Reusable Metallic Seal Using Memory Metal

This invention relates to reusable seals that are virtually perfectly leakproof at very high to very low pressures, very low (cryogenic) to very high (500.degree. C.) temperatures, and in extremely hostile environments such as space or harsh, corrosive chemicals such as acids and other chemically reactive substances.

Conventional contact seal materials are generally of two different classes: metallic and non-metallic. The non-metallic seals are capable of excellent sealing at low cost over limited pressures and temperatures, and for a limited lifetime. However, they must be compatible with the fluid to be sealed and the environment the seal is in. The pressure, temperature and fluid compatibility issues are the greatest obstacles against using these seal materials.

Metallic seals are used to overcome the shortcomings of nonmetallic seals. However, conventional metallic seals, while allowing wider temperature and pressure capability than non-metallic seals, require a very smooth seal face against which to seal, and are susceptible to degradation caused by chemical incompatibility with the sealed fluid. Moveover, the permanent set induced in the metallic seal when it is deformed to make the seal makes this type of seal worthless for reuse.

Thus, there has long been a need in industry for a seal that remains effective over long periods of time regardless of temperature, pressure or the chemical environment, and which is economical for those reasons. This long sought seal would also have reduced need for extremely smooth machining of the seal faces and the close tolerances that have been necessary for good sealing surfaces for conventional seal materials, and would be capable of reused many times over and over again without any degradation in sealing effectiveness.

Accordingly, it is an object of this invention to provide a static seal for sealing an interface such as a coupling or the like, which can be reused many times without losing its sealing effectiveness.

It is another object of the invention to provide a static seal that can survive high temperatures and pressures and a severe chemical environment.

It is yet another object of the invention to provide a static seal that remains effective virtually indefinitely in sealing against leaks of fluid under high or low pressure.

It is still another object of the invention to provide a dynamic seal between relatively moving surfaces that can operate at high or low pressures, can survive hostile environmental environments, and is compatible with most fluids.

It is yet still another object of the invention to provide a seal that is insensitive to the surface finish of the sealing surfaces.

It is another object of the invention to provide a seal that provides a means to maintain the initial preload at the seal interface as the temperature changes.

These and other objects of the invention are attained in a continuous annulus of Nitinol that can be used like a conventional O-ring seal between two seal faces, and then can be reused after removal from the seal interface by heating the seal above its transition temperature to remove the surface indentations and irregularities, which returns it to its original shape and surface condition for reuse as a new seal.

The invention and its many attendant objects and advantages will become clear upon reading the following detailed description of the preferred embodiment in conjunction with the following drawings, wherein:

FIG. 1 is a cross-sectional elevation of two coupling members with a seal according to this invention at the seal interface between the coupling members;

FIG. 2 is an enlarged section of the seal and sealing surfaces of the coupling shown in FIG. 1, showing the surface irregularities of the sealing surfaces;

FIG. 3 is a cross-sectional elevation of the coupling shown in FIG. 1, with the coupling members clamped closed on the seal;

FIG. 4 is an enlarged section of the seal and the sealing surfaces of the coupling members shown in FIG. 3, showing the conformance of the seal with the surface irregularities in the sealing surfaces;

FIG. 5 is a plan view of one embodiment of a seal according to this invention;

FIG. 5A is a sectional elevation of a portion of the seal shown in FIG. 5;

FIG. 6 is a plan view of a second embodiment of a seal according to this invention;

FIG. 6A is a sectional elevation of a section of the seal shown in FIG. 6;

FIG. 7 an schematic elevation of a mandrel on which Nitinol wire can be wound to make Nitinol seals in mass production;

FIG. 8 is a schematic elevation of the mandrel in FIG. 7 with the Nitinol helix in the mandrel cut, shifted and welded to make a series of parallel rings of continuous Nitinol wire;

FIG. 9 is an elevation of a test configuration for testing the integrity of the seal provided by a seal as shown in FIG. 5;

FIG. 10 is an exploded sectional elevation of a dynamic seal in accordance with this invention; and

FIG. 10A is a sectional elevation of the dynamic seal installed between two relatively moving coupling members in accordance with this invention.

Turning now to the drawings, wherein like reference characters designate identical or corresponding parts, and more particularly to FIG. 1 thereof, two members of a typical coupling for a fluid-tight system are shown, a male member 30 and a female member 32. Each member has a projecting distal portion 34 to which a fluid line (not shown) is connected by permanent secure means such as pressure fittings, silver solder or the like. A through bore 36 through each member 30 and 32 communicates with the fluid line when it is connected to the distal end 34 of the members. The proximal end of each member 30 and 32 is formed in a flange 38 and 40, respectively. Clamping means of conventional design (not shown) are provided in the usual manner for exerting a clamping force to hold the flanges 38 and 40 together under pressure.

The material of the coupling members 30 and 32 is typically a hard material that is immune to the chemical effects of the fluid that the fluid-tight system is to contain. A typical material for coupling members for use in a corrosive or other hostile chemical environment would be Inconel or stainless steel. The material should also have a yield strength greater than 100 KSI for a purpose which will become clear presently.

Each flange 30 and 32 has a sealing face 44 and 46 on its faying surface 52 and 54, respectively. The seal faces are typically flat, but there will always be some microscopic surface irregularities, shown (greatly enlarged for purposes of illustration) in FIG. 2. These irregularities would prevent the two sealing surfaces 44 and 46 from coming into intimate continuous contact across the entire surface of the seal faces because the material is too hard to deform sufficiently under the pressure exerted by the clamping means, so minute spaces would always exist between the two sealing faces 44 and 46 through which fluid under pressure could leak.

To fill the space between the hard sealing faces 44 and 46 and thereby establish a continuous region of material from face to face that will prevent the flow of fluid between the faces, the method of this invention interposes an O-ring 50 of memory metal such as Nitinol in its Martensitic state between the seal faces 44 and 46, and the flanges are then clamped together with sufficient force to deform the memory metal under the pressure exerted on the two flanges 38 and 40, such that the O-ring conforms to the surface topography of the seal surfaces 44 and 46 to fill all the surface irregularities in the sealing faces.

The Nitinol O-ring seal in the Martensitic state of the Nitinol has a yield strength of about 20-30 KSI whereas the material of the coupling members 30 and 32 is typically on the order of 100 KSI or higher. The coupling members are normally made of stainless steel or high strength steel, depending on the fluid that will be carried in the system in which they are to be used. The yield strength of the coupling members is substantially greater than the yield strength of the Nitinol O-ring seal, so that when the flanges of the mating coupling members are tightened together, as shown in FIGS. 3 and 4, it is the Nitinol that yields and conforms to the surface topography of the seal faces.

The faying surfaces 52 and 54 of the coupling members 30 and 32, respectively, abut the corresponding faying surface 54 and 52 of the coupling members 32 and 30, respectively, when the flanges 38 and 40 are clamped together. In this condition, the seal surfaces 44 and 46 are slightly separated as shown in FIG. 3, with a spacing less than the thickness of the seal 50 so the seal is deformed when the coupling members 30 and 32 are clamped together. The interference is about 5% to provide optimum conformance of the Nitinol with the surface irregularities of the sealing surfaces 44 and 46, but not enough strain in the Nitinol to cause it to undergo a permanent set or change state. This would not be detrimental to the sealing ability of the seal, but could adversely affect its ability to resume its original shape after heating to Austenitic state during the restoration step before reuse of the seal, as will be explained in detail below.

Memory metals of which the seal of this invention are made are materials that "remember" the shape they had when they were last formed in their Austenitic state. When the memory metal is transformed to its Martensitic state by reducing its temperature below the transition temperature, its yield strength drops substantially. It can be deformed relatively easily in its Martensitic state and conforms to the surface topography of the seal faces of the coupling members 30 and 32 readily under pressure exerted by the clamping members on the flanges 38 and 40. The most interesting property of memory metals is their "memory." When they are heated to a temperature above their transition temperature, the memory metal spontaneously reverts to the shape it had before it was deformed in the Martensitic state.

Nitinol is a type of memory metal made primarily from nickel and titanium. Nitinol is commercially available in various formulations to provide different characteristics, primarily various transition temperatures. One particularly attractive characteristic of Nitinol is its high corrosion resistance and resistance to hydrogen embrittlement. This feature makes Nitinol an excellent choice for a seal in fluid systems for fluids that are incompatible with common seal materials. Likewise, Nitinol seals are unaffected by temperatures in the deep cryogenic range, at which temperatures conventional seal materials would be brittle and crack. This feature is of great importance in cryogenic gas storage where the practical limits on such storage systems ar imposed by the pressure and temperature capabilities, and the associated working life, of the seals in the system.

Although a Nitinol seal will last indefinitely in a coupling, indeed as long or longer than the fluid system itself, it is the nature of fluid couplings that they are intended to be decoupled sometime in their life, sometimes frequently. Conventional metal seals can be used reliably only once and must be discarded when the coupling is opened. Memory metal seals according to this invention, on the other hand, can be restored to their original undeformed condition by heating them above their transition temperature, to their Austenitic state. At the transition temperature, the energy stored in the crystal structure of the material when it was deformed in the Martensitic state is released and the part spontaneously reverts to the shape it had before it was deformed. It can then simply be cooled back below its transition temperature and it is ready to be reused, in as good a condition as when it was new. Thus, the expense of using Nitinol seals in fluid systems can be significantly lower than conventional seals because, even though the initial cost of the seal itself may be higher than some other seals, the fact that it does not degrade with age and therefore does not need to be periodically replaced, and that it can be reused over and over again make it far more economical in terms of labor saved and additional procurement obviated.

One method of making the Nitinol seals according to this invention includes forming the Nitinol wire in a loop, and securing the ends of the loop together to form a continuous, uninterrupted annulus of precisely predetermined diameter and uniform thickness. The cut ends of the loop should have no burrs or distortion that would cause an irregularity in the annulus of the final seal, and the joint at the cut ends if the loop should have no chemical contamination that would cause the joint to behave differently from the main part of the annulus in operation or when the seal is heated above its transition temperature to restore it to its original shape after use.

FIGS. 7 and 8 illustrate a method for forming these seals in mass production with the requirements given above. A long length of Nitinol wire is wrapped in a helix 112 around the length of a mandrel 111, and the helix 112 is cut longitudinally along a line 113 to sever each loop of the wire along the length of the mandrel by a single pass of a stressless cutter such as a laser beam, EDM head, or a water jet cutter having an abrasive in the cutting fluid. These cutting methods provide clean cut ends on the loops and no distortion or burrs that would have to be ground off the final product. The ends of the loops 114 are then aligned as shown in FIG. 8 by shifting the wound helix at the cut by one wire thickness, and the aligned ends of each loop is then welded with a laser to make a clean weldment that is close to the same diameter as the Nitinol wire, so no grinding is necessary.

The integrity of the seal provided by a Nitinol seal was tested in a test rig illustrated in FIG. 9. A "coupling" 70 having two seal members 72 and 74 is held together by six bolts 76, which function as "clamping members." The seal members 72 and 74 each has a faying surface 78 and 80, respectively, which is finished flat to serve as facing sealing surfaces of the seal members.

An annular groove 82 is machined in the sealing surface 80 of coupling member 74 to receive a circular Nitinol seal 84. The thickness of the Nitinol wire of the seal 84 is 0.062" and the depth of the groove 82 is 0.059" to provide an interference of 0.003" when the clamping members 76 are fully tightened down, causing a 5% strain in the Nitinol seal. This degree of strain is sufficient to compress the Nitinol into the microscopic surface irregularities in the sealing surfaces 78 and 80 of the coupling members to fill all the possible leak avenues that fluid could take to escape between the sealing surfaces on the coupling members 72 and 74.

To test the seal integrity, a vacuum is established in the center of the coupling 70 between the coupling members 72 and 4 within the seal 82. The vacuum is pulled by a vacuum pump 86 through a vacuum manifold 88 which is connected to the coupling 70 by a pipe system 90. Helium is sprayed into the test chamber 68 through a pipe 92 connected to a supply 94 of helium so that any leak in the seal 84 would draw helium into the center of the coupling and thence through the pipe system 90 into the vacuum manifold 88. A helium detector 96 in the vacuum manifold 88 would detect any helium within the sensitivity of the instrument.

The Nitinol seal was tested in the test rig shown in FIG. 9 to ascertain what rate of leakage would be observed at extreme vacuum. Helium leakage at less than 2.times.10.sup.-10 cc/sec at room temperature was detected.

The Nitinol seal used in the test described above was a conventional formulation of 50% nickel and 50% titanium, and has a transition temperature of 85.degree. C. To test the reusability of the seal, it was removed from the coupling 70 and heated above its transition temperature to restore it to is original shape and surface condition. It was then returned to the coupling, tightened in place and the helium leak test was rerun. The results were substantially identical, demonstrating that the seal did not suffer a degradation from being reused. Since the reusability feature is based on a crystalline transformation mechanism rather than a mechanical mechanism, no work hardening takes place that would limit the number of reuse cycles. It is expected, therefore, that the Nitinol seal will have an almost unlimited reusable life as long as the strain and temperature limitations are observed. Indeed, Nitinol has been subjected to more than 2.times.10.sup.8 cycles without any apparent work hardening or reduction in physical properties.

The surface finish and flatness tolerance of the seal faces of the coupling need not be as smooth and fine as those of the couplings using a conventional seal. For example, a conventional coupling using a conventional metal O-ring seal would normally require a surface no rougher than 16 microinches for liquid and 8 microinches for gas. A conventional coupling using a conventional elastomeric seal would require a surface no rougher than 32 microinches for liquid, and 16 microinches for gas. The seal of this invention, on the other hand, can provide a near-zero leakage seal in a coupling having a surface finish of 100 microinches or greater. Because of the ability to conform to the surface topography and the very high surface pressure at the seal interface which the Nitinol can tolerate, the seal of this invention can provide superior leak resistance with a coupling that is manufactured with less demanding requirements, and hence which is more economical.

Turning now to FIG. 10, a dynamic seal is shown to prevent leakage of fluid between a housing 100 and an axially movable mating housing or shaft 102. At least one, and preferably several O-ring seals 104 are disposed on a reduced diameter section 106 of the shaft 102. The thickness of the seals 104 is slightly greater than the difference between the inside radius 108 of the housing 100 and the outside radius 110 of the shaft 102 so that the O-ring seals will be compressed about 2-5% when the shaft 102 is received within the housing 100.

In operation, the seal shown in FIG. 10 permits the shaft and the housing 100 to move relative to each other in the axial direction while the O-rings 104 roll on the shaft and the interior wall of the housing 100. The seal interface does not experience a sliding motion as in conventional dynamic seals, but instead the seal material is pressed against the seal faces of the shaft and the housing and conforms to the surface topography to fill and seal all the microscopic irregularities through which fluid could otherwise escape.

The material of the seals 104 is an Austenitic form of Nitinol known as Superelastic Nitinol. Superelastic Nitinol is made by extensive cold working of Austenitic Nitinol, such as drawing Austenitic Nitinol wire through a series of 50-80 ever smaller dies. The cold working transforms the Austenitic Nitinol to a highly elastic form of Austenitic Nitinol having a relatively high yield strength of about 80-90 KSI. This Superelastic Nitinol is ideal for dynamic seal applications because of its flexibility. This material can undergo an elastic deformation of over 7% without permanent set.

The Superelastic Nitinol can also be used in the couplings shown in FIGS. 1 and 9, but the coupling material must be fabricated from material with a yield strength of at least 100 KSI, so the coupling deforms the seal rather than being deformed by the seal. An advantage of using Superelastic Nitinol in these couplings is that it can be reused without heating to restore it to its original shape and surface condition. The Superelastic Nitinol seal, when squeezed to a 5-8% strain in the seal interface, deforms elastically rather than plasticly like the seal shown in FIG. 1, so that it springs back elastically to its original shape and surface condition when it is removed from the coupling, and is immediately ready for reuse.

The Nitinol seals can be used in very high temperature applications and its high temperature capabilities make it ideal for such applications. Indeed, the seal of this invention makes it possible to provide take-apart couplings in high temperature applications that always in the past have required hard connections that required cutting to open a line to remove a component or the like. However, in these applications the seal will take a permanent set then the temperature goes above the transition point while the seal is in place. The seal will continue to function as well as it did below the transition point, but the material will be transformed to the Austenitic state and the "memory" of the original shape will be lost. The seal in this application will remain effective above the maximum temperature of most systems, up to about 1000.degree. C.

Thus, a seal has been described that can be reused indefinitely and each time provides a virtually perfect seal, able to remain effective at low temperatures and at very high or very low pressures. The inert nature of the seal material makes it immune to the deleterious effects of corrosive and other reactive fluids and chemical environments. This seal provides an effective seal in economical couplings that are manufactured to less demanding conditions of surface finish and flatness tolerance, and by reason of its low CTE, maintains the preload on the seal during temperature changes.

Obviously, numerous modifications and variations of the preferred embodiment disclosed herein will occur to those skilled in the art in light of this disclosure.