Page 136 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

Before opening the IG system, it should, if possible, be gas freed and any
enclosed space in which the system is opened up should be ventilated to avoid
---ใช้เพื่อการศึกษาเท่านั้น---
any risk of oxygen deficiency.

Continuous positive ventilation must be maintained before and during the work.
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7.2 Venting Systems

7.2.1 General
Venting systems are required to meet the requirements of SOLAS. They are
necessary for achieving safety on board a tanker and it is essential that they
are operated to meet their design intent and that they are properly maintained.
To facilitate dilution of the hydrocarbon vapours into the atmosphere clear of
the tanker’s deck, venting systems allow vapours to be released either:
At a low velocity, high above the deck from a vent riser; or

At high velocity from a high velocity valve closer to the deck. This facilitates
dilution of the hydrocarbon vapours in the atmosphere clear of the tanker’s
deck.
Vents are sited in selected locations to prevent the accumulation of a
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flammable atmosphere on the tank deck or around any accommodation or
engine room housings (see Section 2.5.4).
Ship’s personnel should be fully conversant with the operation and
maintenance of all components of the venting system and should be aware of
its limitations in order to prevent over or under-pressurisation of the tank(s) the
system is serving (see Section 7.2.2 below).

7.2.2 Tank Over-Pressurisation and Under-Pressurisation

7.2.2.1 General

Over-pressurisation of cargo and ballast tanks is due to compression of
the ullage space by the inadequate release of vapour or by the overfilling
of the tank. Under-pressurisation can be caused by not allowing inert gas
vapour or air into the tank when liquid is being discharged. The resulting
over or under-pressure in the tank may result in serious deformation or
catastrophic failure of the tank structure and its peripheral bulkheads,
which can seriously affect the structural integrity of the ship and could
lead to fire, explosion and pollution. (See also Section 7.1.8.)

Structural damage can also be caused by not allowing inert gas, vapour
or air into a tank whilst liquid is being discharged. The resulting
under-pressure in the tank can result in deformation of the ship’s
structure, which could result in fire, explosion or pollution.

To guard against over and under-pressurisation of tanks, owners/
operators should give serious consideration to fitting protection
devices as follows:

Individual pressure sensors with an alarm for each tank.
Individual full flow pressure/release devices for each tank.

102 © ICS/OCIMF/IAPH 2006