I plan to post more about process safety fundamentals in the future. In this post I want to share about process safety fundamentals which is important terms regarding characterization of fires and explosion.
Disclaimer: I am not a process safety expert. I am still learning about process safety topics. So, your feedback is appreciated.
Based on statistics, the main cause of accidents in the chemical industry is fires and explosions. To prevent fires and explosions, we need to:
- Learn the basics of fires and explosions
- Conduct experiments to assess flammable and explosive materials
- Implement these concepts in plant operations effectively
Understanding the basics of fires and explosions begins with familiarizing yourself with key terms related to these phenomena. You will learn through this post.
Fire Triangle
The fire triangle is shown in the figure below.
Fire occurs when fuel, an oxidant, and an ignition source are present. In practice, the concentration of fuel and oxidant must be within specific ranges, and the ignition source must be sufficiently strong to initiate the fire.
The fire triangle is applicable to gases, liquids, and solids.
Oxygen in the air is typically the oxidizer, but other substances such as fluorine gas, chlorine gas, peroxides, and chlorides can also act as oxidizers.
Ignition sources include static electricity, sparks, hot surfaces, open flames, and electric circuits.
If one component of the fire triangle is removed, a fire will not occur. The traditional method for fire control was to eliminate ignition sources. Currently, efforts are increasingly focused on preventing flammable mixtures.
Autoignition Temperature
Autoignition temperature is fixed temperature above which adequate energy is available in the environment to provide an ignition source. In other words, it is the temperature at which a material ignites in air or some other oxidant at specified pressure without the aid of a spark or flame.
Boiling-Liquid Expanding-Vapor Explosion (BLEVE)
A BLEVE occurs if a vessel containing a liquid at a temperature above its atmospheric pressure boiling point ruptures. This leads to explosive vaporization of a large fraction of the vessel contents, possibly followed by combustion or explosion of the vaporized cloud if it is combustible. This type of explosion occurs when an external fire heats the contents of a tank of volatile material. As the tank contents heat, the vapor pressure of the liquid within the tank increases, and the tank’s structural integrity is reduced because of the heating, possibly leading to combustion or an explosion of the vaporized cloud.
Confined Explosion
Confined explosion is an explosion of fuel and oxidant mixture inside a closed system, such as vessel or building.
Deflagration
In this explosion, the reaction front moves at a speed less than the speed of sound in the unreacted medium.
Detonation
In this explosion, the reaction front moves at a speed greater than the speed of sound in the unreacted medium.
Figure below illustrates the difference between deflagration and detonation based on maximum resulted overpressure due to release of mechanical or chemical energy.
Flammability Limits
Vapor-air mixtures ignite and burn only within specific composition ranges. Below the lower flammable limit (LFL), the mixture is too lean to burn. Above the upper flammable limit (UFL), it is too rich to burn. Combustion occurs only when the mixture is between these limits. Volume percent of fuel is the common unit used.
Lower explosion limit (LEL) and upper explosion limit (UEL) are equivalent to LFL and UFL.
Flash Points
The flash point of a liquid is the lowest temperature at which it produces enough vapor to ignite with air, burning only briefly due to insufficient vapor for continuous combustion. In general, the flash point increases with increasing pressure.
Minimum Ignition Energy
Minimum ignition energy is the lowest energy needed to ignite a combustible vapor, gas, or dust cloud.
Shock Wave
A shock wave is a transient change in the gas density, pressure, and velocity around an explosion. This change can be discontinuous (shock wave) or gradual (pressure wave).
Unconfined Explosion
Unconfined explosions occur in the open area, usually due to flammable gas spill. The gas mixes with air and ignites. These explosions are less common than confined ones since wind often dilutes the gas below the LFL. However, they can be highly destructive because they involve large amounts of gas over extensive areas.
Figure below shows plot of concentration and temperature, and relationship between several definitions above.
References:
- Perry’s Chemical Engineers’ Handbook 8th Edition Section 23 Process Safety
- Introduction to Process Safety for Undergraduates and Engineers – A CCPS Concept Book
- Process Safety Calculations, Second Edition. Renato Benintendi