OHS Code Explanation Guide

Published Date: July 01, 2009
Bookmark this page

Part 10 Fire and Explosion Hazards

Section 162 Prohibitions

Subsections 162(1) and 162(2)

Employers must ensure that flammable and combustible substances at the workplace do not ignite and harm workers or damage equipment. No worker, other than a competent worker responding in an emergency, must enter or work at a work area in which the atmosphere exceeds 20 percent of a substance’s lower explosive limit (LEL). Above this limit the safety margin, or margin for error, is small.

Before performing work involving an atmosphere that may contain an explosive gas, the atmosphere may need to be tested to determine if a flammable mixture is present. Where atmospheric testing is required, it must be done before work begins and may be required at regular intervals while work continues. The use of electronic gas detection equipment is recommended as it allows for the continuous monitoring of gas or vapour concentration in air. The most common unit of measurement is the percentage of the lower explosive limit (% LEL).

The LEL is the minimum amount of fuel that must be present in air to ignite. If the air/fuel mixture is below the LEL, it is considered too “lean” and will not ignite. The upper explosive limit (UEL) is the maximum amount of fuel that can be present in air for ignition to occur. If air/fuel mixture is above the UEL, it is considered too “rich” and will not ignite (see Figure 10.2). In this situation there is insufficient oxygen to support combustion. The wider the explosive range, the more difficult it is to manage the potential of an ignition resulting in a fire or explosion.

Figure 10.2 Graphic explanation of LEL and UEL

Using methane as an example, a 5 percent mixture of methane in air is the minimum concentration that will ignite and explode in the presence of an ignition source. When the concentration of methane in air reaches its LEL of 5 percent, a gas monitor calibrated for methane will read 100 percent LEL. If the concentration of methane in the air is 0.5 percent, the instrument will read 10 percent LEL. Table 10.1 shows LEL and UEL limits for selected hydrocarbon gases and liquids.

Table 10.1 LEL and UEL limits for selected hydrocarbon gases and liquids

Flammable Substance

Lower Explosive Limit (LEL)

Upper Explosive Limit (UEL)

Methane

5%

15%

Ethane

3%

12.5%

Propane

2.3%

9.5%

Butane

1.9%

8.5%

Hydrogen sulphide

4.0%

46%

Toluene

1.27%

7.0%

Gasoline

1.3%

6.0%

Cutter oil

1.1%

6.0%

Envirovert (drilling fluid)

0.7%

6.0%

Crude Oil

1%

7%

To ensure the health and safety of workers, gas monitor readings in work areas should not exceed 20 percent of the LEL for the following reasons:

(a) gas monitors may be calibrated for a flammable gas or vapour other than the one being tested for;
(b) the atmosphere being tested may contain a mixture of unknown flammable gases or vapours;
(c) the gas monitor’s correction factors may be inaccurate or unreliable;
(d) worker sampling techniques may not be the best e.g. it may be difficult to get to the bottom of a vessel where gases that are heavier than air can pool; and
(e) to provide an added safety factor that reduces the likelihood of an explosion.

Subsection 162(3) and 162(3.1)

Smoking materials and open flames are a potential source of ignition. They may not be present during the storing, handling or processing of a flammable substance. If open flames are unavoidable during these activities, the “hot work” requirements of section 169 must be met.

Subsections 162(4) and 162(5)

Exposing flammable or combustible liquids which are at a temperature at or above their flash point to the air can result in explosive mixtures in the air. Equipment in the area that is not designed to prevent it from becoming an ignition source could cause an explosion or fire.

Flammable liquids are those that flash at temperatures below 37.8°C (100°F), while combustible liquids flash at temperatures above 37.8°C (100°F). Different liquids flash at different temperatures. Some “flammable liquids”, such as gasoline (flash point approximately -46°C), flash at very low temperatures and should be considered flammable at all temperatures.

Material Safety Data Sheets (MSDSs) provide information such as a particular liquid’s flash point. MSDSs also describe any precautions that need to be taken when handling the liquid.

A flammable or combustible liquid at a temperature above its flash point presents a potential fire and explosion hazard, particularly if a potential source of ignition is present. The restrictions on mixing, washing, cleaning, and other uses of a flammable or combustible liquid at a temperature at or above its flash point are intended to prevent a fire or explosion.

Subsection 162(6)

Rags contaminated with flammable or combustible substances can heat up and burst into flames under the right conditions. Such rags must therefore be stored in containers with a lid that keep out air. Without air, a fire quickly smothers itself. Temporary storage containers should be emptied frequently and used as recommended by the manufacturer. The container must be clearly labelled as being for the storage of contaminated rags (see Figure 10.3).

Figure 10.3 Examples of temporary storage container for contaminated rags