Chemicals present a substantial hazard in the form of fire and explosion.
Combustion of one gallon of toluene can destroy an ordinary chemistry laboratory in minutes, potentially resulting in fatalities.
The potential consequences of fires and explosions in pilot plants and plant environments are even greater.
The three most common chemical plant accidents are fires, explosions, and toxic releases.
Organic solvents are the most common source of fires and explosions in the chemical industry.
To prevent accidents resulting from fires and explosions, engineers must be familiar with
- The fire and explosion properties of materials.
- The nature of the fire and explosion process.
- Procedures to reduce fire and explosion hazards.
The Fire Triangle
Essential elements for combustion fuel, oxidizer, and an ignition source are Illustrated by the fire triangle.
Definition of Fire: Fire, or burning, is the rapid exothermic oxidation of an ignited fuel.
Fuel can be solid, liquid, or vapor form, with vapor and liquid fuels generally easier to ignite.
Combustion of fire: It is a chemical reaction in which a substance combines with an oxidant and releases energy. Part of the energy released is used to sustain the reaction.
Combustion occurs in the vapor phase, liquids are volatilized, and solids are decomposed into vapor before combustion.
Conditions for fire
- Fuel, oxidizer, and an ignition source are present at necessary levels.
Fire will not occur if
- Fuel is not present or not in sufficient quantities.
- The oxidizer is not present or not in sufficient quantities.
- The ignition source is not energetic enough to initiate the fire.
Examples of fire triangle components
- Wood, air, and a match.
- Gasoline, air, and a spark.
Fuels, Oxidizers, and Ignition sources
Fuels
Wood, Paper, Coal, and Gas are just a few of the products commonly thought as fuels.
However, from a chemical standpoint, the common fuel elements are carbon (C), and Hydrogen (H).
Carbon (C) is found in coal, coke lignite, and peat.
Other carbon fuels include fat, petroleum, and natural gas.
Hydrogen is commonly found in conjunction with these carbon compounds.
Common examples of fuels are as follows
Liquids: gasoline, acetone, ether, pentane.
Solids: plastics, wood dust, fibers, metal particles.
Gases: acetylene, propane, carbon monoxide, hydrogen.
Oxidizers
Quantities of oxidizer or air must be available for complete combustion to occur; otherwise, a fire will smolder.
The amount of oxygen required to sustain a fire may depend on the form and characteristics of the substance that burns.
A liquid or solid as it is heated evolves into vapor. As the concentration of vapor increases, it forms a flammable mixture with the oxygen in the air.
As a result, it may not be necessary to remove all the oxygen to extinguish such a fire.
Liquid fires can generally be put out by reducing the oxygen concentration below 12 to 16% (by volume).
Solid fires may require a greater reduction of oxygen concentration below 5% for surface smoldering and as low as 2% for deep-seated smoldering.
Common oxidizers are as follows
Gases: oxygen, fluorine, chlorine.
Liquids: hydrogen peroxide, nitric acid, perchloric acid.
Solids: metal peroxides, ammonium nitrite.
Ignition Sources
Ignition: Ignition of a flammable mixture may be caused by a flammable mixture coming in contact with a source of ignition with sufficient energy or the gas reaching a temperature high enough to cause the gas to autoignition.
Autoignition temperature (AIT): A fixed temperature above which adequate energy is available in the environment to provide an ignition source.
Fuel will not burn until it reaches a certain temperature, which depends on the type of fuel and factors such as the exposed surface, the vapor present, and the presence or absence of other fuels.
Common ignition sources are
Sparks, flames, static electricity, heat.
Mixing
The chemical union of fuel and oxygen requires the proper mixing of these compounds.
For example, if the ratio of oxygen to fuel is either too high or too low, a fire will be extinguished. The proper oxygen-to-fuel mix must be maintained to sustain a fire.
Past methods for controlling fires and explosions elimination or reduction of ignition sources. Not robust enough due to low ignition energies and plentiful ignition sources.
Current practices prevent fires and explosions by continuing to eliminate ignition sources while focusing on preventing flammable mixtures.
Flammability Characteristics
Flammability limits
Flammability limits for a flammable gas define the concentration range of a gas-air mixture within which an ignition source can start a self-propagating reaction.
The minimum and maximum fuel concentration in air that will produce a self-sustaining reaction are called the lower flammability limit (LFL) and the upper Flammibility limit (UFL) .
It is also called LEL (lower explosivity limit) and UEL (upper explosivity limit).
The flammability limits are functions of
- Ignition energy
- Ignition pressure
- Ignition temperature of the mixture or substance
- Inert gas concentration
- Relative humidity of the mixture or substance
Flash Point (FP)
The flash point of a liquid is the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air. At the flash point the vapor will burn but only briefly; inadequate vapor is produced to maintain combustion. The flash point generally increases with increasing pressure.
Fire Point
The fire point is the lowest temperature at which a vapor above a liquid will continue to burn once ignited; the fire point temperature is higher than the flash point.
Autoignition Temperature (AIT)
Autoignition temperature (AIT): A fixed temperature above which adequate energy is available in the environment to provide an ignition source.
Difference Between Fire and Explosion
Fire is defined as the rapid exothermic oxidation of an ignited fuel.
The fuel can be in solid, liquid, or vapor form, but vapor and liquid fuels are generally easier to ignite.
The explosion is referred to as the expansion of gases, which results in a rapidly moving pressure or shock wave.
The distinction between fire and explosion lies in the rate of energy release.
- Fire releases energy slowly.
- The explosion releases energy rapidly.
Fires can happen because of explosions, and explosions can happen because of fires.
Types of Fire
Classifications of fire based on classes
Class A
Fires involving ordinary combustible materials such as wood, paper, cloth, and plastics.
This type of fire is extinguished by bringing the burning materials below their ignition temperatures with the quenching and cooling effects of water.
Under certain circumstances, these fires may be extinguished by the blanketing or smothering effects of dry chemical and carbon dioxide fire extinguishers.
Class B
Fires are fueled by flammable liquids or gases, such as gasoline, oil, grease, and propane.
These type of fires are most successfully extinguished by limiting the air that supports combustion.
Fire extinguishers dispersing dry chemicals, carbon dioxide, foam, halogenated hydrocarbon agents, and fog streams are recommended for class B fires.
Class C
Fires involve energized electrical equipment, where the primary hazard is the electrical current.
The extinguishing agents recommended are dry chemicals, carbon dioxide, compressed gas, and vaporizing liquid.
Class D
Fires are fueled by combustible metals, such as magnesium, titanium, sodium, and potassium.
Class D fires require special extinguishing methods and agents, such as the graphite-based type.
Class K
Fires involving cooking oils and fats are commonly found in commercial kitchens.
The most effective way to extinguish a Class K fire is by using a wet chemical fire extinguisher specifically designed for these types of fires. These extinguishers work by spraying a fine mist of potassium acetate, potassium carbonate, or other agents that react with the cooking oil to create a foam-like layer, suppressing the fire and preventing reignition.
Detonation and Deflagration
Explosions from the combustion of flammable gas or liquid are of two kinds.
- Detonation
- Deflagration
Detonation
Detonation is extremely rapid, self-propagating explosive combustion with a velocity exceeding the speed of sound.
Initiated by mechanical, friction, or heat.
Occurs in solid, liquid, and reactive gas explosives.
Examples include acetone, ethylene, acetylene, benzene, methane, etc.
Deflagration
Deflagration is the very rapid auto combustion of particles at the surface with a speed less than the speed of sound.
Initiated by contact with a flame or spark, continued by impact or friction.
Examples are internal combustion engines, gas stoves, fireworks, and gunpowder.
Burns outward and requires fuel to spread.
Difference between Detonation and Deflagration
Deflagration burns at subsonic speeds (typically around 300 m/s).
Detonation flame front travels as a shock wave followed closely by a combustion wave velocity equals the velocity of sound in the hot products of combustion.
Detonation generates greater pressures and is more destructive than deflagration.
Peak pressure deflagration 8 bar, detonation 20 bar.
The transition from deflagration to detonation can occur, resulting in temporarily higher velocities (‘Over driven’ condition).
Types of Explosion
It is referred to as the combustion of a ‘ combustible mixture (gas cloud), causing a rapid increase in pressure.
It releases sudden and violet energy and this violence depends on the rate at which energy is released.
There are various types of explosions such as Confined gas explosion, Vented confined explosion, Partially confined vented explosion, Unconfined gas explosion, Confined Vapour Cloud Explosion (CVCE), Boiling Liquid Expanding Vapour Explosion (BLEVE).
Unconfined Vapor Cloud Explosion (UVCE)
Unconfined Vapor Cloud Explosion ( occurs when a cloud of flammable vapor, gas, or mist mixes with air in a concentration range that is capable of ignition and encounters an ignition source.
This ignition could be anything from a spark to a flame, electrical equipment, or even hot surfaces.
When the vapor cloud ignites, it rapidly combusts, releasing a significant amount of energy.
The unconfined vapor cloud explosion ( is a fire accident that could cause severe damage and it sometimes leads to a high potential of casualties.
Process of UVCE
Formation of Vapor Cloud
Flammable liquids or volatile chemicals evaporate into the air, forming a cloud of vapor This vapor can be denser than air and can spread over a considerable distance, depending on factors such as wind speed and direction.
Vapor Cloud Dispersion
The vapor cloud disperses in the atmosphere, gradually mixing with air This dispersion can be influenced by factors such as weather conditions, terrain, and the properties of the substance involved.
Ignition Source
An ignition source comes into contact with the flammable vapor cloud Ignition sources can be external, such as sparks from electrical equipment or flames from nearby processes, or internal, such as hot surfaces within the cloud.
Combustion
When the vapor cloud encounters the ignition source, it ignites rapidly, leading to combustion This combustion releases a large amount of energy in the form of heat and pressure.
Explosion
The rapid combustion of the vapor cloud generates a shockwave and thermal radiation, resulting in an explosion The shockwave can propagate outward, causing damage to surrounding structures, equipment, and personnel in the vicinity.
Effects
The effects of a UVCE can be severe, including structural damage, fires, and potential injury or loss of life The explosion can also trigger secondary hazards, such as the release of toxic gases or the spread of fires to adjacent areas.
Measures for the prevention of UVCE incidents
Proper handling and storage of flammable liquids and chemicals to minimize the risk of vapor release.
Implementation of safety protocols and procedures to control ignition sources and prevent accidental ignition.
Adequate ventilation and control measures to minimize the formation and dispersion of flammable vapor clouds.
Training of personnel on safety practices and emergency response procedures to effectively respond to UVCE incidents and minimize their impact.
Boiling Liquid Expanding Vapour Explosion (BLEVE)
A Boiling Liquid Expanding Vapor Explosion (BLEVE) is a type of explosion that can occur when a vessel containing a pressurized liquid, typically a flammable substance like propane or liquefied natural gas (LNG) is subjected to an external fire or heat source.
It is also known as a fireball.
It is a combination of fire and explosion with an extreme radiant heat emission within a very short time.
Process of BLEVE
Pressurized Vessel
The substance is stored in a vessel under pressure, typically in a liquid state The pressure keeps the substance in its liquid form at ambient temperatures.
Exposure to Fire
If the vessel is exposed to an external fire, such as in a fire incident or accident, the heat from the fire causes the temperature of the liquid inside the vessel to rise rapidly.
Boiling
As the temperature of the liquid rises, it eventually reaches its boiling point, causing it to vaporize into gas This rapid vaporization leads to an increase in pressure inside the vessel.
Structural Failure
If the pressure inside the vessel exceeds its structural integrity, the vessel may fail catastrophically, resulting in a sudden release of the pressurized vapor.
Explosion
The sudden release of pressurized vapor can result in an explosion. The explosion is characterized by a rapid expansion of the vapor, which can generate a powerful shockwave and thermal radiation.
BLEVEs can be extremely hazardous due to the potential for widespread damage and the release of flammable gases, which can ignite and lead to secondary fires or explosions.
Prevention measures for BLEVEs
It includes proper storage and handling of pressurized liquids, as well as implementing safety protocols to prevent fires and control heat exposure to pressurized vessels.
Additionally, emergency response plans should be in place to mitigate the impact of a BLEVE if one occurs.
Conclusion
In conclusion, it’s crucial to take serious steps to prevent fires and explosions in places where chemicals are handled. This means making sure everyone knows how to stay safe, using the right equipment, and following strict rules. By working together, using the latest safety technology, and always trying to do better, we can greatly reduce the chances of accidents. This protects not only the people working with chemicals but also the surrounding areas and the environment. Safety should always come first.
FAQ’s
Fires and explosions in chemical plants are often caused by factors such as equipment malfunctions, chemical reactions, human error, improper handling of hazardous materials, electrical faults, and inadequate safety procedures.
Prevention measures include proper storage and handling of chemicals, regular equipment maintenance, implementing safety protocols, providing comprehensive training to personnel, using appropriate personal protective equipment (PPE), and conducting thorough risk assessments.
Immediately evacuate the area, activate the fire alarm, notify emergency services, and use the appropriate fire extinguisher if it’s safe to do so. Do not attempt to extinguish a large fire without proper training and equipment.
Safety measures include implementing strict operating procedures, conducting regular inspections and maintenance of equipment, providing adequate ventilation, using explosion-proof electrical fixtures, and ensuring personnel are properly trained in emergency response protocols.
Yes, chemical engineers must adhere to regulations and standards established by government agencies such as OSHA (Occupational Safety and Health Administration), EPA (Environmental Protection Agency), and NFPA (National Fire Protection Association), as well as industry-specific guidelines and best practices.
Common types include water, foam, dry chemical, carbon dioxide (CO2), and wet chemical extinguishers. The type used depends on the nature of the fire; for example, Class B fires involving flammable liquids require a different extinguisher than Class C fires involving electrical equipment.
Potential ignition sources include open flames, hot surfaces, electrical equipment, sparks from mechanical operations, static electricity, and frictional heat. Conducting a thorough hazard analysis can help identify and mitigate these risks.
Immediately alert others in the area, evacuate if necessary, contain the spill using appropriate absorbent materials, and notify emergency response personnel. Avoid contact with the spilled material and follow established spill response procedures.
Proper ventilation can be achieved through the installation of mechanical ventilation systems, including exhaust fans, ductwork, and explosion-proof equipment. Regular monitoring of air quality and compliance with ventilation standards are also essential.
Personnel should receive training in fire safety, emergency response procedures, hazard recognition, proper use of fire extinguishers, evacuation protocols, and first aid/CPR. Regular drills and refresher training sessions are also recommended to ensure readiness in case of emergencies.
References
Chemical Process Safety Fundamentals with Applications, by Daniel A. Crowl, Joseph F. Louver, Prentice Hall International Series in the Physical and Chemical Engineering Sciences.
Health, Safety, and Accident Management in the Chemical Process Industries, by Ann Marie Flynn, Louis Theodore, Marcel Dekker Inc.
Safety and Accident Management in the Chemical Process Industries Ed. by H. Heinemann, M. Dekker.
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