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Asphalting with nitrogen – case study Valero refinery, USA

US Chemical Safety Board

CSB is an independent US agency that investigates chemical accidents in various industries.
The agency does not have the role to impose fines, but to make recommendations to factories involved in accidents, to the institutions that regulate the industry, environmental and occupational health agencies, in order to prevent other possible accidents and to improve the practices at work, its reports being public.

In November 2005, at the Valero Refinery in Delaware, USA, two employees of a subcontractor died due to inhaled nitrogen during a maintenance operation (connecting a pipeline to a pressure vessel while it was purged with nitrogen).
In an attempt to recover a fallen object in the vessel, the first employee fanned inside the vessel and died, and the second died asphyxiated as he tried to save his colleague.


Every year, workers are killed by breathing “air” that contains too little oxygen. Because 78% of the air we breathe is nitrogen, it is assumed that nitrogen is not dangerous. However, nitrogen is breathable when mixed with sufficient oxygen.

Neither nitrogen nor oxygen can be detected by smell alone. In a closed enclosure, an atmosphere enriched with nitrogen, an oxygen-empty atmosphere, can only be detected by special measuring instruments.
If the concentration of nitrogen is high or low in oxygen, the human body is deprived of oxygen and asphyxiation occurs.

  • Warnings and recommendations in the CSB Report are valid in any nitrogen application in a closed space and possible exposed people:a low-oxygen atmosphere in a dark space may be deadly with some inspiration;
  • access to a low-oxygen enclosure should not be allowed without equipment providing additional oxygen;
  • warnings and access restrictions on indoor hazard must be permanently maintained;
  • the human instinct to jump on someone in need (especially a friend or colleague) caused too many casualties in the case of closed casualties;
  • saving or attempting to save must be done with special equipment in order not to endanger their own lives.

Additional information about the danger of nitrogen asphyxiation can be found in the CSB Safety Bulletin.

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