GB2244104A

GB2244104A - Mechanical seals

Info

Publication number: GB2244104A
Application number: GB9109702A
Authority: GB
Inventors: Lange Jacobus Anthon Martin De
Original assignee: Flexibox Ltd
Current assignee: Flexibox Ltd
Priority date: 1990-05-15
Filing date: 1991-05-03
Publication date: 1991-11-20
Anticipated expiration: 2011-05-03
Also published as: GB9109702D0; GB9010845D0; NL9100831A; GB2244104B

Abstract

An outboard mechanical seal of a tandem shaft seal arrangement has a spring retention sleeve 18 surrounding the rotating sealing ring thereof. A spring plate 16 against which a loading spring 12 acts is adapted to abut the load retention sleeve when the mechanical seal is nearly worn. A barrier liquid vessel 23 in conjunction with a level alarm 27 and a pressure alarm 28 monitors the seal to determine when it is worn viz excess flow past the seal and, when the primary seal fails viz there will be a pressure rise in the barrier liquid. <IMAGE>

Description

SHAFT TANDEM SEAL ARRANGEMENTS This invention relates to shaft tandem seal arrangements.

Such arrangements generally comprises two mechanical seals, each including a stationary seal member and a rotatable seal member with the faces of the seal members in rubbing contact and with one of the seal members being resiliently loaded, usually by a mechanical spring or bellows, into contact with the other seal member.

This spring or bellows loading also serves to take-up or compensate for wear in the rubbing faces to prevent leakage.

In a tandem seal arrangement there is a main mechanical seal which performs the full sealing duty and a secondary or outboard mechanical seal which is non-active when the primary mechanical seal is functional and which takes over the full sealing duty if a massive failure of the main mechanical seal occurs.

However, the functional condition of the outboard seal is an unknown factor until the full sealing duty is imposed thereon. It may, for various reasons, be in a worn or failed condition or only be partially functional i.e. a severely although not totally leaking condition, at the time it is called upon to perform full sealing duty.

A consequence of such outboard seal inadequacy is usually at least damage to the pump or other machine incorporating the tandem seal arrangement.

It is an object of the present invention, therefore, to provide a shaft tandem seal arrangement wherein the operational condition of the outboard mechanical seal is signalled to an operator.

According to the present invention, therefore, the outboard mechanical seal of a shaft tandem seal arrangement is provided with a seal load retention device adapted to become effective when the outboard seal is nearly worn.

Preferably the seal load retention device is a sleeve surrounding the resiliently loaded seal member and adapted to be engaged by a spring plate against which the seal load acts while a sufficient axial movement of the seal member is available before failure occurs due to wear.

An embodiment of the present invention will now be described with reference to the accompanying drawing which is a fragmentary half longitudinal section of an outboard seal of a shaft tandem seal arrangement.

The outboard seal is a mechanical seal comprising a rotatable sealing ring 10 in face-to-face rubbing contact with a stationary sealing ring 11.

The rotatable sealing ring 10 is loaded by a mechanical spring 12 to pressure it into contact with the stationary sealing ring 11.

The spring 12 is supported in an annular spring carrier 13 which is drivingly connected to a shaft sleeve 14 which, in turn, is keyed to a rotatable shaft 15. The spring 12 abuts against an annular spring plate 16 which in turn abuts against the rotatable sealing ring 10.

The spring carrier 13 at its end nearer the stationary sealing ring 11 is fitted internally with an abutment ring 17 which locates between it and the annular spring plate 16 a spring retention sleeve 18 also rotatable with the shaft sleeve 14 and shaft 15.

O-ring seals 19 and 20 are provided between the stationary sealing ring 11 and a housing 21 and the rotatable rotating sealing ring 10 and the shaft sleeve 14.

A rotatable axial pumping ring indicated at 22 surrounds the spring carrier 13.

Failure of the outboard mechanical seal occurs when the spring pressure generated by the spring 12 is lost.

The spring retention device in the form of the sleeve 18 against which the annular spring plate 16 eventually abuts ensures that there is no sudden loss of spring pressure.

The spring retention sleeve 18 becomes operative when the interface between the rotatable and stationary sealing rings 10 and 11 is at position A and there is a reserve axial movement of this interface between position A and position B before it is necessary to change the outboard mechanical seal.

The sealing ring interface is at position A when the annular spring plate 16 abuts the retention sleeve 18. At this position A spring pressure begins to be lost and as a result the outboard mechanical seal will start slowly to leak.

Associated with the mechanical seal is a barrier liquid vessel indicated at 23 with barrier liquid flow to and from the mechanical seal being indicated by arrows C and D respectively, the axial pumping ring 22 ensuring flow between C and D.

The barrier liquid vessel 23 incorporates maximum and minimum levels sensors 24, 25 and is provided with a level gauge 26.

The sensors 24,25 are coupled to a microswitch 27 of a level alarm (not shown). Also the vessel 23 is fitted with a pressure alarm 28.

The vessel 23 has a filling plug 29 at its top and a drain 30 at its bottom.

A vent/flare arrangement 31 is provided at the top of the vessel 23.

Leakage from the outboard mechanical seal will initially be very gradual and can easily be ascertained either by monitoring the barrier liquid level in the customary barrier liquid vessel 23 via the level gauge 26 or by a signal from the level alarm 27 associated with the vessel 23.

When such situation is noted or signal is given the operator will know that the outboard mechanical seal has passed its useful life and has to be changed.

The advantage of this invention is that the operator is signalled by the slow leakage in the outboard mechanical seal when the latter is still perfectly safe and operational.

If, just at this moment, the main mechanical seal fails completely, the pressure on the outboard mechanical seal will rise and will provide sufficient axial force on the outboard mechanical seal to close the gap due to wear and stop leakage. In this situation a pressure increase on the barrier liquid vessel 23 will be indicated to the operator by the pressure alarm 28 that the main mechanical seal is failing.

As a consequence of the present invention an operator is given a warning in time when there is doubt about the operational integrity of the tandem seal arrangement.

It is to be noted that if the barrier liquid vessel 23 is still under normal pressure and the outboard mechanical seal is performing its sealing function an operator not only knows that the tandem seal arrangement is working properly but can also be sure that the outboard mechanical seal is still in a condition to take over the duty of the main mechanical seal if required.

Claims

CLAIMS:

1. An outboard mechanical seal for a shaft tandem seal arrangement comprising, in operative association, with a resilientlyloaded seal member of the mechanical seal, a seal load retention device adapted to become effective when the outboard seal is nearly worn.

2. An outboard mechanical seal as claimed in claim 1, in which the resiliently-loaded seal member is a rotatable sealing ring adapted to be rotatably fast with a rotating shaft or sleeve surrounding and rotatable with the latter and in face-to-face contact with a stationary sealing ring mounted in a housing traversed by the rotating shaft.

3. An outboard mechanical seal as claimed in claim 2, in which the rotating sealing ring is surrounded by a sleeve against which a resiliently-loaded spring plate is adapted to abut while a sufficient axial movement of the rotatable sealing ring is available before failure of the mechanical seal occurs due to wear.

4. An outboard mechanical seal as claimed in claim 3, in which the rotatable sealing ring is acted upon by an axially-directed mechanical spring or bellows disposed between an annular carrier and the spring plate which abuts the end of the rotating sealing ring remote from the face contact with the stationary sealing ring.

5. An outboard mechanical seal as claimed in claim 4, in which the load-retention sleeve is disposed within the annular carrier with its end remote from the spring or bellows butting against an abutment ring provided in the annular carrier.

6. An outboard mechanical seal as claimed in claim 4 or 5, comprising an axial pumping ring rotatably fast with the annular carrier for pumping a barrier fluid through the mechanical seal.

7. An outboard mechanical seal as claimed in any one of claims 3 to 5, arranged to provide a predetermined axial movement of the sealing rings interface after abutment of the spring plate with the load-retention sleeve and before the mechanical seal requires replacement consequent upon wear of the sealing rings.

8. An outboard mechanical seal as claimed in claim 6 or 7 comprising in flow and return communication with the axial pumping ring a barrier liquid vessel containing maximum/minimum level sensors and/or a level gauge whereby an operator can monitor the operational condition of the mechanical seal.

9. An outboard mechanical seal, substantially as hereinbefore described with reference to the accompanying drawing.

10. A shaft tandem seal arrangement including an outboard mechanical seal as claimed in any one of claims 1 to 9.