Bilge Water - Storage, Treatment and DischargeWater is used in variety of ways in a ship's engine room. It is obvious that some part of this water is lost either thorough leakages or operational faults. So from where does all this water exactly come from and where does it go? What is this water called? Let's find answers to these questions.What is bilge?To understand what is bilge water, first we need to know what exactly a bilge is.A bilge is the lowest space of the ship. It is the area where two sides of the ship meet. The corners of the lowest compartment of the ship constitute a bilge. You can see a bilge compartment if you go to any of the corner of lowest platform of ship's engine room and peep beneath the floor plates. It's a space which is often filled up with an opaque black liquid.These bilge compartments are also known as bilge wells, where all the bilge water gets accumulated. The depth and the number of bilge wells depends on the ship's size, its capacity and the amount of bilge it generates. A bilge well is the most important residual collection tank of the entire engine room. We will learn about the importance of a bilge well later in the article.

What is bilge water?Bilge water is not exactly water but a mixture of variety of substances. Its a mixture of fresh water, sea water, oil, sludge, chemicals and various other fluids.So where does all these substances come from?Sea water and fresh water can find its way to the bilge wells due to leakage in the pipe lines, leaky pump and valve glands, from machinery, propulsion system, over flowing of tanks and even due to accidental spills. All these substances get accumulated in the bilge wells and the mixture formed is known as bilge water.A significant amount of sea water can seep into the bilge well if there is a leakage in the shaft gland or stuffing box. Sea water can also enter from leaky main sea chest valves. Fresh water from heat exchangers, fresh water generator and boilers often contribute to the filling of bilge wells. Oil finds it way to the bilge well mainly from the fuel oil purifiers, leakage in fuel line and oil spills.

What if the bilge wells overflow?It is a governmental law that any liquid that has oil particles suspended in it should not be discharged in the open sea. Bilge wells are helpful to store this contaminated water but can sometimes be a threat to the engine room. If the bilge wells overflow it could cause a rise in the level of water up to or above the floor plates. This could lead to accidents, emergency situations or even disturbance in the stability of the ship. For this reason, bilge wells are periodically emptied by pumping out the bilge with the help of bilge pumps.MARPOL states that bilge water cannot be directly pumped out into the sea. For this reason, bilge water is first passed through an Oily water separator where the level of suspended oil particles in the mixture is significantly brought down. When the PPM (parts per million) of oil particles in the mixture comes down to the permissible limit, it is allowed to be thrown overboard but only when the ship is en route.Engine Room Bilge SystemBilge wells are always provided with strainers to prevent solid particles from entering. This is done to prevent choking of the solid particle in oily water separator or bilge pump. Absence of strainers might lead to drop in bilge pump outlet pressure and also reduction in outlet flow of oily water separators.

The propeller of the ship is fitted at the aft and attached to a crankshaft coming from the main engine. This is done so that the rotating motion of the Main Engine can be converted into thrust to propel the ship. The propeller shaft or tail shaft is supported by a bearing arrangement which acts as an intermediate phase between the sea and the ship.The stern tube is a hollow tube passing at the lower stern part of the ship carrying tail shaft and connecting it to the propeller out at sea, bearing for the tail shaft, lubrication arrangement and most importantly the sealing arrangements.

Stern tubeWithstand loadThe propeller which hangs at the aft end exerts load on the shaft, which is supported and withstand by the stern bearing. The bearing is a cast iron bush lined with a white metal having excellent load handling and lubricating property.The stern tube is fitted at the stern frame and internal framing of vessels hull at aft peak.This allows the tail shaft to rotate smoothly in the bearing area for uninterrupted propulsion.SealingThe stern tube bearing consists of sealing arrangement to prevent ingress of water and to avoid the lubricating oil to escape into the sea.Sealing arrangementThe lubrication system for ships with variable draught (due to loading and unloading of cargo) consists of header tanks located at around 2 to 3 meters above the water line so that the differential pressure ensures no water ingress.Different sealing arrangements are used to prevent water ingress and oil leakage. They are as follows: Stuffing boxes consisting of packing material. Lip seals in contact with shaft to prevent passage of oil or water along the shaft. Radial face seals supported with springs fitted radially around the shaft, aft bulkheads and after end of the stern tube.Out of these, the lip seal arrangement is most popularly used.