Shaft seal device

Details Shaft seal device Shaft seal device

2001-02-09

2002-09-24

Knight, Anthony (Department: 3676)

Seal for a joint or juncture

Seal between relatively movable parts

Relatively rotatable radially extending sealing face member

C277S400000

Reexamination Certificate

active

06454268

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a shaft seal device to be used in for example a pump or other fluid machine, more particularly relates to a shaft seal device suitable when the sealed fluid is a fluid containing slurry or a high viscosity fluid etc.
2. Description of the Related Art
In a pump or other fluid machine, a shaft is provided rotatably in a casing. A shaft seal device is provided to prevent the fluid in the casing from leaking outside from the clearance between the shaft and the casing.
As a shaft seal device used in such a fluid machine, a mechanical seal is known. Sealing the clearance between a rotating shaft and a casing holding the shaft rotatively requires that a rotating seal ring rotating together with the shaft and a non-rotating seal ring fixed to the casing be brought into contact at the seal surfaces. At the seal surfaces, the wear between the rotating seal ring and the non-rotating seal ring becomes a problem. Therefore, recently, as shown in for example U.S. Pat. Nos. 5421593 and 4290611 and German Patent Publication DE3834214A1, a non-contact type mechanical seal provided with hydrodynamic grooves at the seal surface of the rotating seal ring and preventing the seal surfaces from contact together with rotation of the shaft has been proposed.
As the shape of the hydrodynamic grooves of the non-contact type mechanical seal, there are known spiral shaped ones as shown in U.S. Pat. No. 5071141 and T-shaped ones as shown in U.S. Pat. No. 5,092,612.
However, the dynamic pressure generating grooves of the non-contact type mechanical seal of the related art are formed at the outer diameter side of the rotating seal ring. Therefore, when using this non-contact type mechanical seal as a seal structure between the sealed fluid region and the purge gas region, the sealed fluid easily penetrates into the dynamic pressure generating grooves, so th at various problems occur to the seal surface induced by the sealed fluid. In particular, when the sealed fluid was a slurry or high viscosity liquid, there was the problem that the seal easily became incomplete due to wear at the seal surface.
Note that, as shown in U.S. Pat. No. 5,092,612, a non-contact type mechanical seal where hydrodynamic grooves are formed not only at the outer diameter side of the rotating seal ring, but also the inner diameter side is itself known. In the structure combining a plurality of mechanical seals into a system through purge region, using a mechanical seal formed with hydrodynamic grooves at only the inner diameter side of the rotating seal ring as the seal structure between the sealed fluid region and the purge gas region, however, was first proposed by the present inventors.
Further, U.S. Patent No. 5,421,593 proposes a seal structure using a contact type mechanical seal as the seal structure between the sealed fluid region and the purge gas region and making the pressure of the purge gas region lower than the pressure of the sealed fluid region. When using a contact type mechanical seal as the seal structure between the sealed fluid region and the purge gas region, however, the partial wear at the seal surfaces becomes severe and the durability becomes inferior. In particular, when slurry fluid is to be sealed, the wear at the seal surface becomes a problem. The sealed fluid leaks to the purge gas region having a low pressure and as a result may leak outside the casing. When the sealed fluid is a corrosive liquid, the seal components arranged in the purge gas region may be corroded and the durability of the seal device remarkably reduced.
In the seal structure described in this specification (U.S. Pat. No. 5,421,593), by just replacing the contact type mechanical seal with a conventional non-contact type mechanical seal, the pressure of the sealed fluid act as a back pressure onto the seal ring directly. Further, the sealed fluid penetrates into the hydrodynamic grooves formed at the outer diameter side of the seal ring, so that it induces the problem that the sealed fluid enters into the purge region having low pressure.
SUMMARY OF THE INVENTION
The present invention was made in consideration of this situation and has as its object to provide a shaft seal device with little wear at the seal surface, superior in durability, resistant to penetration of the sealed fluid into the purge region, and superior in sealability.
To achieve the above object, according to the present invention, there is provided a shaft seal device comprising:
a casing having a hollow portion,
a rotatable shaft extending inside the casing from a sealed fluid region to an outer region,
a non-contact type first mechanical seal arranged between the casing and the shaft so as to divide the hollow portion into the sealed fluid region and a purge region located between the sealed fluid region and the outer region, and
a second mechanical seal arranged between the casing and the shaft so as to divide the hollow portion into the purge region and the outer region, wherein
the first mechanical seal comprises a first rotating seal ring having a first rotating seal surface rotating along with the shaft and a first non-rotating seal ring held by the casing and having a first non-rotating seal surface able to be pushed axially and slidable against the first rotating seal surface,
a plurality of first hydrodynamic grooves communicating with the purge region formed at the first rotating seal surface or the first non-rotating seal surface so as to extend from an inner circumferential edge thereof to an outer circumferential edge direction, and
a purge fluid higher in pressure than the pressure of the sealed fluid fed into the purge region.
In the shaft seal device according to the present invention, it is possible to effectively prevent the entry of the sealed fluid into the purge region by the co-action between the purge fluid having a higher pressure than the pressure of the sealed fluid and the plurality of the first hydrodynamic grooves communicating with only the inner circumferential edge in the seal surface. That is, a sliding surface clearance is formed between the first rotating seal ring and the first non-rotating seal ring of the first mechanical seal by the action of the first hydrodynamic grooves. The first hydrodynamic grooves communicate only with the inner circumferential edge in the seal surface, so the purge fluid in the purge region is drawn into the first hydrodynamic grooves by the centrifugal force. Therefore, a barrier layer formed by the purge fluid is formed in the sliding surface clearance and it is possible to effectively prevent the sealed fluid from entering the purge region from the outer diameter side of the first mechanical seal.
Further, in the present invention, the first mechanical seal is non-contact type seal structure, so there is little wear of the seal ring and the durability is excellent. Further, due to an action of purge fluid spouting into the sealed fluid region etc., even if the sealed fluid is a slurry etc., an effective seal becomes possible.
Preferably, the pressure of the purge fluid in the purge region acts as a back pressure into a back surface of the first non-rotating seal ring located at an opposite side of the first non-rotating seal surface, and the first hydrodynamic grooves communicate with the purge region through an inner diameter hollow portion located at an inner circumferential side of the first non-rotating seal ring.
Because a back pressure of the purge fluid acts into a back surface of the first non-rotating seal ring, the first non-rotating seal surface of the first non-rotating seal ring is pushed axially against the first rotating seal surface of the first rotating seal ring. As a result, when a shaft doesn't rotate, sealability between the seal surfaces improves.
Further, purge fluid acts into the inner circumferential surface of the first seal ring, so it is possible to make a configuration that easily introduces the purge fluid into the first hydrodynamic grooves.
Prefarably, the first rotating seal

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