WO1989002555A1

WO1989002555A1 - A mechanical seal

Info

Publication number: WO1989002555A1
Application number: PCT/GB1988/000726
Authority: WO
Inventors: John Anthony Knight
Original assignee: Flexibox Limited
Priority date: 1987-09-11
Filing date: 1988-09-02
Publication date: 1989-03-23

Abstract

A mechanical seal (9) to seal a rotating shaft (7) that extends from a housing (3) which contains a product fluid to be retained within the housing (3), comprises a first seal (11) element coupled to the shaft (7), and a second seal element (13) coupled to the housing (3). The mechanical seal (9) is characterised by the provision of an elastomeric bellows (19) connected at one end to the second seal element (13) and at the other end to the housing (3), the bellows (19) having a radially disposed surface responsive to the pressure of the product fluid, whereby the closing force acting on the second seal element (13) is a combination of the spring action of the bellows (19) and the pressure of the product fluid acting on the surface. As a result of this arrangement the bellows (19) is responsive to changes in pressure in the product fluid to vary the area of the surface thereby to vary the closing force.

Description

A MECHANICAL SEAL

The invention relates to a mechanical seal to seal a rotating shaft that extends through a stationary housing which contains a fluid product.

International Patent Application No. PCT/G3 83/00093 in the name of the applicant relates to a mechanical seal of the type described above, and insofar as it relates to a conventional mechanical seal, discloses:

(a) A first annular seal element 40 secured to and rotating with a shaft 10,

(b) a second annular seal element 52, supported by a housing 12 for sliding movement in the axial direction of the shaft 10, and

(c) a metal spring 64 positioned to urge the seal element 52 into contact with the rotatable seal element 40 thereby to form a mechanical seal.

Conventional spring loaded mechanical seals of this type generally perform well when sealing clean fluids. However, they are often subject to problems when sealing liquids containing solids in suspension. By way of explanation, all mechanical seals allow the passage of product fluid to some degree. The leakage arises since the product fluid forms a film between the seal elements, and in time the product fluid flows to the atmosphere side of the mechanical seal. Such leakage usually is not serious and does not adversely affect the seal characteristics. However, when the product fluid contains solids in suspension the solids deposit on the atmosphere side of the seal elements and tend to jam up the sliding mechanism, restrict axial flexibility of the seal, and cause loss of contact of the seal elements. Typically, the result is heavy leakage and subsequent failure of the mechanical seal.

Australian Patent Application No. 61559/86 in the name of Borg-Warner Corporation describes an alternative mechanical seal which aims to overcome the above-described problem by replacing the metal spring with an annulus of rubber connected to the seal elements by adhesive and loaded in shear.

One disadvantage with the mechanical seal is that the stresses created can result in failure, a point that Application 61559/86 acknowledges and attempts to overcome. Moreover, it has been observed that using rubber in shear as a spring results in very high spring rates, with either excessive seal loading, or alternatively, a very limited ability to accommodate normal wear. Another disadvantage with the mechanical seal is that it is susceptible to failure at the joins between the seal elements and the rubber.

A further disadvantage of conventional spring loaded mechanical seals relates to the need to arrange the mechanisms of the seal such that the sealing members are kept in positive contact at all times regardless of pressures and other forces acting on the mechanical seal. By way of explanation, with all mechanical seals there are forces acting to bring the seal elements into close contact and forces acting to separate the seal elements. The forces acting to close the seal elements comprise the initial spring load and a hydraulic force resulting from the product fluid acting against exposed surfaces of the seal elements. The forces acting to open the seal elements comprise a hydraulic component resulting from the fluid between the seal elements such as the film of product fluid and lubrication. Obviously, for a mechanical seal to be stable and in practice workable the closing forces must always be greater than the opening forces.

The main disadvantage of the need to have the closing forces dominant arises at lower product pressures where the spring load component of the closing forces tends to force the hydrostatic lubrication from between the sealing elements thereby resulting in dry running and thus reduced life. Obviously, reducing the spring load assists to obviate this problem, but there are factors that limit the extent to which the spring load can be reduced. The factors include the practical need to allow for full take up at normal wear and the need to overcome O-ring drag.

Another approach is to reduce the area of the radially disposed surface of the seal elements exposed to product fluid but in practice a cut-off point, known as the balance point, is reached where the opening forces equal the closing forces. Any further reduction of the closing forces leads to an unstable situation where the opening forces are greater than the closing forces and the only thing holding the mechanical seal closed is the spring force. Once the opening forces are greater than the spring force the seal will in effect fail. Typically, this will occur with an increase in the pressure of the product fluid.

' There is therefore a need for a mechanical seal in which the hydraulic force component of the closing forces is relatively low at low pressures, leaving the spring to do the work, but which automatically increases at higher pressures to counter the corresponding increase in the opening forces. According to the present invention there is provided a mechanical seal to seal a rotating shaft that extends from a housing which .contains a product fluid to be retained within the housing, comprising:

(a) a first seal element coupled to the shaft,

(b) a second seal element coupled to the housing,

(c) and an elastomeric bellows connected aι one end to the second seal element and at the other end to the housing, the bellows having a radially disposed surface responsive to the pressure of thf product fluid, whereby the closing force acting on the second seal element is a combination of the spring action of the bellows and the pressure of the product fluid acting on the surface, and whereby the bellows is responsive to changes in pressure in the product fluid to vary the area of the surface thereby to vary the closing force.

The arrangement is such that at relatively low pressure the surface area responsive to the pressure of the product fluid is relatively low. AS a consequence the closing force is also relatively low. However, as the pressure of the product fluid increases the bellows expands radially inwardly to reduce the inner diameter of the bellows. The expansion of the bellows causes an increase in the surface area responsive to the pressure of the product fluid so that the closing force increases. The inner diameter of the bellows, the material from which the bellows is formed and the thickness are selected to allow a predetermined variation in inner diameter and surface area over a range of pressures of product fluid. It is preferred that the surface is annular.

It is also preferred that the mechanical seal further comprises . a support sleeve to limit the inward expansion of the bellows.

In a preferred arrangement the first seal element is connected to a sleeve which is mounted by an annular collar onto the shaft. The second seal element is connected to one end of the bellows by mechanical means such as press fits, clamp rings or adhesives. The other end of the bellows is clamped to the housing thus allowing reaction torques to be transmitted from the second seal element to the housing and thereby effectively keeping the second seal element from rotating whilst still allowing second seal element to axially and angularly travel to accommodate normal wear, floats and runouts.

A detailed description of a preferred embodiment of the present invention is now provided with reference to the accompanying drawings in which:-

Pig. 1 is a cross-section through a preferred embodiment of a mechanical seal in a closed position to seal a rotating shaft extending from a housing containing a product fluid; and

Fig. 2 is a section showing the effect of varying pressures on the u-shaped bellows shown in Fig. 1.

In Fig. 1 is shown a housing 3 which contains a volume 5 of product fluid under pressure, a rotating shaft 7 extending from the housing 3 and a mechanical seal generally identified by the numeral 9 positioned to prevent the escape of the product fluid through the generally annular gap 10 between the shaft 7 and the housing 3.

The mechanical seal 9 comprises two seal elements 11 and 13 having respective annular faces 15 and 17. in the position shown in the figure the two annular faces 15 and 17 are in contact thereby to define the seal.

The seal element 11 is connected to and rotates with the shaft 7 by means of a sleeve 18 which is mounted to the shaft 7 by a collar 20. The collar 20 comprises two halves (not shown) which act to press the sleeve 18 onto the shaft 7 to provide the necessary drive. The sleeve 18 is split at its end for this reason.

The seal element 13 is supported by an elastomeric bellows 19 which is generally o-shaρed in longitudinal half-section. The seal element 13 is positively fixed to the bellows at 21 so that a static seal is arranged at the interface and also so that frictional torque on seal element 13 effectively is transferred to the bellows 19. In the preferred arrangement a metallic support ring 20 is moulded to the bellows 19 so that the bellows 19 is circumferentially supported to allow radial compression of the elastomeric material between the support ring 20 and the seal element 13 thereby to effect a leak free joint. It is also envisaged that the leak free joint could be effected by the use of separate clamp rings or adhesive bonding.

The mechanical seal bellows 19 also comprises a flanged section 22 which allows the bellows 19 to be positively clamped to the housing 3 by a clamp ring 23 pulled into correct position by set screws 24.

Thus, the seal element 13 (hereinafter referred to as the stationary seal element) , is elastically fixed to the housing 3 and elastically urged against the seal element 11 (hereinafter referred to as the rotary seal element) by preloading the bellows 19 by bending the elastomer into the u-shaped configuration. The seal elements may be formed from any suitable materials selected in accordance with a number of parameters which include the speed of rotation of the shaft 7 and the nature of the product fluid 5. Suitable materials for the rotary and stationary seal elements 11 and 13 include sintered ceramics, alloy metals and carbon. Suitable materials for the bellows 19 include any natural and synthetic rubbers and other elastomeric materials such as polyurethane and these may if required be reinforced by fibrous or woven material.

Referring now to Fig. 2, the mechanical seal further comprises a support sleeve 25 which has its outside diameter selected to clear by a predetermined spacing the inside of the bellows at 26. The support sleeve 25 defines a limit stop for the bellows 19, as will be described hereinafter.

In use, as the bellows 19 is pressured by product fluid 5 within the housing 3 it will elastically deform and expand radially inwardly towards the support 25. At zero pressure there will be no inwards stretching effect but as the pressure of the product fluid 5 increases the bellows 19 will reduce in diameter at 26 effectively increasing its radially disposed area. As a consequence, the closing force acting against the stationary element 13 increases. Thus, it is possible, to select the closing forces per unit pressure of product fluid by reference to the nature of the elastomeric material and the thickness of the bellows 19.

As a consequence, the seal can be arranged for very low initial hydraulic closing forces without the risk of overbalancing occurring at higher pressures.

Claims

CLAIMS ;

1. A mechanical seal to seal a rotating shaft that extends from a housing which contains a product fluid to be retained within the housing, comprising:

(a) a first seal element coupled to the shaft,

(b) a second seal element coupled to the housing,

(c) and an elastomeric bellows connected at one end to the second seal element and at the other end to the housing, the bellows having a radially disposed surface responsive to the pressure of the product fluid, whereby the closing force acting on the second seal element is a combination of the spring action of the bellows and the pressure of the product fluid acting on the surface, and whereby the bellows is responsive to changes in pressure in the product fluid to vary the area of the surface thereby to vary the closing force.

2. A mechanical seal as claimed in claim 1, in which the radially disposed surface of the bellows is annular.

3. A mechanical seal as claimed in claim 1 or 2, comprising a support sleeve to limit the inward expansion of the bellows.

4. A mechanical seal as claimed in claim 3, in which the support sleeve is integral with the housing and lies radially inboard of the bellows.

5. A mechanical seal as claimed in claim 1, in which the first seal element is connected to a sleeve which is mounted by an annular collar onto the shaft. the second seal element is connected to one end of the bellows by mechanical means and the other end of the bellows is clamped to the housing thus allowing reaction torques to be transmitted from the second seal element to the housing and thereby effectively keeping the second seal element from rotating whilst still allowing second seal element to axially and angularly travel to accommodate normal wear, floats and runouts.

6. A mechanical seal as claimed in claim 5, in which the mechanical means connecting the second seal element to the bellows comprises a press fit therebetween, clamp ring means or adhesive.