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Hazardous Area Classification in Oil & Gas Industry
A Fire Prevention Tool
SFPE ENGINEERING TECHNOLOGY CONFERENCE
Oct 4, 2010
New Orleans, LA
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Overview
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Oil and gas plants handle flammable materials
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Flammable Range
Air
Flammable
Too lean to burnToo rich to burn
100%
0%LFL UFL
LFL Lower Flammable Limit
UFL Upper Flammable Limit
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Flammable Range of Few Materials
Material LFL (Vol %) UFL (Vol %)
Methane 5 15
Ethane 3 15.5
Propane 2 9.5
Butane 1.5 8.5
Hydrogen 4 75.6
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Flammable Range - Methane
0%Lean – will not burn
Flammable
Too rich – will not burn
UFL – 15%
LFL – 5%
100%
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Source of Release
• A point from which a flammable gas, vapor or liquid may be released into atmosphere.
ex. flanges, instrument connections, pump seals, etc
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Sources of Release
Instrument Connection
Flange
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Fire Triangle
Simultaneous occurrence of all three sources causes fire.
Cutting off any one, extinguishes fire. 8
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Ignition Source
Electrical Apparatus
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Aim of Area Classification
• To avoid ignition of releases that may occur from time to time
• The approach is to reduce to an acceptable level the probability of coincidence of a flammable atmosphere and an electrical or other source of ignition.
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Hazardous Area
• A three dimensional space in which a flammable atmosphere may be expected to be present at such frequencies as to require special precaution for the design and construction of equipment, and the control of potential ignition sources.
Leak Source
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Hazardous Area
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Hazardous Area
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Extent of Hazardous Area
• Direct Example Approachlimited to common facilities
• Point Source Approachrelease rates are dependent on process conditions
• Risk-based Approachan option methodology which may reduce the hazardous area determined by the point source approach
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Grade of Release
• Continuous: A release that is continuous or nearly so.
• Primary: A release that is likely to occur periodically or occasionally in normal operation. (Vents, Sample points, etc)
• Secondary: A release that is unlikely to occur in normal operation and, in any event, will do so only infrequently and for short periods. (Flanges, Instrument connections, etc)
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Subdivision of Hazardous Area
• Zone 0: flammable atmosphere is continuously present or present for long periods.
• Zone 1: flammable atmosphere is likely to occur in normal operation.
• Zone 2: flammable atmosphere is not likely to occur in normal operation and, if it occurs, will exist only for short period.
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Zone and Division
Continuous Release
Primary Release
Secondary Release
Codes and Standards
Zone 0 Zone 1 Zone 2IEC 60079, IP 15, API 505, NFPA 497
Division 1 Division 2API 500, NEC Article 500, NFPA 497
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Grade of Release and Zone (Division)
Under unrestricted ‘open air’ conditions:
• Continuous grade normally leads to Zone 0 (Division 1)
• Primary grade normally leads to Zone 1 (Division 1)
• Secondary grade normally leads to Zone 2 (Division 2)
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Effect of Ventilation
• The term ‘grade of release’ and ‘zone’ are not synonymous.
• Poor ventilation may result in a more stringent zone while, with high ventilation provision, the converse will be true.
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• Cone Roof Storage Tank
Zone Classification
Zone 1
Zone 0
Zone 2
Zone 1
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• Floating Roof Storage Tank
Zone Classification
Zone 1
Zone 2
Zone 1
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Classification of Zone (Division)
• Zone (Division) classification is based on the frequency of release.
• Each zone (division) is further classified into Groups and Temperature Class based on the properties of flammable material processed in that zone (division).
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Gas Group
Typical Gas Hazard NEC Article 500 IEC
Acetylene A IIC
Hydrogen B IIC
Ethylene C IIB
Propane D IIA
• Flammable materials are sub-divided into groups based on minimum ignition energy required for igniting them.
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Auto-ignition Temperature (AIT)
• Minimum temperature required to initiate or cause self-sustained combustion of the fluid independent of the heating.
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Temperature Class
Temperature Class Auto-ignition Temperature (o C)
T1 >450T2 >300T3 >200T4 >135T5 >100T6 >85
• Temperature Class is assigned to flammable material based on its auto-ignition temperature.
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Auto-ignition Temperature (AIT)
Material AIT (o C) Temperature Class
Methane 595 T1
Ethane 515 T1
Propane 470 T1
Butane 365 T2
Pentane 285 T3
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Area Classification
Zone 2 IIA T3
Zone 2 IIC T1
Zone 1 IIA T3
Zone 1 IIC T1
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Battery Room
• Lead acid batteries emit H2 during charging and create hazardous area
• Battery rooms are classified as Zone 1 Group IIC Temperature Class T1
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Factors to be considered:• Zone in which the equipment will be
used.• Sensitivity to ignition of the material
likely to be present, expressed as a gas group.
• Sensitivity of the material present to ignition by hot surfaces, expressed as a temperature class.
Selection of Electrical Equipment
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Selection According to Zone Classification
• Equipment suitable for Zone 0 can be used in Zones 0, 1 or 2.
• Equipment suitable for Zone 1 can be used in Zones 1 or 2.
• Equipment suitable for Zone 2 can be used only in Zone 2.
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Selection According to Gas Groups
• Grouping becomes more severe in going from IIA to IIB to IIC.
• Group IIB equipment may be used in place of Group IIA equipment.
• Group IIC equipment can be used in place of equipment for both Group IIA and IIB.
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Selection According to Temperature Class
Temperature Class Maximum Surface Temperature (o C)
T1 450T2 300T3 200T4 135T5 100T6 85
• Equipment having a lower maximum surface temperature class may be used in place of that a higher maximum surface temperature (lower T class), but not conversely.
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• Oil Immersed ProtectionHeavy current switchgear and transformers
• Pressurization or Continuous DilutionAnalyzers, control boxes, etc
• Powder-filled ProtectionElectronics, chokes, etc
• Flame-proof EnclosureMotors, lighting, junction boxes, etc
Equipment Protection
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Summary
• Identify the flammable materials.• Identify sources of release and
determine grade of release (and hence Zone 0,1, 2 or Division 1, 2).
• Determine extent of hazardous area.• Assign Gas Groups and Temperature
Class.• Select appropriate Electrical apparatus.
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Thank you
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