Risk Assessment

In the Fall, 2013 edition (Volume 28, No. 4) of Industrial Fire World we discussed industrial silo fires; one of several locations in an industrial plant which has the potential for a dust explosion. Is this article we will extend that discussion to the remainder of the facility. We will also repeat some of what was discussed on silos.

There are two basic scenarios that might be encountered:

● a dust explosion(s) that has already occurred; often with only residual remaining fire and,

● fire not initially involving dust but dust becomes involved in an explosion during the fire.

 Preventing the first scenario is beyond the capabilities of operational firefighters. Fire marshals, OSHA, insurance carriers and company safety department try to prevent such explosions through engineering, housekeeping, explosion suppression, isolation, and/or venting systems. If prevention measures fail and a dust explosion occurs in an occupied area, the fire service should expect:

 ● mass casualties, including multiple victims with severe burns,

● structural instability with possible secondary collapses

● hazardous materials release from ruptured pipes and vessels.

Although these are major challenges, they are typically evident on arrival. A more insidious hazard is the dust explosion that occurs during a more typical fire; perhaps inadvertently initiated by the fire service responders.

Everyone in the fire service is familiar with the fire triangle. Heat, fuel and oxygen are all necessary to have a fire. Take away one or more of these legs and the fire cannot continue. Commonly, a forth component -- the chemical chain reaction -- is added to create the fire tetrahedron.

For a dust explosion to occur, five elements are needed to create the so-called dust pentagon:

● Heat

● Fuel

● Oxygen

● Dispersion of dust particles

● Confinement.

The first three are shared with the fire triangle. The fifth, confinement, is present inside of process equipment, silos or created by the building itself. The fire service needs to avoid anything that might create the fourth element, dispersion. This can include kicking up dust during agent application, ventilation, operations that involve heavy pounding during forcible entry or creating a water hammer in the sprinkler system.

The fire service needs to be aware of occupancies and processes where combustible dust is expected to exist. There are some obvious occupancies like grain elevators, coal silos and occupancies that produce sawdust. There are many other combustible dusts such as rubber, plastics, paper, textile flock, powdered chemicals and various foodstuffs like sugar, starch, and flour. This is just a partial list for example purposes. Metal dusts like aluminum might not readily come to mind but they can produce some of the most severe explosions.

There are engineering techniques to determine how much dust is too much. Two common methods for screening during the preplanning session are: 1) if you can no longer discern the surface color below the dust it is too much, and 2) if it is deep enough to write your name in it, it is too much. These are simplistic methods that should be followed with an engineering evaluation but it is a starting point.

Because silos are such a common hazard, we are repeating the earlier information here for completeness.

 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.

General silo guidance, which is adapted from the Swedish Civil Contingencies Agency1

● “Do not open the silo! Entrained air will oxygenate the fire, leading to increased smoldering intensity, which in turn may contribute to rapid fire escalation in conveyor systems before and after the silo … 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 two to four 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 it needs 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 this out and try to assemble the components.

● Nitrogen is preferred over carbon dioxide because it could break down into carbon monoxide that is explosive. 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.

Dust explosions are a severe industrial hazard. The fire service certainly needs to be prepared for them but even more importantly does not want to initiate one during firefighting operations.

Feel free to contact the author at [email protected]

i Based on research conducted by the SP Technical Research Institute of Sweden and titled Silo Fires; Fire Extinguishing and Preventive and Preparatory Measures The publication is available at no cost at https:// www.msb.se/sv/Produkter--tjanster/Publikationer/Publikationer-fran-MSB/Silo-fires-fire-extinguishingand-preventive-and-preparatory-measures/:

John Frank is Senior Vice President of the XL Catlin’s Property Risk Engineering / GAPS Loss Prevention Center of Engineering / GAPS team provides property loss prevention consulting and delivers individually tailored solutions to protect and enhance property, production, and profit. With approximately 220 engineers and consultants in 18 countries, the team brings clients occupancy specific experience as well as deep knowledge of specific hazards across industries.