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 consist 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 sea water into dedicated bildge tanks to maintain ships stability; while de ballasting is pumping water out of those bildge tanks for the same reason.
Both the process of ballasting and de ballasting are undertaken when a ships get in or out of the coastal waters. Similarly bilge suctions are connected to the bildge and ballast system; to pump out accumulated water in bilges through a oil content monitor or altogether in emergency condition such as flooding.
Both bilge and ballast system are interconnected in the ships to perform each other operation in event of any emergency. This is done by operating cross over valve which separate the two systems.
Need And Purposes of Bilge And Ballast System
All water leakages from heat exchanger, pumps and pipelines collect in the lower most deck of the ship in 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 accumulation of water or oil water mixture in engine room or cargo space.
The bildge 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 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 ship. If the upward buoyancy is just too low its likely to sink too often; while if its 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 is maintained by ballasting and de ballasting 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 consists of a general service pump, bilge and ballast pump and a dedicated bilge pump. The two system take suction from their dedicated valve manifold; and are separated from each other using non return dedicated suction valve as displayed in above diagram.
The output from the bilge pump is then discharged overboard through oil content monitor for cargo holds bilges; or via 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 bilge well; but are only to be used under emergency condition. This ensure 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 be 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 separate power supply; connected to the bilge suction.
Key Parts of The System And Their Function
1 ) Bilge Suction Manifold
A bilge suction manifold connect all bilge wells with the bilge pumping system and general service pump. This enable pumps to de-water ship compartments; at the same time as the fire fighting is in progress. These 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 to choose particular bilge well for the bilge suction. A bilge suction manifold have 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 to the bottom most position at various position in engine room space.
2 ) Bilge And Ballast Pump
The bilge and ballast pump consists of three pairs of pumps namely; bilge pump, ballast pump and general service pump. All three pumps have separate system but are interconnected to be used as whole in emergency. These pumps are vertically mounted self primed centrifugal pumps; with 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 audio visual indication of high bilge well level. The capacity of these bilge and ballast pumps depends upon the diameter of bilge main, the length between perpendiculars and modular depth of the tank or compartment.
A ballast pump is generally of higher capacity then the bilge pump and are controlled remotely from the cargo room; while a bilge pump can be remotely controlled from fire control station. The dedicated controllers of both pumps are located in the main switch board in the engine control room.
For moderate to low capacity output as required on small boats and light shore based application; small yet powerful submersible bilge pumps are used for the purpose. Typically these pumps costs you about a 100 dollars; although i see a drop of 20 to 40 dollar 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- prevents ships from any oil discharge whatsoever. The only exception is given under the clause 14 of 2 provision is that; it allows for 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 picture. It is an device which helps separates the oil from given water oil mixture; to provide a 15ppm mixture output. The device is divided into two main parts; primary filter and the secondary filter. The primary compartment or filter is equipped with baffle plates and heating arrangement.
While the secondary or second stage filter is equipped with Coalecer Filter. These filters absorbs 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 ensure the content is below 15 ppm mark while it is discharged overboard. You can read in details about the oily water separator and how it works from one of our old post; “Oily Water Separator Construction Working & Dismantling” from link over here.
Associated Regulations And Working Procedure
All ships are required by law to have dedicated system for bilge, ballast and fire fighting with interconnected lines; so in emergency they can be used to play the role of other. The arrangement of such 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 bilge pipeline to avoid flooding of compartment even when; one of the bilge suction line is chocked or damaged. Other than that ships are also require to have a separate emergency bilge pumping system. Oily water separator is used under normal condition to discharge bilge water overboard ship.
Proper arrangement should be there to bypass the O.W.S or O.C.M in 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 bilge suction valve is closed during operation of ballast pump. Similarly ensure the suction valve of ballast manifold is closed while operating 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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