This unit consists of catch plates that are inside a coarse separating compartment and an oil collecting chamber.
Here the oil has a density that is lower than that of the water, which makes the former rise into the oil collecting compartment and the rest of the non-flowing oil mixture settles down into a fine settling compartment after passing between the catch plates.
After a period of time, more oil will separate and collect in the oil collecting chamber. The oil content of water that passes through this unit is around 100 parts per million oil.
A control valve (pneumatic or electronic) releases the separated oil into the designated OWS sludge tank.
The heater may be incorporated into this unit for smooth flow and separation of oil and water.
A heater may be incorporated in this unit either in the middle or sometimes in the bottom part of the unit (depending upon the area of operation and capacity of the separator equipment) for smooth flow and separation of oil and water.
The first stage helps in removing some physical impurities to achieve fine filtration in the later stage.
2. The Filter unit
This is a separate unit whose input comes from the discharge of the first unit.
This unit consists of three stages – filter stage, coalescer stage, and collecting chamber.
The impurities and particles are separated by the filter and are settled at the bottom for removal.
In the second stage, the coalescer induces a coalescence process in which oil droplets are joined to increase their size by breaking down the surface tension between oil droplets in the mixture.
These large oil molecules rise above the mixture in the collecting chamber and are removed when required.
The output from this unit should be less than 15 ppm to fulfill legal discharge criteria.
If the oil content in water is more than 15 ppm then maintenance work such as filter cleaning or renewal of filters is to be done as required.
A freshwater inlet connection is also provided to the filter unit to clean and flush the filter. This is usually done before and after the operation of an oil separator unit.
3. Oil Content Monitor and Control Unit
This unit functions together in two parts – monitoring and controlling.
The ppm of oil is continuously monitored by Oil Content Monitor (OCM); if the ppm is high it will give an alarm and feed data to the control unit.
The control unit continuously monitors the output signal of OCM and if an alarm arises, it will not allow the oily water to go overboard by means of operating a 3-way solenoid valve.
There are normally 3 solenoid valves commanded by the control unit. These are located in the first unit oil collecting chamber, second unit oil collecting chamber, and one on the discharge side of the oily water separator which is a 3-way valve.
The 3-way valve inlet is from the OWS discharge, where one outlet is overboard and the second outlet is to the OWS sludge tank.
When OCM gives an alarm, the 3-way valve discharges the oily mixture in the sludge tank.
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A small pipe connection of fresh water can be provided to the OCM unit for flushing. Whenever this line is in use, an alarm is sounded and recorded in the OCM log, ensuring a record to check the discharge valve was shut during this period.
As in most shipping companies, the OWS is meant to be operated only by the chief engineer, and the training levels on OWS systems for other crew members are found to be very low. The ship operators should ensure onboard guidance and training are included in the training schedule of the ship.