Volute casing for a centrifugal pump and centrifugal pump

Abstract

A volute casing includes a chamber, an outlet passage, and a cutwater to direct fluid to the outlet passage. The cutwater includes an inner surface, an outer surface and a leading edge joining the inner and outer surfaces and has a cross-sectional contour in a midplane perpendicular to the axial direction. The cross-sectional contour includes a starting point at the leading edge, and a minimum point on the inner surface, the starting point defined by a tangent to the leading edge, the tangent intersecting the central axis, and the minimum point defined by a location, at which the inner surface has a minimum distance from the central axis. A straight profile chord located in the cross-sectional contour, and extending from the cutwater starting point to the cutwater minimum point, has a maximum orthogonal distance from the inner surface being at most 15% of the length of the profile chord.

Claims

1. A volute casing for a centrifugal pump, the volute casing having a central axis defining an axial direction, comprising: a volute chamber configured to receive an impeller for rotation about the axial direction; an outlet passage configured to discharge a fluid; and a first cutwater configured to direct the fluid to the outlet passage, the cutwater comprising an inner surface facing the central axis, an outer surface facing away from the central axis and a leading edge joining the inner surface and the outer surface, the cutwater having a cross-sectional contour in a midplane perpendicular to the axial direction, the cross-sectional contour comprising a cutwater starting point at the leading edge, and a cutwater minimum point on the inner surface, the cutwater starting point being defined by a tangent to the leading edge, the tangent intersecting the central axis, and the cutwater minimum point being defined by a location, at which the inner surface has a minimum distance from the central axis, the cutwater being configured such that a straight profile chord located in the cross-sectional contour, and extending from the cutwater starting point to the cutwater minimum point, has a maximum orthogonal distance from the inner surface, the maximum orthogonal distance being at most 15% of the length of the profile chord, and an angular distance between the cutwater starting point and the cutwater minimum point measured on the midplane by an angle between the tangent to the leading edge through the cutwater starting point and a straight line connecting the cutwater minimum point with the central axis is at least 5.5.

2. The volute casing in accordance with claim 1, wherein the maximum orthogonal distance of the profile chord from the inner surface is approximately 13% of the length of the profile chord.

3. The volute casing in accordance with claim 1, wherein the inner surface of the cutwater is curved such that the orthogonal distance of the profile chord from the inner surface first increases when moving from the cutwater starting point to the cutwater minimum point, reaches the maximum orthogonal distance, and then decreases to zero at the cutwater minimum point.

4. The volute casing in accordance with claim 1, wherein the angular distance between the cutwater starting point and the cutwater minimum point is approximately 6.5.

5. The volute casing in accordance with claim 1, wherein the inner surface of the cutwater is configured such that the cross-sectional contour and a basic circle are tangent to each other at the cutwater minimum point, the basic circle being centered on the central axis and having a radius equal to a distance between the central axis and the cutwater minimum point.

6. The volute casing in accordance with claim 1, further comprising a second cutwater configured to direct the fluid to the outlet passage, the second cutwater comprising an inner surface facing the central axis, an outer surface facing away from the central axis and a leading edge joining the inner surface and the outer surface, and the inner surface of the second cutwater being analogously designed as the inner surface of the first cutwater at least between the leading edge and the cutwater minimum point.

7. A centrifugal pump comprising: a volute casing, according to claim 1; and an impeller arranged in the volute casing.

8. The volute casing in accordance with claim 1, wherein the maximum orthogonal distance is at most 13% of the length of the profile chord.

9. The volute casing in accordance with claim 1, wherein the angular distance between the cutwater starting point and the cutwater minimum point is at least 6.5.

10. The volute casing in accordance with claim 1, wherein an inclination angle measured on the midplane between the profile chord and a straight line connecting the cutwater minimum point with the central axis is at least 114.

11. The volute casing for a centrifugal pump, the volute casing having a central axis defining an axial direction, comprising: a volute chamber configured to receive an impeller for rotation about the axial direction; an outlet passage configured to discharge a fluid; and a first cutwater configured to direct the fluid to the outlet passage, the cutwater comprising an inner surface facing the central axis, an outer surface facing away from the central axis and a leading edge joining the inner surface and the outer surface, the cutwater having a cross-sectional contour in a midplane perpendicular to the axial direction, the cross-sectional contour comprising a cutwater starting point at the leading edge, and a cutwater minimum point on the inner surface, the cutwater starting point being defined by a tangent to the leading edge, the tangent intersecting the central axis, and the cutwater minimum point being defined by a location, at which the inner surface has a minimum distance from the central axis, the cutwater being configured such that a straight profile chord located in the cross-sectional contour, and extending from the cutwater starting point to the cutwater minimum point, has a maximum orthogonal distance from the inner surface, the maximum orthogonal distance being at most 15% of the length of the profile chord, and an inclination angle measured on the midplane between the profile chord and a straight line connecting the cutwater minimum point with the central axis is at least 110.

12. The volute casing in accordance with claim 11, wherein the inclination angle is approximately 114.

13. The volute casing in accordance with claim 11, wherein an inclination angle measured on the midplane between the profile chord and a straight line connecting the cutwater minimum point with the central axis is at least 114.

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 schematic view of an embodiment of a volute casing according to the invention,

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

(4) FIG. 3 is an enlarged view of the upstream end of the cutwater in a cross-sectional view in the midplane of the cutwater.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) FIG. 1 is a cross-sectional schematic view of an embodiment of a volute casing according to the invention, which is designated in its entity with reference numeral 1. FIG. 2 is a cross-sectional view of an embodiment of a centrifugal pump according to the invention, which is designated in its entity with reference numeral 100, and which comprises the volute casing 1 shown in FIG. 1. The centrifugal pump 100 comprises an inlet 101 through which a fluid, in particular a liquid, for example water, can enter the pump 100 as well as an outlet 102 for discharging the fluid. The pump 100 further comprises at least one impeller 103 for acting on the fluid. The impeller 103 is arranged within a volute chamber 2 of the volute casing 1. During operation, the impeller 103 is rotating about a rotational axis extending in an axial direction A. The volute casing 1 comprises a central axis C coinciding with the rotational axis of the pump 100. Thus, the axial direction A is defined by the central axis C of the volute casing 1 orwhat is the sameby the rotational axis about which the impeller 103 rotates during operation.

(6) A direction perpendicular to the axial direction A is referred to as radial direction. The term axial or axially is used with the common meaning in the axial direction or with respect to the axial direction. In an analogous manner the term radial or radially is used with the common meaning in the radial direction or with respect to the radial direction.

(7) FIG. 2 shows the pump 100 in a cross-section parallel to the axial direction A, more precisely the central axis C lies in the section plane. FIG. 1 shows the volute casing 1 in a cross-section perpendicular to the axial direction A as it is indicated by the cutting line I-I in FIG. 2.

(8) The impeller 103 is mounted on a shaft 104 in a torque proof manner. By the shaft 104 extending in axial direction A, the impeller 103 is driven during operation of the pump 100 for a rotation about the axial direction A. The shaft 104 is driven by a drive unit (not shown), for example an electric motor or any other type of motor, to which the shaft 104 is coupled. In a manner known as such, the shaft 104 and the impeller 103 are supported by a bearing unit 105. A sealing unit 106 seals the shaft 104 against leakage of the fluid along the shaft 104.

(9) As shown in FIG. 1, the volute casing 1 comprises the volute chamber 2 for receiving the impeller 103 and an outlet passage 3 for guiding the liquid to the outlet 102. The flow of liquid coming from the inlet 101 enters the volute chamber 2 generally in the axial direction A and is then diverted by the impeller 103 in a circumferential direction. As it is characteristic for a volute casing, the distance between the inner wall delimiting the volute chamber 2 and the central axis C of the volute casing 1 increases when viewed in the flow direction towards the outlet passage 3, thus building a flow channel for the liquid which flow channel that widens in flow direction. The volute casing 1 further comprises at least a first cutwater 4 for directing the liquid into the outlet passage 3, i.e. the first cutwater 4 divides the flow channel such that the liquid flows along both sides of the cutwater 4. The cutwater 4 is also referred to as splitter rib or as cutwater tongue or simply as tongue. The embodiment shown in FIG. 1 is configured with two cutwaters and comprises, a part from the first cutwater 4, a second cutwater 4 which is arranged at a location 180 displaced with respect to the location of the first cutwater 4 when viewed in the circumferential direction of the volute chamber 2. The design with two cutwaters 4, 4 as such is known in the art and therefore does not require a more detailed explanation. The main reason for providing two cutwaters 4, 4 in the volute casing 2 is the balancing of the radial thrust acting upon the impeller 103.

(10) Although the embodiment described here, comprises a first and a second cutwater 4, 4 it has to be understood that the invention also comprises such embodiments in which the volute casing 1 is designed with only one cutwater.

(11) Each cutwater 4, 4 comprises an inner surface 41 facing the central axis C, an outer surface 42 facing away from the central axis C and a leading edge 43 which is the axially extending edge of the cutwater 4, 4 facing the flow of liquid, i.e. at the leading edge 43 the flow of liquid is split. The leading edge 43 constitutes the upstream end of the cutwater 4, 4. Thus, the inner surface 41 of the respective cutwater 4, 4 is that lateral surface of the cutwater 4, 4 which is closer to the central axis C and the outer surface 42 of the respective cutwater 4, 4 is that lateral surface of the cutwater 4, 4 which is farer away from the central axis C. The leading edge 43 is joining the inner surface 41 and the outer surface 42.

(12) Referring now to FIG. 3 the design of the cutwater 4, 4 and in particular the design of the inner surface 41 near the leading edge 43 will be described in more detail. It goes without saying that this description applies both for the first cutwater 4 and for the second cutwater 4 FIG. 3 shows an enlarged view of the upstream end of the cutwater 4, 4, which is the end comprising the leading edge 43 of the cutwater 4, 4. FIG. 3 represents a cross-section through the cutwater 4, 4 perpendicular to the axial direction A in a section plane coinciding with a midplane of the cutwater 4, 4 The midplane is perpendicular to the axial direction A and represents the geometrical center plane of the cutwater 4, 4 with respect to the axial direction A. In FIG. 3 the drawing plane coincides with the midplane. The design of the cutwater 4, 4 in the midplane is represented by the cross-sectional contour 44 of the cutwater 4, 4 in the midplane. The midplane and more precisely the cross-sectional contour 44 comprise a cutwater starting point CS and a cutwater minimum point CM.

(13) The cutwater starting point CS is located on the leading edge 43, and the midplane (or the cross-sectional contour 44, respectively). The cutwater starting point CS is defined by that point of the cross-sectional contour 44 at which a tangent T to the leading edge 43 exists, that orthogonally intersects the central axis C.

(14) The cutwater minimum point CM is located on the inner surface 41, more precisely at the intersection of the inner surface 41 and the midplane (or the cross-sectional contour 44, respectively). The cutwater minimum point is defined by that point located both in the midplane (or the cross-sectional contour 44, respectively) and on the inner surface 41, at which the inner surface 41 has a minimum distance D from the central axis C, as measured in the midplane.

(15) As can be seen in FIG. 3 the inner surface 41 of the cutwater 4, 4 is designed such that the distance D of the inner surface 41 from the central axis C continuously decreases when moving along the inner surface 41 from the cutwater starting point CS to the cutwater minimum point CM. At the cutwater minimum point CM, the distance D reaches its minimum and increases upon further moving away from the leading edge 43 beyond the cutwater minimum point CM. The inner surface 41 is designed as a smooth and curved surface having a minimum distance D from the central axis C at the cutwater minimum point CM.

(16) FIG. 3 further shows a profile chord P defined as a straight line in the cross-sectional contour 44 extending from the cutwater starting point CS to the cutwater minimum point CM. The length L of the profile chord P is the distance between the cutwater starting point CS and the cutwater minimum point CM. Due to the curved design of the inner surface 41 the orthogonal distance between the straight profile chord P and the inner surface 41 varies between the cutwater starting point CS and the cutwater minimum point CM. The inner surface 41 is designed and curved in such a manner that the orthogonal distance of the profile chord P from the inner surface 41 first increases when moving from the cutwater starting point CS to the cutwater minimum point CM, reaches a maximum orthogonal distance DM, and then decreases to zero at the cutwater minimum point CM.

(17) According to the invention, the maximum orthogonal distance DM between the profile chord P and the inner surface 41 is at most 15%, and preferably at most 13% of the length L of the profile chord P. In the embodiment shown in FIG. 3 the maximum orthogonal distance DM equals approximately 13% of the length L of the profile chord P.

(18) Another preferred feature of the design of the cutwater 4, 4 is related to the distance between the cutwater starting point CS and the cutwater minimum point CM. The distance is determined by an angular distance that is measured on the midplane by an angle . The angle is the angle between the tangent T to the leading edge 43 and a straight line W perpendicular to the axial direction A, or the central axis C, respectively, wherein the straight line W connects the cutwater minimum point CM with the central axis C. This angle measuring the angular distance between the cutwater starting point CS and the cut water minimum point CM is at least 5.5 and preferably at least 6.5. In the embodiment shown in FIG. 3 the angle measuring the angular distance between the cutwater starting point CS and the cut water minimum point CM equals approximately 6.5.

(19) Still another preferred feature of the design of the cutwater 4, 4 is related to the inclination of the profile chord P. The inclination is measured on the midplane by an inclination angle which is defined as the angle between the profile chord P and the straight line W, i.e. the line perpendicular to the axial direction A and connecting the cutwater minimum point CM with the central axis C. Preferably, the inclination angle is at least 110 and more preferred at least 114. In the embodiment shown in FIG. 3 the inclination angle equals approximately 114.

(20) According to a further advantageous measure, the inner surface of the cutwater 4, 4 is designed in such a manner that the cutwater minimum point CM constitutes an absolute minimum in the distance D of the cross-sectional contour 44 from the central axis C, i.e. there is no other point on the cross-sectional contour 44 at which the distance D of the inner surface 41 from the central axis C is smaller than or equals the distance D at the cutwater minimum point CM. That is, the cross-sectional contour 44 and a basic circle BC are tangent to each other at the cutwater minimum point CM, wherein the basic circle BC is defined by having its center on the central axis C and a radius that equals the distance D between the central axis C and the cutwater minimum point CM, which is the minimum of the distance D. The basic circle BC lies in the midplane. The outer surface 42 of the cutwater 4, 4 may be designed in any known manner.

(21) The volute casing according to an embodiment of the invention, and in particular the configuration of the inner surface 41 of the cutwater 4, 4 in the area adjacent to the leading edge 43, considerably reduces the risk of cavitation at the cutwater 4, 4 where the flow velocity of the liquid conveyed by the centrifugal pump 100 is very high. Especially when the centrifugal pump 100 is operated in a part-load region, i.e. away from the pump's 100 best efficiency point, and the pump 100 is generating a smaller flow rate than the flow rate the pump 100 is designed for, the configuration of the inner surface 41 avoids the occurrence of local velocity peaks or at least considerably reduces the velocity peaks, which usually exist in known designs. It has been found that such local velocity peaks in known designs predominantly occur at the inner surface of the cutwater in a region downstream of the leading edge of the cutwater.

(22) By the volute casing 1 according to the invention with the new design of the inner surface 41 downstream of the leading edge 43 the local velocity of the fluid is reduced in the critical areas of the inner surface 41 of the cutwater 4, 4 in particular in a part-load operation of the pump 100. Reducing the velocity of the fluid, or avoiding the local velocity peaks, increases the local static pressure of the fluid in these locations. More precisely, the difference between the suction pressure at the inlet 101 of the pump 100 and the local static pressure at the inner surface 41 of the cutwater 4, 4 is increased. Consequently, the local static pressure at the inner surface 41 of the cutwater 4, 4 falling below the vapor pressure of the liquid can be avoided (or at least the risk is considerably reduced). Thus, cavitation is efficiently avoided without the need to increase the suction pressure. This results in safer and better operation of the pump 100 by avoiding cavitation induced effects such as increased vibrations, noise, instabilities in the head performance curve, reduced differential head and severe erosion effects reducing the lifetime of the volute casing.