Centrifugal pump with coalescing effect, design method and use thereof

Abstract

The invention provides a centrifugal pump, distinctive in that the pump comprises two or more stages; the last stage in the direction of flow has been adapted so that it provides a larger equilibrium droplet size than the upstream stages. Method of designing the pump and use of the pump.

Claims

1. A centrifugal pump, the centrifugal pump configured to operate in two or more stages, each stage comprising: at least one combined impeller and a diffusor, the impeller being arranged upstream relative to the diffusor; an impeller of the last stage of the two or more stages in the direction of flow, relative to an upstream stage impeller, comprising at least one of the following features: an impeller-flow-conduit inlet at equal distance from a rotation shaft; and a reduced impeller outer diameter compared to upstream stage impellers combined with an impeller-flow-conduit outlet having an increased axial flow direction component and a reduced radial flow direction component relative to the upstream stage impellers; and a step down gear upstream the last stage for providing reduced rotational speed; and a diffusor of the last stage of the two or more stages in the direction of flow, relative to an upstream stage diffusor, comprising at least one of the following features: increased diffusor flow-conduit length combined with increased or increasing diffusor flow conduit cross-sectional area.

2. The centrifugal pump according to claim 1, comprising: a housing having a constant diameter; a last stage impeller-flow-conduit inlet at equal distance from the rotation shaft as for upstream impellers combined with a last stage impeller-reduced-outer diameter; and a diffusor having increased diffusor-flow-conduit length extending from the upstream impeller-flow-conduit outlet outwards to the housing from where the diffusor flow conduit turns and extends inwards to the next impeller flow conduit inlet or to a pump outlet.

3. The centrifugal pump according to claim 1, wherein the impeller outer diameter decreases for each subsequent stage.

4. The centrifugal pump according to claim 1, comprising: an impeller with a flow-conduit outlet having an increased axial-flow-direction component and a reduced radial-flow-direction component in the last stage relative to an upstream stage impeller-flow-conduit outlet.

5. A method of designing or modifying a pump for a given pressure head to mitigate downstream separation processes, the method comprising: dividing the pump into two or more stages, each stage comprising at least one combined impeller and a diffusor, the impeller being arranged upstream relative to the diffusor; designing or modifying a last stage of the pump in a direction of flow to comprise at least one of the following features: an impeller having an impeller-flow-conduit inlet at equal distance from a rotation shaft and a reduced impeller outer diameter compared to upstream stage impellers combined with an impeller-flow-conduit outlet having an increased axial flow direction component and a reduced radial flow direction component relative to the upstream stage impellers; and a step down gear upstream the last stage for providing reduced rotational speed; and designing or modifying the last stage of the pump in the direction of flow to comprise at least one of the following features: a diffusor having increased diffusor flow-conduit length combined with increased or increasing diffusor flow conduit cross-sectional area compared to upstream stage diffusors.

6. The method according to claim 5, wherein the pump comprises: a housing having a constant diameter; a last stage impeller-flow-conduit inlet at equal distance from the rotation shaft as for upstream impellers combined with a last stage impeller-reduced-outer diameter; and a diffusor having increased diffusor-flow-conduit length by arranging the diffusor-flow-conduit from the upstream impeller-flow-conduit outlet outwards to the housing from where the diffusor-flow-conduit turns and extends inwards to the next impeller-flow-conduit inlet or is directed to a pump outlet.

7. A centrifugal pump, the centrifugal pump configured to operate in two or more stages, each stage comprising: at least one combined impeller and a diffusor, the impeller being arranged upstream relative to the diffusor; an impeller of the last stage of the two or more stages in the direction of flow, relative to an upstream stage impeller, comprising at least one of the following features: an impeller-flow-conduit inlet at equal distance from a rotation shaft and a reduced impeller outer diameter compared to upstream stage impellers; an impeller flow-conduit outlet having an increased axial flow direction component and a reduced radial flow direction component relative to the upstream stage impellers; and a diffusor of the last stage of the two or more stages in the direction of flow, relative to an upstream stage diffusor, comprising at least one of the following features: increased diffusor flow-conduit length; and increased or increasing diffusor flow conduit cross-sectional area.

8. The centrifugal pump according to claim 7, comprising: a housing having a constant diameter; a last stage impeller-flow-conduit inlet at equal distance from a rotation shaft as for upstream impellers combined with a last stage impeller-reduced-outer diameter; and a diffusor having increased diffusor-flow-conduit length extending from the upstream impeller-flow-conduit outlet outwards to the housing from where the diffusor flow conduit turns and extends inwards to the next impeller flow conduit inlet or to a pump outlet.

9. A method of designing or modifying a pump for a given pressure head to mitigate downstream separation processes, the method comprising: dividing the pump into two or more stages, each stage comprising at least one combined impeller and a diffusor, the impeller being arranged upstream relative to the diffusor; designing or modifying a last stage of the pump in the direction of flow to comprise at least one of the following features: an impeller having an impeller-flow-conduit inlet at equal distance from a rotation shaft and a reduced impeller outer diameter, compared to upstream stage impellers; an impeller having an impeller-flow-conduit outlet having an increased axial flow direction component and a reduced radial flow direction component, relative to the upstream stage impellers; and designing or modifying the last stage of the pump in the direction of flow to comprise at least one of the following features: a diffusor having an increased diffusor flow-conduit length compared to upstream stage diffusors, and a diffusor having increased or increasing diffusor flow conduit cross-sectional area compared to the upstream stage diffusors.

10. The method according to claim 9, wherein: the pump is designed with a constant diameter housing having a last stage impeller-flow-conduit inlet at equal distance from a rotation shaft as for upstream impellers combined with a last stage impeller-reduced-outer diameter; and a diffusor having increased diffusor-flow-conduit length by arranging the diffusor-flow-conduit from the upstream impeller-flow-conduit outlet outwards to the housing from where the diffusor-flow-conduit turns and extends inwards to the next impeller-flow-conduit inlet or is directed to a pump outlet.

11. A centrifugal pump configured to operate in two or more stages, the pump comprising: a housing having a constant diameter; wherein each stage comprises at least one combined impeller and a diffusor, the impeller being arranged upstream relative to the diffusor; an impeller of the last stage of the two or more stages in the direction of flow, relative to an upstream stage impeller, comprises an impeller-flow-conduit inlet at equal distance from a rotation shaft and a reduced impeller outer diameter compared to upstream stage impellers; and a diffusor of the last stage of the two or more stages in the direction of flow, relative to an upstream stage diffusor, comprises increased diffusor-flow-conduit length extending from the upstream impeller-flow-conduit outlet outwards to the housing from where the diffusor flow conduit turns and extends inwards to the next impeller flow conduit inlet or to a pump outlet.

12. A centrifugal pump configured to operate in two or more stages, the pump comprising: a housing having a constant diameter; wherein each stage comprises at least one combined impeller and a diffusor, the impeller being arranged upstream relative to the diffusor; wherein a last stage of the two or more stages in the direction of flow comprises: an impeller having an impeller-flow-conduit inlet at equal distance from a rotation shaft and a reduced impeller outer diameter compared to upstream stage impellers; and a diffusor having increased diffusor-flow-conduit length extending from the upstream impeller-flow-conduit outlet outwards to the housing from where the diffusor flow conduit turns and extends inwards to the next impeller flow conduit inlet or to a pump outlet.

Description

FIGURES

(1) The invention is illustrated with six figures, of which:

(2) FIG. 1 illustrates a prior art pump,

(3) FIG. 2 illustrates a pump of the invention,

(4) FIG. 3 illustrates another pump of the invention,

(5) FIG. 4 illustrates optimal pump design according to the invention,

(6) FIG. 5 illustrates the technical effect of the invention, and

(7) FIG. 6 illustrates the effect of droplet size for a downstream separator.

DETAILED DESCRIPTION

(8) Reference is first made to FIG. 1, illustrating a prior art multi stage centrifugal pump 100, comprising an inlet 101, an outlet 102, six impellers 103 and diffusors 104 arranged between the impellers and downstream the last impeller. The impellers 103, having identical diameters, are hatched with one type of filling for all impellers. Likewise, the diffusors 104 are hatched with one type of filling for all diffusors. With this typical design, all impellers are identical and all diffusors between the impellers are identical. Dotted lines and arrows indicate the fluid path through the pump.

(9) Reference is then made to FIG. 2, illustrating a centrifugal pump 1 of the invention, comprising six impellers 2 and diffusors 3 arranged between the impellers, and after or downstream the last impeller a diffusor section is arranged toward the outlet 5. The further parts of the pump 1, such as inlet 4, outlet 5, housing 6 and connection to a driving shaft 7, are according to prior art and assumed to be well known for persons skilled in the art, for which reason only the novel features will be described in detail. The distinctive feature of the pump of the invention is that the ultimate stage, step or impeller in the direction of flow provides a larger equilibrium droplet size than the upstream stage, step or impeller, by providing pressure boosting with coalescing effect and low shear. In the illustrated embodiment, the impellers decrease in diameter toward the outlet, whilst the diffusors between the impellers increase in size/volume toward the outlet. The impellers become successively smaller in diameter, the diffusors increase correspondingly, filling up the increased space between the housing and shaft whilst enhancing coalescence by prolonging the residence time of fluid in said diffusor.

(10) Reference is made to FIG. 3, illustrating a further embodiment of a pump 1 of the invention. More specifically, this embodiment also comprises successively smaller diameter impellers 2 for each stage, and successively larger diffusors 3 for each stage. The housing diameter, impeller diameters and diffusor diameters are larger than for the embodiment illustrated in FIG. 2, which can allow a higher coalescing effect for each stage. The ultimate diffusor, which is the diffusor coupled to the outlet, has significantly increased residence time of the pumped fluid, by increased outlet channel cross section area and length. The pump illustrated in FIG. 3 provides an enhanced equilibrium droplet size over the embodiment illustrated in FIG. 2, by enhanced coalescence due to increased number of droplet collisions in the diffusors because of longer fluid residence time.

(11) The impellers, diffusors or both, can be modified or selected in many ways for providing a pump of the invention, as described above and below.

(12) Reference is made to FIG. 4, illustrating a method of optimal pump design of the invention, for designing a pump of the invention by varying the impeller diameter. The Y-axis denotes the actual inlet droplet size in the continuous phase, in this case oil droplets in produced water. The X-axis denotes pump stage pressure head. In this example, the inlet droplet size is 7 m, as indicated by a lower continuous line start point and text on the Y-axis. When the fluid flows through the first stage, pressure is built up while at the same time the droplet size increases up to a certain level, seen as the continuous line starting from 7 m on the Y-axis and increasing to a top of the line or curve, corresponding to about 9 m droplet size at higher pressure. The top point indicates the optimal stage pressure head A, corresponding to a particular first stage impeller diameter A as indicated. The first stage impeller is the largest diameter impeller. The outlet from the first stage is produced water with oil droplet size of 9 m, corresponding to a new line in FIG. 4, starting at 9 m on the Y-axis and providing a further droplet size increase and further pressure head, as found at the top point B of the curve, and corresponding to a smaller diameter impeller B of the second stage, also indicated in the figure. More specifically, each subsequent stage comprises a smaller diameter impeller, delivering reduced pressure head but increased equilibrium droplet size. An optimal head curve indicates how this is related for pumps of the invention by varying the impeller stage diameter for a specific type of impeller. Similar methods can be used, alone or in combinations, for varying other parameters, such as diffusor length or width or residence time, impeller design (from radial toward axial from inlet toward outlet) and other methods, which are discussed in this document and also represents embodiments of the invention.

(13) Without wishing to be bound by theory, it is assumed that each pump stage or pump provides an equilibrium droplet size for a particular type of inlet fluid mixture. If the inlet droplet size is sufficiently small, the droplet size will increase whilst the pressure increase. If the inlet droplet size is larger than the equilibrium droplet size, the pressure will increase but the droplet size will decrease. If the inlet droplet size is equal to the equilibrium droplet size, the pressure will increase but the droplet size will remain equal. The droplet size is the average or median droplet size.

(14) Reference is made to FIG. 5, illustrating comparison results for pumps of the invention compared to prior art pumps. More specifically, the applicant has tested conventional pumps in the laboratory, pumps which are typically used in various produced water applications. FIG. 5 is a diagram showing the effects on oil droplet sizes from the various pumps at different pump differential pressures. In this comparative study the following pumps were used: 1. New Pump: A centrifugal pump according to the invention. 2. Standard Pump: A conventional single-stage centrifugal pump.

(15) The diagram of FIG. 5 shows the various pumps' outlet droplet sizes in m on the y-axis, represented by Dv(50), as a function of inlet droplet sizes on the x-axis for three different pump differential pressures; 7, 10, and 13 bars, respectively. The black, dotted diagonal line illustrates when outlet droplets equal inlet droplets in size. Again, this signifies that results above the dotted line imply that the net effect of the pump is oil droplet enlargement while results below the line dotted line means that the net effect is oil droplet breaking. The results may be summarized as follows: A pump according to the invention clearly provides the best oil droplet performance when compared to the single-stage centrifugal pump. The outlet oil droplet sizes are always larger for the pumps of the invention.

(16) Not illustrated, extensive comparative testing against prior art multi stage pumps and also screw pumps has been undertaken. Standard multi stage centrifugal pumps or single stage centrifugal pumps are never close in performance, only screw pumps are comparable for some embodiments, but only for the large inlet droplet sizes 15 and 20 m where downstream separation processes usually will function as intended anyway.

(17) Reference is made to FIG. 6, indicating typical separation effect of a deoiling hydrocyclone. At droplet sizes from about 13 m to 9 m, the separation effect drops dramatically, from about 95% to about 17%. If the inlet pressure to a hydrocyclone must be raised for effective operation, using a pump of the invention can be essential for a good result. Compared to a screw pump, the multi stage centrifugal pump of the invention is small and energy effective.

(18) The pumps of the invention provides the required pressure head by modifying the pump so as to have a decreasing pressure head toward the outlet, by one or more of the features: decreasing the impeller diameter, enlarging the diffusor, reducing speed for subsequent inpeller stages, modifying subsequent impellers toward more axial flow on behalf of radial flow, or additional features discussed herein. The result is a droplet coalescense, if the inlet fluid droplet size is smaller than the quilibrium droplet size, or less droplet break up, if the inlet fluid droplet size is larger than the equilibrium droplet size. Some multiphase pumps or gas tolerant pumps, as well as compressors, may have a smaller flow bore at subsequent impellers, or even a smaller impeller size, however, this has only to do with the gas being compressed and requiring less space, it has nothing to do with coalescence, reduced droplet break up or facilitating subsequent separation.