Centrifugal Purifier

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Centrifugal PURIFIERS Basic principles : the purpose of the separation is to To free a liquid of solid particles. To separate two mutually insoluble liquids with difference in density , removing any solid at the same time. Separation by gravity : continuous separation & sedimentation can be achieved in the settling tank having the outlet arranged at levels suitable to their density ratio of two liquid phases. Separation of heavy & solid particles are also achieved over here. Liquids with specific gravity difference can be separated by gravity , the equation like this Separating force F = (/6) D ( w o) gas

centrifugal separation : In a rapidly rotating vessel the gravity is replaced by the centrifugal force, which can be thousand times greater. Separation and sedimentation are more continuous and faster. As the separation is achieved by the gravity has a smaller separating force differential than the centrifugal force . the separating force , F= (/6) D (w o) R where R is effective radius and is the angular velocity. Development of a separator The heavier phase (water) had only a short distance to travel before coming to the bowl wall where solids were deposited and the heavy phase liquid (water) was guided to the water discharge. However, the sludge retention volume and the liquid dwell time for a given throughput could only be increased by lengthening the bowl. This gave rise to bowl balancing and handling problems. The wide bowl type was able retain more sludge before its performance was impaired and was much easier to dean. On the other hand settling characteristics in a wide bowl machine are relatively poor towards the bowl centre and the distance the water has to travel before reaching the wall is great. To overcome these problems a stack of conical discs (Figure 2.15) spaced about 2-4 mm apart is arranged in the bowl. The liquid is fed into the bottom of the stack and flows through the spaces between adjacent plates. The plates then act as an extended settling surface, with the heavy impurities impinging on the under surfaces of the discs. As the particles impinge of the disc surfaces. they accumulate and eventually slide along the discs towards the periphery. At the disc stack periphery, water globules and solid particles continue to move out towards the bowl wall with the water being sandwiched between the solids and the oiL which orientates itself towards the bowl centre. The boundaries at

which substances meet are known as interfaces. The oil/water interface is very distinct and is known as the e-Iine. To gain tne fullest advantage from the disc stack the e-line should be located outside of it. On the other hand if the e-Iine is located outside the water outlet baffle (top disc) discharge of oil in the water phase will take place. Referring back to gravity separation in a settling tank, if the tank is partitioned as shown in Figure 2.16 continuous separation will take place. Since the arrangement is a very crude V-tube containing two liquids of different specific gravities, the height of the liquid in the two legs will have the relationship

oil ( e-l) = wtr {e-h} in case of centrifuges o(e -- l) = wtr {e-h}The mechanical design of the centrifuge requires that the e-Iine is confined within certain strict limib. However variation~ in gravity will be found depending upon the port at which the vessel takes on bunkers. It is necessary therefore to provide means of varying h or l to compensate for the variation in specific gravity. It is usually the dimension h which is varied. and this is done by the use of dam rings (sometimes called gravity discs) of different diameters. Normally a table is provided in the instruction book for the machine, giving the disc diameter required for purifying oils of various specific gravities. Alternatively the disc diameter Dh may be calculated from the following formula which is derived from (5) Dh =2 [ l (l / h) + e{1-(l /h)} ] The dimension e can be taken as the mean radius of one thin conical plate and the heavy top conical plate (outlet baffle). If oil is discharging in the water outlet the gravity.disc is too large.

Difference between a clarifier & purifier: 1. A clarifier is intended to separate only the solids where as the purifier is intended to separate both solids and water from the oil. 2. In a purifier a dam ring/ gravity disc is provided at the top of the purifier which has an aperature to discharge water and oil separately. In case of a clarifier the dam ring is replaced by a blank disc. 3. No sealing water is required for a clarifier. Basics: Here the fluids with difference in characteristics can be handled by one machinery. Liquid is fed at the bottom of the stack of discs. Centrifugal force tends to separate the two liquids with difference in specific gravity. This force is very high due to high rotational speed.A purifier is fitted with a dam ring which controls the position of the separation line or interface between the water and oil when the bowl is rotating.:; If the inside of the dam ring is too large in diameter the separation line or interface moves outward towards the outer periphery of the bowl and some oil will be discharged with the water from the water outlet. If the inside diameter of the dam ring is too small the interface moves inwards and some water will be discharged with the oil. The diameter of the dam ring is governed by the density of the oil being treated. If the diameter of the hole in the dam ring is increased, the interface between

the oil and the water contained in the bowlmoves outwards. If the diameter increased excessively, oil globules will bedischarged with the sludge and water. If the hole diameter is reduced unduly, the interface moves inwards and particles of water will be discharged with the clean oil. Holes are piaced in the conical plates making up the plate stack to allow the oil to feed upwards into the clearance spaces between the conical discs. Water and heavy stable emulsions are discharged through the dam ring and spin off the bowl or are removed from the rotating bowl by a paring disc. Clean oil is discharged from the bowl and spins off or may be removed by another paring disc. Heavy solid matter is held in the bowl. (Fig. 3.3(a) ) Clarifiers do not have a dam ring, a plain ring (also called a sealing ring) JS fitted in its place, and the water outlet is sealed off. Solid matter and water that has passed through the separator are retained within the clarifier bowl until it is opened and the water and sludge are discharged. On modern machines surveillance and control devices watch out for the build-up of water within the bowl or when a very small amount of water is discharged with the clean oil. The bowl is then caused to open. Modern centrifuges are operated as clarifiers and controlled in this manner. The conical disc stack mav not be fitted with feed holes, but if they are fitted a blank conical disc without feed holes will be fitted at the bottom of the conical disc stack. When a centrifugal separator is started up it must be filled with water to

establish a seal which prevents oil leaving the separator at the water outlet. Clarifier bowls do not have to be filled with water after start up. The main purpose of the clarifier is to remove traces of foreign material not removed when the oil passed through the separators, and to act as a second line of defence against the accidental passage of contaminant material into the clean side of the oil storage system. Clarifiers are not normally used to clean lubricating oil unless the oil is almost free of any water content.

'botch treatment' and 'continuous treatment' Originally the terms continuous treatment and batch treatment applied only to the purification of main engine crankcase lubricating oil. In continuous treatment the separator was operated for the whole of the time the main engine was in operation. Lubricating oil was supplied to the separator by its own pump taking oil from the main engine drain tank, or was bled from the pressure supply to the main engine. The separator was never shut down except for short periods when it was being cleaned. Batch treatment referred to the system whereby the engine was shut down and the whole of the sump lubricating oil charge was pumped up to the dirty lubricating oil tank in the upper part of the engine room. The lubricating oil was heated in the tank and left as long as possible to settle out solids, sludge and any water. It was then slowly purified in one batch, hence the name. Today the terms have different meanings. Batch treatment centrifugal separators and clarifiers must be shut down for cleaning after treating a batch of oil. C6ntinuous treatment machines are capable of being cleaned or sludged without being shut down. These machines are also termed automatic self cleaning separators and clarifiers or automatic separators and clarifiers. This is necessary to heat fuel and lubricating oil prior to treatment in a centrifugal separator to reduce the viscosity of the oil so that it flows easily into and out of the separator and does not cause high pumping loads. Heating the fuel or lubricating oil, which is a mixture of oil, water and solids, lowers the specific gravity of the constituent parts. The specific gravity of the oil Is reduced at a greater rate than the specific gravity of the water and solids; thus the difference in the specific gravities of the constituent parts is greater when the mixture is heated.

centrifugal separator used for the purification of fuel and lubricating oils

The main part of a centrifugal separator is the bowl, which is mounted on the top of vertical spindle supported in two bearings. On the spindle between the bearings is a multiple-tooth helical worm. This worm is driven by a helical'toothed gear wheel connected to a driving motor through a centrifugally

operated clutch. The bowl is