For many years, the fire investigation industry has seen numerous fires involving livestock buildings. These fires are often high profile, especially when masses of animals are lost or injured. Many times these losses are in older structures with neglected electrical and/or heating systems. Given the interior environment, corrosion and degradation to electrical and heating components are common. Today, we are seeing these in new state-of-the-art buildings less than five years old. Needless to say, these incidents are extremely costly for the insurance carrier, as many of the new state-of-the-art buildings are into the seven-figure range. The most prevalent and well known of these losses are in the swine production arena.
As of March 1, 2013, according to USDA records, the United States has 65.9 million hogs.[i] The majority of these animals are raised throughout the corn belt in states such as Iowa, Illinois, Indiana, Ohio, North Carolina, and Minnesota, just to name a few. The method of raising these animals is in climate-controlled confinement structures.
In recent years, the industry trend is to erect a structure over a concrete pit 8 to 10 feet deep with slatted floors above for the collection of waste. The waste is then pumped out of the pit once or twice a year and applied to crop ground. During the time the waste is in the pit, certain chemical changes take place during the decomposition of the waste. Gases that are released during this process include but not limited to hydrogen sulfide, carbon dioxide, ammonia, and methane. Of these, each can be of significant consequence, but one is a major issue when confined: methane. Methane, CH4 , or natural gas, as it is more widely known, is the most prevalent of the gases. The typical waste pit configuration allows for several high-volume fans to be placed along the perimeter of the building to ventilate the pit. The fans disperse the gases generated during the decomposition process to the atmosphere. These fans also keep the noxious and combustible gases out of the occupied space of the building.
In recent years (since about 2008), formulation of swine feed has changed to include recycled corn products such as dried distillers grain with soluble (DDGS) and the addition of other processed food items. Since about 2010, sporadic reports of a foam layer forming atop numerous confinement swine manure pits have circulated. This foam layer has been blamed or linked to over a dozen significant explosions/fires in the upper Midwest.
People often ask, “How does the trapping of methane gas in a foam layer in a confinement swine building manure pit cause an explosion?” The foam layer that forms atop the liquid in the waste pit is comprised of bubbles containing methane gas. Studies at some major Midwest universities indicate that these bubbles are from 40 to 60% methane gas by volume. When this foam layer is stagnant or un-agitated, the methane is released very slowly and does not accumulate in a confined space or enclosed area. The pit ventilation fans are able to disperse the gas. Two of the losses that Donan has investigated have resulted after a time when the structure was idle or unoccupied. When some type of action takes place, repopulating the pens, cleaning of the pens, or similar, the foam layer is disrupted, and large amounts of methane are released.
If the methane can be evacuated to the atmosphere without reaching a concentration in air of greater than 4%, no potential for explosion or fire exists. However, if the methane release is greater or the ventilation system for the pit is inactive or reduced (as is intentionally done during idle times), the concentration of methane to air can be above the 4% or lower explosive limit. Also, methane gas has a vapor density of 0.5, which indicates it is lighter than air and will rise.
In the first illustration of Figure 2, left, the high methane concentration in the undisturbed foam is contained within the waste pit and away from potential ignition sources. In the second illustration showing the foam being agitated or disturbed, the methane release is throughout the occupied portion of the structure, where many potential ignition sources are present.
In one such instance that Donan investigated, the damage was limited to a localized flash fire, injuring the building owner and destroying a number of pigs. The building had been empty for at least two weeks while some ventilation upgrades and cleaning were being performed. Minimal pit fans were in operation, and the weather was in the mid to upper 20s. During the filling of the structure with grower pigs, the explosion and flash fire occurred. Two standing pilot, liquefied petroleum gas- (LPG) fueled heaters were operating. As the pens were being populated, the discharge from the animals entered the pit and began to agitate the foam layer. The escaping methane gas rose to the level of the standing pilot light heater in a concentration to air of greater than 4%, causing a flash fire.
In another such loss that Donan investigated, the structure had been without livestock for nearly a week while routine repairs and cleaning were being accomplished. During the day, the temperature was warmer than the set point on the thermostats for the gas heaters. Crews worked in the facility most of the day, power-washing and cleaning, undoubtedly agitating the foam layer. The heaters in this building wire not of the standing pilot type. Sometime the next morning, the ambient temperature had fallen below the set point on the heaters, one of the heaters called for heat, and the explosion ensued. This explosion ripped the metal ceiling and the metal roof open, sending a fireball through the attic space. The blast also dislodged all of the pit fans from their mountings. Firefighters reported the pit “burning with a blue flame” throughout much of the incident. In the days following the explosion and fire, crews were removing debris from the site. It was noted that the foam layer on the pit had rapidly increased. During removal of the metal roofing, a fireball about 15 feet in diameter was witnessed as a worker lit a cigarette. No one was injured, but it attests to the danger of the methane gas being produced.
The investigation into commercial agriculture buildings has taken on a whole new twist in the last two to three years because of the pit foam phenomenon . The reason for the foaming of the waste pit is still being sought out, and remedies are being tested. It is known that the foam layer on the pits contains high concentrations of methane gas, and multiple ignition sources are present within the structure. Updates to this article will be presented as more information becomes available.
[i] United States Department of Agriculture