9.5.3 Overflows Discharging Overboard
In general, overflow pipes discharging through the vessel’s sides are to be led above the freeboard deck and are to be fitted with a non-return valve (not to be of cast iron, see 4-6-2/9.13.2) at the shell.
Where the overflow discharging overboard cannot be led above the freeboard deck, the opening at the shell is to be protected against sea water ingress in accordance with the same provisions as that for overboard gravity drain from watertight spaces described in 4-6-4/3.3. In this connection, the vertical distance of the ‘inboard end’ from the summer load water line may be taken as the height from the summer load water line to the level that the sea water has to rise to find its way inboard through the overboard pipe.
Where, in accordance with the provisions of 4-6-4/3.3, a non-return valve with a positive means of closing is required, means is to be provided to prevent unauthorized operation of this valve. This may be a notice posted at the valve operator warning that it may be shut by authorized personnel only.
9.5.4 Overflow Common Header
Where overflows from tanks in more than one watertight subdivision are connected to a common header below the freeboard or bulkhead deck, the arrangement is to be such as to prevent fore-and-aft flooding from one watertight subdivision to another in the event of damage.