These amendments have been consolidated into FSS Code Chapter
15
Chapter 15 Inert Gas Systems
The text of existing chapter 15 is replaced by the following:
This chapter details the specifications for inert gas systems
as required by chapter II-2 of the
Convention.
2
Engineering specifications
For the purposes of this chapter:
2.1.1
Cargo tanks means those cargo
tanks, including slop tanks, which carry cargoes, or cargo residues,
having a flashpoint not exceeding 60ēC.
2.1.2
Inert gas system includes inert
gas systems using flue gas, inert gas generators, and nitrogen generators
and means the inert gas plant and inert gas distribution together
with means for preventing backflow of cargo gases to machinery spaces,
fixed and portable measuring instruments and control devices.
2.1.3
Gas-safe space is a space in
which the entry of gases would produce hazards with regard to flammability
or toxicity.
2.1.4
Gas-free is a condition in
a tank where the content of hydrocarbon or other flammable vapour
is less than 1% of the lower flammable limit (LFL), the oxygen content
is at least 21%, and no toxic gases are presentfootnote.
2.2
Requirements for all systems
2.2.1.1 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-flammablefootnote.
2.2.1.2 The system shall be capable of:
-
.1 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;
-
.2 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;
-
.3 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;
-
.4 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
-
.5 delivering inert gas with an oxygen content
of not more than 5% by volume to the cargo tanks at any required rate
of flow.
2.2.1.3 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.
2.2.1.4 The inert gas supply may be:
-
.1 treated flue gas from main or auxiliary boilers,
or
-
.2 gas from an oil or gas-fired gas generator,
or
-
.3 gas from nitrogen generators.
The Administration may accept systems using inert gases
from one or more separate gas generators or other sources or any combination
thereof, provided that an equivalent level of safety is achieved.
Such systems shall, as far as practicable, 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.
2.2.2.1 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.
2.2.2.2 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.
2.2.2.3 Suitable shutoff arrangements shall be
provided on the discharge outlet of each generator plant.
2.2.2.4 The system shall be designed to ensure
that if the oxygen content exceeds 5% by volume, the inert gas shall
be automatically vented to atmosphere.
2.2.2.5 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, their
air inlets shall be provided with blanking arrangements.
2.2.2.6 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.
2.2.3.1
Non-return devices
2.2.3.1.1 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.
2.2.3.1.2 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:
-
.1 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
-
.2 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.
2.2.3.1.3 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.
2.2.3.1.4 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.
2.2.3.1.5 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.
2.2.3.1.6 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.
2.2.3.1.7 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.
2.2.3.1.8 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.
2.2.3.1.9 The non-return devices shall be located
in the cargo area on deck.
2.2.3.2.1 The inert gas main may be divided into
two or more branches forward of the non-return devices required by
paragraph 2.2.3.1.
2.2.3.2.2 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.
2.2.3.2.3 Each cargo tank not being inerted shall
be capable of being separated from the inert gas main by:
-
.1 removing spool-pieces, valves or other pipe
sections, and blanking the pipe ends; or
-
.2 arrangement of two spectacle flanges in series
with provisions for detecting leakage into the pipe between the two
spectacle flanges; or
-
.3 equivalent arrangements to the satisfaction
of the Administration, providing at least the same level of protection.
2.2.3.2.4 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.
2.2.3.2.5 Piping systems shall be so designed
as to prevent the accumulation of cargo or water in the pipelines
under all normal conditions.
2.2.3.2.6 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.
2.2.3.2.7 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.
2.2.3.2.8 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.
2.2.3.2.9 Inert gas piping systems shall not pass
through accommodation, service and control station spaces.
2.2.3.2.10 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 Organizationfootnote.
2.2.4
Indicators and
alarms
2.2.4.1 The operation status of the inert gas
system shall be indicated in a control panel.
2.2.4.2 Instrumentation shall be fitted for continuously
indicating and permanently recording, when inert gas is being supplied:
2.2.4.3 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.
2.2.4.4 In addition, meters shall be fitted:
-
.1 in the navigating bridge to indicate at all
times the pressure referred to in paragraph 2.2.4.2.1 and the pressure
in the slop tanks of combination carriers, whenever those tanks are
isolated from the inert gas main; and
-
.2 in the machinery control room or in the machinery
space to indicate the oxygen content referred to in paragraph 2.2.4.2.2.
2.2.4.5
Audible and visual alarms
2.2.4.5.1 Audible and visual alarms shall be provided,
based on the system designed, to indicate
-
.1 oxygen content in excess of 5% by volume;
-
.2 failure of the power supply to the indicating
devices as referred to in paragraph 2.2.4.2;
-
.3 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;
-
.4 high-gas pressure; and
-
.5 failure of the power supply to the automatic
control system.
2.2.4.5.2 The alarms required in paragraphs 2.2.4.5.1.1,
2.2.4.5.1.3 and 2.2.4.5.1.5 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.
2.2.4.5.3 An audible alarm system independent
of that required in paragraph 2.2.4.5.1.3 or automatic shutdown of
cargo pumps shall be provided to operate on predetermined limits of
low pressure in the inert gas main being reached.
2.2.4.5.4 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 systemfootnoteThe manuals
shall include guidance on procedures to be followed in the event of
a fault or failure of the inert gas system.
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
2.3.1.1
Inert gas generators
2.3.1.1.1 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.
2.3.1.1.2 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.
2.3.1.2
Gas regulating valves
2.3.1.2.1 A gas regulating valve shall be fitted
in the inert gas main. This valve shall be automatically controlled
to close, as required in paragraph 2.2.2.2. 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.
2.2.1.2.2 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.
2.3.1.3
Cooling and scrubbing arrangement
2.3.1.3.1 Means shall be fitted which will effectively
cool the volume of gas specified in paragraph 2.2.1.2 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.
2.3.1.3.2 Filters or equivalent devices shall
be fitted to minimize the amount of water carried over to the inert
gas blowers.
2.3.1.4.1 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 2.2.1.2. 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 2.2.1.2 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.
2.3.1.4.2 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.
2.3.1.4.3 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
2.3.1.5
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.
2.3.1.6
Prevention of flue gas leakage
2.3.1.6.1 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.
2.3.1.6.2 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
2.3.2.1 In addition to the requirements in paragraph
2.2.4.2, means shall be provided for continuously indicating the temperature
of the inert gas at the discharge side of the system, whenever it
is operating.
2.3.2.2 In addition to the requirements of paragraph
2.2.4.5, audible and visual alarms shall be provided to indicate:
-
.1 insufficient fuel oil supply to the oil-fired
inert gas generator;
-
.2 failure of the power supply to the generator;
-
.3 low water pressure or low water flow rate to
the cooling and scrubbing arrangement;
-
.4 high water level in the cooling and scrubbing
arrangement;
-
.5 high gas temperature;
-
.6 failure of the inert gas blowers; and
-
.7 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
2.4.1.1 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 2.2.1.2.
2.4.1.2 A feed air treatment system shall be fitted
to remove free water, particles and traces of oil from the compressed
air.
2.4.1.3 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.
2.4.1.4 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
2.4.2.1 In addition to the requirements in paragraph
2.2.4.2, instrumentation is to be provided for continuously indicating
the temperature and pressure of air at the suction side of the nitrogen
generator.
2.4.2.2 In addition to the requirements in paragraph
2.2.4.5, audible and visual alarms shall be provided to include:
-
.1 failure of the electric heater, if fitted;
-
.2 low feed-air pressure or flow from the compressor;
-
.3 high-air temperature; and
-
.4 high condensate level at automatic drain of
water separator."