Last Updated on September 2, 2020 by Amit Abhishek
A bilge and ballast system is an interconnected network of valve manifold; pipelines, ballast tanks, dedicated valves with bilge and ballast pumps. The system is used to fill tanks, transfer water from one tank to another and prevent possible accumulation of water; in cargo holds or machinery space by discharging overboard.
The whole process consists of two key steps of ballasting and de-ballasting the ship; along with the added purpose to pump out bilges from cargo and machinery space. Ballasting is the process of taking in seawater into dedicated bilge tanks to maintain ships stability; while deballasting is pumping water out of those bilge tanks for the same reason.
Both the process of ballasting and deballasting are undertaken when ships get in or out of the coastal waters. Similarly, bilge suctions are connected to the bilge and ballast system; to pump out accumulated water in bilges through an oil content monitor or all together in emergency conditions such as flooding.
Both bilge and ballast systems are interconnected in the ships to perform each other operation in event of any emergency. This is done by operating a cross over valve which separates the two systems.
Need And Purposes of Bilge And Ballast System
All water leakages from the heat exchanger, pumps, and pipelines collect in the lowermost deck of the ship in the engine room; known as bilge well. Similarly, all cargo tanks have dedicated bilge well on both sides of the tank; i.e port and starboard. These bilges are connected to the bilge pumps; to maintain dryness avoiding the accumulation of water or oil-water mixture in the engine room or cargo space.
The bilge line is independent of any line used to ballast or de ballast tanks in ship and covers; engine room bilges, bilges from bow thruster room, recess bilge well, cargo holds bilge drain well, bilges in the cargo pump room, shaft alley, and other drains. Together they consist of four key parts; bilge suction, primary bilge tank, holding tank, and bilge injection valve.
Both having too high or too low buoyancy is dangerous for the safe operation of the ship. If the upward buoyancy is just too low it’s likely to sink too often; while if it’s high the ship will have problem with propulsion and can topple by its side in bad weather. Thus its stability and structural stress on the ship are maintained by ballasting and deballasting operation.
If interested in ballasting as a process and its system you can read in detail about the same from one of our old post ” What is Ballasting And De-ballasting ? – Methods & Procedure ” from link over here.
General Arrangement of Bilge And Ballast System
A typical bilge and ballast system consist of a general service pump, bilge, and ballast pump, and a dedicated bilge pump. The two systems take suction from their dedicated valve manifold; and are separated from each other using a nonreturn dedicated suction valve as displayed in the above diagram.
The output from the bilge pump is then discharged overboard through an oil content monitor for cargo holds bilges; or via an oily water separator for bilges from engine room bilge wells. Their role is to ensure the discharged water should not contain oil more than 15 ppm; under Marpol regulation 1 but can maintain oil content as low as 5 ppm.
As shown in the above diagram the ballast pump can also take suction from the bilge well but are only to be used under emergency conditions. This ensures that the ballast pump lines remain free from any oil or harmful chemicals. Typically bilge water contains traces of oil, emulsifier, various solvents, and other liquids; which needed to be minimized before discharging overboard the ship.
All bilge suction on board whether being in cargo holds or engine space are fitted with mud holds or pair of strainers; followed by the screw down non return valve. In larger ships, an additional submersible pump is installed above the bulkhead deck with a separate power supply; connected to the bilge suction.
Key Parts of The System And Their Function
1 ) Bilge Suction Manifold
A bilge suction manifold connects all bilge wells with the bilge pumping system and general service pump. This enables pumps to de-water ship compartments; at the same time as the fire fighting is in progress. This bilge suction manifold consists of a number of valves with common rail output.
Situated at the inward side of the forward skid; the manifold allows us to choose a particular bilge well for the bilge suction. A bilge suction manifold has valves leading to bilge wells at engine room starboard; port, aft, chain locker, v.s.p room, and forward tank void. The connection lines run under the engine room deck plate; to reach the bottom-most position at various positions in engine room space.
2 ) Bilge And Ballast Pump
The bilge and ballast system pump consists of three pairs of pumps namely; bilge pump, ballast pump, and general service pump. All three pumps have a separate system but are interconnected to be used as a whole in an emergency. These pumps are vertically mounted self primed centrifugal pumps; with a double-acting motor-driven unit.
The float switch and sensor situated at the bilge well trigger the bilge pump to start discharging via O.C.M or O.W.S. Other than that they provide an audio visual indication of high bilge well level. The capacity of these bilge and ballast pumps depends upon the diameter of the bilge main, the length between perpendiculars, and the modular depth of the tank or compartment.
A ballast pump is generally of higher capacity than the bilge pump and is controlled remotely from the cargo room; while a bilge pump can be remotely controlled from the fire control station. The dedicated controllers of both pumps are located in the main switchboard in the engine control room.
For moderate to low capacity output as required on small boats and light shore-based applications; small yet powerful submersible bilge pumps are used for the purpose. Typically these pumps cost you about 100 dollars; although I see a drop of 20 to 40 dollars on sales and discounts. So keep your eye out on amazon and check the current price.
3 ) Oily Water Separator
MARPOL 73/78, Annex 1-Regulations for the Prevention of Pollution by Oil prevents ships from any oil discharge whatsoever. The only exception is given under clause 14 of 2 provisions is that; it allows for the discharge of bilge water containing oil as long as the oil content is below 15 ppm concentration.
This is where oily water separator came into the picture. It is a device that helps separates the oil from a given water-oil mixture; to provide a 15ppm mixture output. The device is divided into two main parts; the primary filter and the secondary filter. The primary compartment or filter is equipped with baffle plates and heating arrangements.
While the secondary or second stage filter is equipped with Coalescer Filter. These filters absorb the oil molecules further reducing the oil content as low as 15 ppm concentration. The mixture is then passed through an oil content sensor which ensures the content is below the 15 ppm mark while it is discharged overboard. You can read in detail about the oily water separator and how it works from one of our old posts; “Oily Water Separator Construction Working & Dismantling” from the link over here.
Associated Regulations And Working Procedure
All ships are required by law to have a dedicated system for the bilge, ballast, and fire fighting with interconnected lines; so in an emergency, they can be used to play the role of others. The arrangement of such a system should be such; that the water should not pass from sea to the bilge well under any situation.
Further provisions should be made in the bilge pipeline to avoid flooding of the compartment even when; one of the bilge suction lines is chocked or damaged. Other than that ships are also required to have a separate emergency bilge pumping system. The oily water separator is used under normal condition to discharge bilge water overboard ship.
The proper arrangement should be there to bypass the O.W.S ( oily water separator ) or O.C.M ( oil content monitor ) in an emergency situation. In large ships there must be two sets of each pump; so one should always be in standby or serviceable condition. The bilge lines are generally kept out of the double bottom and only two such lines; are permitted to pass through the collision bulkhead.
Ships stability is ensured and checked before and after operation of the system. Soundings of tanks are taken and tanks openings are closed to ensure unnecessary connected lines are closed. Use dedicated pumps and manifold to ballast, de-ballast, bilge transfer, and bilge discharge operation.
Ensure the bilge suction valve is closed during the operation of the ballast pump. Similarly, ensure the suction valve of the ballast manifold is closed while operating the bilge pump. Operate the system and maintain proper records of associated operation; i.e ballasting, de-ballasting, and bilge discharge.
Note: This article is produced on request from Mariston lerins. If you find any mistake or have advice or opinion let us know by the comment down below.
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