The scope of the bilge system is the removal of water from each compartment of the ship; this system is present on all types of vessels and is essential for the safety.
The system consists of several pits located at the lowest points of each compartment, a series of pipes that connect the pits to the bilge pumps and a series of pipes that allow the pumping of water outboard (either directly or through filtration systems).
The pits collect the drains of the machinery and the infiltration of water.
The bilge pumps are therefore used in two different situations:
- in normal operation they drain the pits from the water that collects inside; usually the amount of water is reduced and the operation of the pump is very short
- in emergency situations they must remove water from a flooded compartment; in this case large quantities of water are pumped and the operation can take several hours
The bilge pump capacity is calculated according to the rules of Classification Society and depends on the size of the vessel. They are therefore sized to guarantee the emptying of large masses in the event of a leak to the hull. In normal operation, the pumps are therefore always very oversized for the mass of water contained in a pit; their operation may be irregular and intermittent. This situation can be solved by installing pumps with reduced flow (daily bilge pumps) dedicated to the daily emptying of the pits, avoiding the use of centrifugal pumps.
Depending on the type of vessel, the pits are located very far from the pumps. In addition, the pits are always lower than the pump. The pump must then remove the air contained in the suction pipe and sucks the water from pits that can be at a considerable distance. The pumps must be SELFPRIMING in order to evacuate the air contained in the suction line. The suitable types are as follows:
- piston pumps: high suction capacity, high complexity of construction, high cost, generally used for low/medium flow rates
- eccentric screw pumps: high suction capacity, low flow rates
- side channel pumps: only for very low flow rates and limited suction lines
- 2 screws pumps: good suction capacity, low flow rates, high cost
- centrifugal pumps with AIR EJECTOR: good suction capacity, medium/high flow rates, low cost, simplicity of construction
For medium/large bilge systems, centrifugal pumps with air ejector are the most used pumps. The use of the air ejector has the following advantages:
- makes any centrifugal pump self-priming
- low cost
- allows the removal of air before starting the pump, avoiding dry running
- high vacuum capacity
- quick priming