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Oily Water Separator and Sewage Plant

Guide Book for Marine Engineers

OILY WATER SEPARATOR:

Principle of Operation:

The principle of operation of OWS is the gravity difference between oil and water.

The force acting on an oil globule causing it to move in the water is proportional to the difference in weight

between the oil particle and a particle of water of equal volume.Fs = separating force

P = density of water

P = density of oil globule

d = diameter of oil globuleg = acceleration due to gravity

The resistance to the movement of the globule depends on its size and viscosity of the fluid.

For small particles moving under stream line flow the relationship between these properties is expressed by

Strokes Law: Fr = resistance to movement

= viscosity of fluid

Fr = 3d = terminal velocity of particle

d = diameter of oil particle

When separation of an oil globule in water is taking place,

Fs = Fr

High rate of separation is favored by:

Large size of oil globule.

Elevated temperature of the system ( i.e. lowers viscosity of oil, density of oil-water but oil is higher). Residence time.

Avoidance of turbulence or agitation. Laminar or stream line flow.

Low viscosity of oil globule.Power supply

DiaphragmControl SV

Oil Sensor Vent

Vent & filling To Oil

Collecting Test

SV Oil Tank CocksO/B

Test

TC cocks

To Bilge OilTank

Filter Filter FromBilge

Drain Pump

Second stage and Coalescer First Stage

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Control Box

2)(18

dPoPwg

v =

gPoPwdFs )(6

3=

Filters used in OWS are 2 types

1. Coalescence type:

Hydrophilic material i.e. glass wool.

Absorbs water rather than oil.

Has to be renewed after certain periods.

2. Absorption type: Oleophilic material

Absorbs oil rather than water.

Fre uent re lacement is not re uired.


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Non-return spring loaded water discharge valve prevents syphoning as the seperator positioned above the overboardvalve. Syphoning is permitted once pumping has stopped.

Operation of OWS:

The oily water separator 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 and the purityof this stage is 100 ppmor less.

The remaining oily 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 untilit 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 15 ppmor 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 theseparator.

The water flows in turn through two filter stages and the oil-removed pass 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 and the oil from the collecting

spaces is drained away manually as required. Coalescer is the break down of surface tension between oil droplets in an oily water mixture, which causes

them to join and increase in size.

With respect to Oily Water separation state how following conditions affect separation:

a) Oil Density

b) Mixture Temperaturec) High Pumping Rate

d) Oily water interface bellow Test Cocks.

a) Oil Density: The principle of separation by which an oily-water separation function is based on difference indensity between water and oil. If other separating conditions are constant then high-density difference between

oil and water will accelerate the separation.

Separation 0. 95 s.g. of oilVelocity 0. 99 s.g. of oil

Diameter of oil particle (mm)

b) Mixture Temperature: Mixture temperature affects the specific gravity differential of oil and water, and the

surface temperature of oil particles. The speed at which oil droplets rise depends on size, the difference indensity and the surface tension. Generally, a high rate of separation is favored at elevated temperature of the

system.

High oil density / low mixture temperature causes:1) Low density differential between water and oil.

2) Increasing surface tension, thus discourage coalescence.

For this reason, low mixture temperature results oil discharged with water.

c) High Pumping Rate: The performance of separator depends on residence time of oil-water mixture in the

separator, the degree of agitation (affects the droplet size of oil). High pumping rate will cause higher velocityof water in separator, which

- Reduce the residence time of water in the separator.

- Increase turbulence and disintegration of oil globules.

Therefore, high pumping rate can result in oil being discharged.

d) Oily water interface bellow Test Cocks: Oil-water interface below test cock indicates that oil-collecting space

is full of oil. At this point, oil is collected and discharged to oil collecting tank automatically or manually. If not

discharged, further separated oil will be collected up to the water collected point of the separator. after thiscondition, more collected oil will be discharged with the water.

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Internal Baffles are fitted in OWS due to:

Baffles plates are fitted in the 1st stage of oily water separator to improve separation of oil from water.

Oily water has to pass through a series of dashed plates before leaving the 1st stage separator.

Oil droplets from the plates tend to travel upwards, being collected at the baffles and funneled up to the oilcollection space.

Coalescers are generally incorporated in OWS due to:

Coalescers are generally incorporated in the 2nd stage for bilge water purification towards 15 ppm requirement.

The coalescing materials are usually porous and granular or fibrous membra. The coalescer device encourages

formation of large oil droplets from the dispersed phase. Small oil droplets come in contact with the coalescersurface and as they congregate, thus join up to form large oil droplets of oil.

The oil droplets remain in the coalescer until they have grown large enough to achieve the buoyancy required

to break away and float upward.

Grill

Large oil dropletsSmall oil droplets

Coalescer Element

How and why maximum throughput of bilge and ballast water is restricted?The ability of oil to settle in a gravimetric separator will obviously depend on its density, the residence time and the

degree of agitation. Throughput of bilge water relates the velocity and residence time. Velocity will be higher due tohigher throughput; but higher throughput causes turbulence or agitation, which will disrupt the separation process.

Agitation causes re-entrainment.Therefore, maximum throughput of bilge or ballast water is restricted to achieve the optimum velocity and

residence time for high rate of separation. High throughput can result in oil discharged with water.

Temperature changes: Density / specific gravity / surface tension.

Throughput changes: Velocity / turbulence / agitation / residence time

Why positive displacement pump is preferred to centrifugal pump for supply purpose?

Centrifugal pump has various disadvantages on separator performance, such as

Centrifugal pump churns (moves violently) the supply and produce small oil droplets throughout the water sothat 15 ppm requirement can not be met.

But a slow speed positive displacement pump enables a much better performance to be achieved from the

separator as they do not produce large quantities of small oil droplets.

Leakage from seal ring.

Re-circulation of liquid in the impeller.

Excessive speed of fluid in the volute casing (15 ~ 25 m/sec).

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SEWAGE PLANT:

Vent

Wash waterC = Clorinator

CF = Course Filter Soil inlet

C CFFloat switch

Settling tank

Aeration tank

Collecting tankOver 60minutes

Board Air

Port sideAir Comp

Stbd side SeaShore connection Pump Water

Biological Sewage Treatment Plant

BOD is a measure of the total amount of oxygen, which will be taken up by the chemical and organic matter in theeffluent. This is usually associated with a specific period and that normally taken is 5 days period. The value is

written as BOD5, and is determined by incubating one litre sample of sewage at 20oC diluted in sufficiently well

oxygenated water. The amount of oxygen absorbed over the 5 days period is measured.

Suspended Solids are unsightly and over a period of time can give rise to silting problems. They are usually a sign

of malfunctioning of sewage plant. Suspended solids are measured by filtering a sample through a pre-weighedasbestos pad, which is the dried and reweighed.

The Ecoliform Count is a family of bacteria, which lives in the human intestine. The can be quantified easily in a

laboratory test, the result of which is indicative of the amount of human waste preset in a particular sewage samplead the count is expressed per100 ml. Standard sample incubating 48 hrs at 35oC.

MARPOL 73/ 78 Annex IV: Regulation for the prevention of pollusion by sewage from ship.

This regulation is applicable to

Ship of 200 GRT and above.

Ship of carrying 10 passenges and above.

Sewage can be discharged from ship:

When the ship is en-route and not less than 4 knots speed.

Comminuted and disinfected sewage 4 nitical miles away from the nearest land.

Not comminuted and disinfected seawge notical miles away from the nearest land.

Shore Connection for Sewage Plant: Permissible Discharge Condition:

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210 mm OD x 16 mm flange Suspended solids : 50 mg / liter 4 holes x 18 mm dia 170 mm PCD Biochemical Oxygen Demand [BOD] : 50 mg / liter 4 x 16 mm bolts and nuts. E-Coleform count : 250 / 100 ml Pipe diameter 100 mm acceptable : 6 kg / cm2.

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