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Keywords:

  • smoldering fire;
  • agricultural silo;
  • carbon monoxide;
  • self-heating;
  • explosion

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED

Many organic granular materials are susceptible to self-heating when stored in silos. Under the right environmental conditions self-heating can lead to spontaneous ignition and smoldering combustion. This is a case study of an explosion that occurred in a grain storage silo with a smoldering fire. Although the hazards of combustible dust are relatively well recognized, the explosion hazard presented by a smoldering fire is less well known. There were two explosions in this incident: a primary explosion involving carbon monoxide and smoke generated by the smoldering fire, and a secondary explosion fueled by combustible dust. The explosion caused both injuries to personnel and significant property damage. This article discusses the causal factors for the explosion and the lessons learned for effectively monitoring and responding to a smoldering fire in a silo. The potential explosion hazard of the carbon monoxide and smoke generated by a smoldering fire is emphasized. © 2013 American Institute of Chemical Engineers Process Saf Prog 33: 94–103, 2014


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED

Many organic granular materials are susceptible to self-heating when stored in silos. Under the right environmental conditions self-heating can lead to spontaneous ignition and smoldering combustion. This is a case study of an explosion that occurred in a grain storage silo with a smoldering fire. Although the hazards of combustible dust are relatively well recognized, the explosion hazard presented by a smoldering fire is less well known. There were two explosions in this incident: a primary explosion involving carbon monoxide (CO) and smoke generated by the smoldering fire, and a secondary explosion fueled by combustible dust. The explosion caused several injuries to personnel and significant property damage.

The subject facility stored agricultural grain products in a concrete grain elevator comprised of several individual conventional silos. Material was moved into the grain elevator silos from railcars, trucks, or barges using unloading stations, conveyors, and a bucket elevator. Below the grain elevator was an access tunnel with discharge chutes to a conveyor. The silos were provided with aeration fans that could be used to provide a flow of air through the material to cool and dry hot, moist processed material.

One of the operators discovered the fire by detecting a distinctive burnt odor in the access tunnel. Several silos were examined before it was determined which silo was the source of the odor. Subsequent investigation the next day revealed several unmistakable signs of combustion: burnt odor, light smoke, elevated temperatures measured by an optical pyrometer, some charred and discolored grain product material sampled from the silo, decreased oxygen concentration, and elevated levels of CO inside the silo. That evening one of the operators opened the roof hatch to measure the temperature of the silo contents and noted glowing coals on the surface of the grain product. Unsure of how to proceed, the facility owner attempted to suffocate the fire by preventing the intrusion of air into the silo. No attempt was made to consistently and effectively monitor the progress of this action.

Eventually, after several weeks, a grain salvage company was hired to remove the grain product from the silo. This was a process that was to take several days. During this time the rooftop hatch and the tunnel chute were often both open creating a chimney effect which accelerated product combustion. Water was frequently sprayed on smoking material from both the top of the silo and from the tunnel. On the day of the explosion, small flames were observed at an aeration vent. The tunnel hatch was closed but the rooftop hatch remained open. The explosion occurred half an hour later. Almost two months had elapsed after the discovery of the smoldering fire.

This article discusses the causal factors for the explosion and the lessons learned for effectively monitoring and responding to a smoldering fire in a silo. The potential explosion hazard of the CO and smoke generated by a smoldering fire is emphasized in the context of the special hazards of silo fires. Finally, strategies for prevention and suppression of silo fires are presented.

ACCIDENT SUMMARY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED

The subject grain facility is located on the bank of a river and is adjacent to a railroad line. The facility stores various agricultural grain products. The facility consists of an office and a grain elevator. The exact age of the facility is unknown, but some construction drawings date back to the early 1960s. The elevator is a concrete structure with several conventional silos (not oxygen-limiting). The facility can receive or ship grain via truck, rail, or barge. The grains are conveyed to the rooftop of the elevator where they are distributed to the various silos via a drag conveyor. The silos have a nominal diameter of 25 feet and are approximately 80 feet in height. Two separate dust collection systems are used throughout the facility. The first system handles the barge conveyor and truck pit conveyor. The second system handles the drag conveyors on the rooftop, the tunnel and the elevator as well as the silos. Schematics of the silos can be seen in Figure 1 and 2.

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Figure 1. Top view of grain elevator and silos.

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Figure 2. Side view of grain elevator and silos.

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Facility personnel first documented evidence of a fire in Silo 1 early in the year. This evidence consisted of smoke, an unusual odor, an elevated temperature measurement, discolored grain, and glowing embers. Low oxygen levels and high CO levels were also measured in the headspace of the Silo 1. After taking these measurements, facility personnel intermittently monitored the temperature of the silo for 3 weeks. Several facility employees observed sporadic smoke and odor. Within a week or two of taking the initial temperature measurements, the smoke and temperatures declined. Facility employees then noticed an increase in both temperature and smoke emission several weeks after the initial observations.

The facility then contracted a salvage company to remove the material from the silo as well as agglomerated grain from the walls of the silo. Material was being removed from the bottom of the silo through a hatch in the access tunnel. As the crew for the salvage company began removing grain from the silo, they noticed both an unpleasant odor as the grain was being removed in large chunks. During the grain removal process, water was frequently sprayed on smoking material from both the top of the silo and from the tunnel. Within several days of the contractor beginning to remove the material, the crew noticed an increase in temperature and odor. The crew was equipped with a 4-gas monitor that did alarm on CO when the monitor was placed inside the hatch in the access tunnel from which they were removing the grain. This hatch was internal to the concrete elevator structure.

Within a week after the salvage company started removing grain from the silo, the removal process slowed due to the presence of hardened material. After returning from a break one day, the salvage contractor employees noticed that the smoke emission from Silo 1 significantly increased. The contractor employees then opened a manway on the side of the silo, near the aeration fan, and attempted to dislodge the hardened material inside the silo while smoke continued to pour out of the top of the silo. The salvage company then applied water to the grain inside the silo from a hatch on the roof using a garden hose. A timeline of the key events is seen below (Table 1).

Table 1. Timeline of Key Events
DateEvent
Early MayFacility personnel noticed grain leaving Silo 1 was coming out discolored
Middle of MaySmoke, an unusual odor and elevated temperature measurement were coming from Silo 1
Middle of MayLow oxygen levels and high CO levels were measured in the headspace of Silo 1
Middle of May– Early JuneFacility personnel intermittently monitored the temperature of the silo. Water was frequently sprayed on smoking material from both the top of the silo and from the tunnel. Sporadic smoke and declining temperatures were noted throughout this period
Early JuneFacility personnel noticed an increase in both Silo 1 temperature and smoke emissions for several weeks
Middle of JuneFacility contracted a salvage company to remove material from the Silo
End of JuneSalvage company personnel 4-gas meter alarmed on CO when placed inside the silo. Large chucks of grain were being removed.
Early JulySmoke emissions significantly increase. Salvage company opened manway near aeration fan trying to dislodge material while smoke poured out of the top of the silo. A garden hose was used to apply water to the silo from a hatch on the roof.

Just prior to the explosion, the salvage company was instructed to remove all the tools from the interior of the elevator access tunnel. To accomplish this, workers entered the access tunnel to remove the tools. During this process, two explosions occurred inside the grain elevator injuring several people, including the workers in the access tunnel. The first explosion occurred in Silo 1 and a secondary explosion propagated through one of the two bucket elevator legs and was released through the elevator boot into the access tunnel where the workers were located. Upon arrival to the scene by the Fire Department, they noted smoke venting from the northwest silo and roof structure.

FIRE AND EXPLOSION DYNAMICS ANALYSIS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED

Combustible Dust Fires and Explosions versus Smoldering Fires

Combustible grain dust fires and explosions in silos have been a significant concern for over a hundred years. NFPA 61 is the standard for the prevention of fires and dust explosions in agricultural and food processing facilities and was developed in 1923 to prevent dust explosions in grain terminals and flour mills [1]. NFPA 61 provides numerous requirements for preventing combustible dust explosions. However, NFPA 61 does not provide specific guidance on the control and extinguishment of smoldering fires.

Occupational Safety and Health Administration regulation 29 CFR 1910.272 covers grain handling facilities and provides requirements for the control of grain dust fires and explosions. While the CFR regulation provides requirements for issues such as providing training to employees regarding grain silo fire hazards, hot work requirements, and housekeeping, the CFR does not provide guidance on fire-fighting or how to suppress or control a fire in a grain silo, including a smoldering fire.

Furthermore, most of the standards and literature related to silo fires and explosions focus on the combustible dust hazards and provide limited guidance or requirements on dealing with the explosion hazards that are generated by a smoldering fire. Extensive guidance is available on limiting combustible dust emissions and accumulations, limiting ignition sources, and protecting against the results of a deflagration by isolation, suppression, and/or venting. However, the unique hazards that are present as a result of the gases generated during a smoldering fire receive far less attention in the standards and literature. Some case studies of smoldering gas explosions are reported by Eckhoff [2].

The Smoldering Fire

The two most common causes of smoldering fires in silos are (1) self-heating that results in spontaneous combustion and (2) the transport of a burning ember generated during material processing. For a material to undergo self-heating it must be capable of undergoing an exothermic reaction and for solid materials it usually needs to be in a porous or granular form. Most agricultural products and organic materials can typically undergo some form of self-heating when they are in a granular form. Material self-heating resulting in a smoldering silo fire requires the following [3]:

  • The material must exhibit self heating,
  • Self-heating must reach a runaway, or critical condition, that is, the reaction must rapidly accelerate so that a high temperature is reached,
  • The runaway self-heating must start sustained smoldering,
  • The sustained smoldering must reach the outside surface of the material and possibly erupt into flaming.

The development of a self-heating reaction that transitions into a fire is presented in Figure 3. . Whether or not a self-heating material can reach a thermal runaway condition is dependent on several factors including the overall size of the material mass, the material and ambient temperatures, the boundary conditions (the walls), ventilation and availability of oxygen, size of the particles, moisture content of the material, as well as other factors and is beyond the scope of this discussion. According to Krause et al., “The characteristic range of temperature for a self-ignition process turning into a smoldering fire is from 150 to 400°C. A range of temperature from 250 to 500°C indicates a propagating smoldering fire with the temperature being the higher the more oxygen is available for combustion” [3]. Once initiated, a smoldering fire within a silo of material can go unnoticed for weeks or even months.

image

Figure 3. Stages of a fire in bulk material triggered by self-ignition (adapted from [3]).

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Burning embers entering the silo can typically be prevented through the use of fast-acting spark/ember detection and extinguishing systems that extinguish the hot ember before they can be introduced into the stored material.

Self-heating must be prevented or controlled through appropriate material processing, transport and storage to control the material temperature and moisture content. Detection of smoldering fires can be achieved through a combination of CO or hydrocarbon vapor detection within the silo, interior material temperature measurements, and infrared (IR) monitoring of the silo/collector wall temperature.

In the case study presented, there was a fire in the silo for several weeks leading up to the explosion. The evidence, observed by the facility and contract personnel, consisted of the following: smoke, elevated temperature measurements of the material bed, discolored material (brown and black as shown in Figure 4.) accompanied by a burnt odor, a reduced oxygen concentration in the silo, and an elevated CO concentration in the silo. In general, these observations are indicators of fire as defined by both the National Fire Protection Association (NFPA) and American Society of Testing and Materials [4, 5]. More specifically, these observations (plus the detection of an unusual or burnt odor) are specific indicators of smoldering combustion in a bed of agricultural grain product [3, 6].

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Figure 4. Smoldered material in the silo. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Smoldering will typically produce glowing embers. Glowing will occur when the smoldering reaction reaches the outer surface of the material or when the material is penetrated to the location where the reaction front is located [3]. Therefore, glowing embers may not be visible if the smoldering wave is below the surface of the fuel and the material has voids or channels present due to material agglomeration (“rat holes”). However, glowing embers, which are indisputable evidence of smoldering combustion, were observed by facility personnel on at least one occasion prior to the explosion. An example of smoldering material and glowing embers in a silo after a silo explosion are shown in Figure 5. In the weeks before the explosion, communications between facility personnel acknowledged that a smoldering fire was present in the silo. These conditions were observed for several weeks prior to the explosion.

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Figure 5. Smoldering material and glowing embers after a silo explosion. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Conditions in the silo described in this case study were conducive for the development of self-heating and a smoldering fire. One important factor was the caking of agglomerated material to the walls of the silos. The material was sticking to the walls of the silos and “clumping” (agglomerating). This was a common occurrence in the silos and was attributed to the elevated moisture content and temperature of the material during production and transfer of the material to the silos.

Clumping is caused by loading the material into storage at too high of a temperature and with too much moisture. The high temperature and moisture conditions of the material not only increase their propensity for agglomeration, they also promote the growth of mold. For many organic materials such as agricultural grains, the initial self-heating step is microbiological growth. In one investigation it was found that pelletized wheat caked on the side of a silo contained on the order of at least 10 to 1,000 times the amount of mold species as the uncaked pellets [7]. Mold growth leads to biological heating which is the first step toward a silo fire [8]. Controlling temperature and moisture of materials during production, transfer, and storage can be essential to reducing the risk of smoldering fires in silos. The process of self-heating can only occur if the material were allowed to remain idle in the silo for an extended period of time, on the order of several weeks, which was what occurred in this case study.

Conditions for an Explosion

There were essentially two explosions in this accident: the primary explosion in the silo and a secondary explosion that propagated through one of the two elevator legs and was released through the elevator boot into the access tunnel. In evaluating the source of the primary explosion, three factors had to be considered: the fuel, the ignition source, and ventilation (the source of air/oxygen). Together these three factors comprise the fire triangle.

For the secondary explosion, two additional factors had to be considered: dispersion of the fuel and confinement. These additional factors combined with the factors comprising the fire triangle make up the combustible dust pentagon.

Fuel

There were four potential fuels involved in the primary explosion: CO, smoke, “water gas,” and combustible dust from the stored material. Efficient burning that has the proper ratio of fuel to oxygen (air) to allow a stoichiometric reaction typically produces carbon dioxide (CO2) and water (H2O) as the primary products of combustion. On the other hand, smoldering combustion tends to manifest itself as an under-ventilated fire that does not have sufficient oxygen available for efficient combustion. Under-ventilated fires generate large quantities of CO. CO can form a flammable atmosphere over a wide concentration range: between 12.5% and 74% by volume. In addition, CO is a toxic gas and even at relatively low concentrations can create an asphyxiation hazard within the silo and adjacent confined spaces. In the incident presented above, the presence of CO was demonstrated by measurements of the atmosphere within the silo. The measurement indicated that the CO concentration was significantly above the normal atmospheric levels and was increasing. This smoldering combustion process was also evident as an oxygen-deficient atmosphere–the oxygen concentration in the silo was measured to be approximately 30–40% below normal atmospheric concentrations.

The generation of smoke, the second potential fuel, was evident on several occasions. Smoke is a product of combustion, and smoke from incomplete combustion can contain carbon particles and unburned fuel gases that can serve as fuel in a deflagration. The existence of smoke within the silo was detected by plant personnel, both visually and by smell, for several weeks leading up to the explosion.

An additional potential fuel was “water gas.” The term “water gas” refers to the generation of CO and hydrogen (H2) due to the application of water to a smoldering fire in a silo. According to the National Grain and Feed Association (NGFA) Emergency Preplanning and Firefighting Manual for grain elevators [9]:

Although the most common approach to a fire is to apply water, there is some hazard in using water, particularly in bin fires. A danger of using water on a hot grain fire in a confined space is the possibility of the water reacting with carbon, a product of combustion, to form a potentially explosive concentration of carbon monoxide and hydrogen known as “water gas.”

NFPA 61 provides the following explanation of the water gas reaction:

Application of small amounts of water on glowing grain in a partially confined space, such as a grain silo, and in the presence of air can generate a water gas reaction. The glowing grain must be at temperatures of at least 700°C to 800°C (1290°F to 1470°F), and initial water contact may not cool the mass of glowing grain below 600°C (1110°F).

The partial oxidation reduction between carbon and water forms carbon monoxide and hydrogen as follows:

  • display math

In the presence of oxygen (air), the carbon monoxide and hydrogen burn, immediately releasing heat as follows:

  • display math

In a partially confined space, the combustion energy will rapidly pressurize the space beyond what the silo walls or tops can withstand, causing destruction of the silo.

The fourth potential fuel present in the silo was combustible grain dust within the silo. Agricultural products such as that in the incident described above have the potential to generate small particles as they are processed, handled, and transferred. An “agricultural dust” is defined by NFPA 61 as, “Any finely divided solid agricultural material 420 microns [0.42 µm] or smaller in diameter (material passing a U.S. No. 40 Standard Sieve) that presents a fire or explosion hazard when dispersed and ignited in air” [1]. A more general definition for a combustible dust, as presented in NFPA 654, is “A finely divided combustible particulate solid that presents a flash fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations” [10]. To be an explosion hazard, combustible dusts need to be in a dispersed form, which can occur during the pneumatic conveying and discharging of material into a silo. However, any dust generated would tend to settle to the lower depths of the product mass. To participate as a fuel in an explosion, the dust must first be suspended before it is ignited. Just prior to the explosion described in this case study, there were no reports of actions or plant activities that could lead to dust suspension within the silo. Thus, although dust may have played a role in the secondary explosion within the bucket elevator, it is unlikely that it played a role in the primary explosion.

It is concluded that the fuel for the primary explosion was some combination of CO, smoke, and hydrogen (in the form of water gas) forming a flammable atmosphere due to the smoldering material within the silo. It is typical for accidents of this nature to be attributed to combustible dust, and therefore, the hazards of a smoldering fire, especially one that persists for some time, are often overlooked. For example, a method for minimizing dust cloud formation is to wet the surface of the material, as was done in this case study. However, the application of a relatively small amount of water to a smoldering organic material will often not cool the heated mass and can cause the material to undergo a biological degradation that can generate flammable gasses.

Ignition Source

As a result of the incident investigation, it was determined that there were few competent ignition sources inside the silo. There were no moving parts that could produce mechanical friction, no electrical components that could malfunction, no hot work activities, and no signs of anyone using smoking materials in the vicinity. The only plausible ignition source was the self-heating and spontaneous ignition of the stored material. The self-heating process begins with biological heating caused by respiration of molds in the material. As the temperature of the bulk mass increases, chemical oxidation begins and generates heat at a faster rate. Eventually, if the temperature continues to rise, a smoldering combustion wave is formed in the product mass. Smoldering produces glowing embers with a typical temperature between 600 to 750°C (roughly 1100 to 1400°F) [11]. Glowing embers can become exposed to the atmosphere within the silo if the smoldering wave approaches the surface of the pellet mass or due to the removal of material covering the glowing mass. The glowing embers may persist or they may transition to flaming combustion. Once the smoldering combustion wave reaches the fuel surface, transition to flaming combustion can occur.

Ventilation

The progress of the smoldering wave in the stored material was influenced by the ventilation conditions in the silo in the weeks, days, and hours leading up to the explosion. It is possible to quench a smoldering reaction by starving it of oxygen. In this incident, the facility took steps to try and seal the silo, but the construction of the silo was not gas-tight and there was interconnection between the silos–the silos were connected to one another by vents at the junction between the top of the silo and the concrete roof. Therefore, some infiltration of fresh air and exfiltration of combustion products (observed as smoke and odor) occurred. Furthermore, during the multi-week duration of the fire, the facility had run several pieces of process equipment and pneumatically conveyed product to the adjacent silos, thereby, introducing additional air into the silo structure. The process also involved the running of a dust collection system connected to the silos, including the subject silo that would have pulled fresh air into the silos. Although the dust collection pipe to the subject silo had been blocked off, the operation of the dust collector would have increased the ventilation through the adjacent silos due to the interconnection of the silos.

Furthermore, some of the silos, including the subject silo, were provided with an aeration system to reduce the moisture content of the material. The system consisted of a perforated pipe that extended onto the lower portion of the silos which was connected to an external fan and air inlet. The investigation revealed that the aeration system was rarely used to dry the materials. As part of the attempts to seal the silo, the air inlet was covered with a sheet of plastic, however, a nearby auxiliary air inlet had not been covered. During the weeks leading up to the explosion the plastic sheeting had been observed to melt and the seal had become compromised allowing fresh air into the silo and for the smoldering reaction in the material to continue.

Finally, just prior to the explosion, the lower manway on the bottom of the silo was opened in order to dislodge agglomerated material. The opening of the manway increased the natural draft (chimney effect) of air into the silo and accelerated the fire.

Although the facility had attempted to seal the silo, various circumstances allowed air to infiltrate the silo and provide oxygen to the smoldering material. These circumstances are attributed to continuing to operate the plant and convey material to nearby silos, limited knowledge of the silo construction, and an ineffective fire suppression strategy, which is addressed further in the Suppression Strategies section.

Explosion Dynamics

The primary explosion occurred when the flammable atmosphere inside the silo ignited. The resulting deflagration damaged the concrete roof of the grain elevator and the concrete structure of the bucket elevator and caused a secondary explosion in the elevator boot. The most probable cause of the primary explosion within the silo was the ignition of a flammable atmosphere of CO, hydrogen, and smoke ignited by either glowing embers or flames at the material bed surface. The concrete roof of the entire silo structure was damaged at the edges and concrete was cracked and spalled at locations of piping penetrations. This damage indicated that the roof was at least partially lifted by the impulse of the primary explosion and then dropped as the combustion product gases vented from the silo. The mass of the concrete roof was estimated to be between 80,000 and 120,000 pounds. The deflagration pressure within the silo could easily achieve a peak pressure between 80 and 150 pounds per square inch gauge (psig). Neglecting the effects of venting, this translates into a peak lifting force of between 1.5 and 2.8 million pounds force–more than sufficient potential to lift the entire roof of the silo.

The primary explosion propagated a pressure wave along two different paths through connected vent ducts. One path was the ducting for the silo dust collector. The pressure wave caused the explosion relief panel on the dust collector to function. The second path entered the vent ducts for the bucket elevator, fueled by combustible duct in the ducts and in the elevator itself dispersed by the primary explosion. The pressure wave triggered a secondary explosion in the boot of one of the bucket elevators resulting in a deflagration overpressure that ruptured the elevator boot allowing flames and hot gasses to enter the access tunnel, where the injured workers were standing. The overpressure was caused by the secondary explosion in the elevator leg.

Secondary explosions are a common feature of explosion events in a combustible dust environment. Grain elevator legs are especially susceptible to grain dust explosions due to the potential for combustible dust accumulation in the elevator boot. Despite the clear evidence of the overpressure in the elevator boot, no evidence of heat damage to the rubber belt or any of the plastic buckets was observed indicating an extremely rapid flame front velocity that did not have time to damage the plastic components. Similarly, no explosion damage was evident in the tunnel.

Following the explosion, the material in the silo continued to smolder and burn as is evidenced by the continued emission of smoke for several hours. Firefighters were finally able to suppress the fire by removing the material, spraying the exposed material with water, and spraying water through the open manway at the bottom of the silo.

PREVENTION AND MONITORING

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED

The first step to preventing a smoldering fire gas explosion is to prevent a smoldering fire from occurring in the silo. As mentioned above, the two most common causes of smoldering fires in silos are self-heating that results in spontaneous combustion and the transport of a burning ember generated during material processing. Burning embers entering the silo can typically be prevented through the use of fast-acting spark/ember detection and extinguishing systems that extinguish the hot ember before they can be introduced into the material.

Self-heating must be prevented or controlled through silo cleaning, material turnover (i.e., removal of old material), as well as appropriate material processing, transport and storage to control the material temperature and moisture content. Detection of smoldering fires can be achieved through a combination of CO or hydrocarbon vapor detection within the silo, interior material temperature measurements, and IR monitoring of the silo/collector wall temperature.

Early detection of a smoldering fire is desirable, but difficult to achieve using conventional methods. By the time that smoke or elevated concentrations of (for example) CO are detected at the top of the silo, the pyrolysis process has probably been going on for several days. Once a smoldering fire starts in a silo, special suppression strategies need to be implemented, as traditional fire-fighting strategies have been shown to be ineffective and can increase the hazard of an explosion.

SUPPRESSION STRATEGIES FOR SILO FIRES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED

While fires and explosions are somewhat common in agricultural silos, the bulk of the literature regarding the hazards tends to focus on the combustible dust hazard. The smoldering fire hazards are less commonly addressed. This section provides a summary of effective strategies and good practices for dealing with smoldering fires in silos as well as a discussion of ineffective practices. Some of these practices, while similar in basic concept, can produce significantly different results in the way in which they are implemented.

Due to the unique nature of a smoldering fire in a silo, special fire suppression strategies need to be utilized. In addition, some traditional fire suppression methods have been determined to have limited effectiveness or can exacerbate the hazardous condition within the silo, and should be avoided. Education, training and a robust emergency response plan with regard to smoldering silo fires is extremely important to the safe mitigation of the hazards.

Water

A traditional fire suppression strategy employed by fire departments and fire brigades is to flood a fire with large volumes of water. The application of water to a flaming fire involving normal combustible materials (wood, paper, grains, etc.) is extremely effective in extinguishing the fire as the heat from the fire is removed by converting the liquid water to huge volumes of steam in a ratio of approximately 1,700 gallons of steam for each gallon of water. The steam also acts to displace oxygen from the fire to assist in extinguishment. Because of its abundance, simple application and tremendous effectiveness, water has been the fire suppression media of choice for most fires, especially those involving ordinary (e.g. Class A) combustible materials.

However, the application of small amounts of water and water spray systems such as sprinklers are completely ineffective against a smoldering fire in a silo. Dousing a silo fire with water is only effective if the water reaches the fire, as with a fire near the surface of the material that can be extinguished with a simple hose stream. For a deep-seated smoldering fire in a silo the application of water, even huge volumes of water, can be ineffective or even cause a more hazardous situation. In addition, the deeper the fire is in the material, the more difficult it will be to suppress with water. Penetration of water into the material is weak. For water that does penetrate the material, the water may evaporate due to the heated material as well as the flow of hot gases, smoke, and combustion products without ever reaching the source of the fire. The application of water may wet the top surface of the material; however, the water can find routes and channels through the material to the bottom of the silo without ever reaching the burning material. Continued application of water will fill the silo with water and can lead to the structural integrity of the silo being compromised. The application of water to smoldering material or glowing embers inside a silo can also increase the fermentation process or generate flammable CO and hydrogen gas (water gas) as described above.

The use of water on silo fires can have additional negative effects:

  • The water spray can disturb and disperse combustible dust, leading to a potential combustible dust flash fire or explosion,
  • Swelling of the goods stored in the silo which in turn are pressing open the cell walls,
  • The fire extinguishing water does not reach the fire source because the goods stored in the silo are sticking together,
  • The injection of an inerting agent at a later date is hindered or even made impossible, and
  • The silo cells become statically overloaded due to hydrostatic pressure from the accumulated water.

Foam

There is no evidence to support using foam suppression agents to extinguish a fire in a silo, particularly a smoldering fire. While there is experimental data that indicates that foam can be effective, practical applications during actual fire scenarios have shown a variety of results. The purpose of foam is to provide a barrier that blocks the flow of air into the material from above. However, there may be other routes of air infiltration into the silo and/or there may be an extensive amount of air within the material itself. As with the application of water, foam will most likely not extinguish the fire and could generate an atmosphere within the silo that causes an explosion.

Sealing the Silo

Because of the dangers associated with the application of water on a hot grain fire in a confined space, some silo fires can be more effectively controlled and suppressed by the reduction of the oxygen supply. One method of eliminating the oxygen is to seal the silo. Sealing a silo containing a smoldering fire can limit the amount of air that can reach the smoldering reaction.

In the weeks leading up to the explosion, the facility attempted to seal the smoldering silo to extinguish the fire by consuming the oxygen in the silo while preventing the inflow of fresh air. They attempted to seal the aeration fan inlet, exhaust, and duct collector vents and sealed other places where smoke was exiting the silo or where air could enter. However, the design of the silos prevented the achievement of a completely air-tight seal because each silo was vented to its adjacent silos at the junction between the silo top and the roof. Thus, air could flow between adjacent silos. Spread of smoke and CO between the adjacent silos was evident in smoke staining observed in the upper portions of the silos as shown in Figure 6. . There is a soot line at a fairly uniform elevation within each of the silos. This is consistent with smoke flowing from the smoldering silo into the remaining silos. Smoke was transported from the silo and accumulated in the other silos via the hydraulic connection. Because the warm smoke was more buoyant (less dense) than the ambient air, the smoke filled each silo from the top down.

image

Figure 6. Smoke staining on the upper portions of the interconnected silos. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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To determine the efficacy of a silo sealing effort, the facility needs to monitor the silo atmosphere. Successful sealing of the silo would be demonstrated by the depletion of oxygen without an increase in CO or temperature. In this case study, the facility relied on temperature monitoring instead of atmospheric monitoring to check on the condition of the silo. However, the temperature monitoring strategy that was implemented was unable to determine that the silo sealing effort failed to extinguish the fire.

Oxygen Displacement/Inerting with Gaseous Agents

The principal of inerting a silo is similar to that of sealing the silo and is designed to dilute and displace the available oxygen within the silo. The smoldering reaction consumes the oxygen in the air filling the voids between the particles. The reaction progresses by fresh air being introduced to the smoldering reaction zone. Sources for the fresh air are from within the material (away from the reaction zone) or from outside the material (i.e., from the empty space above the material, through leaks in the silo wall, through unsealed vents, etc.). The flow of air through the material is typically by diffusion or convection.

The concept of inerting a smoldering silo fire is to replace and/or reduce the available oxygen with a gaseous agent that is incapable of undergoing an oxidation reaction and will not react with the stored material. The most common gaseous agents used for inerting a silo fire are carbon dioxide (CO2) and nitrogen (N2), mostly due their availability and cost. The primary difference between the two gases is the density. CO2 has a density greater than air while N2 has a density much closer to air, which is comprised of approximately 79% nitrogen. Because of the density differences, CO2 is best applied from the top of the silo where it can sink down through the material while N2 can be applied from the top or the bottom. The two gases can also be applied simultaneously. Based on the size of the silo and the quantity of organic, granular material, the inert gas may need to be applied at multiple locations. However, the application of the inert gas must be applied at a pressure and flow sufficient to penetrate the granular material but without the generation of potentially hazardous clouds of combustible dust.

Note that the mechanism by which inert gases suppress fires can also create an asphyxiation hazard to personnel in adjacent confined spaces. The use of inert gases should be performed using typical safe practices such as monitoring for hazardous and low oxygen atmospheres.

The most important factors for the application of a gaseous agent into the silo is sealing of openings, reduction of oxygen within the entire silo, and continuous application of the gas to control and/or suppress or the fire prior to removal of the material. Some references have indicated that it is safe to begin emptying the silo only when the oxygen concentration is reduced to below 8%; however, no scientific basis for this claim has been established [12]. Intermediate-scale experiments involving the extinguishment of smoldering fires in silos conducted by Tuomisaari et al. concluded the following [12]:

  • Extinguishing smoldering fires in silos is an extremely complicated issue primarily due to the inhomogeneity of the material,
  • The application of water should not be used to extinguish silo fires,
  • When suppressing smoldering silo fires with an inerting gas, monitoring the suppression progress inside real silos is extremely difficult because the oxygen concentration in the space above the material is not necessarily representative of the concentration within the material,
  • Emptying a silo is only safe after the fire has been extinguished, and failed attempts to extinguish silo fires by inerting may be due to the silo being opened too early,
  • Other inerting gases, or combinations of gases such as commercially available suppression agents, can be used to suppress silo fires; however, the use of CO2 and N2 are the most practical,
  • CO2 was found to be a more effective agent than N2 due to the higher gas density which causes CO2 to leak from the silo more slowly and remain in the silo longer than N2,
  • The most effective way of injecting a gaseous agent is from the bottom of a silo due to the upward buoyant flow in the silo,
  • Inerting with a gaseous agent is more effective during the early stages of the fire due to the increased buoyant flow that is created by a larger fire, and
  • No quantitative guidelines can be recommended to verify the extinguishment of a smoldering fire.

To enable effective inerting, silos should be provided with ports where an inerting gas can be introduced into the silo easily. In addition, means to completely seal the silo as tightly as possible need to be provided to prevent leakage of the inerting gas and to prevent the inflow of fresh air. Plates and/or doors can be used to cover any openings at the bottom of the silo. A source for large volumes of carbon dioxide and/or nitrogen should be identified before a fire occurs. Some facilities have contractors identified that can quickly respond with tankers of inert gas and can begin the introduction of gas into the silo quickly. Controlled emptying of the silo can be performed when temperature measurements and gas analyses indicate that the smoldering reaction has substantially reduced. As mentioned above, there is no quantitative guidance to determine exactly when a smoldering fire in a silo has been completely extinguished.

Examination of historical examples of smoldering fires in silos have demonstrated that inerting the material using a gaseous agent such as CO2 and/or N2 is the most effective means of controlling and suppressing a smoldering fire in a silo. The best method of extinguishment is to smother the fire with inert gas (in the gas phase) delivered as close to the bottom of the silo as possible [2, 3]. Unfortunately, the use of inerting gases is not accepted by all emergency responders. A 2010 article featured on the Fire Chief magazine website states the following [13]:

Using carbon dioxide or nitrogen gas to extinguish conventional silo fires is a questionable practice. This is because the structure is open to atmosphere and oxygen usually can still reach the fire. Some departments have had success with these gases, but more experimentation is needed before a recommendation can be made regarding their use.

Monitoring

Accurate measurements of the concentration of CO and oxygen within the storage silo can provide information regarding the smoldering fuel and the effectiveness of suppression activities. Smoldering generates CO and smoke, both of which can be fuels in gas explosions. In the case study presented, the facility attempted to institute a monitoring program for the silo temperature. Properly done, temperature measurements using a temperature probe could be an effective way to probe the location and extent of smoldering combustion [14]. By allowing the smoldering fire to continue to burn and by failing to control and monitor the ventilation into the silo, the facility allowed CO and smoke to accumulate and achieve a flammable concentration in the silo headspace. If the formation of a flammable atmosphere within the silo had been prevented, there would have been no explosion. Additionally, if the silo had been properly sealed, the fire would have consumed all of the oxygen in the silo and the fire would have self-extinguished. During the weeks leading up to the explosion the facility attempted to seal the smoldering silo to starve the fire of oxygen. However, facility personnel did not adequately monitor the progress of the fire to evaluate the effectiveness of these actions. The most effective method to judge the progress of smoldering combustion in a confined space is to perform atmospheric monitoring within the silo. Instead, the facility monitored the silo temperature using inadequate instrumentation.

The temperature of the material in the silo can also provide information regarding the extent of the smoldering mass of material. However, temperature measurements can be extremely difficult, especially, if there is a significant amount of material in the silo. According to the NFGA emergency preplanning and firefighting manual [14]:

A grain or grain product fire in a concrete bin or silo is one of the most difficult to extinguish. Although heat or smoke may be present in the grain over space, rarely will there be significant flame present. Thus, the fire may not be located easily within the bin. Since grain is a good insulator, temperatures extremes also may not always be detected by temperature cables.

Two effective methods for measuring the temperature of the material in the silo and locating hot spots are through the use of thermographic photography and a thermometer probe [14].

The simplest means for obtaining temperature measurements is through the use of a noncontact thermometer, but this type of measurement can only measure surface temperatures (the outer silo wall, the top surface of the material, the external surfaces of piping, etc.) at a single point. This type of device is typically not suitable for temperature measurements in the material interior (the region that is smoldering). Furthermore, surface temperatures measured with this type of device may be inaccurate because smoke can interfere with the device's optics, resulting in the device measuring the average smoke temperature, thus reporting a lower temperature than the true surface temperature of the material. A noncontact thermometer cannot measure interior temperatures deep inside the material which is where smoldering combustion typically occurs, as in the case study described. Therefore, the device could not “see” the smoldering combustion. The surface temperature would have been much cooler than the interior temperature where smoldering is occurring.

IR thermal imaging cameras can be used to measure larger surfaces of the silo including the outer walls, ducts, manyways, etc. and may provide better indications of where hot spots are located and/or the progression of the fire. Unfortunately, like the noncontact thermometer, thermal imaging cameras only provide surface temperatures. To obtain temperatures within the material, thermometer probes can be inserted into the mass of material. A thermometer probe is a long, hollow probe made with pipe with a sharp tip that can be inserted into the material to locate hot spots caused by smoldering or burning material in a storage silo. Some of the drawbacks to a thermometer probe is that it has to be physically inserted into the material (which has inherent hazards), the probe must be inserted at the correct location to measure the temperature of the hot spot, and the level of the material may not allow the insertion of such a probe from an accessible location.

Rapid Response

When a smoldering fire is detected in a silo, a rapid response becomes very important. A delayed response will allow the fire to grow and spread through the material making the fire more difficult to suppress. As a smoldering fire grows, the temperature increases causing the buoyancy forces within the material to increase. An increased buoyant flow will carry inerting gases away from the smoldering material at a greater rate. A delayed response also allows flammable gases and smoke to build up within the silo.

The delay in material removal during the presented case study increased the hazard of the situation in two ways: (1) the time delay allowed the smoldering combustion region to grow in size, making it more difficult to remove the material, and (2) during the time delay, the facility periodically increased the ventilation to the silo, thus, prolonging the life of the fire and increasing the generation rates of CO and smoke containing flammable unburnt gases and particulates.

The agglomerated pellets would have been difficult to remove without the complication of smoldering. Smoldering complicates the problem because it hardens the agglomerated material by charring it. This has been observed in other silo fires where smoldering leads to bridging of the particulate material.

The facility had the salvage contractor dump the material recovered from the silo into the tunnel because if the material was dumped outdoors onto the ground it would interfere with the operation of the truck scale and essentially shut down the shipping and receiving function of the facility. Removal of the material through the manway onto the ground outside the silo would have allowed the material to be removed in a quicker, safer way. In addition to the delay in beginning the removal of the material, the slow removal of the material through the tunnel increased the overall hazard of the situation. Dumping the pellets into the cramped confines of the tunnel put the salvage workers at greater risk to personal injury.

Guidance Documents

Section 12.6 of NFPA 61 provides requirements for fire-fighting operations. Among these requirements, NFPA 61 states that, “Fires, when discovered, shall be reported promptly to facility management and emergency responders, including the fire department.” NFPA 61 also recommends that, “If possible, incipient fires shall be manually extinguished or burning materials removed.” For fires that cannot be controlled promptly in their incipient stage, NFPA 61 requires that the endangered structure(s) be evacuated. For the case study presented above, the facility failed to follow these requirements.

To fight incipient fires in a concrete bin or silo, NFPA 61 recommends the following:

  • Concrete bin or silo fires should be extinguished by removing burning material from the bin or silo directly to the outside after wetting the top surface of the material with gentle application of water at a low flow rate directly to the burning materials.
  • Water fog should be applied to walls and to the underside of the roof to reduce airborne dust.
  • Fire should be located by thermometer probes, thermographic photography, or feeling heat on bin or silo surfaces.
  • Openings to the bin or silo should be sealed to limit oxygen entry.
  • Material flow to and from the bin or silo should be stopped.
  • Fire-fighting operations should be done from outside the bin or silo.

Furthermore, NFPA 61 cites the NGFA Emergency Preplanning and Firefighting Manual for guidance on firefighting actions at grain elevator facilities. The NGFA manual indicates that, “Procedures for extinguishing bin fires vary with each fire” [9]. The document discusses three options for fighting fires in concrete bins: (1) starve the fire of oxygen, (2) flood the bin with inert gas to displace the oxygen in the bin, or (3) apply water gently to the walls, grain surface, and hotspots. The document also recommends consulting with a structural engineer to get advice on how to cut access holes into the bin if the hotspots cannot be reached through the bin's existing openings. The recommended practices for fighting a concrete bin or silo fire as presented in the NGFA guidance document are (emphasis provided by NGFA):

  • Stop all equipment operating in the area, particularly the system to the troubled bin.
  • Seal all openings into the bin, particularly at the bottom and sides to reduce the supply of oxygen to the fire.
  • Locate and determine the extent of the fire. Never try to fight a fire from inside the bin.
  • Monitor temperature in adjoining bins.
  • Consider displacing oxygen by injecting carbon dioxide or nitrogen into the bin overspace.
  • Gently wet down interior exposed walls at the bin top and the grain surface.
  • If water is used, apply it to hot spots slowly with a probe or through access holes cut in the bin walls.

If the smoldering material cannot be extinguished using this procedure the NGFA guidance document gives the following instruction for removing the smoldering material [9]:

  • Wet down the exposed bin bottom in the tunnel area of the facility (or side doors if applicable) and all surrounding equipment to inhibit ignition if smoldering grain or flames erupt from the bin.
  • Unload the material directly outside through manhole ports or loadout spouts by auger or other means. Never run hot or burning grain from a troubled bin through an elevator leg or into another bin.
  • If necessary, slowly allow grain to exit the bottom onto the floor (in the tunnel), wetting all material exiting the bin. This can be accomplished by:

    • ○ Using a temporary chute to direct grain flow onto the floor;

    • ○ Cutting or otherwise removing the belt to allow the grain to flow directly onto the floor;
    • ○ If it is not possible to have the grain flow directly onto the floor, allow it to flow onto a stationary belt, wetting the grain as it exits the bin and shoveling it off the belt.

Note that the NGFA manual recommends removing the material directly to the outside when possible. Examples of burning material being removed from two silos are shown in Figure 7. and 8Figure 8. . In the case study presented above, the material was not removed directly to the outside, due to the use of the truck scale. When removing material into an enclosed tunnel, as was done in the case study, all of the material exiting the bin should be wetted. This was not the procedure that was followed.

image

Figure 7. Removal of stored material through silo wall to expose smoldering material. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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image

Figure 8. Removal of burning material. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED

The storage in silos of organic granular materials that are prone to self-heating must be managed carefully. It is important to understand the self-heating capability of the granular material in question, and to define safe (subcritical) storage conditions. The storage conditions within the silo should be controlled to avoid supercritical storage temperatures or excessive humidity. Since the propensity for self-heating often increases with material age, other useful safety measures include inventory control (frequent material turnover) and inspection of silo contents to detect caked solids on the silo wall or granular flow anomalies within the silo.

If evidence of self-heating or smoldering is observed in a silo, it merits immediate, serious attention. The hazardous situation should be monitored with appropriate technology while a strategy for suppressing the smoldering fire is developed. Suppressing a smoldering fire requires specialized skills and should not be attempted by inexperienced personnel. The most effective suppression strategy is to flood the silo with an inert gas that can extinguish the fire, but there may be site-specific circumstances that make this strategy infeasible. If inert gas flooding is not feasible, then the only other options are to either let the fire burn itself out in the silo (a process with may take months) or to rapidly remove the entire contents of the silo so that the burning material can be extinguished outside the silo. The latter option may require substantial structural damage to the silo, and should, therefore, be implemented under the guidance of a structural engineer.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. ACCIDENT SUMMARY
  5. FIRE AND EXPLOSION DYNAMICS ANALYSIS
  6. PREVENTION AND MONITORING
  7. SUPPRESSION STRATEGIES FOR SILO FIRES
  8. CONCLUSIONS
  9. LITERATURE CITED
  • 1
    NFPA 61, “Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities,” 2008 edition, National Fire Protection Association, Quincy, Massachusetts.
  • 2
    R. Eckhoff, Dust Explosions in the Process Industries, 3rd Ed., Gulf Professional Publishing, Elsevier Science, Amsterdam (2003).
  • 3
    U. Krause, (Editor), Fires in Silos, Wiley-VCH, Berlin (2009).
  • 4
    NFPA Glossary of Terms, National Fire Protection Association, Quincy, MA (2008).
  • 5
    ASTM E176-10a Standard Terminology of Fire Standards, American Society of Testing and Materials, West Conshohocken, PA (2011).
  • 6
    J.T. Mills, Spoilage and Heating of Stored Agricultural Products: Prevention, Detection, and Control, Agriculture Canada, Ottawa (1989).
  • 7
    D.A. Blasi, G.L. Kuhl, J.S. DrouillardS, C.L. Reed, D.M. Trigo-Stockli, K.C. Behnke, and F.J. Fairchild, Wheat Middlings Composition, Feeding Value, and Storage Guidelines, Kansas State University Agricultural Experimental Station, Kansas State University, Manhattan, KS (1998).
  • 8
    P.C. Bowes, Self-Heating: Evaluating and Controlling the Hazards, Elsevier, Amsterdam (1984).
  • 9
    K.J. Goforth, et al., Emergency Preplanning and Firefighting Manual: a Guide for Grain Elevator Operators and Fire Department Officials, National Grain and Feed Association, Washington, DC (1987).
  • 10
    NFPA 654, “Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids”, 2013 ed., NFPA, Quincy, MA.
  • 11
    D. Drysdale, Introduction to Fire Dynamics, 2nd Ed., Wiley, Chichester (1999).
  • 12
    M. Tuomissaari, D. Baroudi, and R. Latva, Extinguishing smouldering fires in silos, BRANDFORSK project 745–961, VTT Building Technology, Technical Research Centre of Finland, Espoo (1998).
  • 13
    T. Halpin, A Different Animal: Agricultural silo fires present unique challenges and dangers to firefighters, Fire Chief, available at: www.firechief.com, accessed on October 1, 2010.
  • 14
    K.J. Goforth, and C.R. Ostrowski, Emergency Preplanning and Firefighting Manual: a Guide for Grain Elevator Operators and Fire Department Officials, National Grain and Feed Association, Washington, DC (1987).