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Reactivity hazards are part of the safety, health, and environment (HSE) topic for the Fundamentals of Engineering (FE) Chemical exam. Runaway reactions are associated with reactivity hazards and are also an important safety concern in the industry. Past major industrial accidents and fatalities have occurred due to runaway reactions, and they are still a factor for consideration today. Thermal runaway is an out-of-control exothermic reaction (releases heat) that can lead to devastating consequences. These reactions can become uncontrollable rapidly and are not easily contained by firefighters and first responders.
1. The Fire Triangle
The fire triangle is the science behind stopping a flammability hazard and stopping a runaway reaction. Heat, fuel, and an oxidizing agent (oxygen) are the three (3) components that comprise the fire triangle. At least one of the components must be eliminated to stop the reaction and extinguish the fire, and while you only need to eliminate one of the three components, this can prove to be a very difficult and daunting task. Care must be taken to mitigate and prevent the fire triangle from coming to fruition. The fire triangle is also established by the General Combustion Stoichiometric Equation:
Fuel + Oxygen -> Carbon Dioxide + Water
Different fuel sources (e.g., methane, propane) will have different stoichiometric quantities, but this is the general formula. Nitrogen (N2) is inert and does not react, so it is often excluded from stoichiometry. But remember that air is approximately 78% nitrogen and 21% oxygen, so while nitrogen may not be relevant, it is still present. Combustion is also an exothermic reaction (releases heat) process that shares similarities with thermal runaway (I think of thermal runaway as being excessive combustion). Incomplete combustion can also cause health hazards by generating carbon monoxide (CO) poisoning due to inadequate oxygen (CO vs CO2). Because of potential CO exposure, it is critical to ensure you have good ventilation and unblock any obstructions since incomplete combustion can occur anywhere (home, business, plant, etc.) at any time.
2. Assessing Thermal Runaway Firefighting Techniques
Thermal runaway occurs when surplus heat speeds up a reaction, so the reaction rate increases, more heat is released, and the reaction happens faster and with more severity in a vicious cycle. This can become uncontrollable and cause dangerous fires and explosions that can be damaging and increasingly difficult to contain. Heat removal can slow the reaction (eliminating one of the Fire Triangle components), but this is difficult to achieve as a thermal runaway reaction rapidly becomes more powerful. Fires and explosions due to thermal runaway can easily overwhelm firefighting techniques, so location is a factor when assessing facilities where thermal runaway can occur. Fires can spread rapidly, to begin with, and adding reactivity hazards will only be more disastrous.
3. Texas City Disaster
One past industrial accident of thermal runaway severity was the Texas City Disaster (1947). The SS Grandcamp was docked at the port of Texas City in Galveston Bay. Ammonium nitrate (NH4NO3) detonated on a cargo ship that triggered subsequent runaway reactions, which led to fires and explosions of other cargo ships and oil facilities, ultimately leading to the deadliest industrial accident in United States history (still stands today). You can see from my description that a fire started the detonation and evolved into the severe vicious cycle that I mentioned earlier. The ammonium nitrate was also stored at higher temperatures, increasing both its chemical activity and likelihood of generating an explosion.
4. Noticing Smoke
Crew members first noticed smoke in the SS Grandcamp cargo hold and tried using water and fire extinguishers to stop the fire (eliminate the fire triangle components). When this proved to be ineffective, the captain authorized sealing all hatches to smother the fire by removing its oxygen content and filling the cargo hold with steam. On a smaller scale, lids are often used to smother skillets so you can prevent kitchen fires. By eliminating the oxygen, the fire cannot "breathe", so it ends before a kitchen fire can become more severe. However, nitrate (NO3) is an oxidizer, so not only was this ineffective on the SS Grandcamp, but it may also have strengthened the fire (steam contributed more heat to the fire). Scientific data and technology have certainly improved over the years since 1947, so it is possible that the SS Grandcamp crew may not have realized this mistake. Closing the hatches would have also prevented ventilation for the oxidizing agent.
5. Effect of Pressurized Steam
The pressurized steam blew off the cargo hatches, and the SS Grandcamp detonated shortly after, due to the excessive heat and pressure. Thermodynamics (8-12 Questions) is another key topic on the FE Chemical exam, with steam tables listed in the FE Reference Handbook (v 10.0.1, p. 157-158). Remember that exam topics tend to overlap, so you can use knowledge of certain exam content to help with other exam questions. Water becomes steam at its 100° C (212° F) boiling point for standard pressure conditions (1 atm = 14.7 PSIA). Since steam is already at 212° F when entering the volatile cargo hold, you can see how the pressurized steam heat was contributing towards the fire intensity.
6. Effect of Other Volatile Components
The state of Texas is one of the largest industrial producers in the United States (oil, gas, chemicals, etc.), so there were other volatile components impacted, furthering the catastrophe. Because the SS Grandcamp was docked in port, other vessels and watercraft were destroyed too; this was essentially a total loss in the port of Texas City. The explosive blast levelled buildings and destroyed chemical plants; debris flying at supersonic speeds punctured infrastructure, causing leaks of other volatile components. The nearby town of Galveston also suffered infrastructure losses and fatalities. Because the fires were so ferocious, first responders from other locations were unable to immediately reach the disaster site; again, the severity of thermal runaway should not be taken lightly. The vicious cycle of reactions can produce a fire that can become almost unstoppable (there were two explosions at the port, with the second explosion occurring due to the first explosion, succeeding the cycle).
7. The New London School Explosion
The New London School explosion (1937) was another industrial accident caused by the fire triangle and combustion. Texas was growing as a prosperous industrial hub with the discovery and exploration of oil wells, so businesses and communities were booming despite the Great Depression in the United States. The New London School was erected as a new educational institution; however, the school board made two (2) mistakes that contributed to the explosion. The first decision was opting for the installation of gas heaters throughout the building rather than choosing a steam distribution system. It is not practical or safe to install individual heaters throughout a building; it was probably considered cheaper at the time, but a distribution system is more effective. The individual gas heaters were all devices producing volatile fuel (fire triangle component).
8. Born from a Spark
The second fatal mistake was the school board cancelling their natural gas contract in favor of plumbers installing a residual gas line to feed the building and its gas heaters. Residual gas was a by-product of extracted oil, so major oil companies were not bothered by this practice; it did not impact their operations or profits. However, the residual gas line was tapped and installed poorly, causing a gas leak that was building up inside the school. With the gas fuel and oxygen, there were now two (2) components of the fire triangle. The last component was when Lemmie R. Butler (workshop instructor) incurred a spark from an electric sander (an ignition source can count as heat in the fire triangle). The gas-air mixture was volatile and ignited, causing widespread damage and structural collapse at New London School. Fortunately, unlike the Texas City disaster, there were no other volatile facilities nearby, so the school explosion did not lead to subsequent reactions and explosions. The reaction explosion occurred at the school only, and there were no other additional sources of heat to strengthen the frequency or severity of combustion.
Conclusion
In the aftermath of industrial accidents, the Texas Engineering Practice Act has since been enacted and amended to reflect legislation from public backlash. This act describes that practicing engineers must have professional engineering registration with the state of Texas to have the engineer title (most recent rules are effective as of June 2017). Ethics and Professional Practice (3-5 Questions) are another topic on the FE Chemical exam, and again, you should note how the exam topics overlap. There have been many other industrial accidents that were encountered by our engineering predecessors. While very unfortunate, they always provide a good learning experience to improve upon processes and safety. You can certainly find more industrial information online and check back with School of PE for more blog posts about industry discussion.
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