hazardous area classification

© 2015 – Hervé Baron

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Engineering Training

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It shows how Hazardous area classification is made.

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Hazardous area classification

The purpose of area classification is to identify the extent of the area made hazardous due to the presence of explosive atmosphere and to classify the identified hazardous area on the basis of likelihood and type of flammable release expected to be present.

Outcome:

Specify explosion protection of instruments and electrical apparatus according to their location.

Ensure that sources of ignition area are segregated from sources of flammable gas,

Select the location of clean air inlets for ventilation systems or other safety requirements,

Design the Plant layout to minimize classification of access roads.


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The following information is required to enable correct selection of electrical equipment and instruments suitable to the area in which they are to be located: Zone of hazardous area, Gas group of material, Maximum permissible surface temperature (Temperature Class).


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The following information is required to enable correct selection of electrical equipment and instruments suitable to the area in which they are to be located: Zone of hazardous area,

Zone 0 location is a location at which ignitable concentration of flammable gases or vapours

are present continuously or for long periods of time.

Zone 1 location is a location at which ignitable concentration of flammable gases or vapours are likely to exist under normal operating conditions,

Zone 2 location is a location at which ignitable concentrations of flammable gases or vapours are not likely to occur in normal operation and if they do occur will exist only for a short period

Ref: API RP 505 Recommended Practices for Classification of Locations for Electrical Installations at Petroleum Facilities classified as Class I, Zone 0, Zone 1 and Zone 2 (1997 Edition).


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The following information is required to enable correct selection of electrical equipment and instruments suitable to the area in which they are to be located: Zone of hazardous area, Gas group of material,

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Commentaires de présentation

Gas Group: example of gas group IIC or IIB+H2 is battery room and analyser shelters


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The following information is required to enable correct selection of electrical equipment and instruments suitable to the area in which they are to be located: Zone of hazardous area, Gas group of material, Maximum permissible surface temperature (Temperature Class).

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Temperature class = maximum surface temperature, selected as function of ignition temperature of the concerned flammable mixture


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Hazardous area classification drawings

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Electrical/Instrumentation apparatus in the dashed area must be provide with explosion protection such as flame proof enclosure shown.


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Hazardous area classification How are the hazardous area classification drawings established?


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Hazardous area classification How are the hazardous area classification drawings established? 1) Identification of hazardous fluids


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1) Identification of hazardous fluids a) Product & mixtures properties table

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Gas flammability is assessed Liquid flash point is determined


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b) Stream conditions and hazardous area extent determination

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Flammable gases as well as liquids at temperature above their flash point create hazardous area


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Hazardous area classification ADEQUATELY VENTILATED PROCESS LOCATION WITH HEAVIER THAN AIR GAS OR VAPOR SOURCE LOCATED ABOVE GRADE API RP 505 Fig 21

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Extent of hazardous area is defined in code


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b) Stream conditions and hazardous area extent determination This is issued as part of the Hazardous Equipment Table document

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Flammable gases as well as liquids at temperature above their flash point create hazardous area


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Hazardous area classification extent of hazardous area (API RP 505)

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The type of zone depends on likeliness of discharge


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The extent depends if the gas is heavier (left) or lighter (right) than air


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Extent of hazardous area depends on quality of ventilation


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Combustible liquid storage tank


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Flammable liquid storage tank


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Why is the inside of the storage tank zone 1 for the tank on the left and zone 0 for the tank on the right?

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Left side: tank contains combustible liquid, i.e., liquid at a temperature below flash point Right side: tank contains flammable liquid, i.e., liquid at a temperature above flash point


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WRAP-UP

Hazardous area classification is done in 3 steps:

Define the classification philosophy, i.e, specify which code will be applied,

e.g., API RP 505 (this is usually done part of the Safety Concept)

Issue of Hazardous Equipment table identifying properties of fluids handled (flammability of gases, flash point of liquids) as well as extent of hazardous area created by the various process streams as a function of the nature of fluid handled and its conditions (pressure, temperature)

Draw the hazardous area envelopes


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