AP48A

AP48A - Centrifugal seal.

Info

Publication number: AP48A
Application number: APAP/P/1987/000052A
Authority: AP
Inventors: Denis James Martin; Anthony Grzina; Charles Harold Warman; Philip Neil Mitchel
Original assignee: Res And Development Pty Limited
Priority date: 1986-01-09
Filing date: 1987-01-02
Publication date: 1989-09-03
Also published as: JPS63502607A; PH26362A; AU591546B2; US5078410A; IN166621B; NZ218822A; WO1987004223A1; HUT46956A; KR880700907A; CN87100110A; AP8700052A0; AU6842987A; DE3778229D1; EP0253839B1; JPH0643870B2; OA08571A; MY101134A; ZA8733B; EP0253839A1; CA1317329C

Abstract

The invention provides a centrifugal sealing member for preventing leakage between a rotating member and a stationary member with an outer cylindrical surface of circular cross-section, said sealing member comprising an annular member and an annular engaging member, said support member being adapted to be sealingly mounted on said rotating member, with said annular engaging member being adapted to sealingly engage around the said outer cylindrical surface of the stationary member when said rotating member slows below a predetermined speed and is adapted to disengage from around the said outer cylindrical surface when the rotating member rotates faster than the predetermined speed, to provide a running clearance between the centrifugal sealing member and the stationary member.

Description

THIS invention relates to sealing means for the prevention of fluid leakage between the rotating and stationary members of centrifugal fluid pumps, turbines, agitators and the like.

Conventionally most centrifugal pumps prevent or control fluid leakage both during operation and while stationary, by the use of interfacial seals. These seals contain stationary and rotating sealing surfaces which are held in sliding contact with a controlled surface pressure to minimise fluid leakage past the sealing surfaces. Interfacial seals may be subdivided into two classes dependent upon the disposition of the sealing surfaces relative to the rotating shaft axis, viz axial seals and radial seals. Both types of Seal are widely used in centrifugal fluid pumps.

Axial seals encompass the broad class of mechanical end face seals in which the sealing surfaces are flat, rigid, and disposed normally to the shaft axis. These seals have been widely applied to centrifugal pump, turbine and agitator shaft sealing applications with-^-gas, liquid and slurries

as the pumped product, and have been developed in an extensive range of variant designs.

Radial seals define the class of shaft seals in 5 which the sealing contact surface are cylindrical and concentric with the pump shaft axis. The stuffing box is a commonly used seal of this type in which impermeable and compliant packing material is contained and pressurised in an annular cavity surrounding the shaft by an axially adjusted gland member. This type of seal is extensively applied in both liquid and slurry centrifugal pumps. Seals of this type belong to the class of radial seals.

The abrasive nature of pumped fluid in centrifugal slurry pumps commonly results in severe wear of shaft seal elements resulting in increased fluid leakage and maintenance requirements. Even with clean liquid pumps, sealing elements wear and require adjustment. These problems have resulted in the development of hydrodynamic shaft seals which are incorporated in many centrifugal slurrypumps. These invariably comprise auxiliary pumping vanes on the impeller which develop a hydrodynamic fluid pressure to eliminate leakage from the pump casing. During pump operation fluid leakage is eliminated or substantially reduced with this type of non contacting 6eal. Supplementary sealing means are required additional to the hydrodynamic seal to prevent leakage from the pump when stationary. Such static seals are commonly interfacial seals of the radial type. Although fluid leakage in operation is effectively prevented or controlled by the hydrodynamic seal, the continuous sliding contact at the static seal surfaces results in abrasive wear at these surfaces with resultant loss of static sealing performance.

The aim of this invention is to overcome by novel means a number of limitations associated with conventional centrifugal fluid pump seals and to improve current sealing technology, particularly in relation to, but not limited to, centrifugal slurry pumps (where abrasive wear is greatest), and in accordance with the following objectives:

AP 0 0 0 0 4 8 (a) to increase the effective operating life. of pump seals between maintenance overhauls;

(b) to reduce abrasive wear rate at the sealing surfaces;

(c) to provide a seal suitable for use under the most adverse fluid conditions, including heterogeneous fluids typical of coarse abrasive slurries;

(d) to permit substantial axial displacement between the stationary and rotating sealing surfaces; and (e) to provide a seal with automatic loading of sealing surfaces and requiring no manual adjustment of loading either during installation or in operation.

The present invention in one broad form comprises a centrifugal sealing member for preventing leakage between a rotating member and a stationary member with an outer cylindrical surface, said sealing member comprising an annular support member and an inner annular engaging member, said support member being adapted to be mounted on said rotating member, with said annular engaging member being adapted to sealingly engage around the said outer cylindrical surface of the stationary member when said rotating member slows below a predetermined speed and is adapted to disengage from around the said outer cylindrical surface when the rotating member rotates faster than the predetermined speed, to provide a running clearance between the centrifugal sealing member and the stationary member.

Preferably seals according to embodiments , of the present invention are used for preventing fluid leakage through the running clearance between stationary and rotating members of centrifugal pumps and the like, which incorporate hydrodynamic means for preventing such leakage until their speed *

of rotation decreases by some predetermined

AP 0 0 0 0 4 8 fraction of normal operating speed. The seal comprises a stationary member with an outer cylindrical surface substantially concentric with the axis of rotation of the rotating members and adapted to be engaged in sealing contact by a surrounding annular sealing member detachably mounted on an axially adjacent rotating member by fluid tight means. The mass and the composite modulus of radial elasticity of the annular sealing member is so proportioned that at a rotational speed greater than that at which leakage would cease to be prevented by the said hydrodynamic means the annular sealing member extends radially outwards due to centrifugal force sufficiently to provide a running clearance between the ₍rotating annular sealing member and the stationary cylindrical surface.

The invention will now be described by way of example with reference to the accompanying drawings, in which :

Figure 1 is a partial cross sectional view of a preferred embodiment of the present invention with the pump shaft stationary;

Figure 2 illustrates another embodiment . of the . present invention in partial cross section with the pump shaft stationary;

and

Figure 3 is partial cross sectional view taken on line I-I of Figure 2.

Like parts are illustrated by like characters throughout the specification and drawings.

Figure 1 shows one preferred embodiment of the invention in a simple form as applied to a centrifugal slurry pump. Pump impeller 1 is attached to shaft 2 which contains the pressurised fluid discharging from the periphery of rotating impeller 1. A secondary sealing impeller or expeller 3 is mounted on shaft 2 adjacent to impeller 1 and contained within a separate sealing chamber 4 formed by the external surface of casing

AP 0 0 0 0 4 8 member 5 and expeller chamber 6 which are clamped together in sealing contact. Expeller 3 contains multiple vanes 7 of substantially radial form attached to a substantially plane disc rotatably driven by shaft 2 and substantially concentric therewith. The annular sealing member 8 comprises an outer annular support member 9 and an integral engaging member 10, made of a suitable elastomeric material. A stiffening reinforcement ring 11 may be embedded in the sealing member. The sealing ring 8 engages in a fluid tight manner in the annular spigot 12 in the expeller 3.

The dimensions of the engaging means 10 and its modulus of elasticity are chosen such that at rotational speeds greater than that at which leakage would cease to be prevented by hydr^dynamic means (i.e. the rotation of the expeller 3), the engaging means 10 moves under the influence of the centrifugal force out of sealing engagement with the outer cylindrical sealing surface 13, to provide a running clearance between the rotating sealing member 8 and the cylindrical sealing surface 13.

The main elements of the hydrodynamic sealing means in the embodiment shown in Figure 1 comprise the vanes 7 of expeller 3 and auxiliary leakage vanes 14 of impeller 1 operating in concert with the adjacent surfaces of casing 5, expeller chamber 6, and stationary spoiler vanes 15.

The principal elements of the seal, formed by utilizing the sealing member of the present invention, of which one embodiment is shown in

Figure 1, comprise the annular sealing member 8, expeller 3, and stationary member 16.

Another embodiment of the sealing member is shewn 15 in Figures 2 and 3. With reference to Figure 2, the rotating sleeve 17 is mounted on shaft 2 adjacent and in driving contact with expeller 3.

Stationary member 16 is mounted with sealing . engagement in expeller chamber 6 with its outer 20 cylindrical sealing surface 13 concentric with the axis of shaft 2. The annular sealing member 8 is the principal component of the seal, and lemprises a helical tension spring 18 disposed in toroidal form with its rectilineal axis of symmetry colinear with the axis of shaft 2 and enclosed by and in intimate driving contact with an elastomeric sheath 19, which is termed the annular engaging member, separate multiple ballast weights 20 mounted with working radial clearance within the toroidal core Of spring 18, and a helical tension spring 2l disposed in toroidal form and moulded within an annular mounting bead of elastomer 22, which is termed the annular support member, concentric with shaft 2. The multiple ballast weights 20 are configured to provide maximum density of the toroidal core of spring 18. They are typically constructed in high density material, and in the embodiment shown in Figure 3 have a barrel shaped form with conical ends, and are packed with minimum radial and end clearance within the toroidal core of spring 18 to permit relative movement, between the spring coils and ballast weights 20. Spring 18 contains driving means 23 to rotatably drive ballast weights 20 with annular sealing member 8. In the embodiment shown in Figure 3 the driving means 23 is combined with a toroidal connector element for spring 18 which locally reduces the toroidal core diameter of spring 18 and provides a rotational driving surface for ballast weights 20. The mounting bead 22 is integral with elastomeric sheath 19 and displaces axially from helical tension spring 18. Mounting bead 22 engages with static sealing contact a matching grooved recess 24 in rotation sleeve member 17, which determines the axial and radial location of annular sealing member 8 relative to rotating shaft 2. Annular sealing member 8 is rotatably driven by multiple radially disposed projections 25 at its outer periphery which are integral with elastomeric sheath 19, and which engage with corresponding surfaces 26 of vanes 7 of expeller 3.

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At its inner radial extremity, and displaced axially remote from mounting bead 22, annular sealing member 8 incorporates a radial inward projection of elastomeric sheath 19 to form a continuous sealing lip 27 of limited axial width, concentric with shaft 2, and having an inner cylindrical sealing surface 28 in radial sealing contact with outer cylindrical surface 13 of stationary member 16. The geometry of seal lip 27 is configured to provide for substantial radial wear without detriment to sealing performance. Sealing lip 27 may typically comprise a harder grade of elastomer than the elastomeric sheath 19, to which it is integrally formed, to provide more favourable wear characteristics. With pump shaft 2 stationary the outer periphery of the annular sealing member 8 has a small radial clearance, with the inner radial extremity 29 of vanes 7 of expeller 3.

The principal elements of the seal, formed by utilising the sealing member of the present invention, of which one embodiment is shown in Figure 2, comprise the annular sealing member 8, rotating sleeve 17, expeller 3, and stationary member 16.

Operation of the pump seal will be described with reference to Figures 1, 2 and 3. Effective sealing against fluid leakage is required for the two conditions of stationary and rotating shaft systems. With shaft 2 stationary, and in the low speed range during pump start-up, sealing xs provided wholly by the seal of this invention. In this condition sealing chamber 4 is normally flooded with fluid, immersing the outer surface of annular sealing member 8 between seal lip 27 and the outer support member 9 in (Figure 1) or elastomer mounting bead 22 (in Figure 2). . Sealing Surface 13 of stationary sleeve member 16 and annular sealing member 8 are in sealing contact with a radial pressure predetermined to exclude fluid leakage past their common contact surfaces.

The inner surface area of annular sealing member 8 is normally exposed to the atmosphere and subject to ambient pressure. If the outer periphery of annular sealing member 8 is subjected to, a fluid pressure elevated above ambient, as may occur by supercharging of the pump inlet, the increased fluid pressure acting upon annular sealing member 8 causes increased contact pressures at seal lip 27 (in Figures 1 and 2), and at mounting bead 22 (in

Figure 2 only), thereby providing increased resistance to fluid leakage.

At normal pump operating speed, and in the upper speed range during pump start-up, sealing against fluid leakage between stationary and rotating members is provided wholly by hydrodynamic effects. The combination of expeller vanes 7 and auxiliary leakage vanes 14 of impeller 1 develops a centrifugal fluid pressure in excess of the impeller discharge pressure, hence fluid leakage from casing 5 via the sealing chamber 4 is prevented. In normal seal operation an equilibrium condition is established with fluid in sealing chamber 4 driven by expeller vanes 7 in an annular vortex at the periphery of the chamber, surrounding inner zones of air on both sides of expeller 3. Annular sealing member 8 occupies a fluid free region within the annular vortex, and leakage from sealing chamber 4 across sealing faces 13 and 28 is thus prevented by hydrodynamic fluid effects. At full operating pump speed annular sealing member 8 is in its fully radially expanded position, with its outer periphery in supporting contact with surfaces 29 of expeller 3 (for embodiment shown in Figure 2), with corresponding separation of sealing surfaces 13 and 28.

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In Figure 1, radial expansion of annular sealing lip 10 of member 8 is caused by centrifugal force acting on the mass of the lip. This radial expansion is resisted at low speeds by elastic circumferential tension in seal lip 10.

Γη the speed range approaching normal pump operating speeds the combined centrifugal loading predominates over the combined elastic circumferential tension, resulting in radial expansion of annular sealing member lip 27 with a corresponding radial separation of sealing surfaces 13 and 28. The critical rotational speed of annular sealing member 19 at which sealing surfaces 13 and 28 separate is an important seal design parameter, and is a function of the axial spacing relationship of seal lip 27, spring 18 and mounting bead 22; the masses of ballast weights 20, spring and elastomeric sheath 19; and the radial stiffness of spring 18, seal lip 10 stiffness and mass and elastomeric sheath 19. Typically these design parameters are configured to produce separation of sealing surfaces 13 and 28 at a rotational speed lower than the lowest limit of normal pump operating speeds.

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Sealing surfaces 13 and 28 are thus in sliding contact only during the brief starting and stopping process. During normal pump operation the surfaces are separated with consequent absence of abrasive wear. This is of particular relevance in centrifugal slurry pumps in which sliding surfaces are unavoidably contaminated by abrasive solid particles with resultant rapid abrasive wear. Abrasive wear at sealing surfaces j 3 and 28 is limited only to the acceleration and deceleration periods during starting and shut-down of the pump during which the pump speed is below the critical seal separation speed.

It is a common feature of centrifugal slurry pumps that axial adjustment of shaft 2 and associate/· rotating components is normally provided to imprcvt deteriorating pump performance resulting from increased impeller end clearances caused by abrasive wear. The present invention can tolerate large axial displacement of seal lip of the sealing member due to its cylindrical contact geometry and axial extent of the matching stationary sealing surface. Axial shaft adjustment is beneficial to

seal operation with this invention as the localised worn stationary surface adjacent to the surface of annular sealing member 16 is replaced with unworn surface by axial adjustment of the shaft.

AP 0 0 0 0 4 8

Claims

A centrifugal sealing member for preventing leakage between a rotating member and a stationary member with an outer cylindrical surface of circular cross-section, said sealing member comprising an annular support member and an annular engaging member, said support member being adapted to be sealingly mounted on said rotating member, with said annular engaging member being adapted to sealingly engage around the said outer cylindrical surface of the stationary member when said rotating member slows below a predetermined speed and is adapted to disengage from around the said outer cylindrical surface when the rotating me.mb· r rotates faster than the predetermined speed, to provide a running clearance between the centrifugal sealing member ano the stationary member.

A centrifugal sealing member according to claim 1 adapted to seal between stationary and rotating members winch incorporat· hydrodynamic means for preventing leakage bad ORIGINAL therebetween at normal operating speeds wherein the annular engaging member has its mass and its composite modulus of radial elasticity so proportioned that at a

5 rotational speed less than normal operating speed, the said annular engaging member extends radially outwards due to centrifugal force sufficiently to provide a running clearance between the rotating centrifugal

10 sealing member and the outer cylindrical surface of the stationary member.
3. A centrifugal sealing member according to claim 2 wherein the annular engaging member

15 has its mass and its composite modulus of radial elasticity so proportioned that at a rotational speed greater than that at which leakage would cease to be prevented by the said hydrodynamic means, the said annular

20 engaging member extends radially outwards due tc centrifugal force sufficiently to provide a running clearance between the rotating centrifugal sealing member and tnc out‘_c cylindrical surface of the stationary member.

AP 0 0 o 0
4 8

BAD ORIGINAL

A centrifugal sealing member according to anyone of the preceding claims wherein in use the running clearance between the centrifugal sealing member and the stationary member results from outward radial extension of the annular engaging member due to centrifugal force acting thereon, and such outward radial extension is limited by contact with opposing surrounding surfaces on the rotating member so as to provide a predetermined running clearance between the annular engaging member and the outer cyclindrical surface of the stationary member.

15 5. A centrifugal sealing member according to any one of the preceeding claims wherein said annular support member has embedded -.herein a reinforcing ring.

20 6. A centrifugal sealing member according to anyone of the preceeding claims wherein said annular support member and said annular engaging member project in sulstautialiy binsame axial direction.

bad ORIGINAL

7. A centrifugal sealing member according to anyone of claims 1 to 4 inclusive wherein the annular engaging member includes a helical tension spring disposed in substantially
5 toroidal form with its rectilineal axis of , symmetry substantially collinear with the axis of the outer cylindrical surface of the stationary member.

10 8. A centrifugal sealing member according to claim 7 wherein the helical tension spring encloses, within its toroidal core, multiple ballast masses adapted to be driven by and rotated with the said helical tension spring.
9. A centrifugal sealing member according to claim 7 or 8 wherein the helical tension spring is enclosed by and in intimate driving contact with an elastomeric sheath.
10. A centrifugal sealing member according to claim 9 wherein the enclosing elastomeric sheath is provided with a projecting lip, which in use, is adapted to make sealing

AP 0 0 0 0 4 8

BAD ORIGINAL 5 contact with the outer cylindrical surface of the stationary member, said projecting lip being configured for substantial radial wear.
11. A centrifugal sealing member according to claim 9 or 10 wherein the support means comprise an annular lateral extension of the enclosing elastomeric sheath terminating axially in a mounting bead moulded about a toroidal helical spring, said bead being adapted to detachably sealingly engage in a matching grooved recess in the said rotating member .

15
12,

A centrifugal sealing member according to claim 9, 10 or 11 wherein the enclosing elastomeric sheath is provided on its outer peripheral surface with projections adapted for driving engagement with the rotating member.
13. A centrifugal seal assembly - comprising a centrifugal sealing member according to o'11, or 12 when appended to claim 11, a stationary member and a rotating member, wherein the grooved recess has a contiguous converging conical surface to facilitate the radial expansion and engagement of the

5 mounting bead in the grooved recess during assembly of the seal.
14. A centrifugal seal assembly according to claim

13 wherein the internal diameter of the unworn

10 projecting lip is before extension less than the unworn diameter of the cylindrical surface of the stationary member on which it seals by such amount that sealing engagement is maintained by the elasticity of the annular
15 sealing member after significant wear of both the projecting lip and cylindrical surface hj.occurred.

15. A centrifugal seal assembly according to claim

20 13 and 14 wherein the cylindrical su.. face o f the stationary member has et contiguor converging conical surface to facilitate the radial expansion required for installation the annular sealing member in sealing

ΛΡ0 0 0 0 4 8

BAD ORIGINAL $ engagement uith the 11 a t i uC ?. a ; e - l i a d r i u . surface during assembly of tee s-: :-.1.

15. A pump including a centrifugal sealing me ator substantially as claimed in any one of claims 1 to 12.
17. A pump including a centrifugal assembly substantially as claimed in any one of claims 13 to 15.