On many ships with RORO decks there are above-deck valves known as scuppers for drainage. The aim is that any water flowing onto the deck, for example when extinguishing a fire, is able to drain off. Scuppers can be closed using maneuverable valves, but in the event that these are not closed in time there are also non-return valves to prevent water from entering from the outside.
It is possible that poorly functioning non-return valves can accelerate the process of capsizing, such as when the Finnbirch sank two years ago. These non-return valves must be checked in accordance with rules for the class, but since the valves are often inaccessible these inspections can be difficult to carry out.
One method of checking the valves is to externally attach a plug with a hose connection. By applying water pressure to the non-return valve from the outside, any water that passes through is visible on the deck. If this is the case, the non-return valve is not functioning correctly.
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