BILGE SYSTEM:-
The bilge system fitted in dry cargo and container ships is used to remove loose water from machinery, boiler and cargo spaces. Tanks for liquid cargo and ballast are served by cargo discharge systems and ballast systems respectively. They are not connected to the bilge system, unless they have a double function. Accommodation spaces are served by scuppers with non-return valves which are fitted at the ship's side.
Bilge system regulation:-
The number of power operated bilge pumps (usually 3 or 4 ) that are required in the machinery spaces is governed by the size and type of ship. For smaller vessels, one of the pumps may be main engine driven but the other must be independently driven. A bilge ejector is acceptable as a substitute provided that, like the pumps, it is capable of giving an adequate flow rate. At least 120m/min(400ft/mis) through the pipe is required.
The diameters of bilge main and branch pipes, pump capacity and minimum water speed in the line are given by the following formulae :-
Bilge main dia, d1= 1.68 +25 mm
Branch dia, d2 = 2.16 +25 mm. 6.2 not to be less than 50mm and need not exceed 100 mm. D1 must never be less than d2.
Where, L= length of ship in m;
B = Breadth of ship in m.
D = moulded depth at bulkhead deck in m.
C = length of compartment in m.
Each pump should have sufficient capacity to give a water capacity of 122m/min through the Rule size mains of this bore.
Furthermore each bilge pump should have a capacity of not less than 0.565\ . d1 m/hr (where d1 in mm)
The fire pumps, excluding any emergency fire pump fitted, must be capable of delivering a total quantity of water at a defined head not less than two-thirds of the total bilge pumping capacity. The defined head ranges from 3.2 bar in the case of passenger ships of 4000 tons gross or more to 2.46 bar for cargo ships of less than 1000 tons gross.
Pumps installed for bilge pumping duties must be serf-priming or able to be primed. Positive displacement pump or centrifugal pump with an air pump are also suitable.
The bilge pumps may be used for other duties such as general service, ballast and fire-fighting which are intermittent. The statutory bilge pumps may not be used for continuous operation on other services such as cooling, although bilge injections can be fitted on such pumps and are a requirement on main sea water pump or stand-by circulating pumps.
Common suction and discharge chests permit one pump to be used for bilge and ballast duties. The pipe systems for these services must, however, be separate and distinct. The ballast piping has screw lift valves, so as to be able to both fill and empty tanks as required. Bilge system valves are screw-down non- returns to prevent any flooding back to the compartment served. At the pump suction valve chest, the bilge valve must be of screw-down non-return type to prevent water from entering the bilge line from sea water or ballast suctions.
Normally there are three pumps connected to the bilge main. These are fire and bilge pump, general service pump and the auxiliary bilge pump. The ballast pump and main sea water circulating pump have also emergency bilge suction. The ballast pump being self priming can serve as one of the required bilge pump as well as being the stand-by sea-water circulating pump. The auxiliary bilge pump, is normally a low capacity, smooth flow pump which is suited for use in conjunction with the oily/water separator. All bilge suctions must have screw down non -return valves with strainers or mud boxes at the bilge well.
Ballast Water System Requirement :-
The design of the ballast system is governed by the requirements of the type of ship the system is serving.
When a vessel must proceed between two ports without cargo or only partially loaded, it may become necessary to take on ballast for any one of or combination of the following reasons:-
(a)To reduce the vessels freeboard and give sufficient immersion of the propeller to reduce ship and propeller-excited vibration.
(b)To give better rudder action due to the greater depth of water flow around the rudder.
(c)To trim the vessel so the depth of water forward is such that the possibility of pounding (slamming) damage is reduced to a minimum when heavy weather is experienced.
(d)To provide satisfactory stability conditions,
(e)To assist in better weight distribution, thereby reducing hull stresses.
(f) To give the ship better sea-keeping qualities in heavy weather.
In some cases, the cargo loading requirements are such that a ship may be tender on sailing from, a loading port, and as fuel is consumed from the double bottom tanks, the stability may decrease until the ship takes up an angle of loll. In this condition, the ship may 'flop' from side to side and possibly become dangerous. Double bottom tanks must then be filled to increase the bottom weight and obtain the desired degree of stability by lowering the position of centre of gravity(G) such that metacentric height can improve(made more positive).
Capacity of ballast pumps :-
The designed capacity for the ballast pumps will be governed by the minimum time requirement of filling or discharging ballast prior to, during or after cargo operations.
Cargo liners and general traders with comparatively low rates of cargo loading and discharge do not require as large a ballast pumping capacity as a bulk carrier or tanker. Vessels with tanks available for either ballast or oil fuel are fitted with change-over chest or cock designed to prevent mistakes. An oily water separator of adequate capacity on the ballast pump discharge would prevent discharge of oil with ballast from a tank that had been used for fuel or oil cargo.
In oil tankers and OBO's, segregated ballast tanks and separate ballast pumps for clean ballast other than cargo pump are normally provided. Ballast tank air and overflow pipes must be of the required size relative to the filling lines, i.e. 25% greater in area and in any case, not less than 50mm bore. They are fitted at the highest part of the tank or at the opposite end to the filling connection.
Ballast tank Capacity:-
The increase of welding and replacement of riveting in vessel construction has greatly decreased the light weight displacement of ships. Modern cargo liners and general traders of all welded construction require, for satisfactory operation in ballast condition, a ballast displacement of approximately twice the lightweight displacement. Some of the weight requirement to obtain the ballast displacement is sometimes obtained by taking oil fuel. In bulk carriers and similar classes of vessel, where cargo may only be carried in one direction, the ballast capacity is often greater than that for cargo liners and general traders. In tankers when cargo tanks are used for the carriage of ballast the only limit to the quantity of ballast water carried is the loaded displacement of the vessel.
The arrangement and capacity of the ballast tanks in bulk carriers should be such that stability in the fully or partially ballasted condition does not result in a ship that is excessively stiff.
It must also be remembered that the carriage of excess amounts of ballast results in a slower passage and an attendant waste of fuel.
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