In fire and explosion prevention engineering, inerting refers to the introduction of an
inert (non-combustible) gas into a closed system (e.g. a container or a process vessel) to make a flammable atmosphere oxygen deficient and non-ignitable.
[NFPA 69. Standard on Explosion Prevention Systems. National Fire Protection Association.][NFPA 77. Recommended Practice on Static Electricity. National Fire Protection Association.]
Inerting relies on the principle that a combustible (or flammable) gas is able to undergo combustion (explode) only if mixed with air in the right proportions. The
flammability limits
Mixtures of dispersed combustible materials (such as gaseous or vaporised fuels, and some dusts) and oxygen in the air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to a ...
of the gas define those proportions, i.e. the ignitable range. In combustion engineering terms, the admission of inert gas can be said to dilute the oxygen below the
limiting oxygen concentration
The limiting oxygen concentration (LOC), also known as the minimum oxygen concentration (MOC), is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel. It is expressed in ...
.
Inerting differs from
purging. Purging, by definition, ensures that an ignitable mixture ''never forms''. Inerting makes an ignitable mixture safe by introduction of an inert gas.
Certain inert gases are unsuitable for inerting
Because the mixture by definition is ignitable before inerting commence, it is imperative that the inerting procedure does not introduce a potential source of ignition, or an explosion will occur.
NFPA 77 states
[ that carbon dioxide from high-pressure cylinders or fire extinguishers should never be used to inert a container or vessel. The release of carbon dioxide may generate static electricity with enough energy to ignite the mixture, resulting in an explosion.][
] The release of {{CO2 for fire fighting purposes has led to several accidental explosions of which the 1954 Bitburg explosion may be the most devastating.
Other unsafe processes that may generate static electricity include pneumatic transport of solids, a release of pressurized gas with solids, industrial vacuum cleaners, and spray painting operations.[TRBS 2153. Vermeidung von Zündgefahren infolge elektrostatischer Aufladungen. revention of electrostatic ignition sources. Gemeinsame Minist. 15/16 (2009) 278.]
Other uses
The term ''inerting'' is often loosely used for any application involving an inert gas
An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...
, not conforming with the technical definitions in NFPA standards. For example, marine tankers carrying low-flash products like crude oil, naphtha
Naphtha ( or ) is a flammable liquid hydrocarbon mixture.
Mixtures labelled ''naphtha'' have been produced from natural gas condensates, petroleum distillates, and the distillation of coal tar and peat. In different industries and regions ' ...
, or gasoline
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...
have inerting systems on board. During the voyage, the vapor pressure of these liquids is so high, that the atmosphere above the liquid (the headspace) is too rich to burn, the atmosphere is unignitable. This may change during unloading. When a certain volume of liquid is drawn from a tank, a similar volume of air will enter the tank's headspace, potentially creating an ignitable atmosphere.
The inerting systems use an inert gas generator
Inert gas generator (IGG) refers to machinery on board marine product tankers. Inert gas generators consist distinctively of a gas producer and a scrubbing system.
Process
Diesel is burned using atmospheric air in a combustion chamber and the ex ...
to supply inert make-up gas instead of air. This procedure is often referred to as ''inerting''. Technically, the procedure ensures that the atmosphere in the tank's headspace remains unignitable. The gas mixture in the headspace is not ''inert'' per se, it's just unignitable. Because of its content of flammable vapors, it will burn if mixed with air. Only if enough inert gas is supplied as part of a '' purge-out-of-service'' procedure, will it be unable to burn when mixed with air.
See also
* ATEX
The ATEX directives are two EU directives describing the minimum safety requirements for workplaces and equipment used in explosive atmospheres. The name is an initialization of the French term ''Appareils destinés à être utilisés en ATm ...
* Flammability limits
Mixtures of dispersed combustible materials (such as gaseous or vaporised fuels, and some dusts) and oxygen in the air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to a ...
* Limiting oxygen concentration
The limiting oxygen concentration (LOC), also known as the minimum oxygen concentration (MOC), is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel. It is expressed in ...
* Purging (gas)
In fire and explosion prevention engineering, purging refers to the introduction of an inert (i.e. non-combustible) purge gas into a closed system (e.g. a container or a process vessel) to prevent the formation of an ignitable atmosphere. Purging ...
* 1954 Bitburg explosion (jet fuel storage tank)
* The 1966 explosion of the naphtha tanker MV Alva Cape
External links
* Fighting Smoldering Fires in Silos – A Cautionary Note on Using Carbon Dioxide. Guest post at
www.mydustexplosionresearch.com blog
Nov 27, 2017
References
Explosion protection
Fire
Safety