8.4.1 Cargo tanks designed
to withstand a maximum external pressure differential exceeding 0.25
bar and capable of withstanding the maximum external pressure differential
which can be attained at maximum discharge rates with no vapour return
into the cargo tanks, or by operation of a cargo refrigeration system,
need no vacuum relief protection.
8.4.2 Cargo tanks designed
to withstand a maximum external pressure differential not exceeding
0.25 bar, or tanks which cannot withstand the maximum external pressure
differential that can be attained at maximum discharge rates with
no vapour return into the cargo tanks, or by operation of a cargo
refrigeration system, or by sending boil-off vapour to the machinery
spaces, should be fitted with:
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.1 two independent pressure switches to sequentially
alarm and subsequently stop all suction of cargo liquid or vapour from the cargo
tank, and refrigeration equipment if fitted, by suitable means at a pressure
sufficiently below the maximum external designed pressure differential of the
cargo tank; or
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.2 vacuum relief valves with a gas flow capacity at
least equal to the maximum cargo discharge rate per cargo tank, set to open at a
pressure sufficiently below the external design differential pressure of the cargo
tank; or
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.3 other vacuum protection systems acceptable to the
Administration.
8.4.3 Subject to the requirements
of chapter 17, the vacuum relief valves
should admit an inert gas, cargo vapour or air to the cargo tank and
should be arranged to minimize the possibility of the entrance of
water or snow. If cargo vapour is admitted, it should be from a source
other than the cargo vapour lines.
8.4.4 The vacuum protection
system should be capable of being tested to ensure that it operates
at the prescribed pressure.