There are basically two types of upright silos: conventional silos and oxygen-limiting or “sealed” silos. Conventional silos (Figure 1) are typically used for the preservation and storage of feedstock, such as corn, hay or other livestock feed. Conventional silos are typically loaded from the top. Note in Figure 1, there are a round-domed top and an enclosed unloading chute on the right side of the silo, next to the barn.
Oxygen-limiting silos (Figure 2) are constructed of steel, concrete or fiberglass and have tightly sealed openings and hatches through the use of rubber gaskets. This prevents — or limits — oxygen from entering. Note in Figure 2 — the oxygen-limiting silo — the roof is less rounded than the conventional silo; there is also no chute or unloading doors along the length of the silo. When an oxygen-limiting silo has a gasket failure or a hatch is left open, oxygen can enter the storage area. Spontaneous heating is a major cause of ignition in these types of silos. As a result of the tightly packed materials and limited oxygen introduction, smoldering fires can occur which produce large amounts of combustible gases as a result of incomplete combustion.
When the fire department arrives, typically more hatches are opened and often fire streams are directed into the oxygen deficient atmosphere within the silo. If we think back to our basic firefighter training, we have heat, fuel, and all we are missing is the oxygen. By opening additional hatches on the silo or injecting air into the silo through the use of water streams, we complete the fire tetrahedron. This is akin to kicking in the front door of a structure fire that is presenting potential backdraft conditions. The result at a silo fire is similar. Often a violent explosion occurs. In the 1985 case, the concrete roof of the silo was lifted approximately four feet in the air.
Where prevention is an important consideration in much of what we do, strangely, it is largely ineffective in the case of silo fires. Occupational Safety and Health Administration (OSHA) estimates that over 90 percent of farms in the United States are not subject to OSHA regulations. Code enforcement efforts are typically not pointed at farms and other privately owned occupancies that utilize vertical silos. However, identification and educational measures can be helpful in prevention efforts. For example, the silo operator should be educated in the fact that when not being filled or emptied, oxygen-limiting silos should be kept closed. This will reduce the chances of ignition by insuring that insufficient oxygen is present for even limited combustion.
Proper maintenance of the silo according to manufacturer’s recommendations is also essential. The moisture content of the stored materials should be monitored to limit the possibility of spontaneous heating. Above all, silos, especially oxygen-limiting silos, must be identified and pre-incident surveys completed on the occupancy. The pre-incident survey should include mitigation recommendations. Placards can also be placed on oxygen-limiting silos which will warn firefighters of the dangers.
As in all things, despite our best prevention efforts, silo fires are going to occur. There are a few fire control recommendations to remember. It helps to think of a burning oxygen-limiting silo as a ticking time bomb, with a very long fuse. First, no water or foam — which is mostly water — should be introduced into the oxygen-limiting silo. This will increase the amount of oxygen present and could result in an explosion. If roof hatches are open, DO NOT close them if there is smoke and/or steam coming from them. The silo may also be vibrating or banging noises may be heard through the walls. This indicates product movement or pressure differentials as a result of combustion. Closing the roof hatches will only allow the silo to build more pressure. If the silo is quiet and there has been no smoke or steam coming from the hatches for several days, the roof hatches may be closed. However, they should not be latched. They should be left unlocked so that subsequent pressure can escape should combustion be causing an increased pressure deep within the silo.
That was not a mistyped statement above, the hatches should not be closed until smoke and/or steam is not visible for SEVERAL DAYS. If you are able to close the hatches (without latching or locking them), you should leave them closed for up to three weeks, or until the fire consumes all the oxygen in the silo and self extinguishes. It is important to establish a collapse zone around the silo that is at least one and one-half times the height of the silo. Some silos are equipped with carbon dioxide or liquid nitrogen injection systems that are activated by external valves. Information on the operation of these systems and the location of the activation valves should be collected in pre-incident surveys. You can also talk with the silo manufacturer. They can provide assistance with the proper procedures for extinguishment. As a general rule, if an oxygen-limiting silo is burning, no water should be directed inside and attempts at entry should never be made.
Conventional silos do not represent an explosion hazard since they are not sealed. However, there are a number of other dangers. It is possible to enter a conventional silo. This requires a firefighter in full personal protective equipment and self-contained breathing apparatus, wearing a full body harness with a lifeline attached, to enter the confined space (silo) usually from the top. These are highly technical operations.
Figure 2 — Oxygen-Limiting Silo
The firefighter entrant must be trained in confined space entry, as a hazardous materials technician, high angle rescue and its associated ropes and knots, as well as a little bit of trench rescue. The silo is a larger than average permit-required confined space which requires air monitoring during entry. Aerial apparatus may be needed to access the roof of the silo and to serve as an anchor point.
Once inside, hidden fires may create void spaces within the contents. If a firefighter attempts to stand on the contents, they could fall into one of these hidden void spaces. Therefore, ground ladders or sheets of plywood — similar to that used on trench rescues — should be used to prevent the firefighter from falling under the surface of the contents. This entry can aid in sizing up the fire within the conventional silo and a thermal imaging camera (TIC) can greatly help with this size up.
It may be difficult to fully extinguish a fire in a conventional silo. Even carbon dioxide and nitrogen may be ineffective as the silo is open to the atmosphere and is not sealed. Oxygen may enter the space and displace the extinguishing agent before it can smother the fire. Elevated large caliber water streams may also be ineffective as they may not reach deeply into the silo contents. It may be necessary to partially or completely unload the silo to extinguish the smoldering contents. In any case, you should anticipate committing significant time and resources to this incident.
In both conventional and oxygen-limiting silos, use time to your advantage. Treat these like hazmat calls and don’t get in a big hurry. You can’t do much at the oxygen-limiting silos and the conventional silos require so many technical things to come together at once, there is no reason to rush. Consider your risk versus reward when making an entry into a conventional silo.
We all want to avoid expending countless time and resources when we will end up unloading the entire silo anyway. But take time to identify the oxygen-limiting silos in your jurisdiction — and your neighbor’s. Keep in mind that your neighbor may be tied up with something else one day and you may be first in to one of these incidents in their jurisdiction. Use identification and education to your advantage in prevention efforts. Where those fail and a silo fire occurs, avoid changing the configuration of oxygen-limiting silos and use extreme caution when operating on or in conventional silos.
Be safe and do good.
NIOSH Fire Fighter Fatality Investigation 2010-14
NIOSH Publication Number 86-118
USFA Technical Report Series 096