Page 82 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

2.7.2 Nature of Hazard
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Residual fuel oils are capable of producing light hydrocarbons in the tank
headspace, such that the vapour composition may be near to or within the
flammable range. This can occur even when the storage temperature is well
below the measured flashpoint. This is not normally a function of the origin or
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manufacturing process of the fuel, although fuels containing cracked residues
may show a greater tendency to generate light hydrocarbons.

Although light hydrocarbons may be present in the headspaces of residual fuel
oil tanks, the risk associated with them is small unless the atmosphere is within
the flammable range and an ignition source is present. In such a case, an
incident could result. It is therefore recommended that residual fuel oil
headspaces be regarded as being potentially flammable.

2.7.3 Flashpoint and Headspace Flammability Measurement

2.7.3.1 Flashpoint
Fuel oils are classified for their safety in storage, handling and
transportation by reference to their closed cup flashpoint (see also
Section 1.2.5). However, information on the relationship between the
calculated flammability of a headspace atmosphere and the measured
flashpoint of the residual fuel oil has shown that there is no fixed
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correlation. A flammable atmosphere can therefore be produced in a tank
headspace even when a residual fuel oil is stored at a temperature below
its flashpoint.

2.7.3.2 Headspace Flammability

Traditionally, gas detectors such as explosimeters have been used to
check that enclosed spaces are gas free, and they are entirely suited to
this purpose (see Section 2.4.3). They have also been used to measure
the “flammability” of headspaces in terms of percentage of the Lower
Flammable Limit (LFL). Such detectors rely on a calibration carried out
normally on a single hydrocarbon, such as methane, which may have LFL
characteristics that are far removed from the hydrocarbons actually
present in the headspace. When using an explosimeter to assess the
degree of hazard in non-inerted residual fuel oil tank headspaces, it is
recommended that the instrument is calibrated with a pentane/air or
hexane/air mixture. This will result in a more conservative estimate of the
flammability, but the readings should still not be regarded as providing a
precise measurement of the vapour space condition.

When taking measurements, the manufacturer’s operating instructions for
the instrument should be closely followed and the instrument’s calibration
should be checked frequently as oxidation catalyst detectors (pellistors)
are likely to be susceptible to poisoning when exposed to residual fuel oil
vapours. For information on poisoning of pellistors, see Section 2.4.3.2.

In view of the problems associated with obtaining accurate
measurements of the flammability of residual fuel tank headspaces using
readily available portable equipment, the measured % LFL only ranks
fuels broadly in terms of relative hazard. Care should be exercised
therefore in interpretation of the figures obtained by such gas detectors.

48 © ICS/OCIMF/IAPH 2006