Page 52 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

2.3.7 Mercaptans
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Mercaptans are colourless, odorous gases generated naturally by the
degradation of natural organisms. Their smell has been likened to rotting
cabbage. Mercaptans may occur on ships where sea water has remained
beneath an oil cargo or where oil residues are left in tanks that contain water,
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such as in a dirty ballast tank after it has been incompletely drained. They are
also found in water treatment plants and ballast treatment facilities.

Mercaptans are also present in the vapours of pentane plus cargoes and in
some crude oils. They are also used as an odorising agent in natural gas.

Mercaptans can be detected by smell at concentrations below 0.5 ppm,
although health effects are not experienced until the concentration is several
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times higher than this.

The initial effects of mercaptans on people are similar to those caused by H 2
exposure, i.e. irritation to the lungs, eyes, nose and throat. If the concentration
is very high, unconsciousness may occur and it may be necessary to
administer oxygen.

2.3.8 Gasolines Containing Tetraethyl Lead (TEL) or Tetramethyl
Lead (TML)
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The amounts of Tetraethyl Lead (TEL) or Tetramethyl Lead (TML) normally
added to gasolines are insufficient to render the gases from these products
significantly more toxic than those from unleaded gasolines. The effects of the
gases from leaded gasolines are therefore similar to those described for
petroleum gases (see Section 2.3.3).

2.3.9 Inert Gas

2.3.9.1 General
Inert gas is principally used to control cargo tank atmospheres, thus
preventing the formation of flammable mixtures. The primary requirement
for an inert gas is low oxygen content. Its composition can, however, be
variable. (Table 7.1 in Section 7.1.3 provides an indication of typical inert
gas components expressed as a percentage by volume.)

2.3.9.2 Toxic Constituents
The main hazard associated with inert gas is its low oxygen content.
However, inert gas produced by combustion, either in a steam raising
boiler or in a separate inert gas generator (flue gas), will contain trace
amounts of various toxic gases that may increase the hazard to
personnel exposed to it.

Precautions necessary to protect personnel against the toxic components
of inert gas during tank entry are given in Section 7.1.6.12. However,
these precautions do not include requirements for the direct measurement
of the concentration of the trace constituents of flue gas. This is because
gas freeing the atmosphere of a cargo tank from a hydrocarbon gas
concentration of about 2% by volume to 1% LFL, and until a steady 21%
by volume oxygen reading is obtained, is sufficient to dilute these toxic
constituents to below their TLV-TWA.

18 © ICS/OCIMF/IAPH 2006