Goal
To enable monitoring of the integrity of the containment system and to ensure
that the atmosphere within the system and hold spaces is maintained in a safe
condition at all times that the ship is in service.
9.1 Atmosphere control within the cargo containment
system
9.1.1 A piping system shall be arranged to enable each
cargo tank to be safely gas-freed, and to be safely filled with cargo vapour from a
gas-free condition. The system shall be arranged to minimize the possibility of
pockets of gas or air remaining after changing the atmosphere.
9.1.2 For flammable cargoes, the system shall be
designed to eliminate the possibility of a flammable mixture existing in the cargo
tank during any part of the atmosphere change operation by utilizing an inerting
medium as an intermediate step.
9.1.3 Piping systems that may contain flammable cargoes
shall comply with 9.1.1 and 9.1.2.
9.1.4 A sufficient number of gas sampling points shall
be provided for each cargo tank and cargo piping system to adequately monitor the
progress of atmosphere change. Gas sampling connections shall be fitted with a
single valve above the main deck, sealed with a suitable cap or blank (see
5.6.5.5).
9.1.5 Inert gas utilized in these procedures may be
provided from the shore or from the ship.
9.2 Atmosphere control within the hold spaces (cargo
containment systems other than type C independent tanks)
9.2.1 Interbarrier and hold spaces associated with
cargo containment systems for flammable gases requiring full or partial secondary
barriers shall be inerted with a suitable dry inert gas and kept inerted with
make-up gas provided by a shipboard inert gas generation system, or by shipboard
storage, which shall be sufficient for normal consumption for at least 30 days.
9.2.2 Alternatively, subject to the restrictions
specified in chapter 17, the spaces referred to in 9.2.1 requiring only a partial
secondary barrier may be filled with dry air provided that the ship maintains a
stored charge of inert gas or is fitted with an inert gas generation system
sufficient to inert the largest of these spaces, and provided that the configuration
of the spaces and the relevant vapour detection systems, together with the
capability of the inerting arrangements, ensures that any leakage from the cargo
tanks will be rapidly detected and inerting effected before a dangerous condition
can develop. Equipment for the provision of sufficient dry air of suitable quality
to satisfy the expected demand shall be provided.
9.2.3 For non-flammable gases, the spaces referred to
in 9.2.1 and 9.2.2 may be maintained with a suitable dry air or inert
atmosphere.
9.3 Environmental control of spaces surrounding type
C independent tanks
Spaces surrounding cargo tanks that do not have secondary barriers shall
be filled with suitable dry inert gas or dry air and be maintained in this condition
with make-up inert gas provided by a shipboard inert gas generation system,
shipboard storage of inert gas, or with dry air provided by suitable air drying
equipment. If the cargo is carried at ambient temperature, the requirement for dry
air or inert gas is not applicable.
9.4 Inerting
9.4.1 Inerting refers to the process of providing a
non-combustible environment. Inert gases shall be compatible chemically and
operationally at all temperatures likely to occur within the spaces and the cargo.
The dew points of the gases shall be taken into consideration.
9.4.2 Where inert gas is also stored for firefighting
purposes, it shall be carried in separate containers and shall not be used for cargo
services.
9.4.3 Where inert gas is stored at temperatures below
0°C, either as a liquid or as a vapour, the storage and supply system shall be
designed so that the temperature of the ship's structure is not reduced below the
limiting values imposed on it.
9.4.4 Arrangements to prevent the backflow of cargo
vapour into the inert gas system that are suitable for the cargo carried, shall be
provided. If such plants are located in machinery spaces or other spaces outside the
cargo area, two non-return valves or equivalent devices and, in addition, a
removable spool piece shall be fitted in the inert gas main in the cargo area. When
not in use, the inert gas system shall be made separate from the cargo system in the
cargo area except for connections to the hold spaces or interbarrier spaces.
9.4.5 The arrangements shall be such that each space
being inerted can be isolated and the necessary controls and relief valves, etc.,
shall be provided for controlling pressure in these spaces.
9.4.6 Where insulation spaces are continually supplied
with an inert gas as part of a leak detection system, means shall be provided to
monitor the quantity of gas being supplied to individual spaces.
9.5 Inert gas production on board
9.5.1 The equipment shall be capable of producing inert
gas with an oxygen content at no time greater than 5% by volume, subject to the
special requirements of chapter 17. A continuous-reading oxygen content meter shall
be fitted to the inert gas supply from the equipment and shall be fitted with an
alarm set at a maximum of 5% oxygen content by volume, subject to the requirements
of chapter 17.
9.5.2 An inert gas system shall have pressure controls
and monitoring arrangements appropriate to the cargo containment system.
9.5.3 Spaces containing inert gas generation plants
shall have no direct access to accommodation spaces, service spaces or control
stations, but may be located in machinery spaces. Inert gas piping shall not pass
through accommodation spaces, service spaces or control stations.
9.5.4 Combustion equipment for generating inert gas
shall not be located within the cargo area. Special consideration may be given to
the location of inert gas generating equipment using a catalytic combustion
process.