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Beware of Methane Fires

29 January 2013

Correct ventilation of pig buildings will prevent the build-up of methane that recently injured two people removing manure from a barn. Robert Chambers and Doug Richards of the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) and Sam Bradshaw of Ontario Pork offer two solutions for venting to minimise the risks of explosion and fire.

Recently, an owner and his employee were removing manure from a finishing barn. The barn is a mechanically ventilated, 1,200-head grow–finishing barn approximately seven years old. There was a central hallway with rooms on each side with several motors, light switches and lights present in the hallway. Although there are inlets present, there are no exhaust fans or ducts leading from the hallway to an exhaust fan. The floor of the hallway is solid concrete, but there was an access hole covered with a steel plate with approximately on-inch diameter drain hole open to the transfer pit below. The pits in the room are eight feet deep and are drained to the 10-foot deep transfer pit by slider gates. The transfer pit is connected to a 12-foot deep pump out pit located on one end of the barn. There is an approximately eight-foot deep baffle that is located between the transfer pit and the pump out pit.

When the pits are emptied, the slider gate is raised a bit and the manure flows out into the transfer pit. As soon as the manure level reaches the bottom of the baffle between the transfer pit and the pump out, the headspace becomes pressurised. As more manure flows in, the pressure builds. Methane is only slightly soluble in liquid so just by moving, it is released into the headspace. It has a specific gravity of 0.5 compared to air, or half the weight of air, so it rises and collects in unvented spaces. The pressure in the transfer pit headspace forced the collected methane through the drain hole and up into the hallway.

As there was no ventilation to remove this methane, it collected under the ceiling. As more and more methane built up the layer of collected methane would have moved further down from the ceiling.

Methane is in the flammable range at four per cent to 15 per cent by volume. Higher than this, it burns only on the interface as there is not enough air to support combustion. In these types of situations when this flammable layer comes in contact with a spark from a motor, light switch or open flame, the flames spread rapidly along the underside of the methane layer creating a flash over. It was this flash- over that critically burned the two victims. The remaining methane then burns off more slowly. If the methane is distributed in a large enough flammable layer or throughout the entire space, then a violent explosion can result.

The solution in this case is two-fold. The first is to provide exhaust ventilation for the hallway. An exhaust fan will be placed in the gable end and ducted to the ceiling so that any methane that is present in the hallway is removed. An inlet at the opposite end ensures that the entire hallway is ventilated.

The second is to vent the transfer pit under the hallway with a chimney or pit fan at one end and an inlet at the other end. Again, this will eliminate the pressure build up in the headspace and ensure that any manure gases will be collected and exhausted before they enter the hallway. Having the transfer pit vented will also aid in emptying the pits as the manure being drained to the transfer pits does not have to fight the back pressure caused by the trapped gasses. The chimney vent must be terminated above the roof line, like a stove pipe, so as not to be affected by back pressure from the wind blowing against the barn. The size of this vent is site-specific but as air moves through a pipe much easier than manure, the required diameter of the vent pipe is smaller than the diameter of the manure pipe.

In order to prevent such tragedies, producers should look for unvented spaces throughout their facilities. With liquid manure systems, always assume that methane is being produced.

Research out of Quebec shows that even in cold weather, methane bacteria, though slower, will still produce methane, up to 16 litres per cubic metre of manure per day. In warm weather, the bacteria produces methane at a faster rate - up to 30 litres per cubic meter of manure per day.

As the methane is lighter than air, it will always rise up. It is colourless and odourless as well. If there is an unvented area such as a closet, feed room, office or hallway as described above, and any ignition source such as a switch, motor or a pilot light will trigger a flash over or explosion and a fire. All barns or rooms within barns with liquid manure storage or transfer pits should be ventilated at all times. Even if the barn is empty of animals but still has liquid manure present, producers should provide at least three air changes or more per day.

The authors encourage all producers to take a few minutes to walk through their facilities and verify that all spaces, even closets, are being vented. Producers should be especially concerned if they notice floor drains gurgling, air blowing up from drain holes during manure transfer, manure shooting up into the pit or room from the pull plug holes, or poor emptying of the manure pits. These may be signs that the transfer pit or pipe is unvented or the vent is blocked. The cost to retrofit these situations is minimal compared to the cost of a flash over fire or explosion.

January 2013

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