WO1990010166A1

WO1990010166A1 - Seal for shaft

Info

Publication number: WO1990010166A1
Application number: PCT/GB1990/000325
Authority: WO
Inventors: Lawrence William Gatehouse; John David Hooley
Original assignee: Baker Hughes Ltd
Current assignee: Baker Hughes Ltd
Priority date: 1989-03-02
Filing date: 1990-03-02
Publication date: 1990-09-07
Anticipated expiration: 1991-09-02
Also published as: AU5192090A

Abstract

A seal for a rotatable shaft (1) projecting into an enclosure (4) comprises inter alia a first, non-rotating, axially movable, part (9) surrounding the shaft (1) and having a first sealing face (10) and a second part (11) fast with the shaft (1) and having a second sealing face (12). When the shaft is stationary excess pressure in the enclosure (4) causes axial movement of the first part (9) to effect a seal between the faces (10, 12), thereby preventing leakage of vapour from the enclosure (4). When the shaft is rotating a rotating impeller (13) in cooperation with a stationary stator (14) raises the pressure of liquid in a chamber (15) relative to that in the enclosure (4), causing a diaphragm (8) to flex and break the seal between the faces (10, 12), thus preventing wear on the sealing faces (10, 12) while still preventing vapour leakage.

Description

SEAL FOR SHAFT

BACKGROUND OF THE INVENTION

The present invention relates to a seal for a shaft of the type that projects through a bearing into an enclosure such as a reaction vessel. The seal is to prevent vapour from the enclosure reaching the atmosphere.

There is a labyrinth seal such as a helical grooved windback seal around the shaft. This seal provides a barrier to the vapour from the enclosure to the atmosphere so that diffusion of the vapour into the atmosphere is prevented. A windback seal will prevent that escape while the shaft is rotating.

However, when rotation ceases the vapour will reach the atmosphere, especially if the enclosure is under high pressure, so that it contaminates the atmosphere to detrimental effect.

SUMMARY OF THE INVENTION

In order at least to ameliorate this, we provide a sealing assembly for a rotatable shaft which passes into an enclosure. A two part seal is provided which seals the enclosure at least under certain conditions. The two part seal comprises a first non-rotating part which surrounds the shaft, which is axially movable under the influence of pressure and which has a first sealing face thereon, and a second part which rotates with the shaft and has a second sealing face which when contacting the first, substantially seals a potential leakage path between the first part and the shaft.

In one embodiment the axial movement of the first part of the seal depends on the pressure difference between a liquid in a chamber and the contents of the enclosure, so that an overpressure in the enclosure forces the sealing faces together. The chamber is defined by an impeller mounted on the shaft and a stationary stator, the impeller and stator cooperating to form means for raising the pressure of the liquid in the chamber with respect to the enclosure under a first predetermined condition, e.g. when the shaft rotates, so that the movement of the first part is controlled by a pressure sensitive member, e.g. a diaphragm, which is responsive to the pressure of the liquid in the chamber.

The first part of the seal may be a cylindrical member around the centre of the diaphragm which is a wall between the enclosure and the chamber and whose central part moves axially in response to the pressure difference between the chamber and the enclosure. In this case the first sealing face may be the end edges of the cylinder.

The second sealing face is preferably and conveniently on the same member which forms the impeller. An additional seal, e.g. a labyrinth seal, is preferably provided for sealing the enclosure from the shaft when there is a clearance between the sealing faces. This additional seal may be a helical groove windback seal. The function of this seal will be assisted by the overpressure of the liquid in the chamber during rotation.

This shaft is preferably a vertical shaft and may be a driving or driven shaft of any vertical machine, such as a pump, agitator, autoclave or similar.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the invention will now be described, by way of example, with reference to the accompanying Figure 1 which is a diagrammatic diametrical section through the embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Figure 1 shows a vertical rotatable shaft 1, a bearing assembly 2 for the shaft 1 below which is a volume

3. The shaft 1 enters an enclosure 4 e.g. a reactor vessel, the contents of which give off vapour. A liquid reservoir 5 is annular, surrounds the shaft 1 and is partially filled with liquid. This may be an oil having lubricant properties.

A helical grooved windback labyrinth seal ^"7 is provided on a sleeve 6 surrounding the shaft 1, the labyrinth seal being close fitting to the shaft 1. The labyrinth seal 7 helps to seal the volume 3, particularly during rotation of the shaft.

Between the reservoir 5 and the enclosure 4 is a flexible diaphragm 8 which surrounds the shaft. The central part of the diaphragm 8 adjacent the shaft is formed into an axially movable cylinder 9 whose further end face from the diaphragm is a first sealing face 10.

A member 11 is attached to the shaft 1, part of which has a second sealing face 12 which can contact the first sealing face 10 so as to seal the potential leakage path between the cylinder 9 and the shaft 1. Another part of the member 11 is an impeller 13 which, with a stationary stator member 14, defines a chamber 15 between the reservoir 5 and the sealing faces 10,12. On rotation of the impeller 13 with the shaft 1, the impeller 13 cooperates with the stator member 14 and liquid in the reservoir 5 is driven into the chamber 15, so raising the pressure of liquid therein.

The function of the seal thus described is as follows: when the shaft 1 is stationary, there is an overpressure in the enclosure 4 as compared with the reservoir 5 and chamber 15. The diaphragm flexes towards the chamber 15 and the cylinder 9 is pushed towards the member 11. The first sealing face 10 contacts the second sealing face 12 so as to seal the chamber 15 and reservoir 5 from the enclosure 4. The greater the overpressure in the enclosure 4, the harder the sealing faces are forced together.

When the shaft 1 rotates, pressure is developed in the chamber 15 by the impeller 13 and stator member 14. As this pressure is greater than that in the enclosure 4, the diaphragm flexes towards the enclosure 4 and the sealing faces 10,12 no longer contact. In fact there is a clearance of typically 0.05 cm (0.02 inches) between the faces 10,12. In this way, the shaft can rotate without wearing the sealing faces 10,12. The overpressure in the chamber 15 additionally prevents or slows the rate of passage of gas through the labyrinth seal 7. The present invention is particularly applicable to vertical pumps and agitator shafts.

Claims

CLAIMS :

1. A seal for a rotatable shaft (1) projecting into an enclosure (4), characterized by:

(i) a first non-rotating part (9) which surrounds the shaft (1), is axially movable relative thereto under the influence of pressure and has a first sealing face (10) thereon;

(ii) a second part (11) which rotates with the shaft (1) and has a second sealing face (12) which when contacting the first sealing face (10) substantially seals a potential leakage path between the first part (9) and the shaft (1); wherein in a first predetermined condition the pressure in the enclosure (4) is greater than the pressure influencing the first part (9) so that the first part is moved axially into sealing contact with the second part (11) and in a second predetermined condition the pressure influencing the first part (9) is greater than the pressure in the enclosure (4), so that the first part (9) moves axially to provide clearance between the first (10) and the second (12) sealing faces.

2. A seal according to Claim 1, wherein the means for exerting pressure on the first part is a liquid contained in a chamber (15) defined by an impeller (13) mounted on the shaft and a stationary stator (14), the impeller (13) and stator (14) cooperating to form means for raising the pressure of the liquid in the chamber (15) with respect to the pressure in the enclosure (4), so that in the second predetermined condition the liquid in the chamber (15) is forced towards a pressure-sensitive member (8) which controls the axial movement of the first part (9).

3. A seal according to Claim 2 wherein the second sealing face (12) is formed by a part of the impeller (13).

4. A seal according to Claim 2 or Claim 3 wherein the pressure-sensitive member (8) is a diaphragm between the enclosure (4) and the chamber (15), the central part of the diaphragm (8) moving axially in response to a pressure difference between the chamber (15) and the enclosure (4).

5. A seal according to any one of Claims 2 to 4, wherein the first part (9) is a cylindrical member around the centre of the pressure-sensitive member (8).

6. A seal according to Claim 5, wherein the first sealing face (10) is formed by one end of the cylindrical member.

7. A seal according to any preceding claim wherein the first predetermined condition is when the shaft (1) is stationary and the second predetermined condition is- hen the shaft (1) is rotating.

8. A seal according to any preceding claim wherein the liquid in the chamber (15) is a lubricating oil.

9. A sealing assembly (2) for a rotatable shaft (1) projecting into an enclosure (4), comprising as a first seal a seal according to any preceding claim.

10. A sealing assembly (2) according to Claim 9, further comprising a labyrinth seal (7) in series with the first seal.

11. A sealing assembly according to Claim 10, wherein the labyrinth seal (7) is a helical groove windback seal.

12. A method of sealing an enclosure (4) into which projects a rotatable shaft (1), characterized by use of a seal comprising:

13. A method according to Claim 12, wherein the enclosure (4) is a reaction vessel, the contents of which give off vapour, whereby leakage of the vapour from the vessel is substantially prevented in both the first and second predetermined conditions.