GB2307276A

GB2307276A - Multi-phase fluid compressor

Info

Publication number: GB2307276A
Application number: GB9704440A
Authority: GB
Inventors: Andries Otter
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1996-03-06
Filing date: 1997-03-04
Publication date: 1997-05-21
Also published as: GB9704440D0

Abstract

A multi-phase fluid compressor comprises a rotating trumpet-shaped shroud (7) which co-axially surrounds the impeller (1) such that, besides the primary fluid passage (11) formed by the impeller (1) in which predominantly gaseous components are compressed, an annular secondary fluid passage (10) is formed through which predominantly liquid and solid constituents are transported. Thus, the shroud (7) will reduce contact between said constituents and the impeller (1) which rotates in use at high speed to compress the gaseous components, thereby reducing frictional forces exerted to, and wear of, the impeller.

Description

MULTI-PHASE FLUID COMPRESSOR The invention relates to a multi-phase fluid compressor.

More particularly, the invention relates to a multi- phase fluid compressor of the roto-dynamic type. Such a roto-dynamic compressor comprises an impeller which in use rotates at high speed within a casing.

In a radial version of a roto-dynamic gas compressor, also known as a centrifugal compressor, the circumferential velocity of the tip of the impeller is typically in the order of 400 m/s. If such a compressor is used to compress a multi-phase fluid comprising natural gas and liquid droplets, the liquid droplets and solid particles, such as sand, generally cannot follow the streamlines of the gas phase as a result of centrifugal forces. In such case as a result of prerotation of the fluid mixture at the entrance of the impeller the liquid and solid constituents will be deposited on the internal surface of the casing at the entrance of the impeller.

The collision of the liquid and any solid constituents with the casing and impeller may lead to significant friction and damage of the compressor. Hence, conventional roto-dynamic gas compressors cannot cope with gas containing more than a few per cent free liquids on a mass basis. Even if damage to the compressor by liquid deposition could be prevented by use of wearresistant materials, the solution would not be very attractive as the liquid imposes significant friction on the impeller, which leads to a high energy consumption of the compressor.

It is an object of the present invention to provide a multi-phase fluid compressor which is able to compress a multi-phase fluid such that transportation of liquid and any solids contained in the fluid does not lead to large frictional forces within the compressor and/or damage thereof.

It is a further object of the present invention to provide a multi-phase fluid compressor in which contact is reduced between the fast rotating impeller and any liquid and solids contained in the compressed fluid.

The compressor according to the invention thereto comprises: - a fluid inlet section which is formed within a substantially co-axial tubular part of the casing, in which section centrifugal forces imposed on the multiphase fluid flowing therethrough towards the entrance of the impeller will induce a concentration of any liquid and solid constituents of the fluid near the outer circumference of the fluid inlet section; - a substantially tubular shroud which is rotatable relative to the central axis of the casing and which at least partly surrounds the impeller; - a primary fluid passage which is at least partly formed by the impeller and via which in use predominantly gaseous constituents of the fluid are induced to flow from the fluid inlet section towards a fluid outlet section of the compressor; and - a secondary fluid passage which is at least partly formed by the shroud and via which in use predominantly liquid and solid constituents of the fluid are induced to flow from the fluid inlet section towards the fluid outlet section of the compressor.

The presence of the secondary fluid passage and the rotatable shroud which may in a specific embodiment of the compressor rotate at a lower speed than the impeller allows an efficient transport of solid and liquid constituents at a moderate speed via the secondary fluid passage whereas the gaseous fraction passes at a high speed through the primary fluid passage.

Preferably, the compressor is of the centrifugal type and comprises an impeller which includes a series of vanes that are rigidly secured between a central body which is mounted on a rotatable shaft and a tubular tapered outer body, wherein the shroud surrounds and is secured to said outer body.

It is also preferred that the tubular outer body of the impeller comprises a ring-shaped end section which extends beyond the tips of the vanes and the shroud comprises, when seen in a plane in which the central axis lies, a U-shaped end section whereby the legs of this Ushaped end section surround the ring-shaped end section of the outer body of the impeller to create in use a siphon-type liquid lock between said sections.

The presence of such a siphon-type liquid lock prevents that in the event that the compressed fluid has a varying liquid content gas would flow back from the outlet towards the inlet via the secondary fluid passage.

It is also preferred that the shroud surrounds the outer body of the impeller and at least part of the tubular inlet of the compressor.

Furthermore, if the shroud is not rigidly connected to the impeller, the shroud may be driven by drive means to rotate at a lower speed of rotation than the impeller.

Such drive means may adjust the angular speed of the shroud in response to variation of the liquid content of the fluid. Such adjustment of the angular speed of the shroud may also be used to exert such a centrifugal force on the liquid which passes through the secondary fluid passage that a liquid film is formed against the inner surface of the shroud which liquid film has no or only limited contact with the outer surface of the fast rotating impeller. This will reduce frictional forces between the impeller and the liquid film and reduce the power consumption of the compressor.

These and other features, objects and advantages of the multi-phase fluid compressor according to the invention will become apparent from the accompanying claims, abstract and drawings.

In the drawings Figures 1, 2 and 3 show schematic sectional views of three embodiments of the compressor according to the invention. In these Figures the central axis I of each embodiment shown lies in the plane of the drawing and only the part of the compressor at one side of the central axis I is shown. In each Figure the part of the compressor at the other side of the central axis I, which is not shown, is, apart from the scrolled volute at the outlet of the compressor, a substantially similar mirror image of the illustrated side of the compressor.

Referring now to Figure 1, there is shown a rotodynamic centrifugal compressor comprising an impeller 1 which is mounted on a shaft 2. The shaft 2 is rotatably supported by means of a bearing unit 3, such that the shaft 2 and impeller 1 are rotatable about the central axis I of casing 4.

The casing 4 consists of two sections that are interconnected by bolts (not shown). The casing 4 is equipped with diffuser vanes 5 which guide the fluid into a scrolled volute that forms the outlet 6 of the compressor.

The impeller 1 comprises a series of vanes 1B which are mounted between a disk-shaped central body 1A which is fixed to the shaft 2 and a tubular tapered outer body 1C. A trumpet-shaped shroud 7 co-axially surrounds the tubular outer body 1C of the impeller 1 and a tubular inlet 8 of the compressor. The shroud 7 is mounted within a recess formed within the inner surface of the casing 4 and is rotatably supported by the casing 4 via a bearing assembly 9.

Sealing rings 9A prevent contact between the compressed fluid and the bearing assembly.

An annular space is formed between the tubular tapered outer body 1C of the impeller 1 and the inner surface of the shroud 7. The annular space forms a secondary fluid passage 10 whereas the openings formed between the vanes 1B and the central and outer body 1A and 1C, respectively, form a primary fluid passage 11 of the compressor.

In use, rotation of the impeller 1 will induce the fluid mixture to prerotate within the inlet 8. As a result of this prerotation centrifugal forces will cause any liquid and solid constituents of the fluid to be swept against the inner surface of the shroud 7, as shown by the dots.

Frictional forces will cause the shroud 7 to rotate about the central axis I and centrifugal forces will induce said liquid and solid constituents to flow as a thin film from the inlet 8 via the secondary fluid channel 10 towards the outlet 6.

The gaseous components of the multi-phase fluid, which are lighter than the liquid and solid constituents, will tend to concentrate at the centre of the inlet 8 and will then enter into the primary fluid passage 11.

In the embodiment shown in Figure 1 the shroud 7 is not equipped with any driving motor and frictional forces exerted by the bearing assembly 9 and sealing rings 9A on the shroud 7 will generally cause the shroud 7 to rotate at a lower angular speed than the impeller 1.

The tubular outer body 1C of the impeller comprises a ring-shaped end section 1D which is surrounded by a U-shaped end section 7D of the shroud 7 to form a U-shaped fluid passage therebetween. In use this U-shaped fluid passage will fill up with liquid and form a siphontype liquid lock which prevents reverse flow of gas via the secondary fluid passage 10 in the event that the compressed fluid temporarily contains no liquid constituents.

Referring now to Fig. 2 there is shown a compressor .

comprising an impeller 21 which is mounted on a shaft 22.

The shaft is rotatably supported by a bearing unit 23 such that the shaft 22 and impeller 21 are rotatable about a central axis I of casing 24. The casing consists of two sections and comprises diffuser vanes 25 which guide the compressor fluid into a scrolled volute that forms the outlet 26 of the compressor.

The impeller 21 comprises a central body 21A, a series of vanes 21B, an outer body 21C and a shroud 21D which is secured to the outer body 21C by a series of spacer elements 27 such that an annular space is formed between the outer body 21C and the shroud 21D. The openings between the vanes 21B form a primary fluid passage 28 whereas said annular space forms a secondary fluid passage 29. Seals 30 serve to prevent reverse flow of fluid from the outlet 26 towards the inlet 31 of the compressor.

In use rotation of the impeller 21 will cause prerotation of the fluid mixture within the inlet 31.

This prerotation will cause separation of liquid and solid constituents towards the outer circumference of the inlet 31.

Hence liquid and solid constituents will predominantly flow into the secondary fluid passage 29 whereas gaseous components of the multiphase fluid will predominantly flow into the primary fluid passage 28.

The compressor is provided with a liquid lock 32 which prevents reverse flow of gas via the secondary fluid passage in the event that the compressed fluid temporarily comprises no liquid.

An advantage of interconnecting the shroud 21D and the outer body 21C of the impeller 21 is that no bearings are required for supporting the shroud 21D within the casing 24 and that vanes (not shown) can be arranged within the secondary fluid passage 29. Such vanes within the secondary fluid passage 29 will generally have a smaller pitch angle than the vanes 21B within the primary fluid passage 28 such that the rotational speed of the solid and liquid constituents within the secondary fluid passage 29 will be lower than the rotational speed of the gaseous components within the primary fluid passage 28.

Referring now to Fig. 3 there is shown a compressor comprising an impeller 41 which is mounted on a shaft 42.

The shaft is rotatably supported by a bearing unit 43 such that the impeller 41 and shaft 42 are rotatable about a central axis I of the casing 44.

The casing comprises a series of fixed diffuser vanes 45 which guide the compressed fluid into a scrolled volute that forms an outlet 46 of the compressor.

The impeller 41 comprises a central body 41A on which a series of vanes 41B are mounted.

The vanes 41B are surrounded by a shroud 47 which comprises a co-axial inner and outer walls 47A and 47B, respectively. Between these walls 47A and 47B an annular space is present which forms a secondary fluid passage 48 through which in use liquid and solid constituents are transported from an inlet 49 towards the outlet 46 of the compressor.

The openings that are formed between the vanes 41B of the impeller form a primary fluid passage 50 through which in use predominantly gaseous components of the fluid are transported.

The shroud 47 is rotatably supported within a recess in the inner surface of the casing 44 by a bearing unit 51. Sealing rings 52 prevent passage of fluid via the annulus formed between the shroud 47 and the casing.

In use centrifugal forces imposed on the fluid within the inlet 49 will induce liquid and solid constituents of the fluid to move towards the outer circumference of the,.

inlet 49 and into the secondary fluid passage 48.

Gaseous components will predominantly flow into the primary fluid passage 50 between the vanes 41B of the impeller.

Preferably the shroud 47 is provided with a drive motor (not shown) which induces the shroud to rotate at a lower speed of rotation than the impeller. The motor may adjust the angular speed of the shroud 47 in response to variation of the liquid content of the fluid.

If the fluid compressor is installed in a flowline in which large variations of the liquid content of the fluid occur, as is the case in a severe slug flow regime, it is desirable to install a slug catcher upstream of the inlet of the compressor in order to alleviate the slugging regime.

Claims

1. A multi-phase fluid compressor comprising: - an impeller which is mounted within a casing such that the impeller is rotatable relative to a central axis of the casing; - a fluid inlet section which is formed within a substantially co-axial tubular part of the casing, in which section centrifugal forces imposed on the multiphase fluid flowing therethrough towards the entrance of the impeller will induce a concentration of any liquid and solid constituents of the fluid near the outer circumference of the fluid inlet section; - a substantially tubular shroud which is rotatable relative to the central axis of the casing, and which at least partly surrounds the impeller;; - a primary fluid passage which is at least partly formed by the impeller and via which in use predominantly gaseous constituents of the fluid are induced to flow from the fluid inlet section towards a fluid outlet section of the compressor; and - a secondary fluid passage which is at least partly formed by the shroud and via which in use predominantly liquid and solid constituents of the fluid are induced to flow from the fluid inlet section towards the fluid outlet section of the compressor.

2. The compressor of claim 1, wherein the compressor is of the centrifugal type and comprises an impeller which includes a series of vanes that are mounted between a central body on a rotatable shaft and a substantially tubular tapered outer body and wherein the shroud surrounds said outer body, and the secondary fluid passage is formed by an annular space between the shroud and said outer body.

3. The compressor of claim 1, wherein the compressor is of the centrifugal type and comprises an impeller which includes a series of vanes that are rigidly secured between a central body which is mounted on a rotatable shaft and a tubular tapered outer body and wherein the shroud surrounds and is rigidly secured to said outer body.

4. The compressor of claim 1, wherein the compressor is of the centrifugal type and comprises an impeller including a series of vanes that are rigidly secured to a central body which is mounted on a rotatable shaft and wherein the shroud surrounds the vanes of the impeller and comprises co-axial inner and outer walls.

5. The compressor of claim 2 or 3, wherein the outer body of the impeller comprises a ring-shaped end section which extends beyond the tips of the vanes and the shroud comprises, when seen in a plane in which the central axis lies, a U-shaped end section whereby the legs of this Ushaped end section surround the ring-shaped end section of the outer body of the impeller to create in use a liquid lock between said sections.

6. The compressor of any preceding claim, wherein the shroud surrounds the outer body of the impeller and at least part of the tubular inlet of the compressor.

7. The compressor of any preceding claim, wherein the shroud is rotatably mounted within a recess formed within the inner wall of the casing by a bearing assembly.

8. The compressor of claim 2 or 4, wherein the shroud is provided with drive means which are able to induce the shroud to rotate at an angular speed of rotation about the central axis which is lower than the angular speed of rotation of the impeller.

9. The compressor of claim 8, wherein the drive means are able to vary the angular speed of the shroud in response to variation in the liquid content of the fluid.

10. The compressor of any preceding claim, wherein the fluid inlet comprises vanes for inducing the fluid to follow a helical flow path through the inlet.