Patent Application
20170037971
1. Field
This invention relates generally to a metal gasket seal and, more particularly, to a metal gasket seal made of, for example, titanium, vanadium, zirconium, erbium or alloys thereof that each provides a low elastic modulus, a high yield strength, corrosion resistance and a low coefficient of thermal expansion (CTE) to be suitable for preventing helium gas leaks in cryogenic applications on, for example, aerospace vehicles.
2. Discussion
Various systems on board a spacecraft require cryogenic cooling of an object or device. For example, various types of sensors, such as infrared radiation sensors, need to be cryogenically cooled so as to reduce or eliminate thermal noise and allow the sensor to provide the degree of sensitivity desired. In order to provide cryogenic cooling, it is known to provide a storage tank of helium on the spacecraft that delivers the helium to a chiller to provide a super-cooled helium gas to the sensor or other device. Typically, various pipes and tubes direct the helium gas from the storage tank to the chiller and to the sensor, which often requires various fittings to couple ends of the pipes together and to couple the pipes to the storage tank, chiller and the sensor. These fittings need to be leak tight to perform correctly, and as such usually employ seals.
In order to provide gas-tight sealing, elastomeric gasket seals, such as rubber gasket seals, are usually employed that provide a significant compensation to sealing surface irregularities through a large elastic deformation of the seal under a compressive load. A rubber gasket seal meets these objectives because under pressure, rubber gasket seals deform and fill microscopic valleys at the sealing surface, thus drastically reducing the leak rate of gas molecules through the sealed area. However, elastomeric materials cannot be used for cryogenic helium-tight joints. Particularly, elastomeric materials have high out-gassing rates that often contaminate the helium gas. Further, elastomeric materials are highly permeable to helium gas and cannot meet most stringent leak tight requirements. Also, elastomeric materials often become rigid and brittle at cryogenic temperatures.
Metallic gasket seals are therefore the most popular type of gasket material for use in cryogenic helium-tight applications. Typically, these metallic gasket seals are made of stainless steel, however other metals are known to be used, such aluminum, nickel, copper, or alloys thereof. Although stainless steel gasket seals are effective in providing gas-tight sealing for cryogenic applications, often times these gasket seals become less effective over time. For example, under normal use as a result of system vibrations, fitting cycling, temperature expansion, etc., the stainless steel gasket seal may lose its effective sealing making it undesirable for the particular application.
Further, the vast majority of metallic materials do not have a low enough elastic modulus and exhibit poor elastic deformation when used as a gasket seal. As such, known metals for gasket seals are generally not robust enough for cryogenic helium-tight joints.
Low yield strength of a material results in plastic deformation under a high contact pressure at a sealing surface. Known metallic gasket materials typically exhibit a large plastic deformation at the seal surface and as such an irreversible deformation does not ensure long duration gas tightness. Under temperature excursions, plastic deformation cannot recover, thus gradually losing sealing surface contact, where a gasket seal must not be reused after plastic deformation.
Corrosion resistance of a metal gasket seal is required since the seal material is in intimate contact with a seal surface, which is often made of a different metal, such as stainless steel. Galvanic potential differences can lead to localized corrosion to either the gasket or the seal surface.
A proper selection of material CTE for a metal gasket seal is also important, especially for cryogenic applications, since all of the fittings are made at a temperature higher than cryogenic temperatures before being placed into service. Gasket seal materials with a high CTE, such as aluminum and copper, do not ensure a gas-tight joint at cryogenic temperatures because large magnitude shrinkage of the gasket seal in relation to the sealing surfaces occurs, which causes loss of seal.
The following discussion of the embodiments of the invention directed to a metal gasket seal having a low elastic modulus, a high yield strength, suitable corrosion resistance and a low coefficient of thermal expansion (CTE) is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the metal gasket seal of the invention will be discussed below as having specific application for cryogenic, helium-tight applications on spacecraft. However, as will be appreciated by those skilled in the art, the gasket seal will have other applications.
As will be discussed in detail below, the present invention proposes a metal gasket seal that has particular application for cryogenic fittings and systems, and particularly cryogenic systems that use helium gas for cooling. The gasket seal of the invention has a number of highly desirable properties including a low elastic modulus, a high yield strength, suitable corrosion resistance and a low CTE. Specific metals that satisfy these requirements include, but are not limited to, titanium (Ti), vanadium (V), zirconium (Zr), erbium (Er), and alloys thereof. Specific titanium alloys having aluminum (Al), V, chromium (Cr) and/or tin (Sn) include, but are not limited to, Ti-6Al-4V, Ti-13V-11Cr-3Al, Ti-15V-3Cr-35n-3Al, and zircalloy, which is an alloy of zirconium, tin and other metals. Each of these metals has an elastic modulus in the range of 0.5-0.7 relative to the coupling material, such as stainless steel, and a CTE in the range of 0.5-0.8 also relative to the coupling material, such as stainless steel. Other possible metals, but less suitable as a result of being chemically unstable in an ambient environment, include the rare earth elements lanthanum, cerium, gadolinium and praseodymium.
A metal gasket seal 66 having an annular shape with a central circular opening 68 is positioned between the seal surfaces 42 and 46. With the seal 66 in this position, the extension 62 of the male clamping portion 60 is threaded into the opening 58 of the female clamp portion 56 so that an inside structural feature of the female clamp portion 56 pushes against the annular flange 50 and an inside structural feature of the male clamp portion 60 pushes against the annular flange 52 compressing the gasket seal 66 between the seal surfaces 42 and 46 and providing a hermetic helium-tight connection. The size of the gasket seal 66 would be specific for a particular joint and clamping device for the specific application. For example, the gasket seal 66 may have an outer diameter of one quarter of an inch, the opening 68 may be one eighth of an inch in diameter and the thickness of the seal 66 may be 10-30 mils. It is stressed that the coupler 30 is illustrated merely as an example in that the gasket seal 66 of the invention discussed herein can be employed in combination with any suitable coupler.
The gasket seal 66 is fabricated of metal by any suitable metal fabrication process, such as stamping. The metal of the gasket seal 66 can be any of the metals discussed above and/or have any of the properties discussed above for the applications referred to herein. For example, the metal will have a desirable elastic modulus, such as between 0.5 and 0.7 relative to stainless steel, to provide a desirable elastic deformation, and will have a suitable CTE, such as between 0.5 and 0.8 relative to stainless steel, to reduce relative contraction of the seal 66 at cryogenic temperatures. Further, the gasket seal 66 may be fabricated in a particular manner, depending on the particular alloy, to have further desirable properties, such as a high yield strength to minimize plastic deformation and be corrosion resistant. For example, the seal 66 may be heat treated, cold worked or a combination thereof to enhance its yield strength, and may be subjected to a surface treatment, such as passivation, to enhance corrosion resistance, specifically resisting galvanic corrosion. Further, the gasket seal 66 may be mechanically, chemically or electrochemically polished to reduce surface irregularities.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
