The Swedish Club details the steps to deal with flammable liquid cargo.
Flammable liquid cargo
Flammable liquid cargoes present risks of explosions in cargo tanks and other compartments. These explosions are
often followed by fire. Sometimes fire and/or explosions propagate to other tanks.
In that case, to investigate the cause it is necessary to work back to the origin tank by plotting the direction of the first displacement of bulkheads between tanks. At the first tank, all of the bulkheads will most probably have been displaced outwards initially.
Inert gas system
In many tankers the basis of safety is avoiding flammable/explosive mixtures by using inert gas (IG). In this case it is obviously essential to ensure that the IG system is working properly
and is used correctly.
In other tankers, usually smaller ones, there may be no IG facility. There are periods of higher risk such as when discharging a volatile flammable cargo and air is drawn into the tanks, when the resulting mixture of residual cargo vapour and air may be flammable/ explosive.
Instruments, pumps and other relevant items need to be kept in good condition to avoid producing ignition sources. Procedures such as cleaning should be carried out strictly in accordance with the relevant guidance to avoid producing ignition sources.
The International Safety Guide for Oil Tankers and Terminals (ISGOTT) provides detailed guidance on how tanker operations should be carried out.
Before maintenance or repair work is started, all relevant spaces need to be properly gas-freed and all applicable safety procedures complied with. Explosions have occurred because residual sludge or other material in tanks has evolved flammable vapour after gas-freeing, so that an explosive mixture has developed later.
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Source: The Swedish Club