A bilge system is a piping system installed onboard a vessel for the purpose of removing water that accumulates in enclosed spaces (holds, machinery spaces, cofferdams) as a result of condensation, leakage, washing, or fire fighting. It is also to be capable of controlling flooding in the Engine Room as a result of minimal piping system damage.
In general, the vessel’s bilge system consists of three main sub-systems:
- Engine room bilge system
- Cargo hold bilge system
- Rudder trunk or bow thruster bilge system
The engine room bilge system and cargo hold bilge system are interconnected and an isolation valve is fitted between these two systems. This isolation valve should normally be closed and sealed and seal recorded into engine room seal book. The rudder trunk or bow thruster bilge system is a separate system.
The bilge system is served by three pump sets:
- The engine room bilge pump
- The cargo bilge pump
- The fire and general service pumps
Under normal operating conditions the engine room bilge pump is used to pump from the engine room bilge main to either the clean bilge water tank or the oily water tank. Bilge water from the engine room system can be discharged overboard , while vessel is underway, only via the oily water separator.
The cargo bilge pump is used to pump from the cargo hold bilge main to the cargo bilge tank if the ship is designed as such. If there is no designated cargo bilge tank, the bilge can be pumped directly overboard after has been visually checked for presence of oil, otherwise should be pumped into engine room bilge tank via interconnection valve.
The fire and general service pumps can only discharge water overboard or to the fire and general service system. These pumps should only be used to pump bilge water in an emergency.
It is very important that the overboard discharge is not to be used for discharging bilges unless under emergency conditions.
The rudder trunk or bow thruster bilge system is operated by an ejector, which is driven by water, the driving water being supplied from the fire and wash deck system.
The engine room bilge pump is, usually, of a self priming type which can draw water from the various bilge suctions via the engine room bilge system and discharge to the oily water tank or clean bilge tank. It can take, also, suction from oily bilge tank, clean bilge tank, cofferdams, engine pit, void spaces, stern tube cooling tank etc. In general, all the bilge pump suction points are connected to the bilge pump via the engine room bilge system, except for the clean bilge tank, the oily water tank and the oily water separator. The engine room bilge pump is equipped with a suction strainer and, on modern vessels, the pump motor can be set to stop automatically if the pump runs dry. The pump can be stopped locally or from the shore connection stations on main deck close to the bilge & sludge shore discharge manifold.
The oily water tank receives water from the engine room bilge pump, and has different capacity according with the vessel size. The tank is equipped with a steam heating coil, and has, usually, three suction lines:
- Sludge transfer pump suction, high
- Sludge transfer pump suction, low
- Engine room bilge pump
The oily water tank allows oil to separate from the bilge water by gravity and the oil can then be removed by the sludge transfer pump, which has a high and low suction. Water is removed from the oily water tank and transferred to the clean bilge tank by the engine room bilge pump.
The clean bilge tank receives water from the oily bilge tank and various scuppers and save-alls in the engine room. Similar with oily bilge tank, its capacity is strictly connected with the vessel size and is equipped with a steaming out connection. The clean bilge tank has two suction lines, one from engine room bilge pump and the other from oily water separator supply pump.
Only water with an oil content of less than 15ppm is discharged overboard. A feed pump supplies bilge water from the clean bilge tank to the oily water separator which is used to treat bilge water from the clean bilge tank before it is discharged overboard. About this machinery will discuss on a later post as the subject is very comprehensive.
As mentioned above, bilge wells connected to the engine room bilge system are normally pumped to the oily water tank using the bilge engine room pump. The bilge pump may also pump from the engine room bilge system to the clean bilge tank, cargo bilge holding tank or the shore connections. The procedure to pump from the engine room bilge wells to the oily bilge tank is, generally, as follow:
- Check and ensure that the engine room bilge pump suction strainer is clean.
- Check and ensure by taking a sounding measurement that there is sufficient space in the oily water tank for the bilge water, before starting the transfer operation . If the oily water tank is full, bilge water can be transferred to the clean bilge tank.
- Open the bilge pump discharge valve to the oily water tank and ensure that all the other bilge pump discharge valves are closed.
- Open the bilge pump suction valve to the main engine room bilge system as this connects all the suction points to the bilge pump.
- Check the suction strainer on the bilge suction to be pumped and open the suction valve.
- Start the bilge pump. Ensure that the bilge pump does not run dry. Usually, bilge pumps have a sea water suction connection which can be used for priming. don’t keep the sea water valve connection open for too long time, as there is a risk of filling the oily bilge tank with too much sea water.
- Close the bilge suction valve before the bilge is completely empty to prevent the pump to run dry.
Under normal operating conditions the cargo hold bilge system is served by the cargo bilge pump which is, usually, a positive displacement pump but the pump suction is also equipped with a priming unit because of the length and large volume of the cargo hold bilge system. The pump will draw water from the various hold bilge suction points via the forward and aft cargo hold bilge systems and discharge to the cargo bilge holding tank or directly overboard.
The fire and general service (GS) pumps are vertical centrifugal pumps and each pump is equipped with a priming unit. A cross-connection line is fitted between the pump suction and discharge and this line is arranged to ensure that bilge water cannot be discharged into the fire and wash deck main.
The emergency bilge suction is provided to deal with serious flooding of the machinery spaces. Under such circumstances when the situation threatens the safety of the vessel, it is permissible to use this means to pump the bilge water directly overboard. The emergency bilge suction valve is part of the ship’s safety equipment and must be maintained operable with testing and greasing at intervals not exceeding one week.
The bilge system, especially the cargo hold bilge system must be periodically pressure and vacuum tested (6 monthly period) in order to check the valve proper sealing and pipe integrity. Failure to do so can result in cargo hold flooding and damaging of cargo.
Usually, cargo hold bilge wells are equipped with non return flaps which prevents the water backflow into the bilge wells. Checking the integrity of these flaps is one of the reason of the system pressure test that must be periodically done. The vacuum test is done in order to check the remote butterfly valves sealing property.
The procedure for vacuum and pressure testing will follow into a later post.
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