The following guidelines are recommended for optimum goaf gas drainage:
Surface goaf holes for gas drainage provide the highest capacity and lowest cost option for goaf gas drainage under most circumstances.
Goaf holes should be drilled on the return side of the goaf, preferably at 30-70m from the gateroad depending on the longwall caving conditions.
Goaf gas drainage hole diameter should be about 250-400mm for optimum flow rates and goaf holes may be drilled at 100-300m spacing, depending on the goaf gas emissions and other conditions.
The total capacity of the goaf gas drainage plants should be about two to three times the expected goaf gas emissions to cater for deep goaf holes gas drainage, shifting of goaf plants or goaf hole connection changes and reduced plant efficiencies due to high pressure losses. The provision of a high capacity gas drainage system allows optimisation of goaf gas drainage strategies, improves the overall efficiency, and provides better gas control on the face.
The goaf gas drainage system should include a combination of goaf holes near the face and deep holes in the panel in order to improve the overall gas drainage efficiency and to reduce the effects of barometric pressure changes on tailgate gas levels.
The strategy of continuous operation of deep goaf holes at moderate capacity should be implemented (ie. intermittent operation of deep goaf holes at high capacity may not improve the overall efficiency and may lead to problems).
Goaf gas drainage should be carried out from around three to four goaf holes in the panel (including deep holes), instead of the standard practice of gas drainage from just two holes closest to the face. Application of increased suction pressure to drain more gas from goaf holes closest to the face might result in increased air dilution, without any net increase in gas drainage flow rates.
The ventilation system in the panel should be designed to minimise oxygen ingress into the goaf, including immediate sealing-off of all the cut-throughs behind the face, in order to improve overall gas drainage efficiency.
Oxygen concentration levels in the goaf holes should be less than 5% to reduce sponcom risk in the longwall goafs.
Gas drainage from adjacent old goafs should also be carried out wherever possible, depending on the goaf gas emission flow rates.