WO2016100509A1

WO2016100509A1 - A diffuser for a multiphase fluid compressor pump

Info

Publication number: WO2016100509A1
Application number: PCT/US2015/066084
Authority: WO
Inventors: Ismail Hakki SEZAL; Siddharth Navinchandra ASHAR; Vittorio Michelassi; Rene DU CAUZE DE NAZELLE; Davide Giacche
Original assignee: General Electric Company
Priority date: 2014-12-16
Filing date: 2015-12-16
Publication date: 2016-06-23

Abstract

A diffuser (1) for a multiphase fluid pump is disclosed, the diffuser comprising a central hub (8) coaxially aligned with a surrounding shroud (6). The hub and shroud forms together an annular passage (10) for fluid flow (F) through the diffuser. A set of diffusor blades (7) are arranged angularly spaced about the hub, the blades connecting the hub and the shroud while projecting a pressure side (16) towards the flow (F). The diffusor blades (7) are saddle- shaped such that in any radial plane (IV-IV; V-V) intersecting a diffusor blade, the diffusor blade projects a convexly curved pressure side (16) towards the flow, whereas in any tangential plane (III-III) intersecting the diffuser blade the diffusor blade projects a concavely curved pressure side ( 16) towards the flow.

Description

A DIFFUSER FOR A MULTIPHASE FLUID COMPRESSOR PUMP

TECHNICAL FIELD OF THE INVENTION

The present invention relates to multiphase fluid pumps and more specifically to a diffuser for a compressor pump adapted for pumping mixed fluids containing gas and liquid.

BACKGROUND AND PRIOR ART

Compressor pumps can be used in oil and gas production for moving fluids having a comparatively high gas content. The compressor pump raises pressure in the fluid to remove or reduce the vapour phase before discharging a homogenized fluid to downstream equipment, usually to a downstream located production pump which is less tolerant to free gas in the fluid, such as a centrifugal pump, e.g. One type of compressor pumps used in hydrocarbon production is the helico-axial pump. Helico-axial multiphase pumps can be designed and employed for downhole applications, wherein the pump is installed inside a well bore. The diffusor of the present invention is however not limited to use in downhole applications but is rather suitable for implementation in any standalone multiphase pump for subsea or topside application.

The helico-axial pump typically comprises a cylinder shroud in which a shaft is journaled centrally and driven in rotation by a submersible electric motor. A helical impeller is rotationally fixed to the shaft. The helico-axial pump is typically composed of several successive stages wherein each stage includes an impeller section followed by a diffuser section. The diffuser includes stationary blades which connect the shroud with a central hub through which the impeller shaft passes, rotationally journalled. The impeller and the diffuser each provide an annular flow passage defined on one hand by the shroud and on the other hand by the impeller shaft and the hub respectively. In the impeller the sectional flow area decreases towards the diffuser as the result of an increasing impeller shaft diameter, whereas in the diffuser the sectional flow area increases towards the following impeller as the result of a decreasing hub diameter. The impeller compresses the fluid towards the diffuser, imparting an axial and a rotational component to the flow. The stationary blades in the diffuser eliminate the rotational component and return the flow to axial. In effect of a radial expansion of the flow through the diffuser the flow velocity is reduced, resulting in increase of the static pressure in the fluid. A typical example of a helico-axial compressor pump can be found in US 6547514 B2.

Another example of prior art multiphase compressor pumps can be found in US 2005/0186065 Al . The document discloses a two phase flow conditioner with a gas conditioning section including a series of impellers and diffusers arranged in a cylinder housing. In the diffuser a set of diffuser blades stationary connect a diffuser hub with the surrounding housing. The diffuser blades are curved, comprising a concave side which receives fluid that enters the diffuser from a downstream impeller, the concave side defining an upstream side of the diffuser blade with respect to the impeller rotation and the rotational component in the flow that enters the diffuser. The opposite and convex side is conversely defined as the downstream side of the diffuser blade. The blades are similar to a cut out portion of a sphere thus having a scoop-shaped profile facing the flow, concave in both radial and axial directions along the upstream side of the blades.

SUMMARY OF THE INVENTION

An overall object for the present invention is to provide improvements in the design of the diffuser of a multiphase fluid compressor pump, thus aiming for enhanced diffusion and homogenization aiming for enhanced overall efficiency and operation of the pump.

In a first aspect of the invention this object is met in the novel design of a diffuser comprising a central hub which is coaxially aligned with a surrounding cylinder shroud, the hub and shroud together forming an annular passage for fluid flow through the diffuser. A set of stationary diffuser blades are angularly spaced about the hub and connect the hub and the shroud. Each diffuser blade projects an upstream side towards the flow entering the diffuser. The diffuser blades are saddle-shaped such that in any radial plane intersecting a diffuser blade the diffuser blade projects a convexly curved pressure side towards the flow, whereas in any tangential plane (parallel to the diffuser axis) intersecting the diffuser blade the diffuser blade projects a concavely curved suction side towards the flow. Expressed in other words, the diffuser blade is saddle-shaped in such way that the pressure side of the diffuser blade which faces the rotational direction of the impeller, and thus faces the rotational component in the flow that enters the diffuser section, is formed by a convex radial generatrix and a concave axial generatrix.

The above specified 3-dimensional blade geometry provides the advantage and technical effect of a better control of the load distribution through the blade, and improves the transfer of load from the blade to the shroud and to the hub respectively. This shape of the blade further suppresses or avoids completely flow separation, which in turn leads to an increased pressure recovery and overall efficiency and may also lead to a reduced swirl in the flow that exits the diffuser, as compared to a 2 -dimensional, planar diffuser blade geometry e.g.

The diffuser blades extend in the flow direction from a leading edge to a trailing edge, wherein in one embodiment a shroud-connecting edge of the diffuser blade is located forward of a hub-connecting edge as seen in the direction of the rotational component in the flow entering the diffuser. Leaning the leading edge provides an improved matching between the impeller blade and the diffuser blades, which in turn may result in the technical effect and advantage of efficient transition of the fluid and increased operating range and overall performance for the multiphase compressor pump. The forward displacement of the shroud-adjacent edge of the diffuser blade is preferably equal to or less than one half of the angular distance between adjacent diffuser blades. In one advantageous embodiment the diffuser blade leading edge and / or the trailing edge is toothed or wave-shaped (scalloped) . Applying a wave-shape to the leading or trailing edges enhances mixing of gas and liquid, and may additionally be effective to control or avoid a potential stall in situations where large gas pockets are entrained in the multiphase fluid.

In a second aspect of the invention the object is met in the novel design of a hub in a diffuser for a multiphase fluid compressor pump, wherein the hub comprises a rotation- symmetric hub wall having a hub wall portion of reducing radius that forms a slope in the hub wall, wherein the onset of said slope is located in a downstream end of a cylindrical hub wall portion which is parallel or nearly parallel with the shroud from the entry to the diffuser.

In other words, as compared to the conventionally applied continuous curve in the hub wall, the curvature of the hub wall is shifted downstream this way displacing a diverging diffuser section towards the exit of the diffuser, while maintaining a diffuser section of continuous or almost unchanged radius and flow section in an upstream portion of the diffuser.

One effect achieved by applying this specific design to the hub wall is that it allows for separation of blade-to-blade diffusion and meridional diffusion, as well as control of the load applied to the blades. By proper choice of slope angle and slope angle distribution (here referring specifically to the location of the slope between entry and exit to the diffuser), the load on the diffuser blades can be controlled: strong blade loading is generated where no or weak meridional channel change occurs, i.e. in the upstream and parallel or nearly parallel diffuser section, whereas weak blade loading is applied in the region of high meridional diffusion, in the diverging diffuser section. In embodiments of the invention, the extension of the cylindrical hub wall portion may deviate from perfect parallelism with the shroud within a range of +/- 10°, preferably within a range of +/- 5°. In one embodiment of the invention, the onset of the slope in the hub wall is located within a range of about 10-70 % of the hub length as seen from the entry to the diffuser.

In one embodiment of the invention, the cylindrical hub portion has a length counted from the entry amounting to 10-70 % of the total length of the hub.

In one embodiment of the invention, a slope angle and the location of the slope onset point may be determined such that a tangent applied to the slope intersects the shroud downstream of the entry to the diffuser.

In embodiments of the invention the slope comprises a convex-concave curve and wherein a midpoint on that curve is located within 20-80 % of the diffuser length as counted from the entry to the diffuser. In one embodiment of the invention the midpoint of the slope may be located downstream of a longitudinal centre of the hub.

Further details and embodiments of the invention will be discussed below.

SHORT DESCRIPTION OF THE DRAWINGS

In the following detailed description of embodiment examples reference will be made to the accompanying drawings. In the drawings,

Fig. 1 shows a prior art diffuser blade in a 3-dimensional view,

Fig. 2 shows a partially sectioned cut out detail above a centre line through a multiphase pump incorporating a diffuser according to the present invention, Fig. 3 is a tangential section III-III parallel with the centre line through a diffuser blade in the diffuser of Fig. 2

Fig. 4 is a radial section IV-IV through the diffuser of Fig. 2

Fig. 5 is a radial section V-V through the diffuser of Fig. 2

Fig. 6 is a 3- dimensional view of the novel diffuser blade, and

Fig. 7 shows the diffuser blade in elevational view.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Initially it should be noted that the drawings are only schematic and not true to scale.

With reference to prior art, Fig. 1 shows the spherical diffuser blade which has been discussed above in the background description.

Fig. 2 is a partially sectioned view above a centre line C.L., showing in longitudinal section a diffuser 1 incorporated in a compression stage of a pump for compression of multiphase fluid. The pump may include a number of successive stages each of which comprises a diffuser 1 following upon an impeller 2 with respect to the direction of flow F of fluid through the pump. The flow F is generated by the driven rotation (motor not shown) of a helico-axial pump blade 3, the flow direction F resulting from an axial movement A and a rotational movement R imparted to the fluid by the rotating pump screw. An impeller shaft 4 reaches through the diffuser 1 to the next compression stage. The impeller shaft 4 may be journaled for rotation in bearings 5 accommodated in the diffuser. A cylindrical shroud 6 encloses the impeller and diffuser sections of the pump. Albeit Fig. 2 shows the pump in a horizontal orientation it should be noted that a pump including the diffuser 1 is suitable for operation at other orientations, such as in a downhole application e.g., when the pump is inserted in a well bore and submerged in fluid.

The diffuser 1 comprises a set of stationary diffuser blades 7 which connect the shroud 6 with a hub 8 in the centre of the diffuser (the blades 7 thus covering the radial distance r between the hub and the shroud) . The hub 8 is a rotationally symmetric structure with a hub wall 9 that defines, with the inner surface of the shroud 6, an annular flow passage 10 through the diffuser from an upstream located entry 1 1 to a downstream located exit 12. The sectional area of the passage 10 grows as the passage diverges towards the exit 12 as the result of the hub wall 9 being formed with decreasing radius and, in sectional view, a curvature sloping towards the exit end 12. In particular, the curvature of the hub wall includes a slope 13 that commences a certain distance downstream of the entry end 1 1. Within the scope of the present invention the distance from the entry 1 1 to the onset of the slope 13 may be varied, and likewise may the slope angle γ be varied with respect to the longitudinal axis A. As used here, onset refers to a point 14 in the hub wall where a straight hub portion 9' transfers into a curved hub portion 9" sloping towards the longitudinal axis (see Fig. 7). The length of the cylindrical hub portion 9' may amount to approximately 10-70 % of the total length of the hub, as counted from the entry 1 1 to the diffuser. In other words, moving the curvature in the hub wall further downstream results in a cylindrical hub portion 9' wherein the hub wall is parallel or nearly parallel with the shroud. In this connection "nearly" indicates a deviation from perfect parallelism with the shroud preferably within the range of +/- 10°, or even more preferred within a range of +/- 5°.

In one specific embodiment within the scope of the invention, illustrated in Fig. 2, the slope angle γ and the location of the slope onset point 14 may be chosen such that a tangent TAN applied to the slope 13 intersects the shroud 6 downstream of the entry 1 1 to the diffuser.

By moving the curvature in the hub wall downstream as disclosed, the present invention provides a mechanism for controlling the loading of the diffuser blades. The blade loading comes from the pressure applied from the fluid and is related to the curvature of the blade and the change in flow direction with respect to the incoming flow angle. More precisely, in the upstream portion of the diffuser where the radius of the flow passage is substantially continuous and the flow velocity is maintained, the load in a blade-to-blade direction is the strongest where load can be transferred short distance to the hub and the shroud. In the downstream portion of the diffuser, where the flow passage through the diffuser diverges from the onset of the slope, the velocity in the flow decreases causing a raise in the static pressure. However, in this region where load is transferred long distance to the hub and the shroud, the blade-to-blade load is weak as compared to the blade loading in the upstream portion of the diffuser.

The shape of the slope and the location of the slope in the hub wall can be varied. The slope 13 may for example be realized in the form of a continuous S-curve from the onset point 14 towards the exit 12 of the diffuser. The slope may alternatively include a straight cone section between a convex curve in the upstream end of the slope and a concave curve in the downstream end of the slope. In all cases the slope has a midpoint 15 located between the convex and concave end curves, see Fig. 7. The exact location of the midpoint 15 between the entry 1 1 and the exit 12 of the diffuser may vary. In some embodiments it may be preferred to locate the midpoint 15 downstream of a longitudinal centre C of the diffuser, this way extending the region of strong blade-to-blade loading and weak meridional channel change. In alternative designs the slope 13 may be displaced both upstream or downstream, thus shifting the location of the midpoint 15 towards the entry or towards the exit respectively. In terms of the axial length of the diffuser the position of the midpoint

15 can be varied preferably within a range of 20-80 % of the diffuser length as counted from the entry 1 1. In correspondence herewith, the position of the slope onset 14 in the hub wall is preferably set within a range of about 10-70 % of the diffuser length as seen from the entry 1 1.

The 3-dimensional shape of the diffuser blade 7 will now be described with reference to the drawings of Figs. 3-7. As is best understood from the perspective view of Fig. 6 the diffuser blade 7 has the general shape of a horse saddle. In particular, the diffusor blade 7 is a convexo-concave element having a pressure side

16 which faces the rotational component R in the flow F generated by the impeller. The opposite side of the blade can correspondingly be characterized as a suction side of the diffusor blade. The pressure side 16 is formed with a composite curvature generated by a convex radial generatrix G_r, and a concave axial generatrix G_a. In correspondence herewith the suction side 17 of the diffuser blade is generated by a concave radial generatrix and a convex axial generatrix. In the diffuser, the blade 7 is oriented with an angle of attack a relative to the flow which decreases towards the downstream end of the blade, as is illustrated schematically in Fig. 3 which shows a section through the diffuser blade along the tangential plane III-III in Fig. 2. Thus, in the upstream portion of the blade where the meridional flow path is straight (or nearly straight) the blade is shaped with a curvature that causes a stronger change in the flow direction, whereas once that the flow path starts to diverge the loading on the blade is reduced by changing the curvature of the blade for a weaker turning of the flow.

In the axial dimension the diffuser blade 7 extends through the diffuser, in the direction of flow, from a leading edge 18 to a trailing edge 19. As seen in the axial direction from leading edge to trailing edge the blade 7 projects a concave pressure side 16 facing the flow. In the radial dimension the diffuser blade extends between the hub and the shroud from a hub-connecting edge 20 to a shroud-connecting edge 21. As seen in the radial direction from hub to shroud, the blade 7 projects a convex pressure side 16 facing the flow.

In the diffuser 1 the blades 7 are distributed at equal angular spacing around the hub 8. In particular, the blades 7 are arranged leaning towards the rotation R of the impeller, and thus towards the rotational component R in the flow generated by the impeller. As best seen in the radial planes IV-IV and V-V, see Figs. 4 and 5 respectively, the shroud-connecting edge 21 is displaced forward of the hub- connecting edge 20 with respect to the flow. In a preferred embodiment the forward displacement D of the shroud- connecting edge 21 is equal to or less than half the angular distance β between adjacent diffusor blades (see Fig. 4).

A potential problem in connection with pumping and compression of multiphase fluid is the risk of flow separation. This situation occurs mostly in operation at low mass flow (i.e. at the left side of the operation curve of compressors or pumps). If flow separation is extended to the full flow passage area then no flow can pass through the machine, a situation which is also named stall. If a stationary component, such as the diffuser, stalls completely, it makes no difference whatever the conditions are in the upstream impeller: the flow cannot go through the machine and it loses all pumping power.

In one embodiment the present invention provides a solution which reduces or eliminates the problem of stall in a multiphase fluid pump. To this purpose, see Fig. 7, at least the leading edge 18 of the diffuser blade 7 is toothed or well- shaped (scalloped). Shaping the edge or edges 18, 19 as disclosed provides improved mixing of the fluid phases and thus counteracts and reduces the risk for separation.

It should be understood that the novel designs of the diffuser blade and the hub wall can be applied separately or in combination in a diffuser, each contributing beneficially to improved operation of the diffuser and of a multiphase fluid pump incorporating the diffuser. It should also be understood that the compressor part with the impeller 2 is inserted in the disclosure for purpose of illustration only and form no parts of the invention, and are thus not limiting the scope of the invention as defined in the specification of embodiments and by the accompanying claims.

Claims

A diffuser (1) for a multiphase fluid pump, the diffuser comprising

a central hub (8) coaxially aligned with a surrounding shroud (6), the hub and shroud together forming an annular passage (10) for fluid flow (F) through the diffuser,

a set of diffusor blades (7) angularly spaced about the hub and connecting the hub and the shroud, the blades projecting a pressure side (16) towards the flow (F),

characterized in that the diffusor blades (7) are saddle-shaped such that in any radial plane (IV-IV; V-V) intersecting a diffusor blade the diffusor blade projects a convexly curved pressure side (16) towards the flow, whereas in any tangential plane (III-III) intersecting the diffuser blade the diffusor blade projects a concavely curved pressure side (16) towards the flow.

The diffuser of claim 1 , wherein the diffuser blades (7) extend in the flow direction from a leading edge (18) to a trailing edge (19), and wherein a shroud- connecting edge (21) of the blades is located forward of a hub- connecting edge (20) of the blades as seen in the direction of a rotational component (R) in the flow (F) entering the diffuser.

The diffuser of claim 2, wherein the forward displacement (D) of the shroud- connecting edge (21) amounts to one half or less of the angular distance (β) between adjacent diffuser blades.

The diffuser of any previous claim, wherein the leading edge (18) and/ or the trailing edge (19) of the diffuser blade (7) is toothed or wave-shaped.

The diffuser of any previous claim, wherein the hub (8) comprises a rotation- symmetric hub wall (9) having a hub wall portion (9") of reducing radius that forms a slope (13) in the hub wall, wherein the onset (14) of said slope (13) is located in a downstream end of a cylindrical hub wall portion (9') which is parallel or nearly parallel with the shroud (6) from the entry ( 1 1) to the diffuser.

6. The diffuser of claim 5, wherein the cylindrical hub wall portion (9 may

deviate from perfect parallelism with the shroud (6) within a range of +/- 10°, preferably within a range of +/- 5°.

7. The diffuser of claim 6, wherein the cylindrical hub portion (9 has a length counted from the entry ( 1 1) amounting to 10-70 % of the total length of the hub.

8. The diffuser of any of claims 5-7, wherein the onset ( 14) of the slope ( 13) in the hub wall is located within a range of 10-70 % of the hub length as seen from the entry ( 1 1) to the diffuser.

9. The diffuser of claim 8, wherein the slope ( 13) comprises a convex- concave curve and wherein a midpoint ( 15) on that curve is located within 20-80 % of the diffuser length as counted from the entry ( 1 1) to the diffuser.

10. . The diffuser of claim 9, wherein the midpoint ( 15) of the slope ( 13) is located downstream of a longitudinal centre (C) of the hub.

1 1. The diffuser of any of claims 5- 10, wherein a slope angle (γ) and the location of the slope onset ( 14) is determined such that a tangent (TAN) applied to the slope ( 13) intersects the shroud (6) downstream of the entry ( 1 1) to the diffuser.