Wednesday, 20 June 2012

Oily Water seperators

Gravitational

The most common type of oily water seperators found on ships are of the gravitational type. these rely on the difference in SG of the mix to seperate out the oil from the water
Oily Water seperators
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Shown above is a gravitational type seperator capable of very good quality effluent discharge. A safety valve (not shown) is mounted on the unit as are test cocks and a drain valve.

Operation

The unit is initially filled with purge water. The discharge from the pump has a sample line take of to a 15ppm 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 15pp monitor is lined up to the pump discharge sample line
As oil coalescers 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.

Alarms and shutdowns

Should the 15ppm equipment detect discharge with oil content over 15ppm it shuts the unit down and activates and alarm
When the pump cut out probe is activated a timer starts, should the oil interface not reach the pump cut in probe within a set time a alrm is sounded and the unit shutdown.
Should the oil interface reach the alarm and shut down probe and alarm is activated and the unit shut down

Centrifugal seperators

Centrifugal seperators have been propsed for the use as oily water seperators. The quality of the output is determined by the throughput rate. The slower the flow of oil through the seperator the better quality output. A question mark exists over their ability to cope with fine emulsions and chemical pre-treatment is recommended.

Seperators capable of emulsion treatment

Gravitational seperators are not capable of operation with oil emulsions , or mixtures containing oil of high sg. Approaching 1 or above. The latter may be improved by the preheating of the mixture before or during the gravitatinal process. The former is more difficult, current regualtion requires the careful control of detergents capable of effecting the operation of the fitted seperator. This means that modern efficient detergents containing surfactants may be only used in restricted quantities or not at all.
Alternatives to gravitational seperation are now becoming available capable of dealing with these mixtures. The most common at the moment involves the use of Polyaluminium Chloride. This causes the emulsified oil to join to gether (flock). The emulsion is thus broken and the water and oil seperated. 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 gravtiational seperators.
An alternative method is the useof Electrocoagulation. This relies on the three factors of a stable emulsion
-Ionic Charge
-Droplet or Particle Size
-Droplet or particle density
An electrical charge is passed through a scarificial anode made of aluminium. The released ions are attracted to the negatively charge fine droplets of contaminants. The overal 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 bouyancy. The seperated droplets rising to the surface may be removed. This is a very efficient process and large volumes can be coped with.

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