Fire And Explosion Prevention
Fire, or combustion, occurs when a substance quickly oxidizes and releases energy. The four elements of the fire tetrahedron must be present for a fire to begin: fuel, oxidizing agents, an ignition source, and a chemical chain reaction. A fire cannot occur unless all portions of the tetrahedron are present.
Common fuels include gasoline, hydrocarbons, and dust. The most common oxidizing agents are oxygen, halogens, and strong acids. Ignition sources include open flames, sparks, static electricity, and hot surfaces. The chemical chain reaction involves the chemical oxidation pathways, including the formation of free radicals.
An explosion occurs when there is a sudden conversion of potential energy (chemical or mechanical) into kinetic energy with the production and release of gases under pressure. These high-pressure gases then do mechanical work such as moving, changing, or shattering nearby materials. An explosion can be the result of a chemical reaction (propagating deflagration) or mechanical explosion (rupture of pressurized container). Five factors must be present for a chemical explosion to occur: fuel, oxidizing agents, an ignition source, confinement, and fuel dispersion. Explosions can be either chemical or physical: chemical explosions occur when materials decompose or combine exothermically, producing energy and oxygen that serves as fuel for the explosion. Physical explosions are those that occur without chemical reaction; physical explosions often occur when materials stored at high pressure are suddenly and unexpectedly released into the atmosphere, forming a shock wave.
The highest risks involve activities related to startup, shutdown, and maintenance. Risk is also increased when piping and equipment have been open to the atmosphere and during initial filling or charging of systems.
The oxidizing agents and ignition source in explosions are similar to those of fires, but the fuel for an explosion is most commonly dust or confined gases. For a dust explosion to occur, the explosive material must first be dispersed into a dust cloud large enough to generate a deflagration. If enough congestion or confinement is present, this may lead to an explosion. Explosions from other fuels often result from runaway reactions. Runaway reactions, which occur when an exothermic reaction has its temperature increased, produce additional heat and cause uncontrollable rises in temperature.
Case Study: 2015 Tianjin Explosions
In 2015, a fire broke out in a freight container storage facility owned by Ruihai Logistics in Tianjin, China. After the fire occurred, a series of explosions followed, causing 173 deaths and hundreds of injuries. The fire began when a container of nitrocellulose, a volatile chemical that can ignite in warm air, reached a high temperature. The combustion released oxygen and, since nitrocellulose burns in the presence of water, initial firefighting efforts only worsened the fire. Had firefighters understood the properties of the chemicals, they could have used halon or a dry chemical to fight it and possibly prevent the explosions from occurring. The element of the fire tetrahedron that was easiest to remove was the chemical chain reaction. Once the fire had broken out, it ignited calcium carbide, releasing acetylene into the surrounding area. When the acetylene entered a container of ammonium nitrate, the ammonium nitrate rapidly oxidized and detonated within the container. The explosion could have been prevented or mitigated by storing smaller quantities of ammonium nitrate per container or proper storage in a cool area.
CASE STUDY: T2 Laboratories Explosion and Fire
In 2007, an explosion occurred at T2 Laboratories in Jacksonville, Florida in a 2,500-gallon batch reactor for the production of MCMT, a common additive to gasoline. A single reactor exploded, killing four T2 workers and injuring thirty-two additional individuals. The explosion occurred because the cooling jacket surrounding the reactor failed to operate before the temperature in the reactor increased beyond the temperature of no return and because the pressure relief system was not capable of relieving the pressure from a runaway reaction. At the increased temperature and pressure, a secondary reaction occurred which created a greater risk of runaway. The cooling water had no reserve tanks or backup valves, so any single failure with the cooling water system would result in loss of temperature control for the reactor. The rupture disk was set to initialize the pressure relief system once the reactor pressure passed 400 psig, but the runaway reaction had already occurred by that point; the runaway reaction increased the pressure in the reactor by 32,000 psig per minute, causing an immediate tank rupture and subsequent explosion. Had T2 installed redundancies within their process control system the cooling medium could have activated even though the primary water release valve failed.
Fire And Explosion Prevention Methods
Three common techniques to prevent fires and explosions are inerting, controlling ignition sources (including static electricity), and installing proper ventilation. Other prevention methods include preventing dust accumulation, using explosion-proof equipment, and installing sprinkler systems. In most chemical plants, layers of protection exist so that if one or more pieces of equipment fail a disaster can still be averted. A process with layers of protection will avoid accident or loss if a single-point equipment failure occurs. Various process controls are more suitable to some equipment and environments than others, as described below.
When handling flammable materials, the upper and lower flammability limits should be taken into account. The lower flammability limit (LFL) of a material is the lowest concentration in air at which it can ignite, while the upper flammability limit (UFL) of a material is the highest concentration in air at which it can ignite.
Inerting refers to adding an inert gas to a potentially flammable system to reduce the oxygen concentration below the limiting oxygen concentration (LOC). All systems have a LOC at which point the chemical chain reaction cannot generate enough energy to produce fire. Many gases have LOCs at or near 10%, while the LOCs of dusts are usually around 8%. There are exceptions to this principle; hydrogen, for example, has a LOC of 5%. Most inerting systems attempt to keep the atmosphere around a flammable substance at four percentage points below the substance's LOC by introducing gases such as nitrogen or carbon dioxide. Common inerting techniques include vacuum purging, pressure purging, sweep-through purging, and siphon purging.
Vacuum purging is the most common technique for inerting reactors because most reactors are designed to operate at a wide range of pressures. This technique involves creating a vacuum within a vessel by greatly reducing the pressure, re-pressurizing the vessel with an inert gas, and repeating the previous steps until the desired oxygen concentration is achieved. As the pressure in the reactor is lowered, the concentration of oxygen in the reactor remains constant while the total number of moles of oxygen (and other species) is reduced. Re-pressurizing the vessel with an inert species reduces the oxygen concentration; this process is repeated until the oxygen concentration is significantly below the LOC. Vacuum purging is preferred in industry because it uses the lowest quantity of inert gas out of the four methods.
Pressure purging is in essence the reverse of vacuum purging. Pressure purging is used when a vessel can withstand pressures greater than atmospheric but cannot withstand pressures significantly lower than atmospheric. This process involves pressurizing a vessel with an inert gas, thus lowering the concentration of oxygen within the vessel. The vessel is then vented to atmospheric pressure, decreasing the number of moles of gas within the vessel while maintaining constant oxygen concentration. Pressure purging uses more inert gas than vacuum purging, but will generally require fewer stages because pressure in most vessels can be increased by a greater amount than it can be decreased.
Sweep-through purging is a system of altering the concentrations of species within a vessel without changing the vessel's pressure. This technique is common when a vessel's pressure tolerance is unknown or it is known that a vessel can only safely operate at atmospheric pressure. Siphon purging involves simultaneously adding inert gas into a vessel and removing the mixed gas from the vessel. Since only inert gas is added to the vessel while both oxygen and inert gas are removed from the vessel, the concentration of oxygen in the vessel decreases over time. One disadvantage of this technique is that because inert species are simultaneously added and removed from the vessel, much of the inert species is wasted rather than used. Another disadvantage of this technique is that it is difficult to evenly mix the inert species in the vapor space. If the inert gas is not evenly mixed, there can be pockets of the environment where the oxygen concentration is still too high and a risk of fire still exists.
Siphon purging is a technique to mitigate the quantity of inert species wasted by sweep-through purging. Siphon purging involves filling the vessel with a liquid, then draining the liquid from the vessel while inert gas is added. This allows the pressure in the vessel to remain constant while inert species are added.
Controlling Static Electricity
A common hazard in chemical plants is the buildup of static charge within an environment. Sparks and ignition resulting from charge buildup can create an environment where fires and explosions can occur. Altering the rates of charge buildup and charge relief and separating fuels from high-charge environments are common practices to control static electricity.
A large voltage gradient between two bodies will result in a large static electricity buildup between the two materials. However, this voltage gradient is negated when the two bodies are joined by a conducting material such as steel to form one body through bonding.
Grounding, a common method of eliminating static buildup, occurs when a body is connected to a large, neutrally charged object that serves as a near-infinite electron source or sink. Often, this object is the ground itself, but any neutrally-charged object orders of magnitude larger than the charged body will effectively ground the body. Static charges can be generated when material is transferred. When filling vessels from top nozzles, dip pipes are often used to help prevent splashing and subsequent buildup of static charge. For those reasons, it is inherently safer for a vessel to be filled from the bottom when possible. This is especially important for liquids with slow dissipation of static charges such as toluene.
Installing Proper Ventilation
The purpose of ventilation is to limit the concentration of hazardous fumes within a chemical plant. If a plant is designed in the open air, then the concentration of dangerous fumes in the air will likely be negligible. Furthermore, the wind can disperse any hazardous chemicals in the air at such low concentrations so as to not be a hazard. Outdoor or open-to-air facilities are inherently safer in this regard, as their environment provides natural ventilation.
However, many chemical plants are located within buildings, which confine hazardous gases in a defined space. For confined chemical plants, there are two common types of ventilation to dilute and disperse flammable or explosive gases: local exhaust ventilation and dilution ventilation.
Local exhaust ventilation is the preferred method of ventilation because it uses less energy and replacement air than dilution ventilation. A local exhaust ventilation system consists of three parts: a hood, ducts to transport fumes from the plant to the atmosphere, and fans to disperse the chemicals as they are transported.
Two types of hoods are typically used to capture fumes in a local exhaust ventilation system: Slot exhaust hoods face the operator and contain a duct at the back to pull gases away from the operator and enclosed hoods are installed over an area of hazardous chemicals and pull gas away through ducts into the atmosphere. Enclosed hoods require less energy than slot exhaust hoods and are preferable when they can be installed.
Ducts are simply tubes that carry contaminated air, uncontaminated air, and dusts to or from a facility. Laminar flow is preferred to turbulent flow for ducts, as duct resistance is greater in turbulent flow. As a result, round ducts are preferred to square ducts. If dusts are transported with a duct, the fan must be sufficiently powerful to prevent dust from accumulating within the duct, to prevent fires and explosions.
Dilution ventilation is the process of lowering the concentration of contaminants in a facility by introducing clean air into the environment while removing air within the facility without discretion. Contaminated air is pushed out of the facility by an exhaust fan while clean air is introduced to the facility by a makeup air fan and duct. Because dilution ventilation does not specifically remove chemicals through a hood like local exhaust ventilation does, it should not be used in facilities where a large amount of contaminants are produced.
Centrifugal fans and propeller fans are commonly used in ventilation systems. Propeller fans are generally used for systems without ducts while centrifugal fans are more generally placed at the end of a duct.
Prevention of Dust Accumulation
Dispersion and confinement of dust can result in a dust explosion, so preventing the accumulation of dust within an enclosed facility is critically important. Dust can often accumulate on surfaces such as ceiling beams or light fixtures. Furthermore, even if a facility is well ventilated there can still be areas within a facility that are enclosed. In these cases, enclosed spaces should be equipped with dust collection and disposal equipment. Facilities that handle explosive dusts should be regularly inspected and cleaned.
Using Explosion-Proof Equipment
Choosing the right type of explosion-proof equipment is important in the prevention of explosions. Areas within a chemical facility may be designated as explosion-proof (XP) or non-explosion-proof (non-XP). Flammable materials are then only used in XP areas. This designation only applies to the type of materials present: ignition sources or open flames may be present in a non-XP area. Explosion-proof materials are graded based on Class and Division.
A material's class specifies the types of explosive agents the material is designed to withstand.
A material's division specifies the probability of the presence of a flammable atmosphere. Division 1 equipment is designed to operate in areas where flammable atmospheres are likely to occur, and Division 2 equipment is designed to operate in areas where flammable atmospheres can only occur in abnormal circumstances.
Installing Sprinkler Systems
Sprinkler systems consist of numerous sprinkler heads attached to the ceiling of a facility that are connected by a water main. When smoke, flame, or high temperature is detected, a sprinkler system will disperse water throughout the facility to extinguish a fire and create an environment where a fire cannot spread. A downside of sprinkler systems is that water may damage equipment within a facility. Note that sprinkler systems contain fires rather than prevent them. The two main types of sprinkler arrays are wet pipe systems and deluge systems.
Smaller plants and facilities typically employ a wet pipe sprinkler system. When a fire breaks out, a link keeping the water from the sprinkler system will melt and the sprinkler will activate. Each sprinkler is activated individually because each sprinkler is controlled by a different link. This allows for smaller, localized fires to be extinguished with the least possible collateral damage. The sprinklers cannot be shut off unless the water supply to the facility is halted. In colder facilities, a risk of water pipes freezing exists.
Larger plants and facilities typically employ a deluge sprinkler system . This system differs from a wet pipe system because in a deluge system all sprinklers are connected to a common control point. The upside to a deluge system is that it is more likely to contain large fires because it sprays water over an entire facility the moment smoke or fire is detected. A downside of a deluge system is that it will cause more collateral damage than a wet pipe system. Furthermore, containment or disposal systems of runoff water is necessary in larger systems.
Shock sensitivity refers to how an explosive material behaves when rapidly compressed. Chemicals have a wide range of shock sensitivity; for example, nitroglycerin can explode when shaken or even firmly touched. Chemicals are typically categorized as primary, secondary, or tertiary explosives. Primary explosives explode when exposed to small shocks. Typically a chemical is deemed a primary explosive if it will explode if its container is struck with a hammer. Primary explosives that explode upon any physical contact are designated as contact explosives. Secondary explosives are those that are less shock-sensitive than primary explosives but can still explode if handled unsafely. They present a significantly lower, but not negligible, risk of explosion. TNT is an example of a secondary explosive. Tertiary explosives are extremely shock insensitive; these chemicals will typically not explode even if detonated by a primary explosive. An example of a tertiary explosive is ammonium nitrate fuel oil; ANFO will only explode if involved in a large explosion containing both primary and secondary explosives. These chemicals are often deliberately detonated in mining and construction projects.
Acetylene is a commonly used explosive and shock-sensitive material. At pressures greater than two atmospheres, acetylene will rapidly and exothermically decompose if subjected to a shock wave or heat source. Acetylene also has an extremely low LFL of 2.5%. Acetylene is often dissolved in another compound for transportation and reacts explosively with common piping materials such as copper.
Industrial insurance companies are third-party agencies that assess fire and explosion risks, among other potential issues with safety, at chemical facilities. Industrial risk insurers provide expertise as to how a facility can reduce risk of fires and explosions.
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