Oily water separator working principles and guidance

Requirement of oily water separators for cargo ships

Oil/water separators are used to ensure that ships do not discharge oil when pumping out bilges, oil tanks or any oil-contaminated space. International legislation relating to oil pollution is becoming more and more stringent in the limits set for oil discharge.

Clean water suitable for discharge is defined as that containing less than 15 parts per million of oil. Oil/water separators using the gravity system can only achieve 100 parts per million and must therefore be used in conjunction with some form of filter.

A complete oil/water separator and filter unit for 15 parts per million purity is shown in Figure above. The complete unit is first filled with clean water; the oily water mixture is then pumped through the separator inlet pipe into the coarse separating compartment. Here some oil, as a result of its lower density, will separate and rise into the oil collection space. The remaining oil/water mixture now flows down into the fine separating compartment and moves slowly between the catch plates.

More oil will separate out onto the underside of these plates and travel outwards until it is free to rise into the oil collecting space. The almost oil-free water passes into the central pipe and leaves the separator unit. The purity at this point will be 100 parts per million or less. An automatically controlled valve releases the separated oil to a storage tank. Air is released from the unit by a vent valve. Steam or electric heating coils are provided in the upper and sometimes the lower parts of the separator, depending upon the type of oil to be separated. Where greater purity is required, the almost oil-free water passes to a filter unit.

Fig:Oily water separator

The water flows in turn through two filter stages and the oil removed passes to oil collecting spaces. The first-stage filter removes physical impurities present and promotes some fine separation. The second-stage filter uses coalescer inserts to achieve the final de-oiling. Coalescence is the breakdown of surface tension between oil droplets in an oil/water mixture which causes them to join and increase in size. The oil from the collecting spaces is drained away manually, as required, usually about once a week. The filter inserts will require changing, the period of useful life depending upon the operating conditions. Current legislation requires the use of a monitoring unit which continuously records and gives an alarm when levels of discharge in excess of 15 parts per million occur.

Cargo ships guideline for oily water separator

All vessels are required to carry the above in order to comply with MARPOL regulations. It is essential that this item of machinery is operated and maintained correctly and strictly in accordance with international regulations and manufacturer’s instructions. It is to be noted that it is illegal to sail from a port without a functioning OWS and it is a requirement that sufficient spares for the unit are carried onboard. Guidance on MARPOL equipment is contained within the Critical equipment section.

The Chief Engineer is fully responsible for the maintenance and operation of the OWS and its operation must only be carried out by officers fully familiar with the equipment and under direct supervision of the Chief Engineer. When the equipment is not in use, the Chief Engineer is to ensure that a system is in place to lock-out the equipment to avoid unauthorised operation with keys being held by the C/E.

An operation manual for the OWS must be onboard. Crew members involved in the use of the equipment must be familiar with its contents. In addition, operating instructions and an operating diagram are to be posted on or next to the OWS unit. These are to be as clear and simple as possible.

Regular operational tests and checks of the OWS and associated equipment must be carried out in accordance with the PMS and also no later than 24 hours before arrival in port where possible. Minimum recommendations are that full inspection and cleaning of the OWS is to be carried out at least every two months with filters cleaned on a monthly basis. Such tests must be recorded in the engine log book and oil record book. The tank from which the OWS takes suction should be cleaned on a two monthly basis.

A spare set of OWS filter elements must be available onboard at all times and included in the Minimum Critical Spares List .

A sample is to be landed yearly for testing in a shore laboratory; the test standard is to be USEPA 1664. The Oil Content Monitor is to be calibrated annually.

Any defects discovered with the OWS and associated equipment must be rectified without delay. Where a defect cannot be repaired on board for want of spare parts or the need for specialist service, the management office must be notified and a remedial action plan agreed upon. If the vessel is about to enter port, knowing that the OWS is defective, the fact must be reported in the pre-arrival notices via agents according to local regulations. Defects remaining on the system must be reported to Class / Flag by the management office prior to leaving port limits.

All entries entered into the Oil Record Book (ORB) must be up-to-date, accurate, truthful and in full compliance with MARPOL requirements. An example of a typical ORB entry regarding OWS maintenance could be. “Opened and inspected OWS unit. Filters cleaned/renewed as required. Valves and piping on discharge side of OWS opened, inspected and cleaned. After maintenance, OWS and oil content meter checked for proper operation and found in order.”

An interlock should be fitted in order to prevent the improper use of flushing water diluting the oily water mixture running through the measuring cell. In other words, when flushing water is used to clean the measuring cell, the interlock stops any oily water mixture from being discharged overboard.

All pipe work associated with the OWS, the coil bilge system, emergency bilge suction line, bilge shore discharge connection, overboard connections for the GS system and boiler blow-down overboard connection must be correctly identified and labelled. A drawing showing the layout of the above should be available. On this drawing, all valves and flanges are identified and given a number. This drawing is to be attached to the Oil Record Book (ORB). See below Documents for a generic drawing for your reference. We would reiterate that only the flanges as marked 1 to 6 will require labelling.

Fig: TYPICAL SINGLE HULL VLCCs OWS PIPING LAYOUT

All numbered flanges identified in the drawing should have had a small hole (about 5mm) drilled through in order to accommodate the fitting of a security seal. Each seal has a unique number and a form should be available to identify the flange number against the seal number. The date and time of fitting the seal should also be recorded as well as the date and time of the seal being broken including the reason for same. No seal should be broken without the joint permission of the Master and Chief Engineer.

Any flanges located at or near the OWS equipment and overboard discharge valves not and must be blanked off. These may exist as original or modified construction. Flanges should be removed from any flexible hoses maintained on board in order to avoid creating wrongful suspicion of an illegal by-pass of the OWS equipment.

The overboard discharge of the OWS should be coated internally with a suitable light colour epoxy coating. The reason for this is that uncoated pipes can react to corrosion and can result in what looks like a black oily residue adhering to the walls of the pipe. Some inspectors jump to the conclusion that this is oil. A light epoxy coating will prevent such corrosion and any discolouration caused by other sources will be readily apparent if the pipe is opened up for inspection.

The Bridge must be advised before the OWS is used to ensure that the operation is in compliance with MARPOL and so that they can keep a watch astern to ensure that no oily sheen is sighted on the water. If such a sheen is sighted, the operation must be stopped immediately. Where practicable, the oil water separator should only be used in daylight hours.

Function of oil in water monitoring system

Current regulations with respect to the discharge of oily water from ships set limits of concentration 15 parts per million. A monitor is required in order to measure these values and provide both continuous records and an alarm where the permitted level is exceeded.

The principle used is that of ultra-violet fluorescence. This is the emission of light by a molecule that has absorbed light. During the short interval between absorption and emission, energy is lost and light of a longer wavelength is emitted. Oil fluoresces more readily than water and this provides the means for its detection.

A sample is drawn off from the overboard discharge and passes through a sample cell. An ultra-violet light is directed at the sample and the fluorescence is monitored by a photoelectric cell. The measured value is compared with the maximum desired value in the controller/recorder.

Where an excessive level of contamination is detected an alarm is sounded and diverting valves are operated. The discharging liquid is then passed to a slop tank.

Fig : Oil in water monitoring system

The oil is automatically bled from the evaporator to a heat exchanger in which liquid refrigerant mixed with the oil is vaporized. The heat for vaporizing the refrigerant is obtained by passing warm liquid refrigerant from the condenser, through the heat exchanger. Vapour and oil are passed to the compressor where the oil returns to the sump while the freon passes to the compressor suction. The regulator is a thermostatically controlled valve which operates in the

International Convention for the Prevention of Pollution from Ships 73/78 - Circular sent to all Members

The MARPOL Regulations contain limits on the amount of oil which ships can legitimately discharge into the sea. Where discharge from bilge tanks is permitted it is a requirement that an oil discharge monitoring and control system together with oil filtering equipment (Oily Water Separator) be fitted so as to ensure that the oil content of any discharge does not exceed the maximum permitted under MARPOL (15ppm).

Any residue or sludge should then either be incinerated or discharged into reception tanks in port. Owners are required to ensure compliance with these Regulations by inspection of log books, oil record books, incinerator logs and records of port discharges. There is an irreducible minimum of residue or sludge which a superintendent should expect to see accounted for.