WO2000071259A1

WO2000071259A1 - Centrifugal separator with a by-pass hole in the distributor and a method of separating by means of such a centrifugal separator

Info

Publication number: WO2000071259A1
Application number: PCT/SE2000/000965
Authority: WO
Inventors: Gunnar ÅSTRÖM
Original assignee: Alfa Laval Ab
Priority date: 1999-05-21
Filing date: 2000-05-15
Publication date: 2000-11-30
Also published as: SE514219C2; SE9901855L; AU4966600A; SE9901855D0

Abstract

A centrifugal separator with a centrifugal rotor (2) comprises a distributor (3) dividing the interior of the centrifugal rotor into an inlet chamber (6) for receiving a liquid mixture to the separator and a separation chamber (8). The distributor (3) has at least one distribution passage (12) for conducting a main flow of the liquid mixture from the inlet chamber to the separation chamber. There is a device (16, 18) for discharging a separated relatively light liquid phase and a device (26, 28) for intermittent discharge of the substantially entire liquid content of the separation chamber and the inlet chamber. According to the invention the distributor (3) has at least one minimal by-pass hole (34) for passing a flow of the liquid mixture from the inlet chamber to the separation chamber. The by-pass hole (34) is situated radially inside the distribution passage (12) and preferably radially outside the radial level (38) of the free liquid surface in the separation chamber (8), suitable radially outside the inner edge of the separation plate (30A) situated nearest to the distributor (3).

Description

CENTRIFUGAL SEPARATOR WITH A BY-PASS HOLE IN THE DISTRIBUTOR AND A METHOD OF SEPARATING BY MEANS OF SUCH A CENTRIFUGAL SEPARATOR

The present invention relates to a centrifugal separator with a centrifugal rotor rotatable about a rotation axis, comprising a distributor having a central part and an external part connected with and extending radially outwardly from the central part, the central and external parts surrounding the rotation axis, the distributor dividing the interior of the centrifugal rotor into an inlet chamber and a separation chamber, and an inlet conduit for supplying a liquid mixture to be separated centrally into the inlet chamber. The inlet chamber communicates with the separation chamber through at least one distribution passage in the distributor for conducting a main flow of the liquid mixture during operation from the inlet chamber to the separation chamber. The centrifugal separator further comprises a device for discharging a separated relatively light liquid phase, and a device for intermittent discharge of substantially the entire liquid content of the separation chamber and inlet chamber. Centrifugal separators of this kind are used among other things for cleaning of liquid mixtures in the form of polluted oil from sludge and/or water. The cleaned oil obtained by the separator in this case constitutes the light liquid phase. Sludge and/or water in the polluted oil separate in the separation chamber and accumulate in the radially outermost portion of the separation chamber. Traditionally, the sludge is discharged from the separation chamber by the device for intermittent discharge usually comprising a slide valve, which momentarily opens a discharge passage between the separation chamber and peripheral outlet openings, so that the sludge is thrown radially out of the separation chamber together with at least a portion of separated water. To prevent valuable cleaned oil in the separation chamber from following, the cleaned oil is pushed radially inwardly before the discharge operation takes place by supplying a heavier liquid, usually water, to the separation chamber. Often the separation chamber is only emptied in part at these intermittent discharges in order to reduce the loss of oil .

In certain circumstances, however, particles in the sludge may tend to deposit on various elements in the separation chamber, above all on radially outer parts of the internal side of the separation chamber, but also on the conical separation plates existing in the separation chamber. In consequence, all the particles are not discharged from the separation chamber when the device for intermittent discharge is activated. This is a disadvantage resulting in an unbalanced centrifugal rotor but also in a deteriorated function of the separation plates, so that the separation efficiency of the centrifugal separator is reduced. To eliminate this disadvantage a so called total emptying of the separation chamber is applied, which means that the device for intermittent discharge keeps the discharge passage open long enough to discharge at least substantially the entire content of the liquid mixture from the separation chamber and inlet chamber. This total emptying results in that occurring sludge particles are more efficiently rinsed from the separation chamber and the inlet chamber during each discharge operation.

The process of intermittent total emptying of the separation chamber comprises certain preparatory steps in order to prevent valuable oil from being discharged from the separation chamber together with the water and sludge. Thus, in an initial stopping step the supply of the polluted oil to the centrifugal separator is stopped. In a subsequent displacement step water is supplied to the separation chamber to displace separated oil radially inwardly in separation chamber, so that the device for discharge of the light liquid phase (usually comprising a paring chamber formed in the rotor, in which paring chamber the light liquid phase forms a liquid body rotating with the rotor, and a stationary paring disc arranged in the paring chamber) feeds the oil out of the separation chamber. At an optimal displacement all oil is displaced out of the separation chamber without the displacement liquid, the water, flowing into the device for discharge of the oil. After these preparatory steps the device for intermittent discharge is activated so that the separation chamber is substantially totally emptied.

However, a problem of the above described intermittent total discharge process is that after the first preparatory steps, when the supply of polluted oil to the centrifugal separator is stopped, a minor amount of unseparated oil is trapped in the inlet chamber radially inside the distribution passages of the distributor and then is inevitable discharged from the centrifugal separator together with sludge and water at each intermittent discharge operation. Since the centrifugal separator usually is intermittently emptied at inter- vals of about 1 hour and often is in operation during a long total operation time per year the total loss per year of valuable oil in the discharged sludge can be significant.

The object of the present invention is to at least substantially reduce loss of light liquid phase, in particular in the form of valuable oil, in connection with intermittent total emptying of a centrifugal separator. According to one aspect of the present invention, a centrifugal separator of the kind initially described is provided, which is characterised in that the inlet chamber also communicates with the separation chamber through at least one minimal by-pass hole in the distributor for passing by a flow of the liquid mixture from the inlet chamber to the separation chamber, the by-pass hole being situated radially inside said distribution passage. As a result, the water supplied during the displacement step will displace at least the major portion of the amount of unseparated light phase, which is trapped in the inlet chamber, to the radially inner part of the separation chamber via the by-pass hole. This is of great importance when separating polluted oil.

Of course, during normal operation a "short circuit flow" of polluted oil will appear from the inlet chamber through the by-pass hole to the radially inner part of the separation chamber. However, this short circuit flow will give rise to a insignificant reduction of the separation efficiency of the centrifugal separator. Thus, since the short circuit flow only constitutes a small fraction of the main flow through the distribution passages in the distributor the separation efficiency for most applications will be satisfactory. Consequently, the expression "minimal by-pass hole" is intended to mean that the by-pass hole is not larger than necessary to permit that the portion of the trapped polluted oil in the inlet chamber to be displaced through the by-pass hole has time to flow through the latter during the dis- placement step.

The device for discharging the light liquid phase is adapted to keep a free liquid surface thereof at a radial level in the separation chamber, during operation of the centrifugal separator. Preferably, the by-pass hole is situated radially outside said radial level, so that the flow which during normal operation flows through the by-pass hole into the separation chamber is subjected to a certain separation therein. As a result, the negative influence on the separation efficiency of the centrifugal separator caused by the flow through the by-pass hole is reduced.

In a centrifugal separator according to the invention typically at least the external part of the distributor is conical and a set of truncated conical separation plates are normally arranged in the separation chamber, wherein one separation plate situated nearest to the distributor has a radially inner edge. Advantageously, the by-pass hole is situated radially outside the inner edge of the distributor, so that the flow through the by-pass hole flows into the separation channel defined by the distributor and the separation plate situated nearest thereto. In the separation channel, the liquid mixture is relatively efficiently separated, which means that the flow through the by-pass hole only has such an insignificant negative influence on the separation efficiency of the centrifugal separator that this influence can be disregarded in almost every applications, in particularly when separating polluted oil .

In accordance with a second aspect of the present invention, there is provided a method of separating a liquid mixture by means of a centrifugal separator with a rotating centrifugal rotor, which includes a distributor dividing the interior of the centrifugal rotor into an inlet chamber and a separation chamber. The method includes a separation step in which the liquid mixture is centrally fed into the inlet chamber, from which a main flow of the liquid mixture is conducted through at least one distribution passage in the distributor, so that the main flow enters the separation chamber at a determined radial level therein, and a separated relatively light liquid phase is discharged from the separa- tion chamber, and a displacement step in which the feed of the liquid mixture into the separation chamber is stopped and displacement liquid is supplied to the separation chamber for displacing the light liquid phase radially inwardly in the separation chamber, whereafter substantially the entire liquid content of the separation chamber and the inlet chamber is discharged from the centrifugal separator. The method is characterized by also conducting a by-pass flow of the liquid mixture during the separation step from the inlet chamber through at least one minimal by-pass hole in the distributor, so that the by-pass flow enters the separation chamber at a radial level therein which is situated radially inside said determined level at which the main flow of the liquid mixture enters the separation chamber, and also supplying liquid during the displacement step to the inlet chamber for displacing light liquid phase separated in part therein radially inwardly, so that the light liquid phase separated in part passes through the by-pass hole into the separation chamber. A free liquid surface of the light liquid phase may be maintained during the separation step at a radial level in the separation chamber, and the liquid mixture may be conducted from the inlet chamber through the minimal by-pass hole, so that it enters the separation chamber radially outside the radial level of the free liquid surface.

The centrifugal separator and method according to the invention are explained in more detail in the following with reference to the accompanying drawing showing an axial cross- section through a centrifugal separator according to the invention.

The drawing shows a centrifugal separator according to the invention which comprises a centrifugal rotor 2 rotatable about a rotation axis 7, and a distributor 3 with a central conical part 4 and a external conical part 5, which is connected to and extends radially outwardly from the central part 4. The parts 4 and 5 of the distributor 3 surround the rotation axis 7 and divide the interior of the centrifugal rotor 2 into an inlet chamber 6 and a separation chamber 8. There is an inlet conduit 10 for supplying a liquid mixture to be separated centrally into the inlet chamber 6. The inlet chamber 6 is divided by radially spacing wings, not shown, into a number of radial inlet passages and communicates with the separation chamber 8 via distribution passages 12 extending through the distributor 3 from the inlet passages, respectively, for conducting a main flow of the liquid mixture from the inlet chamber 6 to the separation chamber 8. A device for discharge of a separated relatively light liquid phase comprises an annular light phase paring chamber 14, formed by a U-shaped wall portion 16 connected to the centrifugal rotor 2, and a stationary paring disk 18 extending radially into the light phase paring chamber 14. The embodiment shown in the drawing also has a device for discharge of a separated relatively heavy liquid phase comprising an annular heavy phase paring chamber 20 formed by a U-shaped wall portion 22 connected to the centrifugal rotor 2, and a stationary paring disk 24 extending radially into the heavy phase paring chamber 20. A device for intermittent discharge of the separation chamber 8 and the inlet chamber 6 of substantially the entire liquid content therein comprises an axially movable slide valve 26 adapted to intermittently open a discharge passage to a number of sludge outlets 28 in the radially outermost portion of the centrifugal rotor 2. A set of truncated conical separation plates 30 is arranged in the centrifugal rotor 2 in the separation chamber 8 and is supported by the radially outer conical part 5 of the distributor 3. The separation plates 30 are provided with upwardly conducting passages, here in the shape of upwardly conducting holes 32, with form axial rows of upwardly con- ducting holes 32 situated radially in front of respective distribution passages 12 in the distributor 3. The distributor 3 is also provided with a minimal by-pass hole 34 for conducting a minimal flow of the liquid mixture from the inlet chamber 6 to the separation chamber 8. The by-pass hole 34 is situated radially inside said distribution passages 12 and radially outside the radially inner edge of the separation plate 30A situated nearest to the distributor 3. Since the inlet chamber 6 is divided into radial inlet passages by said radial spacing wings a circular recess must be formed on the under side of the distributor 3 or, which is preferred, on the under side of the spacing wings radially at level with the by pass hole 34, in order to provide liquid communication between all inlet passages and the by-pass hole 34. Alterna- tively, the distributor 3 may be provided with several minor by-pass holes 34, which communicate with respective inlet passages. The risk of the by-pass holes 34 clogging decreases with fewer by-pass holes 34, however, since each of the bypass holes 34 then may be designed with a larger through-flow area.

An important application of the centrifugal separator according to the invention is cleaning of polluted oil containing undesired components in the form of sludge and water. Therefore the function of the above centrifugal separator according to the invention is described in the following in connection with separation of such polluted oil.

During operation the polluted oil is pumped through the inlet conduit 10 to the inlet chamber 6, in which a prior separation of the polluted oil takes place, so that rela- tively heavy sludge particles separate radially outwardly.

The major portion of the polluted oil is distributed through the distribution passages 12 of the distributor 3 and the upwardly conducting holes 32 of the separation plates 30 to separation channels 36 defined by adjacent separation plates 30. In the separation channels 36 the pollutions in the form of a heavy liquid phase consisting of water and sludge are separated, which pollutions flow radially outwardly while the cleaned oil in the light liquid phase flows radially inwardly and is discharged from the centrifugal separator via the light phase paring chamber 14. The heavy liquid phase of water is discharged from the centrifugal separator via the heavy phase outlet 20. The sludge is accumulated in the radially outermost portion of the separation chamber 8. The U-shaped wall portion 16 of the light phase paring chamber 14 works as an overflow and thereby maintains a free liquid surface of the cleaned oil in the separation chamber 8 at a given radial level 38. A flow of polluted oil flows through the by-pass hole 34 into the separation channel 36A nearest to the distributor 3 and is subjected to a certain limited separation in this separation channel 36A.

Regularly, for instance once an hour, accumulated sludge is discharged by operating the slide valve 26 to temporarily open the discharge passage between the separation chamber and the sludge outlets 28, so that the sludge is thrown out of the separation chamber 8 via the sludge outlets 28. To ensure that the separation chamber 8 is efficiently cleaned during the sludge discharge operation, so that possible sludge particle deposited on for instance the separation plates 30 are rinsed away, the discharge passage, however, is open long enough to permit discharge of substantially the entire content of liquid of the separation chamber 8. At the same time also the liquid content of the inlet chamber 6 is discharged. Prior to a sludge discharge operation the supply of polluted oil is stopped by closing a valve 40. Then a valve 41 is opened for supplying displacement liquid, suitable in the form of water, through the inlet conduit 10 to the separation chamber 8, so that the oil therein is displaced radially inwardly and can be discharged via the light phase paring chamber 14. The displacement liquid also displaces the amount of polluted oil existing in the inlet chamber 6 radially inside the distribution passages 12 of the distributor 3, so that this polluted amount of oil passes through the by-pass hole 34 to the separation chamber 8. When substantially all the oil in the centrifugal separator has been discharged therefrom via the light phase paring chamber 14 the displacement is finished by closing the valve 41, whereafter the slide valve 26 is operated to open the discharge passage to substantially totally discharge sludge and displacement liquid from the separation chamber 8 and inlet chamber 6.

Claims

C-T .ATMS

1. A centrifugal separator with a centrifugal rotor (2) rotatable about a rotation axis (7) , comprising a distributor (3) having a central part (4) and an external part (5) connected with and extending radially outwardly from the central part, the central and external parts surrounding the rotation axis, the distributor dividing the interior of the centrifugal rotor into an inlet chamber (6) and a separation chamber (8) , an inlet conduit (10) for supplying a liquid mixture to be separated centrally into the inlet chamber, the inlet chamber communicating with the separation chamber via at least one distribution passage (12) in the distributor for conducting a main flow of the liquid mixture from the inlet chamber to the separation chamber during operation, a device (16, 18) for discharging a separated relatively light liquid phase, and a device (26, 28) for intermittent discharge of substantially the entire liquid content of the separation chamber and inlet chamber, characterized in that the inlet chamber (6) also communicates with the separation chamber (8) via at least one minimal by-pass hole (34) in the distributor (3) for passing by a flow of the liquid mixture from the inlet chamber to the separation chamber, the by-pass hole being situated radially inside the distribution passage (12) .

2. A centrifugal separator according to claim 1, in which the device (16, 18) for discharging the light liquid phase is adapted to maintain a free liquid surface thereof at a radial level (38) in the separation chamber (8) during operation of the centrifugal separator, characterized in that the by-pass hole (34) is situated radially outside said radial level (38) .

3. A centrifugal separator according to claim 2, in which at least the outer part (5) of the distributor (3) is conical and a set of truncated conical separation plates (3) is arranged in the separation chamber (8) and is supported by the outer part of the distributor, a separation plate (30A) situated nearest to the distributor (3) having a radially inner edge, characterized in that the by-pass hole (34) is situated radially outside the radially inner edge of the distributor (3) .

4. A method of separating a liquid mixture by means of a centrifugal separator with a rotating centrifugal rotor (2), which includes a distributor (3) dividing the interior of the centrifugal rotor into an inlet chamber (6) and a separation chamber (8) , which method comprises a separation step in which the liquid mixture is centrally fed into the inlet chamber, from which a main flow of the liquid mixture is conducted through at least one distribution passage (12) in the distributor, so that the main flow enters the separation chamber at a determined radial level therein, and a separated relatively light liquid phase is discharged from the separa- tion chamber, and a displacement step in which the feed of the liquid mixture into the separation chamber is stopped and displacement liquid is supplied to the separation chamber for displacing the light liquid phase radially inwardly in the separation chamber, whereafter substantially the entire liquid content of the separation chamber and the inlet chamber is discharged from the centrifugal separator, characterized by also conducting a by-pass flow of the liquid mixture during the separation step from the inlet chamber through at least one minimal by-pass hole (34) in the distri- butor (3), so that the by-pass flow enters the separation chamber (8) at a radial level therein which is situated radially inside said determined level at which the main flow of the liquid mixture enters the separation chamber, and also supplying liquid during the displacement step to the inlet chamber for displacing light liquid phase separated in part therein radially inwardly, so that the light liquid phase separated in part passes through the by-pass hole into the separation chamber.

5. A method according to claim 4, characterized by maintaining a free liquid surface of the light liquid phase during said separation step at a radial level in the separation chamber (8) , and conducting the liquid mixture from the inlet chamber (6) through the minimal by-pass hole (34) , so that it enters the separation chamber (8) radially outside the radial level of the free liquid surface.