The move to a new mining area and a subsequent increased demand on the ventilation systems was the motivating factor behind a decision to install underground booster fans at BHP Billiton’s West Cliff longwall mine in New South Wales. At 1600 kilowatts of combined power, the two fans are believed to be the biggest ever installed in an underground Australian coal mine.
Recently, the transfer of longwall operations to another section of the mine resulted in the main entry and bleed ventilation functions being provided by the same set of intake and return airways. West Cliff’s ongoing production targets of 15,000 tonnes per day, representing a substantial increase on historical figures, combined with the shift to the new mining area, has led to a greater demand on gas and ventilation management systems.
West Cliff mine management considered several options to improve the efficiency and capacity of the gas drainage and ventilation systems. Already being implemented is a range of post-drainage capture efficiency measures. These include upgrading the surface vacuum plant exhausters and the underground pipe reticulation system. Efficiency will also improve as a result of changes to cross measure drilling patterns and density beneath the longwall extraction.
In addition, small-scale ventilation solutions, such as improvements to stopping leakage resistance and removal of stowage from airways were evaluated. None of the routine initiatives available were however capable of yielding the required large-scale lift in methane dilution capacity of the ventilation system.
A range of options was examined, including: a main mine fans upgrade; building an upcast shaft, either inbye or outbye of LW5 tailgate; building a downcast shaft; or a downcast shaft with booster fans; and finally, installing underground booster fans only.
Investigation of all options showed that a booster fan installation could deliver the desired ventilation system performance at a fraction of the capital cost of sinking a new shaft. Combined with an underground ventilation appliance upgrade, the ventilation and gas drainage strategy will serve for the life of mine, estimated at approximately 15 years.
Booster fans are widely prevalent overseas, however, they have not been used in the Australian coal industry for almost 20 years. A worldwide literature search was conducted to help with the understanding of hazards associated with such installations. The NSW Department of Mineral Resources has given necessary approvals for the installation and operation of a booster fan.
The fans will be supplied by Melbourne company ABB Fans and are double inlet 1900DY centrifugal fans. Centrifugal fans were selected because of their increased impellor to fan casing clearances as well as providing the ability to locate the motors in intake air without the necessity for a right angle gearbox, as would be the case with an axial fan.
The fan motors are ABB-HXR 770kW, six-pole, 11kV AC induction motors. Motors in the intake are to be connected by a 4m-long cardon shaft to the fans in the return.
The booster fan duty will be 300cu.m per second at 3kPa, with the main fan duty of 320cu.m/sec at 3.3kPa. A 20% factor of safety has been integrated into the design to give it flexibility and so it can accommodate any future mine plan changes. A ventilation quantity of approximately 125-135cu.m/sec will be supplied to the longwall circuit.
The installation will involve two fans located outbye of the longwall split in separate roadways, operating in parallel. The principle of operation of the fans is to boost the pressure available to overcome the resistance of the longwall ventilation circuit.
A set of bypass doors will be used to maintain inbye airflow in the event of a booster fan stoppage. These doors are designed to open under gravity in order to maintain ventilation, albeit at a reduced level.
An independent power supply will be provided for the fans via a surface to seam borehole. This will avoid booster fan stoppage due to underground power supply interruption and allow the booster fans to remain in operation when the underground power is isolated. The borehole will also incorporate a fibre optic cable for condition and atmospheric monitoring and a back-up twisted copper core cable for monitoring in a power off situation.
A comprehensive three-stage risk assessment process was undertaken in order to identify hazards and develop a rigorous risk reduction strategy.
The monitoring system incorporates redundancy of monitors as well as two stage alarm levels. The level one alarms will initiate corrective action and level two alarms will trip power. Items to be monitored include methane, carbon monoxide, differential pressure (ventilation appliances and fan), bearing temperature and vibration, bypass door status, and holding brake status.
West Cliff colliery believes the successful installation and operation of the booster fan is the most cost effective means of ventilating the mining operations for the life of mine. Planned commissioning for the booster fan installation is December this year.
