GB1561454A

GB1561454A - Devices for pumping a fluid comprising at least a liquid

Info

Publication number: GB1561454A
Application number: GB5308576A
Authority: GB
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1976-12-20
Filing date: 1976-12-20
Publication date: 1980-02-20

Description

(54) DEVICES FOR PUMPING A FLUID COMPRISING AT LEAST A LIQUID (71) We, INSTITUT FRANCAIS DU PETROLE, a body corporate organised and existing under the laws of France, of 4, Avenue de Bois-Preau, 92502 Rueil-Malmaison, France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to devices for pumping a fluid comprising at least a liquid.

Embodiments of the present invention provide an anti-cavitation device whereby a liquid can be pumped even when the pressure and temperature conditions of the liquid are close to those where the liquid flow becomes a flow of diphasic fluid comprising a liquid phase and a gaseous phase. Embodiments of the present invention are also suitable for pumping a diphasic fluid, i.e. a fluid which at the intake of the pumping device, under the prevailing pressure and temperature conditions, is formed by a mixture of a liquid and a gas which is not dissolved in the liquid, which liquid may or may not be saturated with gas.

Pumping a diphasic fluid, such as, for example, but not exclusively, a diphasic oil effluent formed by a mixture of oil and gas, raises at the present time some problems whose solution is particularly difficult, when under the thermodynamic conditions of the diphasic fluid to be pumped, and more particularly such conditions at the intake of the pumping device, the volumetric ratio of gas to liquid is high.

The volumetric ratio of gas-to-liquid, which will be referred to in the following as 'the volumetric ratio', is defined as the ratio of the volume of fluid in the gaseous state of the volume of fluid in the liquid state, the value of this ratio being defined under the thermodynamic conditions of the diphasic fluid.

As is well known, the components of a diphasic fluid are likely to pass from one phase into the other. Thus, if the liquid is subjected to a sudden pressure drop, at least a portion of the gas dissolved in the liquid becomes nearly instantaneously vapourised, causing a sudden rise of the volumetric ratio. On the other hand, if the pressure of the diphasic fluid is increased, then even if the pressure rise is abrupt saturation of the liquid is reached only very slowly by re-dissolution of the gas through the surface of contact of the two phases, although this phenomenon of re-dissolution is relatively fast at the beginning. Consequently, a decrease in the value of the volumetric ratio resulting from a sudden pressure rise occurs more slowly than an increase in the volumetric ratio resulting from a sudden pressure drop.

Conventional pumping devices have the disadvantage or producing, near some of their components, marked local pressure drops which, for the above-indicated reasons, result in an effective increase of the value of the volumetric ratio of the diphasic fluid.

When the value of the volumetric ratio of the diphasic fluid is zero, i.e. when the whole gas content is dissolved in the liquid, the pumped liquid behaves as a monophasic liquid fluid and conventional pumping means give good results irrespective of their type (reciprocating pumps, rotary pumps or suction pumps), as long as their conditions of use do not produce cavitation phenomena or local negative pressures which are likely to cause vaporization of a substantial portion of the gas contained in the liquid.

Experience shows that conventional pumping devices may be used with an acceptable efficiency, as long as the value of the volumetric ratio at the intake of the pump is small, i.e. in practice smaller than 0.2. Above this value the efficiency decreases very rapidly and conventional pumping devices then become practically unusable.

In order to improve the operation of the conventional pumping devices, the gaseous phase is separated from the liquid phase before pumping by means of a special device, such as a static or dynamic separator. This separator withdraws a substantial portion of the gas prior to introducing the fluid into the active part of the pump, and it then becomes necessary to discharge the gas through an outflow circuit separate from the circuit of the pumped fluid, to prevent the gas from entering the pump. The operation of such a separate circuit is not always possible and it will in any way make the pumping operations more difficult.

According to the invention there is provided a device for pumping a fluid comprising at least a liquid, the device having at least one fluid compression element, said element comprising a hollow casing provided with inlet means and outlet means for the fluid, an impeller housed in said casing and drive means for rotating said impeller, said impeller comprising a hub and at least one blade fixed in position with respect to said hub, said blade having a leading edge facing said inlet means and a trailing edge facing said outlet means, said blade having a profile defined as the intersection of the blade with a cylindrical surface coaxial with said impeller, said profile being such that its angle of inclination to the axial direction of the impeller decreases continuously from said leading edge to said trailing edge, said profile being of substantially zero curvature in the immediate vicinity of the leading edge, and a curve of variation of the curvature of the blade profile, as a function of the distance along said profile from the leading edge, having a slope whose value increases progressively from the leading edge to the trailing edge of the blade.

Embodiments of the invention described in more detail below, in addition to having an anti-cavitation property whereby monophasic fluid can be pumped, have the advantage of maintaining high efficiency when used for substantially increasing the pressure of a diphasic fluid whose volumetric ratio may be equal to or greater than 0.9, under the thermodynamic conditions prevailing at the intake of the device.

Another advantage of the embodiments of the invention described below is that of delivering a fluid whose columetric ratio is lower than the intake volumetric ratio, the volumetric ratio of the delivered fluid being preferably equal to zero.

The invention will now be further described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a diagrammatic axial section of a pumping device embodying the invention and suitable for pumping a diphasic effluent from an oil well; Figures 2A and 2B are perspective views of two embodiments of a fluid compression stage for the device of Figure 1; Figure 3 shows the general configuration of the developed profile of a blade carried by a hub in the compression stages of Figures 2A and 2B; Figure 3A shows the curvature variation of the blade along the profile; Figure 4 shows more in detail the leading edge of a blade; Figure 5 illustrates in more detail a possible law of variation of the profile of the hub in the compression stages illustrated in Figure 2B;; Figure 6 is a cross-section of a flow straightener for use with a compression stage having a hub of varying diameter; Figure 7 is a developed view of the profile of a blade of the flow straightener of Figure 6; and Figures 8 and 9 illustrate further embodiments of the invention.

Owing to their anti-cavitation properties, the pumping devices embodying the invention now to be described can be advantageously, but not exclusively, used for pumping a fluid formed by a mixture of at least one liquid and at least one gas, the mixture having under the thermodynamic conditions prevailing at the inlet of the pumping device, a volumetric ratio substantially equal to zero, corresponding to a complete dissolution of the gas in the liquid.

This liquid may be more or less saturated with the gas, such that a greater or lesser pressure decrease causes vaporisation of at least one part of the gas dissolved in the liquid.

The pumping devices now to be described are also suitable for pumping diphasic fluids having. under the thermodynamic conditions at the intake of the device, a volumetric ratio different from zero and whose value might reach or exceed 0.9.

In the following, the term 'fluid' will either designate a monophasic fluid wherein a gas is fully dissolved, or a diphasic fluid comprising a liquid phase and a gaseous phase.

Figure 1 diagrammatically shows in axial section a particular non limitative embodiment of the invention in the form of a pumping device designed for pumping a diphasic oil effluent.

The pumping device shown in Figure 1 is so designed as to be compatible with existing equipment and to be introduced into the bottom of a producing oil well. The pumping device comprises a hollow casing 1 which, in this embodiment, is cylindrical so as to be more easily introduced into a well. The casing 1 is provided with at least one intake port 2 for admitting fluid and an outlet port 3 which communicates with a discharge circuit for the pumped fluid, this circuit being diagrammatically represented by a pipe 4 at the end of which the casing 1 is secured by any suitable means, such as by threading 5.

In the embodiment illustrated in Figure 1 there are a plurality of intake ports 2 constituted by openings in the wall of the casing 1 and the pumping device comprises, at the level of said openings, a deflector 14, fixed in Position with respect to the casing 1, to deflect the fluid after it has entered the casing and produce a flow having a substantially axial direction, i.e. parallel to the axis of the pumping device.

Inside the casing 1 is housed an impeller having a shaft 6 rotated by a motor 7 (such as, for example, but not exclusively, an electric motor whose electric supply cables are now shown in the drawing) and (optionally) a transmission device, diagrammatically shown at 8, for adapting the speed of rotation of the shaft 6 to the speed at which the shaft must be rotated.

The transmission device 8, which may be of any suitable known type and may comprise gears, will not be described in detail, since its design requires only ordinary technical skill.

The shaft 6 is held in position by at least two bearings 9 and 10.

The bearing 10 comprises an annular member 10' secured in position with respect to the housing 1 by radial arms 11, spaces between the radial arms enabling the fluid to flow in the direction of arrows F. The bearing 10 further comprises a roller bearing assembly 12 which is located between the shaft 6 and the member 10'. An inner ring or race of the roller bearing assembly 12 is axially secured in position with respect to the shaft 6, whereas an outer ring or race of the bearing assembly 12 is axially displaceable with respect to the member 10', to accommodate any variation in the length of the shaft 6.

Optionally, depending on the kind of pumped fluid, the roller bearing assembly 12 may be a sealed roller bearing assembly, but it is possible to use an ordinary roller bearing assembly by providing sealing flanges on both sides of the bearing 10 and sealing it with a lubricating material, such as grease, when it is mounted on the device.

The bearing 9 comprises at least one axial thrust bearing arrangement, such as a ball thrust bearing, capable of withstanding axial forces applied to the pumping device, and at least one centering device, such as a ball bearing, a taper-roller or straight roller bearing.

The bearing 9 also includes a sealing device 13 and communicates with a lubricating device 15, comprising, for example, an oil tank having at least a part of its wall deformable in order to equalise the oil pressure and the hydrostatic pressure at the location of the pumping device.

Whenever required, a second oil tank 16 is provided for lubricating the motor 7 and/or the transmission device 8.

The components 7, 8, 15 and 16 are secured in an extension of the casing 1, for example by means of a securing flange 17a.

Between the intake ports 2 and the outlet port 3 of the pumping device and inside the casing 1 is located at least one compression element defining a compression stage operative to increase the fluid pressure. Three of such compression elements or stages, designated 17 to 19, are shown in Figure 1. This number is not limitative, the number of compression stages depending on the pressure increase which must be obtained.

The compression elements 17 to 19, which will be described hereinafter in more detail, are secured in position with respect to the shaft 6, on which they may, for instance, be tightly fitted, the proper distance between consecutive elements being maintained by spacing members 20 to 23.

A respective one of a plurality of flow straighteners 24 to 26 is located at the outlet of each compression stage, each flow straightener being secured to the casing 1, such as by means of securing screws 27 (indicated by chain-dotted lines in Figure 1) For the sake of clarity, in Figure 1 the respective clearances between the spacing members 20 to 23 and the flow straighteners 24 to 26, between the compression elements 17 to 19 and the casing 1 and between the compression elements 17 to 19 and the flow straighteners 24 to 26 have been considerably exaggerated. In practice, such clearances should be reduced to the minimum values compatible with proper machanical operation of the pump, so that fluid leakages are minimised and expansion of the respective components at the operating temperature does not cause any jamming.

Figure 2A and 2B show in perspective two non-limitative embodiments of a compression element or stage for increasing the fluid pressure. In each case the compression element essentially comprises a hub 28 secured in position with respect to the shaft 6 which, during operation of the device, is rotated in the direction shown by an arrow w. (The symbol w is also used hereinafter to indicate rotational speed).

The hub 28 carries at least one blade or vane. In the embodiments illustrated in Figures 2A and 2B, the hub 28 carries two blades 29 whose characteristics are given hereinunder. This number of blades is by no way limitative, but it will generally be preferable to select a number of blades facilitating static and dynamic balancing of the impeller. The outer radius of the blades 29 is such that the volume which they generate during their rotation substantially corresponds to the interior of the casing, which is cylindrical in the present embodiment.

The blades 29 may be inserted into the hub 28 and secured thereto by welding, but the blades are preferably manufactured integrally with the hub 28 by machining the blades and hub from a piece of metal of suitable size, or by integrally moulding the blades and hub.

It is possible to integrally manufacture the shaft 6 and the different compression elements 17 to 19 by machining or moulding, the flow straighteners 24 to 26 and the flow deflector 14 being made in several parts which are thereafter assembled. Alternatively, it is possible to manufacture in only one piece each compression element and a corresponding part of the shaft 6, the different parts of the shaft 6 being connected to one another when assembling the device.

Figure 3 shows the developed profile of a blade formed by the intersection of a 'mean line' means the line where the blade would intersect the cylindrical surface if the blade had a thickness equal to zero. The inventors have discovered that, for optimum operation, the mean line of each blade shall be such that: a) the angle of inclination of the mean line to the axial direction of the impeller defined as the angle 8 formed between said axial direction and the tangent to the mean line of the blade at any point M thereof, decreases continuously as said point M moves from the leading edge 30 of the blade toward the trailing edge 30a thereof; b) the curvature lp of the blade is small - in other words substantially zero - in the vicinity of the leading edge 30, i.e. the radius of curvature p, which is inversely proportional to the curvature, is very high; c) the slope of a curve Y (Figure 3A), representing the value of the curvature at a point of the mean line of the blade as a function of the distance separating this point from the leading edge 30, (i.e. either the curvilinear abscissa of this point along the profile of the blade, or the distance between this point and a plane P perpendicular to the axis of rotation XX' and passing through the leading edge 30), increases progressively with said distance; and d) the pumping device being designed for a given flow rate, considering what is known in the art as the 'operating point' (corresponding to those conditions which are generally selected for theoretical computations), the angle of inclination a at any point of the leading edge 30 is defined by the following relationship: wR tana=V co being the speed of rotation of the impeller in radians/second, R being the distance in meters between the point of the leading edge 30 and the axis of rotation XX'; and V being the velocity of the fluid immediately upstream of this point with respect to the direction of flow of the fluid in the compression stage, this velocity being expressed in meters/second.

The blade preferably has a thin profile. The outer and inner faces of the blade are nearly parallel. In practice, however, the blade should have a high mechanical strength; to this end, as shown in Figure 4, a minimum thickness e is required. Thus, in the vicinity of the leading edge 30, the inner and outer faces of the blade will be given respective angles of inclination a' and a" having values respectively smaller and greater than the value of the above-defined angle a, the difference between the angles ' and a" being smaller than 10 and preferably smaller than 6".

In other words, the inner and outer faces of the blade form in the immediate vicinity of the leading edge an angle at most equal to 10 and preferably smaller than 6".

Moreover, as illustrated in Figure 4, the inner and outer faces of the blades are connected by a rounded portion having a small radius of curvature.

It will generally be preferable to rotate the hub 28 at a speed s such that, at the operating point. wRm is equal to or greater than 4, Rm being the r.m.s. value of the radius R, V defined by the relationship: 7 Rm2 = R2e + R2i where Re is the outer radius of the blade and Ri is the radius of the hub 28 or inner radius of the blade, both measured in the plane of the leading edge of the blade.

In the embodiment illustrated in Figure 2A, the hub 28 has a constant diamater 8 and the length D of the hub is preferably selected to be at least equal to 50% of the value of the diameter, and preferably from 50% to 300% of the value of the diameter.

In the particular embodiment of Figures 1 and 2B, the diameter of the hub 28 is not uniform along its length. The hub 28 for this embodiment is shown in Figure 5.

In Figure 5, only the hub 28 has been illustrated. The diameter AA' of the cross-section of the hub 28 at its end located nearer the intake port 2 is smaller than the diameter CC' of the cross section of the hub at its end nearer the outlet port 3. Between these end cross-sections the diameter of the hub 28 increases progressively in continuous manner. With respect to the direction of flow F of the fluid in the compression stage, the diameter of the hub 28 along a first portion of the hub whose length is d varies less than along the remaining portion of the hub, and the chord AB subtending the profile of the first hub portion of length d has a slope selected to be at most equal to 35 % (i.e. BD/AD= 0.35) while the slope of the hub profile at the cross-section AA' is smaller than the slope of the AB and is at most equal to 20%.

Using such a hub, the inventors have discovered that in addition to the above-indicated characteristics, each blade should have on the first hub portion a length at most equal to 70% of the overall length of the blade, or the length d of the first hub portion should be at most equal to 60% of the overall length D of the hub, the lengths d and D being measured along a direction parallel to the axis of rotation XX'.

In the application illustrated in Figure 1, i.e. for a device designed for pumping a diphasic oil effluent, good results have been achieved by selecting for the hub 28 of each compression stage a length D of between 50 and 100% of the value of the average diameter m of of the hub 28 (Figure 5).

The so-determined blade profile causes practically no disturbance in the fluid flow over the first portion of the blade profit while the pressure rise of the pumped fluid is mainly achieved over the remaining portion of the blade profile, i.e. over the portion of the blade profile having the smallest radius of curvature, this pressure rise being enhanced by the increase in the hub diameter, if the latter is of varying diameter. The other design characteristics of the device, such as the minimum diameter of the hub 28, the inner diameter of the casing 1 etc can be determined by those skilled in the art for each application, in relation to the maximum allowed size, the liquid flow rate, the desired pressure rise, the maximum volumetric ratio desired at the device inlet. etc.

At the outlet of each compression stage, the fluid has a velocity whose direction is not axial.

As is well known in the art, the use of a flow straightener permits an increase in the static pressure by nullifying, or at least reducing, a rotary component of the fluid flow.

This flow straightener may be of any known type, but when the compression element comprises a hub of increasing diameter it is preferable to use a flow straightener of the type illustrated in Figures 6 and 7. In Figures 6 and 7, the flow straightener is shown by solid lines and the compression stage is shown by chain-dotted lines.

When the impeller hub 28 has a non-uniform diameter, the flow straightener is formed by a sleeve 31 (Figure 6) carrying fins 32. A ring 33 secured to the fins 32 enables the flow straightener to be secured to the casing 1, for example by the screws 27.

The external diameter of the sleeve 31 decreases progressively from the inlet to the outlet of the device, with respect to the direction of flow of the fluid indicated by the arrow F, so that the fluid cross-section increases progressively.

In a preferred embodiment of the invention, the cross-section of the fluid flow at the inlet of the flow straightener is substantially equal to the cross-section of the fluid flow at the outlet of the compression stage, upstream of the flow straightener with respect to the direction of flow of the fluid, while the cross-section of the fluid flow at the outlet of the flow straightener is substantially equal to the cross-section of the fluid flow at the inlet of the compression stage located downstream of the flow straightener.

The fins 32 have a suitable profile enabling deflection and straightening of the fluid flow.

At the inlet of the flow straightener the fins 32 are substantially tangential to the fluid flow at the outlet of the compression stage, whereas at the outlet the fins are tangential to a plane passing through the axis of the pumping device. Between these two ends the angle of inclination of the profile of the fins 32 with respect to the axis of the device varies in continuous manner.

Such a flow straightener can be manufactured by moulding, preferably by moulding under pressure. facilitate In order to facilitate the manufacture of the flow straightener it is possible to impart to the fins 32 (whose developed profile is illustrated in Figure 7) a radius of curvature of constant value between the inlet and the outlet of the flow straightener. In this case the fins are, for example, constituted by portions of tubular elements. Such a flow straightener has given excellent results during tests, wherein the axial speed of the fluid at the outlet was observed to be substantially equal to the axial speed at the inlet of the compression stage located upstream, thus permitting the use of a series of identical compression stages.

Changes can be made to the above-described embodiments without departing from the scope of the present invention as defined by the appended claims. The hub 28 of a compression stage may have a more pronounced increase in its diameter along its second part as shown in Figure 8, so that the centrifugal effect becomes important. The height of the blades can be substantially constant, the casing la having fixed therin an annular element 1b having an internal shape complementary to the volume defined by rotation of the blades.

This type of pump, associated with a flow straightener constituted by one or more stages, may be designed for high flow rates such as those required for collecting oil and gas (diphasic mixture) from a plurality of underwater well.s It is also possible to insert intermediate blades of smaller radius between above-described blades of the impeller. Such intermediate blades are located on the second hub portion, as diagrammatically shown in Figure 9 which is a developed view of the impeller. In this case, the intermediatb blades will have a profile identical to that of the other blades on the second hub portion.

It is possible, for pumping diphasic fluids, to associate a pumping device embodying the invention with a pump according to the prior art, when the volumetric ratio at the outlet of the pumping device is equal to zero or very small.

WHAT WE CLAIM IS: 1. A device for pumping a fluid comprising at least a liquid, the device having at least one fluid compression element, said element comprising a hollow casing provided with inlet means and outlet means for the fluid, an impeller housed in said casing and drive means for rotating said impeller, said impeller comprising a hub and at least one blade fixed in position with respect to said hub, said blade having a leading edge facing said inlet means and a trailing edge facing said outlet means, said blade having a profile defined as the intersection of the blade with cylindrical surface coaxial with said impeller, said profile being such that its angle of inclination to the axial direction of the impeller decreases continuously from said leading edge to said trailing edge, said profile being of substantially zero curvature in the immediate vicinity of the leading edge, and a curve of variation of the curvature of the blade profile, as a function of the distance along said profile from the leading edge, having a slope whose value increases progressively from the leading edge to the trailing edge of the blade.

2. A device according to claim 1, wherein the thickness of said blade does not substantially vary over the greater part of its length, and opposite faces of the blade delimit between each other, in the immediate vicinity of the leading edge, an angle at most equal to 10 , the faces being connected by a rounded portion.

3. A device according to claim 2, wherein said angle is at most equal to61"adius to 60.

4. A device according to claim 2 or claim 3, wherein the root mean square radius Rm of the blade is such that during operation of the device the value of the ratio wren at any V point of the leading edge is at least equal to 4, Rm being expressed in metres and being defined by the relationship: 3 D ?- . wherein 7R,2 Re-fl\l Re is the outer radius of the blade and Ri is its inner radius, both measured at the leading edge, ", is the speed of rotation of the impeller in radians/second, and V is the axial velocity in metres/second of the fluid immediately upstream of said point with respect to the direction of flow of the fluid in the compression stage.

5. A device according to claim 4, wherein said hub is of uniform diameter along its length.

6. A device according to claim 5, wherein the length of the hub is at least equal to 50% of its diameter.

7. A device according to claim 4, wherein the diameter of a first end cross-section of the hub is smaller than the diameter of a second end cross-section of the hub that is nearer to the outlet means than the first cross-section, the external diameter of the hub varies continuously from said first to said second cross-sections, and the variation of the external diameter of the hub along the length of the hub is slower on a first hub portion, immediately adjacent said first cross-section, than on a second hub portion immediately adjacent said second cross-section.

8. A device according to claim 7, wherein the external diameter of the hub varies less along said first hub portion than along said second hub portion.

9. A device according to claim 7, wherein a chord subtending the hub profile on said first hub portion has a slope, with respect to the axis of the impeller, at most equal to 35%.

10. A device according to claim 9, wherein the slope of the hub profile at the first cross-section has a smaller value than the slope of said chord subtending the profile of said first hub portion.

11. A device according to claim 10, wherein said slope of the hub profile has a value of at most 20% at the first cross-section.

12. A device according to claim 11, wherein the length of the hub, measured parallel to the axis of the impeller, has a value at least equal to 50% of the value of the average diameter of the hub.

13. A device according to claim 12, wherein the length of said first hub portion is at most equal to 60%of the overall length of the hub, these lengths being measured parallel to the axis owt the impeller.

14. A device according to claim 13, wherein the length of said blade over said first portion of the hub is at most equal to 70% of the overall length of said blade.

15. A device according to any one of the preceding claims, wherein the interior of the casing has a shape complementary to the volume generated by the rotation of the blade.

16. A device according to claim 15, wherein the blade is of substantially uniform outer

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Claims

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blades of the impeller. Such intermediate blades are located on the second hub portion, as diagrammatically shown in Figure 9 which is a developed view of the impeller. In this case, the intermediatb blades will have a profile identical to that of the other blades on the second hub portion.

It is possible, for pumping diphasic fluids, to associate a pumping device embodying the invention with a pump according to the prior art, when the volumetric ratio at the outlet of the pumping device is equal to zero or very small.

WHAT WE CLAIM IS:

1. A device for pumping a fluid comprising at least a liquid, the device having at least one fluid compression element, said element comprising a hollow casing provided with inlet means and outlet means for the fluid, an impeller housed in said casing and drive means for rotating said impeller, said impeller comprising a hub and at least one blade fixed in position with respect to said hub, said blade having a leading edge facing said inlet means and a trailing edge facing said outlet means, said blade having a profile defined as the intersection of the blade with cylindrical surface coaxial with said impeller, said profile being such that its angle of inclination to the axial direction of the impeller decreases continuously from said leading edge to said trailing edge, said profile being of substantially zero curvature in the immediate vicinity of the leading edge, and a curve of variation of the curvature of the blade profile, as a function of the distance along said profile from the leading edge, having a slope whose value increases progressively from the leading edge to the trailing edge of the blade.

2. A device according to claim 1, wherein the thickness of said blade does not substantially vary over the greater part of its length, and opposite faces of the blade delimit between each other, in the immediate vicinity of the leading edge, an angle at most equal to 10 , the faces being connected by a rounded portion.

3. A device according to claim 2, wherein said angle is at most equal to61"adius to 60.

4. A device according to claim 2 or claim 3, wherein the root mean square radius Rm of the blade is such that during operation of the device the value of the ratio wren at any V point of the leading edge is at least equal to 4, Rm being expressed in metres and being defined by the relationship: 3 D ?- . wherein 7R,2 Re-fl\l Re is the outer radius of the blade and Ri is its inner radius, both measured at the leading edge, ", is the speed of rotation of the impeller in radians/second, and V is the axial velocity in metres/second of the fluid immediately upstream of said point with respect to the direction of flow of the fluid in the compression stage.

5. A device according to claim 4, wherein said hub is of uniform diameter along its length.

6. A device according to claim 5, wherein the length of the hub is at least equal to 50% of its diameter.

7. A device according to claim 4, wherein the diameter of a first end cross-section of the hub is smaller than the diameter of a second end cross-section of the hub that is nearer to the outlet means than the first cross-section, the external diameter of the hub varies continuously from said first to said second cross-sections, and the variation of the external diameter of the hub along the length of the hub is slower on a first hub portion, immediately adjacent said first cross-section, than on a second hub portion immediately adjacent said second cross-section.

8. A device according to claim 7, wherein the external diameter of the hub varies less along said first hub portion than along said second hub portion.

9. A device according to claim 7, wherein a chord subtending the hub profile on said first hub portion has a slope, with respect to the axis of the impeller, at most equal to 35%.

10. A device according to claim 9, wherein the slope of the hub profile at the first cross-section has a smaller value than the slope of said chord subtending the profile of said first hub portion.

11. A device according to claim 10, wherein said slope of the hub profile has a value of at most 20% at the first cross-section.

12. A device according to claim 11, wherein the length of the hub, measured parallel to the axis of the impeller, has a value at least equal to 50% of the value of the average diameter of the hub.

13. A device according to claim 12, wherein the length of said first hub portion is at most equal to 60%of the overall length of the hub, these lengths being measured parallel to the axis owt the impeller.

14. A device according to claim 13, wherein the length of said blade over said first portion of the hub is at most equal to 70% of the overall length of said blade.

15. A device according to any one of the preceding claims, wherein the interior of the casing has a shape complementary to the volume generated by the rotation of the blade.

16. A device according to claim 15, wherein the blade is of substantially uniform outer

radius over its whole length.

17. A device according to claim 15, wherein the casing is cylindrical.

18. A device according to any one of the preceding claims, comprising, downstream of the fluid compression element with respect to the direction of flow of the fluid, a flow straightener operative to reduce a rotary component of velocity of the fluid flowing out of the compression element.

19. A device according to claim 18, wherein said flow straightener is provided with fixed rigid fins each having a profile which, at an end of the fin forming a leading edge, is substantially tangential to the fluid flow leaving the fluid compression element, and which, at the other end of the fins, forming a trailing edge, is tangential to a plane passing through the impeller axis.

20. A device according to claim 19, wherein the profile of each fin of the flow straightener has a constant radius of curvature.

21. A device according to claim 6 or claim 7, comprising, downstream of the compression element, a flow straightening element operative to reduce a rotary component of the velocity of fluid flowing out of the compression element and designed such that the cross-section of flow of the fluid through the flow straightening element increases progressively in the direction of flow of the fluid.

23. A device according to claim 22, comprising a plurality of sequentially arranged compression elements and flow straightening elements, the arrangement being such that the cross-section of flow of the fluid at the intake end of a flow straightening element is substantially equal to the cross-section of flow of the fluid at the discharge end of a compression element located immediately upstream of said flow straightening element, and the fluid cross-section at the outlet end of said flow straightening element is substantially equal to the cross-section of the intake end of a compression element located immediately downstream of said flow straightening element.

24. A device according to claim 1, wherein the hub carries a plurality of the said blades and wherein further blades are carried by the hub between the said blades exclusively over a portion of the hub nearer said outlet means, said further blades each having a profile identical to a profile of the first-mentioned blades on said hub portion.

25. A device for pumping a fluid comprising at least a liquid, the device being substantially as herein described with reference to Figures 1, 2A or 2B and 5, 3,4, 6 and 7, or Figures 1, 2A or 2B and 5, 3,4, 6 and 7 as modified by Figure 8 and/or Figure 9, of the accompanying drawings.