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At one time or another, most refineries experience spontaneous ignition of iron sulfide either on the ground or inside equipment. When this occurs inside equipment like columns, vessels, and tanksand exchangers containing residual hydrocarbons and air, the results can be devastating. Most commonly, pyrophoric iron fires occur during shutdowns when equipment and piping are opened for inspectionor maintenance. Instances of fires in crude columns during turnarounds, explosions in sulfur, crude or asphalt storage tanks, overpressures in vessels, etc., due to pyrophoric iron ignition are notuncommon.
Often the cause of such accidents is a lack of understanding of the phenomenon of pyrophoric iron fires. This article aims to explain the basics of pyrophoric iron fires and to provide ideasfor developing safe practices for handing over equipment for inspection and maintenance.
What is Pyrophoric Iron Oxidation?
The word "pyrophoric" is derived from the Greek for "fire-bearing". According to Webster's dictionary, "pyrophoric material" means "any material igniting spontaneously or burning spontaneously in air when rubbed, scratched, or struck, e.g. finely divided metals".
Ironsulfide is one such pyrophoric material that oxidizes exothermically when exposed to air.  It is frequently found in solid iron sulfide
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scales in refinery units.  It makes no difference whetherthese pyrophoric sulfides exist as pyrite, troilite, marcasite, or pyrrhotite.  It is formed by the conversion of iron oxide (rust) into iron sulfide in an oxygen-free atmosphere where hydrogen sulfidegas is present (or where the concentration of hydrogen sulfide (H2S) exceeds that of oxygen).  The individual crystals of pyrophoric iron sulfides are extremely finely divided, the result of which isthat they have an enormous surface area-to-volume ratio.
When the iron sulfide crystal is subsequently exposed to air, it is oxidized back to iron oxide and either free sulfur or sulfur dioxide gas...