• (a)inerting empty cargo tanks and slop tanks by reducing the oxygen content of the atmosphere in each tank to a level at which combustion cannot be supported; and
• (b)maintaining—
  • (i)the atmosphere in any part of any cargo tank or slop tank at an oxygen content not exceeding 8% by volume; and
  • (ii)a positive pressure at all times in port and at sea, except when it is necessary for such a tank to be gas-free; and
• (c)eliminating the need for air to enter a tank during normal operations, except when it is necessary for such a tank to be gas-free; and
• (d)purging empty cargo tanks and slop tanks of hydrocarbon gas, so that subsequent gas-freeing operations will at no time create a flammable atmosphere within the tank.

Inert gas delivery rate and content

The inert gas system shall be capable of delivering inert gas—

• (a)to the cargo tanks and slop tanks at a rate of at least 125% of the maximum rate of discharge capacity of the ship expressed as a volume; and
• (b)with an oxygen content of not more than 5% by volume in the inert gas supply main to the cargo tanks and slop tanks at any required rate.

The inert gas supply may be treated flue gas from the main or auxiliary boilers, from one or more separate gas generators or other sources or from any combination of sources. The Chief Executive Officer or a surveyor may accept systems using inert gases other than flue gas, provided that the Chief Executive Officer or surveyor is satisfied that an equivalent standard of safety is achieved. Systems using stored carbon dioxide are not permitted unless the Chief Executive Officer or a surveyor is satisfied that the risk of ignition from generation of static electricity by the system is acceptable.

Flue gas isolating valves shall be fitted in the inert gas supply mains between the boiler uptakes and the flue gas scrubber. These valves shall be provided with indicators to show whether they are open or shut, and precautions shall be taken to maintain them gastight and keep the seating clear of soot. Arrangements shall be made so that the boiler soot blowers cannot be operated when the corresponding flue gas valve is open.

• E7.5.1A flue gas scrubber shall be fitted which will—
  • (a)effectively cool the volume of gas specified in paragraph E7.2; and
  • (b)remove solids and sulphur combustion products.
• E7.5.2The cooling water arrangements shall be such that an adequate supply of water will always be available without interfering with any essential services on the ship. Provision shall also be made for an alternative supply of cooling water.
• E7.5.3Filters or equivalent devices shall be fitted to minimize the amount of water carried over to the inert gas blowers.
• E7.5.4The flue gas scrubber shall be located aft of all cargo tanks, slop tanks, cargo pump rooms, and cofferdams separating these spaces from machinery spaces.

• E7.6.1At least 2 inert gas blowers shall be fitted that together are capable of delivering to the cargo tanks and slop tanks at least the volume of gas specified in paragraph E7.2.
• E7.6.2In a system provided with a gas generator, the Chief Executive Officer or a surveyor may permit only one blower if the system is capable of delivering the total volume of gas required by paragraph E7.2 to the protected cargo tanks and slop tanks, provided that sufficient spares for the blower and its prime mover are carried on board to enable any failure of the blower and its prime mover to be rectified by the ship’s crew.
• E7.6.3Two fuel oil pumps shall be fitted to the inert gas generator, provided that the Chief Executive Officer or a surveyor may permit only one fuel oil pump to be fitted if sufficient spares for the fuel oil pump and its prime mover are carried on board to enable any failure of the fuel oil pump and its prime mover to be rectified by the ship’s crew.
• E7.6.4The inert gas system shall be so designed that the maximum pressure which it can exert on any cargo tank or slop tank will not exceed the test pressure of any cargo tank or slop tank. Shut-off arrangements shall be provided on the suction and discharge connections for each blower. Arrangements shall be made to enable the functioning of the inert gas plant to be stabilised before commencing cargo discharge. If the blowers are to be used for gas-freeing, their air inlets shall be provided with blanking arrangements.
• E7.6.5The blowers shall be located aft of all cargo tanks, slop tanks, cargo pump rooms, and cofferdams separating these spaces from machinery spaces.

• E7.7.1The design and location of the scrubber and blowers and their piping and fittings shall be such as to prevent flue gas leakages into enclosed spaces.
• E7.7.2To permit safe maintenance, an additional water seal or other effective means of preventing flue gas leakage shall be fitted between the flue gas isolating valves and scrubber, or incorporated in the gas entry to the scrubber.

• E7.8.1A gas regulating valve shall be fitted in the inert gas supply main.

  This valve shall be automatically controlled to close as required in subparagraphs E8.9.3 and E8.9.4. It shall also be capable of automatically regulating the flow of inert gas to the cargo tanks and slop tanks unless means are provided to automatically control the speed of the inert gas blowers required by paragraph E7.6.

• E7.8.2The gas regulating valve shall be located at the forward bulkhead of the most forward gas-safe space through which the inert gas supply main passes.

• E7.9.1At least 2 non-return devices, one of which shall be a water seal, shall be fitted in the inert gas supply main to prevent the return of hydrocarbon vapour to the machinery space uptakes or to any gas-safe spaces under normal conditions of trim, list and motion of the ship. These devices shall be located between the gas regulating valve required by paragraph E7.8 and the aftermost connection to any cargo tank, slop tank, or cargo pipeline. The non-return devices shall be located on the cargo area on deck.
• E7.9.2The water seal required by subparagraph E7.9.1 shall be capable of being supplied by 2 separate pumps, each of which shall be capable of maintaining an adequate supply at all times.
• E7.9.3The arrangement of the seal and its associated fittings shall be such that it will prevent back-flow of hydrocarbon vapours and will ensure the proper functioning of the seal under normal operating conditions.
• E7.9.4Provision shall be made to ensure that the water seal is protected against freezing, in such a way that the integrity of the seal is not impaired by overheating.
• E7.9.5A water loop or other arrangement approved by the Chief Executive Officer or a surveyor shall be fitted to all associated water supply piping, dry piping and all venting or pressure sensing piping leading to gas-safe spaces. Means shall be provided to prevent such loops from being emptied by vacuum.
• E7.9.6The deck water seal and all loop arrangements shall be capable of preventing the return of hydrocarbon vapours at a pressure equal to the test pressure of the cargo tanks.
• E7.9.7The second non-return device shall be a non-return valve or equivalent that—
  • (a)is capable of preventing the return of vapours or liquids or both;
  • (b)is fitted forward of the deck water seal required by subparagraph 7.9.1; and
  • (c)has—
    • (i)positive means of closure; or
    • (ii)an additional valve with positive means of closure that is located forward of the non-return device to isolate the deck water seal from the inert gas main to the cargo tanks and slop tanks.
• E7.9.8As an additional safeguard against the possible leakage of hydrocarbon liquids or vapours back from the deck main, means shall be provided to permit the section of the line between—
  • (a)the valve having positive means of closure required by subparagraph E7.9.7; and
  • (b)the gas regulating valve required by subparagraph E7.8.1, to be vented in a safe manner when the first of those valves is closed.