Section 1 Cargo Containment System Atmosphere Control

1.1.1 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 minimise the possibility of pockets of gas or air remaining after changing the atmosphere.

1.1.2 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 utilising an inerting medium as an intermediate step.

1.1.3 Piping systems that may contain flammable cargoes shall comply with Pt 11, Ch 9, 1.1 Atmosphere control within the cargo containment system 1.1.1 and Pt 11, Ch 9, 1.1 Atmosphere control within the cargo containment system 1.1.2.

1.1.4 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 also Pt 11, Ch 5, 2.3 Cargo transfer arrangements 2.3.5.(e).

1.1.5 Inert gas utilised in these procedures is to be provided onboard the ship unit.

1.2.1 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.

1.2.2 Alternatively, subject to the restrictions specified in Pt 11, Ch 17 Special Requirements, the spaces referred to in Pt 11, Ch 9, 1.2 Atmosphere control within the hold spaces (cargo containment systems other than Type C independent tanks) 1.2.1 requiring only a partial secondary barrier may be filled with dry air provided that the ship unit 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.

1.2.3 For non flammable gases, the spaces referred to in Pt 11, Ch 9, 1.2 Atmosphere control within the hold spaces (cargo containment systems other than Type C independent tanks) 1.2.1 and Pt 11, Ch 9, 1.2 Atmosphere control within the hold spaces (cargo containment systems other than Type C independent tanks) 1.2.2 may be maintained with a suitable dry air or inert atmosphere.

1.3.1 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.

1.4.1 Inerting refers to the process of providing a non-combustible environment. Inert gases should 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 and be sufficiently low to alleviate the formation of ice or hydrates within the cargo tank or liquid pipework.

1.4.2 Where inert gas is also stored for fire-fighting purposes it shall be carried in separate containers and shall not be used for cargo services.

1.4.3 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 structure of the ship unit is not reduced below the limiting values imposed on it.

1.4.4 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.

1.4.5 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.

1.4.6 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.

1.4.7  Inert gas systems are to be so designed as to minimise the risk of ignition from the generation of static electricity by the system itself.

1.5.1 The equipment shall be capable of producing inert gas with an oxygen content at no time greater than 5 per cent by volume. 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 per cent oxygen content by volume.

1.5.2 An inert gas system shall have pressure controls and monitoring arrangements appropriate to the cargo containment system.

1.5.3 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.

1.5.4 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.