Introduction
In chemical plants, oil & gas facilities, pharmaceutical factories, and other process industries, explosion protection is one of the most important safety considerations.
The purpose of dividing hazardous areas and non-hazardous areas is to prevent combustible gases, vapors, or dust from mixing with air and forming an explosive atmosphere.
A simple way to understand the difference:
- Hazardous Area: The main concern is controlling potential ignition sources. Electrical equipment installed in these areas must comply with appropriate explosion protection requirements.
- Non-Hazardous Area: The key is preventing flammable substances from entering the area through proper distance, ventilation, pressurization, sealing, and maintenance.
However, hazardous area classification is not simply based on whether a location is called a “pump area” or “tank area”. The correct approach starts with identifying possible release sources.
1. Start With Identifying the Release Source
Before classifying an area, engineers must determine:
Where can flammable materials potentially escape?
Different equipment has different potential leakage points.
Typical examples:
Pumps
For centrifugal pumps, possible release sources include:
- Mechanical seals
- Drain points
- Vent connections
- Pressure gauge connections
- Instrument interfaces
- Flanges at inlet and outlet pipelines
Storage Tanks
Important points include:
- Breathing valves
- Level instrument connections
- Drain outlets
- Loading and unloading connections
Reactors
Potential release sources include:
- Feeding ports
- Sampling points
- Vent lines
- Agitator shaft seals
The same equipment may contain different risk levels at different locations.
For example:
- Mechanical seals may leak due to wear, vibration, temperature changes, or loss of flushing.
- Flange leakage depends on gasket condition, bolt tightening, pipeline stress, and thermal expansion.
- Sampling points depend on operating frequency.
Therefore, hazardous area drawings should identify actual release points rather than simply marking an entire area as “pump zone” or “tank zone”.
2. Hazardous Area Classification: Zone 0, Zone 1, and Zone 2
For explosive gas atmospheres, hazardous areas are commonly classified into:
Zone 0
An area where an explosive gas atmosphere is continuously present, for long periods, or frequently.
Examples:
- Inside storage tanks
- Inside process vessels
- Inside equipment containing flammable gases or vapors
Zone 1
An area where an explosive atmosphere is likely to occur during normal operation.
Examples:
- Frequently used sampling points
- Loading and unloading areas
- Venting locations
Zone 2
An area where an explosive atmosphere is not likely during normal operation, and if it occurs, it exists only for a short time.
Examples:
- Around mechanical seals
- Around valves
- Around flanges
- Around instrument connections
For combustible dust environments, similar classifications are used:
- Zone 20
- Zone 21
- Zone 22
The classification depends on:
- Frequency of release
- Duration of presence
- Dust or gas characteristics
3. Explosion Risk Depends on Gas Dispersion, Not Only Location
Hazardous area classification is not only a two-dimensional drawing.
Gas and vapor movement in real plants is three-dimensional.
Lightweight gases
Examples:
- Hydrogen
- Methane
These gases tend to rise and may accumulate in:
- Roof spaces
- Ceiling areas
- Structural beams
- High platforms
Heavy vapors
Examples:
- LPG vapor
- Solvent vapor
These may stay close to the ground and travel through:
- Drain channels
- Trenches
- Cable pits
- Low areas
Additional structures can also change the hazardous area:
- New shelters
- Temporary walls
- Equipment congestion
- Poor ventilation
A location originally classified as safe may require reassessment after site modifications.
4. Why Non-Hazardous Areas Still Need Protection
A non-hazardous area does not mean “zero risk”.
Many control rooms, electrical rooms, and analyzer shelters use ordinary electrical equipment because they are protected by design conditions.
Typical protection measures include:
- Adequate separation distance
- Positive pressure systems
- Continuous ventilation
- Proper sealing
- Cable penetration protection
- Automatic door closing systems
For example, a pressurized control room remains safe only when:
- Room pressure meets design requirements
- Fresh air intake is located in a safe area
- Doors remain closed
- Cable penetrations are properly sealed
- Pressure alarms and interlocks function correctly
A running ventilation fan does not automatically mean the room is protected.
Actual protection should be verified through:
- Pressure monitoring
- Alarm records
- Functional testing
- Inspection records
5. Selecting Explosion-Proof Instruments
Explosion-proof equipment selection should consider more than just the “Ex” mark.
Engineers need to verify:
- Hazardous area classification
- Gas group
- Temperature class
- Equipment Protection Level (EPL)
- Explosion protection method
- Ambient temperature
- IP protection rating
- Cable gland requirements
For example:
Ex db IIC T4 Gb
Meaning:
Ex
Equipment designed for explosive atmospheres.
db
Flameproof enclosure protection.
The enclosure can withstand internal explosions and prevent flames or hot gases from igniting the surrounding atmosphere.
IIC
Gas group classification.
IIC represents the highest gas risk group and includes gases such as:
- Hydrogen
- Acetylene
T4
Maximum surface temperature:
≤135°C
Gb
Equipment Protection Level for gas environments.
Suitable selection depends on the hazardous zone classification.
Simply selecting equipment with an “Ex” marking is not enough.
The complete combination of:
- Zone classification
- Gas group
- Temperature class
- Protection type
- EPL level
must match the application.
6. Explosion Protection Depends on Correct Installation
Even certified explosion-proof equipment can lose its protection capability if installed incorrectly.
Common installation requirements include:
Cable Entry
Cable glands must match:
- Cable diameter
- Armored cable type
- Equipment protection method
Incorrect cable glands may cause:
- Poor sealing
- Damaged cable insulation
- Loss of explosion protection
Unused cable entries must use approved explosion-proof plugs.
Temporary solutions such as:
- Plastic plugs
- Tape
- Ordinary bolts
cannot replace certified sealing components.
Enclosure and Flameproof Joints
Do not:
- Drill holes into explosion-proof enclosures
- Modify flameproof joints
- Polish sealing surfaces with improper tools
Any modification may affect the original certification.
7. Intrinsically Safe Circuits: Check the Complete Loop
Intrinsic safety is not determined only by whether an instrument has an “Ex ia” marking.
The entire loop must be evaluated.
A typical intrinsically safe circuit includes:
- Field instrument
- Safety barrier
- Cable
- Terminals
- Grounding system
When replacing an instrument, engineers should check:
- Maximum voltage
- Maximum current
- Power parameters
- Safety barrier output parameters
- Cable capacitance
- Cable inductance
- Cable length
- Shielding and grounding method
An intrinsically safe transmitter connected to an unsuitable barrier may no longer satisfy intrinsic safety requirements.
8. Practical Examples in Industrial Instrumentation
Explosion protection requirements are common for:
Flow Measurement
Examples:
- Magnetic flow meters
- Coriolis flow meters
- Vortex flow meters
Typical requirements:
- ATEX / IECEx certification
- Ex d or Ex ia protection
- Suitable temperature class
Level Measurement
Examples:
- Radar level transmitters
- Guided wave radar
- Vibrating fork switches
Selection depends on:
- Tank type
- Process media
- Hazardous zone
- Gas group
Pressure Measurement
Examples:
- Pressure transmitters
- Differential pressure transmitters
Important factors:
- Process connection
- Hazardous area classification
- Explosion protection type
Conclusion
The difference between hazardous and non-hazardous areas is not simply whether explosion-proof equipment is installed.
A complete explosion protection strategy requires:
- Identifying release sources
- Classifying hazardous zones correctly
- Understanding gas dispersion behavior
- Maintaining non-hazardous area protection conditions
- Selecting suitable explosion-proof instruments
- Ensuring correct installation and maintenance
In industrial facilities, even a small opening, an unsealed cable penetration, or a permanently open door can compromise the original safety design.
Explosion protection is not only about equipment selection — it is a complete system approach involving design, installation, operation, and maintenance.