocarbons with nitrogen blanketing. When such a tank is being filled up the blanketing gases containing hazardous vapours will be expelled throughout the filling operation. Obviously filling operation in such a tank could be quite regular and hence it is classified as Zone 0.
• A process might be so sensitive to control that relief valves frequently open which might be considered normal and location adjacent to  the point of release should be classified as Zone 0. However, if the relief valves operate infrequently under usual conditions, it is not to be considered normal

ZONE 1 AREAs:

• Zone 1 locations may be distinguished when any of the following conditions exist:
• Flammable atmospheric concentration is likely to occur frequently because of maintenance, repairs or leakage.
• Failure of process, storage or other equipment is likely to cause an electrical system failure simultaneously with the release of flammable gas or liquid.
• Flammable liquid or vapour piping system(containing valves, meters or screwed or flanged fittings) in an inadequately ventilated area.
• The area below the surrounding elevation or grade is such that the flammable liquids or vapours may accumulate therein.
• Imperfectly fitting peripheral seals on floating roof tanks.
• Inadequately ventilated pump rooms for flammable gas or for volatile, flammable liquids.
• Interiors of refrigerators and freezers in which volatile flammable materials are stored in lightly stoppered or easily ruptured containers.
• API separators
• Oily waste water sewers/basins

ZONE 2 AREAS:

• Zone 2 locations may be distinguished when any one of the following conditions exist:
• The system handling flammable liquid or vapour is in an adequately ventilated area and is so designed and operated that the explosive or ignitable liquids, vapours or gases will normally be confined within closed containers or closed systems from which they can escape only during abnormal conditions such as accidental failure of a gasket or packing,
• The flammable vapours can be conducted to the location as through trenches, pipes or ducts.
• Consider a process pump with a mechanical seal. The leakage is possible only in case of failure of a mechanical seal. The failure of mechanical seal can be considered as an abnormal event and hence area surrounding the pump can be classified as Division 2.

METHODS OF PROTECTION

                                                                 Ignition energy

                                                           Oxidiser                  Fuel                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           For any reaction like combustion, oxidation and explosion to take place the following three components in due proportions should be present simultaneously:

Fuel: gas,vapours or powdered form

Oxidiser: generally air or oxygen

Ignition energy: thermal or electrical

All the protection methodologies aim at eliminating one or more of the triangle components to reduce the risk of igniting an explosion to an acceptable level.

Following are some of the methods of protection:

1. Explosion-proof enclosure: Ex “d”

In this method, explosion is allowed to occur but is confined to an enclosure built to resist the excess pressure created by an internal explosion. The flame generated is prevented from propogating to the outside atmosphere by choosing a flame path which also cools down any products of the internal explosion to such a level that the flammable environment outside cannot be ignited.

• An explosion proof enclosure need not be gas tight
• Material of construction is generally metallic generally aluminium
• Opening of an enclosure is not permitted while the device inside is powered on.

       2. Pressurization: Ex”p”

A protective gas (air or inert gas) is kept inside the enclosure with a pressure slightly greater than the one of the external atmosphere.

• Generally used for large enclosures such as panels
• Opening of enclosure is not allowed while the device inside is powered on

      3. Encapsulation: Ex “m”

This protection method is based on segregation, of those electrical parts that by means of sparks or heating can ignite a dangerous mixture, by means of a resin resistant to the specific ambient conditions.

Generally, this method is used to protect electrical circuits of modest dimension that do not contain moving parts unless these are already inside an enclosure that prevents the resin from entering (ie: reed relays)

    

    4. Oil immersion : Ex “o”

By immersing the ignition source in oil, contact between the source and the combustible atmosphere is prevented.

     5. Powder filling Ex “q”

Powder material is filled into the enclosure to prevent the flammable atmosphere from coming in contact with the ignition source.

       6. Increased safety Ex “e”

Electrical apparatus is designed in such a way that it does not produce any sparks in normal functioning and also so as to reduce chances of abnormal occurrence which could cause sparks or temperatures with sufficient energy to ignite a flammable atmosphere.

        7. Normally non-sparking circuits (Non-incendive): Ex “n”

Non-incendive circuits may contain components which may spark under normal operating conditions but the energy released by such components is limited in normal operation to a value incapable of causing ignition. This method of protection is used in Zone II only.

         8. Special protection: Ex “s”

This method, of German origin, is not covered by any CENELEC or IEC standard and is not recognized in North America. It was developed to allow certification of apparatus which even if not protected as per any of the common protection methods, can be considered safe by means of appropriate tests and/or detailed analysis of a design.

          9. Intrinsic safety: Ex “I”

It is based on the principle that to ignite an explosive atmosphere a certain minimum amount  of ignition energy is required. An IS circuit is virtually incapable of generating archs or sparks or thermal effects that are able to ignite an explosion of a dangerous mixture both in normal functioning and specific fault conditions.

According to CENELEC, there are two categories:

Category “ia” allows upto 2 independent faults and can be used for Zone 0 application while category “ib” allows only one fault and can be used only in Zone 1.

SURFACE TEMPERATURE CLASSIFICATION:

An apparatus directly located in HAZARDOUS LOCATION must also be classified for maximum surface temperature that can be generated by the instrument either in normal functioning or fault condition.

The maximum surface temperature must be lower than the minimum ignition temperature of the gas present.

SURFACE TEMPERATURE CLASSIFICATIONS IN EUROPE AND NORTH AMERICA

MAX SURFACE TEMP °F	MAX SURFACE TEMP °C	EUROPE	USA/CANADA
842	450	T1	T1
572	300	T2	T2
536	280		T2 A
500	260		T2 B
446	230		T2 C
419	215		T2 D
392	200	T3	T3
356	180		T3 A
329	165		T3 B
320	160		T3 C
275	135	T4	T4
248	120		T4 A
212	100	T5	T5
185	85	T6	T6

It is evident that an apparatus classified for a temperature class can be used in the presence of all gases with an ignition temperature higher than the temperature class of the instrument. For example, an apparatus classified as T5 can be used with all gases having an ignition temperature greater than 100°C