An acceptable equivalent to one automatic non-return valve
with a positive means of closing from a position above the freeboard
deck would be one automatic non-return valve and one sluice valve
controlled from above the freeboard deck. Where two automatic non-return
valves are required, the inboard valve should always be accessible
under service conditions, i.e. the inboard valve should be above the
level of the tropical load waterline. If this is not practicable,
then, provided a locally controlled sluice valve is interposed between
the two automatic non-return valves, the inboard valve need not be
fitted above the load waterline.
Where sanitary discharges and scuppers lead overboard through
the shell in way of manned machinery spaces, the fitting to shell
of a locally operated position closing valve together with a non-return
valve inboard, is considered to provide protection equivalent to the
requirements of regulation 22(1).
The requirements of regulation 22(1) for
non-return valves should be applicable only to those discharges which
remain open during the normal operation of a ship. For discharges
which must necessarily be closed at sea, such as gravity drains from
topside ballast tanks, a single screw-down valve operated from the
deck is considered to provide efficient protection.