EP1272761B1

EP1272761B1 - Centrifugal pump with a bearing arrangement

Info

Publication number: EP1272761B1
Application number: EP01910257A
Authority: EP
Inventors: Arne Jonsson
Original assignee: Hamworthy KSE AS
Current assignee: Wartsila Oil and Gas Systems AS
Priority date: 2000-02-24
Filing date: 2001-02-23
Publication date: 2006-09-27
Anticipated expiration: 2021-02-23
Also published as: DK1272761T3; ES2270985T3; NO311105B1; EP1272761A1; ATE340936T1; KR20030009366A; JP4035326B2; KR100471537B1; DE60123410T2; DE60123410D1; AU3783301A; NO20000912L; EP1272761B8; NO20000912D0; JP2003533643A; WO2001088380A1

Abstract

A centrifugal pump capable of being coupled to a drive motor, where the pump has an impeller (3) for radial direction of flow and having a discharge edge (13) at its outer diameter, and where the pump has a radially split pump casing (15) and is provided with at least one shaft seal (2). The impeller (3), for absorption of the forces acting on the impeller (3), is supported in a bearing device advantageously consisting of two axially adjacent bearings (1, 1'), where the width of the bearings or their respective bearing raceways (12, 12'; 14, 14') forms a characteristic distance a. A centre point (4) on the discharge edge (13) of the impeller (3) has, seen axially, a position which is at a distance b from another centre point between the bearings (1, 1') and in the direction of or from the inlet end (21) of the pump, where b </= 2a.

Description

The present invention relates to a centrifugal pump for transport of fluid as specified in the preamble of Claim 1. Such a pump is eg known from EP-A-0 013 942, which discloses a centrifugal pump for liquids mixed with solids, comprising a pump impeller supported by bearing means in a pump housing placed around it, and a driving shaft positively joined to the pump impeller.
The object of the invention is to obtain a marked improvement of a motor-dnven pump unit which has a centrifugal pump of primarily a so-called single suction type and having a drive motor placed in direct connection with the pump. The unit may preferably be of the vertical type, i.e., having a vertical axis of rotation, and so constructed that the pump components essential for maintenance can be dismantled without either pipelines or drive motor having to be moved.
Centrifugal pumps that are incorporated in a pump unit of this kind are known in a multitude of different embodiments. Usually, there is a shaft supported in two shaft bearings that are spaced apart by several hundred millimetres. At one of its ends, the shaft supports an impeller and also has a shaft seal placed between the impeller and one of the bearings. In this embodiment the impeller has a substantial overhang of about the same magnitude as the distance between the bearings. Normally, a radially directed force of a magnitude that varies with the fluid flow through the pump acts on the impeller. The force results in a buckling (deformation) of the shaft, which in turn calls for a sufficiently large clearance at the inner seals of the impeller and causes a significant reduction in the function of the shaft seal. This factor has been the subject of attention in specifications for the minimum quality of pumps. The specifications API 610 (American Petroleum Institute) and ISO 5199 (International Organization of Standardization) both prescribe a maximum permissible value of 0.05 mm for the buckling close to the shaft seal. In commercial and technical assessment another way of expressing the extent of the buckling is used, namely the expression L³/D⁴. In this expression L represents the overhang of the impeller and D is the diameter of the shaft.
To avoid a support of the impeller and overhang, the impeller can be placed with a bearing on each side. However, this requires two shaft seals, i.e., one on each side. At least one of the bearings can also be placed in the pumped fluid, lubricated and cooled thereby. In special cases, such as in hermetically sealed pumps, both the bearing and the drive mechanism are more or less placed in the fluid. Consequently, this embodiment has no shaft seal.
Furthermore, an impeller can be mounted directly on a shaft of a drive motor, usually an electromotor, or on a shaft extension that is rigidly connected to the electromotor shaft. However, conventional electromotors of standard design, at motor outputs of more than 10-30 kW, do not have sufficient bearing strength in their shaft bearing. Thus, there are pump designs where the shaft extension is equipped with at least one extra bearing.
To allow the internal components of the pump unit such as the impeller, shaft seal etc to be dismantled easily without the pipelines or drive motor having to be disassembled, the pump casing may be split along the pump shaft (axial split) or split transverse to the shaft (radial split). In connection with a radial split, it is now common to use an opening on the drive motor side (so-called "back pull out"). Dismantling is made possible by first disassembling a part of the shaft or a part (spacer) of an extended shaft coupling. In this case, these parts are of such a length that the internal components can then be taken out. In this type of pump unit there is normally one or more torsion-rigid, but otherwise elastic shaft couplings to allow slight deviation from a precisely concentric location of the centrifugal pump and drive motor. Shaft bearings can then be placed according to the earlier method, or if one of the shaft couplings is only angle-elastic, one of the bearings can be placed on the pump shaft, the rest of the bearing function being carried out through the drive motor bearing.
According to the invention it is an object of the new centrifugal pump to remedy a number of the disadvantages that are associated with the prior art.
According to the invention, a centrifugal pump is provided with the features of Claim 1.
According to a further embodiment, a rotatable sleeve is arranged between at least one of the bearings and the said at least one shaft seal for transport and/or diversion of any leakage from the shaft seal to the pump surroundings, and the sleeve divides the hub section of the impeller into two chambers, where the chamber in which the bearings are placed is separated from the pump fluid and/or from any fluid which leaks out of the shaft seal.
According to another embodiment, a drive shaft for transmission of torque from the drive motor to the impeller is at its end fitted to at least two contact faces in a shaft end-receiving recess in the impeller hub, e.g., by using a so-called spline or key connection, the drive shaft at its other end being made for connection with the drive motor shaft via a coupling. The coupling may consist of a first flange secured to or in one piece with the drive shaft and a second flange secured to or in one piece with the motor shaft, said first and second flanges being joinable by means of nut and bolt connections, adhesive bonding, shearing pins or the like.
The bearings in the bearing device advantageously have their respective outer bearing raceway bodies placed in the impeller hub and their respective inner bearing raceway bodies mechanically connected to the pump casing.
Advantageously, the shaft seal is placed in an annular part of the impeller hub, and the respective outer bearing raceway bodies of the bearings can also be fastened via the impeller to the drive shaft and be rotatable together therewith.
The shaft of the centrifugal pump is preferably arranged to be substantially vertical, and the drive motor may be mounted on a stand that is connected to the pump casing.
The said rotatable sleeve is preferably constructed having double walls and/or having vanes designed for producing, on rotation of the sleeve with the impeller, a pumping effect in order to convey any leakage fluid from the shaft seal past the chamber for the shaft bearing device and out into the pump surroundings.
The invention will now be described in more detail with reference to the attached drawings, wherein:

Fig. 1 is a cross-section through a pump; and
Figs. 2 and 3 shows sections of two variants.

One of the special features of the invention is that shaft bearings 1, 1' and shaft seal are placed in an impeller 3 with no overhang or a minimal overhang. The centre line of the impeller is indicated by the reference numeral 21. The radial forces and moments acting on the impeller are taken up by the bearings without the occurrence of tilting moments or bending moments and accompanying deformations. The expression L³/D⁴ is practically equal to zero. The shaft bearings are thus made having bearing raceways (bearings faces) that in at least one bearing have a function corresponding to several spaced-apart bearings. The bearings can, for example, be made as several single or double angular contact ball bearings, conical ball bearings or so-called four-point bearings. At least one shaft seal 2 for the pumped medium is placed immediately next to or at a limited distance to one of the bearings 1, and separated therefrom by a rotatable dividing sleeve or cup 5 of relatively little wall thickness. The sleeve 5 thus divides an area in the impeller hub section into two separate chambers 6, 7. The axial extent of the sleeve 5 from the shaft seal 2 is sufficiently great to transport and/or divert, as a result of its shape or its rotation, any leakage from the shaft seal chamber 7 to the pump unit surroundings 16 via an opening 17 in the pump casing, thus preventing leakage from penetrating into the chamber 6 where the shaft bearings 1, 1' are located. Because of the limited space inside the impeller hub section for bearings and seals, a drive shaft 8 for transmitting torque to the impeller 4 will be relatively heavily loaded and is at one of its ends connected to the impeller 3 via a positive connection by using at least two fasteners or shaft keys, preferably several of these in the form of so-called splines, indicated by the reference numeral 9 in the figure. The drive shaft 8 is advantageously separated or sealed from the pumped fluid by using the sleeve 5 and relevant parts 19 of the impeller hub section. The drive shaft 8 is also at its other end joined permanently to or made in one piece with driving flange 10, 10' to which an optionally flexible shaft coupling can be connected for delivery of torque. The impeller hub section may also be made as bearing raceways 12, 14; 12', 14' or as bearing seats for the shaft bearings 1, 1', which then causes forces and moments on the impeller 3 other than torsional moments to be transmitted to the bearings. This in turn means that the drive shaft 8 is only loaded by torque and its design can be optimised therefor. The placing of the shaft bearings 1, 1' and shaft seal inside the impeller results in a great change in the total size and weight of the pump unit. In particular in the case of vertical pumps where the drive motor (not shown) is supported by a stand (not shown) placed on or immediately next to the pump casing of the centrifugal pump 15, 15', 15'', 15''', a great reduction in the total height of the unit is obtained. This leads to a reduction in the size of and risk of normal motor vibrations. In the case of pump units located on board a vessel, this also reduces the risk of disturbing movement of the unit on the motion of a vessel at sea and vibrations caused via its propulsion machinery.
The outer bearing rings or bearing raceways 12, 12' of the shaft bearings are preferably rotatable with the impeller 3 and placed in the impeller hub 3'. This involves the advantage that only static stresses occur and thus no dynamic loads that can cause fatigue.
The bearing device is also essentially placed in the impeller hub or in immediate connection with this hub. The axial dimensions of the bearing device are thus normally less than or at most of the same magnitude as the total axial length of the impeller.
To obtain optimum conditions as regards moments, the impeller 3, for absorption of the forces acting thereon, is supported in a bearing device consisting normally of two axially adjacent bearings 1, 1', where the respective bearing raceways (12, 12'; 14, 14') of the bearings form a characteristic distance a, and the centre point 4 on the discharge edge 13 of the impeller 3, seen axially, will have a position that has a distance b from a centre point between the bearings 1, 1' and in the direction of or from the inlet end 20 of the pump, wherein the condition b ≤ 2a must be satisfied.
For the actual shaft bearing, the width of the bearing, i.e., the axial dimensions of the bearing ring or several bearing rings, constitutes a first value a1 for said characteristic distance a. A second value a2 is the distance between the centres of force 30, 30' on the centre line 21 of the bearing device. In the case of, for instance, pairwise mounted single angular contact ball bearings in "back-to-back" assembly, double angular contact ball bearings and a single angular contact ball bearing of the so-called four-point type, the value of a2 is given in the catalogues of ball bearing manufacturers, and this value is normally two to three times greater than a1. Of course, analogous conditions apply in the case of conical ball bearings and also other bearing types. The distance a that is characteristic for the invention is the largest of the values a1 and a2. Advantageously, the bearing device is made having bearings of the aforementioned type where 2a is greater than a1 so as to be able in the best possible way to absorb bending moments and tilting moments. An embodiment having distance a1 can be used for simpler structures where, for example, only one single step ball bearing is used.
Formally, the distance a1 constitutes a definition for the distance a for the cases where the distance a2 is impossible to define.
The rotatable sleeve 5 is advantageously disposed between at least one of the bearings and at least said shaft seal 2 for transport and/or diversion of any leaks from the shaft seal 2 to the surroundings 16 of the pump. The sleeve 5 can expediently divide the impeller hub section into the two chambers 6, 7 where the chamber 6 in which the bearings 1, 1' are placed will be separated from the pump fluid and/or from any fluid which leaks out of the shaft seal 2. The flange coupling 10, 10' between the shafts 8 and 18 can be secured by means of nut and bolt connection, adhesive bonding or use of shearing pins or the like.
As can be seen from the figure, the bearings 1, 1' have their respective, outer bearing raceway bodies 12, 12' placed in the impeller hub 3', whilst the respective inner bearing raceway bodies 14, 14' of the bearings are mechanically connected to the pump casing 15"'. It will also be seen that the shaft seal 2 is placed on an annular part 3" of the impeller hub. Furthermore, it will be seen that the outer bearing raceway bodies 12, 12' of the bearings are connected via the impeller 3 to the drive shaft 8 and are rotatable together therewith.
The rotatable sleeve 5 can be made having double walls and/or having vanes, so designed that on the rotation of the sleeve 5 together with the impeller 3 a pumping action will be generated in order to convey any leakage fluid that has moved beyond the shaft seal 2 past the chamber 6 for the shaft bearing device and out into the surroundings 16 of the pump via an opening 17 in the pump casing. This is shown in Figs. 2 and 3.
In Fig. 2 it is shown how the sleeve 5 has an additional part 22 in the form of a surrounding sleeve part, having radial bores 23. In Fig. 3 the sleeve is provided with a vane ring 24.

Claims

A centrifugal pump for transport of fluid, which centrifugal pump can be coupled to a drive motor, comprising a casing (15), an impeller (3) having an axial extent and an outer diameter with a discharge edge (13), said discharge edge (13) being arranged for a substantially radially outward directed discharge, a drive shaft (8) having a first end connected to said impeller (3) and a second end which can be coupled to said drive motor, a bearing device for rotational supporting of said impeller (13) in said casing (15), said bearing device (1,1') being arranged within the axial extent of said impeller (3) and consisting of one or more bearings (1,1') and a bearing carrier (19,15"') connected to said casing (15) and extending into said impeller (3), characterized by said bearing carrier (19,15''') having a central throughgoing bore accomodating said drive shaft (8), at least one shaft seal (2) between said impeller (3) and the said casing, thus separating the bearing device (1,1') from the transported fluid,
the width and/or respective bearing raceways (12,12',14,14') of the said bearing or bearings (1,1') forming a characteristic distance a which constitutes the largest of the total axial width a1 of the bearings and a distance a2 between the centres of force of the bearings at a centre line (21) of the bearing device, a centre point (4) on the discharge edge (13) of the impeller (3), seen axially, having a position which is at a distance b from an axial centre point for the bearing device, where b ≤ 2a.
A centrifugal pump as disclosed in claim 1, wherein a sleeve (5) is connected to the impeller (3) and is disposed between the bearings (1, 1') and the said at least one shaft seal (2) for transport and/or diversion of any leakage from the shaft seal (2) to the surroundings (16) of the pump, the sleeve (5) forming two chambers (6,7), where the chamber (6) in which the bearings (1, 1') are located is separated from pump fluid and/or from any fluid which leaks out of the shaft seal.
A centrifugal pump as disclosed in claim 1 or 2, wherein the drive shaft (8) for transmission of torque from the drive motor to the impeller (3) is at one of its ends fitted to at least two contact surfaces in a shaft end-receiving recess in the impeller (3,3'), e.g., by using a so-called spline or key coupling; and that the drive shaft at its other end is designed for connection with the drive motor shaft (17) via a coupling (10, 10').
A centrifugal pump as disclosed in claim 3, wherein the coupling consists of a first flange (10) secured to or in one piece with the drive shaft (8) and a second flange (10') secured to or in one piece with the motor shaft, and that said first and second flanges are joinable by means of nut and bolt connection, adhesive bonding, shearing pins or the like.
A centrifugal pump as disclosed in claim 1 or 2, wherein the bearings (1, 1') have their respective outer bearing raceway bodies (12, 12') located in the impeller (3,3') and that the respective inner bearing raceway bodies (14, 14') of the bearings (1, 1') are mechanically connected to the pump casing.
A centrifugal pump as disclosed in claim 1 or 2, wherein the shaft seal (2) is placed on an annular part (3") of the impeller; and that respective outer bearing raceway bodies (12, 12') of the bearings are fastened via the impeller (3) to the drive shaft (8) and are rotatable together therewith.
A centrifugal pump as disclosed in one or more of the preceding claims, wherein the centrifugal pump has an essentially vertically arranged axis of rotation (21), and the drive motor is mounted on a stand that is connected to the pump casing.
A centrifugal pump as disclosed in claim 2, wherein the sleeve (5) is made having double walls and/or having vanes which are designed to produce, on rotation of the sleeve together with the impeller, a pumping action in order to convey any leakage fluid from the shaft seal (2) out into the surroundings of the pump.