Preventing and Extinguishing Silo Fires

Key Points:

  • Most silo fires occur because the silage is too dry
  • Silo fires frequently smoulder undetected for a while; the forage bursts into flames when it comes in contact with the air
  • Call the fire department immediately if you suspect a silo fire

What Causes Fires?

  • Silage and haylage that is too dry (below 40% moisture content) will enter the spontaneous combustion cycle
  • A large mass of forage can allow heat to build up (fuel + heat)
  • A slow trickle of air moving into the material adds oxygen (fuel + oxygen)
  • Old silage (2+ years) in the silo is at risk of drying down to the critical level

The process of forage heating up and then burning is typically called spontaneous combustion. In silos, forage can dry down if air enters the silo through leaks in walls or doors; this results in the potential for fires throughout the whole year.

Spontaneous heating and combustion occur when sufficient moisture (above 25% and below 45% moisture content for forages), oxygen (air) and organic matter are present together to support the growth of bacteria and moulds. This growth results in an initial temperature peak of 54°C–65°C. When the forage reaches this temperature range, a chemical process called the Maillard Reaction may occur, causing additional heat generation. This reaction can be self-sustaining and does not require oxygen (air) to continue. The gases produced will ignite if they have reached a high enough temperature and are exposed to oxygen (air).

Figure 1. The Fire Triangle. Stored forage is fuel. If it heats and is exposed to oxygen, the result is a fire.

Fire Prevention

The moisture of the silage or haylage is the main factor in forage fires. To reduce this burning issue, know the crop moisture content. Buy a moisture tester or use your microwave oven to determine the moisture content.

Silo TypeRecommended Storage Moisture (%)Recommended Dry Matter Content (%)
Horizontal (Bunker) Silo60-7030-40
Tower Silo60-6535-40
Oxygen-limiting Tower Silo45-5545-55
Bagged (Chopped)60-7030-40
Table 1. Recommended Storage Moisture Contents for Silage. Source: H. Bellman, S. Clarke, B.Stone

The key to fireproofing silos lies in eliminating the combination of dry silage and fresh air. The following components of a silage storage are necessary to make silage and to prevent fires:

  • Check doors on tower silos regularly and if loose-fitting or damaged, repair so air does not enter the silage.
  • Air can also enter through damaged silo walls. Empty silo completely every 2 years. Check walls for damage and repair if necessary.

Most silo fires occur because the silage is too dry. Silage should be between 45% and 65% moisture content. Silage below 40% moisture content, coupled with extra air from poor packing or leakage of air into the silo creates a risk of heat damage or fire.

The Fire Danger Zone

  • If the silo warms up.
  • If you see wisps of water vapour (steam).
  • If you smell a slight caramel odour.
  • If you smell a pungent, scorched stench like burning baler twine call the fire department.
  • If you see smoke or flames call the fire department.
65 CCheck temperature daily. Below this point, temperatures are not considered abnormal, although anything above 52 C is unusual.
70 CDanger! Check the temperature every 4 hours.
80 CCall the fire department! Keep all doors and other openings closed to prevent drafts which add oxygen to the fire. Forage above 82 C may burst into flame when exposed to air.
Table 2. Critical temperatures for spontaneous combustion of stored forage.

For silage, temperature readings above 82°C indicate that the material will eventually char, smoulder or burn. Probing silage is mainly used to find the location of the fire, since in most cases a fire is burning before anyone notices the signs of heating. Typically, the fire will be in the top 3 m (10 ft) or around poorly sealed silo doors. Infrared scanning can also be used to indicate the hot spot location. Access to this equipment may be obtained through your farm safety officer, the Ontario Fire Marshal’s Office in Toronto or local insulation companies.

Extinguishment Procedures

Call the fire department and let them put the fire out. Firefighters will require full turn-out gear including self-contained breathing apparatus due to dangerous silo gases.

Conventional open-top towers and horizontal (bunker) silos

Do not attempt to extinguish a silo fire by pumping large quantities of water onto the surface of the silage, hoping it will soak in and cool the fire. This will not work. Water will not penetrate the silage well enough to control the fire. In addition, upright silos cannot withstand the higher lateral pressure created by the water and structural damage may result.

In conventional open-top silos and bunker silos, firefighters will locate the hot spots with a probe and put it out by injecting small streams of water through the probe right to the seat of the fire. Gases may be produced when injecting water into a hot silage fire. However, there is no containment of gases since the silo is open and not sealed, which practically precludes the occurrence of an explosion.

Oxygen-limiting tower silos

In oxygen-limiting silos, a fire is potentially very dangerous, since there is containment of explosive gases. The method of control will be to inject liquid nitrogen or carbon dioxide into the silo to cool the fire. See Table 2 for the amounts needed. Every oxygen-limiting silo should have valves specifically designed to inject gases for fire control. Check the owners’ manual for specific information about extinguishing a silo fire.

DO NOT USE WATER OR FOAM to fight a fire in an oxygen-limiting silo. Opening the top hatch to apply water or foam will allow oxygen to enter. The stream of water or foam will carry large amounts of air and oxygen into the silo. This can produce an explosive atmosphere which can be ignited by the burning silage. In addition, the steam formed when the water reaches the fire may also contribute to an explosion.

Silo Size (diameter x height)Carbon Dioxide (CO2)Liquid Nitrogen (N2)
6 m x 18 m (20 ft x 60 ft)20 cylinders40 cylinders
6 m x 21 m (20 ft x 70 ft)22 cylinders44 cylinders
6 m x 24 m (20 ft x 80 ft)30 cylinders60 cylinders
7 m x 18 m (24 ft x 60 ft)30 cylinders60 cylinders
7 m x 21 m (24 ft x 70 ft)35 cylinders70 cylinders
7 m x 24 m (24 ft x 80 ft)40 cylinders80 cylinders
9 m x 18 m (30 ft x 60 ft)45 cylinders90 cylinders
9 m x 21 m (30 ft x 70 ft)50 cylinders100 cylinders
9 m x 24 m (30 ft x 80 ft)60 cylinders120 cylinders
Source: Murphy, Dennis J. and William C. Arble. Extinguishing Silo Fires and (NRAES-18). Pennsylvania State University.Note: gas cylinders are size large
Table 3. Estimate amounts of carbon dioxide or liquid nitrogen for control of silo fires

Unloading fire-damaged forage

If safe to do so, unloading of fire-damaged forage is recommended:

  • Since there is very little nutritional value left
  • Water has leached the acids out of the forage, allowing moulds to grow and spoil the silage
  • Re-ignition of silage material may occur

Call the fire department before unloading any forage. Only attempt to remove forage when the fire department is standing by with a water truck, since the material may flame when exposed to air.

Removal of forage from a horizontal silo may be accomplished by using a high-hoe, front-end loader, or a logging clam. Upright silos can be emptied using the silo unloader.  Run the unloader intermittently, to minimize risk of motor overheating. In conventional open-top silos, thoroughly douse the top layers of forage before unloading to reduce re-ignition risk.

Remove forage and place a good distance away from the barn in case it re-ignites. Dispose of damaged forage in an appropriate manner. Spreading in a safe area, such as a plowed field, is one possibility.

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