Managing the Threat of Gas and Vapor Explosions

Managing the Threat of Gas and Vapor Explosions

Loss-of-containment incidents pose one of the greatest process safety risks to people working in industrial settings. This is because gases and vapors can easily form ignitable mixtures with the very air we breathe, leading to several hazardous conditions, including flash fires, pool fires, jet fires, vapor cloud explosions, and under certain conditions, even detonations.

Managing this threat requires an understanding of where, and under what conditions, exposure exists and taking the necessary steps to control it. Whether you store, handle, or process flammable/combustible materials, it is necessary that you have all of the critical data needed to assess risk at both ambient and process conditions.

Some of the properties that process safety leaders would need to evaluate the risks and assure the safety of the people and operations involving flammable gases or liquids include:

  • Vapor Pressure – The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. Vapor pressure is an indication of a liquid's evaporation rate. A substance with a high vapor pressure at normal temperatures is often referred to as volatile.
  • Limiting Oxygen Concentration (LOC) – This is the minimum concentration of oxidant (oxygen) that is capable of supporting combustion. This data can be used to study explosion prevention (elimination of flammable atmosphere by oxidant depletion or decreasing ignition sensitivity of the flammable atmosphere) or explosion severity reduction involving the use of inert gases.
  • Flash Point – The flash point of a liquid is the lowest temperature at which sufficient vapor is evolved to form a flammable mixture in air at standard atmospheric pressure. The flash point provides a simple, convenient index for assessing the flammability of a wide variety of materials.
  • Auto-ignition Temperature – This is the lowest temperature at which a material will spontaneously ignite in the absence of an external ignition source, such as a spark or flame. The auto-ignition temperature can be used to specify operating, storage, and handling procedures for a material.
  • Explosion Severity (Maximum Explosion Pressure, Pmax, and maximum rate of pressure rise, Kg) –Pmax and Kg are explosive properties measured in the laboratory to quantify the severity of a gas/vapor cloud explosion. The Pmax and Kg data can then be used to design explosion protection measures for process equipment and buildings.
  • Flammability Diagram – Under various conditions of temperature and pressure, liquids will evolve a certain quantity of vapors. For pure substances, the vapors evolved in air at atmospheric pressure may be flammable over a certain range of concentrations defined by the upper and lower flammability limits. If the oxygen (oxidant) concentration in air is reduced the range of flammability is generally reduced. The effect of oxygen (oxidant) concentration on the flammability properties is best represented by plotting a three-component flammability diagram showing the effect of oxygen concentration, and inert gas (N2) at atmospheric on the flammability of the vapors or gases.

The table below shows the effect of increasing ↑ and decreasing ↓ pressure, temperature, and oxygen concentration on Flash Point and Limits of Flammability.

flash point and limits of flammability

As can be appreciated, it is important to determine the flammability of fuels/atmospheres under process conditions in order to properly assess the hazards and take measures for ensuring the safety of the people, the facility, and the community.

Our accredited state-of-the-art flammability laboratories can provide testing according to the American Society for Testing and Materials (ASTM) and the United Nations/Department of Transportation for all process conditions and scales. To learn more, please visit our page on Process Safety Lab Testing.

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Friday, 25 September 2020