Ballast system is provided on ships for:
- To ensure the stability of the ship in all loading conditions of the ship.
- To adjust the centre of gravity of the ship in all loading conditions.
- To adjust the trim by adding ballast water weight in the required tank and d) to ensure distribution of load, shear forces and bending moments due to self-weight and cargo are withing designed limitations.
- To ensure distribution of load, shear forces and bending moments due to self-weight and cargo are within designed limitations.
The ballast pumps are generally centrifugal pumps. The pumps take suction from the sea chest or the cross sea suction main connecting sea chests. The ships are provided normally with 2 ballast pumps.
The sea water is pumped into tanks from the main line running through pipe tunnel or duct keel (on big ships) or through separate pipelines for individual tanks. The individual branch lines are provided with remotely operated pneumatic or hydraulic valves. The de-ballasting or emptying the tanks is also carried out by same pipe system and hence valves should be of a type which allows reverse flow. Butterfly valves are frequently employed for this service in all modern ships and the ballast operation is carried out and controlled by a ballast control panel. This ballast control valve operations centralised and will carry a mimic panel for indicating the tanks which are being ballasted or de-ballasted and for indicating whether a valve is in open or closed position.
Where the pipelines of one ballast tank pass through other ballast tanks or through oil tanks extra precautions are to be taken by providing heavy gauge pipes for withstanding corrosion for a longer time. In such cases flange joints are avoided for pipelines and only sleeve joints are provided.
The top side wing ballast tanks are filled by ballast main which is run on the open deck with individual valves provided for each tank. Normally the topside tanks are pumped out by opening the shipside discharge valves provided for each tank with operation provided from the open deck.
The galvanised steel pipes are used for the ballast system. Class approved synthetic/plastic pipes are also used for the ballast system. The velocity of water ( where galvanised steel pipes are used) is normally 3m/sec.
The ballast tanks are prone to high level of corrosion and hence when ships are built special care is taken to thoroughly clean the tank surfaces before application of paint system.