How Hazardous Areas Work | Fires & Explosions

hazardous area classification guides us in preventing disasterEver wondered how big your hazardous zone really is? What determines its size? How far away do you have to put sensors or control boxes that don’t have the appropriate certifications?

We’re going to pull back the curtain and show you how hazardous locations really work. It all starts with a fundamental understanding of fire and explosion hazards.

The primary factors in a fire or explosion are fuel, oxygen, and a heat source (ignition). Remove any of these and you’ll prevent or stop the fire. There are secondary factors for explosions: gas or dust suspended in the air, the concentration, and explosivity limits.

Knowing the relationship between these factors at any given point in your hazardous area is the key to assigning the location and extent of your zones. Control these factors, and you can engineer a safer work area and open up more options for instrumentation.

Explosive Limits

There are a few important explosive limits to understand: Lower and Upper Explosive Limits, and the Flash Point.

The concentration of the gas and air has to be within a certain range for the possibility of an explosion to exist. The point at which there is too much air and not enough gas is called the Lower Explosive Limit (LEL). The point at which there is too much gas and not enough air is called the Upper Explosive Limit (UEL). Only when the concentration of gas and air is between the LEL and the UEL is there a chance of ignition.

The flash point is another limit to fire and explosion. The flash point is the lowest temperature at which enough vapor is given off a liquid to form a flammable mixture with the air. Different liquids have different flash points. For example, gasoline has a low flash point at – 45° C, so it produces flammable vapors almost all the time. On the other hand, diesel has a high flash point of 55° C, so it doesn’t produce flammable vapors until its quite hot.

These factors will play a major role in what areas are hazardous, what zones are assigned to those areas, and how large the hazardous area is.

Grade Of Release

The point where gasses are released into the air is extremely critical. This is called the source of release. The rate and likelihood of the release is known as the grade of release.

The grades are labeled in order of decreasing frequency and likelihood:

  • Continuous Grade of Release – release is continuous, or is expected to occur either frequently or for long periods of time
  • Primary Grade of Release – release is expected to occur periodically during normal operation
  • Secondary Grade of Release – release is not expected to occur during normal operation. If it does occur, it is expected to be infrequent and for short periods

When the source of release is in open air, there is a typical relationship with the zone that goes like this:

  • Continuous grade = zone 0
  • Primary grade = zone 1
  • Secondary grade = zone 2

The grade of release directly relates to the zone, but it is not the same thing. Ventilation affects how stringent and how large the zones may be.


The availability of good ventilation, in relation to the release of hazardous gases, is what will determine the zone classification of the area and the radius of the zone. The zone will have a radius as large as the distance required for the hazardous mixture of gas and air to fall below the Lower Explosive Limit (LEL). The classification of the zone will depend on the degree and availability of the ventilation and the grade of release.

There are three degrees of ventilation:

  • High – will reduce the concentration of the hazardous gas/air mixture below LEL at the source of release – virtually instantaneously
  • Medium – will control the concentration
  • Low – cannot control the concentration or cannot prevent persistence

The ventilation and grade of release work together to determine the classification and size of hazardous zones.

Hazard Radius

There are really 3 ways to determine the radius of the hazardous zone.

For common or typical facilities, you can refer to direct examples. For example, a typical wellhead in open-air conditions will have a zone 2 radius of about 3 meters from all pipe, and a small zone 0 around a ventilation pipe (source of release).

For all other situations, a point source approach is better. Here, all the variables are considered, including the flash point of the liquid, the source of release, grade of release, and ventilation. You may be able to match up with a radius table provided in the standard you follow (IP 15, API 505, etc). If not, then specific dispersion calculations must be done.

Finally, for secondary releases with an unknown release rate, a risk-based approach is the last option. This approach attempts to determine a leak rate based on the hole size and pressure. Once this is determined as closely as possible, calculations are carried out.

Production and Storage

We deal a lot with the upstream Oil & Gas industry, which is largely outdoors. Here, zone radii are small due to ventilation. Zone 0 equipment may be necessary on a tank or wellhead, but only a few feet away, the hazard can drop to zone 1 of 2, or even disappear.

This is a big deal as remote sensors and control panels are increasingly deployed to monitor distant tanks and facilities. Often, battery packs and communication equipment is not certified for hazardous zones. All is not lost in these circumstances. It’s common for this equipment to be set up very near by, just outside the classified zones.

The zone radius will vary greatly depending not only on the circumstances of the application, but on the standard that you adhere to and the approach used to determine the zone radius. In reality, no two individuals will interpret the code and standards exactly the same way.

Rest assured, though, that many of our sensors are certified for use in zone 0, 1, and 2 locations to meet your safety needs. Hazardous areas are not to be taken lightly. Seemingly small oversights have cost lives before. We’ll help you get the right level and pressure sensors for your application, no matter how hazardous.

Let us know if you have questions about how hazardous areas work, or about what sensors work best in your hazardous area.

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photo credit: Josh13770 via photopin cc

Primary source of information:
Leroux, Patrick. "Area Classification. Why ? Where ? How ? Who ? When ?" 2012
IECEx International Conference. Dubai, United Arab Emirates. Mar. 2012. IEC-IECEx.
Web. 11 Mar. 2014.>.


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