EP0879089B1

EP0879089B1 - Method and device for the supply of control liquid to a centrifugal separator

Info

Publication number: EP0879089B1
Application number: EP97902780A
Authority: EP
Inventors: Per Gunnar Karlsson
Original assignee: Alfa Laval AB
Current assignee: Alfa Laval AB
Priority date: 1996-01-29
Filing date: 1997-01-24
Publication date: 2002-06-12
Anticipated expiration: 2017-01-24
Also published as: CN1209760A; JP4253040B2; JP2000504269A; DE69713277T2; KR100478425B1; RU2196641C2; DE69713277D1; EP0879089A1; CN1079295C; SE9600313D0; AU1678897A; SE505099C2; KR19990082082A; SE9600313L; WO1997027945A1

Description

The present invention concerns a method of supplying a control liquid to a rotor of a centrifugal separator by means of a supplying device having a cylinder, in which an axially displaceable wall element is arranged to axially delimit a storing chamber in the cylinder, in which storing chamber a predetermined volume of control liquid is stored, and out of which at least a portion of this volume of control liquid is pressed during operation out of the storing chamber into the rotor by axial displacement of the wall element.

The invention also concerns a device for supplying of control liquid to a rotor of a centrifugal separator. The device is of the kind comprising a cylinder, in which an axially displaceable wall element axially delimits a storing chamber, the storing chamber being arranged to store a predetermined volume of control liquid. To the storing chamber there is connected an inlet for the supply of control liquid to the storing chamber upon displacement of the wall element in a first direction, and an outlet passage arranged in the supplying device for at least a portion of the volume of control liquid stored in the storing chamber to be pressed out of the storing chamber into the rotor via a conduit connected to the outlet passage, when control liquid is supplied to the rotor upon axial movement of the wall element in a second direction. Supplying devices of this kind also comprise means to press the wall elements in the second direction with a predetermined force, which is higher than the resulting force of a liquid pressure prevailing in the storing chamber when control liquid is supplied to the rotor.

Furthermore, the present invention concerns a centrifugal separator, which is provided with a device to supply control liquid to its rotor.

US 4 510 052 shows a centrifugal separator, which is provided with a device of this kind to supply control liquid to its rotor in a manner described above to intermittently open and re-close a peripheral passage when needed or at equal time periods to empty the separator chamber of the rotor of all its contents of product or a desired portion thereof. The passage is kept closed by pressing the valve slide in a direction closing the passage by a pressure force of a control liquid, which is present in a so called closing chamber, which is provided with an outlet valve at its outermost portion. This outlet valve is opened for discharge of more or less of the content of control liquid in the closing chamber by supplying a certain volume of control liquid by means of an injector driven by pressurized air at a high flow into a so called opening chamber, which is provided with a throttled outlet at its outermost portion. The control liquid present in the opening chamber influences the outlet valve with a pressure force in a direction opening the outlet valve against the action of a force resulting from a number of springs.

Gradually, the opening chamber is filled up with control liquid, which is entrained into the rotation of the rotor. The free liquid surface of the control liquid is then displaced radially inwardly and reaches rapidly a certain radial level, at which the pressure force from the control liquid accumulated in the opening chamber, upon lasting conditions when the control liquid is fully entrained into the rotation of the rotor, would exceed the resulting spring force and open the outlet valve whereby the control liquid would flow out of the closing chamber. However, the supply of control liquid takes place at such a high flow that the opening chamber is filled up radially inwardly to a level far inside the first mentioned radial level with control liquid, which only partly has been entrained into the rotation of the rotor, before the outlet valve opens and lets control liquid out of the closing chamber.

When the outlet valve has opened this rapidly results in so much control liquid flowing out of the closing chamber that the pressure from the remaining control liquid in the closing chamber is no longer able to keep the passage closed against the liquid pressure from the product present in the separation chamber and instead the passage is opened and product flows out of the separation chamber. How large the portion of the contents of the separation chamber that flows out through the passage is, is determined by for the length of time the control liquid present in the opening chamber, which gradually flows out through the throttled outlet, is able to keep the outlet valve open, that is the volume of and the flow at which control liquid is supplied to the opening chamber before the outlet valve opens.

In the known supplying device the amount is regulated at each discharge occasion by adjusting the time, during which the high air pressure is acting on the injector and presses control liquid out of the same. The amount of the time in these connections is very short, which makes it very difficult to discharge a desired volume of product with sufficient accuracy and repeatability.

The closing chamber and the injector are refilled with control liquid from a source of control liquid, which often consists of a tank located at a limited level above the centrifugal separator, or of the mains water system. The refilling of the product, which is to be separated in the separation chamber after a discharge has taken place, cannot be done quicker than refilling of the closing chamber without having product leaking out through the passage. Since the supply of product to the separation chamber is not normally shut off during the discharge procedure, the supply of control liquid to the closing chamber in these cases has to be regulated and adapted to the supply of the product to the separation chamber.

Since the available liquid pressure of the source of control liquid is low and, furthermore, if it consists of the mains water system varies heavily varying, it is very difficult to adjust a suitable supply flow. The fact that the available liquid pressure is low also means that conduits and valve having large throughflow area have to be used, which makes it even more difficult to adjust a suitable supply flow.

The object of the present invention is to accomplish a method and a device of the kind initially described for the supply of control liquid to a rotor of a centrifugal separator, which makes it possible first to supply a very well defined volume of control liquid at a high flow with a high repeatability and then to supply a further volume of control liquid at a flow, which is independent of outer circumstances which are difficult to control, such as the liquid pressure in the mains water system.

According to the present invention this is accomplished by a predetermined portion of the volume of the control liquid stored in the storing chamber, during a first step after the supply has been initiated and upon displacement of the wall element through a first distance with an adjustable pressure, is pressed out of the storing chamber through an outlet passage in the supplying device and against the influence of a first flow resistance in this outlet passage, and the control liquid being pressed out at a first flow higher than a predetermined minimum flow, by at least a portion of the first outlet passage being closed by an opening and closing element connected to the wall element (26) when the wall element has been displaced the first distance, and by at least a portion of the remaining volume of control liquid in the storing chamber, during a second step following the first step upon continuing displacement of the wall element a second distance, being pressed out of the storing chamber, against the influence of a second flow resistance higher than the first flow resistance, through a second outlet passage arranged in the supplying device, and at a second flow substantially lower than the first flow.

To accomplish this a device to supply a control liquid to a rotor of a centrifugal separator according to the present invention comprises a first outlet passage for connecting the storing chamber with said conduit and arranged to admit a first flow, higher than a predetermined minimum flow of control liquid, out of the storing chamber against the influence of a first flow resistance upon displacement of the wall element through a predetermined first distance in the second direction, and a second outlet passage also for connecting the storing chamber with the conduit and arranged to admit a second flow, substantially lower than the first flow, out of the storing chamber against the influence of a second flow resistance higher than the first flow resistance, upon displacement of the wall element through a second distance in the second direction. Furthermore, the device according the the invention comprises an opening and closing element connected to the wall element and arranged to open and to keep open the first outlet passage during the displacement of the wall element through the first distance in the second direction, to at least partly close the first outlet passage when the wall element is displaced through the first distance and to keep the first outlet passage at least partly closed upon the continued displacement of the wall element in the second direction.

In an embodiment of the invention the first and the second outlet passages are kept closed during displacement of the wall element a third distance, which keeps the first and second distances apart, the device also comprising a third outlet passage, for connecting the storing chamber to the conduit, and through which control liquid is brought to flow at least during the displacement of the wall element the third distance against a third flow resistance higher than the second flow resistance, whereafter the second outlet passage is opened and is kept open during the displacement of the wall element through the second distance.

Preferably, control liquid is pressed out of the storing chamber during the first step with a flow which is at least double the flow at which control liquid is pressed out of the storing chamber during the second step.

In a preferred embodiment of the invention the wall element is pressed in the second direction by means of pressurized air, the wall element also delimiting axially a pressure chamber arranged to be filled up with pressurized air at an adjustable pressure during displacement of the wall element in the second direction, and to be emptied of air during displacement of the wall element in the first direction.

In the following the invention will be described more closely with reference to the attached drawings, in which

figure 1 schematically shows an axial cross section through a rotor of a centrifugal separator, which is provided with a device, by means of which control liquid can be supplied to the rotor in accordance with the present invention,

figure 2 schematically shows in more detail a longitudinal section through a supplying device according to one embodiment of the invention, and

figure 3 shows a longitudinal section through a supplying device according to an other embodiment of the invention.

The rotor shown in figure 1 has an upper part 1 and a lower part 2, which are kept together by a locking ring 3. Inside the rotor there is an axially movable valve slide 4, which together with the upper part 1 delimits a separation chamber 5. Together with the lower part 2 the valve slide 4 delimits a closing chamber 6, which is arranged to open and close an annular passage 7 which surrounds the rotational axis of the rotor and connects the radially outermost part of the separation chamber 5 with outlet openings 8 arranged in the lower part 2 of the rotor and through which more or less of the product content in the separation chamber 5 is let out intermittently. The closing chamber 6 is provided with a central inlet 9 for the supply of control liquid and at its radially outermost portion it is provided with an outlet valve 10 for the discharge of control liquid from the closing chamber. The control liquid present in the closing chamber 6 acts during operation with a pressure force on the valve slide 4 in a direction closing the passage 7 against the influence of the pressure force from the product present in the separation chamber 5.

Centrally in the rotor a stationary inlet tube 11 opens for the supply of the product, which is to be centrifugally treated. The inlet tube 11 is surrounded by a distributor 12, which distributes the supplied product out to the separation chamber 5, in which the components in the product are separated. The main separation then takes place in interspaces between a stack of separation discs 13 arranged in the separation chamber 5. On the central inlet tube 11 a stationary discharge device 14 is arranged for centrally discharging a separated specifically light component, which is present in the supplied product.

The axially movable outlet valve 10 is pressed, by the pressure force from a number of helically coiled springs 15 distributed around the rotational axis, in a direction closing the outlet valve 10. The outlet valve 10 delimits together with the lower part 2 of the rotor an opening chamber 16, which has a central inlet 17 for the supply of controlling liquid to the opening chamber 16, and a throttled outlet 18 through which control liquid present in the opening chamber 16 flows out of the same. Control liquid is supplied to the inlet 17 of the opening chamber 16 via conduit 19, which also supplies control liquid to the inlet 9 of the closing chamber 6, in which inlet 9 the conduit 19 opens. The inlet 17 of the opening chamber 16 communicates with the inlet 9 of the closing chamber 6 via an overflow outlet 20, over which control liquid flows into the inlet 17 of the opening chamber 16 when the closing chamber 6 is filled up radially inside this overflow outlet 20. Control liquid, which is present in the opening chamber 16, influences the outlet valve 10 with a pressure force in the opening direction against the influence of the pressure force from the helically coiled springs 15.

To the conduit 19 a device 21 is connected for the supply of control liquid to the rotor via the conduit 19. This supplying device 21 is connected to a conduit 22 for the supply of control liquid and to a presssurized air conduit, through which air of a predetermined, preferably adjustable, pressure can be supplied or evacuated out of the supplying device 21 by turning a three-way valve 23. In the examples in the figures the three-way valve is electromagnetically controlled but in installations in which explosion security is required, it is suitable to choose a pneumatically controlled three-way valve. The conduit 22 for the supply of control liquid is in the shown examples provided with a back flow valve 24. In many cases the conduit system for the supply of control liquid has such a high resistance against back flow that this valve can be done without.

In figure 2 an embodiment of a supplying device according to the invention is shown in more detail. This has a cylinder 25, in which an axially movable wall element 26 is arranged. The wall element 26 is sealed against a cylindrical inner surface in the cylinder 25, during displacement of the wall element 26, by means of two

sealing rings

27 and 28, which are located at such a big axial distance from each other that an inclination of the wall element 26, which can make the axial displaceability of it more difficult, is prevented.

Axially on one side of it the wall element 26 delimits a storing chamber 29 and on the opposite side of it a pressure chamber 30. A preferably circular cylindrical element 31 is fixedly connected to the wall element 26 on the side of it facing the storing chamber 29. The longitudinal axis of the elelment 31 is parallel to, preferably coaxial with, the axis of the cylinder 25. The wall element 26 is pressed towards the end wall 32 of the cylinder, which like the wall element 26 delimits the storing chamber 29 axially in one direction, when the three-way valve 23 is set in the position in which the pressure chamber 30 is connected to the source of air at the high pressure. In another position of the three-way valve 23 the pressure chamber 29 is connected to the surrounding atmosphere for evacuating of the air contents of the pressure chamber 30.

In the end wall 32 is a first outlet passage 33 which connects the storing chamber 29 to the conduit 19 and is arranged to admit a first flow, higher than a predetermined minimum flow of control liquid, out of the storing chamber against the influence of a first low flow resistance. The minimum flow is higher than the maximum flow, at which control liquid flows out of the opening chamber through the throttled outlet 18.

The conduit 22 for the supply of control liquid from a source of control liquid is in the example shown in figure 2 connected to the conduit 19. The first outlet passage 33 is coaxial to the element 31 and has the same cross section as the element 31. The element 31 has an axial extension, which is shorter than the maximum axial extension of the storing chamber 29. Hereby, all of the element 31 will, during displacement of the wall element 26 a first distance from its end position adjacent a second end wall 34 of the cylinder 25, which is located to the left in the example shown in the figure, be located inside the storing chamber 29.

When the wall element 26 has been displaced this first distance the element 31 reaches the end wall 32 and closes the first outlet passage 33, which is provided with a sealing ring 35. This sealing ring seals against the outer contour of the element 31 during continuing displacement of the wall element 26 and the element 31 through a second distance in the same direction as during the first distance. In figure 2 the wall element 26 is shown in a position in which it is located during displacement of it through this second distance.

In the element 31 there is arranged a second outlet passage 36, which also connects the storing chamber to the conduit. Upon displacement of the wall element 26 and the element 31 through the second distance, this second outlet passage 36 admits a second flow, which is substantially lower than the first flow, out of the storing chamber against the influence of a second flow resistance on this second flow, which is higher than the first flow resistance.

How long this second distance is, through which the wall element 26 is displaced and presses control liquid out of the storing chamber 29 and consequently how large the volume of control liquid, which during this displacement is supplied to the rotor at said second flow, is determined by the time during which air of high pressure is connected to the pressure chamber 30 via the three-way valve 23. The pressure chamber 30 is connected to the source of air of the high pressure when the three-way valve is set in one position, whereas in an other position of the three-way valve 23 it is connected to the surrounding atmosphere for evacuating of the air contents of the pressure chamber 30. The wall element 26 can not be displaced further than to the end position of it towards the end wall 32. In the shown example the maximum displacement of the wall element 26 is delimited by the end wall 32. To control the volume of control liquid supplied to the rotor during the displacement of the wall element through the second distance, a stop element having a suitable axial length can for instance be inserted in the storing chamber and be connected to the end wall 32.

The embodiment of a supplying device according the invention shown in figure 3 differs from the embodiment shown in figure 2 in that the conduit 22 for the supply of control liquid from a source of control liquid in this example is connected directly to the storing chamber at the end wall 32 of the cylinder 25 and not via the conduit 19. Furthermore, the end of the second outlet passage 37 turned from the wall element 26 does not open in the axial end surface of the element 31 but in its circumferencial surface at a distance from its axial end. During its displacement axially in the second direction, which is to the right in the figure, the element 31 will close the first outlet passage 33 for a while before the second passage 37 is opened and control liquid can flow out of the storing chamber 29 through the same. Hereby, when needed, you may obtain a clear separation between the supply flow during displacement of the wall element 26 through the first distance and the supply flow during the displacement of the wall element 26 through the second distance.

To make a displacement of the wall element 26 and the member 31 possible the short distance, during which displacement the first outlet passage 33 and the second outlet passage 37 are closed, a third outlet passage 38 is arranged in the element 31. This third outlet passage 38 is provided with a restriction 39 with such a high flow resistance that only a low flow can flow out of the storing chamber 29 during the displacement of the wall element 26 this short distance. Besides, in figure 3 the wall element 26 is shown in its end position, in which the storing chamber 29 has its largest volume.

The supply of control liquid to the rotor of a centrifugal separator by means of the device 21 which is connected to the rotor via the conduit 19 takes place according to the present invention in the following way:

Before a supply of control liquid is initiated in order to, for instance, as in the embodiments shown in the figures, open and re-close a passage 7 to let out more or less of the product contents of the separation chamber 5 when needed or at equal time periods, the three-way valve 23 has been set in the position so that the storing chamber 29 is evacuated of its air contents, for a time long enough for the pressure of the control liquid in the conduit to press the wall element 26 in a first direction to its end position towards the end wall 34 of the cylinder 25 and completely refill the storing chamber 29.

When a supply of control liquid has been initiated the three-way valve 23 is turned so that the pressure chamber 30 is connected to the source of air of a pressure, which is essentially higher than the pressure of the control liquid in the storing chamber 29. Hereby, the wall element 26 is displaced in a second direction towards the end wall 32. Upon displacement of the wall element 26 a first distance it presses control liquid out of the storing chamber 29 through the first outlet passage 33, which admits a high first flow through the same against the influence of a low first flow resistance, and further through the conduit 19 which opens in the annular inlet 9 of the closing chamber 6 in the rotor. In the inlet 9 the control liquid rotates with a radially inwardly directed free liquid surface, which in figure 1 is marked with a triangle. During normal conditions of operation the conduit 19 opens at a radial level, which is located so much radially outside the radial level of the free liquid surface that there is equilibrium at the opening between the pressure from the rotating liquid body and the pressure from the conduit 22 for the supply of control liquid. Since the wall element 26 presses control liquid out of the storing chamber 29 at a substantially higher pressure the free liquid surface in the inlet 9 will be displaced radially inwardly and be located radially inside the radial level of the overflow outlet 20. This results in that control liquid flows via the overflow outlet 20 into the opening chamber 16 at a flow, which is higher than the flow, which passes out through the throttled outlet 18 whereby the opening chamber 16 gradually is filled up radially inwardly. Control liquid flows into the opening chamber 16 as long as the outlet valve 10 keeps the closing chamber 6 closed. The supply of control liquid takes place at such a high flow that the opening chamber 16 will be filled by control liquid, which only partly has been entrained into the rotation of the rotor, radially inwardly to a radial level which is far inside the radial level at which the pressure force from the control liquid accumulated in the opening chamber 16 during lasting conditions with fully entrained control liquid would exceed the resulting pressure force from the springs 15 and open the outlet valve 10, before the outlet valve 10 opens and control liquid flows out of the closing chamber 6.

When the outlet valve 10 has opened control liquid flows out of the closing chamber 6, which in turn means that the passage 7 is opened and more or less of the product contents of the separation-chamber 5 flows out through the outlet openings 8. The fact that control liquid flows out of the closing chamber 6 also means that no more control liquid flows into the opening chamber 16 and control liquid in the opening chamber 16 is gradually drained out through the throttled outlet 18. Eventually the opening chamber is only filled up radially to a radial level, at which the pressure force from the control liquid accumulated in the opening chamber 16, even if it is fully entrained, no longer exceeds the pressure force of the springs 15. This results in that the outlet valve 10 closes the outlet out of the closing chamber 6.

How much control liquid flows out of the closing chamber 6, and indirectly how great a portion of the product content of the separation chamber flows out through the outlet openings 8, depends on how long control liquid in the opening chamber 16 is able to keep the outlet valve 10 open.

In close connection to this the wall element 26 has been displaced the first distance and the element 31 connected to the wall element 26 reaches the first outlet passage 33 and closes it.

During continuing displacement of the wall element 26 the first outlet passage 33 is closed while a

second outlet passage

36 or 37 is open during displacement of the wall element 26 through a second distance

This

second outlet passage

36 or 37, which during displacement of the wall element 26 the second distance connects the storing chamber 29 to the conduit 19, admits a second flow through the same against the influence of a second flow resistance, which is higher than the first flow resistance. This second flow of control liquid flows into the closing chamber 6 via the conduit 19 and the inlet 9 of the closing chamber and gradually fills up the closing chamber radially inwardly. This second flow is then adapted to the supply flow through the inlet tube 11 of the product, which shall be centrifugally treated. The volume of control liquid, which is supplied to the closing chamber 6 during displacement of the wall element the second distance is regulated by adjusting the time during which the pressure chamber is connected to the pressurized air source of high pressure, or by inserting a mechanical stop into the storing chamber 29, which limits the displacement of this second distance of the wall element 26. The volume is adapted in a such way that the opening of the conduit 19 after that the wall element 26 has been displaced the second distance is located partly immersed radially outwardly in the rotating liquid body in the inlet 9.

After that the three-way valve 23 is turned in a way such that the pressure chamber 30 is connected to the surrounding atmosphere. Hereby, the air in the pressure chamber 30 can be evacuated and the storing chamber 29 be refilled with control liquid by the wall element 26 being brought back by the pressure of the control liquid in the conduit 22 towards the end wall 34 of the cylinder 25.

The supply flow during the displacement of the wall element 26 through the first distance and the supply flow during the displacement of the wall element 26 through the second distance can, as described above and shown in the embodiment according to figure 3, when needed be kept apart in time.

Claims

Method of supplying a control liquid to a rotor of a centrifugal separator by means of a supplying device having a cylinder (25), in which a displaceable wall element (26) is arranged to axially delimit a storing chamber (29) in the cylinder (25), in which storing chamber (29) a predetermined volume of control liquid is stored, and out of which at least a portion of this volume of control liquid during operation is pressed out of the storing chamber (29) into the rotor by axial displacement of the wall element (26), characterized in that

a predetermined portion of a volume of control liquid stored in the storing chamber (29) is pressed out during a first step after a supply has been initiated, by displacement of the wall element (26) a first distance, the control liquid being pressed out at an adjustable pressure of the storing chamber (29) through a first outlet passage (33) in the supplying device (21) and against the influence of a first flow resistance in this outlet passage (33), and the control liquid being pressed out at a first flow higher than a predetermined minimum flow,

at least a portion of the first outlet passage (33) is closed by an opening and closing element connected to the wall element (26) when the wall element (26) is displaced the first distance, and

at least a portion of the remaining volume of control liquid in the storing chamber (29) is pressed out of the storing chamber (29), during a second step following the first step upon continuing displacement of the wall element (26) a second distance, against the influence of a second flow resistance higher than the first flow resistance, during the second step the liquid being pressed out through a second outlet passage (36,37) in the supplying device (21) at a second flow substantially lower than the first flow.
Method according to claim 1, characterized in that the first and the second outlet passages (33 and 36,37 respectively) are kept closed during the displacement of the wall element (26) through a third distance, which keeps the first and second distances apart, the supplying device (21) also comprising a third outlet passage (38) which connects the storing chamber (29) with the conduit (19) and through which control liquid is brought to flow, at least during displacement of the wall element through the third distance, against a third flow resistance which is higher than the second flow resistance, whereafter the second outlet passage (36,37) is open and kept open during displacement of the wall element through the second distance.
Method according to claim 1 or 2, characterized in that control liquid is pressed out of the storing chamber (29) during the first step with a flow which is at least double the flow at which control liquid is pressed out of the storing chamber (29) during the second step.
Device for supplying control liquid to a rotor of a centrifugal separator, which supplying device comprises

a cylinder (25), in which a wall element (26) is axially displaceable in the cylinder (25) and axially delimits in the cylinder (25) a storing chamber (29) arranged to store a predetermined volume of control liquid,

an inlet connected to the storing chamber (29) for the supply of control liquid to the storing chamber (29) upon displacement of the wall element (26) in a first direction,

an outlet passage connected to the storing chamber (29) and arranged in the supply device (21) for at least a portion of the volume of control liquid stored in the storing chamber (29), during supply of control liquid to the rotor, to be pressed out of the storing chamber (29) and into the rotor, via a conduit (19) connected to the outlet passage, upon axial displacement of the wall element (26) in a second direction, and

means to press the wall element (26) in the second direction, during supply of control liquid, with a predetermined force higher than the force resulting from a liquid pressure prevailing in the storing chamber (29),

characterized in that the device comprises

a first outlet passage (33) for connecting the storing chamber (29) to said conduit (19) and arranged to admit a first flow, higher than a predetermined minimum flow, therethrough against the influence of a first flow resistance upon displacement of the wall element (26) through a predetermined first distance in the second direction,

a second outlet passage (36, 37) for connecting the storing chamber (29) to said conduit (19) and arranged to admit a second flow, substantially lower than the first flow, therethrough against influence of a second flow resistance higher than the first flow resistance, when the wall element (26) is displaced through a second distance in the second direction, and

an opening and closing member (31) connected to the wall element (26), and arranged to open and keep open the first outlet passage (33) during displacement of the wall element (26) through the first distance in the second direction, to at least partly close the first outlet passage (33) when the wall elemlent (26) is displaced through the first distance, and to keep the first outlet passage (33) at least partly closed during continuing displacement of the wall element (26) in the second direction.
Device according to claim 4, characterized in that it is arranged to keep the first and the second outlet passage (33 and 36,37 respectively) closed upon the displacement of the wall element (26) a third distance, which keeps the first and second distances apart, the device also comprising a third outlet passage (38) for connecting the storing chamber (29) to the conduit (19) and arranged to admit, at least upon displacement of the wall element (26) through the third distance, a flow therethrough against a third flow resistance higher than the second flow resistance.
Device according to claim 4 or 5, characterized in that said means to press the wall element (26) in the second direction is arranged to press the wall element (26) by means of pressurized air, the wall element (26) also delimiting axially a pressure chamber arranged to be filled with pressurized air at an adjustable pressure during displacement of the wall element (26) in the second direction, and to be emptied of air during displacement of the wall element (26) in the first direction.
Centrifugal separator, characterized in that it is provided with a device according to any of the claims 4, 5 or 6.