where water tanks refer to freshwater and sea water tanks; oil tanks refer to fuel oil, lubricating oil, hydraulic oil and other oil tanks. Small water or oil tanks of less than 1 m3 (36 ft3) may have vent pipe of 38 mm (1.5 in.) minimum internal diameter.
9.3.3(b) Vent sizing. Where a separate overflow is not fitted, the aggregate area of the vent pipe(s) provided for the tank is to be at least 125% of the effective area of the filling line.
Where overflow pipe(s) are fitted, and the aggregate area of the overflow pipes is at least 125% of the effective area of the filling line, in such cases, vent pipes need not exceed the minimum sizes in 4-6-4/9.3.3(a).
Where high capacity or high head pumps are used, calculations demonstrating the adequacy of the vent and overflow to prevent over- or underpressurization of the ballast tanks are to be submitted. See also 4-6-4/9.5.2.