One wrong line on your plant layout can change the safety of your entire facility.

Hazardous Area Classification: From Theory to Practical Implementation

Whenever we discuss about Hazardous Area Classification (HAC), everyone in the manufacturing sector talks about zones. Most professionals are aware that these zones are directly linked to the selection of electrical equipment. But the real challenge begins when we ask: How do we decide which part of a process comes under which zone?

To answer this, let’s go step by step and decode the fundamentals of HAC.

What is Hazardous Area Classification?

Hazardous Area Classification is the process of identifying areas where flammable gases, vapors, or dusts may be present in sufficient quantities to create an explosion hazard. The main purpose is to categorize areas into zones, which then guide the selection of electrical equipment and installation methods to minimize ignition risks.

Understanding the Zones

Hazardous areas are divided into zones based on the frequency and duration of the presence of explosive atmospheres:

• Zone 0: Explosive atmosphere present continuously or for long periods (typically >1,000 hours/year).

• Zone 1: Explosive atmosphere likely to occur occasionally during normal operation (10–1,000 hours/year).

• Zone 2: Explosive atmosphere unlikely under normal operation, but if it occurs, it exists only for a short time (<10 hours/year).

For dusts, the equivalents are Zone 20, 21, and 22.

Step 1: Identifying Sources of Release

The first step is identifying all possible release sources: pumps, compressors, flanges, vents, storage tanks, sampling points, drains, etc. Each source is categorized as:

• Continuous: Releases expected frequently or for long durations (e.g., inside a tank vapor space).

• Primary: Releases likely during normal operation (e.g., pump seals, vent lines).

• Secondary: Releases unlikely but possible under abnormal conditions (e.g., flange leaks).

Step 2: Determining Zone Boundaries

Factors to Consider

The extent of each hazardous zone depends on:

• Rate of release (small leak vs. major venting)

• Ventilation (natural or forced air movement)

• Properties of the gas/vapor/dust (heavier-than-air gases may settle, lighter ones may rise)

• Equipment configuration (indoors vs. outdoors, enclosed vs. open-air plant)

Horizontal & Vertical Extent

• Horizontal zones: Spread outward from the source, radius depending on release rate and ventilation.

• Vertical zones: Defined by gas density—lighter gases form hazards above equipment, heavier gases near the ground.

Typical Tools and Approaches

1. Standards-based Nomographs & Tables

   • IEC 60079-10-1 Annex B provides tabulated guidance for zone distances in different release conditions.

   • NFPA 497 offers recommended practices for petroleum and chemical industries.

   • OISD guidelines (India) for petroleum storage/handling provide zone maps for specific equipment.

2. Software Tools

   • PHAST, FLARESIM, DNV GL Safeti: Used for consequence modeling and dispersion studies.

   • CFD (Computational Fluid Dynamics): For complex geometries and indoor scenarios, helps visualize how gases accumulate or disperse.

   • AutoCAD / SmartPlant P&ID overlays: To mark hazardous zones directly on plant layouts.

3. Analytical Calculations

   • Using equations for release rate, lower flammable limit (LFL), and ventilation velocity to estimate cloud dispersion.

   • Standards like IEC 60079-10-1 provide formulas for estimating extent in meters.

Step 3: Applying Engineering Judgment

Standards always note that engineering judgment is critical because:

• Real plant conditions (weather, congestion, process complexity) rarely match textbook examples.

• Engineers must consider worst-case scenarios (loss of ventilation, simultaneous failures).

• Field experience, past incident data, and industry practices are applied to fine-tune zone boundaries.

For example:

• A vent stack discharging above roof level may theoretically need a large Zone 1, but with proper height and wind velocity, it can be downgraded to Zone 2 or even non-hazardous.

• A flange in open air with strong cross-ventilation may be limited to a very small Zone 2 boundary, whereas the same flange indoors may extend to a larger Zone 1.

Step 4: Marking Zones on Layouts

Once zones are finalized, they are:

• Drawn as shaded/hatched areas on plant layouts (AutoCAD, MicroStation, etc.)

• Clearly differentiated (Zone 0 in red, Zone 1 in orange, Zone 2 in yellow)

• Shared with design, operations, and safety teams for equipment selection and maintenance planning

Standards and References

Some key standards and guides:

• IEC 60079-10-1 & 10-2 – Classification of gas and dust hazardous areas

• NFPA 497 & NFPA 499 – Recommended practices for classification of gases and dusts

• API RP 505 – Classification of hazardous locations for petroleum facilities

• IS/IEC 60079 series – Adopted Indian standards for explosive atmospheres

• OISD-STD-113 & 116 – Indian guidelines for petroleum and natural gas industry

Final Note

Hazardous Area Classification is a blend of science, standards, and engineering judgment. Tools and software assist, but final responsibility rests with competent engineers who understand both the theory and the real-world process conditions.

👉 To validate your existing classification or for support in your upcoming projects, contact us at agnirakshaniti@gmail.com.