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Bilge and ballast systems for general cargo ships

Arrangements of bilge main:

The bilge system is used to remove small quantities of fluid that have leaked or condensed into a dry space. The system serves the machinery spaces, cargo holds, cofferdams, voids, stores, tunnels and pump rooms. Each space has its own piping but the pump is likely to be shared.

The bilge main is arranged to drain any watertight compartment other than ballast, oil or water tanks and to discharge the contents overboard. The number of pumps and their capacity depend upon the size, type and service of the vessel. All bilge suctions must be fitted with suitable strainers, which in the machinery space would be mud boxes positioned at floorplate level for easy access. A vertical drop pipe would lead down to the bilge.

The emergency bilge suction or bilge injection valve is used to prevent flooding of the ship. It is a direct suction from the machinery space bilge which is connected to the largest capacity pump or pumps. An emergency bilge pump is required for passenger ships but may also be fitted as an extra on cargo ships. It must be a completely independent unit capable of operating even if submerged.

Bilge and ballast systems are interconnected so that each can perform the other's function in an emergency, ie a ballast pump could be used to pump out a flooded engine room. They are connected by means of a cross­over valve.

A centrifugal pump with a priming device is usually used, driven by an electric motor housed in an air bell. The power supply is arranged from the emergency generator. A typical system is shown in Figure . The various pumps and lines are interconnected to some extent so that each pump can act as an alternative or standby for another.

The capacity of a bilge system is defined by the diameter of the bilge main and pump capacity for the volume of the enclosed space. In passenger and cargo ships where the engine room provides bilge pumping, the whole ship is the ‘enclosed space’.
The diameter of the bilge main is:
d = 25+1.68vL(B+D)
where, d = internal diameter of bilge main, in millimetres
L = length between the ship’s perpendiculars, in metres
B = extreme breadth, in metres
D = moulded depth, in metres

In a tanker with a separate cargo pumping and piping system, the ‘enclosed space’ is the engine room and the diameter of the bilge main is:
d = 35+3vLo (B+D)
Lo = length of the engine room, in metres

Cargo ships are required to have two bilge pumps with non-return valves fitted to prevent back-flow or cross-flow.

The pumping system in a passenger ship must be able to drain water from any dry space when one or more of the ship’s other compartments are flooded. However, the system is not required to empty the flooded space. A flooded passenger ship is required to have at least one bilge pump, with its own power supply, available for pumping. Bilge suctions must have remotely operated suction valves. The minimum number of pumps required is three or four, depending on the ship’s design.

Mud boxes and strum boxes (line filters) are fitted at the ends and in bilge lines to stop debris being sucked into the pipe. The requirements for bilge systems on ships carrying dangerous goods are basically the same as for cargo ships. However, systems drawing fluids from gas-dangerous spaces are kept segregated with their own pumps and pipes, where appropriate, from systems serving gas-safe spaces.

Find out more on bilge and ballast system sketches

The ballast system

The ballast system is arranged to ensure that water can be drawn from any tank or the sea and discharged to any other tank or the sea as required to trim the vessel. Combined or separate mains for suction and discharge may be provided. Where a tank or cargo space can be used for ballast or dry cargo then either a ballast or bilge connection will be required. The system must therefore be arranged so that only the appropriate pipeline is in service; the other must be securely blanked or closed off. Where tanks are arranged for either oil or ballast a change-over chest must be Fitted in the pipeline so that only the ballast main or the oil transfer main is connected to the tank.

Bilge and ballast systems

Fig: Bilge and ballast systems on board

Ballast pumps - Electric-driven pumps, usually vertically mounted and fitted with separate motor-driven priming systems. Close-coupled designs have the pump rotor mounted on an extended motor shaft. This can cause difficulties when there is a need to open up the pump, as the motor may also have to be dismantled to gain access. With owners expecting to shorten port turn round times; the need to get the ballast in or out of the tanks can take on a sense of urgency.

Container ships are case in point. With containers stacked perhaps six high they cannot leave port until the ballasting is correct. This means that ballast pumps have to move impressive amounts of seawater. If the operator says the ballast tanks have to be filled or emptied in a certain time, it is possible to look at this simplistically and divide their volume by the time to calculate the rate. However, as the tank empties, the head will reduce, and so will the effective flow rate. This in turn means the safety margin built in by the pump manufacturer is reduced, and friction losses in the pipework can take the flow out of specification.

A priming system with an adequate air-handling capacity is another important need. The pump/priming system not only has to contend with the depth of the tanks in the double bottom but also with the height of the pump above the tanktop. Air ejectors have limited capacity, so for the larger pumps separate motor driven pumps are required.

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