EP2401087A1

EP2401087A1 - Centrifugal separator and method for separating

Info

Publication number: EP2401087A1
Application number: EP10714387A
Authority: EP
Inventors: Lasse Hurnasti
Original assignee: Alfa Laval Corporate AB
Current assignee: Alfa Laval Corporate AB
Priority date: 2009-02-24
Filing date: 2010-02-18
Publication date: 2012-01-04
Anticipated expiration: 2030-02-18
Also published as: RU2480291C1; EP2401087B1; WO2010098714A1; CN102413940B; SE0950101A1; SE533360C2; BRPI1008001A2; BRPI1008001B1; CN102413940A; CA2753349A1; CA2753349C

Abstract

The invention relates to a centrifugal separator and method for controlling a radial level of an interface layer (22) formed during operation between a light and a heavy liquid in a separating chamber (7) in said centrifugal separator. The control comprises means (26) for detecting the interface layer level in the separating chamber, and a device (21, 23, 24) for supplying/discharging a control liquid to/from the separating chamber (7) by means of a variable speed pump (23), wherein said means (26) for detecting the interface layer level is adapted to communicate with means (25) for controlling the speed of the pump in such a way that supply and discharge of the liquid take place in such amount per unit of time that the interface layer (22) in the separating chamber (7) is within predetermined radial inner and outer levels.

Description

CENTRIFUGAL SEPARATOR AND METHOD FOR SEPARATING

FIELD OF THE INVENTION

The present invention relates to a centrifugal separator according to the preamble of claim 1 and a method for controlling a radial level of an interface layer in a centrifugal separator according to the preamble of claim 6.

BACKGROUND TO THE INVENTION AND PRIOR ART

A centrifugal separator according to the preamble of claim 1 and a method for controlling a radial level of an interface layer in a centrifugal separator according to the preamble of claim 6 are known from

WO 00/37177 A1 , which describes control equipment for a centrifugal separator for separating a light liquid of relatively low density and a heavy liquid of relatively high density from a mixture containing these two liquids. The liquids may, for instance, be oil and water. The control equipment is intended for a centrifugal separator comprising a rotor, which is rotatable around a rotational axis and forms an inlet for said mixture and a separating chamber, which communicates with the inlet and which has a radially inner zone and a radially outer zone, said zones being adapted during a separating operation to contain separated light liquid and separated heavy liquid respectively.

A centrifugal separator of this kind may have outlets configured in several different ways for the separated liquids. Thus, the rotor may be provided with so-called overflow outlets for both of the liquids or an overflow outlet for one liquid and another kind of outlet for the other liquid. An outlet of such other kind may be constituted by, for instance, a non-rotatable so- called paring member or by nozzles situated in the surrounding wall of the rotor. Nozzles are used as a rule where the mixture supplied contains not only said two liquids but also a relatively large amount of solids which are heavier than the two liquids. The separated solids together with part of the separated heavy liquid may thus be discharged through the nozzles situated at the periphery of the rotor, whereas the separated light liquid is removed from a central part of the rotor via an overflow outlet or a paring member. In these cases the rotor may also form a space which communicates with the radially outer zone of the separating chamber in such a way that during a separating operation it will contain separated heavy liquid (but not separated light liquid). An excess of separated heavy liquid which does not leave the separating chamber via said nozzles can therefore be removed from the rotor via this space.

Another type of centrifugal separator in which solids as well as two different liquids can be separated is a so-called decanter centrifuge. In a centrifugal separator of this kind there is in the rotor a so-called sludge conveyor, which is adapted to transport separated solids along the surrounding wall of the rotor to a sludge outlet. The sludge outlet is often situated at a level in the rotor which is radially within the level of the outlets for the two separated liquids.

During a separating operation in a nozzle centrifuge of the kind described above or in a decanter centrifuge having a sludge conveyor, it may be difficult to maintain at a predetermined radial level an interface layer formed in the rotor between the separated liquids therein. The reason for this is that an uncontrollable amount of separated heavy liquid per unit of time often leaves together with the separated solids via the sludge outlet of the rotor. If this uncontrollable amount of heavy liquid would exceed the amount of heavy liquid, which per unit of time is introduced into the rotor with the mixture to be treated therein, the interface layer in the separating chamber between light liquid and heavy liquid will move radially outwards, and separated light liquid will in the end be lost together with the separated solids when the latter leave the rotor via the sludge outlet.

A particular separating operation in which this has caused problems is the removal of sand and water from oil in connection with oil recovery from so-called oil sands. In this context, nozzle centrifuges are used in at least two separating steps.

In a first separating step a mixture of oil, water, solvent and sand residues is introduced into a nozzle centrifuge and, in addition to the mixture, an amount of water is supplied to the centrifuge. The water supplied is added to ensure that the interface layer formed, in the rotor's separating chamber, between oil and water shall not be displaced radially outwards. Such a radial displacement of the interface layer might otherwise take place after a number of hours of operation because of said nozzles becoming worn by the outflowing sand and therefore releasing more water per unit of time than at the beginning of the separating operation. After the first separating step, the oil still contains not only solvent but also residues of sand and water. This oil is conducted out from a central part of the rotor via a paring member and is pumped on to another nozzle centrifuge to undergo a second separating step.

To achieve as good separating results as possible, special control equipment has been developed for controlling the separating operation in the first and second separating steps. The special control equipment is described as prior art in WO 00/37177 A1. That control equipment makes it possible to avoid continuous addition of excess water to the mixture being introduced into the centrifugal rotor. Instead, water is introduced into the separating chamber of the rotor — only when needed and only in a required amount — via a space of the kind previously described, i.e. a space communicating only with the radially outer zone of the separating chamber. Water is also removed from the rotor via the same space during periods when an excess of water enters with the oil which is to be cleaned, which excess cannot leave the rotor through the sludge outlet nozzles.

Said control equipment includes a pressure vessel for water, the lower part of which communicates via a conduit with a liquid transfer member, situated in said space in the rotor of the centrifugal separator, for the introduction of water into or discharge of water from the rotor. In the upper part of the pressure vessel a gas pressure is maintained (usually by means of nitrogen gas) and its magnitude is continuously controlled on the basis of the amount of water present at the time in the pressure vessel, so that the liquid pressure at the bottom of the pressure vessel, and therefore in the conduit via which the pressure vessel communicates with said space in the centrifugal rotor, is always kept constant at a predetermined value.

This constant value of the liquid pressure in said conduit corresponds to a desired radial level in the separating chamber of the rotor for the interface layer therein formed between separated oil and separated water. If the interface layer moves radially outwards from the desired level, the pressure drops in said space in the rotor, with the result that water is pushed from the pressure vessel into the rotor via the conduit until the interface layer returns to the desired radial level. A level-detecting member in the pressure vessel is adapted to initiate when necessary a supply of new water to the pressure vessel so that it never becomes empty of water.

If the interface layer in the separating chamber of the rotor starts moving radially inwards from the desired level, the pressure in said space in the rotor increases and excess water is pushed from this space into the pressure vessel via said conduit. When the liquid level in the pressure vessel has risen to an upper limit level, a bottom outlet of the pressure vessel opens to release water therefrom.

According to WO 00/37177 A1 , said control equipment is expensive, complicated and bulky. Therefore WO 00/37177 A1 describes a less expensive and a more simple control equipment for a centrifugal separator of the kind described above.

That control equipment includes a device for supply of a control liquid which is of higher density than said light liquid. The supply device has a pressure source for delivering pressurised control liquid, and a supply conduit which at its one end is connected to the pressure source in order to receive pressurised control liquid and at its other end is connected to a liquid transfer member in order to introduce pressurised control liquid into the rotor. The supply device is adapted upon need to supply control liquid to the rotor in only such amount per unit of time as is required to prevent an interface layer formed in the separating chamber between separated light liquid on the one hand and separated heavy liquid or control liquid on the other hand from moving radially outwards from a predetermined radial supply level. The control equipment also includes a device for discharge of separated heavy liquid and/or control liquid from said space in the rotor. The discharge device has a discharge conduit which is adapted, when the rotor is supplied with an excess of heavy liquid, to discharge separated heavy liquid and/or control liquid from the rotor through said discharge conduit in such amount per unit of time as is required to prevent said interface layer from moving radially inwards from a predetermined radial discharge level.

The control equipment described in WO 00/37177 A1 is characterised in that the discharge device is arranged to discharge liquid from said space in the rotor a different way than through said supply device. This control equipment differs from the control equipment previously described in that the pressure source in the supply device is not integrated in the discharge device, i.e. the control equipment has respective separate conduits for supply and discharge of liquid. The separated heavy liquid and/or control liquid leaving the rotor therefore need not be accumulated at an elevated pressure and no pressure vessel is needed. Consequently, there is also no need for a system for compression of gas and for control of the pressure of such gas. Instead, the pressure source may be constituted by a simple liquid pump and the whole control of the supply of control liquid and discharge of separated heavy liquid and/or control liquid may be by means of so-called constant pressure valves. Any container needed for a buffer amount of control liquid may be free of pressure and common to several centrifugal separators.

Although the control equipment is thus simplified, it still has certain disadvantages. The equipment includes a double set of components in the form of respective separate supply and discharge conduits with constant pressure valves. There is therefore a need to further simplify the control equipment for the centrifugal separator. SUMMARY OF THE INVENTION

An object of the present invention is to provide a centrifugal separator and a method of the initially described kind which comprise simple, inexpensive and compact control equipment.

This object is achieved with the initially described centrifugal separator which is characterised in that the pressure source is a variable speed pump, wherein said means for detecting the interface layer level is arranged to communicate with means for controlling the speed of the pump in such a way that the supply and discharge respectively of the liquid in said space take place in such amount per unit of time that the interface layer in the separating chamber is within said predetermined inner and outer radial levels.

The control liquid used may for example be the heavy liquid (water). The invention therefore does not require the control liquid to be in some form other than the heavy liquid, although that is also possible. All that is required is that the control liquid be of higher density than the light liquid.

The object is also achieved by the method initially described which is characterised in that the liquid is supplied to and discharged from said space by means of a variable speed pump as said pressure source, the speed of the pump being controlled, by the interface layer level detected in the separating chamber, in such a way that the supply and discharge respectively take place in such amount per unit of time that said interface layer in the separating chamber is within said predetermined inner and outer radial levels. The invention differs from the first of the previously described known forms of control equipment in that the separated heavy liquid and/or control liquid need not be accumulated at an elevated pressure, i.e. there is no need for any pressure vessel with systems for compression of gas and control of the pressure of such gas.

The invention differs from the second of the previously described known forms of control equipment in that neither the separate discharge conduit nor said constant pressure valves are necessary. According to the invention, supply and discharge of control liquid take place via the same conduit. The conduit with the variable speed pump is thus adapted to allow liquid flow in both directions, i.e. in both a supply and a discharge direction through the pump. The speed control of the pump eliminates the need for said constant pressure valves, since the speed control is itself adapted to control the liquid pressure in said conduit in such a way that the amount of control liquid supplied and discharged respectively per unit of time is such that said interface layer is within said predetermined interface layer levels.

A further object of the present invention is to provide a centrifugal separator with control equipment which in a simple and effective way controls said level of the interface layer in the separating chamber.

A further object of the present invention is to provide a centrifugal separator with control equipment which in a simple and effective way detects the interface layer level in the separating chamber.

A further object of the present invention is to provide a centrifugal separator with control equipment which in a simple and effective way controls the pump for delivery of pressurised control liquid. These further objects are achieved by the embodiments defined below according to claims 2 - 5 and 7 - 10 respectively.

According to an embodiment of the present invention, the pump is a reversible pump. Reversing the pump makes it possible for the liquid to be also pumped from the space in the discharge direction, thereby achieving quicker control when the level of the interface layer is within the inner predetermined radial level. It should be noted, however, that the pump need not be reversible for discharge of control liquid. The pump may for example be a variable speed rotary pump, e.g. a centrifugal pump, adapted to allow backflow of liquid. The level of the interface layer in the separating chamber depends on the speed of the pump, wherein a lowering of the speed of the pump (or having the pump at standstill) will cause the interface layer to move radially outwards as a result of the liquid pressure from the separating chamber overcoming the counterpressure from the pump, thus discharging the liquid from said space.

According to a further embodiment of the invention, said means for controlling the pump comprises a frequency converter. The frequency converter makes it possible for the speed of the pump to be controlled in an energy-efficient and simple way.

According to a further embodiment of the invention, said means for detecting the interface layer level in the separating chamber comprises a pressure sensor arranged to detect a change in the liquid pressure in the space, which liquid pressure depends on the level of the free liquid surface in the space. As the space is in liquid-transferring communication with the separating chamber via its radially outer zone, the level of the interface layer in the separating chamber will affect the free liquid surface in the space. If the interface layer moves radially outwards, the free liquid surface in the space will also move radially outwards, and vice versa. If the free liquid surface in the space moves radially outwards, the liquid pressure in the space will drop (at a given radial level). In this way the interface layer level in the separating chamber can easily be determined via the free liquid surface of the space. Alternatively, a device may be provided for detecting the radial position of the free liquid surface in the space. In all cases a detection operation of this kind refers to detecting the radial position of the interface layer formed in the separating chamber.

According to a further embodiment of the invention, said conduit is in liquid pressure transmitting communication with the space via the liquid transfer member, and the pressure sensor is arranged to detect a change in the liquid pressure in the conduit between the pump and the liquid transfer member. In this case the liquid pressure in the conduit provides a measure of the radial level of the free liquid surface in the space.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained in more detail below by a description of an embodiment with reference to the attached drawing.

Fig. 1 discloses schematically a longitudinal section through a rotor which forms part of a centrifugal separator, and control equipment according to the invention. DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

Figure 1 discloses, in an example of an embodiment of the invention, a centrifugal separator which comprises a rotor having a lower part 1 and an upper part 2, which parts are connected to one another by means of a locking ring 3. The rotor is situated uppermost on a vertical driveshaft 4 which is connected to the lower rotor part 1 and rotatable around a rotational axis R.

Within the rotor there is a distributor 5 which divides the rotor interior into a central inlet chamber 6 and an annular separating chamber 7 extending around the distributor. The distributor 5 rests on a central portion of the lower rotor part 1 via radially and axially extending wings (not shown) which are distributed around the rotational axis R of the rotor. The inlet chamber 6 communicates with the separating chamber 7 via channels 8 situated between said wings. A stationary inlet pipe 9 extends from above axially into the rotor and opens in the inlet chamber 6. Within the separating chamber 7, a stack of conical separating discs 10 is kept axially in place between the upper part 2 of the rotor and the lower part of the distributor 5. Each separating disc 10, like the lower part of the distributor, has at its outer periphery a number of recesses 11 distributed around the rotational axis R and situated axially in line with one another. Although the embodiment disclosed comprises separating discs and a distributor with recesses at its outer peripheral edge, it should be noted that rotors also exist which exhibit separating discs with a distributor which has recesses or holes radially within its outer peripheral edge (i.e. holes a short distance in on the conical surface of the separating disc), which recesses or holes are likewise distributed around the rotational axis R and situated axially in line with one another. At the radially outermost part of the separating chamber 7 the lower rotor part 1 carries several nozzles 12 distributed around the rotational axis R of the rotor. Each nozzle 12 has a through channel via which liquid and finely divided solids can be ejected from the separating chamber 7.

The upper rotor part 2 carries a central annular cap 13, which on its inside delimits an annular outlet chamber 14 open radially inwardly towards the rotational axis of the rotor. The outside of the stationary inlet pipe 9 supports an outlet member 15 in the form of a paring disc which extends radially outwards into the outlet chamber 14.

A radially inner zone 7a of the separating chamber 7 communicates with the outlet chamber 14 via an overflow outlet 16 formed by an annular flange which is supported by, and situated on the inside of, the upper rotor part 2. The overflow outlet 16 is not necessary for the function of the rotor and might, if desired, be dispensed with. Alternatively, the outlet member 15 might be dispensed with, in which case liquid flowing out from the separating chamber 7 might leave the rotor directly.

An annular space 17 delimited in the lower part 1 of the rotor is open radially inwardly towards the rotational axis R of the rotor. The space 17 communicates with a radially outer zone 7b of the separating chamber 7 via channels 18 and 19 and a plurality of pipes 20 distributed around the rotational axis R. Instead of the pipes 20, the rotor may of course be provided with other means for creating said connection between the radially outer zone 7b and the annular space 17. For example, the rotor may be provided with channels integrated in the lower part 1 of the rotor, in which case the channels will constitute extensions of the channels 18 and 19. Alternatively, the pipes 20 may be replaced by a separate plate arranged at the lower part 1 of the rotor and provided with channels which connect to the channels 18 and 19 of the rotor part and which create said connection between the radially outer zone 7b and the annular space 17.

A stationary liquid transfer member 21 extends into the space 17 and is adapted to conduct liquid into the space 17 or conduct liquid out therefrom.

A vertical broken line 22 in the separating chamber 7 represents a certain radial level therein.

The centrifugal rotor is suitable for treating a mixture of oil and water and of solids therein suspended. The mixture is supplied to the rotor via the inlet pipe 9 and is forwarded from the inlet chamber 6 to the separating chamber 7 via the channels 8. The mixture is distributed, via distribution channels formed by the recesses 11 in the separating discs, to the various interspaces between the separating discs 10, in which the various mixture components are separated from one another. Separated oil flows radially inwards and further out from the rotor via the outlet chamber 14 and the outlet member 15, whereas separated solids and water leave the rotor via the nozzles 12.

If the amounts of water and oil which respectively leave the rotor via the nozzles 12 and the outlet member 15 equal the amounts of water and oil contained in the mixture supplied to the rotor, a state of equilibrium occurs on the interface layer between separated oil and separated water. Such a state of equilibrium is shown schematically in the figure at the radial level 22 in the separating chamber 7. In a state of equilibrium of the kind described it is presumed that free liquid surfaces establish themselves in the various chambers and spaces of the rotor at the radial levels indicated in the figure by small triangles. If the state of equilibrium coincides with a desired predetermined radial level, no adjustment of the radial level 22 of the interface layer takes place, i.e. no liquid flows out from the rotor or into the rotor via the liquid transfer member 21. It should be noted, however, that the radial position of the radial level 22 shown in the figure need not be the state of equilibrium formed in practice, nor the desired predetermined radial level. To achieve as good separating results as possible, the desired predetermined radial level may be somewhat elsewhere in the separating chamber (e.g. at or somewhat radially outside the distribution channels formed by the recesses 11 in the separating discs). It is further presumed that separated solids leave the rotor via the nozzles 12 without blocking them for outflowing separated water.

Depending on wear of the nozzles 12 and/or variations in the amounts of water and oil in the mixture supplied to the rotor, however, it is in practice impossible, without using special control equipment, to maintain said interface layer between oil and water in the separating chamber 7 at the desired predetermined level, which is hereinafter assumed to correspond to the radial level 22 shown in the figure. Control equipment of this kind is connected to the liquid transfer member 21 and is adapted either to supply via the latter to the rotor a variable amount of control liquid (e.g. water) if said interface layer in the rotor tends to move radially outwardly from the level 22, or to remove a variable amount of water from the rotor if the interface layer tends to move radially inwardly from the level 22.

The control equipment comprises a device which has a pressure source in the form of a variable speed pump 23 and a conduit 24 which is connected at its one end to the pump 23 and at its other end to the liquid transfer member 21. The pump 23 is connected to a so-called VFD (variable frequency drive), i.e. a frequency converter 25 for controlling the speed of the pump 23, which frequency converter 25 communicates with a sensor 26 in the form of a pressure sensor P adapted to detect a change in the liquid pressure in the conduit between the pump 23 and the liquid transfer member 21.

The liquid transfer member 21 may within the scope of the invention be of various kinds. If it is stationary, i.e. non-rotating, as illustrated in the figure, it may with advantage comprise an annular disc surrounding the rotor's rotational axis R and extending into the space 17. The liquid transfer member may form one or more radially extending channels, or form one or more annular channels extending around the rotational axis R (see SE 76670). In either case the channels lead into the liquid which is present in the space 17. Rotation of the rotor causes a liquid pressure, the magnitude of which depends on the position of the free liquid surface of the liquid body which rotates together with the rotor in the space 17. The position of the liquid surface in the space 17 is itself influenced by any movement of the radial position 22 of the interface layer in the separating chamber 7 between separated oil and separated water. Thus, if the interface layer in the separating chamber 7 moves radially outwards, the free liquid surface in the space 17 also moves radially outwards, whereupon the pressure in the conduit 24 drops. Upon movement of the interface layer radially inwards, the pressure in the conduit 24 rises.

If the pressure in the conduit 24 tends to drop below a predetermined first value which corresponds to an outer predetermined radial level (somewhat radially outside the level 22) for the interface layer between oil and water in the separating chamber 7, the speed of the pump 23 increases to raise the pressure in the conduit 24 and pump water into the space 17 and further via the channels 18, 19 and the pipes 20 to the separating chamber 7. The speed increase will be greater or lesser depending on how low the pressure in the conduit 24 drops, and the amount of water pumped in per unit of time will be such that the interface layer between oil and water in the separating chamber is kept radially within said outer predetermined radial level.

If instead the pressure in the conduit 24 tends to rise above a predetermined second value which corresponds to an inner predetermined radial level (somewhat radially within the level 22) for the interface layer between oil and water in the separating chamber 7, the speed of the pump 23 is reduced (or the pump may even be reversed) to reduce the pressure in the conduit and thus discharge (or pump) water from the space 17. Water will then flow through the pump 23 in an opposite direction (relative to the supply direction described above). The speed reduction will be greater or lesser (or the pump may even be reversed) depending on how much the pressure in the conduit 24 increases, wherein the amount of water discharged (or pumped) per unit of time will be such that the interface layer between oil and water in the separating chamber 7 is kept radially outside said inner predetermined radial level.

The control equipment may also comprise a container 27 for control liquid (water) which is maintained in a desired quantity and at a desired temperature. To this end, the container 27 is provided with an inlet conduit, an outlet conduit, a float, valves controlled by the float, and a warming device (not shown in Fig. 1). Such a container is known per se from WO 00/37177 A1 (see Fig. 6). The pump 23 is arranged to use the container 27 as a control liquid reservoir in such a way that control liquid is both supplied to and discharged from the container 27 upon adjustment of the interface layer level 22 in the separating chamber 7. Control liquid supplied to the container 27 may thus be reused.

Any suitable pump desired may be used for the transport of water from and to the space 17, subject to the pump being adapted to allow flow in both directions. Such pumps are well known to one skilled in the art and are therefore not described here.

In a similar way, the pressure sensor 26 for detecting the pressure in the conduit may be replaced by other means for detecting the level of the interface layer. Instead of said pressure detection, means may be provided for detecting the radial position of the free liquid surface in the space 17. Also conceivable are other indirect means of detecting the interface layer level which detect at least one parameter related to the position of the interface layer and which, on the basis of said parameter or parameters, calculate or otherwise determine the radial position of the interface layer. In all cases, a detection operation of this kind refers to detecting the radial position of the interface layer formed between oil and water in the separating chamber 7. If so desired, it is of course also possible to provide means for direct detection of the radial position of the interface layer.

Control equipment according to the invention will of course also work in conjunction with a hermetically closed centrifugal rotor, i.e. a centrifugal rotor in which a space 17 is intended to be completely filled with liquid and to communicate with the interior of a stationary liquid transfer member which seals against the rotatable centrifugal rotor.

The invention is not limited to the embodiment disclosed, but may be varied and modified within the scope of the claims set out below.

Claims

1. A centrifugal separator for separating a light liquid of relatively low density and a heavy liquid of relatively high density from a liquid mixture containing these two liquids and solids therein suspended, which centrifugal separator comprises

a rotor (1 , 2) which is rotatable around a rotational axis (R) and which has an inlet (6, 9) for said mixture of liquids, wherein the rotor (1 , 2) delimits a separating chamber (7) which communicates with said inlet (6, 9) and which has a radially inner zone (7a) and a radially outer zone (7b), which zones are adapted, during operation of the centrifugal separator, to contain separated light liquid and separated heavy liquid respectively, which liquids form between them an interface layer (22) in the separating chamber (7), and such that the rotor has a space (17) which communicates with said radially outer zone (7b) of the separating chamber in such a way that during operation said space (17) will contain separated heavy liquid which fills the space radially inwards and forms a free liquid surface at a level related to the radial level of the interface layer (22) in the separating chamber, and

a control equipment comprising

- means (26) for detecting the interface layer level (22) in the separating chamber (7),

a device (21 , 23, 24) for supplying a control liquid to the radially outer zone (7b) of the separating chamber via said space (17), which control liquid is of higher density than the light liquid, wherein the device has a pressure source (23) for delivering pressurised control liquid, and a conduit (24) for supplying the control liquid, which conduit is connected at its one end to the pressure source (23) for receiving pressurised control liquid and is connected at its other end to a liquid transfer member (21) for introducing pressurised control liquid into said space (17), the device being adapted upon need to supply control liquid in only such amount per unit time as is required to prevent the interface layer (22) formed in the separating chamber (7) between separated light liquid on the one hand and separated heavy liquid or control liquid on the other hand from moving radially outwardly from an outer predetermined radial level, and

said device (21 , 23, 24) for supply of control liquid being also adapted to discharge liquid from the radially outer zone (7b) of the separating space through the same conduit (24) as the supply of control liquid, which device is adapted, when the separating chamber is supplied with an excess of heavy liquid, to discharge through the liquid transfer member (21) at least one out of the separated heavy liquid and the control liquid from said space (17) in such amount per unit of time as is required to prevent said interface layer (22) from moving radially inwardly from an inner predetermined radial level,

c h a r a c t e r i s e d i n

that the pressure source (23) is a variable speed pump, wherein said means (26) for detecting the interface layer level is arranged to communicate with means (25) for controlling the speed of the pump in such a way that the supply and discharge respectively of the liquid in said space (17) take place in such amount per unit of time that the interface layer (22) in the separating chamber (7) is within said predetermined inner and outer radial levels.

2. A centrifugal separator according to claim 1 , characterised in that the pump (23) is a reversible pump.

3. A centrifugal separator according to any one of the preceding claims, characterised in that said means (25) for controlling the pump comprises a frequency converter.

4. A centrifugal separator according to any one of the preceding claims, characterised in that said means (26) for detecting the interface layer level in the separating chamber comprises a pressure sensor arranged to detect a change in a liquid pressure in the space (17), which liquid pressure depends on the level of the free liquid surface in the space.

5. A centrifugal separator according to claim 4, characterised in that the conduit (24) is in liquid pressure transmitting communication with the space (17) via the liquid transfer member (21 ) and that the pressure sensor (26) is arranged to detect a change in the liquid pressure in the conduit (24) between the pump (23) and the liquid transfer member (21 ).

6. A method for controlling a radial level of an interface layer (22) formed during operation between a light liquid of relatively low density and a heavy liquid of relatively high density in a centrifugal separator for separating said liquids from a liquid mixture containing said liquids and therein suspended solids, which method comprises the steps of rotating a rotor (1 , 2) belonging to a centrifugal separator around a rotational axis (R), said liquid mixture being supplied through an inlet (6, 9) to the rotor and subjected to rotation therein,

separating said liquids under the influence of centrifugal force in a separating chamber (7) which is delimited by the rotor and which communicates with said inlet, which liquids are caused by the centrifugal force to form between them said interface layer (22) at said level inside the separating chamber which has a radially inner zone (7a) and a radially outer zone (7b), which zones are caused, by the rotation of the rotor, to contain separated light liquid and separated heavy liquid respectively,

supplying separated heavy liquid during rotation of the rotor to a space (17) which communicates with said radially outer zone (7b) of the separating chamber in such a way that the heavy liquid fills the space (17) radially inwards and forms a free liquid surface which has a radial level related to the radial level of the interface layer in the separating chamber (7),

detecting the interface layer level in the separating chamber,

supplying, upon need, a control liquid to the radially outer zone (7b) of the separating chamber via said space (17), which control liquid is of higher density than said light liquid, the control liquid being supplied to the space (17) through a conduit (24) which at its one end is connected to a pressure source (23) for receiving pressurised control liquid and at its other end is connected to a liquid transfer member (21) for introducing pressurised control liquid into said space (17), the control liquid being supplied in only such amount per unit of time as is required to prevent said interface layer (22) formed in the separating chamber between separated light liquid on the one hand and separated heavy liquid or control liquid on the other hand from moving radially outwardly from an outer predetermined radial level, and

discharging, upon need, liquid from the radially outer zone (7b) of the separating chamber via said space (17) by means of the liquid transfer member (21 ) and the same conduit (24) as the supply of control liquid, at least one out of the separated heavy liquid and the control liquid being discharged in such an amount per unit time as is required, when the separating chamber (7) is supplied with an excess of heavy liquid, to prevent said interface layer (22) from moving radially inwardly from an inner predetermined radial level,

c h a r a c t e r i s e d i n

that the liquid is supplied to and discharged from said space (17) by means of a variable speed pump (23) as pressure source, the speed of the pump being controlled, by the interface layer level detected in the separating chamber, in such a way that the supply and discharge respectively take place in such amount per unit of time that said interface layer (22) in the separating chamber is within said predetermined inner and outer radial levels.

7. A method according to claim 6, characterised in that the liquid is supplied to and discharged from said space (17) by means of a reversible pump (23).

8. A method according to claim 6 or 7, characterised in that the pump (23) is controlled by means of a frequency converter (25).

9. A method according to any one of claims 6 - 8, characterised in that the detection of the interface layer level (22) is by pressure measurement (26) in the space (17), which pressure measurement detects change in a liquid pressure which depends on the level of the free liquid surface in the space.

10. A method according to claim 9, characterised in that the pressure measurement takes place in said conduit (24) between the pump (23) and the liquid transfer member (21), which conduit is in liquid pressure transmitting communication with the space (17).