Code L1,5BN:

L = Type of tank:

Tank for substances in the liquid state (liquids or solids handed over for carriage in the molten state).
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1,5 = Calculation pressure:

Minimum calculation pressure: 1,5 bar (see 6.8.2.1.14)
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B = Openings (see 6.8.2.2.2):

Tank with bottom-filling or bottom-discharge openings with 3 closures
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N = Safety valves/devices:

Tank without a breather device according to 6.8.2.2.6 and not hermetically closed.