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Any cargo transfers conducted during the loaded passage should conclude with the instruction to 'close all valves'. This is particularly important with multi-grade cargoes. Most cargo contaminations result from a valve inadvertently left open after a previous cargo operation.
In other parts of the tanker cycle, open valves may contribute to slop tank overflows, ballast contaminations, discovery of ROB in tanks which had previously been stripped and pumproom bilges filled with gasoline.
The best way to prevent this type of incident is to conclude each cargo operation with the instruction to 'close all valves'.
Every vessel has a number of valves which must remain closed at all times during parts of the tanker's operating cycle. These valves should be sealed and a red-letter, standing order issued that the chief officer is the only person on board authorised to break a valve seal.
Other critical valves which must remain closed for the current operation should be marked in some way as a reminder to the cargo watch officer and the pumpman. After operating any of these valves, the cargo watch officer should log their setting in the deck log, indicating the time they were opened/closed.
On a tanker with manually controlled valves 'closing all valves' at the end of one operation, only to turn around and open many of the same valves for the next operation seems like a great waste of effort and time. Nevertheless, it is the one foolproof way of ensuring that a valve is not left open by mistake.

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