Drain lines from toilets, bathrooms, galley, etc.
The requirements as to the number and type of valves and their closing appliances are dependent on the lowest inboard opening of each discharge system in relation to the summer water line. The inboard opening (inboard end) is the lowest point where water could enter and flood the vessel if back-flooding was to occur due to valve failure in the line. This will normally be water closet, washbasin, drain from the showers, etc.