OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 10 Fire and Explosion Hazards

Section 169 Hot work

Subsection 169(1)

Section 1 of the OHS Code defines hot work as work in which a flame is used or sparks or other sources of ignition may be produced. This includes

(a) cutting, welding, burning, air gouging, riveting, drilling, grinding, and chipping,
(b) using electrical equipment not classified for use in a hazardous location, and
(c) introducing a combustion engine to a work process.

Work activities that meet the definition of “hot work” must be carried out in accordance with the requirements of this section when:

(a) they are carried out in a work area that is itself a hazardous location (as defined);
(b) they are carried out in a work area (defined as part of a work site) that is not a defined hazardous location (normally) but is one where an explosive atmosphere exists temporarily because

(i) a flammable substance is or may be in the atmosphere of the work area;
(ii) a flammable substance stored, handled, processed or used in the location may be released into the air in flammable concentrations during the work process;
(iii) the hot work is on or in an installation or item of equipment that contains a flammable substance or its residue; or
(iv) the hot work is on a vessel that contains residue that may release a flammable gas or vapour when exposed to heat.

Subsections 169(2) and 169(3)

The employer must ensure that hot work is not started until:

(a) a “hot work” permit is issued in accordance with the employer’s permit system (see Figure 10.9 for an example of a hot work permit). This permit must indicate the nature of the hazard considering

(i) the presence of flammable materials,
(ii) the presence of combustible materials that burn or give off flammable vapours when heated, and
(iii) the presence of a flammable gas in the atmosphere, or gas entering from an adjacent area, such as a sewer that has not been properly protected. (Portable detectors for detecting the presence of combustible gases can be placed in the area to warn workers of the entry of these gases.);

Figure 10.9 Example of hot work permit

(b) any combustible material that is close enough to be ignited by sparks or heat from the work process is cleared from the work location or shielded or otherwise isolated from potential ignition;

(c) procedures are implemented that make sure that the hot work is done safely. Section 8 of the OHS Regulation requires the procedures to be in writing and available to workers;

(d) testing shows that the atmosphere does not contain

(i) a flammable substance, in a mixture with air, in an amount exceeding 20 percent of that substance’s lower explosive limit (LEL) for gas or vapours, or
(ii) more than 20 percent of the minimum ignitable concentration of dust in air or more than moderate accumulations of dust on surrounding surfaces. Accumulations of dust can be considered moderate if the colour of the surface beneath the surface is visible through the dust layer.

The LEL is the minimum concentration of gas or vapour that must be present in order for ignition to occur. Below that concentration, the air/fuel mixture is too “lean” and will not ignite. A limit of 20% LEL provides a safety factor to account for the delay in response of gas detectors or the variation of the gas or vapour concentration over the area of the hot work.

Using gasoline as an example, a 1.3 percent mixture of gasoline vapour in air (100 percent of its LEL) is a high enough concentration to ignite and explode in the presence of an ignition source. When gasoline vapour reaches its LEL of 1.3 percent, a properly calibrated gas monitor (specifically calibrated for gasoline) will report the reading as 100 percent of the LEL. If the amount of gasoline vapour in air is 0.13 percent, then the instrument will report the reading as 10 percent of the LEL. Table 10.1 shows LEL and UEL limits for selected hydrocarbon gases and liquids.

Test instruments must be calibrated and used according to the manufacturer’s specifications. Workers using test instruments must be made aware through training that the reading on the instrument depends on the chemical used to calibrate the instrument. For example, a test instrument calibrated using methane will read correctly for atmospheres containing methane, but may not give correct readings for atmospheres containing a flammable substance that is not methane.

Sometimes manufacturers publish correction factors that can be used when working with different flammable substances. Test instruments may sometimes give a “false” reading if the atmosphere being tested is deficient in oxygen. Many combustible gas sensors need a minimum percentage of atmospheric oxygen to function properly. The employer must ensure that workers are trained in how to correctly use the test instrument, including understanding the instrument’s limitations.

While hot work is being done, atmospheric tests must be repeated at regular intervals appropriate to the hazard associated with the work being done. Regular testing confirms that a flammable or explosive atmosphere, capable of causing a fire or explosion, has not built up over time. Continuous monitoring is often required to accomplish this. Recent improvements in portable electronic combustible gas detection systems make continuous monitoring more practical. Use of the older manual sampling systems is not recommended.

For more information
Safe Work Permits
Bulletin SH013
Hot Work on Drums and Tanks
Department of Labour, New Zealand