Seal for a joint or juncture – Process of dynamic sealing – Peripheral radially sealing flexible projection
Reexamination Certificate
2000-08-21
2003-04-15
Knight, Anthony (Department: 3676)
Seal for a joint or juncture
Process of dynamic sealing
Peripheral radially sealing flexible projection
C277S311000, C277S471000, C277S553000, C277S932000
Reexamination Certificate
active
06547250
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to a seal assembly and is more particularly directed to a seal assembly with two separate sealing mechanisms that cooperate with one another to provide static and dynamic sealing.
BACKGROUND OF THE INVENTION
A seal assembly for providing sealing between a movable cylindrical body and a housing body comprising a bore requires both static and dynamic sealing mechanisms. The seal assembly is normally associated with either the movable body or the housing body. That is, the seal assembly is normally in a fixed relationship with one of the bodies and provides a static seal with that body. A dynamic seal is normally provided against the other body, which is movable relative to the seal assembly.
Such conventional seal assemblies are employed for example, between reciprocating or rotating shafts disposed within a bore provided in a housing, or between a reciprocating piston and a cylinder. Conventional seal assemblies are disposed in an annular space between the movable body and the housing body and typically employ the same sealing mechanism for providing both static and dynamic sealing.
U-shaped spring-energized shaft seals are well known as disclosed, for example, in U.S. Pat. Nos. 5,163,692, 5,799,953, 5,979,904, 5,984,316 and 5,992,856.
The '692 Patent, the '953 Patent, and the '904 Patent all disclose U-shaped spring-energized seals that rely upon a spring disposed between two flanges to bias the flanges from one another to enable both static and dynamic sealing. The '316 Patent discloses a seal assembly with a spring that energizes a dynamic seal and a static seal that relies upon an interference fit of a flange between the housing and a metal band. The '856 Patent discloses a seal assembly with independent static and dynamic sealing mechanisms, but both sealing mechanisms are spring-energized.
A particularly difficult application for such seal assemblies is cryogenic applications. A seal assembly employed in a cryogenic apparatus is subjected to an operating environment that is distinct from the operating environment of a non-cryogenic apparatus. A cryogenic operating environment is unique in many ways, including, for example, the effects of thermal contraction, the distinctive physical properties of cryogenic liquids, such as their high compressibility and volatility, and the effect of low temperatures on material properties and sealing capabilities. “Low” temperatures in the context of cryogenic applications are defined herein as temperatures below 190 degrees Kelvin where fluids such as nitrogen, oxygen, argon, methane, hydrogen and natural gas are in the liquid state.
At such low temperatures, fluoropolymers such as, for example polytetrafluoroethylene or polychlorotrifluoroethylene, which are typical materials used for the U-shaped body of conventional seal assemblies, shrink more than the typical metallic materials employed for housings and shafts, thereby often resulting in a tight seal against the shaft or piston and leakage problems between the seal and the housing bore or cylinder.
In cryogenic apparatus such as pumps, the seal assembly may also be subjected to very high differential pressures. For example, for a seal between the housing and the shaft of a cryogenic reciprocating pump, the differential pressure acting on the seal assembly may be higher than 5000 psi (34 MPa).
U.S. Pat. No. 5,996,472 discloses a cryogenic reciprocating pump that employs a U-shaped spring-energized seal assembly for sealing between a piston and the piston cylinder. The seal assembly is employed in combination with a plurality of separate piston rings that have underlying expander rings to press the piston rings against the interior surface of the cylinder. The U-shaped spring-energized seal, like other conventional spring-energized seals, relies upon a spring disposed between two flanges to press the seal against the interior surface of the housing. As temperature decreases and the effects of differential thermal expansion coefficients cause the seal material to shrink more than the housing, and the effectiveness of the seal is reduced.
The term “thermal expansion coefficient” is defined herein as the ratio of the change of size of an object to its original size per unit temperature rise. In the context of annular seal assemblies, changes in the “size” of an annular seal member result in a change in the inner and outer diameter. That is, as the temperature of an annular object is decreased, the inner and outer diameters will also decrease. Such dimensional changes will be greater for objects with higher thermal expansion coefficients.
Accordingly, there is a need for a seal assembly suitable for providing sealing between a movable body such as a shaft or piston and a relatively stationary housing or cylinder. In addition, there is a need for a seal assembly that is suitable for providing sealing in a cryogenic apparatus that compensates for thermal effects at low temperatures and thus reduces the potential for fluid leakage at both the static and dynamic sealing surfaces.
SUMMARY OF THE INVENTION
A seal assembly for providing fluid sealing between a movable inner body and a housing body employs two separate sealing mechanisms for providing static and dynamic sealing. The seal assembly is fixedly associated with one of the bodies, and the seal assembly comprises:
(a) a static seal for providing a seal between the seal assembly and the associated body; the static seal comprises a metallic member in the shape of a continuous ring wherein the static seal is temperature-activated by the metallic member having a thermal expansion coefficient that is different from that of the associated body. The metallic ring is thereby urged towards the associated body to activate the static seal when the seal assembly is within a predetermined operating temperature range; and
(b) a dynamic seal for providing a seal between the seal assembly and the body not in a fixed position relative to the seal assembly; the dynamic seal comprises a dynamic seal member in the shape of a continuous ring with at least one flange that cooperates with the static seal to provide a seal between the static and dynamic seal.
In preferred embodiments, when the temperature of the seal assembly is within the predetermined operating temperature range the metallic member forms a static seal by being pressed into direct contact with the associated body.
In one embodiment the associated body is the housing body and the movable inner body is a cylindrical body, such as a reciprocating piston, a piston rod, or a rotating shaft, which is disposed within a hollow cylinder or bore formed within the housing. In this embodiment, the operating temperature range, for example, may be in a range less than 190 degrees Kelvin. Within this operating temperature range the housing shrinks more than the metallic member to cause a temperature-activated static seal. The metallic member may be an alloy that has a thermal coefficient of expansion of less than 1.5×10
−6
/°F. and the housing body is made from a material such as stainless steel that has a thermal expansion coefficient of about 9.9×10
−6
/°F. The alloy selected for the metallic member may a nickel-iron alloy. In a preferred embodiment the alloy comprises between 34 and 36 per cent nickel, a maximum of 0.12 per cent carbon, a maximum of 0.50 manganese, and a maximum of 0.50 silicon, with the remainder being iron.
In an alternative arrangement, the associated body may be the movable body. In this arrangement, when the operating temperature range is in a range less than 190 degrees Kelvin, within the operating range the metallic ring shrinks more than the movable body to cause a temperature-activated static seal between the seal assembly and the movable body.
In a preferred embodiment the dynamic seal member comprises at least one flange that cooperates with a flange of the metallic member. Preferably at least a portion of the static sealing force applied between the flanges of
Brook Thomas
Muehlchen Bruce
Noble Stephen D.
Knight Anthony
McAndrews Held & Malloy Ltd.
Patel Vishal
Westport Research Inc.
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