Pump for conveying a highly viscous fluid

Abstract

A pump for conveying a highly viscous fluid includes a casing with at least a first inlet and an outlet for the fluid, and an impeller for conveying the fluid from the inlet to the outlet. The impeller is arranged on a rotatable shaft for rotation around an axial direction, and includes a front shroud facing the first inlet of the pump. The casing includes a stationary impeller opening for receiving the front shroud of the impeller and has a diameter. The front shroud and the stationary impeller opening form a gap having a width in a radial direction perpendicular to the axial direction, and the ratio of the width of the gap and the diameter of the impeller opening is at least 0.0045.

Claims

1. A pump for conveying a highly viscous fluid, comprising: a casing with at least a first inlet and an outlet for the highly viscous fluid; an impeller configured to convey the highly viscous fluid from the first inlet to the outlet, the impeller being arranged on a rotatable shaft for rotation about an axial direction, and comprising a front shroud facing the first inlet of the pump, the casing including a stationary impeller opening configured to receive the front shroud of the impeller, the stationary impeller opening having a diameter, the front shroud and the stationary impeller opening forming a gap having a width in a radial direction perpendicular to the axial direction, and the gap extending parallel to the rotatable shaft, a ratio of the width of the gap and the diameter of the stationary impeller opening being at least 0.0045, and the gap having a length in the axial direction which is at least 0.092 times the diameter of the impeller opening, wherein the length extends between an outer circumferential surface of the front shroud and an inner circumferential surface of the stationary impeller opening.

2. A pump in accordance with claim 1, wherein the ratio of the width of the gap and the diameter of the stationary impeller opening is at least 0.0050.

3. A pump in accordance with claim 1 wherein the ratio of the width of the gap and the diameter of the stationary impeller opening is at most 0.0070.

4. A pump in accordance with claim 1, wherein the gap comprises a plurality of lands consecutively arranged with respect to the axial direction and two adjacent lands of the plurality of lands are separated by a groove.

5. A pump in accordance with claim 1, wherein the stationary impeller opening comprises a wear ring delimiting the gap with respect to the radial direction, the wear ring being arranged stationary with respect to the casing.

6. A pump in accordance with claim 1, wherein the impeller comprises a wear ring delimiting the gap with respect to the radial direction, the wear ring being arranged stationary with respect to the impeller.

7. A pump in accordance with claim 1, wherein the pump is a double suction pump having a second inlet for the fluid being arranged oppositely to the first inlet of the pump, and the impeller is a double suction impeller comprising vanes for conveying the fluid both from the first inlet and from the second inlet to the outlet.

8. A pump in accordance with claim 7, wherein the front shroud is a first front shroud, and the impeller comprises a second front shroud facing the second inlet of the pump, the casing includes a second stationary impeller opening configured to receive the second front shroud of the impeller and having a diameter, wherein the second front shroud and the second stationary impeller opening form a gap having a width in the radial direction perpendicular to the axial direction, and the ratio of the width of the gap formed by the second front shroud and the second stationary impeller opening and a diameter of the second stationary impeller opening is at least 0.0045.

9. A pump in accordance with claim 8, wherein the ratio of the width of the gap formed by the second front shroud and the second stationary impeller opening and the diameter of the second stationary impeller opening is at least 0.0050.

10. A pump in accordance with claim 8 wherein the gap formed by the second front shroud and the second stationary impeller opening has a length in the axial direction which is at least 0.092 times the diameter of the second stationary impeller opening.

11. A pump in accordance with claim 8, wherein the second stationary impeller opening comprises a wear ring delimiting the gap formed by the second front shroud and the second stationary impeller opening with respect to the radial direction, the wear ring being arranged stationary with respect to the casing.

12. A pump in accordance with claim 8, wherein the gap formed by the first front shroud and the first stationary impeller opening and the second gap formed by the second front shroud and the second stationary impeller opening are substantially identical.

13. A pump in accordance with claim 1, wherein the pump is a centrifugal pump.

14. A method comprising: operating a pump in accordance with claim 1 in the oil and gas industry.

15. A pump in accordance with claim 1, wherein the pump is a single stage centrifugal pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail hereinafter with reference to the drawings.

(2) FIG. 1 is a cross-sectional view of an embodiment of a pump according to the invention,

(3) FIG. 2 is an enlarged representation of detail I in FIG. 1,

(4) FIG. 3 is a sketch of the front shroud and a wear ring as part of the stationary impeller opening,

(5) FIG. 4 is as FIG. 3, but for a variant of the embodiment,

(6) FIG. 5 is a second variant for the design of the gap between the front shroud and the stationary impeller opening, and

(7) FIG. 6 is an illustration of a comparison of a pump according to the invention with prior art pumps.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(8) FIG. 1 shows a cross-sectional view of an embodiment of a pump according to the invention which is designated in its entity with reference numeral 1. FIG. 2 shows an enlarged representation of detail I in FIG. 1. The pump 1 is designed for conveying a highly viscous fluid, whereas the term highly viscous has the meaning that the kinematic viscosity of the fluid is at least 10.sup.4 m.sup.2/s, which is 100 centistokes (cSt).

(9) In this embodiment the pump 1 is designed as a double suction single stage centrifugal pump. This design is one preferred embodiment which is in practice useful for many applications. Of course, the invention in not restricted to this design. A pump according to the invention may also be designed as a single suction centrifugal pump or as a multistage centrifugal pump or as any other type of centrifugal pump. Based upon the description of the embodiment shown in FIG. 1 and FIG. 2 it is no problem for the skilled person to build a pump according to the invention, that is designed as another type of pump, especially centrifugal pump, for example a single suction pump.

(10) The double suction pump 1 comprises a casing 2 with a first inlet 3, a second inlet 3 and an outlet 4 for the fluid to be pumped. The fluid may be for example crude oil, oil or any other hydrocarbon fluid being highly viscous. The pump 1 has an impeller 5 with a plurality of vanes 51 for conveying the fluid from the first inlet 3 and the second inlet 3 to the outlet 4. The impeller 5 is arranged on a rotatable shaft 6 for rotation around an axial direction A. The axial direction A is defined by the axis of the shaft 6 around which the impeller 5 rotates during operation. The shaft 6 is rotated by a drive unit (not shown).

(11) The direction perpendicular to the axial direction A is referred to as the radial direction.

(12) The first inlet 3 and the second inlet 3 are arranged oppositely to the first inlet with respect to the axial direction A. Thus, according to the representation in FIG. 1, the fluid is flowing both from the left side and from the right side in axial direction A to the impeller 5, whereas the fluid from the first inlet 3 is flowing in opposite direction to the impeller as the fluid from the second inlet 3. The impeller 5 conveys both the fluid coming from the first inlet 3 and the fluid coming from the second inlet 3 into the radial direction to the outlet 4 of the pump.

(13) The impeller 5 comprises a front shroud 7 covering the vanes 51 and facing the first inlet 3 of the pump 1. Since in this embodiment the impeller 5 is designed as a double suction impeller 5 it comprises a second front shroud 7 facing the second inlet 3 and covering the vanes 51 on the side of the impeller 5 which faces the second inlet 3.

(14) The casing 2 includes a stationary impeller opening 8 for receiving the front shroud 7 of the impeller 5. The stationary impeller opening 8 is stationary with respect to the casing 2 of the pump 1 and has a circular cross-section with a diameter D, whereas the diameter D designates the smallest diameter of that part of the stationary impeller opening 8 which receives the front shroud 7.

(15) In an analogous manner the casing 2 comprises a second stationary impeller opening 8 for receiving the second front shroud 7 of the impeller 5.

(16) In the mounted state the impeller 5 is arranged coaxially within the stationary impeller opening 8 such that the outer circumferential surface of the front shroud 7 faces the inner circumferential surface of the stationary impeller opening 8. Thus, the front shroud 7 and the stationary impeller opening 8 form a gap 9 (see also FIG. 3) between the front shroud 7 and the stationary impeller opening 8. The gap 9 is also called a labyrinth. It has an essentially annular shape and provides sealing action as will be explained hereinafter.

(17) The gap 9 has a width R in the radial direction between the front shroud 7 and the stationary impeller opening 8. The width R, i.e. the extension of the gap 9 in radial direction, is also referred to as radial clearance R and may be constant along the axial extension of the gap 9. The radial clearance R designates the minimum radial clearance along the gap 9.

(18) The second parameter defining the geometry of the gap 9 is the length L of the gap 9 which is the extension of the gap 9 in the axial direction A. The gap 9 extends parallel to the shaft 6 or parallel to the axial direction A, respectively. Thus, the back flow is flowing through the gap 9 parallel to the shaft 6 and in the opposite direction as the fluid flowing through the respective inlet 3. Thus, viewed in the main flow direction of the fluid entering through the respective inlet 3 the starting position of the gap 9, i.e. the opening through which the fluid enters the gap 9, is arranged behind the ending position of the gap 9, i.e. the opening through which the fluid leaves the gap 9.

(19) In an analogous manner a second gap 9 is formed between the second front shroud 7 and the second stationary impeller opening 8. The second gap 9 has a width R in radial direction and a length L in the axial direction A. The second stationary impeller opening 8 has a diameter D. The gap 9 extends parallel to the shaft 6 or parallel to the axial direction A, respectively. Preferably, but not necessarily, the width R equals the width R and the length L equals the length L and the diameter D equals the diameter D. Since the design and the arrangement of the second gap 9 may be identical as the gap 9 the following description will only refer to the gap 9. It shall be understood that this description applies in an analogously same manner also for the second gap 9.

(20) The gap 9 or the labyrinth 9 seals a side room 10 located on the high pressure side of the impeller 5 against the low pressure side of the impeller 5 which is located at the inlet 3. The side room 10 is located at the high pressure side of the impeller 5 near the outlet 4 of the pump 1 and delimited by the front shroud 7 of the impeller 5 as well as by the casing 2 of the pump 1. During operation of the pump 1 a back flow is generated from the region of the outlet 4 through the side room 10. The back flow passes the gap or the labyrinth 9 flowing essentially in the axial direction A, i.e. parallel to the shaft 6 and reaches the low pressure side of the impeller 5 next to the first inlet 3. It is obvious that the back flow reduces the efficiency of the pump 1.

(21) Thus, it is one of the functions of the gap 9 to provide some sealing action to limit the back flow. That is the reason why the gap 9 is also called labyrinth.

(22) It is the basic idea of the present invention to design the width R (see FIG. 2 and FIG. 3) of the gap 9 in the radial direction bigger or larger as compared to solutions known from the prior art. Although one could expect that a larger width R would result in an increased back flow which in turn reduces the pump efficiency, it has been realized that by making larger the width R of the gap 9 the overall efficiency of the pump 1 may be increased.

(23) Referring to FIG. 2 and FIG. 3 the design of the gap 9 will now be explained in more detail. In the embodiment according to FIG. 1 the stationary inlet opening 8 comprises a wear ring 11 delimiting the gap 9 with respect to the radial direction. The wear ring 11 faces the outer circumferential surface of the front shroud 7 that is inserted in the stationary inlet opening 8. The wear ring 11 is fixedly mounted to the casing 2, thus, the wear ring 11 is stationary with respect to the casing 2.

(24) FIG. 3 shows a sketch of the front shroud 7 and the wear ring 11 as part of the stationary impeller opening 8 to more clearly understand the dimensions of the gap 9.

(25) It shall be understood that in an analogous manner also the second stationary inlet opening 8 may comprise a second wear ring 11 (see FIG. 1) delimiting the second gap 9 with respect to the radial direction. The second wear ring 11 may be arranged stationary with respect to the casing 2 as shown in FIG. 1 or the second wear ring may be stationary with the impeller 5 in the same manner as shown in FIG. 4.

(26) According to the invention the width R of the gap 9 is designed such that the ratio of the width R and the diameter D of the impeller opening 8 is at least 0.0045, i.e. R/D0.0045. As already said, the diameter D designates the smallest diameter of the stationary impeller opening 8, i.e. the diameter at that location were the wear ring 11 comes closest to the outer circumferential surface of the front shroud 7. The width R of the gap 9 is the extension in radial of that region where the stationary impeller opening 8 and the front shroud 7 come closest to each other.

(27) The second parameter defining the geometry of the gap 9 is the length L of the gap 9 in axial direction A between the front shroud 7 and the stationary impeller opening 8 or the wear ring 11, respectively. The length L of the gap 9 is the extension in axial direction A of that region where the stationary impeller opening 8 and the front shroud 7 come closest to each other.

(28) In practice it has been proven as advantageous, when the length L of the gap 9 is at least 0.092 times the diameter D of the impeller opening 8, i.e. preferably the condition L/D0.092 is fulfilled.

(29) The optimal width R of the gap 9 depends on the respective application. There are several factors influencing an appropriate choice of the width R of the gap 9, for example the kinematic viscosity of the specific fluid to be pumped, the pressure increase generated by the pump, the flow through the pump or other operational parameters of the pump 1.

(30) For a given set of operational parameters of the pump 1 the width R of the gap 9 should preferably be increased with increasing viscosity of the fluid to be pumped.

(31) In practice and depending on the application it may be preferred that the ratio R/D is at least 0.0050.

(32) According to the preferred embodiments of the pump 1 the maximum ratio RID is 0.0070, i.e. the width R of the gap 9 is preferably at most 0.0070 times the diameter of the stationary impeller opening 8 or the wear ring 11, respectively. However there might be applications, where it is preferred that the width R of the gap 9 is even larger than 0.0070 times the diameter of the stationary impeller opening 8.

(33) FIG. 4 shows in a similar representation as FIG. 3, a variant of the embodiment of the pump 1. According to this variant the impeller 5 and more particular the front shroud 7 of the impeller 5 comprises a wear ring 11 delimiting the gap 9 with respect to the radial direction. The wear ring 11 is fixedly connected to the impeller 5 and rotating with the impeller 5. In this variant the stationary impeller opening 8 may comprise a wear ring 11, too, but may also be designed without a wear ring.

(34) FIG. 5 illustrates a second variant for the design of the gap 9 between the front shroud 7 and the stationary impeller opening 8. According to the second variant the stationary impeller opening 8 or the wear ring 11, respectively, or as an alternative (not shown) the front shroud 7 is designed such that the gap 9 comprises a plurality of lands 12 consecutively arranged with respect to the axial direction A, wherein two adjacent lands 12 are respectively separated by a groove 13. In such a design, the total length L of the gap 9 is the sum of the individual lengths L1, L2, L3, L4, L5 of all lands 12 in the axial direction. The extension of the grooves does not contribute to the total lengths L of the gap 9, i.e. L=L1+L2+L3+L4+L5. The width R in the radial direction is the distance between the lands 12 and the outer circumferential surface of the front shroud 7 in radial direction. It shall be understood that the number of lands and grooves as well as their geometric design shown in FIG. 5 are only exemplary.

(35) The pump 1 according to the invention has a better pump efficiency as compared to pumps known from the state of the art. The pump efficiency designates the ratio of the power delivered by the pump and the power input for the pump, i.e. the power that is used to drive the pump. The power delivered by the pump is usually the hydraulic power generated by the pump 1.

(36) FIG. 6 illustrates a comparison of a pump according to the invention with prior art pumps. The graph shows the pump efficiency P as a function of the viscosity V of the fluid conveyed by the pump. For the purpose of a better understanding the graph is standardized such that the pump efficiency P of the prior art pumps equals the horizontal viscosity axis V, i.e. the pump efficiency P for the pump according to the prior art lies always on the V-axis for each viscosity. Thus, the graph directly shows the increase of the pump efficiency of the pump 1 according to the invention as compared to a prior art pump. The pump efficiency of the pump according to the invention is represented by the curve K. As can be clearly seen, as soon as the viscosity of the fluid is greater than a specific value V1 the pump 1 according to the invention has an increased pump efficiency compared to the prior art pump. The efficiency gain is increasing with the viscosity of the fluid. The specific value V1 of the viscosity where the pump 1 according to the invention becomes more efficient than the prior art pump is usually smaller than the value of 10.sup.4 m.sup.2/s. Thus, for a highly viscous fluid the pump 1 according to the invention has a higher pump efficiency than the prior art pump.

(37) Although specific reference has been made for the purpose of explanation to an embodiment, where the pump 1 is designed as a double suction single stage centrifugal pump the invention is in no way restricted to such embodiments. The pump according to the invention may also be designed as any other type of centrifugal pump, for example as a single suction pump or as a multistage pump. In particular, the invention is applicable both to centrifugal pumps with a closed impeller, i.e. an impeller having a front shroud and a rear shroud, and to centrifugal pumps with a semi-open impeller, i.e. having a rear shroud but no front shroud. In such designs where the impeller has a rear shroud or a rear shroud only, the design of the gap 9 according to the invention may be used for the rear shroud in an analogously same manner as herein described with reference to the front shroud.