Circular
Letter
MSC/Circ.387
REVISED GUIDELINES FOR INERT GAS SYSTEMS
(03.07.2002)
Revised
guidelines for inert gas systems.
(MSC/Circ.353 dated 4 July 1983)
(This is a
revision of MSC/Circ.282 Guidelines for Inert Gas Systems)
(MSC/Circ.387
Revised guidelines for inert gas systems)
The
International Conference on Tanker Safety and Pollution Prevention held in
February 1978 passed resolution 5 recommending that the Inter-Governmental
Maritime Consultative Organization develop Guidelines to supplement the
requirements of amended regulation 62 of Chapter II-2 of the 1974 SOLAS
Convention* by taking into account the arduous operating conditions of inert
gas systems and the need to maintain them to a satisfactory standard. In
addition regulation 62.1 requires that an inert gas system shall be designed,
constructed and tested to the satisfaction of the Administration. These
Guidelines have accordingly been developed to supplement and complement the
Convention requirements for inert gas systems. They are offered to
Administrations to assist them in determining appropriate design and
constructional parameters and in formulating suitable operational procedures
when inert gas systems are installed in ships flying the flag of their State.
________________
* Any reference
to regulation 62 in these Guidelines means the new text of regulation 62 of
Chapter -2 of the 1983 SOLAS amendments, as adopted by the Maritime Safety
Committee at its forty-eighth session in June 1983.
1.2.1
The status of these Guidelines is advisory. They are intended to cover the
design and operation of:
.1 inert gas
systems that are required on new tankers by regulation 60 of Chapter II-2 of
the 1978 SOLAS Protocol and in accordance with regulation 62;
.2 inert gas
systems that are required on existing tankers by regulation 60 of Chapter II-2
of the 1978 SOLAS Protocol and in accordance with regulation 62.20;
.3 inert gas
systems which are fitted but not required to comply with the requirements of
regulation 60 of Chapter II-2 of the 1978 SOLAS Protocol.
1.2.2 However,
for existing inert gas systems the Guidelines are directed primarily at
operational procedures and are not intended to be interpreted as requiring
modifications to existing equipment other than those which are required on
ships to which regulation 62.20 applies.
1.2.3 The
content of these Guidelines is based on current general practice used in the
design and operation of inert gas systems using flue gas from the uptake from
the ship's main or auxiliary boilers, and installed on crude oil tankers and
combination carriers. The Guidelines do not exclude other sources of inert gas,
such as systems incorporating independent inert gas generators, other designs,
materials or operational procedures. All such divergences should be carefully
assessed to ensure that they achieve the objectives of these Guidelines.
1.3.1 'Inert
gas' means a gas or a mixture of gases, such as flue gas, containing
insufficient oxygen to support the combustion of hydrocarbons.
1.3.2 'Inert
condition' means a condition in which the oxygen content throughout the
atmosphere of a tank has been reduced to 8 per cent or less by volume by
addition of inert gas.
1.3.3 'Inert gas
plant' means all equipment specially fitted to supply, cool, clean, pressurize,
monitor and control delivery of inert gas to cargo tank systems.
1.3.4 'Inert gas
distribution system' means all piping, valves, and associated fittings to
distribute inert gas from the inert gas plant to cargo tanks, to vent gases to
atmosphere and to protect tanks against excessive pressure or vacuum.
1.3.5 'Inert gas
system' means an inert gas plant and inert gas distribution system together
with means for preventing backflow of cargo gases to the machinery spaces,
fixed and portable measuring instruments and control devices.
1.3.6 'Inerting'
means the introduction of inert gas into a tank with the object of attaining
the inert condition defined in 1.3.2.
1.3.7
'Gas-freeing' means the introduction of fresh air into a tank with the object
of removing toxic, flammable and inert gases and increasing the oxygen content
to 21 per cent by volume.
1.3.8 'Purging'
means the introduction of inert gas into a tank already in the inert condition
with the object of:
.1 further
reducing the existing oxygen content; and/or
.2 reducing the
existing hydrocarbon gas content to a level below which combustion cannot be
supported if air is subsequently introduced into the tank.
1.3.9
'Topping-up' means the introduction of inert gas into a tank which is already
in the inert condition with the object of raising the tank pressure to prevent
any ingress of air.
With
an inert gas system the protection against a tank explosion is achieved by
introducing inert gas into the tank to keep the oxygen content low and reduce
to safe proportions the hydrocarbon gas concentration of the tank atmosphere.
2.2.1 A mixture
of hydrocarbon gas and air cannot ignite unless its composition lies within a
range of gas in air concentrations known as the "flammable range".
The lower limit of this range, known as the lower flammable limit is any
hydrocarbon concentration below which there is insufficient hydrocarbon gas to
support combustion. The upper limit of the range, known as the "upper
flammable limit" is any hydrocarbon concentration above which there is
insufficient air to support combustion.
2.2.2 The
flammable limits vary somewhat for different pure hydrocarbon gases and for the
gas mixtures derived from different petroleum liquids. In practice, however,
the lower and upper flammable limits of oil cargoes carried in tankers can be
taken, for general purposes, to be 1 per cent and 10 per cent hydrocarbon by
volume, respectively.