Oily Water Separator treats bilges of machinery spaces, which reduces sea pollution to a marginal level. A major source of oil pollution in the past from the operation of ships was the discharge into the sea of tank washing from tankers. This was reduced by the discharge of tank washing to a slop tank for settling, and discharge overboard of the water While retaining the sludge for pumping ashore to the refinery, with the next cargo. Crude oil washing (COW) eliminates the use of water and enables cargo resides to be pumped ashore during discharge because cleaning is carried out simultaneously with the discharge.
Ballast carried in oil cargo and bunker tanks which is therefore contaminated with oil constitutes another pollution source, unless pumped out via an oily water separator. New regulations require tankers of certain sizes to have segregated or clean ballast tanks.
A third pollution source is from machinery space bilges.
The most common type of oily water separators found on ships are of the gravitational type. these rely on the difference in specific gravity of the mix to separate out the oil from the water.
Centrifugal Oily Water Separator
Centrifugal separators have been proposed for the use as oily water separators. The quality of the output is determined by the throughput rate. The slower the flow of oil through the separator the better quality output. A question mark exists over their ability to cope with fine emulsions and chemical pre-treatment is recommended.
Separators capable of emulsion treatment
Gravitational separators are not capable of operation with oil emulsions , or mixtures containing oil of high specific gravity approaching 1 or above. The latter may be improved by the preheating of the mixture before or during the gravitational process. The former is more difficult, current regulation requires the careful control of detergents capable of effecting the operation of the fitted separator.
This means that modern efficient detergents containing surfactants may be only used in restricted quantities or not at all.
Alternatives to gravitational separation are now becoming available capable of dealing with these mixtures. The most common at the moment involves the use of Poly aluminium Chloride. This causes the emulsified oil to join to gather (flock). The emulsion is thus broken and the water and oil separated. Using this process very high quality effluent can be produced with little of no oil or chemical content. The cost is higher than for more conventional gravitational separators.
An alternative method is the use of Electrocoagulation. This relies on the three factors of a stable emulsion
-Droplet or Particle Size
-Droplet or particle density
An electrical charge is passed through a sacrificial anode made of aluminium. The released ions are attracted to the negatively charge fine droplets of contaminants. The overall effect is one of agglomeration with larger and larger droplet sizes being produced. In addition gas bubbles produced by hydrolysis attach to these droplets increasing there buoyancy. The separated droplets rising to the surface may be removed. This is a very efficient process and large volumes can be coped with.
Gravitational Separator Operation
The unit is initially filled with purge water. The discharge from the pump has a sample line take of to a 15 ppm monitor. This is lined up and in used with flushing water used until the pump is running. The unit activation button is pressed, the oil outlet valve is closed, the suction valve is opened and the discharge pump is started. Bilge water is drawn through the unit over a vertical arranged plate stack. The 15 ppm monitor is lined up to the pump discharge sample line
As oil coalesces it is led to the oil discharge chamber. As the oil here builds up the interface drops until the pump cut in probe is activated, the pump is stopped, the suction valve is closed, the oil discharge is opened and the purge water is opened. Oil is forced out of the oil outlet by the purge water.
When the oil water interface reaches the cut out the oil discharge valve and the purge water valve is closed. The suction valve is closed and the pump started.
A complete Oily water separator and filter unit for 15 parts per million purity is shown in figure. The complete unit is first filled with clean sea water, the oily water mixture is then pumped through the separator inlet pipe into 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 on to 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 (heating reduces viscous drag of oil and thus makes separation of oil and water easier).
Where greater purity is required, the almost oil free water passes to a filter unit. 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 fins separation. The second-stage filter uses coalesces inserts to achieve the final de-oiling. Coalescence breakdown of surface tension between oil droplets in an oil/water mixtures 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.
Oil Content Monitoring
Regulations with respect to the discharge of oily water set limits of concentration upto 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 sample cell (Figure ). 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 Vault.
Alarms and shutdowns
If the 15 ppm equipment detect discharge with oil content over 15 ppm it shuts the unit down and activates and alarm. But in some cases only alarm is there.
Reasons for improper functioning of an Oily water Separator:
- The principle of separation on which this separator function is the gravity differential between oil and water. The force acting on oil globule to move in the water is proportional to the difference in weight between the oil particle and a particle of water of equal volume. The resistance to the movement of the globule depends on its size and the viscosity of the fluid. Thus in general, a high rate of separation is favored by:
- Large size of globule.
- Elevated temperature of the system (which affects both the specific gravity differential of the oil and water and the viscosity of the water)
- The use of seawater.
- Pumping consideration. Since the rate of separation depends on the oil globule size it will be appreciated that any disintegration of oil globules in the oily feed to the separator should be avoided and this factor can be seriously affected by the type and rating of the pump used. A large number of bilge pumps are centrifugal and they are often used as the supply pump to separator. They churn the supply and produce small oil droplets dispersed throughout the water, which in turn may seriously affect the separation efficiency. A positive displacement pump e.g. slow running double vane, screw, reciprocating or gear pump enables a much better performance to be achieved from the separator as they do not produce large quantities of small oil droplets. Using pump after the separator may give a discharge having less than 15 ppm. concentration without using second stage filters.
From above two points it is evident that even if the separator is well maintained and correctly operated following factors can cause improper functioning of the separator.
- Through put of the separator is excessive.
- Excessive rolling and pitching of the ship causing disintegration oil globules.
- Pump or and rating is not matching, causing too much of turbulence.