GB2140102A

GB2140102A - Improvements in shaft seals

Info

Publication number: GB2140102A
Application number: GB08312613A
Authority: GB
Inventors: Leslie John Budgen; David Alan Turrell
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1983-05-06
Filing date: 1983-05-06
Publication date: 1984-11-21
Also published as: FR2545570A1; JPS6040869A; DE3416562A1; IT8420808A0; FR2545570B1; IT1173954B; IT8420808A1; DE3416562C2; GB8312613D0

Abstract

A rotor shaft seal includes at least two sealing elements 10, 12 which make contact with the shaft 2 at two spaced regions and which define a chamber 8 for collecting fluid leaking past one or both seals, the leaked fluid leaving the chamber through an outlet 18. The chamber 8 may be scavenged by a gas introduced through a pipe 22. <IMAGE>

Description

SPECIFICATION Improvements in shaft seals This invention relates to shaft seals; that is to say, seals which are associated with a rotatable shaft passing through the wall of a fluid retaining housing and which are effective to prevent the loss of fluid from the housing.

The invention is particularly though not exclusively concerned with such seals associated with rotor drive shafts which pass through the housing of mechanical vacuum pumps containing a relatively costly pumping fluid.

The use of seals associated with rotating shafts passing through a housing wall is extremely well known and the range of designs available is extensive. It is believed that any reference to such existing seal designs is superfluous since they cover an extremely wide field of application. The compiexity of such known shaft seals does, however, vary with the field of application; in general, the complexity of such seals increases as the need to prevent leakage of fluid at the shaft/housing interface progressively becomes more important and seals providing sliding contact with a shaft at a multiplicity of regions have been proposed for cases where there is a particular need to prevent leakage or where the fluid is at a pressure substantially above atmospheric.

All such known seals do, however, suffer from the disadvantage that in the event of wear or failure or otherwise, leakage of fluid from the housing will occur. In, for example, the case of specialist mechanical vacuum pumps containing inert stable and chemically complex pumping fluids, any leakage and consequential topping-up can involve substantial expense.

It is accordingly one object of the present invention to provide a shaft sealing arrangement in which the disadvantages arising from the potential loss of fluids can be reduced.

The present invention according to its broadest aspect provides a sealing arrangement for a rotatable shaft passing through the wall of a fluid-retaining housing, the seal comprising at least two sealing elements making sealing contact with the shaft at two spaced regions and adapted to define a fluid collecting chamber between the regions and means for transferring from the chamber any fluid leaked from the housing.

In its most elemental form the transfer means may comprise any permanently open or sealable outlet provided at the base of the fluid-collecting chamber. In this way, any fluid leaked from the housing will be gravitationally discharged from the collecting chamber outlet for retention in a suitable vessel before being returned to the housing. Leakage loss is accordingly compensated without the need to provide fresh and potentially costly fluid.

In a preferred embodiment of the invention, however, the transfer means include a pipe connecting the base of the collecting chamber with the fluid-containing housing. In this case, transfer of leaked fluid from the collecting chamber and into the housing suitably is assisted by a flow of gas applied to an inlet provided in the collecting chamber, conveniently at its upper end.

Where the shaft seal of the invention is associated with the rotor drive shaft of a mechanical pump of any known kind, the gas flowing through the collecting chamber may form the whole or a part of the gas ballast for the pump. The gas may be dry nitrogen or any other suitable gas and conveniently is supplied to the collecting chamber inlet at a pressure lying within the range 0.1 to 0.2 bar gauge.

Ideally, means are provided on the shaft surface to inhibit the flow of leaked fluid to the axially outermost seal element and to encourage the discharge of fluid from the shaft surface into the collecting chamber; such means may comprise an O-ring secured around the shaft or a groove machined or otherwise provided in the shaft surface intermediate the seal elements.

An embodiment of the invention will now be particularly described by way of example with reference to the accompanying drawing which is a sectional side view of the driven end of a two-stage mechanical pump of known kind.

Referring to the drawing, the driving shaft 2 of the pump rotor, connectable to an electric motor prime mover (not shown) is illustrated passing through the end wall 4 of the pump housing which contains the pumping fluid such as per fluoropolyether.

The construction of such a mechanical pump whether rotary vane or that conventionally known as a Roots pump or other kind is sufficiently well known not to warrant detailed description. A typical two-stage rotary vane pump with ballasting is, for example, disclosed in United Kingdom Patent Specification No. 1 282 197 (Balzer); a more detailed description is immaterial to the present invention and is accordingly not provided.

Secured to the outer surface of end wall 4 is a part-closed cylinder 6 through which the shaft 2 also passes and which is adapted to form a collecting chamber indicated generally at 8 for receiving pumping fluid leaked from the pump housing.

Suitably secured to the inner surface of cylinder 6 are two spaced resilient seal elements 10, 1 2 of known material such as Teflon (RTM) and of a type well known in the art. Elements 10, 1 2 are arranged to make sliding contact with the shaft surface, with element 10 predominantly being effective to prevent leakage of fluid from housing 4 to chamber 8 and with element 1 2 being effective to prevent ultimate leakage of fluid from collecting chamber 8.

An O-ring 14 secured tightly about and rotatable with the shaft 2 and/or a groove 1 6 machined into the shaft surface are effective to intercept and to prevent fluid leaking past element 10 from reaching element 1 2. The 0ring and/or the groove are, in addition, effective with the shaft rotating to assist to throw intercepted fluid off the shaft and into the chamber 8 by the action of centrifugal force.

Provided in the base of cylinder 6 is an outlet 1 8 enabling leaked fluid collecting in chamber 8 to be retained for return to the pump. In the preferred embodiment described, however, an inlet at the upper end of cylinder 6 is adapted by way of pipe 20 to supply gas at positive pressure into chamber 8; this gas flow will leave the chamber 8 by way of outlet 1 8 and is returned to the pump housing through pipe 22. The gas will act as a vehicle for scavenging the chamber 8 of leaked fluid and for returning to the pump housing any leaked fluid collected in the chamber.

The gas which in this embodiment is dry nitrogen is supplied to the pump through the gas ballast inlet (also not shown) at a positive pressure lying within the range of 0.1 to 0.2 bar gauge. The actual gas pressure and flow rate are selected according to the required pump operating parameters. It will be appreciated that the use of the invention provides mechanically simple and inexpensive means for reducing irrevocable loss of fluid and avoiding the need for shaft seals having complex and elaborate sealing arrangements.

It will also be appreciated that while the invention has been described with reference to a mechanical vacuum pump, it is equally applicable to other apparatus such as engines, turbines and the like involving a rotatable shaft passing through a housing wall which retains a lubricating or other fluid. Gases other than dry nitrogen whether applied at positive pressure or under suction also fall within the scope of the present invention.

Claims

1. A sealing arrangement for a rotatable shaft passing through the wall of a fluidretaining housing, the seal comprising at least two sealing elements making sealing contact with the shaft at two spaced regions and adapted to define a fluid collecting chamber between the regions and means for transferring from the chamber any fluid leaked from the housing.

2. A sealing arrangement as claimed in claim 1, wherein the fluid collecting chamber is formed by a chamber outer wall circumscribing the shaft together with the sealing elements, the radially outer ends of the elements being secured to the chamber inner surface.

3. A sealing arrangement as claimed in claim 2, wherein one end of the chamber wall is secured to the fluid retaining housing.

4. A sealing arrangement as claimed in claim 2, or claim 3 wherein the means for transferring fluid from the chamber comprises an outlet provided at the base of the chamber wall.

5. A sealing arrangement as claimed in claim 4, wherein a conduit is arranged to return fluid from the outlet into the fluid retaining housing.

6. A sealing arrangement as claimed in any preceding claim, wherein an inlet provided in the chamber wall is adapted to receive a flow of gas effective to scavenge the collecting chamber of leaked fluid.

7. A sealing arrangement as claimed in any preceeding claim wherein means are provided on the shaft surface to inhibit the flow of leaked fluid to the outermost sealing element.

8. A sealing arrangement as claimed in claim 7, wherein the means are provided intermediate the sealing elements.

9. A sealing arrangement as claimed in claim 7 or claim 8, wherein the means comprise a groove in the shaft surface.

10. A sealing arrangement as claimed in claim 7 or claim 8 wherein the means comprise an 0 ring secured about the shaft.

11. A sealing arrangement as claimed in any one of claims 6 to 10, wherein the shaft is the drive shaft of a mechanical vacuum pump and gas applied to the collecting chamber inlet to form at least a part of the gas ballast for the pump.

1 2. A sealing arrangement substantially as herein described with reference to the accompanying drawings.

1 3. A sealing arrangement substantially as shown in and adapted to generate substantially as herein described with reference to the accompanying drawings.

14. A mechanical pump embodying a sealing arrangement comprising spaced sealing elements defining a fluid collecting chamber, and as claimed in any preceding claim.