Table of Contents
Introduction
Hazardous Area Classification is must for the process industries, because there are so many type of electrical, electronic and instrumentation systems, equipment, instruments installed for the processing raw material to get final end products. These industries use many type of materials for process operation and control which can be dangerous for health, safety and environment (HSE). These operations / stages for processing, creates different type of areas and zones which determines the risk and criticality of the system, equipments and instruments. The method which determines area classification is required for analyzing environment and basic necessity for HSE. Those are where risk or possibility of fire and explosion might occur, are termed as Hazardous Area. Since we all know that a fire can take place only when there is presence of all 3 elements to complete fire triangle-“Heat, Fuel & Oxygen” and an explosion may occur in presence of 6 elements of the explosion hexagon as shown in figure-
So to avoid any fire, explosion, the prime necessity is hazardous area classification. Other than that design and selection of the electrical and electronic equipments and instruments is performed in such a way that used protection methods eliminate any one or more element from the fire triangle or explosion hexagon. The main advantage of Hazardous area classification are-
- To reduce overall installation risk
- To provide working environment safe.
- To provide required safety & mitigation method.
- Selection of proper instrument capable to sustain in hazardous area.
- To develop standard operating procedure.
Approved Standards
For the Hazardous Area Classification, Several Standards, rules and regulations are followed. These standards are applied all over the world by approval agencies such as-
IECEx-
- CSA—Canadian Standards Association Baseefa
- British Approvals Service for Electrical Equipment in Flammable Atmospheres
- Approval accepted by Internationally
ATEX
- Baseefa—British Approvals Service for Electrical Equipment in Flammable Atmospheres
- Approval accepted by European Union
SAA
- Standards Association of Australia
- Approval accepted by Australia
FM
- Factory Mutual
- Approval accepted by North America
CML
- Certification Management Limited
- Approval accepted by Japan
CSA
- Canadian Standards Association
- Approval accepted by North America
NEPSI
- National Supervision and Inspection Centre for Explosion Protection and Safety of Instrumentation
- Approval accepted by China
TIIS
- TIIS—Technology Institution of Industrial Safety
- Approval accepted by Japan
INMETRO
- INMETRO—National Institute of Metrology, Quality and Technology
- Approval accepted by Brazil
CUTR
- FGUP Certification Centre: SC VSI VNIIFTRI Certification Body: OS VSI VNIFFTRI
- Approval accepted by Russia, Belarus, Kazakhstan, and Armenia
Methodology of Hazardous Area Classification
Currently classification of Hazardous Area is performed by two systems-
- Class/Division system
- Zone system
The Complete classification regarding hazardous area is shown as follows-
Class/Division System
The Class/Division system is used predominately in the United States and Canada. Hazardous locations per the Class/Division system are classified according to the Class, Division, and Group.
Class
The Class defines the general nature (or properties) of the hazardous material in the surrounding atmosphere which may or may not be in sufficient quantities.
- Class I—Locations in which flammable gases or vapors may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
- Class II—Locations in which combustible dusts (either in suspension, intermittently, or periodically) may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
- Class III—Locations in which ignitable fibers may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
Division
The Division defines the probability of the hazardous material being able to produce an explosive or ignitable mixture based upon its presence.
- Division 1 indicates that the hazardous material has a high probability of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically or from the equipment itself under normal operating conditions.
- Division 2 indicates that the hazardous material has a low probability of producing an explosive or ignitable mixture and is present only during abnormal conditions for a short period of time.
Group
The Group defines the type of hazardous material in the surrounding atmosphere. Groups A, B, C, and D are for gases (Class I only) while groups E, F, and G are for dusts and flyings (Class II or III).
- Group A—Atmospheres containing acetylene.
- Group B—Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is less than 0.45 mm or MIC ratio is less than 0.40. Typical gases include hydrogen, butadiene, ethylene oxide, propylene oxide, and acrolein.
- Group C—Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is greater than 0.45 mm but less than or equal to 0.75 mm or MIC ratio is greater than 0.40 but less than or equal to 0.80. Typical gases include ethyl either, ethylene, acetaldehyde, and cyclopropane.
- Group D—Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is greater than 0.75 mm or MIC ration is greater than 0.80. Typical gases include acetone, ammonia, benzene, butane, ethanol, gasoline, methane, natural gas, naphtha, and propane.
- Group E—Atmospheres containing combustible metal dusts such as aluminum, magnesium, and their commercial alloys.
- Group F—Atmospheres containing combustible carbonaceous dusts with 8% or more trapped volatiles such as carbon black, coal, or coke dust.
- Group G—Atmospheres containing combustible dusts not included in Group E or Group F. Typical dusts include flour, starch, grain, wood, plastic, and chemicals.
Zone system
Zone system if followed by all over the world except United States and Canada. However, the United States and Canada are trending more towards the Zone System.
Hazardous locations per the Zone system are classified according to its Zone which can be gas or dust. For gas atmospheres electrical equipment is further divided into Groups and Subgroups.
Zone
The Zone defines the probability of the hazardous material, gas or dust, being present in sufficient quantities to produce explosive or ignitable mixtures.
Zone classification for Gas
- Zone 0—Ignitable concentrations of flammable gases or vapors which are present continuously or for long periods of time.
- Zone 1—Ignitable concentrations of flammable gases or vapors which are likely to occur under normal operating conditions
- Zone 2—Ignitable concentrations of flammable gases or vapors which are not likely to occur under normal operating conditions and do so only for a short period of time.
Zone classification for Dust
- Zone 20—An area where combustible dusts or ignitable fibers and flyings are present continuously or for long periods of time.
- Zone 21—An area where combustible dusts or ignitable fibers and flyings are likely to occur under normal operating conditions.
- Zone 22—An area where combustible dusts or ignitable fibers and flyings are not likely to occur under normal operating conditions and do so only for a short period of time.
Group
Electrical equipment is divided into three groups-
- Group I—Equipment intended for use in mines susceptible to firedamp (flammable mixture of gases naturally occurring in a mine).
- Group II—Equipment intended for use in places with an explosive gas atmosphere other than mines susceptible to firedamp. Group II equipment is subdivided into three subgroups.
-
- Group IIA—Atmospheres containing propane, or gases and vapors of equivalent hazard.
- Group IIB—Atmospheres containing ethylene, or gases and vapors of equivalent hazard.
- Group IIC—Atmospheres containing acetylene or hydrogen, or gases and vapors of equivalent hazard.
- Group III—Equipment intended for use in places with an explosive dust atmosphere. Group III equipment is subdivided into three subgroups.
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- Group IIIA—Atmospheres containing combustible flyings.
- Group IIIB—Atmospheres containing non-conductive dust.
- Group IIIC—Atmospheres containing conductive dust.
Protection Techniques and Methods
Various protection techniques and methods have been developed and employed, thus reducing or minimizing the potential risks of explosion or fire from electrical equipment located in hazardous locations. Not all methods are listed.
Class/Division system
Explosion-proof
A type of protection that utilizes an enclosure that is capable of withstanding an explosive gas or vapor within it and or preventing the ignition of an explosive gas or vapor that may surround it and that operates at such an external temperature that a surrounding explosive gas or vapor will not be ignited thereby.
Intrinsically Safe
A type of protection in which the electrical equipment under normal or abnormal conditions is incapable of releasing sufficient electrical or thermal energy to cause ignition of a specific hazardous atmospheric mixture in its most easily ignitable concentration.
Dust Ignition-proof
A type of protection that excludes ignitable amounts of dust or amounts that might affect performance or rating and that, when installed and protected in accordance with the original design intent, will not allow arcs, sparks or heat otherwise generated or liberated inside the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust.
Non-incendive
A type of protection in which the equipment is incapable, under normal conditions, of causing ignition of a specified flammable gas or vapor-in-air mixture due to arcing or thermal effect.
Zone system
The below concepts are high-level protection concepts. There are also sub-levels of protection that may or not be applicable to each type. Also, some equipment may combine multiple types of protection.
Flame-proof
A type of protection in which an enclosure can withstand the pressure developed during an internal explosion of an explosive mixture and that prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure and that operates at such an external temperature that a surrounding explosive gas or vapor will not be ignited there. This type of protection is referred to as “Ex d”.
Intrinsically Safe
A type of protection in which the electrical equipment under normal or abnormal conditions is incapable of releasing sufficient electrical or thermal energy to cause ignition of a specific hazardous atmospheric mixture in its most easily ignitable concentrations. This type of protection is referred to as “Ex i”.
Increased Safety
A type of protection in which various measures are applied to reduce the probability of excessive temperatures and the occurrence of arcs or sparks in the interior and on the external parts of electrical apparatus that do not produce them in normal service. Increased safety may be used with flame-proof type of protection. This type of protection is referred to as “Ex e”.
Type n
A type of protection applied to electrical equipment such that in normal operation it is not capable of igniting a surrounding explosive atmosphere. This type of protection is referred to as “Ex n”.
Type t
A type of protection in which the electrical equipment is equipped with an enclosure providing dust ingress protection and a means to limit surface temperatures. This type of protection is referred to as “Ex t”.
Type h
Refers to one of three different types of protection: (1) where constructional measures are applied to protect against the possibility of ignition from hot surfaces, sparks and compression generated by moving parts; (2) ignition protection where mechanical or electrical devices are used in conjunction with nonelectrical equipment to manually or automatically reduce the likelihood of a potential ignition source from becoming an effective ignition source; or (3) protection where potential ignition sources are made ineffective or separated from the explosive atmosphere by either totally immersing them in a protective liquid, or by partially immersing and continuously coating their active surfaces with a protective liquid in such a way that an explosive atmosphere which may be above the liquid, or outside the equipment enclosure, cannot be ignited. Non-electrical equipment often apply “Ex h” protection methods.
Equipment Protection Level (EPL) Markings
The EPL marking indicates the level of protection that is given to equipment based on the likelihood of its becoming a source of ignition and distinguishing the difference between explosive gas atmospheres, explosive dust atmospheres, and the explosive atmospheres in mines susceptible to firedamp.
Temperature Code (T Code)
A mixture of hazardous gases and air may ignite by coming into contact with a hot surface. The conditions under which a hot surface will ignite a gas depend on surface area, temperature, and the concentration of the gas present in atmosphere. The same can be said about combustible dusts. The T code of a product denotes the maximum surface temperature that a given product will not exceed under a specified ambient temperature. A list for Temperature code classification is shown as follows-
How to Read Equipment Hazardous Area classification nameplate
To identify whether the equipment is suitable for hazardous area is most important for safety purpose. It should be noted that equipment safety level is up to mark and should not create any risk related to equipment protection.
From the above article, we have known how to classify hazardous area & protection techniques. There are two main methodology i.e. Class/Division system and Zone system. Approval agencies following class/ division system mark equipment according to the following sequence–
“CLASS, DIVISION, GROUP & TEMPERATURE CODE”
While Zone system Approval agencies provide equipment marking according to the protection concept for which it has been designed. Approval agencies following Zone system mark equipment according to the following sequence–
“PROTECTION, GROUP & TEMPERATURE CODE”
To understand it some Examples are listed as follows-
- Class I Division 1 Group B,C,D T5
- CL I Div 2 GP ABCD T5 IS
- CL I,II,III Div 1 GP ABCDEFG
- CL II,III Div 1,2 GP EFG T4
- Ex ia IIC T5
- Ex d IIB+H2 T6 Ex
- nA IIC T6 Class I Zone 2 AEx nC IIC T5
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