In the midst of a scorching heat wave that has been record breaking in some areas, a farm-safety expert in Penn State's College of Agricultural Sciences is warning about the danger of silo fires.
Many crop fields are drought-stressed, meaning that the crop is low in moisture to begin with and will dry down very quickly once it is harvested, according to Davis Hill Sr., extension associate in agricultural and biological engineering.
"This year's silage might be too dry and be more prone to silo fires," he said. "Internal combustion of silage material can occur if the silage is put in when it's too dry for the silo. For anything to burn, you need three ingredients -- a heat source, air and fuel."
With silage, the heat source is the heat generated as the material goes through the fermentation process, Hill explained. This occurs naturally and happens with any material being stored. Proper moisture levels help keep the material from getting too hot.
"Air is trapped in the chopped forage during harvest and when blowing the material into the silo," he said. "The drier the material, the more air that is trapped; conversely, the wetter the material, the less air that is trapped."
The fuel is the forage material itself. Generally, it is not a good source of fuel from a burning standpoint, because even material that is too dry for good silage is too wet to burn quickly.
That's a good thing to keep in mind when discovering a silo fire, Hill noted, because when a silo burns, a farm operator can lose a tremendous investment and be faced with an unmanageable cost to replace ruined feed.
"If you have a 20-foot diameter by 60-foot high silo that contains 400 tons of corn silage, and you had to purchase that 400 tons of feed, it would cost you nearly $20,000 -- $50 per ton," he said. "Good hay-crop silage would be considerably more.
Key to fighting fire
"The key to managing a fire inside a silo is locating the fire area and controlling that area without affecting the rest of the material. Better to lose a few tons than a few hundred tons"
Hill said that limiting the fire while protecting the unaffected silage is difficult but not impossible. "The earlier you detect a fire, the easier it is to control, so it's important to regularly monitor your silos for a good three weeks post harvest. This is a critical time when natural fermentation and heating are taking place inside the silo."
Another common cause of spontaneous combustion in silos is putting new silage on top of old silage. Old silage can be quite dry, Hill explained.
"Remember, the dryer the material, the more air that can be trapped in that material," he said. "When fresh material is put on this older material, the natural heating that the new material will go through could be too hot at this location. It also will not pack down as tightly, leaving more air. This would be the first place to look if a fire does occur.
"Likewise, if you know that some of the material you are blowing into the silo is drier than ideal, you might make a mental note of where in the silo that is placed," he continued. "If your silo was two-thirds full when you put fresh material in it, and a fire is discovered a few weeks later, you and the fire company should concentrate efforts to determine if there is more heating going on at this two-thirds level."
Silo fires occasionally can start from the outside, Hill said. These external blazes most frequently start in the chute from either a shorting-out electrical wire or from an adjacent barn fire. Dried material in the chute then can catch on fire and burn through one or more wooden silo doors.
"Once the fire burns through the doors, the material inside the silo can begin to smolder and burn," he said. "Another less common way that a silo fire can occur is from exposure of the silo to the direct heat of a barn fire. Tremendous heat is generated from a barn fire, and if the side of the silo is exposed to that heat long enough, the heat will transfer into the silage and cause it to ignite.
Hill said there often is not much that can be done in such cases. "By the time a fire company arrives to fight a barn fire, resources for keeping a silo cool are very limited. Once a silo is heated, it is best to keep water away from it."
A silo fire usually is discovered when smoke comes from the top of the silo, when charred silage or burnt silo doors drop down the chute, or when a burning smell is evident. The initial decisions made can mean the difference between salvaging a viable crop or ruining it, according to Hill.
"Remember, a fire inside a stack of silage in a silo does not have adequate air to burn aggressively," he said. "This means you don't need to panic. The fire is not going anywhere in a hurry, so you have time to evaluate what you have, to report it to your local fire company and seek out additional expertise if needed."
Some fire companies around the state have been trained to extinguish silo fires, but many have not. For advice on proper procedures, Hill recommends what he termed "an excellent printed resource," "Extinguishing Fires in Silos and Hay Mows" (NRAES-18, 2000 Revision).
"Several technical experts are available throughout Pennsylvania to help farmers and firefighters think through the many management strategies when dealing with silo fires," he said. "This emergency information can be found by calling (814) 865-2808 during working hours or 814-404-5441 after hours."
Information about managing silo fires also is available at the following website: http://www.farmemergencies.psu.edu.