METHOD AND APPARATUS CONCERNING A CENTRIFUGAL SEPARATOR

Abstract: In a centrifugal separator, the rotor of which has a separation chamber (7) with an inlet (16) for a liquid mixture, a constantly open central outlet (23) for a separated liquid and an intermittently openable peripheral outlet (13) for separated solids, a so called displacement liquid is supplied to the separation chamber (7) before the peripheral outlet (13) thereof is opened. According to the invention it is suggested that displacement liquid is supplied first at a certain amount per unit of time and then at a substantially smaller amount per unit of time. Possible existence of displacement liquid is sensed in the separated liquid leaving the rotor through the central outlet (23), and the peripheral outlet (13) is opened if displacement liquid is sensed.

Description: The present invention relates to centrifugal separators and their operation. The invention concerns particularly centrifugal separators of a kind including a rotor having a separation chamber, which has an inlet for a liquid mixture, a central outlet for a separated liquid and a peripheral outlet for separated solids, means to open and close the peripheral outlet during operation of the rotor, supply means for supply of displacement liquid, which is heavier than said separated liquid, to the separation chamber each time the peripheral outlet is to be opened, and control means arranged to activate the supply means for displacement liquid to be supplied during the last part of that time period for which the peripheral outlet is kept closed prior to being opened. In a centrifugal separator of the above described kind, to be used for applications in which the incoming liquid mixture contains apart from solids two liquids to be separated, the separation chamber has three separate outlets; two constantly opened outlets for the respective liquids and one intermittently openable peripheral outlet for the solids. A centrifugal separator of this particular form is described in US-A 4.343.431. Lubricating oils, as an example, are usually purified by means of such centrifugal separators. A problem with these known centrifugal separators is that they are sensitive to variations in the temperature and flow of the supplied liquid mixture. Such variations result in an interface layer formed within the separation chamber between the separated liquids moving radially inwards or outwards, for which reason it is difficult to determine exactly its position in the separation chamber. Another problem is to make the right choice, for such a centrifuge rotor, of the so-called gravity disk, the size of which determines the desired radial level for said interface layer. Furthermore, to exchange the gravity disk requires the centrifuge rotor to be disassembled. A centrifuge rotor with two constantly open liquid outlets has the added drawback that a large part of its separation chamber has to be filled with the one separated liquid, even if the content of this liquid in the supplied mixture is very small or sometimes is zero. This means that the separation chamber is used inefficiently. In a centrifugal separator of the initially defined kind the separation chamber may alternatively have only two outlets; one constantly open outlet for separated relatively light liquid and one intermittently openable peripheral outlet for separated relatively heavy solids. A centrifugal separator of this form is used above all when a certain liquid is to be freed only from solids. However, such a centrifugal separator may be used even if a supplied mixture contains two different kinds of liquids as well as the solids. In a case like this, separated relatively heavy liquid is removed intermittently from the rotor together with separated solids, through the peripheral outlet of the separation chamber. Then it is usual that the relatively heavy liquid constitutes only a small part of the supplied mixture. A problem with the use of a centrifugal separator having only one central liquid outlet, when the supplied mixture contains two different kinds of liquids, is that the need for intermittent discharge of separated solids does not usually coincide in time with the need for intermittent discharge of separated relatively heavy liquid. This problem is particularly difficult to solve if the content of relatively heavy liquid and/or the content of solids in the supplied mixture varies. Furthermore, it is difficult even in a centrifugal separator of this kind to determine exactly the radial position of an interface layer formed in the separation chamber between the separated liquids. Independent of whether a centrifugal separator has one or two constantly open liquid outlets, a predetermined so-called displacement liquid is normally supplied to the separator chamber before each occasion that the peripheral outlet for solids is to be opened. The reason for supplying displacement liquid is to decrease the amount of separated relatively light liquid in the separation chamber so that no such light liquid will leave the separation chamber through the peripheral outlet when this is opened for a short time. If the heavier one of the separated liquids is water, water is normally used as displacement liquid In this connection it has proved difficult to supply an optimum amount of displacement liquid, which depends upon the above described problem of precisely determining the radial position of the interface layer in the separation chamber between the two separated liquids. Uncertainty thus arises about how much displacement liquid may be supplied without this displacement liquid beginning to leak out through the outlet for separated light liquid. The object of the present invention is to provide a method by which this difficulty can be avoided, so that a centrifugal separator of the kind initially described may be used without risk of having an uncontrolled amount of displacement liquid flowing out through the outlet for the separated relatively light liquid. In accordance with one aspect the invention provides a method of operating a centrifugal separator including a rotor having a separation chamber with an inlet for a liquid mixture, a central outlet for a separated liquid and a peripheral outlet for separated solids, and opening means to effect opening and clos­ ing the peripheral outlet during operation of the rotor, the method comprising the steps of actuating the opening means at intervals, and each time the peripheral outlet is to be opened initiating supply to the separation chamber of a predetermined amount of displacement liquid which is heavier than said separated liquid, displacement liquid being supplied during the last part of that time period for which the peripheral outlet is kept closed prior to being opened, characterised in that a first part (P1) of the displacement liquid is supplied at a predetermined amount per unit of time, after which a second part (P2-P5) of the displacement liquid is supplied at a substantially smaller amount per unit of time, that the presence of any displacement liquid in the separated liquid leaving the rotor through the central outlet is sensed, and that the opening means is actuated to open the peripheral outlet upon displacement liquid being detected in the separated liquid. By this method it is possible, at each occasion when the peripheral outlet of the separation chamber is to be opened and before the opening, to supply an optimum amount of displacement liquid to the separation chamber, a small and controlled amount thereof being allowed to flow out through the outlet for the separated relatively light liquid for determining of the radial position in the separation chamber of the interface layer between displacement liquid and separated relatively light liquid. It is necessary that the separation chamber contains at least a certain known amount of separated relatively light liquid, i.e. that the separating operation is under control such that an interface layer between the separated light liquid and a separated heavier component of the supplied mixture has definitely not had time to move inside a certain radial level in the separation chamber. The use of the invention thus presumes that during normal operation of the centrifugal separation said first part of the displacement liquid may be supplied relatively rapidly without risk of having a large part thereof passing out through the outlet for the separated light liquid. Due to the fact that part of the displacement liquid may be supplied relatively rapidly, i.e. with a relatively large amount per unit of time, the time period during which the separation chamber has to contain displacement liquid and, therefore, cannot be used effectively may be kept very short. Due to the fact that a second part of the displacement liquid is then supplied substantially lower, i.e. with a substantially smaller amount per unit of time a large amount of displacement liquid may be prevented from being allowed to mix with the separated light liquid before presence of displacement liquid in the separated light liquid is sensed. It has proved - even if detecting displacement liquid in the separated light liquid very rapidly results in the peripheral outlet of the separation chamber being opened - that the content of displacement liquid in the separated light liquid, which in connection with and immediately after such an opening operation leaves the separation chamber, becomes unacceptably high if the displacement liquid, throughout its supply, is supplied at a too large amount per unit of time. Preferably, the displacement liquid is supplied batch-wise and after supply of one batch the next batch is supplied only after the result of the effect of the supply of the preceding batch on the separated liquid has been sensed. It has been found that when the interface layer between displacement liquid and separated light liquid has reached a certain critical radial level in the separation chamber, it still takes a certain time from the moment when a batch of displacement liquid has been supplied to the moment when existence of displacement liquid can be sensed in the separated light liquid leaving the separation chamber. A batch-wise supply of displacement liquid can thus avoid an unnecessarily large amount of displacement liquid being mixed with already separated light liquid so that cannot be prevented from accompanying it. Preferably the above said first part of the displacement liquid is supplied substantially continuously in a relatively large batch, after which the rest is supplied in smaller batches. In accordance with a second aspect the invention provides a centrifugal separator comprising a rotor having a separation chamber with an inlet for a liquid mixture, a central outlet for a separated liquid and a peripheral outlet for separated solids, opening means for effecting opening and closing of the peripheral outlet during operation of the rotor, supply means actuable for supply of a predetermined amount of displacement liquid, which is heavier than said separated liquid to the separation chamber each time the peripheral outlet is to be opened, and control means arranged to activate the supply means so that displacement liquid is supplied during the last part of that time period for which the peripheral outlet is closed prior to being opened, characterised in that the supply means is so arranged that the displacement liquid is first supplied at a certain amount per unit of time and after that is supplied at a substantially smaller amount per unit of time, that sensing means is arranged to detect displacement liquid in the separated liquid leaving the rotor through the central outlet, and that the opening means is coupled to the sensing means for the peripheral outlet to be opened upon displacement liquid being detected in the separated liquid. Some embodiments of the invention are described in the following detailed description with reference being made to the accompanying drawings in which: Fig 1 shows a sectional view of a centrifuge rotor and, schematically, parts of control equipment for the operation thereof in accordance with the invention; and Fig 2, a and b, illustrate two different methods of supplying so called displacement liquid according to the invention. Fig 1 shows a centrifugal separator comprising a rotor with an upper part 1 and a lower part 2, which parts are held together by means of a locking ring 3. The rotor is supported at the top of a vertical drive shaft 4. Within the rotor there is an axially movable slide 5, which in its upper position shown in the drawing abuts against an annular gasket 6 arranged in a groove in the upper rotor part 1. In this upper position the slide 5 defines together with the upper rotor part 1 a separation chamber 7 within the rotor. Defined between the slide 5 and the lower rotor part 2 is a so-called closing chamber 8, into which a closing liquid can be introduced during operation of the rotor through a stationary pipe 9, and a groove 10 and a channel 11 in the rotor part 2. At the peripheral portion of the rotor the rotor part 2 has a throttled draining channel 12 extending from the closing chamber 8 to the outside of the rotor body. During operation of the rotor closing liquid is supplied continuously to the closing chamber 8 at a flow rate sufficient to keep the latter filled. Thereby the slide 5 is maintained in its upper position, as shown in the drawing, in which it closes the separation chamber 7 from connection with a number of peripheral outlet ports 13. When desired during operation of the rotor the supply of closing liquid may be interrupted for a short period of time. Then the closing chamber 8 is drained wholly or partly through the channel 12, and the slide 5 is pressed axially downwards from the position shown in Fig 1, so that the outlet ports 13 are uncovered for a shorter or longer period of time. For the supply of a liquid mixture of components to be separated, a stationary inlet pipe 14 extends into the rotor and opens into a central receiving chamber 15. From the receiving chamber 15 several channels 16 lead into the separation chamber 7, in which a stack of frusto-conical separation discs 17 is arranged. In the upper part of the separation chamber 7 a conical partition 18, is arranged, which together with the upper rotor part 1 forms a number of supply channels 19 for a so-called displacement liquid. At its central portion the partition 18 has two inwardly directed and axially spaced annular flanges 20 and 21. Between these flanges a central outlet chamber 23 is formed in the rotor, which chamber communicates with the separation chamber 7 through an overflow outlet formed by the radially innermost edge portion of the flange 21. Between the flange 20, which extends some­ what further radially inwards than the flange 21, and an upper end wall 24 of the rotor part 1, an inlet chamber 25 is formed for displacement liquid. This inlet chamber communicates with the previously mentioned supply channels 19. The supply pipe 14 constitutes a part of a surrounding stationary member 26, which also forms an inlet channel 27 for displacement liquid and an outlet channel 28 for a light component of the supplied mixture separated in the rotor. The inlet channel 27 opens at 29 in the inlet chamber 25, and the outlet channel 28 starts at 30 in the outlet chamber 23. During operation of the rotor free liquid surfaces are formed in the different chambers of the rotor at levels illustrated by means of triangles in Fig. 1. As can be seen, the part of the stationary member 26 forming the outlet channel 28 extends to a level radially outside of the free liquid surface in the outlet chamber 23. The outlet channel 28 is connected to the interior of a conduit 31, in which sensing equipment 32 is arranged. This sensing equipment 32 is arranged to sense when fractions of displacement liquid accompanies separated light liquid out of the rotor. If the light liquid is constituted by oil and the displacement liquid of water, the sensing means 32 may be arranged to sense a change of the dielectric constant of the liquid leaving the rotor. Through a signal line 33 the sensing equipment 32 is connected with an opening unit 34, which in turn through a signal line 35 is connected with a valve means 36 arranged occasionally to interrupt the supply of closing liquid to the closing chamber 8 in the rotor, so that its peripheral outlets 13 are opened. The sensing equipment 32 is arranged, immediately upon sensing a predetermined change of the dielectric constant of the liquid flowing through the conduit 31, to activate the opening unit 34 so that the peripheral outlet 13 of the rotor is opened for a short period of time. The opening unit 34 also is connected through a signal line 37 with a control unit 38 arranged to activate the opening unit 34 at predetermined time intervals so that the peripheral outlet of the rotor is opened for a short period of time. The control unit 38 is connected through a signal line 39 also with a supply unit 40, which in turn, through a signal line 41, is connected with a valve means 42. This valve means is situated in a conduit 43, the interior of which communicates with a pressure source of displacement liquid and with the previously mentioned inlet channel 27. The control unit 38 is arranged to activate intermittently the supply unit 40 for supply of displacement liquid to the rotor. Such activation happens, each time, a predetermined period of time before the opening unit 34 is activated by the control unit 38 as described above. The supply unit 40 is adjustable for actuation of the valve 42 in a desired manner for the supply of displacement liquid to the rotor. According to the present invention, the supply unit 40 should be adjusted in a way such that, when it is activated by the control unit 38, there will be a relatively large followed by a relatively small flow of displacement liquid to the rotor. A signal line 44 connects the opening unit 34 with the supply unit 40, so that any supply of displacement liquid to the rotor may be interrupted by the supply unit 40 when the opening unit 34 causes the peripheral outlet 13 of the rotor to be opened. Fig. 1 illustrates by means of dash-dot lines A, B, C and D four different radial levels in the rotor separation chamber 7. Reference is made to these levels in the following description of the centrifugal separator operation. Fig. 2, a and b, illustrate two of several possible methods of supply of displacement liquid within the scope of the present invention. Fig. 2a illustrates that a relatively large portion P1 of displacement liquid is suppled between two points of time t₁ and t₂ at a rate of L litres per hour, e.g. 60 1/h. During short time intervals thereafter smaller portions P2-P5 of displacement liquid are supplied at points of time t₃, t₄, t₅ and t₆. Each of the last mentioned time intervals, during which the portions P2-P5 are supplied, is very short, e.g. 5 seconds, whereas the periods of time between the points of time t₂ and t₃ and between subsequent portions P2-P5 preferably are somewhat longer, e.g. 30 seconds. The average rate, at which displacement liquid is supplied during the time between the point of time t₂ and the point of time when the last portion P5 has been supplied is obviously substantially smaller than the initial rate of L 1/h. Fig. 2b illustrates that displacement liquid is supplied between the points of time t₁ and t₂ at a rate of L 1/h (the same as according to Fig. 2a), and between the points of time t₂ and t₇ at a substantially smaller rate S litres per hour. In the following it is explained how the centrifugal separator according to Fig. 1 is supposed to work, when displacement liquid is supplied in the way illustrated in Fig. 2a. It is assumed that the liquid mixture to be treated is lubricating oil for a diesel motor, which oil is to be freed from solids and possibly existing water. As displacement liquid water is used. During operation of the rotor oil is supplied continuously through the inlet pipe 14, free liquid surfaces being maintained in the different chambers of the rotors as can be seen from Fig. 1. At the start of the separating operation a small amount of water is supplied through the inlet channel 27, the inlet chamber 25 and the supply channel 19, so that an interface layer between this water and the oil is formed in the separation chamber 7 at the radial level A. The reason for the supply of this small amount of water at the beginning of a separating operation is that experience has shown that the solids later to be separated from the oil may be removed more easily from the separation chamber through the peripheral outlets 13, if they have been caused to pass through water. Such an initial supply of water is not absolutely necessary and has nothing to do with the invention. Thereafter the separating operation proceeds for a period of time, the length of which is predetermined and registered in the control unit 38, cleaned oil leaving the separation chamber 7 continuously through the overflow outlet formed by the flange 21. The cleaned oil leaves the rotor through the outlet chamber 23, the outlet channel 28 and the conduit 31. During this period of time an unknown amount of solids and an unknown amount of water is separated from the oil flowing through the separation chamber 7. It is assumed that at the end of the predetermined time period an interface layer between oil and water - or between oil and solids if no water has been supplied or separated from the oil - is situated at the radial level B. At this moment the supply unit 40 is initiated by the control unit 38 to start supplying displacement water. In accordance with Fig. 2a a relatively large portion P1 of displacement liquid is then supplied between the points of time t₁ and t₂ at a rate of L litres per hour. When the whole portion P1 has been supplied to the separation chamber 7 it is assumed that an interface layer therein between oil and water is situated at the level C. Still only cleaned oil will be flowing out of the rotor and passing the sensing equipment 32. The separation chamber is then charged at intervals with four smaller portions P2-P5 of displacement water, according to Fig. 2a, after which the interface layer between lubricating oil and water in the separation chamber is assumed to have moved to the level D. It is assumed that the sensing equipment 32 neither before nor after the point of time t₆ has sensed any change of dielectric constant of the oil that has left the rotor. When a certain time has passed after the point of time t₆ the control unit 38 activates the opening unit 34, so that the latter effects opening and closing of the peripheral outlets 13 of the rotor. The opening time is sufficiently long to ensure that all separated solids and displacement water are thrown out of the separation chamber 7, but is also sufficiently short to prevent loss of oil. After this discharge operation a new separation period is started, the length of which is determined by the control unit 38. During the second separation period it is assumed that a somewhat larger amount of water and/or solids is separated from the oil flowing through the separation chamber 7 than occurred during the first separation period. It is thus assumed that an interface layer between oil and water or solids is situated at the level C in the separation chamber 7 at the time t₁ , i.e. when the first portion P1 of displacement water starts to be supplied to the rotor. When this portion P1 has entered the separation chamber, the interface layer is situation at the level D, and still only clean oil is sensed in the conduit 31. Even after the portion P2 has been supplied to the rotor only clean oil is sensed in the conduit 31, but some seconds after the portion P3 has been supplied to the rotor, the sensing equipment 32 senses existence of small amounts of water in the cleaned oil, since now the interface layer between separated oil and displacement water has reached a level radially very close to the outer edges of the separation discs 17 and fractions of displacement water are entrained with the oil flowing radially inwards between the separation discs. This immediately results in an activation of the opening unit 34, so that the rotor peripheral outlets 13 are opened. Simultaneously a signal is emitted to the supply unit 40, so that further supply of displacement liquid is prevented. Thus, the portions P4 and P5 will not be supplied and the amount of water which can thus accompany cleaned oil out of the rotor is only a part of the portion P3. It has been assumed above that displacement water has been supplied each time at the same speed, i.e. with an amount of L litres per hour. For obtaining a good control over the exact amount of supplied displacement water, the valve means 42 preferably is constituted by a so called constant flow valve, i.e a valve that in its open positon always lets through liquid with a small predetermined amount of time independent of small variations in the pressure drop across the valve. Within the scope of the invention it is of course possible to add portion P1 at a certain rate (1/h) and each of the portions P2-P5 at a lower rate. According to a further developement of the invention it is possible to change automatically the supply of displacement water during a subsequent separation period, if displacement water has been sensed in the cleaned oil. Thus, the control unit 38 may be arranged, in connection with the subsequent separation period, to bring the supply unit to reduce the amount of displacement water in the first portion P1 and, instead, to increase the number of small portions to for instance five or six. The present total amount of supplied displacement water should not be changed. Hereby a rapid adaption to a gradual and unexpected large increase of the amount of water or solids in the oil may be accomplished, so that there will not be too large an amount of displacement water mixed in with cleaned oil. If upon a subsequent period of supply of displacement water no water is sensed in the cleaned oil, the control unit 38 may be arranged to cause the supply unit 40 again to supply displacement water in the original way after the next separation period. The control unit 38 may also contain alarm means, which in one way or another makes an operator observant of the fact that the sensing equipment 32 during subsequent separation periods is actuated to initiate openings of the rotor periphery outlet as a consequence of displacement water being sensed in the cleaned oil. This may for instance indicate that a water leakage has arisen in the diesel motor through which the lubricating oil is circulating. The sensing equipment is preferably arranged to initate opening of the rotor peripheral outlets 13 upon sensing water in the cleaned oil, even if this happens before displacement water is supplied at the end of a separation period. In the above the terms opening unit, supply unit and control unit have been used. However, there do not have to be separate "units" for the means in question. The said terms have been used only for simplifying the understanding of the invention. In practice the three said units can be conveniently integrated in one single central unit for carrying out all of the functions performed by these units.

Claims: 1. A method of operating a centrifugal separator including a rotor having a separation chamber (7) with an inlet (16) for a liquid mixture, a central outlet (23) for a separated liquid and a peripheral outlet (13) for separated solids, and opening means (34) to effect opening and closing the peripheral outlet (13) during operation of the rotor, the method comprising the steps of actuating the opening means at intervals, and each time the peripheral outlet is to be opened initiating supply to the separation chamber (7) of a predetermined amount of displacement liquid which is heavier than said separated liquid, displacement liquid being supplied during the last part of that time period for which the peripheral outlet (13) is kept closed prior to being opened, characterised in that a first part (P1) of the displacement liquid is supplied at a predetermined amount per unit of time, after which a second part (P2-P5) of the displacement liquid is supplied at a substantially smaller amount per unit of time, that the presence of any displace­ ment liquid (32) in the separated liquid leaving the rotor through the central outlet (23) is sensed, and that the opening means (34) is actuated to open the peripheral outlet (13) upon displacement liquid being detected in the separated liquid. 2. A method according to claim 1, wherein the displacement liquid is supplied batch-wise and after the supply of one batch of displacement liquid the next batch is supplied only after it has been sensed whether there is any displacement liquid present in the separated liquid following the supply of said one batch. 3. A method according to claim 1 or 2, wherein the first part (P1) of the displacement liquid is supplied substantially continuously, after which the second part (P1-P5) is supplied in batches of smaller amount than said first part. 4. A method according to any of the preceding claims, wherein upon sensing presence of displacement liquid in the separated liquid means for controlling the supply of displacement liquid is adjusted to reduce the amount of said first part (P1) of the displacement liquid to be supplied the next time the peripheral outlet is to be opened. 5. A method according to any of the preceding claims, wherein the supply of displacement liquid is interrupted in response to displacement liquid being detected in the separated liquid, whereby the amount of displacement liquid actually supplied is less than said predetermined amount. 6. A method according to any preceding claims, wherein the opening means is actuated to open the peripheral outlet (13) after the predetermined amount of displacement liquid has been supplied in the absence of displacement liquid being detected in the separated liquid. 7. A centrifugal separator comprising a rotor having a separation chamber (7) with an inlet (16) for a liquid mixture, a central outlet (23) for a separated liquid and a peripheral outlet (13) for separated solids, opening means (34) for effecting opening and closing of the peripheral outlet (13) during operation of the rotor, supply means (40) actuable for supply of a predetermined amount of displacement liquid, which is heavier than said separated liquid to the separation chamber (7) each time the peripheral outlet (13) is to be opened, and control means (38) arranged to activate the supply means (40) so that displacement liquid is supplied during the last part of that time period for which the peripheral outlet is closed prior to being opened, characterised in that the supply means (40) is so arranged that the displacement liquid is first supplied at a certain amount per unit of time and after that is supplied at a substantially smaller amount per unit of time, that sensing means (32) is arranged to detect displacement liquid in the separated liquid leaving the rotor through the central outlet (23), and that the opening means (34) is coupled to the sensing means (32) for the peripheral outlet (13) to be opened upon displacement liquid being detected in the separated liquid. 8. A centrifugal separator according to claim 7, wherein the supply means (40) is arranged for the displacement liquid to be supplied batch-wise in such a way that a first relatively large batch (P1) and then, at predetermined time intervals, smaller batches (P2-P5) of the displacement liquid are supplied. 9. A centrifugal separator according to claim 7 or 8, wherein the supply means (40) is connected to the sensing means (32) and is arranged to interrupt the supply of displacement liquid in response to displacement liquid being detected in the separated liquid.

Inventors: PALLMAR, ANDERS
Applicants: ALFA-LAVAL MARINE & POWER ENGINEERING AB
Application year: 1988
Main classification: B04B1
IPC codes: B04B1, B04B11