Description of the Related Art [0002] The basic technology for composite overwrapped pressure vessels with metal liners dates back to the late 60's and early 70's.

[0003] High performance fibers offer very high strength-to-weight ratios and are ideal for making lightweight pressure vessels. However, composite laminates fabricated with these fibers have relatively high permeability and cannot contain high-pressure liquids or gases or low pressure gases for extended periods of time. Therefore, composite pressure vessels typically have a liner to prevent leakage. The tank efficiency, as measured by its pressure multiplied by its volume divided by its weight (PV/W), increases as the liner weight decreases. For a low pressure and/or liquid containment, elastomeric or polymeric liners can be used. For high-pressure or gas containment, metal liners are typically used. The metal liners may be structural or non-structural.

[0004] For lightweight, high-pressure gas containment, there are basically two primary technologies, both of which use fiber/matrix composite materials, (a) graphite/epoxy composite with a yielding aluminum liner, and (b) Kevlar/epoxy with load sharing liners (typically stainless steel, titanium alloyed or inconel).

Summary of the Present Inventions [0005] An aspect of at least one of the inventions disclosed herein includes the realization that although composite pressure vessel design has advanced the art of fluid pressure vessels by providing a means of providing a light-weight pressure vessel, composite design also provides an avenue for providing fluid pressure vessels that benefit from the properties of materials heretofore not used for pressure vessels due to their relatively high weight. For example, as noted above, composite pressure vessels comprised of an inner aluminum liner wrapped in a graphite or carbon fiber laminate have become popular due to the resulting low weight of the final assembly resulting from the use of lightweight carbon fiber which provides the overall structural rigidity to the pressure vessel and the lightweight aluminum liner which provides the lealc-proof barrier on the ilmer surface of the tank. As such, carbon fiber wrapped aluminum pressure vessels have largely replaced steel cylinders for pressure vessel applications where light weight is an important factor.

[0006] However, there are other materials which provide certain desirable characteristics for light weight applications, but heretofore have not been incorporated into pressure vessels due to their high weight. For example, it has long been lcnown that certain alloys have a high resistance to flammability, and thus, such materials are commonly used for components of high-pressure oxygen systems due to their low flammability characteristics.

For example, brass is commonly used for nozzles, valves, and other fittings for high-pressure gas systems. However, heretofore, copper and copper alloys such as brass and bronze have not been used for forming composite pressure vessels.

[0007] However, by using a copper alloy, such as brass, as a liner of a composite pressure vessel, the pressure vessel benefits from the low flammability of the copper alloy as well as the lightweight strength that can be imparted from a second material disposed on the outer surface of the copper alloy.

[0008] Another aspect of at least one of the inventions disclosed herein includes the realization that there are certain applications of pressurized fluid vessels which would be greatly expanded if the fluid vessel itself could be made lightweight and flame resistant in a pure oxygen environment.

[0009] For example, it has been lcnown that aluminum composite oxygen cylinders do not perform satisfactorily in a"gun fire test."In particular, it is known that if an aluminum composite tank pressure vessel is filled with pure oxygen, and shot with a 50- caliber projectile, the cylinder explodes violently. This is due to the flammability of aluminum in a pure oxygen environment, as well as the heat of combustion of aluminum itself. For example, aluminum generally has a heat of combustion of about 7,425 calories per gram. Additionally, aluminum, such as aluminum 6061, will sustain combustion in a pure oxygen environment at pressures above about 250 psig. Thus, where a composite aluminum cylinder was filled with pure oxygen to a pressure of 350 psig, then shot with 50 caliber projectile, the aluminum cylinder exploded violently as the aluminum material itself burned in the. oxygen environment.

[0010] Thus, aluminum composite pressure vessels filled with pure oxygen should not be transported in such a way that there is a danger of impact from a projectile, such as in a combat area. As such, the use of composite aluminum tanks for oxygen transportation has been restricted. However, there continues to be a need for lightweight tanks for oxygen transportation. For example, field hospitals in combat areas and aero evacuation flights greatly benefit from having pressurized oxygen available for treating patients. Additionally, repair facilities in combat areas where, for example, welding is necessary, can also benefit from having oxygen available.

[0011] Thus, in accordance with an aspect of at least one of the inventions disclosed herein, a pressure vessel for containing pressurized fluid comprises a copper alloy liner defining an interior volume for containing the pressurized fluid, and a second structural member disposed on an exterior of the liner.

[0012] By constructing the pressure vessel with a copper alloy liner defining the interior volume, the pressure vessel benefits from the low flammability and low heat of combustion of the copper alloy and the lightweight afforded by a composite design.

Preferably, the pressure vessel is configured to be pressurized to a pressure that is less than the promoted combustion threshold pressure of the copper alloy liner material. For example, such a pressure vessel can be constructed to define an interior volume of approximately 1.1 cubic feet. The copper alloy and a second outer material can be configured to provide a pure oxygen tank capable of containing 10,200 liters of oxygen at 3, 000psi. Thus, the copper alloy used preferably has a promoted combustion threshold pressure greater than 3,000 psi. As such, the pressure vessel can be made sufficiently portable, lightweight, and flame resistant so that it can be transported with less restrictions then those imposed on the transportation of oxygen in aluminum-lined composite tanks.

[0013] A further advantage is provided where the material used as the liner of the pressure vessel has a promoted combustion threshold pressure that is substantially higher than the maximum rated pressure of the vessel. For example, the"maximum pressure rating" of the tank is set at a pressure that is substantially below the promoted combustion threshold pressure of the material used for the liner. This provides a further advantage in preventing combustion of the liner material during a destructive test such as a gunfire test.

[0014] For example, at the initial moment when a projectile, such as a tumbling 50-caliber projectile, impacts a side of a tank containing a compressed gas such as pure oxygen, the side of the tank is initially deflected inwardly, thus reducing the volume of the tank and thereby increasing the pressure within the tank. Thus, a tank that has been filled to its"maximum rated"pressure, will initially be further pressurized when impacted with a projectile. The projectile eventually ruptures the tank and can produce significant friction heating as the projectile passes through the tank wall, providing a potential source of ignition for the contents of the tank.

[0015] By using the material that has a promoted combustion threshold pressure substantially above the maximum rated pressure of the tank, the rise in pressure created from an impact of a projectile is less likely to cause the internal pressure of the tank to rise above the promoted combustion threshold pressure, thereby preventing the liner material from bursting into flames during such an event.

[0016] Further aspects, features, and advantages of the present inventions will be apparent from the following description of the preferred embodiments which follows.

Brief Description of the Drawings [0017] The above-mentioned and other features of the inventions disclosed herein will now be described with reference to the drawings of the preferred embodiments of pressure vessels. The illustrated embodiments of the pressure vessel design are intended to illustrate, but not to limit the invention. The drawings contain four figures, in which: [0018] Figure 1 is a schematic diagram of a pressure vessel having a copper alloy liner; [0019] Figure 2 is a side elevational view of a copper alloy liner of a pressure vessel; [0020] Figure 3 is a side elevational view of a pressure vessel having the liner of Figure 2 and a fiber reinforced material disposed around the outer surface of the liner; [0021] Figure 4 is a sectional view of the pressure vessel shown in Figure 3, taken along line 4-4.

Detailed Description of the Preferred Embodiments [0022] With reference initially to Figure 1, a pressure vessel 10 is illustrated schematically. The pressure vessel 10 includes a liner 12 formed of a copper or copper-based alloy material. As used herein, the term"copper-based alloy"is intended to include any alloy having at least a majority of copper. The liner 12 defines an internal cavity 14 which is configured to contain a pressurized fluid.

[0023] The liner 12 can define any shape. For example, the liner 12 can be in the shape of a cube, prism, sphere, cone or other conical shapes. Further, the liner 12 can be cast, machined, or manufactured from any form of stock material. For example, the liner 12 can be formed from sheet or plate material, and cut and/or bent into various shapes and welded together to provide a custom or non-standard shape. Of course, cylindrical shapes are most common.

[0024] The liner 12 can have any desired thickness. Generally, the thickness of the liner 12 would be detennined by the desired rated maximum pressure of the pressure vessel 10 and the mechanical strength of the material used for the liner 12.

[0025] The pressure vessel 10 also includes a fitting 16 extending through the liner 12. Thus, the fitting 16 provides communication between the interior volume 14 and the exterior of the liner 12. The fitting 16 can have any known construction. For example, the fitting 16 can be in the shape of a tube, duct, or frustoconical conduit defining a fluid passage between the interior volume 14 and the exterior of the liner 12. Depending on the application, other devices, such as, for example, but without limitation, valves, gauges, filters, and regulators may be connected to the fitting 16.

[0026] The pressure vessel 10 also includes a second member 18 disposed around the liner 12. By constructing the pressure vessel 10 with a copper or copper-based alloy, and a second member 18 disposed around the liner 12, the pressure vessel 10 benefits from the low cost yet relatively inflammable characteristics of copper and copper-based alloys and benefits from the added strength of the second member 18. With the second member 18 disposed as such, the thickness of the liner 12 can be reduced, where the mechanical strength of the second member 18 carries the load caused by the radial expansion of the liner 12.

[0027] A further advantage is thus provided where the second member 18 is made from a material which has, or is configured to have a higher stiffness in radial expansion than the liner 12. When the internal volume 14 is filled with a pressurized fluid, the liner 12 will tend to expand against the second member 18. Thus, the second member 18 is configured to have or is made from material that has a higher stiffness or modulus of elasticity than the material forming the liner 12. Thus, when the liner 12 expands in response to a pressurized fluid within the internal cavity 14, the second member 18 will provide greater resistance against the radially outward expansion. Thus, the member 18 will carry more of the load created by the pressurized fluid in the internal volume 14 than the liner 12.

[0028] This configuration provides an additional advantage where copper or copper-based alloys are used for the liner 12. For example, copper and copper-based alloys, such as lead, tin, and yellow brasses, are generally weaker and softer than other materials that are known or considered to be materials that possibly can be used as liners for pressure vessels, such as, for example, aluminum and aluminum alloys, steel and steel alloys, and nickel and nickel alloys. Additionally, copper and copper-based alloys have a significantly higher density than aluminum and aluminum alloy which are commonly used as pressure vessel liners. Thus, by using a second member 18 having a greater resistance to radially outward expansion, the liner 12 can be made thinner and thus lighter, thereby limiting the total weight of the pressure vessel 10.

[0029] Figure 2 illustrates a modification of the liner 12, which is identified generally by the reference numeral 20. As noted above, with reference to the liner 12, the liner 20 is formed of a copper or copper-based alloy. The illustrated configuration of the liner 20 is an example of a configuration that is commonly used in the art of composite fluid tanks.

[0030] The illustrated configuration of the liner 20 is commonly referred to as a <BR> <BR> "mandrel. "The mandrel is generally the shape of gas cylinders that have long been known in the art.

[0031] Preferably, the liner 20 has a fitting 22 at one end. Additionally, the liner 20 preferably has a boss 24 disposed at the end opposite the fitting 22. The boss 24 and the fitting 22 are used in a later step in manufacturing of a completed pressure vessel.

[0032] Preferably, as noted above, an outer member 28 is configured, or is made from a material, having a higher stiffness than the liner 20. Thus, the outer member 28 will carry a substantial portion of the load created by the radially outward expansion of the liner 20 caused by pressurized fluid being introduced into the internal cavity defined by the liner 20.

[0033] For example, but without limitation, a fiber-based material such as a carbon fiber material can be disposed on the outer surface of the liner 20 to provide reinforcement therefor. Figure 3 illustrates a completed pressure vessel 26 having a fiber- based material forming the outer member 28 which provides structural reinforcement for the liner 20. In the illustrated example, the material used for forming the outer member 28 is a carbon fiber material.

[0034] One method that is widely known for forming the outer member 28 as such, is to mount the liner 20, which is in the shape of a mandrel, to rotate about its longitudinal axis 30. As the liner 20 is rotated, a sheet of multi-directional carbon fiber fabric pre-impregnated with a resin is wrapped around the liner 20. However, other types of fiber- based materials or other non-fiber based material, as noted above, can be used to form the outer member 28. Other examples of fiber-based materials include, for example, but without limitation, fiberglass and Kevlar/epoxy. Additionally, the fiber material itself can be applied first, then a resin can be applied afterwards.

[0035] Depending on the material used, the outer member 28 may be subjected to further processes, such as for example, but without limitation, vacuum and heat treatments.

[0036] Figure 4 illustrates a sectional view of the pressure vessel 26 illustrated in Figure 3. Preferably, the thickness L of the liner 20 is made as thin as possible, to minimize the weight of the liner 20. This is beneficial because, copper-based alloys have relatively high density, as compared to the density of aluminum. Thus, by minimizing the thickness L of the liner 20, the total weight of the pressure vessel 26 can be minimized.

[0037] Depending on the intended use of the pressure vessel 26, the thickness S of the outer member 28 is sufficiently large to support the liner 20 under the maximum load conditions. In an illustrative but non-limiting example, the internal volume 14 of the pressure vessel 26 is approximately 1.09 cubic feet. The overall length of the vessel 26 is approximately 29.4". In this illustrative example, the outer diameter of the pressure vessel 26 is approximately 10.15". Preferably, the liner has a thickness L between about 1/32 of an inch to about'4 of an inch. In this example, the thiclmess L of the liner 20 is approximately . 062" and the thickness S of the second member is approximately 0.188". Preferably, the fitting 22 defines a standard l/2"SAE port. As such, the pressure vessel 26 can be used with a variety of standard fluid handling fittings, valves, regulators, gauges, and filters.

[0038] In this configuration, the pressure vessel 26 would have a maximum rated pressure of about 3,000 psig. As such, the capacity of the pressure vessel 26 is approximately 10,200 liters of pure oxygen. These dimensions of materials will provide a proof pressure of about 4, 800 psi and a design burst pressure of about 8, 200 psi : [0039] Copper and other copper-based alloys have a promoted combustion threshold pressure of about 7,000-8, 000 psi in a pure oxygen environment. Thus, when the pressure vessel 26 is filled with pure oxygen to its maximum rated pressure of 3,000 psi, the pressure vessel 26 remains far more explosion resistant than compared to a similarly configured aluminum lined pressure vessel.

[0040] For example, aluminum and aluminum alloys such as aluminum 6061 and aluminum bronze have a promoted combustion threshold pressure of about 250 to 500 psi in a pure oxygen environment. Thus, a pressure vessel with an aluminum liner pressurized to 3,000 psi of pure oxygen would be highly flammable. If such a tank were punctured, the tank will be highly likely to burst into flames, with the aluminum itself becoming a fuel.

However, when the tank 26, sized in accordance with the above-noted illustrative example, is filled with pressurized oxygen to approximately 3,000 psi, and if subjected to a strong mechanical impact such as by gunfire, the liner 20 could be deflected significantly without raising the pressure into the vicinity of the promoted combustion threshold pressure of copper or copper alloys in a pure oxygen environment. Thus, the pressure vessel 26 will not likely combust when subjected to such an event.

[0041] Additionally, because the liner 20 can be made generally thinner than the thickness that would be required if the entire vessel 26 was made from solid copper or copper alloy, the total weight of the pressure vessel 26 can be kept lower, thereby increasing and broadening the feasibility of using such a pressure vessel for transporting fluid such as gaseous oxygen.

[0042] Further, it is possible that, due to the lowered flammability of a pressure vessel such as the pressure vessels 10,26, restrictions on the use of such pressure vessels will be reduced. For example, the reduced flammability of such pressure vessels may be sufficient to allow oxygen to be transported in military aircraft flying into combat zones.

Thus, military field hospitals can be more easily supplied with gaseous oxygen for treating patients.

[0043] Exemplary Embodiment [0044] Set forth below is a description of a further exemplary, but non-limiting, embodiment including at least one of the inventions disclosed herein. This exemplary embodiment is not intended to limit the inventions disclosed herein. Rather, the present exemplary embodiment is intended merely to illustrate one possible embodiment of at least one of the inventions disclosed here. In particular, the exemplary embodiment described below has been developed to ease manufacturability and compliance with certain Department of Transportation (DOT) regulations.

[0045] In this exemplary, but non-limiting embodiment, the pressure vessel can be dimensioned as noted above with reference to the non-limiting, exemplary dimensions noted above with reference to the pressure vessel 26 illustrated in Figures 2-4. As such, the cylinder can be a seamless brass alloy liner wound with carbon fiber reinforced plastic composite layers and subjected to an autofrettage pressure. As such, the carbon filament impregnated with epoxy layers are the predominant pressure load bearing elements.

[0046] The vessel 26 can also include an outer layer consisting of glass filament impregnated with epoxy resin provides damage protection. The liner and the layers are configured such that the outer glass layer will carry less than 10 percent of the total pressure at the minimum required burst pressure.

[0047] The brass liner can also include a thin layer (approximately 0.010 inches) of epoxy resin reinforced glass veil matt disposed on its outer surface to prevent galvanic corrosion. Together the inner and outer glass filament layers should carry less than 15 percent of the total pressure load at the minimum burst pressure.

[0048] The winding pattern of the carbon fiber reinforced plastic composite layers may be a combination of helical (including near longitudinal) and hoop. A layer made up of more than one type of fiber could be, but preferably is not used. The marked service pressure can be as high as 5000 PSI at a reference temperature of 70°F.

[0049] The test pressure is preferably 1.67 times the design service pressure. The cylinder should also have a safety factor (burst/service pressure ratio) of about 3.4. The service life of the vessel can be estimated at about 15 years from the date of manufacture.

[0050] The liner can be a cylinder made of 260 brass. The liner preferably has no more than one circumferential seam approximately at the midpoint of the cylindrical portion of the vessel. The liner can be constructed with a boss at the closed end, for ease of winding and a threaded boss at the open end. The bosses may be welded in place with a seam preferably no larger than 3 inches in diameter.

[0051] The materials composition of the brass are preferably within the ranges as follows: ELEMENT MIN% MAX%.

COPPER 68 72 ZINC 28 32 OTHER-0.5 [0052] The liner interior surface preferably is smooth. Any fold in the domed area due to the forming or spinning process preferably is not sharp, deep, or detrimental to the integrity of the liner. Inner surface defects can be removed by machining or another method.

However, preferably the metal loss is minimal and the minimum required wall thickness is maintained. Additionally, the ends of the liner should be concave to pressure.

[0053] The mechanical properties of brass liner material preferably fall into the following ranges: yield strength 17K-29K psi, tensile strength 47K-70K psi, and elongation (2"gauge) at least 25%.

[0054] The carbon fibers can be polyacrylonitrile (PAN) based carbon fiber tows.

The tensile strength of these tows can be at least about 600,000 psi. The modulus of the elasticity preferably is from about 38 million psi to 46 million psi. Additionally, the strain to failure preferably is not be less than about 1 percent.

[0055] The glass fibers preferably are type E glass fibers. As noted above, the glass over-wrap can be used merely for abrasion protection and as a carrier for the green pigment.

[0056] The resin matrix systems can be an epoxy or a modified epoxy type having a pot life compatible with the filament winding process used. The resin matrix system selected preferably has sufficient ductility so that cracking of the resin matrix system does not occur during the manufacturing of the cylinder or during normal operation for the useful life of the cylinder.

[0057] The composite overwrap preferably is formed by layers of continuous fibers in a matrix. Helical or near longitudinal windings preferably cover the entire surface of the liner. When circumferential layers are interspersed for strengthening the side wall, physical discontinuity between the layers preferably is minimized. The fibers preferably are not co-mingled. Thus, each layer preferably contains only one type of fiber. However, the overwrap can be applied through wet winding or pre-impregnated filament winding.

[0058] The design and stress analysis of a carbon fiber reinforced pressure vessel can be complex because of the varying load bearing layers, the varying orientation and thickness of composite layers, and the potential that the liner is subjected to above yield strains at the time of an autofrettage pressure cycle.

[0059] Thus, a reliable model of the cylinder can be used in order to calculate the maximum stress at any point in the liner and fibers; and load distribution between liner and fibers at zero pressure, service pressure, test pressure, and burst pressure. For these purposes, the model used to analyze the cylinder body can be based on thin shell theory, account for non-linear material behavior and nonlinear geometric changes, and account for both circumferential and longitudinal pressure stresses. In such a design effort, the vessel body J can be analyzed alone. However, maximum stresses in the cylinder ends should always be less than the maximum stresses in the vessel body to pass burst tests.

[0060] Such an analysis is most conveniently performed with finite element techniques to analyze the stresses in the fibers. Preferably, the maximum calculated tensile stress (at service pressure) in any fibers (carbon or glass) do not exceed 30 percent of the fiber stress corresponding to the minimum required burst pressure.

[0061] The maximum calculated tensile stress at any point in the liner at the service pressure preferably does not exceed 60 percent of the yield strength of the liner material. The compressive stress in the sidewall of the liner at zero pressure preferably is at least 60 percent and not more than 95 percent of the minimum yield strength of the liner material.

[0062] The maximum fiber stress at service pressure of the carbon fibers or glass fibers preferably does not exceed 30 percent of the fiber stress corresponding to the minimum required burst pressure. Additionally, the vessel preferably is configured such that in the burst failure mode, failure will start in the cylindrical side-wall portion of the vessel.

[0063] Preferably, openings are on heads only. Thus, the centerline of the openings preferably coincide with the centerline of the vessel.

[0064] Any threads on the liner preferably are clean cut, even, without checlcs, and designed in compliance with the requirements of the Federal Standard FED-STD-H28.

Straight threads having at least 6 threads preferably have a calculated factor of safety in shear of at least 10 at the test pressure for the cylinder.

[0065] Of course, the foregoing description is that of preferred embodiments and other exemplary but non-limiting embodiments including various combinations of the inventions disclosed herein. Various changes, modifications, and combinations may be made to the above-described embodiments without departing from the spirit and scope of the invention, as defined by the dependent claims.