CHAPTER 9

FUEL OIL AND LUBRICATING OIL TREATMENT ON BOARD A SHIP

9.1 General concept

To ensure good combustion in the diesel engines and to reduce wear and corrosion of parts of engines it is necessary to remove certain impurities (residues, dirt, abrasive and deposits) from the oil and lubricating oil.

The impurities may be accounted as residues, sand, dirt, water, ash, various salt, and abrasive attending in the fuel oil and lubricating oil.

The quality of fuel oil and lubricating oil is improved by the treating methods mentioned below.

9.2 Treating methods.

9.2.1 Filtration

This method is implemented by putting a special material (paper, combination of metal net or metal plates etc.) in the flow of the dirty liquid (fuel oil or lubricating oil) to prevent the entering of the impurities. Filtration can usually remove impurities of 80 μm in size. Water is not removed by the filtration.

Filter

Back-wash Filter

1. Gear; 2. Seal ring; 3.Cover; 4.Bearing; 5.Shaft; 6.O ring; 7.Filter element;

8. Filter body; 9.Oil inlet; 10.Oil outlet; 11.Discharge pipe with orifice

12. Key; 13 & 14. Bolt & nut; 15. Element holding plate; 16. Gasket

9.2.2 Homogenization

The impurities found naturally in the crude oil, physically and chemically combined with the carbon atoms, will also remain in the residual oil-The waste from the refinery. Attempts to remove them would be very expensive in comparison to the value of the residual oil; therefore, they remain in the refinery waste.

The dense carbon particles, known as asphaltenes, are cause of fuel sludge and incomplete combustion (asphaltenes and impurities are insoluble in fuel oil). Impurities (vanadium, nickel, iron) and catalytic fines are bound within asphaltene molecules (asphaltenes contribute to increase engine particulate emission).

These impurities and asphaltenes are combustible. When purified they are removed and disposed (an energy is lost). If they are burnt with fuel in combustion chamber this energy is exploited (savaged).

To better utilize today's heavy fuel oil and reduce wasted energy and disposal cost, fuel homogenization is used.

• The homogenization systems in which a first liquid or solid component, such as water or impurities etc., is homogenized with a second liquid component, such as fuel oil, and more particularly to such systems where the homogenization is accomplished using orifices to produce shearing and cavitation.

• Homogenization is a process that physically mills and disperses mixtures of fuel (fuel-impurities, fuel-water...) in the form of segments or pieces into very fine particles. The homogenized fuel oils is so fine that they can be directly injected into the combustion chambers of diesel engine.

• Homogenization is the capability of sheering fuel oil consistently to very fine particles (maybe less than 10 microns in size) while evenly mixing droplets of water with this fuel oil at predetermined ratio.

-The technique known as cavitation or cavitating flow in which a stream of a liquid component, such as fuel oil, is passed through a relatively small orifice at high velocity, resulting in a cavitating free turbulent velocity shear layer, and a second component insoluble in the first component, such as water, is added at the orifice exit in order to intermix the two components into a homogenized mixture or colloidal suspension.

Advantages:

- Turn asphaltenes (C57H32) into burnable fuel

- Reduce sludge discharge from purifiers and filters

- Reduce or eliminate waste disposal cost

- Reduce the emissions from burning the homogenized fuel oil

Homogenizers maybe consists of a stationary stator housing with a motor -driven rotor, which is concentrically mounted inside the stator. The mating surfaces of the rotor and stator have special channeled grinding surfaces.

Rotor/Stator Arrangement

-Conical-shaped

- Concentrically mounted

-Slightly decreasing clearance

During operation, at a speed of 1500 to 2000 r.p.m, fuel passing through the homogenizer is exposed to hydrodynamic power:

- Shearing and frictional forces

- Acceleration power

- High frequency ultrasonic waves

- In combination, these forces act together to shear the asphaltene particles down to 3 to 5 microns. Smaller particle size allows the asphaltenes to be evenly blended with the heavy fuel oil, reducing the sludge formation and associated waste disposal costs and yielding more burnable fuel.

To achieve improved combustion, while reducing noxious exhaust emissions, smoke and NOx, some engine manufacturers have incorporated homogenization to produce the water-fuel emulsion. In process, the heated water is introduced into fuel system, after fuel purification, creating a water in-fuel emulsion. Injected into combustion chambers of engine, the mixture consists of water droplets coated with fuel oil. As the water at the center of the droplets flashes due to high temperature, creating "secondary atomization" (micro-explosion), the fuel coating the water explodes into smaller droplets. This secondary atomization will

- Increase the surface area of the injected fuel, improving the rate of vaporization,

- Absorbent of heat by the vaporization of water will reduce peak of combustion temperature , and

- The resulting lower local ignition temperature and the reduction of partial pressure of oxygen present will reduce the formation of NOx.

Disadvantages:

• However, the dispersion of fuel oil cannot be last for a long time. About 24 hours after being homogenized, the fuel oil must be treated again by the same process. Furthermore, the fuel oil should not be treated by centrifugal separator after being homogenized.

• The service life of parts of fuel system (fuel injection valves, fuel injection pump, parts of the homogenizer) is short. They are worn out quickly.

• Nowadays, homogenization is not a best choice because of its price, maintenance cost, and somewhat uncertain efficiency.

9.2.3 Chemical treatment

Some additives are put in fuel oil or lubricating oil in order to improve properties of fuel oil and lubricating oil.

• It makes impurities in fuel oil separated more easily.

• It is the corrosion inhibitor.

• It improve the quality of combustion process of fuel oil better

9.2.4 Separation by effect of Gravity

9.2.4.1 Sedimentation

Sedimentation means keeping the dirty liquid in tank under static condition (under the influence of the gravitation). After a certain period of time, the impurities which are heavier than the liquid will fall down to the bottom of the tanks.

G=γsVs

A=γlVs

R=3πdµvg

Where:

γs and γl :Specific gravity of solid particle and liquid.

ρ: density of liquid

Vs: Volume of solid.

c: Resistance coefficient.

d: Equivalent diameter of solid particle. Let's consider a particle as a spherical particle.

µ: viscosity of liquid.

g: acceleration of gravity.

vg: terminal (separating) velocity of solid particle.

Re; Reynolds number.

During separation (sedimentation) we have the following fomula:

G= A+R

After transforming above formula we have:

∆γ and ∆ρ: the difference between solid particle and liquid.

vg is proportional directly to ∆γ, d and inversely to µ. The above expression shows that the sedimentation of a particles is determined by the physical characteristics of the particle and the liquid.

- The larger the particle diameter, the greater will be the sedimentation rate.

- The greater the difference in density between the particle and the liquid, the greater will be the sedimentation rate.

- The lower the viscosity of the liquid, the greater will be the sedimentation rate.

Otherwise, the shorter the sedimentation height is, the shorter the time required for separation is.

Sedimentation is applied to fuel oil on board a ship as shown in below figure:

F.O Settling tank

9.2.4.2 Continuous separation by gravity

The above figure illustrates a tank for the continuous removal of a solids phase, consisting of particles of varying diameter, from a process liquid. The Liquid with impurities is introduced at one end of the tank (inlet) and flows forwards the outlet at the other end of the tank with a velocity w. The dispersed particles are separated out and fall to the bottom at a sedimentation velocity of vg (due to gravity). The sedimentation orbit of solid particles is determined by the liquid velocity w and separating velocity vg. The condition of separation whereby the effluent, which is the cleaned process liquid leaving the outlet, does not contain particles of a diameter greater than dlimit (in proportion to vg lim). The throughput Q of the tank is:

Q= vg lim S

Where; S: the sedimentation surface area.

The shorter the sedimentation height is (the sedimentation area is increased by fitting a number of horizontal plates and this gives a number of separation channels), the shorter the time required for separation is and the smaller the diameter of particles are separated.

In the case of continuous separation in a tank fitted with horizontal plates, the channels will eventually become clogged with sediment and separation will cease. If inclined plates are fitted, the sediment will slide down the plates under the influence of gravity and will collect at the bottom of the tank.

If two liquids are to be separated from each other by gravity, while solid particles are to be simultaneously removed, the arrangement shown in below figure may be employed. The mixture of liquids flows into the tank through the inlet. In the tank, gravity acts on the particles in the same manner as in a simple sedimentation tank. Sedimentation takes place in space (A). The lighter liquid phase rises towards the surface where it flows over the upper weir (B). The heavier liquid phase falls to the bottom and flows out across the lower weir, having first flowed underneath the baffle wall (C). The latter prevents the lighter liquid from escaping with the heavier liquid.

9.2.5 Separation in centrifugal field

If we fill the tank with a liquid and cause this tank to rotate. A centrifugal field will be generated. The centrifugal acceleration a is not constant but increases as the distance of the particle from the axis of rotation of the tank increases and as the angular velocity increases.

The sedimentation velocity of the particles suspended in the liquid in the centrifugal field:

In case of centrifugal sedimentation, as the centrifugal force acting upon is several thousand times as large as gravity, its separation rate and purification effect are far bigger than those of gravity sedimentation.

The factor z = rω2/g is known as the "g-value" and specifies how much greater the sedimentation rate of a particle is in a centrifugal field as compared with the gravitational field.

There are three types of the centrifugal separation.

A. Cylindrical bowl type

Cylindrical bowl type centrifuge has larger centrifugal effect and is used for separation of "liquid-liquid" or "solid-liquid".

It has rather large number of revolution of about 10,000-20,000 r.p.m, but centrifugal sedimentation area of this type is small because of the restriction in size from its structure.

The structure is very simple, however it is necessary to remove sludge by stopping its operation.

B. Decanter Centrifuges

Decanter type centrifuge is used for the separation of "solid-liquid" having large density difference and much content of solid particles (the solids concentration can go up to 65% by volume and particle size in the feed can be between 1μm and 30 mm.

Clarifier of this type has such a structure as disc type centrifuge without discs and it is designed that solid particles may be settled and accumulated inside the bowl. These types (conical and cylindrical type) are capable of continuous taking out of sedimentary solid particles after dehydrating them to a certain extent.

They are equipped in the conical or cylindrical bowl with screw conveyor rotating at the speed little different from bowl. The decanter centrifuge can be used for most types of liquid/solid separation. It can be used for the classification of solids in liquid suspension or for the clarification of liquids.

C. Disc type centrifuges

Disc type centrifuge has numerous discs in the shape of truncated cone in the bowl and performs separation in the narrow spaces between discs. The structure of this type is more complicated than that of cylindrical type. Its numbers of revolution of bowl is about 4,000-10,000 r.p.m.

If significant separation is to be achieved in this centrifuge, the concentration of the solids phase in the process liquid should not exceed approximately 25% by volume. In addition, the particle or liquid droplet size to be separated should be between 0,1 and 500 µm.

The centrifugal sedimentation area of this type is far larger than that of cylindrical type. The disc type centrifuge is used very widely on board a ship.

9.2.5.1 Disc type centrifuge

a- Theory of separation

A stack of discs in the shape of truncated cone has been incorporated inside bowl for the purpose of increasing separation efficiency.

Small circular or long rectangular plates known as caulks are fitted between the discs. The caulks separate the discs so that separation channels are formed between the discs. The thickness of the caulks may be varied so as to adjust the height of the separation channels to the particle size and the concentration (normally, the space between two discs is about 0.6mm).

The principle of operation of this centrifugal separator as follows:

The liquid to be separated by centrifuge is fed into the top of the bowl through the feed pipe which projects into the bowl centre tube (distributor). The liquid and suspended solids pass downward through the annular space between the distributor and the bowl (arrows). The liquid flows upward along the outer edge of the bowl discs and passes inward between the discs. The coarser solids are removed from the liquid (and accumulated in the sludge space) before entering the discs while finer solids are separated out through the thin layers of liquid during it passes through the disc stack (through the separation channels). Cleaned oil moves to the center of the bowl, then leaves the bowl through the cleaned oil outlet. Solid particles are separated from oil and settled on the beneath surfaces of discs, then slide along these surfaces to the sludge space.

Path of limit particle through separation channel

Solid particles mixing in liquid will be subjected to a centrifugal force and force by liquid flow. A sum of these velocities gives a resultant velocity vρ, the direction of which determines the path of the particle. So a particle will move about the locus indicated by a dotted line.

The limit particle is that which starts from the most difficult position, i.e. point A and is barely separated out at B'. This particle has a diameter of dlim. All particles of a diameter greater than dlim will be separated out.

b- Treating quantity (separator capacity; throughput; feed rate)

If it is required to separate from the liquid all particles of a diameter greater than or equal to a limit particle of diameter dlim, the separator capacity (treating quantity) may be derived by use of the equation:

This equation may be interpreted as follows. To separate out all particles of a diameter greater than or equal to dlim from a liquid having the properties ρl and μ in a centrifugal separator with given characteristics (ω,z,θ,r1, r2), the capacity must not exceed Q.

c- Types of centrifugal separator

There are two types of separation:

- Separation of a solids and a liquid phase- Clarification. The centrifugal separator used to separate solids from liquid is Clarifier.

- Separation of two liquids with simultaneous separation of a solids phase- Purification. The centrifugal separator used for purification is Purifier.

Classifier Purifier

1-Dirty oil-untreated oil 1-Dirty oil-untreated oil

2- Outlet for cleaned oil 2- Outlet for cleaned oil

3- Operating water flow 3-Water outlet

4-Operating water

During operation of purifier, interface (The more or less cylindrical division between the two liquids is known as the interface) between two phases is formed. The location of this interface impacts the separation. The interface move from its correct position:

- Towards the periphery of the bowl, ultimately resulting in broken water seal, if there is an increase in density, viscosity or flow rate or a decrease in temperature. The light phase flows out at the heavy phase outlet.

- Inwards to the axis of rotation (the interface is located closer to the axis of rotation):

+ The heavy liquid will have a greater chance of being purified while the light liquid may contain quantities of the heavy phase.

+ The disc stack is blocked with water (heavy phase).

+ Real sedimentation area is reduced.

The interface is maintained in the correct position, that is, outside the disc stack (between the outer edge of top disc and the outer edge of disc stack).

The interface must be located at the optimum position inside the bowl. The location of the interface is fixed by using gravity discs of different diameters located in the heavy liquid outlet.

The principle of gravity disc is as follows:

In order to make it easy to understand, let's consider about the U-tube in the gravity field (i.e. gravity sedimentation).

Methods to select the gravity disc

• To ensure best quality of the separating process, the interface must be maintained in a stable position. Movement of the interface inwards or outwards the vertical axis of the purifier may cause bad quality of separation. The selection of proper gravity disc is of most important point in using purifier and full attention must be paid.

• The gravity disc can be chosen by following methods.

- Calculation by using given formula,

- Using "trial and error" method,

- Using tables recommended by manufacturers:

When the specific gravity of the light phase is known at 150C (600F) and the heavy phase is water, the hole diameter of the gravity disc to be tried first can be found from Table I if the separating temperature is 550C (1300F), and from Table II if the separating temperature lies between 80-1000C (175-2120F).

- Using monograph:

Nowadays, normally the gravity disc is selected by chart (monograph). The following parameters must be taken into consideration when selecting gravity disc:

1. Specific gravity of oil

2. Separating temperature

3. Feed rate

+ Example 1:

Treating condition:

• Specific gravity of oil to be treated: 0.903 at 150C

• Treating temperature: 700C

• Treating capacity: 1000 l/h

Selection method:

1. Draw a curve (1) from 0.903 (specific gravity=0.903) and a vertical line from 700C (treating temperature).

2. Draw a parallel line (2) from the intersecting point of curve (1) and the vertical line.

3. Connect the intersecting point of the parallel (2) and the ordinate with point of 1000 (treating capacity:1000 l/h) by a direct line (3)

4. The intersecting point of line (3) and the vertical axis (inside diameter of gravity disc) indicates the inside diameter of desired gravity disc (ϕ 69.5 mm).

+ Example 2:

Treating condition:

• Specific gravity of oil to be treated: 0.942 at 500C

• Treating temperature: 980C

• Treating capacity: 700 l/h

Selection method:

1. Draw a curve (4) from 0.942 (specific gravity=0.942) and a vertical line from 500C (treating temperature).

2. Draw a curve (5) from the intersecting point of curve (4) and the vertical line (500C), and a vertical line from 980C.

3. Draw a parallel from the intersecting point of (5) and the vertical line (980C).

4. Connect the intersecting point of the parallel and the ordinate with point of 700 (treating capacity:1000 l/h) by a direct line (6).

5. The intersecting point of line (6) and the vertical axis (inside diameter of gravity disc) indicates the inside diameter of desired gravity disc (ϕ 63.5 mm).

e- Sealing water (liquid seal)

Supposing that first oil is supplied when bowl is empty, oil will flow out from water outlet. To prevent these, it is necessary to supply water beforehand (before separation and after solid discharge; before the dirty oil is allowed into the bowl) in order to seal water outlet. Once water layer had been formed inside bowl, oil supplied later will be discharged from oil outlet (Water is forced against the periphery of the bowl, forming a liquid annulus, the inner side of which acts as a liquid seal for the light phase -oil).

Water supplied in advance is called as "sealing water". Sealing water is supplied from (4) and, passes through the distributor (A) and is accumulated on the periphery within the bowl.

When water is supplied into bowl, it will flow out from only water side (water outlet) while the flow rate is small. But when the flow rate is increased, water will begin to flow out from oil side (oil outlet) as well as water side at a certain flow rate. The flow rate at this time (when being increased) is called as "the critical discharge capacity of heavy liquid (water) side). It differs according to each Model No. of Purifier or inside diameter of gravity disc to be used.

If sealing water is poured in at the flow rate exceeding the critical discharge capacity for a long period, the quantity of sealing water will get excessive and then sealing water will flow out from both water and oil outlet side. The desirable quantity of sealing water is about 70-80% of the critical discharge capacity.

The time for supplying sealing water is decided upon the relation between the quantity and the velocity of flow.

9.2.5.2 Outline of structure of centrifuge (Selfjector)

Driving power from the motor is transmitted to the horizontal shaft through the fiction clutch. The vertical shaft is accelerated by spiral gear installed on the horizontal shaft and pinion gear on the vertical shaft. The bowl is installed on the top of the vertical shaft and revolves at the revolution speed of the vertical shaft (about 8000 - 9000 r.p.m).

The vertical shaft is supported by upper and lower ball bearings. The upper bearing is supported by springs to absorb the vibration generated by the first critical speed (nearly 1,300 r.p.m).

a. Vertical shaft

The vertical shaft is supported by upper and lower ball bearings . The upper bearing is supported by springs (about 6 pieces) to absorb the vibration generated by the first critical speed at start -up (nearly 1,300 r.p.m). The lower bearing is subjected to the thrust load by the weight of the bowl and others.

b. Horizontal shaft

The horizontal shaft is supported at two points by pump side bearing case and motor side bearing case with ball bearings. Maybe, one end of the shaft is connected to gear pump by safety joint.

c. Bowl construction

The bowl is composed of bowl body, bowl hood and bowl nut which form a container. Dirty oil led to separation chamber from dirty oil inlet through distributor. Solids and water are separated from dirty oil while passing the spaces between discs.

The purified oil and separated water flows out continuously from purified oil & separated water outlet respectively.

Mitsubishi Selfjector SJ-T

Disc stack Disc

d. Other components

Friction clutch Brake

Centripetal pump

- Friction clutch: Since the bowl has a large inertia, in case of the method of start-up by directly connecting the motor to horizontal sharp, the capacity of motor and the strength of sharp should be designed too excessive. Therefore, the method of gradually accelerating the bowl has been applied. The friction clutch is used to effect slow starting and acceleration so as to prevent motor overload. After start-up, the friction block lining is pressed under centrifugal force against the inside of friction pulley, slip occurs between friction pulley and lining, and driving force is transmitted to the friction pulley (horizontal shaft side) and in about three to ten minutes the vertical shaft attains its rated rotation.

- Centripetal pump: The centripetal pump is a spiral impeller provided on the top of bowl to transfer light and heavy liquids out of the centrifuge. This pump is provided with a spiral groove or radial hole in a disc with certain thickness, dipped in liquid revolving with the bowl and the liquid ill be discharged by its own turning force along groove or hole.

- Gear pump: This pump is used as suction pump to feed dirty oil to the purifier.

9.2.5.3 Mechanism of sludge discharge and operating water supply device

a. Mechanism of sludge discharge

Impurities (solid particles) that are separated from the dirty oil are accumulated on the inside wall of the bowl during operation. The sludge (mixture of solids, water and oil) is discharged through ports of periphery of the bowl. The sludge discharge ports are closed and opened by the main cylinder or valve cylinder or sliding bowl bottom.

Since the lower pressure water chamber installed at the lower part of the bowl is filled with water during operation, the valve cylinder is pushed upwards by water

Mitsubishi Selfjector SJ

pressure generated by centrifugal force and sludge discharge ports are closed.

On discharge sludge, when water is supplied into the upper chamber, since it has larger pressure receiving area than the lower chamber, the force of the former will exceed over that of the latter. So the valve cylinder will be opened, and solids accumulated inside the bowl will be discharged by powerful centrifugal force.

b- Operating water supplying equipment

The above right figure shows the operating water supply device. In case low or high pressure operating water is supplied, water level of the operating water chamber will move radially and operating water will be supplied into the lower or upper pressure water chamber. Then valve cylinder will slide up and down and sludge discharge ports will be closed or opened.

During operation, the water level in the operating water chamber is settled at a certain position because of the following reasons.

In the above figure, the pressure P of bowl at the top of operating water disc (point "a" is shown in the following equation:

Where, ri : Radius of operating water disc

rf : Radius up to water level

ω: Angular velocity

γ: Specific gravity of water

g; Acceleration of gravity

While, the pressure P'(water pressure) at the top of operating water disc by low pressure operating water tank is shown in the following equation:

H: water head

Since each value γ and g is constant, and that of ω and ri is fixed, the value of rf will vary until the pressure P reaches the pressure equal to that of operating water P' and the water level will be fixed at a certain position. So, water will neither flow into operating water disc from operating water chamber, nor flow out to the chamber.

Under above condition, since the nose of static operating water disc is dipped in water rotating at high speed, naturally friction will be caused between the operating water disc and water. In that case, by the effect of the friction, water around the operating water disc is put into disorder and become water spray, and as a result water will be consumed. Water consumption will also happen in case there are any failures or damages of "O" rings used in sealing the lower chamber and others. But, in order to keep the pressure P as equal to P', for the reason mentioned, additional water will be supplied automatically corresponding to the amount of consumed water from operating water tank and lower water pressure chamber will never get empty.

At the time of sludge discharge, when high pressure operating water is supplied, as the principle mentioned above, water level will move inwards till the pressure P balances with that of high pressure water (water level will move from rf to rf'). When water level reaches the position of holes leading to the upper pressure water chamber, water will enter into them and they will be filled with water. Accordingly, valve cylinder will be pushed down and solids will be discharged.

Furthermore, though plug screws with nozzle have been provided in the upper chamber, when high pressure operating water is supplied, not all of water will be discharged instantly through nozzles and the remained water will fill the upper chamber.

In resuming the purification after discharge when the supply of high pressure operating water is stopped, water in the upper chamber will be gradually discharged outside of bowl through plug screws with nozzle. Water level will then return to the position of rf and the pressure of upper chamber will be reduced to the zero. The valve cylinder will go upwards by the pressure of lower chamber to close sludge discharge ports and accordingly feeding of dirty oil can be resumed.

c- Sludge discharge interval

Many factors influence the sludge discharge interval. Experience will dictate the interval in specific cases. Some of these factors are:

- Fuel type (for FO purifier)

- Engine operating conditions (for LO purifier)

- Condition of engine (for LO purifier)

- Type and condition of lube oil (for LO purifier)

For FO purifier, conditions may change completely when bunkers are changed. Therefore, great care must be given to the discharge interval setting for each new bunker.

Fuel oil Lubricating oil

Diesel oil Heavy fuel oil Diesel oil Heavy fuel oil

Max Nor Max nor Max Nor Max nor

4 hrs 2hrs 2 hrs 1 hrs 4 hrs 2hrs 2 hrs 1 hrs

When the time interval is too long, sludge discharge becomes difficult due to adhesion of sludge and so on. If the interval time is too short, the operating efficiency is worse.

d- Sludge discharge type

Now, self-cleaning purifiers are used on board ships very broadly. There are two types of self-cleaning centrifuge: Periodical sludge discharge and continuous sludge discharge.

Periodical sludge discharge type: Periodically sludge that is accumulated in sludge space of the bowl is discharged manually or automatically. There are two types of periodical sludge discharge:

- Total discharge type: Purifier designed to totally discharge all contained in the bowl.

The operating water for closing bowl is supplied from part "B" with low pressure of about 0.2-0.4 kgf/cm2. The operating water for opening bowl is supplied from part "A" with high pressure of about 2.5-5.0 kgf/cm2.

Total discharge

- Partial discharge type: Purifier designed to partially discharge water and solids only in the bowl. This type has the total discharge function as well.

The operating water for closing bowl is supplied from part "B" with low pressure of about 0.2-0.4 kgf/cm2. The operating water for opening bowl is supplied from part "B" with high pressure of about 2.5-5.0 kgf/cm2. Sludge discharge interval for partial discharge type: 20-30 min.

Partial discharge

Continuous discharge type: Purifier designed to continuously discharge solids and water in the bowl (Sharples Gravitrol Centrifuge).

Continuous discharge

Water from the recycle tank is pumped to the bowl at a rate greater than that of discharge of the nozzles and the excess is rejected over the ring dam. The greater the quantity of water separated, the less will be required from the recycle tank, and a greater quantity of water will be rejected over the ring dam. All solids separated from the oil are discharged with the recycle water into the recycle tank.

9.2.5.4 Factors influencing the quality of separation

Minimum diameter of solids that is separated out is used to evaluate the quality of separation.

From the equation of Q, there are some factors that affect the quality of separation listed below:

• Density difference: The greater the density difference is, the better the quality of separation (the easier the separation). The density difference increases when the separating temperature increases.

• Viscosity: The lower the viscosity is, the better the separation. The viscosity can be reduced by heating (800 C for LO, 90-950 C for FO).

• Throughput (treating quantity, the centrifuge capacity): The lower the throughput, the better the quality of separation. The throughput is not reduced infinitely.

- FO Purifier: The throughput of FO purifier should be larger by 15% than the fuel consumption at engine MCO (MCR).

- LO purifier: The purpose of the centrifuging process to ensure that the equilibrium condition where the engine contamination rate is balanced by the centrifuge separation rate (i.e. contaminant quantity added to the LO per hour = contaminant quantity removed by the centrifuge per hour) is reached with the oil insoluble content being as low as possible.

- The optimum centrifuge flow rate for a detergent oil is about 20-25% of the maximum centrifuge capacity.

- Whereas, for a straight oil, it is about 50-60%.

- Most system oils of today, which incorporate a certain detergency, the optimum will be at about 30-40% of the maximum centrifuge capacity.

• Treating temperature: the higher treating temperature, the better quality of separation.

• Position of the interface

• Revolution of the bowl

• Time interval between sludge discharge: this interval is not chosen properly, much solids accumulates in the bowl and separation is bad

• Number of discs.

9.2.5.5 Procedure of operation

1. Confirmation before operation

Before operation, ensure that following items are without failure.

• The bowl has been completely assembled. The tally marks on the bowl nut and on the bowl hood must be in line.

• Confirm whether the cap nut has been fixed on the vertical shaft.

• Check that collecting covers are clamped with the hinged bolts.

• Remove gear cover and turn the spiral gear by hand to confirm the smooth rotation of the bowl. The bowl should be easily rotated manually.

• Release the brake.

• Confirm that the prescribed amount of lubricating oil is supplied into the gear case by observing the oil level on the oil gauge.

• Confirm that all valves and cocks are properly opened or closed.

• Confirm that the operating tank is filled with sufficient water if the operating water tank is installed or low pressure operating water is maintained at prescribed water pressure if the operating water tank is not installed. High pressure operating water is maintained at prescribed water pressure.

• Confirm the direction of rotation of the motor by wiring of motor..

2. Start-up

• Start the purifier by pressing push-button on the starter.

• Operate the oil heater.

• The bowl attains its rated revolution in 5-10 minutes

• Close the bowl by controlling the operating water.

• Supply sealing water (heavy phase) into the purifier until it begins to flow out through the outlet for heavy phase.

• After the oil is heated to the predetermined temperature, open the feed valve gradually to supply oil into the purifier.

• Fully open the feed valve and set the feed rate by regulating the pump by-pass valve.

Caution at start-up

• In case of any abnormal noise, instantly stop the operation to investigate the cause. Then after correcting the cause, start the operation again.

• The bowl would temporarily generate vibration when passing the first critical speed before attaining the rated speed, but it is not abnormal vibration. However, if there is any unbalance due to defective assembly of the bowl or abnormality of the vertical shaft, the vibration will be amplified and it will become abnormal vibration.

• In case the bowl does not attain the rated speed after specified time passed, stop the operation instantly and find the cause.

• During feeding oil, confirm that no oil is leaking from sludge outlet.

Sludge discharging (Alfa-laval type)

• Shut off the feed valve; open the bowl by turning the control valve to position 1. Hold this position until the 'shot' is heard, i.e. the noise made when the sludge is thrown out of the bowl.

• Empty the bowl discharge mechanism by turning the control valve to position 2. Hold this position for 5-6 seconds.

• Close the bowl by turning the control valve to position 3. Hold this position until liquid flows out through the indicator pipe, showing that the bowl has closed.

• Set to operating condition by turning the control valve to position 4 to compensate for any operating liquid loss.

• Refill the bowl.

Sludge discharging (Mitsubishi type)

• Shut off the feed valve;

• Supply water into the measuring hopper and pour water into the bowl to push out oil inside the bowl in order to decrease oil loss. This operation is so-called "Replacement".

• Open the high pressure operating water valve and hold it until the 'shot' is heard. Observe an increasing of electric current value, keep it for about 5 -6seconds. Then close this valve

• Close the bowl by opening the low pressure operating water valve. The bowl will be closed in short time.

• After refilling the bowl with sealing water, open the feed valve gradually to feed oil.

3. Stop of operation (Alfa-laval type)

• Close the feed valve to stop fuel oil supply.

• Stop the operation of the heater.

• Supply replacement water.

• Open the bowl for discharging by turning the control valve to position 1.

• Shut off the operating liquid feed - control valve to position 2. Then shut off the supply of operating liquid to the control valve.

• Stop the motor by pushing stop-button on the starter and apply the brake.

• Valves and cocks are properly opened or closed.

• Do maintenance if required.

Stop of operation (Mitsubishi type)

• Close the feed valve to stop fuel oil supply.

• Stop the operation of the heater.

• Supply replacement water.

• Open the bowl for discharging by opening the high pressure water valve to supply water for sludge discharging the bowl until bowl open.

• Valves and cocks are properly opened or closed.

• Stop the motor by pushing stop-button on the starter and apply the brake.

• Do maintenance if required.

FO purification system (Mitsubishi SJ)

A.Dirty oil inlet; B.Purified oil outlet; C.Control air inlet; D. Sludge & water outlet;

E.Water inlet; F. Operating water drain; G. Water drain; H. Oil circulation; 1. Purifier

2. Operating water tank (for closing the bowl); CV1. Three-way valve;

SV4, SV5. Solenoid valve; FR1. Air filter regulator; P. Pressure gauge; C. Compound gauge

V1, V2, V3..... Valve

9.2.5.6 How to treat fuel oil with specific gravity above 0.991 (up to 1.01 at 15 0C)

• As the quality of heavy fuel oil in Diesel engine on board ships and in power stations has changed worldwide, there is an increased demand for efficient cleaning in order to achieve reliable and economical operation of diesel engines burning heavy fuel oil. The most crucial quality change affecting the cleaning of heavy fuel oil is the increase in density. At present, fuel oil with densities above 991 kg/m3 (up to 1010kg/m3) at 150c are currently available on the market.

• When the specific gravity of fuel oil to treat exceeds 0.991 (Density:991 kg/m3 ), the specific gravity range covered by a gravity disc narrows. As the specific gravity nears 0.999, the treatment gets easy to be affected by temperature, flow variation and other operating condition changes, thereby causing abnormal outflow or poor separation.

The following chart shows the relation between density of oil and density of water

Density of fuel and water versus temperature

The fact that oils with densities up to 1010 kg/m3 at 15 0c have densities below that of water between 50 and 1000c makes it possible to separate water from high density oil.

The ALCAP separator system produced by Alfa-laval can solve these problems. Any fuel oil with density up to 1010 kg/m3 at 15 0c and viscosity up to 700 cSt at 50 0c can be efficiently cleaned with respect to water and solid particles.

The separator basically operates as a clarifier. Dirty oil is continuously fed to the separator. The flow of oil is not interrupted when sludge and/or water is discharged

Alcap Separator

The HIDENS system produced by Mitsubishi Selfjector can treat any oil with specific gravity up to 1.01 at 15 0c. The HIDENS system consists of partial discharge clarifier and Water Detector installed in the purified oil outlet, when the Water Detector has detected a certain amount of separated water in the bowl, discharges separated water. This system combines a feature of clarifier or freedom from abnormal outflow and a feature of purifier or continual water separation. Water sealing is unnecessary.

Questions

1. State methods used to treat fuel oil on board.

2. Put the name of parts, state function of those parts and explain operating principle of the backwash filter.

3. Put the name of parts, state function of those parts and explain operating principle of fuel oil purifier Mitsubishi SJ.

4. Put the name of parts, state function of those parts and explain operating principle of fuel oil purifier Mitsubishi SJ-T.

5. Explain the theory of separation (how water and solid particles are separated from oil) in disc type centrifuge.

6. What is the clarification, clarifier, purification and purifier. Draw the simple structure of clarifier and purifier. Explain the basic difference between clarifier and purifier.

7. What is the function of gravity disc of purifier? Explain the methods to choose correctly a gravity disc. How to select a gravity disc properly by using monograph.

8. What is the separating quality of the centrifuge? Explain factors influencing the separating quality.

9. Explain the function of sealing water and replacement water during operating the purifier. Explain the sludge discharge types of the self-cleaning centrifuge.

10. Put the name of parts, state function of those parts and state procedures how to operate a purifier (Mitsubishi SJ) in FO purification system.

11. Explain how to treat fuel oil with specific gravity above 0.991 (up to 1.01 at 150C). Tell something about the ALCAP (ALFA-LAVAL) and HIDEN (MITSUBISHI SELF-JECTOR) separator.