What type of valves are fitted to double bottom or water ballast tanks and why? How
would you proceed to pump out a double bottom tank? Why vents fitted? Why are sounding
pipes fitted and where?
Double bottom and water ballast tanks are fitted with screw lift valves. In these cases screw lift
valves are used in order to pump in and out thus the same valve. Once opened they stay opened
until closed manually. To empty a tank, the appropriate valve for the tank on the distribution
chest in the engine room is opened, next the tank suction valve to pump distribution chest is
opened. The discharge overboard valve on the discharge chest and ships side is opened and the
pump started up. All other valves should be shut. To fill a tank, open appropriate tank valve in
distribution chest in engine room. Open sea suction valve and close overboard discharge. Open
discharge valve to main tank line on discharge chest and start the pump. Air pipes (vents) are
fitted at the forward end of each tank. This end is the highest point of the tank while under
normal trim, therefore all air will be expelled, otherwise air pockets might be formed which
would result in damage being done to the tank by movement of water, also the ship is inclined
to list more readily. Sounding pipes are fitted at the aft end of each tank, usually port and
starboard this end being the lower end under normal trim. Sounding pipes are fitted so the level
of the liquid in the tanks can be ensured. These pipes may also serve as vent pipes when filling
tanks, but it should be noted that it is still essential to also have vent pipes.
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