The word “bilge” is a nautical term which is used to refer to a several concepts. Itbdates from 1513, when “bilge” was first used to refer to the lowest compartmentbinside the hull of a ship, where the two sides meet at the bottom. However, the termmis also used to discuss the matter which collects in the bilge: water drains from the decks of the ship into the bilge, dragging detritus from topside down below. The water that collects in the bilge is usually brackish and also has a foul odor, so bilge has also come to be a term which refers to anything foul or unpleasant. Bilge water can be found aboard every vessel, but its composition is always unique. Because the bilge wells receive fluids from many parts of the ship, bilge water can contain water, oil, detergents, solvents, sweat, chemicals, saliva, etc. A bilge pump is a device which is used to remove water from the bilge.
7.15.2 Oily Water Separator (OWS)
OWS are devices use to separate oil from water. A variety of methods to separate oil from water are involved including gravity separation, filters, coagulation/flocculation and floatation. Gravity separation is not always the most successful at oil removal to meet the regulatory discharge requirements, hence coalescing oil/water separators are essentially enhanced gravity type OWS that are commonly used onboard to achieve the greater separation efficiency.
OWS are designed to separate oil from water to 15 parts per million or less as per Marine Pollution Prevention regulatory requirements. The separated oil is retained onboard for disposal to shore when possible and water pumped overboard as it is extracted. The water and oil mixture is collected from various points around the machinery spaces and stored to the bilge tank for processing through the oily water separator. The oil that has been extracted from the oil and water mixture is stored in the bilge separated oil tank for further processing. OWS is one of the oil pollution prevention equipment onboard and categorized as “Critical Machinery” onboard for environmental protection.
7.15.3 Structure and Operating Principle
a. Taiko Oily Water Separator
With this oily water separator, gravity separation method is adopted, by which the oil is separated by the difference of the specific gravity of the two liquid. Oil water mixed liquid, accumulated in the bilge tank is led into the inlet of oily water separator by a bilge pump. The oil and water mixture is roughly separated in the 1st separation chamber. As fine grain oil is still contained in the oil water mixture which roughly separated in the 1st separation chamber, the oil and water mixture is course grained in the coalescer of the 2nd separated chamber. In the 2nd separation chamber, almost all oil are floated and separated and drained into the bilge separted oil tank by means of the oil discharge valve. The treated water of which oil density is less than 15 ppm is discharged overboard.
b. Oil Content Meter
The oil content meter is continuously measures the oil concentration contained in the treated effluent from the oily water separator and issues an alarm when the oil concentration is more than 15 ppm. In some design, the oil content monitor is used to stop the bilge pump automatically or open the 3-way recirculating valve to avoid high oil concentration discharge to the sea.
In most cases, a combination of alarm and automatic stopping of bilge pump or opening of the 3-way recirculating valve is being adapted. Oil content meter of FOCAS 1500C emulsifies the oil in water by ultrasonic waves and measures the difference taking place in the turbidity of the resulting emulsion by the photoelectric calometric method. In order to eliminate the adverse effects of detergent and suspended solid matter on the measurement, the emulsification is effected twice. The change in turbidity caused by detergent and suspended solid matter is completed in the first process of emulsification and the turbidity is caused to change only by the oil content on the second stage of emulsification, thus making possible to measure the oil concentration accurately simply by determining the amount of change in the second stage of emulsification. In order to minimize the production of air bubbles due to ultrasonic vibration, a constant regulator is used to permit highly accurate and stabilized measurement at all times.
7.15.4 Procedure for Operation
a. Preparation
i. Open all valves from the suction line of the bilge pump up to the overboard
discharge line via the oil water separator. Close the manual drain valve.
ii. Open the clean sea water inlet valve and fill up the OWS separating chamber of clean sea water until full. When filling clean sea water, switch on the OWS control panel and open the cock on the top of the chamber to release air in the
separating chamber.
iii. Open the sea water suction valve of the bilge pump, close the clean s.w. inlet valve, switch on the 15 ppm monitor and start the pump. Check the separating chambers for leakage and any abnormalities.
iv. The OWS is required to be full of sea water prior operation and that no leakage is observed in the casing. Any leakage should be rectified as soon as possible.
b. Running
i. Switch on the automatic oil drain system and the oil content meter.
ii. Open all valves in the bilge pipe line and start the bilge pump. In operating the bilge pump, priming of clean sea water is necessary to ensure that the separating chamber is full of clean sea water. Check that the differential pressure between the 1st and 2nd separating chamber is not more than 0.03 MPa or the separating efficiency will be affected.
iii. If the OWS will be running after a long idle time, ensure that clean sea water is circulating for not less than 10 minutes to ensure that the separating chamber is filled with clean sea water and that the oil deposited in the separating chamber is move back at the upper part position of the coalescer.
Precautions:
i.) Always supply clean sea water in the initial running of the OWS to prevent oil
contamination of the 2nd stage separating chamber.
ii.) Do not supply concentrated oil to the OWS to prevent contamination that will
affect its separating efficiency.
c. Stopping
i. When processing of bilge is completed, open the sea water suction valve, circulate sea water in the chamber for not less than 10 minutes and while running with sea water momentarily open the oil discharge valve to release the accumulated oil to the bilge separated oil tank.
ii. When sea water circulation is completed, stop the bilge pump, close all valves and switch off power switches in the control panel and the oil content meter.
