OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 36 Mining

Section 712 Air velocity

Subsection 712(1)

The minimum air velocity allowed at a working face under normal working conditions is specified as 0.3 metres per second. This minimum velocity is mandated to ensure that methane gas levels and coal dust generation are maintained at acceptable levels in this most hazardous location. Both methane gas and coal dust are liberated by the coal cutting activity.

Subsection 712(2)

Unused or intermittently used roadways can sometimes become pockets for methane accumulation. Minimum air velocity standards ensure that methane is adequately diluted and exhausted through the system. Methane is lighter than air and in low air velocities can accumulate in layers close to the roof. Such layers can, in certain circumstances, even move up gradient against the prevailing ventilation flow. Inadequate velocity and laminar flow in mine roadways can create conditions that lead to methane layering. Such layers can contain potentially explosive mixtures of methane in air which may go undetected. These layers can be removed by mixing the general air body by more turbulent airflow.

Subsection 712(3)

The maximum air velocity allowed in a coal mine must also be restricted in order to control dust. Higher velocities pick up coal particles in correspondingly larger quantities and create coal dust problems.  When coal dust is combined with specific concentrations of oxygen, an explosive atmosphere can develop that can release more energy, if it explodes, than a methane atmosphere.

Apart from explosion hazards, coal dust impacts worker health, visibility, covers equipment, dulls lighting and creates a generally unhealthy atmosphere.  As a countermeasure, coal dust in roadways must be removed and the areas dusted with stone dust to reduce any explosion potential and improve visibility.

To reduce dust problems at transfer points, roadways are sometimes widened to reduce air velocity.  An illustration of this is provided in Figure 36.22.

Figure 36.22 Roadway enlargement

Subsection 712(4)

As indicated, the Director may issue an acceptance if an acceptable alternate has been certified by a professional engineer as providing a level of protection that is equal to or greater than the limits specified in subsections (2) and (3) for the actual mining conditions expected.