MARINE OILY WATER SEPARATOR (OWS)

OILY WATER SEPARATOR

Engine room bilges in motor ships are frequently contaminated by  leakage of fuel or lubricating oil.

Oily water separator is a machine which is used to separate the oil and  the water mixtures from the bilge tank, the oil is retained on board and the water is discharged overboard.

Principle: Main principle of operation is the gravity differential between oil and water.

Pic: 2-Stage OWS

Components: -

• First stage separator
• Second stage coalesce
• Plates
• Filters
• Oil Probe
• Solenoid
• Vents

Pic: OWS with coalescer

Operation: -

• Clean water is delivered to the separator through the oily water inlet until discharge takes place out of the vent valve which is then closed.
• Oil water is now delivered to the separator and when the pressure inside the separator reaches 2 bar water discharge valve opens automatically.
• The mixture circulates and flow across the coarse separating compartment where some oil as a result of its lower density will separate and rise into the oil collection space.
• The oily mixture passes through corrugated baffle plate where oil sticks and forms globules which when become big will rise above due to up thrust on the globules.
• The almost oil free water passes into a central pipe and leaves the separator unit.
• Purity at this point is 100 ppm or less but more than 15 ppm.
• An oil probe is provided which will sense the oil content and operate a solenoid valve, the oil is then sent into the bilge tank.
• Air is released from the unit by a vent valve.
• Heaters are provided in the top of the unit to heat the oil and maintain a viscosity.
• The first stage filter removes physical impurities and promotes some fine separation.
• The water with small amount of oil is now sent to the second stage coalesce where it is passed through filters.
• The remaining oil separates out and the purity gets down below 15 ppm.
• This water is then discharged overboard by following certain marpol rules. 

Factors affecting separation: -

In general, a high rate of separation is encouraged by 

• a large size of oil globule
• elevated temperature of the system (which increases the specific gravity differential of the oil and water and reduces the viscosity of the oil)  
• use of sea water.  

Turbulence or agitation should be avoided since it causes mixing and  re-entrainment of the oil. Laminar or streamlined flow is beneficial.  

Means to improve separation: -

• Heating coils provided to optimize separation
• The entrance area in oil/water separators is made large so that flow is slow and large slugs of oil can move to the surface quickly. 
• A low capacity pump encourages slow and laminar flow. 
• Alternation of flow path in a vertical direction continually brings oil near to the surface, where separation is enhanced by weirs which reduce liquid depth. 
• Angled surfaces provide areas on which oil can accumulate and form globules, which then float upwards. 
• Fine gauze screens are also used as coalescing or coagulating surfaces. 
• Piping system should be designed to avoid turbulence due to sharp bends or constrictions and to calculate correctly liquid flow and pipe size to guarantee laminar flow.

A brief overview of an oil content monitoring system is given below.

Oil content monitoring system: -

• The oil content monitor consists of a small chamber with two photocells and a light source.
• The intensity of light falling on both the surface are nearly same.
• The oily water is passed through the oil content monitoring system which alters the intensity of light source falling on one of the photocell.
• The comparator compares the intensities of light source falling on both the photocell and checks the ppm with respect to the amount of intensity.
• When oil content increases beyond a certain value, comparator compares the intensity and will raise the alarm.

The rules which has to be followed for the discharge of sewage is given in annex I as below.

Annex I : Regulations for the Prevention of Pollution by Oil (entered into force 2 October 1983)

1. The ship is proceeding en route;
2. oily mixture is processed through an oil filtering equipment meeting the
3. requirements of the Annex
4. oil content of the effluent without dilution should not exceed 15 parts per million
5. The oily mixture does not originate from cargo pump room bilges.
6. oily mixture should not be mixed with oil cargo residues.