Handbook for Methane Control in Mining - AMMSA

14good eyesight. 24methane level.Otherwise, the audible alarm on the detector could be set to engage at a lowMitigating methane layers. Methane layers are removed by increasing the ventilation quantityand reducing the gas flow by methane drainage. In instances where the source and layer size arelimited, 25 a less satisfactory, but workable method is to use a (well-grounded) compressed airpoweredventuri air mover or an auxiliary fan at each methane source to blow air at the source ofthe layer and disperse it [Creedy and Phillips 1997]. In either case, an aggressive sampling programis necessary to ensure safe conditions.Keep in mind that methane mixed withair cannot unmix to form a layer.The rib and floor as sources ofmethane layers. Methane layeringoccurs when methane is released atthe mine roof. When methane isreleased at the mine floor or rib, thisgas readily mixes into the ventilationair stream, losing its buoyancy.Figure 1–6, from Bakke and Leach[1962], compares 2-cfm methanesources at the roof, rib, and floor ofthe mine entry. Only the roof sourceproduced a significant methane layerat the 2-cfm rate. 26Figure 1–6.—Methane layering with roof, side, and floorsources (from Bakke and Leach [1962]).WHEN RECIRCULATION OF MINE AIR CAN BE HAZARDOUSRecirculation leads to higher methane levelsonly when recirculated air replaces fresh air.24 See the sampling chapter (Chapter 2) and the sections on methane detection in the continuous miner chapter(Chapter 3).25 For example, the immediate face area in a tunnel boring machine.26 The testing did not rule out the possibility of a layer at higher methane flows.