Page 140 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

7.3.3 Loading Rates
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Masters should be provided with information on maximum permissible loading
rates for each cargo and ballast tank and, where tanks have a combined
venting system, for each group of cargo or ballast tanks. This requirement is
aimed at ensuring that tanks are not over or under-pressurised by exceeding
งสมุด ศูนย์ฝกพาณิชย์นาวี
the capacity of the venting system, including any installed secondary venting
arrangements.

Other considerations will also need to be taken into account when determining
maximum loading rates for oil tankers. Precautions against static electricity
hazards and pipeline erosion are described in Section 7.3.3.2.

7.3.3.1 Venting Arrangements

Venting capacity is based on the maximum volume of cargo entering a
tank plus a 25% margin to account for gas evolution (vapour growth).

When loading cargoes having a very high vapour pressure, gas evolution
may be excessive and the allowance of 25% may prove to be insufficient.
Actions to consider in order to ensure that the capacity of the venting
system is not exceeded include a close monitoring of vapour line
pressures on inerted ships and limiting loading rates on non-inerted ships
throughout the loading period or during crude oil washing during
discharge operations. It should be noted that the vapour growth increases
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when the liquid levels in the tank are above 80%. On inerted ships, close
attention should be given to monitoring inert gas system pressures,
particularly when topping-off during loading operations or on commencing
crude oil washing during discharge operations.

When calculating loading rates, a maximum venting line velocity of
36 metres per second should be considered. This flow rate should be
calculated for each diameter of line used. The volume throughputs may
be aggregated where a common vent riser is used, but the maximum flow
rate should not be exceeded anywhere within the system.

7.3.3.2 Flow Rates in Loading Lines
Depending upon the trade of the tanker, a number of loading rates need
to be determined for each cargo tank. These loading rates will be
dependent on the maximum flow rates in the cargo lines for different
งานห้อ A loading rate based on a linear velocity of 1 metre/second at the tank
products and loading operations. In general, the following flow rates may
need to be calculated for each section of the cargo system.

inlet for the initial loading rate for static accumulator cargoes into
non-inerted tanks.
A loading rate based on a linear velocity of 7 metres/second for bulk
loading static accumulator cargoes into non-inerted tanks.

A loading rate based on a linear velocity of 12 metres/second for
loading non-static accumulator cargoes and also for loading static
accumulator cargoes into inerted tanks. This velocity is provided for
guidance only and is generally considered as a rate above which
pipeline erosion may occur at pipe joints and bends.

Where a number of tanks are loaded through a common manifold, the
maximum loading rate may be determined by the flow rate through the
manifold or drop lines. For this reason, it is important that a constant
check is kept on the number of cargo tank valves that are open

106 © ICS/OCIMF/IAPH 2006