Risk Assessment

From time to time we are asked to discuss the kinds of fires that show up with high frequency on the Industrial Fire World Incident Logs. In this article we will address silo fires. The timing of this article is  good because it coincides with a major English language publication (122 pages) on silo fires published by the Swedish Civil Contingencies Agency. It is based on research conducted by the SP Technical Research Institute of Sweden and titled Silo Fires; Fire Extinguishing and Preventive and Preparatory Measures (referred to as “the publication” for the remainder of this article).  The publication is available at no cost at https://www.msb.se/sv/Produkter--tjanster/Publikationer/Publikationer-fran-MSB/Silo-fires-fire-extinguishing-and-preventive-and-preparatory-measures/ The link may be easier to cut and paste from the on-line version of this article. Please see www.fireworld.com or else search for the name of the publication in a search engine.   

The publication addresses both industrial silos and agribusiness silos. This article does not attempt to replicate the information in the publication and we recommend that you use the publication as the basis of your preparation. Instead, we seek to raise awareness and point out a small amount of the information in the publication.

Industrial silo fires can involve coal, grain, plastic resins, all kinds of powder, wood chips, wood pellets, scrap rubber, just to name some potential fuels. Silo fires can start by spontaneous combustion, bringing in smoldering material by conveyor or duct, or by hot work. There are well established fire protection systems that can prevent embers or sparks in ducts from propagating. Detectors activate water spray nozzles to quench the spark or ember before it can enter downstream equipment.   

Industrial silo fires are every bit as technical as flammable liquid storage tank fires. This requires specialized preparation and tactics and perhaps the response of specialized teams. Contact with specialized teams should be made as part of the pre-planning process. We encourage the use of “blanket order contracts” before the incident occurs so that there is no disagreement about terms and conditions in the middle of an incident. Although storage tank firefighting knowledge is widespread among the industrial community, this knowledge is not as well developed for silo fires.

Silo fires differ from tank fires in that they often start out as smoldering events. They can last days, weeks, or even months. The initial responders should ask themselves, “What happens if we do nothing?” These smoldering fires typically do not pose an immediate risk and an aggressive attack by untrained responders can turn them into a major event by initiating a dust, pyrolysis gas, or decomposition gas explosion.  On the other hand, they will not simply burn themselves out in a day or two. Left alone they could burn for months. The nature of this slow combustion allows time for a methodical approach as well as to summon commercial response teams who specialize in these kinds of fires.

A sampling of the information contained in the publication is:

• “Do not open the silo! Entrained air will oxygenate the fire, which leads to an increased smoldering intensity, which in turn may contribute to rapid fire escalation in conveyor systems before and after the silo etc., as well as serious gas and dust explosions”. This was copied verbatim from the publication for completeness of this critical safety rule.
• Set up logistics for a long-duration event (feeding responders, prolonged media coverage, etc.) Anticipate the incident lasting 2-4 times as long as you think it will.
• Whatever method of extinguishment is chosen (such as nitrogen injection), set up agent application ports and connections. This is analogous to the semi-fixed foam systems for the jet fuel storage tanks discussed in the last article. The publication has good details on how to do this, especially for nitrogen injection. Make sure that the equipment needed to carry out these actions can get to where they need to be and have a place to park.
• If nitrogen is to be used, you need a vaporizer unit, not just a truckload of liquid nitrogen. The middle of an incident is not the time to figure all of this out and try to assemble the components.
• Nitrogen is preferred over carbon dioxide which could break down into carbon monoxide, which is an explosive gas. The publication also describes possible electrostatic ignition induced by carbon dioxide discharge.  Carbon dioxide also has additional physiological effects beyond the asphyxiation properties of nitrogen.  
• Besides nitrogen, the publication discusses proper and improper use of water, compressed air foam, and medium expansion foam. Water is specifically advised against when pellets are present because of swelling and other complications such as potential silo collapse. 

Silos are a common industrial hazard that pose a fire and explosion threat, as evidenced by the Industrial Fire World Incident Logs. Although the tactics are not the same as for storage tank fires, the industrial fire community has the knowledge to prepare for and manage storage tank fires. Similar planning and thought processes can be used to prepare for silo fires, especially using the guidance established in Silo Fires; Fire Extinguishing and Preventive and Preparatory Measures. Next steps for industry might be establishing silo firefighting curricula at industrial firefighting schools and a more robust network of expert silo fire response teams.               

Feel free to contact the author at [email protected]

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