Understanding Hazardous Area Classifications

Introduction

A Hazardous Area Classification (HAC) study is an essential analysis aimed at reducing the chances of igniting flammable materials. This study focuses on identifying sources of gases, vapors, and combustible dust, and implementing ignition control designs. HAC is a type of hazard and risk assessment specifically addressing the probability of explosive atmospheres (flammable dust/air mixtures) in certain areas. The classification is conducted following either the NFPA standard’s two Division system or the ATEX standard’s three Zone system.

Upon completing the HAC, steps are taken to manage and control potential ignition sources effectively. This includes enforcing stringent regulations on possible ignition sources in areas with a high likelihood of explosive atmospheres, thereby substantially lowering the risk of explosions in these classified zones.

The primary motivation for conducting HAC has traditionally been to ensure that electrical systems do not present an ignition hazard in facilities dealing with flammable or combustible materials. In North America, the HAC is mandated under the National Electrical Code (NEC), while in Europe, it adheres to the ATEX standards. However, to achieve an acceptable safety level within a plant, it’s crucial to regulate non-electrical ignition sources with the same rigor as electrical sources.

Material Groups for Hazardous Area Classifications

The decision to classify an area as hazardous should be based on the probability that a combustible mixture could be present. Two division classifications are used to specify the likelihood of a flammable atmosphere being present. It is important to understand that it is always preferable to prevent the formation of combustible atmospheres (combustible dust clouds) and dust layers than to have an area classified as hazardous and not to control combustible dust clouds.

Once it is decided that an area should be classified, the next step should be to determine the likelihood of the hazard occurring: Is the area Division 1 or Division 2?  The division defines the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere.

Generally, for flammable vapors and gasses (Class I) divisions are defined as follows.

Class I, Division 1 location is a location where:

1. Ignitable concentrations of flammable gases or vapors exist under normal operating conditions; or
2. Ignitable concentrations of flammable gases or vapors – or combustible liquids above their flash points – may exist frequently because of repair or maintenance operations or because of leakage; or

• Breakdown or faulty operation of equipment or processes might release ignitable concentrations of flammable gases or vapors and might also cause simultaneous failure of electrical equipment, to become a source of ignition.

Class I, Division 2 location is a location where:

• Flammable liquids or gases are handled, processed, or used, but will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown, or in case of abnormal operation of equipment; or
• Ignitable concentrations of gases or vapors are normally prevented by positive mechanical ventilation and which might become hazardous through failure or abnormal operation of the ventilation equipment; or
• Adjacent to a Class I Division 1 location, and to which ignitable concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

Class I Unclassified areas generally are areas where:

• Process, storage, or handling areas which have adequate ventilation [1 scfm/sq.ft., but not less than 150 SCFM, with alarm] and materials are contained within suitable, well-maintained, closed piping systems; or
• Areas where there is not adequate ventilation, but piping systems are without valves, fittings, flanges, and similar accessories that may be prone to leaks; or
• Where materials are stored in suitable containers [drums, IBCs, etc.] and the area has adequate ventilation [1 scfm/sq.ft., but not less than 150 SCFM, with alarm]; or
• Where Class IIIB materials [Flash Point above 200 °C] are handled; open-flame areas; and where some halogenated materials are handled.

Generally, for combustible dust (Class II) divisions are defined as follows.

Class II, Division 1 location is a location where:

• Where a dust cloud is likely to be present under normal conditions, the location should be classified as Division 1; or
• Where a dust layer greater than 1/8 inch (3.0 mm) thick is present under normal conditions (24 hours accumulation), the location should be classified as Division 1; or
• If quantities of ignitable dust are released as a result of frequent maintenance, the location is Division 1. Similarly, in areas where the buildup of dust layers exists without the presence of visible dust suspensions, good and frequent cleaning procedures or the lack thereof will influence the classification of the location.

Class II, Division 2 location is a location where:

The criterion for a Division 2 location is whether the location is likely to have ignitable dust suspensions only under abnormal conditions or hazardous dust accumulations less than 1/8 inch (3 mm) in 24 hours, and surface colors are not discernable. The term “abnormal” is used herein a limited sense and does not include a major catastrophe.

• Chemical process equipment does not fail often. Furthermore, the electrical installation requirement of the NEC for Division 2 locations is such that an ignition-capable spark or hot surface will occur only in the event of abnormal operation or failure of electrical equipment. Otherwise, sparks and hot surfaces are not present or are contained in enclosures. On a realistic basis, the possibility of process equipment and electrical equipment failing simultaneously is remote.
• The Division 2 classification is applicable to conditions not involving equipment failure. For example, consider a location classified as Division 1 because of the normal presence of ignitable dust suspension. Obviously, one side of the Division 1 boundary cannot be normally hazardous and the opposite side never hazardous. Similarly, consider a location classified as Division 1 because of the normal presence of hazardous dust accumulations. One side of the division boundary cannot be normally hazardous, with thick layers of dust, and the other side unclassified, with no dust, unless there is an intervening wall.
• When there is no wall, a surrounding transition Division 2 location separates a Division 1 location from an unclassified location. Only un-pierced solid walls make satisfactory barriers in Class I locations, whereas closed doors, lightweight partitions, or even partial partitions could make satisfactory walls between Class II, Division 1 locations and unclassified locations. Area classification does not extend beyond the wall, provided it is effective in preventing the passage of dust in suspension or layer form.

Class II, Unclassified Location:

It is usually not necessary to classify the following locations:

• Where dust removal systems prevent the following:
  1. Visual dust clouds;
  2. Layer accumulations that make surface colors indiscernible
• Where excellent housekeeping prevents the following:

1. Visual dust clouds;
2. Layer accumulations that make surface colors indiscernible

Difference Between European & IEC Classification

Temperature Code

A mixture of hazardous gases or combustible dust and air or dust layers may ignite in contact with a hot surface. The condition for ignition depends on several factors, such as surface area, temperature, and concentration of gas or dust. Equipment that is approved for hazardous locations receives a temperature code indicating the maximum surface temperature of the equipment and should be selected according to the MIT (for dust), and AIT (for vapors and gases) (see Table below):

Temperature Codes

Relevant Standards and Regulations

• NFPA 70 (National Electrical Code): Provides guidelines for the classification of hazardous locations and the requirements for electrical equipment in these areas.
• NFPA 497: Offers recommended practices for the classification of flammable gases and vapors and the appropriate electrical equipment.
• NFPA 499: Provides recommended practices for the classification of combustible dusts and the appropriate electrical equipment.
• OSHA 29 CFR 1910.307: Addresses hazardous (classified) locations and electrical safety requirements.
• OSHA Combustible Dust National Emphasis Program (NEP): Outlines OSHA’s inspection procedures and policies for facilities handling combustible dust.
• EN 1127-1; Explosive atmosphere – Explosion prevention and protection. Part 1: Basic concepts and methodology
• EN 60079-10; Electrical apparatus for explosive gas atmospheres. Part 10: Classification of potentially explosive atmospheres
• EN 50281-3; Electrical apparatus for use in the presence of combustible dust. Part 3: Classification of areas where dusts are or may be present.

Services offered by Prime Process Safety Center

Finally, Prime Process Safety Center offers the following Combustible Dust Consulting Services; Combustible Dust Hazard Analysis (DHA), Ignition Sources Assessment, Electrostatic Hazard Assessment, Hazardous Area Classification, Fire and Explosion Hazard Analysis, Explosion Prevention and Protection Consulting Services, Fire and Building Code Services, Incident Investigation, Expert Witness and Litigation. Moreover, Prime Process Safety Center offers the following combustible dust testing services Go/No Go Explosibility Screening, Burn Rate / Fire Train Test, Dust Explosion Severity (Kst/Pmax/dP/dt), Minimum Explosible Concentration (MEC)/Lower Explosible Limit (LEL), Limiting Oxygen Concentration (LOC) Test, Minimum Ignition Energy (MIE), Minimum Autoignition Temperature-Cloud (MAIT – Cloud), Layer Ignition Temperature of Dust (LIT), Volume Resistivity, Surface Resistivity, Charge Decay (Relaxation) Time, Breakdown Voltage, Flexible Intermediate Bulk Containers (FIBC), Electrostatic Chargeability Testing, Basket Self-Heating, Grewer Oven Test, Air Over Layer/Powder Layer Test, Bulk Powder Test, Aerated Powder Test