Underground » Detection and Prevention of Fires and Explosions
The project is concerned with eliminating one of the major potential sources of explosion initiation, namely, 'incendive sparking' originating from the cutter picks of continuous miners. The problem arises when the picks come into contact with rocks that contain significant amounts of hard minerals, most especially quartz.
There is strong and consistent evidence from the World's leading coal mining R&D centres, that the origin of such ignitions is the occasional occurrence of streaks of hot material adhering to the surface of the rock behind, and in the track of, the pick. This hot material can be of sufficiently high temperature and heat energy to ignite a flammable methane-air mixture.
Directing a solid cone water spray immediately behind each pick can effectively combat these hot spots. The spray should have an included angle of about 30° and be directed to cover an area extending from the pick tip to about 40mm behind it. It also suggests what the droplet sizes and minimum spray density should be. One means of translating this important intelligence to practical benefit has been the development of the 'wet-head'. This is a continuous miner cutting drum that has been equipped with individual back sprays of the right shape, disposition and flow density to all of its picks. A few 'wet-heads' have been used over recent years, most notably in the USA, with often strikingly beneficial results. They have not, however, been as widely taken up as might be expected and in some instances have been withdrawn. This is mainly because of reliability and maintenance problems.
The main objective of this project was to make a field evaluation of the 'wet-head' principle by undertaking field trials with one fitted to a continuous miner. The evaluation was to be based on measurements using an Infra-red camera as a means of detecting and ultimately quantifying the presence of hot spots while cutting in quartz based rock under a range of operating conditions.
The first stage of the project was a comprehensive review of published R&D literature on the subject. This consistently demonstrated that frictional ignitions from cutting picks arise from hot streaks of incandescent material left in the tracks of the picks as distinct from the showers of sparks that usually accompany a cutting drum working in rocks that have a high quartz content. The literature also consistently argued that a prospective means of overcoming the problem is to use a cutting drum that directs a water jet or spray at or near the trailing edge of each cutter pick and thereby quench any hot material left in the track.
The original intention for this project was to undertake field trials of a continuous miner fitted with a 'wet-head' drum. A 'wet-head' drum' could not be procured and the investigation proceeded with a Webster 2000 Series Cutter Loader, the cutting drum of which is fitted with both back pick flushing and conventional sprays.
A program of experiments with this machine was undertaken in the conglomerate roof of the Great Northern Seam at Cooranbong Colliery and this was completed in July 1999. It involved remotely measuring the location and temperature of hot spots produced by the drum using an infra-red camera. This was intended to provide data on the size, temperature and duration of high temperature events in and around the drum.
The investigation examined the effectiveness of three different water spray configurations in quenching the hot spots generated when the machine drum was cutting in the conglomerate roof. The comparison was made against the hot spots generated by the drum when cutting dry, with the water sprays turned off. The three spray configurations were:
- Pick back sprays only
- Venturi sprays only
- Back sprays & venturi sprays combined
- No sprays - cutting dry