Hazardous Area Classification

By Venky Viswanathan, Lead-Electrical Product Testing Line, Fauske & Associates, LLC

Why hazardous area classification?

The main purpose of hazardous area classification layout is to facilitate the selection of appropriate equipment and installation procedures to ensure that the equipment can operate safely in that environment and does not cause fires or explosions.

Hazardous areas can be defined as locations in an industrial facility in which an explosive atmosphere can be caused due to the presence of flammable gases, liquids, vapors, dusts, and fibers, under both normal and abnormal operating conditions.

Hazardous area classification is a method of analyzing and classifying the environment based on the type of flammable material present, and the probability of its occurrence.

The hazardous area classification layouts are typically prepared by a team of Process and Electrical engineers, supported by specialists from plant operations and Health, Safety and Environment (HSE).

In the United States, API RP 500, NEC, NFPA 497, are NFPA 499 are most commonly used. The American system is based on Class and Division system of hazardous area classification.

In other parts of the world IEC 60079 and standards based on the same such as EN 60079, BS EN 60079, AS/NZS 60079 etc. are more prevalent. IEC 60079 is based on the Zone system of hazardous area classification.

American Standard API RP 505 is also based on the Zone system.

This article does not address hazardous areas caused by flammable fibers.

Historical Perspective

Origins of hazardous area classification can be traced back to coal mining during the industrial revolution. Methane (firedamp) that was trapped in the coal was released during the process of mining. Methane, being a flammable gas, presented a major fire hazard as it was easily ignited by the flaming torches carried by the miners, and this sometimes led to the ignition of coal dust causing major accidents.

As a safety precaution a person wrapped in a wet blanket and carrying a long pole with a flaming ember at the far end entered the mine before the start of a shift. The intent was to ignite a cloud of methane if it was present, and the "safety officer" would (hopefully) be saved by the wet blanket. However, being saved by the wet blanket depended on the volume of methane. The rationale was that this approach would save many other lives.

Safety improved with inventions such as the safety lamp invented by Humphry Davy around 1815. This was followed by much safer devices such as battery-operated lamps.

In the United States, the concept of hazardous areas was first addressed in the 1923 Edition of the National Electric Code (NEC). In current editions, Chapter 5 (Special Occupancies) addresses hazardous areas.

Fire Triangle

Fire can be defined as a rapid oxidation process (a chemical reaction) which releases heat, light and sound.

In order to start a fire three components are necessary, and these are typically represented as the sides of a fire triangle. A fire can be extinguished by eliminating any one of the three components shown in the fire triangle.

The definitions of each of the three components are as noted below:

Fuel- something that will burn under the right conditions

Heat- source of ignition that will cause the fuel to ignite

Oxygen- required to sustain the fire

Fire Tetrahedron

Recently, a fourth component has been added to the fire triangle resulting in a fire tetrahedron. The fourth component is called the “uninhibited chemical chain reaction”.

The “uninhibited chemical chain reaction” provides the "positive feedback" of heat to the fuel to produce the gaseous/vapor form consumed in the flame. In other words, the chemical chain reaction provides the heat necessary to maintain the fire. The fire tetrahedron helps illustrate the benefit of fire suppression with the use of "clean agents". The clean agents help extinguish fires by interrupting the chemical chain reaction of combustion.

Class and Zone Systems

Class and Zone Systems are summarized in the following table:

Notes

1. There is no direct correlation between Class and Zone systems. The table presents an approximate comparison for the purpose of illustrating the similarities / differences between the two systems. Please refer to applicable standards for definitions and details.

Temperature Class

Temperature class can be defined as the maximum surface temperature at any part of the enclosure under any condition. This temperature should be less than the Auto Ignition Temperature (AIT) of the gases present in that area. The rationale is that if the surface temperature of the enclosure is less than the AIT, a necessary condition to complete the fire triangle (Heat) is not fulfilled and thus ignition of the gas is prevented.

Temperature Class is as defined in the table below:

Gas Groups

Gases and vapors have distinct physical and chemical properties such as smell, color, ignition temperature, explosion pressures etc. Given the number of gases and vapors encountered in industry it is not practical to identify gases individually for the purposes of hazardous area classification. Hence, as a practical method, gases are placed into groups. This method is followed in both American and European systems.

Equipment for Hazardous Areas

Equipment such as circuit breakers, switches, contactors etc., which can cause arcing and sparking shall be enclosed in explosion proof housings. Refer to Article 100 of the NEC for definitions of explosion proof equipment.

For Class I locations, the enclosure should be robust enough to contain the arc/spark or explosion within the enclosure itself and should be designed such that the hot gas produced inside the enclosure is adequately cooled as it escapes the enclosure. The surface temperature of the enclosure shall not increase beyond the specified Temperature Class rating.

For Class II locations, the enclosure shall keep dust out of the interior and operate at a safe surface temperature. The presence of dust inside the enclosure is unlikely and hence the probability of an internal explosion is low. These enclosures may have thinner walls in comparison with enclosures rated for Class I installation. The construction of these enclosures is known as dust-ignition proof. Refer to Article 100 of the NEC for definitions of Dust-Ignition proof equipment.

Purged and Pressurized Systems

In case of purged systems, the enclosure is supplied with a protective gas such as dry instrument air or nitrogen at a suitable pressure and flow rate and this will reduce the concentration of any flammable gas or vapor that may be initially present to a level that it will not support an explosion. The enclosure is purged before starting the equipment.

In case of pressurized systems the enclosure is supplied with a protective gas such as dry instrument air or nitrogen to maintain a pressure slightly higher than atmospheric pressure, and this will prevent the entrance of a flammable gas or vapor or a combustible dust inside the enclosure.

NFPA 496 provides information on the methods for purging and pressurizing electrical equipment. Refer to Article 100 of the NEC for definitions of purged and pressurized systems.

Intrinsically Safe Systems

Intrinsically safe systems are typically used in instrumentation and control applications. Intrinsically safe systems do not release electrical or thermal energy to cause ignition. Intrinsically safe systems typically use Zenner barriers or Galvanic isolation.

ANSI/UL 913 provides information on the design, testing and evaluation of intrinsically safe systems. Installation requirements are covered in Article 504 of the NEC.

The IEC system follows the system as defined in various parts of IEC 60079. The most commonly used ones are as noted below:

1. Ex-d ( Flame Proof Enclosure)
2. Ex-e (Increased Safety)
3. Ex-n (Non-Incendive)
4. Ex-p (Pressurized)
5. Ex-i (Intrinsically Safe)
6. Ex-m (Molded Encapsulation)

Best Practices

Some best practices for hazardous area classification are as noted below:

1. Follow established engineering practices such as API, NEC, IEC and PIP etc.
2. Ensure that hazardous area requirements are established before purchasing equipment.
3. Ensure that hazardous area classification layouts reflect the current status of the facility.
4. Review and update layouts before each modification.
5. Ensure that the addition of new equipment does not impact existing equipment / facility and if so, take corrective action.
6. Ensure that the area classification "cloud" (the 3D space where fire or explosion hazards may exist) does not encroach roads, walkways, occupied buildings, welding / fabrication yards etc.
7. Perform a thorough review when introducing new sources of release in an existing facility.
8. Ensure equipment installed in the classified area is rated for service conditions.
9. Follow the manufacturer’s installation manual, applicable codes and standards.
10. Maintain proper documentation.

Conclusion

Electrical equipment such as motors, circuit breakers, solenoids etc., can cause arcs and sparks under normal and abnormal conditions. This could cause a fire and/or an explosion in an environment where flammable chemicals are present, such as in an oil refinery or chemical manufacturing plant. Hazardous area classification helps us identify these risks so we can select appropriately rated equipment to ensure a safe and reliable operating facility.

Fauske and Associates LLC has the expertise, experience and capabilities necessary to perform hazardous area classification studies for flammable gases, liquids, vapors and dusts. Please contact Venky Viswanathan, venky.viswanathan@westinghouse.com.