Oil Cargoes With High H2S Content Last updated 12-Aug-2014

Hydrogen Sulfide (H2S) is a very toxic, corrosive and flammable gas. It has a very low odour threshold and a distinctive odour of rotten eggs.
International Safety Guide for Oil Tankers and Terminals, 5th edt.,ICS, OCIMF, IAPH, section 2.3.6 (ISGOTT).

H2S Purging?

By definition, purging on tankers is an operation of introduction of inert gas into a tank already in the inert condition with the object of further reducing the existing oxygen content and/or reducing the existing hydrocarbon gas content to a level below which combustion cannot be supported if air is subsequently introduced into the tank.

Definitions

Purging. The introduction of inert gas into a tank already in the inert condition with the object of further reducing the existing oxygen content and/or reducing the existing hydrocarbon gas content to a level below which combustion cannot be supported if air is subsequently introduced into the tank.

Introduction of air in ship’s tanks usually required when it is necessary to gas-free empty from cargo tanks, i.e. to remove toxic, flammable and inert gases and increase the oxygen content to 21% by volume after washing. Due to combustible nature of hydrocarbon gas its mixture with air can be ignited when hydrocarbon content in air is more than 2% by volume. To permanently (emphasis mine) reduce the hydrocarbon content to 2% or less and to ensure that, during the subsequent gas freeing operation, no portion of the tank atmosphere is brought within the flammable range the tanks should first be purged with inert gas.

Thus, in simple words purging is an operation of permanent replacement of hydrocarbon gas in empty cargo tanks by inert gas to reduce risk of combustion and subsequent ventilation with air.

Quasi purging which is usually proposed by the charterers is a kind of ventilation of very small empty volumes of loaded tanks to temporary bring H2S content down. Introduction of inert gas into loaded tanks does not displace or dilute (as in case of purging described above) H2S, but rather blows out vapours of H2S. So far as speed of interchange of gases in tank or tanks exceeds speed of H2S vaporisation, content of H2S decreases, but H2S content rises again as soon as this process stopped or suspended.

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