Tandem seal

Details Tandem seal Tandem seal

2001-11-14

2004-02-03

Knight, Anthony (Department: 3676)

Seal for a joint or juncture

Seal between relatively movable parts

Relatively rotatable radially extending sealing face member

C277S358000, C277S361000, C277S399000

Reexamination Certificate

active

06685192

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a tandem seal for sealing a shaft by means of two mechanical seals arranged in a tandem manner in an axial direction, which is used to seal a vaporizable liquid, particularly a liquefied gas, as an object to be sealed.
PRIOR ART
A tandem seal is a shaft sealing device in which two mechanical seals are arranged in a tandem manner in an axial direction. The tandem seal is preferably used with a liquefied gas, for instance, as a sealed object. In the tandem seal, even if large leakage occurs from a mechanical seal at a side of a primary mechanical seal, namely a side of the sealed liquid, a secondary mechanical seal, namely a side of an atmosphere, will be backup. When providing a collection mechanism in a center room between both mechanical seals for collecting the sealed liquid leaked from the primary mechanical seal to the center room, leakage to the side of the atmosphere can be securely prevented.
In the tandem seal, a sealed slide surface of the secondary mechanical seal, which comprises sealing elements at a rotational side and sealing elements at a stationary side, needs to be protected by liquid lubrication. Thereby a center liquid for lubrication is interposed in the center room that is arranged between both mechanical seals. Further, in order to eliminate slide heat generated at a sealed slide surface of the secondary mechanical seal, generally the center liquid is circulated via the outer cooler in a system.
FIG. 3
is a typical prior art showing a tandem seal used as a means for sealing the rotating shaft of the pump for a liquefied gas. Reference numeral
100
designates a rotating shaft of a pump. Numeral
110
designates a housing of a shaft sealing part so assembled that it surrounds an outer circumference of the rotating shaft
100
by a plurality of housing members
111
~
113
at a shaft sealing position. A shaft center of the rotating shaft
100
and the housing
110
extend almost horizontally. The left side in the figure means the inside of a machine. In the machine, a sealed liquid SL (a liquefied gas) exists in a pump room. The right side in the figure means the outside of the machine, namely, the side of the atmosphere.
In a space around the shaft between the housing
110
of the shaft sealing part and the rotating shaft
100
, mechanical seals
120
,
130
are provided in the axial direction. The seal
120
is a primary mechanical seal, which is provided to the inside of the machine. The seal
130
is a secondary mechanical seal, which is provided to the outside of the machine.
The primary mechanical seal
120
comprises a non-rotating fixed ring
121
and a rotating ring
123
. The fixed ring
121
is supported via an O-ring
122
on the inner circumferential part of the housing member
112
of the housing
110
. The rotating ring
123
rotates together with the rotating shaft
100
. The rotating ring
123
is provided in a state that it is movable in the axial direction and is sealed with a sleeve
101
of the rotating shaft
100
via a retainer
124
and an O-ring
125
. Further, the rotating ring
123
is closely contacted with the fixed ring
121
by an enforcement of springs
126
in the axial direction, which functions seal ability of the shaft on a sealed slide surface
120
S.
Basically, the secondary mechanical seal
130
has the similar structure of the primary mechanical seal
120
. That is, at the inner circumferential part of the housing member
113
of the shaft sealing part
110
, it comprises a non-rotating fixed ring
131
and a rotating ring
133
. The fixed ring
131
is supported via an O-ring
132
. The rotating ring
133
rotates together with the rotating shaft
100
. The rotating ring
133
is provided in a state that it is sealed and is movable in the axial direction via a pumping ring
134
and an O-ring
135
on the sleeve
101
of the rotating shaft
100
. Further, the rotating ring
133
is closely contacted with the fixed ring
131
by enforcement in the axial direction of springs
136
. This functions seal ability of the shaft on a sealed slide surface
130
S.
The primary mechanical seal
120
is provided for the liquefied gas as the sealed object in the pump room in the machine. One part of the sealed liquid SL is sent to an outer circumferential room space A of the primary mechanical seal
120
. Then it flows back to the pump room via a flushing discharge hole F
OUT
. Here the flushing discharge hole F
OUT
opens at the housing member
112
of the shaft sealing part housing
110
. This functions as flushing. Thereby the sealed slide surface
120
S of the primary mechanical seal
120
is lubricated and cooled.
Further, a center liquid CL lies in a center room B from the inner circumferential space of the sealed slide surface
120
S of the primary mechanical seal
120
to the outer circumferential room of the sealed slide surface
130
S of the secondary mechanical seal
130
. The center liquid CL is flown back through a reservoir tank (not shown in the figure) installed outside, by axial flow pump action of the helical groove
134
a
of the pumping ring
134
, via a circulation injection hole CL
IN
and a circulation discharge hole CL
OUT
. The pumping ring
134
is provided on the secondary mechanical seal
130
. The circulation injection hole CL
IN
is opened at the housing member
112
of the shaft sealing part housing
110
. The circulation discharge hole CL
OUT
is opened at the housing member
113
. This circulation lubricates and cools the sealed slide surface
130
S of the secondary mechanical seal
130
. Therefore, the center liquid CL is usually filled in the flow line containing the center room B, the reservoir tank, the circulation injection hole CL
IN
, and the circulation discharge hole CL
OUT
.
In the tandem seal described above according to the prior art, the pumping ring
134
circulating the center liquid CL is arranged in the narrow center room B of the inner circumference of the shaft sealing part housing
110
. Thus, there is a difficulty to raise pumping efficiency due to the size or limitation of the installing space of the sealing elements.
Further, the sealed liquid SL of the liquefied gas in the machine is pressed to a certain pressure, while the pressure of the center liquid CL in the center room B is low. Then, the sealed liquid SL leaked from the primary mechanical seal
120
to the center room B vaporizes in the center room B. When the bubble thereof in the center liquid CL increases, pumping efficiency by the pumping ring
134
decreases extremely. Moreover, near the rotating pumping ring
134
, by the cavitation phenomena generated in the center liquid CL, the pumping ring
134
vibrates, and then this vibration is transmitted to the rotating ring
133
of the secondary mechanical seal
130
. Thus, it has bad influence for sealing ability of the secondary mechanical seal
130
.
Moreover, the bubble mixed in the center liquid CL by vaporization of the sealed liquid SL leaked from the primary mechanical seal
120
to the center room B sometimes forms gas zone as a result of gathering at the inner circumferential side of the center room B. This is because the center liquid CL in the center room B circulates and flows in the same direction as the rotation of the rotating shaft
100
. This is caused by contact with the sealing elements at the rotational side of the secondary mechanical seal
130
. Thus, effect of centrifugal separation by specific gravity occurs in the center liquid CL and the bubble of the leaked gas. Accordingly, the sealed slide surface
130
S of the secondary mechanical seal
130
becomes a dry-slide state, namely a slide state without liquid lubrication by the gas zone. This will cause unusual wear or damage by overheats and will rapidly lower sealing function.
SUMMARY OF THE INVENTION
The object of the invention is to eliminate bad affects or influence for sealing function caused from a vaporized gas of the sealed liquid that is leaked from a primary mechanical seal into an intermediate chamber or center room formed between t

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