Page 125 - International safety guide for oil tankers and terminals

SHIPBOARD SYSTEMS

velocity to penetrate to the bottom of the tank. To ensure this, a limit must be
placed on the number of tanks that can be inerted simultaneously. Where this
---ใช้เพื่อการศึกษาเท่านั้น---
limit is not clearly stipulated in the operations manual, only one tank should be
inerted or purged at a time when using the dilution method.

งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Displacement depends on the fact that inert gas is slightly lighter than
hydrocarbon gas so that, while the inert gas enters at the top of the tank, the
heavier hydrocarbon gas escapes from the bottom through suitable piping.
When using this method, it is important that the inert gas has a very low
velocity to enable a stable horizontal interface to be developed between the
incoming and escaping gas. However, in practice, some dilution inevitably takes
place owing to the turbulence caused in the inert gas flow. Displacement
generally allows several tanks to be inerted or purged simultaneously.

Whichever method is employed, and whether inerting or purging (see
Definitions), it is vital that oxygen or gas measurements are taken at several
heights and horizontal positions within the tank to check the efficiency of the
operation. A mixture of inert gas and petroleum gas, when vented and mixed
with air, can become flammable. The normal safety precautions taken when
petroleum gas is vented from a tank therefore should not be relaxed.

7.1.5 Cargo Tank Atmosphere Control

non-flammable condition at all times. It follows that: ึ
7.1.5.1 Inert Gas Operations
Tankers using an inert gas system should maintain their cargo tanks in a

Tanks should be kept in an inert condition at all times, except when it
is necessary for them to be gas free for inspection or work, i.e. the
oxygen content should be not more than 8% by volume and the
atmosphere should be maintained at a positive pressure.
The atmosphere within the tank should make the transition from the
inert condition to the gas free condition without passing through the
flammable condition. In practice, this means that, before any tank is
gas freed, it should be purged with inert gas until the hydrocarbon
content of the tank atmosphere is below the critical dilution line (line
GA in Figure 1.1).
When a ship is in a gas free condition before arrival at a loading port,
the tanks must be inerted prior to loading.

In order to maintain cargo tanks in a non-flammable condition, the inert
gas plant will be required to:
Inert empty cargo tanks (see Section 7.1.6.1).

Be in operation, or be ready for immediate operation, during cargo
discharge, deballasting, crude oil washing and tank cleaning (see
Sections 7.1.6.6. and 7.1.6.9).
Purge tanks prior to gas freeing (see Section 7.1.6.10).
Top up the pressure in the cargo tanks when necessary during other
stages of the voyage (see Sections 7.1.6.5 and 7.1.6.7).

It must be emphasised that the protection provided by an inert gas
system depends on the proper operation and maintenance of the entire
system.

© ICS/OCIMF/IAPH 2006 91