WO2008036901A1

WO2008036901A1 - Pressure vessel end closure for membrane cartridges

Info

Publication number: WO2008036901A1
Application number: PCT/US2007/079164
Authority: WO
Inventors: Bing Han; Christopher K. Delap
Original assignee: Bekaert Progressive Composites, Llc
Priority date: 2006-09-22
Filing date: 2007-09-21
Publication date: 2008-03-27

Abstract

An end closure for a filament-wound pressure vessel (11) having a full bore opening for the acceptance of cylindrical filtration cartridges, which closure includes a circular end plug (31), circular seal means (71) carried by the end plug, and a locking ring means (73) for disposition in a groove (21) in the entrance bell end (17) of the pressure vessel to secure the end plug in operative position. The end plug (31) is compression-molded of advanced polymer matrix composite material to have a flat front face (35), a flat central rear region (47) and a coaxial passageway (37). The central flat rear region (47) smoothly transitions to a frustoconical surface (51) that extends radially outward to an annular cavity (53) of V-shape, the outer surface of which cavity is defined by a surrounding, axially extending peripheral flange (55).

Description

PRESSURE VESSEL END CLOSURE FOR MEMBRANE CARTRIDGES

This application claims priority from U.S. Provisional Application Serial No. 60/846,782, filed September 22, 2007, the disclosure of which is incorporated herein by reference. FIELD OF THE INVENTION

The invention relates to elongated pressure vessels designed to hold cylindrical filtration cartridges, particularly cartridges of the crossflow filtration type, and to improved end closures for sealing the end of such a pressure vessel having a full bore opening.

BACKGROUND OF THE INVENTION

With the growth of reverse osmosis, nanofϊltration, ultrafiltration, and micro filtration as important commercial separation processes, there have been ever- increasing uses of elongated pressure vessels that will hold multiple such cartridges in end-to-end orientation. These pressure vessels are sometimes referred to as "full- bore-access" vessels because the entrance at at least one end, e.g. the upstream end, of the vessel must have an opening of a diameter such that a cartridge of a size very close to the interior diameter of the interior bore can be supplied to and removed from the pressure vessel through such entrance. Likewise, the pressure vessel must have a removable end closure which can be used to seal this entrance to the pressure vessel during separation or filtration operation.

U.S. Patent No. 6,632,356 is an example of a patent showing such a pressure vessel having a length such as to accommodate a multitude of cylindrical cartridges or elements of the spiral wound membrane design that are designed for use in separation or filtration processes. However, in this patent the end plate closure is only schematically shown. U.S. Patents Nos. 5,720,411; 5,866,001; 6,165,303; and 6,858,541; and published international patent application WO88/03830 show end closures designed to provide full bore access to the interior of such an elongated tubular pressure vessel that will accommodate a plurality of such cylindrical filtration or separation cartridges. The disclosures of all these patents are expressly incorporated herein by reference.

Many of the aforementioned patents disclose end closures that require sophisticated metal inserts or components for incorporation into the entrance end of the pressure vessel, which pressure vessels are now frequently made by filament winding about a cylindrical mandrel, as is well-known in this art. For example, the '001 patent utilizes a metal end cap 30, that may have a side port incorporated as a part thereof, which is then overwound to become a part of the pressure vessel along with a central tubular liner; the end closure is provided as a part of a complex head 60 that incorporates a circular end plug. The '544 patent illustrates the use of a two piece end closure where a seal plate is disposed inward of a bearing plate that is held in place by a snap ring or the like. The '411 patent shows a dome-shaped metal component 39 and a mating face plate 41, with the combination being held in place by a helical locking ring 79 that is received in a groove which is provided in a metal ring 23 that is wound as a part of the pressure vessel. U.S. Patent No. 5,595,651 shows a relatively low pressure, all-plastic pressure vessel suitable for crossflow filtration which utilizes an end plug 124 that is received in an end cap 12, that is adhesively joined to a straight tubular central casing 11 wherein the cylindrical filtration cartridges are received. The end closure comprises a circular end plug 124 which carries a sealing O-ring 125; the plug is secured in place by a retaining ring 126 molded of Delrin plastic or the like which has a peripheral edge that is received in a groove 1231 in the inner wall of the end cap.

Although the foregoing patents show various solutions to the problem of efficiently closing a pressure vessel having a full bore opening that can accept right circular cylindrical filtration cartridges, with the growth of crossflow filtration applications, there is a great deal of interest in such pressure vessels, and improved end closures have accordingly been sought.

SUMMARY OF THE INVENTION The invention provides an improved end closure for a tubular pressure vessel having a full bore opening wherein an end plug, which is compression-molded of an advanced polymer matrix composite material and relatively light in weight, is provided that will effectively seal the end of such a pressure vessel designed to operate at high internal pressures. The end plug design effectively distributes forces generated by internal pressure within the vessel so as to minimize deflection and compression and tension stresses in the plug, compared to a dome or plate design; as a result, end plug rigidity is retained and continuous tight sealing is assured when the end plug is secured in the bell-end of a pressure vessel by suitable annular locking means, e.g. segmental annular segments, snap ring, helical locking ring with pull tab, etc.

In one particular aspect, the invention provides an end closure for a tubular pressure vessel for holding a cylindrical filtration cartridge, which vessel has (a) a full bore opening for accepting passage of a cylindrical filtration cartridge into the bore of a central casing section of circular cross-section, (b) an entrance region having an interior surface that extends inward from one end of the vessel toward the central cylindrical casing which is of a size to accommodate the cartridge, and (c) a groove in the entrance surface for receiving locking means, said end closure comprising an end plug having a circular outer periphery that can be slidab Iy juxtaposed with the vessel entrance interior surface, circular seal means, and locking means for disposition in said groove for securing the end closure in its operable position, said end plug having a recess for accommodating said seal means in a peripheral surface where it will be seated to establish contact with the entrance surface of the vessel, and said end plug having a central passageway, which is coaxial with said circular outer periphery and extends axially therethrough, and an inward-facing rear wall that includes a flat center region which surrounds said central passageway, said inward wall transitioning smoothly along an annular surface to a frustoconical surface that is aligned at an angle of between 50° and 65° to the axis of the central passageway, and said frustoconical surface terminating in a peripheral, annular cavity of generally V-shape, that extends radially outward to a circle of a radius between about 90% and about 95% of the radius of said circular end plug.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a perspective view of the entrance end of a pressure vessel, shown in cross-section, wherein there is installed one preferred embodiment of an end closure illustrating various features of the invention. FIGURE 2 is an elevation view of the FIG. 1 embodiment slightly reduced in size and also shown in cross-section.

FIGURE 3 is a front view of the end plug of the end closure that is shown in FIGS. 1 and 2.

FIGURE 4 is a cross-sectional view taken generally along the line 4-4 of FIG. 3. FIGURE 5 is a front perspective view, reduced in size, of the end plug of FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 illustrate the entrance end of a pressure vessel 11 having a full bore opening. The pressure vessel is one made of fiber-reinforced plastic (FRP) and is designed so as can be produced by conventional winding of polymeric resin- impregnated filaments about a mandrel, as generally described with respect to FIG. 2 of the '303 patent. As such, the vessel will have a central casing section 13 that has a cylindrical interior bore 15 of circular cross-section that is sized to be just slightly greater than the diameter of the cylindrical filtration cartridges that will be accommodated therewithin. Each end region of the pressure vessel 11 is preferably formed as an entrance or bell end section 17 of greater interior and exterior diameter than the central casing section 13; however, the entrance end regions might be formed with a diameter substantially the same as that of the casing section. In its fabrication, a suitable spacer is positioned on the mandrel which is shaped to provide an interior sidewall of the bell end of greater diameter and a short frustoconical transition section between the bell end and the central bore 15. Because such a spacer can be slidably removed from the bell end of the fabricated vessel, such a re-usable spacer can be made of metal which allows a tighter tolerance to be obtained in such FRP vessels. An annular insert 19, generally made of metal, that defines an interior circumferential groove 21 is slidably located on the spacer; this insert 19 then remains as an integral part of the pressure vessel 11. Following its fabrication, removal of the vessel from the mandrel, and then removal of the spacer, the bell end section 17 has an interior cylindrical entrance wall 23 of constant diameter which extends to a short frustoconical transition surface 25 that leads to the interior bore 15 of the pressure vessel. The pressure vessel may be optionally provided with one or more side ports by simply cutting circular passageways through the sidewall of the bell end section 17 in the region between the annular insert 19 and the transition surface 25; a side port fitting 27 can then be suitably installed within such a radial passageway and sealed via an annular seal on a peripheral flange 28 that can be accommodated in the annular region provided by the removed spacer or such can be seated in a counterbore machined in the interior wall surface of the bell end well as known in this art. Two such fittings 27 are shown in FIGS. 1 and 2. FIGS. 3, 4 and 5 illustrate the details of one embodiment of an end plug 31 having preferred features of the invention that is designed to effectively seal the bell end of the pressure vessel 11. The end plug 31 is circular in shape, having a peripheral surface 33 that is a section of a right circular cylindrical surface, and its diameter is such that it is slidably received in the bell end of the vessel, being just slightly less than the diameter of the entrance surface 23, which entrance surface is of constant diameter until it reaches the transition section 25. The end plug 31 has a flat front wall 35 and has an axial passageway 37 that extends centrally through the core of the end plug to accommodate an interconnection with a permeate outflow conduit of a cylindrical, spirally wound filtration cartridge. The central passageway 37 accommodates passage of an axially outer section 39 of a tubular interconnector 41; the axially inner end section 43 thereof serves as a female connector that accepts a permeate discharge conduit extending axially from the end cylindrical filtration cartridge, as well known in this art. Entry of the permeate conduit into the female connector section is guided by a lead-in chamfered entry surface. A pair of circular grooves 45 are provided in the interior surface of the inner section 43 of the tubular connector, which grooves accommodate elastomeric seals that assure a fluidtight joinder to the permeate conduit, as well known in this art.

The rear or inward-facing wall of the end plug 31 includes a central flat section 47 that surrounds the passageway 37; the wall smoothly transitions, in an annular region 49, to a frustoconical surface 51. This frustoconical surface 51 extends to a circular boundary having a radius that is between about 90% and 95% of the radius of the end plug 31 , preferably between about 90% to 93% and more preferably between about 91 % and 92%. The frustoconical surface 51 terminates at the bottom of an annular cavity 53 of V-shaped cross-section, the radially outer wall of which is formed by an axially extending peripheral flange 55 which also provides the peripheral surface 33 of the end plug. The flange 55 has a thickness equal to about 5% to about 10% of the radius of the circular end plug 31. A recess 57 in the flange 55 extends to a shoulder 59; this recess 57 accommodates an elastomeric seal 71 (FIG. 2) which will be seated in contact with the cylindrical entrance surface 23 of the bell end and thereby seal the periphery of the end plug when compressed in place. Twelve generally teardrop-shaped cavities 61 are provided in the flat front wall 35 of the end plug 31 extending rearward therefrom, which are regularly angularly spaced at 30° increments about the central axis of the end plug. Spoke-like radially aligned wall sections 63 extend generally from a central boss region 65 of the core wherein the passageway 37 is formed. As seen in FIG. 4, the thickness T of the main wall between the frustoconical surface 51 and the interior surface of the cavities increases by between about 5% to 10% in the axial direction toward the flat front face 35. The interior wall surface of the cavities (see FIG. 4) and thus the innermost walls thereof is designed to lie at an angle A between 25° and 40° to the peripheral surface 33 of the plug to the axis of the central passageway 37 (and the pressure vessel itself). More preferably, orientation is at an angle of about 30° to about 35° and most preferably between about 31° and about 33°. Because of its thickening, the frustoconical surface should lie at an angle about 2° greater so the frustoconical surface which would then be oriented at between about 27° and 42°, more preferably lies at an angle between about 32° and 37° to the centerline of the pressure vessel and more preferably at about 33° and 35° thereto. This overall construction and orientation of the radially outwardly thickening wall optimizes the section modulus, providing greater stiffness in the central region of the plug where it is most beneficial. The assembled pressure vessel 11 shown in FIGS. 1 and 2 would hold a plurality of cylindrical filtration cartridges (not shown); the permeate conduit extending from the filtration cartridge at the left hand end of such a group of cartridges within the pressure vessel 11 would be received in the tubular interconnector 41. With the group of cartridges in a place and interconnected, the end closure plug 31 having an elastomeric seal 71 installed in the peripheral recess 57, where it would seat against the shoulder 59, is slidably inserted; the lead-in chamfer would guide the entry of the end of the permeate conduit into the female connector 43. Although an O-ring seal 71 is depicted, a chevron seal, a quad seal or one of rectangular or other suitable cross-section might be used. With the end plug 31 in place, locking means would be installed in the groove 21 provided by the annular insert 19 to secure the end closure of the pressure vessel. In the illustrated arrangement, a segmental locking ring 73 is illustrated, which would be preferred for a high pressure, seawater type operation where internal pressures might reach 800- 1200 psi. For lower pressure operation pressure vessels, a snap ring or a helical locking ring, such as one of those shown in the '411 patent and in U.S. Design Patent No. D468,392, may be used. When the pressure vessel 11 is being operated in filtration mode, the interior of the casing 13 will be at a high pressure, for example, 300-1200 psi, and accordingly, the end plug 31 which closes the end of the vessel must be able to withstand such pressure over its lifetime in order to retain its shape and assure tight sealing. Whereas it was previously felt that two-piece end closures and/or metal components made of stainless steel or the like should be employed to withstand such pressures, as the diameter of such pressure vessels has grown, e.g., first from 4" to 8", and now to 12" and 16", the substantial increase in both the weight and expense of such end closures has caused such to be reconsidered. Surprisingly, it is found that the end plug 31 made entirely from compression-molded polymer matrix composite material, if designed in accordance with the foregoing description, can effectively seal a pressure vessel which is, for example, 12" or 16" in diameter and operating at pressures as high as about 1200 psi. It has been found that, by carefully shaping the inward wall surface of the end plug and gradually thickening the wall, the compression and tension forces imposed on the inward surface of the end plug 31 are excellently accommodated without deforming the polymer matrix compression- molded body. The wall is shaped such that a flat generally circular central wall surface 47 extends to the annular transition surface 49, which has its midpoint located at a diameter between about 50% and about 60% of the circular periphery of the end plug, and which extends as a frustoconical surface at an angle of between 50° and 65° to the axis of the pressure vessel to the V-shaped annular cavity 53 located just interior of the periphery of the end plug. As a result, it is found that these very significant internal forces are redirected through the gradually thickened radially outer region of the wall to the radially outer annular region of the end plug 31 where they are effectively borne by the locking ring means in contact therewith. The end plug 31 is compression-molded as an integral structure, preferably from a commercially available chopped glass-filled, thermosetting, vinyl ester, polymeric resin material, wherein glass fibers of 1 inch or greater length, e.g. 1-3 inches, constitute greater than 50% of the volume of the material. Other strong fibers, e.g. carbon fibers, could also be used. As a result of this design and use of such polymeric material, an end plug 31 sized to close a pressure vessel that accepts 16" diameter filtration cartridges may weigh only about 35 pounds, yet because of its design, the compression-molded advanced polymer matrix composite component is assured a long stable lifetime. Although the invention has been described with regard to certain preferred embodiments which constitute the best mode known at this time to the inventors, it should be understood that various modifications and changes that would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is set forth by the claims that are appended hereto. Particular features of the invention are emphasized in the claims that follow.

Claims

1. An end closure for a tubular pressure vessel for holding a cylindrical filtration cartridge, which vessel has (a) a full bore opening for accepting passage of a cylindrical filtration cartridge into the bore of a central casing section of circular cross-section, (b) an entrance region having an interior surface that extends inward from one end of the vessel toward the central cylindrical casing which is of a size to accommodate the cartridge, and (c) a groove in the entrance surface for receiving locking means, said end closure comprising: an end plug having a circular outer periphery that can be slidably juxtaposed with the vessel entrance interior surface, circular seal means, and locking means for disposition in said groove for securing the end closure in its operable position, said end plug having a recess for accommodating said seal means in a peripheral surface where it will be seated to establish contact with the entrance surface of the vessel, and said end plug having a central passageway, which is coaxial with said circular outer periphery and extends axially therethrough, and an inward-facing rear wall that includes a flat center region which surrounds said central passageway, said inward wall transitioning smoothly along an annular surface to a frustoconical surface that is aligned at an angle of between 27° and 42° to the axis of the central passageway, and said frustoconical surface terminating in a peripheral, annular cavity of generally V- shape, that extends radially outward to a circle of a radius between about 90% and about 95% of the radius of said circular end plug.

2. The end closure according to claim 1 wherein said recess of said end plug is located in the exterior surface of a short tubular flange which forms both the circular periphery of said end plug and a radially outer wall of said V-shaped annular cavity.

3. The end closure according to claim 2 wherein said recess is located at the axially inward end of said circular periphery of said end plug.

4. The end closure according to any one of claims 1 to 3 wherein said circular seal means is an O-ring.

5. The end closure according to any one of claims 1 to 3 wherein said annular transition surface has a midpoint diameter equal to between about 50% and 60% of the diameter of said circular outer periphery of said end plug.

6. The end closure according to any one of claims 1 to 3 wherein said end plug has a flat front wall wherein a plurality of radially extending cavities are located, which cavities are equiangularly spaced about said central passageway.

7. The end closure according to claim 6 wherein the wall of the end plug between said frustoconical surface and the inner surfaces of said cavities increases in thickness in a direction radially outward.

8. The end closure according to claim 7 wherein said frustoconical surface lies at an angle of between about 32° and 37° to the centerline of the pressure vessel.