The code specifies an inert gas system. Nitrogen sounds like the obvious choice because it eliminates the oxidizing agent – oxygen.
But iron sulfide (FeS) is created in the absence of oxygen – anaerobic conditions – which would be found in unvented tanks or tanks swept with nitrogen. The iron sulfide is fine in the “dormant” state when covered, but when tanks are opened to atmosphere it can spontaneously catch on fire (it’s pyrophoric). Since sulfur is also combustible, there is potential risk.
So you weigh the risk factor when using nitrogen. Do the benefits offset the safety issues caused by the formation of the pyrophoric iron sulfide.
Would anyone care to share experience when using nitrogen or other gas?
-Paul