2.2 Requirements for all systems
220.127.116.11 The inert gas system referred to in chapter II-2 of the Convention shall be designed, constructed and tested to the satisfaction of the Administration. It shall be designed to be capable of rendering and maintaining the atmosphere of the relevant cargo tanks non-flammable.1
18.104.22.168 The system shall be capable of:
- inerting empty cargo tanks and maintaining the atmosphere in any part of the tank with an oxygen content not exceeding 8% by volume and at a positive pressure in port and at sea except when it is necessary for such a tank to be gas-free;
- eliminating the need for air to enter a tank during normal operations except when it is neccesary for such a tank to be gas-free;
- purging empty cargo tanks of hydrocarbon or other flammable vapours, so that subsequent gas-freeing operations will at no time create a flammable atmosphere within the tank;
- delivering inert gas to the cargo tanks at a rate of at least 125% of the maximum rate of discharge capacity of the ship expressed as a volume. For chemical tankers and chemical product tankers, the Administration may accept inert gas systems having a lower delivery capacity provided that the maximum rate of discharge of cargoes from cargo tanks being protected by the system is restricted to not more than 80% of the inert gas capacity; and
- delivering inert gas with an oxygen content of not more than 5% by volume to the cargo tanks at any required rate of flow.
22.214.171.124 Materials used in inert gas systems shall be suitable for their intended purpose. In particular, those components which may be subjected to corrosive action of the gases and/or liquids are to be either constructed of corrosion-resistant material or lined with rubber, glass fibre epoxy resin or other equivalent coating material.
126.96.36.199 The inert gas supply may be:
- treated flue gas from main or auxiliary boilers, or
- gas from an oil or gas-fired gas generator, or
- gas from nitrogen generators
The Administration may accept systems using inert gases from one or more separate gas generators or other sources of any combination thereof, provided that an equivalent level of safety is achieved. Such systems shall, as far as practibable, comply with the requirements of this chapter. Systems using stored carbon dioxide shall not be permitted unless the Administration is satisfied that the risk of ignition from generation of static electricity by the system itself is minimized.
2.2.2 Safety measures
188.8.131.52 The inert gas system shall be so designed that the maximum pressure which it can exert on any cargo tank will not exceed the test pressure of any cargo tank.
184.108.40.206 Automatic shutdown of the inert gas system and its components parts shall be arranged on predetermined limits being reached, taking into account the provisions of paragraphs 2.2.4, 2.3.2 and 2.4.2.
220.127.116.11 Suitable shutoff arrangements shall be provided on the discharge outlet of each generator plant.
18.104.22.168 The system shall be designed to ensure that if the the oxygen content exceeds 5% by volume, the inert gas shall be automatically vented to atmosphere.
22.214.171.124 Arrangements shall be provided to enable the functioning of the inert gas plant to be stabilized before commencing cargo discharge. If blowers are to be used for gas-freeing, the air inlets shall be provided with blanking arrangements.
126.96.36.199 Where a double block and bleed valve is installed, the system shall ensure upon of loss of power, the block valves are automatically closed and the bleed valve is automatically open.
2.2.3 System components
188.8.131.52 Non-return devices
184.108.40.206.1 At least two non-return devices shall be fitted in order to prevent the return of vapour and liquid to the inert gas plant, or to any gas-safe spaces.
220.127.116.11.2 The first non-return device shall be a deck seal of the wet, semi-wet, or dry type or a double block and bleed arrangement. Two shut-off valves in series with a venting valve in between, may be accepted provided:
- the operation of the valve is automatically executed. Signal(s) for opening/closing is (are) to be taken from the process directly, e.g. inert gas flow or differential pressure; and
- alarm for faulty operation of the valves is provided, e.g. the operation status of "blower stop" and "supply valve(s) open" is an alarm condition.
18.104.22.168.3 The second non-return device shall be a non-return valve or equivalent capable of preventing the return of vapours and liquids and fitted between the deck water seal (or equivalent device) and the first connection from the inert gas main to a cargo tank. It shall be provided with positive means of closure. As an alternative to positive means of closure, an additional valve having such means of closure may be provided between the non-return valve and the first connection to the cargo tanks to isolate the deck water seal, or equivalent device, from the inert gas main to the cargo tanks.
22.214.171.124.4 A water seal, if fitted, shall be capable of being supplied by two separate pumps, each of which shall be capable of maintaining an adequate supply at all times. The audible and visual alarm on the low level of water in the water seal shall operate at all times.
126.96.36.199.5 The arrangement of the water seal, or equivalent devices, and its associated fittings shall be such that it will prevent backflow of vapours and liquids and will ensure the proper functioning of the seal under operating conditions.
188.8.131.52.6 Provision shall be made to ensure that the water seal is protected against freezing, in such a way that the integrity of seal is not impaired by overheating.
184.108.40.206.7 A water loop or other approved arrangement shall also be fitted to each associated water supply and drain pipe and each venting or pressure-sensing pipe leading to gas-safe spaces. Means shall be provided to prevent such loops from being emptied by vacuum.
220.127.116.11.8 Any water seal, or equivalent device, and loop arrangements shall be capable of preventing return of vapours and liquids to an inert gas plant at a pressure equal to the test pressure of the cargo tanks.
18.104.22.168.9 The non-return devices shall be located in the cargo area on deck.
22.214.171.124 Inert gas lines
126.96.36.199.1 The inert gas main may be divided into two or more branches forward of the non-return devices required by paragraph 188.8.131.52.
184.108.40.206.2 The inert gas main shall be fitted with branch piping leading to the cargo tank. Branch piping for inert gas shall be fitted with either stop valves or equivalent means of control for isolating each tank. Where stop valves are fitted, they shall be provided with locking arrangements. The control system shall provide unambiguous information of the operational status of such valves to at least the control panel required in paragraph 2.2.4.
220.127.116.11.3 Each cargo tank not being inerted shall be capable of being separated from the inert gas main by:
- removing spool-pieces, valves or other pipe sections, and blanking the pipe ends; or
- arrangement of two spectacle flanges in series with provisions for detecting leakage into the pipe between the two spectacle flanges; or
- equivalent arrangements to the satisfaction of the Administration, providing at least the same level of protection.
18.104.22.168.4 Means shall be provided to protect cargo tanks against the effect of overpressure or vacuum caused by thermal variations and/or cargo operations when the cargo tanks are isolated from the inert gas mains.
22.214.171.124.5 Piping systems shall be so designed as to prevent the accumulation of cargo or water in the pipelines under all normal conditions.
126.96.36.199.6 Arrangements shall be provided to enable the inert gas main to be connected to an external supply of inert gas. The arrangements shall consist of a 250 mm nominal pipe size bolted flange, isolated from the inert gas main by a valve and located forward of the non-return valve. The design of the flange should conform to the appropriate class in the standards adopted for the design of other external connections in the ship's cargo piping system.
188.8.131.52.7 If a connection is fitted between the inert gas main and the cargo piping system, arrangements shall be made to ensure an effective isolation having regard to the large pressure difference which may exist between the systems. This shall consist of two shutoff valves with an arrangement to vent the space between the valves in a safe manner or an arrangement consisting of a spool-piece with associated blanks.
184.108.40.206.8 The valve separating the inert gas main from the cargo main and which is on the cargo main side shall be a non-return valve with a positive means of closure.
220.127.116.11.9 Inert gas piping systems shall not pass through accommodation, service and control station spaces.
18.104.22.168.10 In combination carriers, the arrangement to isolate the slop tanks containing oil or oil residues from other tanks shall consist of blank flanges which will remain in position at all times when cargoes other than oil are being carried except as provided for in the relevant section of the guidelines developed by the Organization2.
2.2.4 Indicators and alarms
22.214.171.124 The operation status of the inert gas system shall be indicated in a control panel.
126.96.36.199 Instrumentation shall be fitted for continuously indicating and permanently recording, when inert gas is being supplied:
- the pressure of the inert gas mains forward of the non-return devices; and
- the oxygen content of the inert gas.
188.8.131.52 The indicating and recording devices shall be placed in the cargo control room where provided. But where no cargo control room is provided, they shall be placed in a position easily accessible to the officer in charge of cargo operations.
184.108.40.206 In addition, meters shall be fitted:
- in the navigating bridge to indicate at all times the pressure referred to in paragraph 220.127.116.11.1 and the pressure in the slop tanks of combination carriers, whenever those tanks are isolated from the inert gas main; and
- in the machinery control room or in the machinery space to indicate the oxygen content referred to in paragraph 18.104.22.168.2.
22.214.171.124 Audible and visual alarms
126.96.36.199.1 Audible and visual alarms shall be provided, based on the system designed, to indicate:
- oxygen content in excess of 5% by volume;
- failure of the power supply to the indicating devices as referred to in paragraph 188.8.131.52;
- gas pressure less than 100 mm water gauge. The alarm arrangement shall be such as to ensure that the pressure in slop tanks in combination carriers can be monitored at all times;
- high-gas pressure; and
- failure of the power supply to the automatic control system.
184.108.40.206.2 The alarms required in paragraphs 220.127.116.11.1.1, 18.104.22.168.1.3 and 22.214.171.124.1.5 shall be fitted in the machinery space and cargo control room, where provided, but in each case in such a position that they are immediately received by responsible members of the crew.
126.96.36.199.3 An audible alarm system independent of that required in paragraph 188.8.131.52.1.3 or automatic shutdown of cargo pumps shall be provided to operate on predetermined limits of low pressure in the inert gas main being reached.
184.108.40.206.4 Two oxygen sensors shall be positioned at appropriate locations in the space or spaces containing the inert gas system. If the oxygen level falls below 19%, these sensors shall trigger alarms, which shall be both visible and audible inside and outside the space or spaces and shall be placed in such a position that they are immediately received by responsible members of the crew.
2.2.5 Instruction manuals
Detailed instruction manuals shall be provided on board, covering the operations, safety and maintenance requirements and occupational health hazards relevant to the inert gas system and its application to the cargo tank system.3 The manuals shall include guidance on procedures to be followed in the event of a fault or failure of the inert gas system.
1 Refer to Revised standards for the design, testing and locating of devices to prevent the passage of flame into cargo tanks in tankers (MSC/Circ.677, as amended by MSC/Circ.1009 and MSC.1/Circ.1324) and Revised factors to be taken into consideration when designing cargo tank venting and gas-freeing arrangements (MSC/Circ.731).
2 Refer to Revised Guidelines for inert gas systems (MSC/Circ.353, as amended by MSC/Circ.387).
3 Refer to Revised Guidelines for inert gas systems (MSC/Circ.353, as amended by MSC/Circ.387).
2.3 Requirements for flue gas and inert gas generator systems
In addition to the provisions in paragraph 2.2, for inert gas systems using flue gas or inert gas generators, the provisions of this section shall apply.
2.3.1 System requirements
220.127.116.11 Inert gas generators
18.104.22.168.1 Two fuel oil pumps shall be fitted to the inert gas generator. Suitable fuel in sufficient quantity shall be provided for the inert gas generators.
22.214.171.124.2 The inert gas generators shall be located outside the cargo tank area. Spaces containing inert gas generators shall have no direct access to accommodation service or control station spaces, but may be located in machinery spaces. If they are not located in machinery spaces, such a compartment shall be separated by a gastight steel bulkhead and/or deck from accommodation, service and control station spaces. Adequate positive-pressure-type mechanical ventilation shall be provided for such a compartment.
126.96.36.199 Gas regulating valves
188.8.131.52.1 A gas regulating valve shall be fitted in the inert gas main. This valve shall be automatically controlled to close, as required in paragraph 184.108.40.206. It shall also be capable of automatically regulating the flow of inert gas to the cargo tanks unless means are provided to automatically control the inert gas flow rate.
220.127.116.11.2 The gas regulating valve shall be located at the forward bulkhead of the forward most gas-safe space through which the inert gas main passes.
18.104.22.168 Cooling and scrubbing arrangement
22.214.171.124.1 Means shall be fitted which will effectively cool the volume of gas specified in paragraph 126.96.36.199 and remove solids and sulphur combustion products. The 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.
188.8.131.52.2 Filters or equivalent devices shall be fitted to minimize the amount of water carried over to the inert gas blowers.
184.108.40.206.1 At least two inert gas blowers shall be fitted and be capable of delivering to the cargo tanks at least the volume of gas required by paragraph 220.127.116.11. For systems fitted with inert gas generators the Administration may permit only one blower if that system is capable of delivering the total volume of gas required by paragraph 18.104.22.168 to the cargo 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.
22.214.171.124.2 Where inert gas generators are served by positive displacement blowers, a pressure relief device shall be provided to prevent excess pressure being developed on the discharge side of the blower.
126.96.36.199.3 When two blowers are provided, the total required capacity of the inert gas system shall be divided evenly between the two and in no case is one blower to have a capacity less than 1/3 of the total required.
188.8.131.52 Inert gas isolating valves
For systems using flue gas, flue gas isolating valves shall be fitted in the inert gas 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 seatings clear of soot. Arrangements shall be made to ensure that boiler soot blowers cannot be operated when the corresponding flue gas valve is open.
184.108.40.206 Prevention of flue gas leakage
220.127.116.11.1 Special consideration shall be given to the design and location of scrubber and blowers with relevant piping and fittings in order to prevent flue gas leakages into enclosed spaces.
18.104.22.168.2 To 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.
2.3.2 Indicators and alarms
22.214.171.124 In addition to the requirements in paragraph 126.96.36.199, means shall be provided for continuously indicating the temperature of the inert gas at the discharge side of the system, whenever it is operating.
188.8.131.52 In addition to the requirements of paragraph 184.108.40.206, audible and visual alarms shall be provided to indicate:
- insufficient fuel oil supply to the oil-fired inert gas generator;
- failure of the power supply to the generator;
- low water pressure or low water flow rate to the cooling and scrubbing arrangement;
- high water level in the cooling and scrubbing arrangement;
- high gas temperature;
- failure of the inert gas blowers; and
- low water level in the water seal.
2.4 Requirements for nitrogen generator systems
In addition to the provisions in paragraph 2.2, for inert gas systems using nitrogen generators, the provisions of this section shall apply.
2.4.1 System requirements
220.127.116.11 The system shall be provided with one or more compressors to generate enough positive pressure to be capable of delivering the total volume of gas required by paragraph 18.104.22.168.
22.214.171.124 A feed air treatment system shall be fitted to remove free water, particles and traces of oil from the compressed air.
126.96.36.199 The air compressor and nitrogen generator may be installed in the engine-room or in a separate compartment. A separate compartment and any installed equipment shall be treated as an "Other machinery space" with respect to fire protection. Where a separate compartment is provided for the nitrogen generator, the compartment shall be fitted with an independent mechanical extraction ventilation system providing six air changes per hour. The compartment is to have no direct access to accommodation spaces, service spaces and control stations.
188.8.131.52 Where a nitrogen receiver or a buffer tank is installed, it may be installed in a dedicated compartment, in a separate compartment containing the air compressor and the generator, in the engine room, or in the cargo area. Where the nitrogen receiver or a buffer tank is installed in an enclosed space, the access shall be arranged only from the open deck and the access door shall open outwards. Adequate, independent mechanical ventilation, of the extraction type, shall be provided for such a compartment.
2.4.2 Indicators and alarms
184.108.40.206 In addition to the requirements in paragraph 220.127.116.11, instrumentation is to be provided for continuously indicating the temperature and pressure of air at the suction side of the nitrogen generator.
18.104.22.168 In addition to the requirements in paragraph 22.214.171.124, audible and visual alarms shall be provided to include:
- failure of the electric heater, if fitted;
- low feed-air pressure or flow from the compressor;
- high-air temperature; and
- high condensate level at automatic drain of water separator.