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Purifiers and Clarifiers. Effects of modern refining. Yield of residual FO per barrel of crude refined has reduced by 40% in recent years This has resulted in Lower hydrogen content of the fuel Increased concentration of Sulpher, Vanadium, Na, trace metals ,found naturally in crude.
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Effects of modern refining Yield of residual FO per barrel of crude refined has reduced by 40% in recent years This has resulted in • Lower hydrogen content of the fuel • Increased concentration of Sulpher, Vanadium, Na, trace metals ,found naturally in crude. • Increased density of FO making water/impurities separation difficult • Lower ignition quality and making bigger ignition delay • Higher levels of asphaltenes and the related sludge problems.
FO and LO • Treatment of FO and LO usually requires filtration and centrifuging. Some of the concepts apply equally to both. • Source of LO Contaminations;a..Air transmitted, water, dust, sand etc.b..Combustion products, soot, acids, water etcc..Fuel d..Wear particles from working parts
FILTRATION There are a great number of different types of filter, for various applications. Fitted into Fuel, Lubricating and Hydraulic systems. In general, the purpose of a filter is to protect items of machinery downstream of its location. Some filters however, in lubricating oil and hydraulic systems, are fitted solely to condition the oil usually in A bypass circuit. Magnetic Filter:- Protects pump and collects steel debris Bypass Filter:- Conditions oil Full Flow Filter:- Protects engine components Centrifuge:- Conditions oil, works on partial flow system
Purifiers and Clarifiers differ only in that clarifiers are not set up to remove water. Their design are similar to the point that most purifiers found on board can be converted to use as a clarifier with simple alteration of the gravity disc If an oily water mix is placed into a tank then separation of the two parts will begin with the lighter element rising to the top. The rate the separation occurs is governed by several factors including the difference in specific gravities and the force of gravity acting upon it. For mixes placed into a settling tank there is little that can be done about the gravity but the difference in the specific gravities can be increased by heating. This because water density changes at a much reduced rate when compared to oil. The limiting factor to this is that the water cannot be heated above 100'C for obvious reasons.
When a volume of light oil is placed into a tank contain a weir and a quantity of water the fluids will tend to arrange themselves as shown above. The height of the water in the weir rises to a point governed by the volume ( and thereby relative height) and specific gravity of the light oil. Knowing this it is possible to form a rudimentary purification system.
As a oil/water mix is fed into the tank separation begins with heavy particulates falling to the base of the tank along with water which joins the other water excess overflowing the heavy phase weir. Hopefully clear oil passes over the light phase weir. The problem arises that to ensure their is sufficient time to allow for full (separation of the oily mix the flow would have to be very small relative to the size of the tank.
Gravity is replaced by centrifugal force as the bowl is spun at high revolutions thereby creating very high g-forces.A disc stack is incorporated to encourage a laminar flow increasing improving the separation effect. Dirty oil is introduced via a centerline oil feed dip tube. The oil is led to distribution holes which are reflected in the disc stack but not the dam
Most of the purifiers are of the self cleaning type in that they are able to open the bowl to discharge any accumulated sludge. In this a fixed centrifugal style impeller is mounted in the light phase outlet drawing the oil and discharging it at pressure sufficient to deliver it to the receiving tank. A discharge valve is fitted which is adjusted to give a constant back pressure in the bowl. The adjustment of this back pressure tends to move the position of the interface but more importantly increases the oil in the light phase delivery chamber increasing the immersion depth of the lip of the pump. This reduces possibility of air being entrained and removes foaming. In the event of seal failure back pressure will fall, this may be detected by a pressure switch initiating a shut down
Increasing the sg of the oil will tend to push the interface outlet and cause overflow from the heavy phase outlet until the equilibrium is restored. Should the interface be moved so far as to breach the dam oil will be issued from the heavy phase outlet and an alarm will sound. The ideal position for the interface is to lie over the distribution holes • Reducing the sg of the oil will tend to bring the interface towards the axis, this reduces the force of separation on the oil mix and reduces the efficacy of the unit possibly leading to contaminants and water carryover with the light phase outlet the "gravity" disc are changeable on virtually all purifiers. Their centre bore is governed by the sg of the oil being centrifuged. The largest bore should be used without risking overflow. The flow rate of a purifier should be set to optimize removal of whole system impurities. The lower the oil feed the greater the time for impurity removal and the more efficient the purification. The higher the rate the greater the amount of system oil is treated per unit of time. For a system such as main engine oil where contaminants are continuously being added to the system. As a rule of thumb the total volume of the system should pass through the purifier three times every 24 hours, this rate may be vary depending on operational parameters. A similar calculation has to be made with fuel oil to ensure removal of water and sludge which may accumulate over time.
THE PARING DISC This device is fitted at the top of the centrifuge, to ensure a pressurized oil discharge. The arrangement is a stationary pump impeller mounted in a chamber. The blades dip into the rotating ring of oil and scoop it out, converting the kinetic energy of rotation into pressure head.
GENERAL OPERATION OF THE CENTRIFUGE • It is a characteristic of a centrifuge that it will not remove all debris particles from oil. • To understand why this is so, it is necessary to examine what happens inside the disk stack during operation. • In very general terms it is stated that the disks are fitted to meet the needs of "Thin Strata Separation", explained in the settling" tank analogy of the separation of solids from liquids. This means that the disk reduce the distance a solid particle has to move before separation is accomplished • This general statement is not sufficient for the explanation what happens in the dynamic conditions of the centrifuge. The first thing the disks do is to break the flow of oil into clearly defined smoothly flowing streams. The main feature of the smooth streams is that oil in the centre of two disks is moving more quickly than oil in the same disk path, that is nearer the disk surface.
GENERAL OPERATION OF THE CENTRIFUGE The oil very close to the disk surfaces is called the boundary layer and the velocity here is insignificantly small.
Separation Inside the disk path the oil flow is dragging the solid particle along. Due to the additional centrifugal force, the solid takes a path like that shown below. As the solid particle approaches the underside of a disk, the oil drag force, which is related to the velocity of the oil, is diminishing. Once at the underside of the disk the oil drag force is insignificant and the only force acting on the solid is centrifugal. Particles at the disk underside move down the disk surface, the centrifugal force increasing all the time, due to radius increasing. They leave at the outer edge and move to the bowl periphery
Separation The general flow for all particles, both oil and solids is from outside diameter to inside. However all particles are also subjected to the dynamic force due to rotation. If a solid particle and an immediately adjacent oil particle are considered. as above, it is seen that both particles have the same types of force applied. The difference in the value of these forces is significant. The value of the dynamic or centrifugal force is (m.w.r) and. while both adjacent particles have the same radius and angular velocity, the mass of the solid particle is larger. This means that it is subjected to a greater centrifugal force than the adjacent oil particles. Due to this, force difference the "path" of a solid particle is slightly different from the general flow direction of the oil.
GENERAL CENTRIFUGE OPERATION When the centrifuge is operated as a separator, the gravity disk fitted, decides the position of the interface, providing all other factors are correct.
GENERAL OPERATION OF THE CENTIUFUGE • Small changes in Temperature, Viscosity, Oil Density or Flow Rate can have considerable effect on separator operation. • This is particularly true with the effects of temperature, viscosity and flow changes, when the density difference between water and oil is so small that the hydraulic equilibrium in the bowl is unstable, This happens with the very heavy fuels. • The effect of small changes in these variables is to either push the interface into the disk. stack giving a poor separating effect, or to allow the interface to drift to the outside of the bowl, with probable loss of seal
GENERAL OPERATION OF THE CENTRIFUGE Research has shown that the number of particles, above 5 microns in size in a sample of untreated fuel oil, is of the order of two to three millions per liter. The diagram is a histogram of particle distribution in such a sample
GENERAL OPERATION OF THE CENTRIFUGE The treatment of oil is always by centrifuge and filtration, the diagram shows the reduction in numbers of particles of all sizes after centrifuging and filtration
GENERAL OPERATION OF THE CENTRIFUGE To eliminate particles above a certain size, filtration is needed: The particle representation after centrifuging and filtration with a 5 micron filter is shown. The previous diagram assumed that centrifuging was followed by Effective filtration, which accounts for the fact that there is a reduction in the number of particles below 5 microns in the filter. The previous points were applicable to the treatment of both lubricating oil and fuel oil, as they were concerned with general points on operation.
EFFECT ON INTERFACE Small changes in Temperature, Viscosity, Oil Density or Flow Rate can have a considerable effect on separator operation.
Effect of change in parameters Small changes in Temperature, Viscosity, Oil Density or Flow Rate can have a considerable effect on separator operation. This is particularly true with the effects of temperature, viscosity and flow changes, when the density difference between water and oil is so small that the hydraulic equilibrium in the bowl is unstable, This happens with the very heavy fuels. The effect of small changes in these variables is to either push the interface into the disk. stack giving a poor separating effect, or to allow the interface to drift to the outside of the bowl, with probable loss of seal
Parallel Operation (High Water Content) A stand-by centrifuge operating in parallel is recommended in emergencies when the water content in the fuel is greater than 10 percent by volume. The effective throughput of each machine should be reduced to 50 percent. Splitting the feed across two machines and effectively doubling the dwell time of the fuel in the centrifuges allows the machines to remove the excess water effectively and quickly. Recent research by Alfa-Laval appears to disapprove this theory. Their findings, from comprehensive laboratory and shipboard tests, show clearly that two centrifuges, run in series., is the best method The results of this research are available on request from Alfa Laval.
TH E ALCAP OPERATING PRINCIPLE This is the Alfa-Laval system for the treatment of very heavy Fuels. The arrangement is to have a centrifuge fitted similar to a clarifier. Cleaned oil is discharged continuously from the clean oil outlet, while separated water and sludge accumulated at the periphery of the bowl. This accumulation moves the interface inwards, as there is no continuous water discharge. When the interface reaches the disk stack, the effectiveness of separation reduce and, some water goes over with the cleaned oil. This water presence is detected by a water transducer fitted in the clean oil outlet. When water is detected in the clean oil outlet, the water in the bowl is discharged either: -. Through the water drain valve With the sludge through the ports at the side of the bowl Separated water is normally discharged with the sludge, as the sludge discharge is timed to take place approximately once every fifteen minutes. If water is detected during the fifteen minute period since the last sludge discharge, the separated water is discharged through the water drain valve. This would occur if excessive water contamination existed.
Centrifugal separators are the most widespread method of lube oil treatment. The separator is located in a by-pass loop within the lube oil treatment system.
Purifier arrangement bearing Spiral gear Horizontal shaft Friction block bearing
Choosing Gravity Disc The graph shown is one typical of one found in a purifier instruction book for selecting appropriate gravity disc size. Shown on the diagram is an example of an oil of sg 0.93 at 0'C. The sg at 15'C for use with this graph is found by projecting along a horizontal line to 15'C. This step would be omitted if the sg at 15'C was already known. A line is then drawn parallel to the pre-drawn sloping lines. Where the drawn sloping line cuts the appropriate oil supply temperature isothermal then This becomes the selection point for the disc. This is found simply by ascertaining which size band the point lies in.
System Concepts for the Future • The sludge is pumped from the sludge tank by an eccentric screw pump and is fed via a heater to the centrifugal separator. The sludge components of water, oil and solids are separated in the separator by centrifugal force. The recovered oil and water is discharged under pressure by centripetal pumps. The concentrated sludge is discharged intermittently via the sludge transfer unit into a heated sedimentation tank where final concentration takes place. Excess water and oil is allowed to overflow back to the sludge tank. The concentrated sludge is pumped from the unit automatically by a solids discharge pump controlled by a level switch. A microprocessor -based control cabinet supervises and controls the complete concentration process.
