8.3.1 An open tank venting
system is a system which offers no restriction except for friction
losses to the free flow of cargo vapours to and from the cargo tanks
during normal operations. An open venting system may consist of individual
vents from each tank, or such individual vents may be combined into
a common header or headers, with due regard to cargo segregation.
In no case shall shutoff valves be fitted either to the individual
vents or to the header.
8.3.2 A controlled tank venting
system is a system in which pressure- and vacuum-relief valves or
pressure/vacuum valves are fitted to each tank to limit the pressure
or vacuum in the tank. A controlled venting system may consist of
individual vents from each tank or such individual vents on the pressure
side only as may be combined into a common header or headers, with
due regard to cargo segregation. In no case shall shut-off valves
be fitted either above or below pressure- or vacuum-relief valves
or pressure/vacuum valves. Provision may be made for bypassing a pressure-
or vacuum-relief valve or pressure/vacuum valve under certain operating
conditions provided that the requirement of 8.3.6 is maintained and that there is suitable indication to show
whether or not the valve is bypassed.
8.3.3 Controlled tank venting
systems shall consist of a primary and a secondary means of allowing
full flow relief of vapour to prevent over-pressure or under-pressure
in the event of failure of one means. Alternatively, the secondary
means may consist of pressure sensors fitted in each tank with a monitoring
system in the ship's cargo control room or position from which cargo
operations are normally carried out. Such monitoring equipment shall
also provide an alarm facility which is activated by detection of
over-pressure or under-pressure conditions within a tank.
8.3.4 The position of vent
outlets of a controlled tank venting system shall be arranged:
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.1 at a height of not less
than 6 m above the weather deck or above a raised walkway if fitted
within 4 m of the raised walkway; and
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.2 at a distance of at least
10 m measured horizontally from the nearest air intake or opening
to accommodation, service and machinery spaces and ignition sources.
8.3.5 The vent outlet height
referred to in 8.3.4.1 may be reduced
to 3 m above the deck or a raised walkway, as applicable, provided
that high-velocity venting valves of an approved type, directing the
vapour/air mixture upwards in an unimpeded jet with an exit velocity
of at least 30 m/s, are fitted.
8.3.6 Controlled tank venting
systems fitted to tanks to be used for cargoes having a flashpoint
not exceeding 60°C (closed-cup test) shall be provided with devices
to prevent the passage of flame into the cargo tanks. The design,
testing and locating of the devices shall comply with the requirements
of the Administration, which shall contain at least the standards
adopted by the Organization.
8.3.7 In designing venting
systems and in the selection of devices to prevent the passage of
flame for incorporation into the tank venting system, due attention
shall be paid to the possibility of the blockage of these systems
and fittings by, for example, the freezing of cargo vapour, polymer
build-up, atmospheric dust or icing up in adverse weather conditions.
In this context it shall be noted that flame arresters and flame screens
are more susceptible to blockage. Provisions shall be made such that
the system and fittings may be inspected, operationally checked, cleaned
or renewed as applicable.
8.3.8 Reference in 8.3.1 and 8.3.2 to
the use of shutoff valves in the venting lines shall be interpreted
to extend to all other means of stoppage, including spectacle blanks
and blank flanges.