Page 75 - International safety guide for oil tankers and terminals

HAZARDS OF PETROLEUM

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Wind
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X X X X X X

Figure 2.4 – Typical pattern of airflow around an accommodation block
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to the denser combined plume, the flammable zone is in close contact
with the top of the accommodation block.

2.5.5 Minimising Hazards from Vented Gas

The objective of venting arrangements and their operational control is to
minimise the possibilities of flammable gas concentrations entering enclosed
spaces containing sources of ignition, or reaching deck areas where,
notwithstanding all other precautions, there might be a source of ignition. In
previous Sections, means have been described of promoting rapid dispersion
of gas and minimising its tendency to sink to the deck. Although this Section is
concerned with flammability, the same principles apply to dispersion of gas
down to concentrations that are safe to personnel.

The following conditions are required by SOLAS for any operation where
flammable mixtures are displaced to the atmosphere or where mixtures are
displaced which could become flammable on dilution with air, such as on
inerted ships:
An unimpeded vertical discharge at a high efflux velocity.
Positioning the outlet sufficiently high above the deck.

Placing the outlet at an adequate distance from the superstructure and
other enclosed spaces.

When using a vent outlet of fixed diameter, usually designed for 125% of the
maximum cargo loading rate, the efflux velocity will drop at lower loading rates.
Vent outlets with automatically variable areas (high velocity vent valves) may
be fitted to maintain a high efflux velocity under all loading conditions. The
permitted height of the outlet above deck is dependent on whether venting is
by a mast riser or through a high velocity vent valve.

© ICS/OCIMF/IAPH 2006 41