To prevent the return of cargo gases or cargo from the tanks
to the machinery spaces and boiler uptake, it is essential that an
effective barrier is always present between these two areas. In addition
to a nonreturn valve, a water seal and vent should be fitted on the
deck main. It is of prime importance that these devices are properly
maintained and correctly operated at all times.
An additional water seal is sometimes fitted at the bottom of
the scrubber (see also 3.9.6).