Improved boss for a filament wound pressure vessel

Abstract: A boss (16) is disposed in a polar opening (18) in a pressure vessel (10) which has a filament wound outer shell (12) and a non-metallic internal liner (14). The boss has a tubular neck (24) which projects outwardly from the vessel interior and an annular support flange (28) which extends radially from the internal end of the neck and supports the perimeter of the polar opening. An offset attachment flange (32) extends radially from the support flange and has two axially opposed surfaces (34,36) with locking grooves (40,42)formed therein. Each locking groove has a bottom wall intermediate a pair of mutually skewed sidewalls (50) for maintaining positive engagement with and retention of complementary respective tabs on the liner. In an application where the liner is a blow molded component, an injection molded interface member (82) is attached to the support flange and provides a site at which the liner is welded.

Description: The present invention relates to a boss system for a pressure vessel. In many circumstances, the qualities of lightweight construction and high resistance to fragmentation and corrosion damage are highly desirable characteristics for a pressure vessel. These design criteria have been met for many years by the development of high pressure composite containers fabricated of laminated layers of wound fibreglass filaments or various types of synthetic filaments which are bonded together by a thermal-setting epoxy resin. An elastomeric or other non-metal resilient liner or bladder is suspended within the filament wound shell to seal the vessel and prevent internal fluids from contacting the composite material. Filament wound vessels often are constructed in a spherical shape or a cylindrical shape with generally spherical ends for use in high pressure applications. A boss is used to reliably join the internal liner with the outer composite shell at pressurization ports in the outer shell such that fluid is prevented from penetrating between the liner and the shell. In many applications, such as in the aerospace industry, composite pressure vessels are required to contain extremely high pressures, operating at 25,000 p.s.i. (1722 bar) with design burst values in the range of 50,000 p.s.i. (3444 bar). Consequently, as internal pressure increases, the interface of the boss, the liner and the outer shell is subjected to extreme structural loading. More particularly, as pressure within the vessel is increased, bearing stress is generated between the boss and the composite shell, resulting in a steep strain gradient through the shell, with the inner strains being much higher than those at the outer surface. Shearing stress develops between the boss and the internal liner due to relative displacement discontinuities resulting from nonuniform loading during internal pressurization. In addition, radially extending support members on the boss are subjected to unacceptable levels of bending stress which can result in fracture of the boss. Moreover, it is critical that during the pressurization of the vessel the liner and outer shell remain firmly engaged with the boss, despite the adverse loading to which the liner and shell are subjected. In US-A-4360116 there is generally disclosed a boss system for a pressure vessel having a filament wound outer shell and a non-metallic internal liner, the system comprising a boss having a tubular neck projecting outwardly through an opening in the outer shell, and an annular flange extending radially from an end of the neck within the vessel, the annular flange having an outer surface for reinforcing the perimeter of the opening in the shell. It is an object of the present invention to provide a boss system which substantially overcomes one or more of the above-mentioned loading and sealing problems. In accordance with the present invention as claimed this object is accomplished in a boss system as generally disclosed above which is characterized by a first generally dovetail-shaped locking groove in the outer surface of the annular flange, a second generally dovetail-shaped locking groove in an inner surface of the annular flange, the liner being split at the annular flange with an outside portion outside the annular flange and an inside portion inside the annular flange, a first generally dovetail-shaped tab on the outside portion of the liner for locking in the first locking groove in the outer surface of the annular flange, and a second generally dovetail-shaped tab on the inside portion of the liner for locking in the second locking groove in the inner surface of the annular flange. In the preferred embodiment of the invention, an offset attachment flange extends radially from the annular flange, and the two locking grooves are provided on each of two axially opposite offset surfaces of the attachment flange. The locking groove in the outer surface of the attachment flange opens outwardly and the locking groove in the inner surface of the attachment flange opens inwardly, with each of the locking grooves having a bottom wall intermediate a pair of mutually skewed sidewalls for maintaining positive engagement with the liner. The offset characteristic of the attachment flange reduces the risk that the liner will extrude out of engagement with the boss and cause leakage between the outer shell and the liner. In order to reduce shear stress between the boss and the liner during internal pressurization of the vessel, a non-metallic shear stress relieving layer may be interposed between the outer surface of the annular flange and the inner surface of the outer shell. The interposed layer may be made of any plastic, thermosetting elastomeric, or other non-metallic material, and may be manufactured by a moulding process or cut from sheet stock. Pressurization damage is also reduced by the construction of the boss. In a preferred form, the annular flange has a diameter sufficient to prevent damage to the outer shell when the vessel is pressurized and is sufficiently thick to avoid unacceptable levels of bending stress in the annular flange and the attachment flange. The boss may be made from alloys of aluminium, steel, nickel, titanium, or other metals. In order that the invention may be well understood the preferred embodiment thereof, which is given by way of example, will now be described in more detail and with reference to the accompanying drawing the sole Figure of which, FIGURE 1, is fragmentary sectional view of the rounded end of an axisymmetric pressure vessel having a boss system which incorporates the features of the present invention. Referring to Figure 1, there is illustrated in fragmented section the rounded, preferably substantially spherical, end of an axisymmetric pressure vessel, generally designated 10. The pressure vessel 10 is comprised of a fibre reinforced outer shell 12 and a non-metallic internal liner 14. A boss 16 extends outwardly through a polar opening 18 formed in the outer shell 12 and defines a pressurization port 20 through which fluid at high pressure may be communicated with the interior of the pressure vessel 10. It is to be noted, however, that the invention may be used in connection with non-polar openings in vessels, as, for example, an opening in a purely spherical vessel. A thin shear accommodating layer 22 is interposed between the outer shell 12, the boss 16 and the liner 14 to prevent damage to the shell or liner during pressurization of the vessel, as will be described hereafter. The outer shell 12 comprises a generally known composite reinforcement made of fibre reinforcing material in a resin matrix. The fibre may be fibreglass, ARAMID, carbon, graphite, or any other generally known fibrous reinforcing material. The resin matrix used may be epoxy, polyester, vinylester, thermoplastic or any other suitable resinous material capable of providing the fragmentation resistance required for the particular application in which the vessel is to be used. The internal liner 14 may be made of plastic or other elastomers and can be manufactured by compression moulding, blow moulding, injection moulding or any other generally known technique. The boss 16 preferably is composed of an alloy of aluminium, steel, nickel, or titanium, although it is understood that other metal and non-metal materials, such as composite materials, are suitable. The thin layer 22 may be made of plastic, or other non-metallic material and may be manufactured by a moulding process or, alternatively, cut from sheet stock. As shown in Figure 1, the subject boss 16 has an outwardly projecting neck 24 with a tapered throat 26 extending through the polar opening 18 in the outer shell 12. The throat 26 is tapered so as to form a concave peripheral groove for receipt of the fibre and resin matrix which make up the shell so that the latter captures the boss 16 to prevent movement of the boss into or out of the vessel 10. Immediately within the pressure vessel 10, an annular support flange 28 radiates from the neck 24 and defines an outer surface 30 of the support flange 28 by which pressurization loads are distributed about the perimeter of the polar opening 18 in the composite shell 12. The support flange 28 has a width W₁ such that the overall diameter of the support flange 28 is sufficient to prevent damage to the outer shell 12 when the pressure vessel 10 is pressurized. In addition, a portion of the thin layer 22 is interposed between the support flange 28, the liner 14 and the outer shell 12 to further minimize damage as the vessel is pressurized. Specifically, pressurization of the vessel interior results in expansive distortion of the rounded vessel end, such that relative slip between the inner surface of the outer shell 12 and the mating portions of the liner 14 and the support flange 28 may occur. In order to accommodate the relative slip and relieve shear stresses otherwise occurring at this interface, the interposed layer 22 extends across the rounded vessel end a distance substantially equal to the diameter D₁ of the circular section of the pressure vessel 10. An annular attachment flange 32 projects radially outward from the support flange 28 a distance W₂. The attachment flange 32 has an outer surface 34 which is inwardly offset from the outer surface 30 of the support flange 28 by a distance T₁, and the attachment flange 32 has an inner surface 36 which is outwardly offset from an inner surface 38 of the support flange 28 by a distance T₂. Thus, the support flange 28 has a thickness T₃ which is sufficient to limit bending stresses in the boss to an acceptable level when the vessel is pressurized. A pair of annular locking grooves 40 and 42, respectively, are located one in the outer surface 34 of the attachment flange 32 and the other in the inner surface 36 of the support flange 38. Each groove receives a complementary tab 44,46, respectively, on the internal liner 12. The locking groove 40 is an outwardly opening groove having a bottom wall 48 intermediate a pair of mutually skewed sidewalls 50, which is to say the groove 40 is a dove-tailed groove. It is understood that other undercut features effective to mechanically lock the liner to the boss are contemplated by the present invention. The locking groove 42 is formed in the inner surface 36 of the attachment flange 32 and has a bottom wall 52 intermediate a pair of mutually skewed sidewalls 54 to again define a dove-tail groove. The complementary geometry of the skewed sidewalls 50 and 54 and respective liner tabs 44 and 46 ensure positive engagement and retention of the internal liner 14 on the boss 16 such that pressurized fluid is prevented from leaking between the liner and the outer shell 12. The offset characteristic of the attachment flange 32 as defined by the inward offset T₁ of the outer surface 34 and the outward offset T₂ of the inner surface 36 reduces the risk that the liner 14 will extrude out of engagement with the boss 16 when under pressure by providing a sufficient surface area for the liner to seal with the attachment flange and prevent leakage.

Claims: A boss system for a pressure vessel (10) having a filament wound outer shell (12) and a non-metallic internal liner (14), the system comprising a boss (16) having a tubular neck (24) projecting outwardly through an opening (18) in the outer shell (12), and an annular flange (28) extending radially from an end of the neck (24) within the vessel (10), the annular flange (28) having an outer surface (30) for reinforcing the perimeter of the opening (18) in the shell (12), characterized by a first generally dovetail-shaped locking groove (40) in the outer surface (30) of the annular-flange (28), a second generally dovetail-shaped locking groove (42) in an inner surface (38) of the annular flange (28), the liner (14) being split at the annular flange (28) with an outside portion outside the annular flange (28) and an inside portion inside the annular flange (28), a first generally dovetail-shaped tab (44) on the outside portion of the liner (14) for locking in the first locking groove (42) in the outer surface (30) of the annular flange (28), and a second generally dovetail-shaped tab (46) on the inside portion of the liner (14) for locking in the second locking groove (42) in the inner surface (38) of the annular flange (28). A boss system as claimed in claim 1, wherein a shear stress relieving layer (22) separate from the liner (14) is interposed between the outer surface (30) of the annular flange (28) and an inner surface of the outer shell (12) to accommodate relative slip therebetween during pressurization of the vessel (10). A boss system as claimed in claim 1, including a shear stress relieving layer (22) interposed between the outer surface (30) of the annular flange (28) and an inner surface of the outer shell (12) to accommodate relative slip therebetween during pressurization of the vessel (10). A boss system as claimed in claim 2 or claim 3, wherein the shear stress relieving layer (22) is made of thermosetting elastomer. A boss system as claimed in any of the preceding claims, including an attachment flange (32) extending radially from the annular flange (28) and having an outer surface (34) spaced inwardly of the outer shell (12), the first generally dovetail-shaped locking groove (40) being in the outer surface (34) of the attachment flange (32). A boss system as claimed in claim 5, wherein the attachment flange (32) has an inner surface (36) which is offset from the inner surface (38) of the annular flange (28). A boss system as claimed in claim 6, wherein the second generally dovetail-shaped locking groove (42) is in the inner surface (36) of the attachment flange (32). A boss system as claimed in any of claims 5 to 7, wherein the outer surface (34) of the attachment flange (32) is offset from the outer surface (30) of the annular flange (28). A boss system as claimed in any of the preceding claims, wherein the boss (16) comprises a material selected from the group consisting of alloys of aluminium, steel, nickel, and titanium or of composite materials.

Inventors: NEWHOUSE NORMAN L, TILLER DALE B, VEYS RONALD B, NEWHOUSE, NORMAN L., TILLER, DALE B., VEYS, RONALD B.
Applicants: TECHNICAL PRODUCTS GROUP INC, TECHNICAL PRODUCTS GROUP, INC.
Application year: 1992
Main classification: F17C13
IPC codes: F17C1, F16J12