Section 9 Inert gas systems

9.1.1 Where an inert gas system is fitted, the arrangements are to comply with the following requirements and, as far as applicable, with the requirements of Pt 5, Ch 15, 7 Inert gas systems on Tankers of 8,000 tonnes DWT and above of the Rules and Regulations for the Classification of Ships. Consideration will be given to special cases where the arrangements are equivalent to those required by the Rules.

9.1.2 The space containing the Inert Gas generator is to be provided with visible and audible alarms for oxygen concentration. The sensors are to be located in positions where potentially dangerous concentrations may be readily detected.

9.1.3 The permeate outlet is to be led to a safe position as far as possible from any source of ignition. The pipe is of sufficient diameter in order to achieve very low velocities of the oxygen.

9.1.4 Spaces containing inert gas generating plants should have no direct access to accommodation spaces, service spaces or control stations, but may be located in machinery spaces.

9.1.5 Inert gas piping should not pass through accommodation spaces, service spaces or control stations.

9.1.6 When not in use, the inert gas system should be made separate from the cargo system in the cargo area except for connections to the hold spaces or interbarrier spaces.

9.1.7 Flame burning equipment for generating inert gas will be specially considered.

9.2.1 One or more pressure-vacuum breaking devices are to be provided to prevent the cargo tanks from being subject to:

1. a positive pressure in excess of the test pressure of the cargo tank if the cargo were to be loaded at the maximum rated capacity and all other outlets were left shut; and

2. a negative pressure in excess of 5 kPa if cargo were to be discharged at the maximum rated capacity of the cargo pumps and the inert gas blowers were to fail.

Such devices shall be installed on the inert gas main unless they are installed in the venting system required by Pt 5, Ch 13, 5 Cargo tank venting arrangements or on individual cargo tanks.

9.2.2 The set pressure of the vacuum valve required by Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.3.(e) is to be 3.5 kPa.

9.2.3 The inert gas system is to be capable to maintain a pressure, under all conditions of service, of not less than 7 kPa in the tanks to be inerted.

9.2.4 Sufficient quantities of inert gas shall be available on board to compensate the losses during voyage.

9.2.5 Portable instruments for measuring oxygen and flammable vapour concentration are to be provided. In addition, suitable arrangement is to be made on each cargo tank such that the condition of the atmosphere can be determined using these portable instruments.

9.2.6 An Inert gas main is to be provided. The inert gas main may be divided into two or more branches forward of the deck main isolation valve.

9.2.7 The inert gas discharge may be connected with the vapour return system for ships carrying cargoes for which inert gas is not mandatory as per the ADN, Table C, Column 20, additional requirements (see Pt 5, Ch 13, 1.3 Materials 1.3.9 for reference). Two means of isolation as per Pt 5, Ch 13, 9.2 Type C tankers and Type N-closed tankers 9.2.8 or Pt 5, Ch 13, 9.2 Type C tankers and Type N-closed tankers 9.2.9 are to be provided in the connection to the vapour return line. In addition a removable spool piece is to be provided on the cargo tank side of the connection. A notice is to be provided located in a prominent position adjacent to the means of isolation, clearly indicating that the spool piece is to be removed and blanking flanges are to be fitted, when the inert gas system is not in use. The removable spool piece is to be clearly identified (labelled/painted in a distinctive colour) and stowed close to its working position.

9.2.8 At least two non-return devices, one of which shall be a water seal, shall be fitted in the inert gas supply main or in the inert gas discharge connection to the vapour return system as applicable, in order to prevent the return of hydrocarbon vapour to any gas-safe spaces under all normal conditions of trim and list. They shall be located between the automatic delivery valve fitted to the forward bulkhead of the forward most gas-safe space and the aftermost connection to any cargo tank.

9.2.9 If a water seal, as required by Pt 5, Ch 13, 9.2 Type C tankers and Type N-closed tankers 9.2.8, is impracticable, it may be substituted by a double block and bleed system. Details of this system are to be submitted for approval.

9.3.1 The inert gases should be compatible chemically and operationally, at all temperatures likely to occur within the spaces to be inerted, with the materials of construction of the cargo.

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

9.3.3 Arrangements suitable for the cargo carried should be provided to prevent the backflow of cargo vapour into the inert gas system.

9.3.4 The arrangements should be such that each space being inerted can be isolated and the necessary controls and relief valves, etc. should be provided for controlling pressure in these spaces.

9.3.5 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.

9.3.6 The equipment should 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 should be fitted to the inert gas supply from the equipment and should be fitted with an alarm set at a maximum of 5 per cent oxygen content by volume. The above is subject to the special requirements of Table C of Part 3 of the ADN where a lower maximum oxygen content may be specified for specific cargoes (see Pt 5, Ch 13, 1.3 Materials 1.3.9 for reference).

9.3.7 An inert gas system should have pressure controls and monitoring arrangements appropriate to the cargo containment system. Acceptable means, located in the cargo area, of preventing the backflow of cargo gas should be provided.

9.3.8 If the inert gas plants are located in machinery spaces or other spaces outside the cargo area, two nonreturn valves, or equivalent devices should be fitted in the inert gas main in the cargo area as required in Pt 5, Ch 13, 9.3 Type G tankers 9.3.7.