As a further precaution to avoid any backflow of gas from
the cargo tanks, and to prevent any backflow of liquid which may enter
the inert gas main if the cargo tanks are overfilled, regulation 62.10.8 requires a mechanical
nonreturn valve, or equivalent, which should be fitted forward of
the deck water seal and should operate automatically at all times.
This valve should be provided with a positive means of closure
or, alternatively, a separate deck isolating valve fitted forward
of the nonreturn valve, so that the inert gas deck main may be isolated
from the nonreturn devices. The separate isolating valve has the advantage
of enabling maintenance work to be carried out on the nonreturn valve.