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Define Hazardous Area Classification (HAZAC)

The scope of this guide is to drawn the guidelines of several different Recommended practice(s) for the Area Classification of a process plant. The area classification is required for the installation of the electrical equipment with  the related specific protection kind within a process area. The basic definition, and the following modifications is based mainly on the 1996 NFPA 70, The national Electrical Code (NEC) and the API 505 Recommended Practice (API RP 505). Once that a location has been classified, requirements for electrical equipment and associated wiring should be determinate from applicable publications (e.g. NFPA 70 and API Recommended Practice 14F (API RP 14F) and local regulations.
The final scope of the document is to achieve the classification of both permanently and temporarily installed electrical equipment. The application is designed in relation to their potential risk of ignition source in presence of an ignitable mixture of “fuel”, or a flammable/ignitible substance, and Oxygen (Air) under normal atmospheric conditions.

Reference Atmospheric Conditions
Pressure101.3 Kpa14.7 Psia
Temperature20°C (293.15 K)68°F

The document provides that is no relevant changes related to the change of the atmospheric conditions from the reference point. On the basis provided earlier, the guide is developed on the recommended practice based on the petroleum facility zones (where ignitable liquids, gases, and vapors are processed, handled and loaded).

References, Codes and Reference Standards:
Actually, there are many Reference standards and industrial codes as reference for the plant area classification. Part of them are developed on the same basis, others are very particular and applied in specific plant type (e.g. Drilling Facilities, Petroleum and petrol chemical plants).
The Hazardous Area Classification presents in this guide is based on the following items as reference:
 API:
API RP 505  Recommended Practice for Classification of Locations for Electrical Installation at Petroleum Facilities Classified as Class I, Zone 0, Zone 1 and Zone 2 (2002).

API RP 500 Recommended Practice for Classification of Locations for electrical Installation at Petroleum Facilities Classified as Class I, Division 1 and Division 2.

IEC:
IEC 60079-10 Electrical Apparatus for explosive gas atmospheres- Part 10: Classifications of hazardous Area.

IEC 60079-12 Classification of Mixtures of Gases or vapors with air according to their maximum experimental Gaps (MEGs) and minimum ignition currents ratio (MIC).

IEC 60079-20 Electrical Apparatus for explosive gas atmospheres- Part 20: Data for flammable gases and vapors, relating to the use of electrical apparatus.

NFPA:

NFPA 30: Flammable and Combustible Liquids Code

NFPA 70: National Electrical Code

NFPA 325: Guide to fire Hazard Properties of Flammable Liquids, Gases, and volatile Solids

NFPA 497: Recommended practice for the Classification of Flammable Liquids, Gases or Vapors and of Hazardous (classified) Locations for Electrical Installations in Chemical Process Areas.

The complete list of the codes, recommended practice, and standards relevant to the hazardous area classification is present at the end of the chapter.

 Basic Definitions:
The following list of definition is based on the reference codes and practice guideline listed before. The reference standard is assigned to each definition.
Boiling Point – The temperature of a liquid boiling at the reference atmospheric conditions. (IEC 79-10, Mod.)
Area Classification – See Further paragraph named (“Area Classification and Definition”).
Class I, Zone 0 – See Further paragraph named (“Area Classification and Definition”).
Class I, Zone 1 – See Further paragraph named (“Area Classification and Definition”).
Class I, Zone 2 – See Further paragraph named (“Area Classification and Definition”).
Combustible Liquid(s) – See Flammable Liquid(s) definition.
Enclosed Area – A three-dimensional space enclose by more than two-third (2/3) of the possible projected plane surface area and of sufficient size to allow the entry of personnel. For a common building, this would required two-third (2/3) of the walls, ceiling, and/or floor be present.
Explosive gas atmosphere – A mixture with air, under the reference atmospheric conditions, of a flammable material in the form of gas or vapor which, after ignition, combustion spreads throughout the unconsumed mixture. (API 505-3.2.20)
Flammable  – Capable of an easy ignition, burning intensely or spreading flame rapidly.
Flammable (Explosive) limit(s)  – The lower (LFL) and upper (UFL) percentages by volume of concentration of gas in gas-air mixture that will form an ignitible mixture.   (NPFA 325)
Flammable Liquid(s) – See Further paragraph named (“Flammable liquid Classification”).
Flash Point  – The minimum temperature of a liquid at which sufficient vapor is give off to form an ignitible mixture with air, near the surface of the liquid, or within the vessel used, as determinate by the test procedure and apparatus specified in NFPA 30.
Grade of Release  – There are three basic grade of release, as listed below, in order of decreasing likelihood of the explosive gas atmosphere being present.(1)
1-     Continuous
2-     Primary
3-     Secondary
Other grades of release may be possible by combination of the basic ones listed.(IEC 79-10, Mod.)
(1)    It is important to underline that there isn’t any relationship with the type of release discussed earlier like” puff” and “plume”.
Grade of Release: Continuous – See Further paragraph named (“Area Classification and Definition”).
Grade of Release: Primary – See Further paragraph named (“Area Classification and Definition”).
Grade of Release: Secondary – See Further paragraph named (“Area Classification and Definition”).
Gas Group(s) – For the Classification, the ignitible gases or vapors are classified in several different groups. The subdivision of the gases is related to the gases physical and chemical properties.
Hazardous (classified) Location(s) – A location where fire and explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dusts, or ignitible fibers of flyings. (API 505-3.2.10.5)
Heavier-than-air Gases of Vapors – Formally those gases of vapors with a relative density above 1.2 as to be regarded as Heavier-than-air gases. (IEC 79-10, Mod.)
Highly volatile liquid(s) (HVL) – See Further paragraph named (“Flammable liquid Classification”).
Ignitible (Flammable) Mixture  – A gas-air mixture that is capable of being ignited by an open flame, electric arc or spark, or device operating above the ignition temperature of the gas-air mixture.  (See “Flammable (Explosive) Limits”) (API 505-3.2.32)
Ignition (Auto ignition) Temperature  (AIT) – The lowest temperature of a heated surface at which, under specific conditions, the ignition of a flammable substance, or mixture in the form of gas or vapor will occur. (IEC 79-10, Mod.)
Lighter-than-air Gases or Vapors  – Formally those gases or vapor with a relative density below 0.8 as to be regarded as Lighter-than-air substances. (IEC 79-10, Mod.)
Maximum Experimental Safe Gap (MESG)  – The maximum gap of the joint between the two parts of the interior chamber of a test apparatus that, when the internal mixture is ignited  and under specific conditions, prevents the ignition of the external gas mixture by propagating through a 25 mm (984 mils) long joint, for all concentrations of the tested gas or vapor in air. (API 505-3.2.38)
Minimum Ignition Current (MIC) – The minimum current that, in a specified spark test apparatus and under specific condition, is capable of igniting the most easily ignitible mixture. (API 505-3.2.39)
Minimum Ignition Current Ratio (MIC Ratio) – The minimum energy required from a capacitive spark discharge to ignite the most easily ignitible mixture of a gas or vapor divided by the minimum current required from and inductive spark discharge to ignite methane under the same test conditions. (NFPA 497)
Normal Operation(s) – The situation when the equipment is operating within its design parameters. (IEC 79-10, Mod.)
Protected Fire Vessel – Any fired vessel that is provided with equipment (such flame arresters, stack temperature shutdown, forced draft burners, with safety controls, and spark arresters) designed to eliminate the air intake and exhaust as sources of ignition. (API 505-3.2.48)
Release, Source of – A point or location from which a flammable gas, vapor or liquid may be released into the atmosphere such that an ignitible gas atmosphere could be formed. (IEV 426-03.06, Mod.)
Release Rate – The quantity of flammable gas or vapor emitted per unit time from the source of release. (IEC 79-10, Mod.)
Vapor Pressure –  The pressure exerted when a solid or liquid is in equilibrium with its own vapor. It is a substance properties linked to the environment condition and determinate by ASTM D 323-82. (IEC 79-10, Mod.)
Vapor-tight Barrier – Is a wall, or barrier that will not allow the passage of significant quantities of gas or vapor at atmospheric pressure. (API 505-3.2.54)
Ventilation – Natural or artificial movement of air and its replacement with “fresh air”.
Ventilation, Adequate – Ventilation that is sufficient to prevent the accumulation of enough quantities of an ignitible mixture into a specific location.
Volatile Flammable Liquid – A flammable liquid whose temperature is above its flash point, or a Class II combustible liquid having a vapor pressure not exceeding 276 Kpa (40 Psia) at 37.8°C(100°F) whose temperature is above its flash point. (API 505-3.2.58)

Basic Condition for Fire(s) and Explosion(s):
As discussed earlier, to occur, a fire and/or and explosion needs three basic elements, without any of them, or specific conditions for each of them, the event cannot occur. The three main elements are: (1) A fuel, not necessary an common combustible (e.g. Dust, or Mill Dust), (2) a combustible (e.g. Air or Oxygen). (3) An igniter source with enough energy to ignite the flammable mixture (e.g. Electrical equipment, free flames, or hot surfaces). Other than the presence of each of these elements, there are two additional conditions needed to obtain a fire or an explosion: (4) The concentration of the fuel within the mixture must be between its own Upper and Lower Flammable Limit. (5) The three basic elements must be in same location, or they must have a position that allows them to complete their own role.
In classifying a particular location, the likelihood of the presence of a flammable gases or vapor is a significant factor in determinate the zone classification (See Further paragraph named “Area Classification and Definition”). Otherwise a distinction must be made: the presence of the flammable mixture could be distinguished between “normal conditions” and “extraordinary condition”. The term “extraordinary condition” doesn’t mean only a catastrophic event like a violent breakage of an item or similar, but also an ordinary maintenance operation. There is obviously an objection: If an item, or a location, needs a frequent maintenance, the act itself will go under the “normal condition”. (API 505.4.2 refers to these condition adopting the phrase “Normal and Abnormal Condition”).
As said, the mixture, to occur into an explosion and/or a fire, must have a concentration within its range of flammability. It is quite important to know or to reach an approximation of the quantities of flammable mixture are present inside the different location, to determinate the extension of the area. As more the released quantities are high, as more the area affected by the hazard is wide.
Another relevant parameter to take into account is the ventilation. The ventilation of a specific location can reduce sensibly the hazard connected to a ignitible substance release, even in major case. A good ventilation, natural and/or artificial), especially inside enclosed location, is the fist measure to adopt to reduce the risk of Fires.
Especially for preliminary studies, even before the engineering starts, where the knowledge of the plant and the area is almost unknown, found even and approximate form of these parameters (Likelihood, Concentration, and Ventilation of a specific area) could be really hard, and in the best case the approximation is totally aloof from reality. In fact, the hazardous area classification is commonly  made during the entire development of the plant, from the first plot plan revised by the process company to the final general plot plan of the engineering phase, reviewing continuously the data and the area classification.  

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