US3587905A - Pressure vessel - Google Patents

Abstract

A PRESSURE VESSEL SUITABLE FOR LARGE SCALE CHEMICAL PROCESSING COMPRISES AN INNER SHELL INSUFFICIENTLY STRONG TO WITHSTAND THE CIRCUMFERENTIAL STRESSES AT THE INTENDED WORKING PRESSURE OF THE VESSEL, IN COMBINATION WITH SEPARATE EXTERNAL MEMBERS SUPPORTING THE INNER SHELL AGAINST CIRCUMFERENTIAL STRESS AND POSSIBLE ALSO MERIDIONAL STRESS IF THE VOLUME AND WORKING PRESSURE ARE GREAT ENOUGH TO DEMAND IT. THE CIRCUMFERENTIAL STRESS MEMBER MAY BE OUTSIDE OR INSIDE THE MERIDIONAL STRESS MEMBERS. FOR PROCESSING CORROSIVE MATERIALS A DOUBLE INNER SHELL OF CORROSION-RESISTANT METAL OR ALLOY MAY BE USED, THE CAVITY BETWEEN THE TWO SHELLS BEING VENTED SO AS TO DIMINISH OR AVOID CONTACT OF THE CORROSIVE MATERIAL WITH THE STRESS-BEARING MEMBERS OF THE VESSEL.

Description

United States Patent [72] lnventor John McFarland Norton-on-Tees, England [21] Appl. No. 779,715

[22] Filed Nov. 29, 1968 [45] Patented June 28, 1971 [73] Assignee Imperial Chemical Industries Limited London, England [54] PRESSURE VESSEL 8 Claims, 11 Drawing Figs.

2,332,462 10/1943 Nilson 220/3 Primary Examiner-Raphael H. Schwartz Att0rneyCushman, Darby and Cushman ABSTRACT: A pressure vessel suitable for large scale chemical processing comprises an inner shell insufficiently strong to withstand the circumferential stresses at the intended working pressure of the vessel, in combination with separate external members supporting the inner shell against circumferential stress and possible also meridional stress if the volume and working pressure are great enough to demand it. The circumferential stress members may be outside or inside the meridional stress members. For processing corrosive materials a double inner shell of corrosion-resistant metal or alloy may be used, the cavity between the two shells being vented so as to diminish or avoid contact of the corrosive materials with the stress-bearing members of the vessel.

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zw 'M A tlorneys PRESSURE VESSEL This invention relates to a pressure vessel constructed in a manner which aflords a number of advantages in ease of assembly, especially when its volume is very large.

Pressure vessels are required for a number of industrial purposes, such as chemical synthesis and hydrocarbon treating. Usually they are provided with entry and exit ports for fluids and they may whe'nin use contain catalyst particles, as in the synthesis of ammonia or methanol; and/or they may be provided with internal structures affording special gas flow-paths, as in the synthesis of ammonia or methanol or in certain types of noncatalytic hydrocarbon hydrogenator; and for other uses such as hydrocarbon partial oxidation they are used empty but with a refractory lining.

Recent advances in technology have led to the recognition of the economic advantages of processing plants affording very large outputs, for example lO-2000 tons and more per day of ammonia. Such outputs call for pressure vessels larger than established techniques of construction can reliably provide; and consequently a new analysis of the problems of making such vessels is called for.

In pressure vessels according to the invention the functions of withstanding meridional and circumferential stresses are separated. In known pressure vessels the cylindrical shell takes both the meridional and circumferential loads and, when the vessel requirements are extrapolated outside the range of volume and working pressure of present-day pressure vessels, welding and/or successful heat treatment of the metal concerned become difficult. Vessels according to the invention use separate masses of metal for taking the two types of load. These may be in a thin sheet form which can have exceptionally high mechanical properties. The requirements for welding are extremely simple: apart from a thin inner shell which must be leaktight, no welds are absolutely necessary in many vessels according to the invention.

According to the invention a pressure vessel comprises an inner shell having a strength insufficient to withstand the circumferential and meridional stresses at the intended working pressure of the vessel, a set of external reinforcing rings supporting the inner shell and capable together of maintaining it against the circumferential stresses at the intended working pressure, a set of meridional members capable of withstanding the meridional loads on the vessel, and a pair of shell end members each secured to one end of the inner shell.

In one form of the invention the reinforcing rings fit closely over the inner shell, and cover almost its whole area. There can however be small annular open areas between the inner cylindrical faces of the the successive rings, into which open areas the inner shell can expand by elastic deformation; for this purpose the inner edges of the rings should be rounded. In this way the rings are kept tightly in position when the vessel is under pressure. If, as is preferred, the aggregate height of the rings is less than the total height of the inner shell, the rings can during assembly of the vessel be moved towards the firstapplied end-member in order to allow examination of the weld by which the second-applied end-member is to be secured. After this weld has been examined and approved, the rings are moved to their final position in which the intermediate rings are about equally spaced.

The reinforcing rings can be solid-forged, but more con veniently they are built up by winding metal strip or plate. In making any ring a single length (which can be assembled by welding smaller lengths end to end) of such strip or plate can be used, or several layers of coaxial rings can be used at each level instead ofa single integral ring. The cross section of each ring should be such as to present a substantial flat area for contact with the vessel inner portion, but it need not be rectangular. Each end ring should however preferably fit closely with the edge of the adjacent end member. Each ring. if of the builtup type, is preferably held in one piece by welding, at least at the outside end of the metal strip from which it is made up. If more than one coaxial layer of reinforcing rings is used, then the number of rings need not be the same in each layer. In an alternative form of the invention the rings are long in the radial direction and short in the axial direction, such that their shape is that of large washers.

In addition to the reinforcement against circumferential stresses, vessels require external means to maintain the end members against meridional stresses. The external means can take the form of one or more internal bracing members, but is more conveniently external to the vessel, for example in the form of an external framework. A very convenient means comprises a set of bridging members outside the reinforcing rings whose ends engage with the end members and which are disposed in a meridional position with respect to the vessel. There are normally at least three bridging members: how many are used depends inter alia on the number of degrees of are over which each engages. They are preferably kept in position by outer retaining rings, but in a convenient form of the invention are inside the reinforcing rings already mentioned. The methods of construction described above for the reinforcing rings can be used for the bridging members or outer retaining rings or both.

Alternatively the meridional stress members can be inside the reinforcing rings.

One or more of the end-members can be a removable closure, for example, of the convex-inwards type described in U.S. Pat. No. 3,410,447. If a large removable closure is not required one or both of the shell end-members is formed with an edge which can be welded to the inner shell, or attached by mechanical means. Preferably each is formed with an annular flange or set of helical or annular splines with which the external framework or the meridional bridging members can cooperate. Each end reinforcing ring is preferably shaped sectionally in such away that it reinforces also the thicker portions of the end members adjacent to their welding edges. This feature is shown clearly in FIG. 1.

The vessel according to the invention has the advantage that it can be assembled without the use of large threaded connections. Thus in the form of the vessel shown in FIGS. 1 and 4 the reinforcing rings, end member flanges, bridging members and retaining rings are held in their mutual dispositions by massive overlap.

The vessel according to the invention is capable of withstanding high pressures, for example l00500 atmospheres, even when made in very large sizes, for example volumes in the range 10 to 3000 cubic metres the upper part of which range clearly constitutes an extrapolation outside the range of present day vessels. Still higher pressures are possible, as may be seen from FIGS. 2 and 3. The vessel does not depend on the making of very large forgings whose strength tends to be nonuniform unless extreme skill is used. Indeed much of the vessel is made up from readily available standard steel sections. It can in favourable conditions be assembled on the site at which it is to be used, and requires very little heat treatment before and after any welding employed. Using preferred methods of assembly any welds can be examined from both sides and heat treated without obstruction by other parts. The inner shell can be made of metal of high corrosion resistance without incurring excessive cost, since the quantity of such metal is comparatively small. Should the inner shell fail, only a leak, and not a catastrophic failure of the vessel, is to be expected and the invention includes a vessel in which a leak passage is deliberately provided.

The invention is illustrated in the accompanying drawings wherein FIGS. 1 and 2 are sectional elevation views of two different types of pressure vessels,

FIG. 3 is a plan view of the vessel shown in FIG. 2; and

FIGS. 4-8 are sectional elevations of other vessel constructions according to the invention.

FIGS. 4A, 5A and 7A are sectional views through the vessel on line 4A-4A of FIG. 4, line 5A-5A of FIG. 5 and on line 7A-7A of FIG. 7 on the transverse axis showing the arrange- Referring more specifically to the drawings, FIG 1 shows one manner of construction of an ammonia synthesis converter wherein the inner shell is formed of three short cylindrical sections welded together at 12 and welded at 14 to the shell end-members 16, which are of welded, forged or layer construction. The inner shell 10 is reinforced by rings 18 and end rings 20. The rings 18 are slightly spaced from each other and from the inner shell 10 so as to allow the formation of convolutions 22 as the result of elastic distortion during the hydraulic test of the vessel or during use at pressure. The end rings 20 are accurately machined so as to give a good fit with the edge surface of the end member 16. The rings 18 and 20 are built up of layers of wound steel plate suitably inch thick. The end-members 16 are maintained against lengthwise movement, that is, the meridional stress is taken, by meridional bridging members 24, which engageby means of end lugs 26 the flanges 28 of the end-members l6, and which are held in position by retaining rings 30.

In assembling the vessel first the lower closing weld 14 is made between the lowermost inner shell section and the lower end-member 16. This weld can be examined on both sides. Then the successive reinforcing rings 20 and 18 are applied and the successive inner shell sections welded on to the first, the welding of course being carried out before applying the ring at that level. When the upper end ring 20 has been applied the total height of the rings is such that the upper edge of the inner shell is above the top surface of the upper end ring 20. The end-member I6 is then welded on, the weld being examined from both sides. The end ring 20 is then moved into a close-fitting position with the edge of end member 16. The other rings 18 are moved to make the spaces between successive rings substantially equal, spacing pieces (not shown),

being inserted to keep the rings in place and thus to press the end rings into contact with the end-member edges. (These spacing pieces are unnecessary after the hydraulic test has been carried out if the test pressure is high enough in relation to the strength of the inner shell 10 to produce the convolutions 22). Meridional bridging members 24 are then placed in position and retained by rings 30, which can be bolted or alternatively heat-shrunk to keep them in position.

FIGS. 2 and 3 show in sectional elevation and plan, respectively, a pressure vessel for extremely high pressure service, for example 3,000 atmospheres. Large pressure vessels for this type of duty have theretofore been limited to a diameter of approximately 20 inches, and are commonly fabricated in a solid tubelike forging having two heavy closures. A 20-inch internal diameter vessel of this existing type using high strength materials is likely to have an outside diameter of approximately 48 inches. The length of such a forging is limited by the size of ingot that can be handled. The wall thickness of 14 inches very largely precludes high strength since methods of improving this by heat treatment are limited by the heavy mass of metal involved. The method of FIGS. 2 and 3 however allows the fabrication of vessels with a bore of at least 30 inches, with no serious limitations on the overall length of the vessel. Only thin material is used, so that very high strength material can be utilized; and moreover no welding is involved in this high strength material.

The vessel shown in FIGS. 2 and 3 consists of an inner leaktight cylinder A, which should be made preferably of ductile material such as, for example, an austenitic stainless steel. The end closure E, E of the vessel is secured to the cylinder A by a screw thread and sealed with a sealing ring. On the end of the cylinder A are machined a series of circular ridges or corrugations B. In FIG. 2 these are shown with a rectangular cross section although an acme form would also be suitable. These corrugations engage grooves C in the ends of meridional members D which are suitably of rectangular or frustosectorial cross section. Members D are constructed of a high strength carbon or alloy steel and may be heat-treated to obtain the necessary properties. When the members D have been assem bled around the inner cylinder A they are retained in position temporarily by two shrunk-on rings G, one at each end, which maintain the assembly in one piece for subsequent work. The circumferential stresses in the vessel are withstood by annular washers H which correspond to the reinforcing rings of FIG. 1 and are made of high strength material. The inner bore of washers H is slightly smaller than the outside diameter of meridional members D and the rings are heated and shrunk into position.

If welding in the high strength material is not allowed, then the size of the vessel is governed by the outer diameter of the members H, which depends upon the size of sheet available. However, it is possible to make larger vessels if members H are made from plates which have been welded together. It will be noted that in this case these welds before assembly are completely available for inspection of any type.

FIG. 4 shows in sectional elevation a manner of construction of very large pressure vessels, having for example an internal volume of 1,000 meter and a bore of 26 ft. or more and intended for use at l00 atmospheres pressure.

For such a pressure vessel the wall-thickness is such that present technology cannot provide a satisfactory means of joining the elements together by welding, and the physical size is such as to prevent transport of the complete vessel or its subassemblies. In particular, although the cylindrical portion of the vessel can be constructed, difficulties exist in the attachment of the ends of the vessel.

The vessel consists ofa cylindrical shell A which is leaktight and is constructed of a ductile material. This cylinder has machined on it a series of circular male corrugations or splines D.

This shell is closed by two covers B, one at each end, of the type according to US. Pat. No. 3,410,447, the covers being designed to operate without bending stresses and the only stresses present being compressive. In this specific instance, since only membrane forces are present, the cover is in the form of a number of close-fitting laminae, but since the stresses are compressive, it is possible to use materials which in massive form have good compressive strength though they may be weak in tension, for example cast iron, glass or concrete, possibly with a covering membrane of ductile material to produce a leaktight cover.

The meridional stresses are taken by a series of members C which are of frustosectorial or rectangular section and have grooves at their two ends mating with the corrugations on shell A. Additionally, these members C have projections at each end which serve to retain the end-covers of the vessel. It will be noted that some of members C can be extended, as at E, to provide a support for the vessel on its foundation.

The circumferential stresses are taken by rings F formed of spirally wound or coaxial plate material. These rings are lowered into position and can be shrunk thermally onto the vessel. At the level of each cover definite thermal contraction is necessary in order to provide the initial axial load on the cover and thus rings H around the bottom cover are of larger internal diameter than rings H around higher up to allow easier fitting in this region. For simplicity only one step has been shown, but in practice more steps may be employed in order to assist application of heat-expanded rings. It will also be noted that rings G at the top cover and similar rings at the bottom cover, are of greater radial thickness. The greater thickness is to deal with the higher stresses at the level of the covers.

FIG. 5 shows in sectional elevation a detail of an alternative vessel generally similar to the vessel of FIG. 4. In this vessel the meridional members are outside the rings, except at the ends. An optional sealing strip l is welded between the cover and the shell to improve the seal at this junction.

FIG. 6 shows in sectional elevation a detail of a vessel resembling that of FIG. I but in which the positions of the meridional and circumferential members have been reversed. The longitudinal load imposed on cover A is transmitted to the meridional members D through a series of corrugations C on an extension of the cover B. The inner leaktight cylinder E is welded to the cover extension. Two small rings F are shrunk onto members- D to retain them in position preparatory to receiving rings G, which may be spirally rolled or can be annular washers as described above.

FIGS. 7 and 8 show in sectional elevation at pressure vessel in which the principle of the invention is applied to the special requirements of processing corrosive materials. Before describing these it should be explained that pressure vessels are generally made from a carbon or alloy steel since this is the most economical method of producing them and these steels have high strength and relatively low cost but, owing to low corrosion resistance of such steels, are commonly lined, for example with silver, stainless steel, titanium or zirconium. Owing for example to differential expansion, faults occur in liners and consequently the corrosive materials locally attack the vessel shell. When the vessel is made by a previouslyproposed multilayer or strip-winding method, or as described in the preceding drawings, failure of the inner liner allows the corrosive medium to penetrate many layers of metal. The

complete cleaning of a vessel in this condition is virtually impossible without a complete strip-down, which is extremely costly.

Although applicable to any of the vessels mentioned, this aspect of the invention is applied in FIG. 7 and 8 to a vessel according to FIG. 6. The hemispherical covers A are lined with titanium at B and this lining is continued to the outer diameter of the cover. The corrosion resistant cylindrical shell C is welded to the two covers and a secondary liner D is wound around this leaving a small interspace E which is maintained by means of a discontinuous titanium sheet which can be seen in the enlarged detail of FIG. 8. The vessel is completed by means of meridional members F and rings G as previously. Before completion of the bottom cover, however, titanium tubes are set into the cover and welded to the cover liner. These -tubes line up with the interspace E between the two titanium cylindrical shells. After the vessel has been completed the small tube H has been closed at its upper end by the butt welds between the head and the cylindrical titanium shells, but is then cleared with a long shank center drill. The advantage of this system is that if a leak occurs in the inner liner C it is unlikely to penetrate any small cracks in the outer liner D since the interspace is vented to atmosphere and there is no driving force to cause the penetration of the liner D. In addition, any aggressive fluid which has passed through C will not form a corrosion product since it finds itself in an all-titanium en vironment. lfa leak has been detected by this method the corrosion products can be removed by trepanning a hole in the inner liner C at the point of failure and washing them away from the immediate region of the repair. For repair, a patch can then be welded in place: the welding of this patch is made somewhat easier since it is being welded against a titanium background and not a carbon steel background.

lclaim:

l. A pressure vessel comprising an inner shell having a strength insufficient to withstand the circumferential and meridional stresses at the intended working pressure of the vessel, a set of reinforcing rings supporting the inner shell and capable together of maintaining it against the circumferential stresses at the intended working pressure, a set of meridional members capable of withstanding the meridional loads on the vessel, and a pair of shell end members each secured to one end of the inner shell wherein the meridional members are in contact with the inner shell, are secured to the shell end members and are surrounded by reinforcing rings.

2. A pressure vessel according to claim 1 in which the reinforcing rings are ofa strip-wound construction.

3. A pressure vessel according to claim 1 in which the reinforcing rings are washers.

4. A pressure vessel according to claim 1 in which the inner shell is surrounded by a second inner shell so as to provide a cavity of cylindrical section, a discontinuous support member is provided between the two shells, and at least one vent is provided from the cavity.

5. A pressure vessel according to claim 4 in which the supporting rings and bridging members are made of steel and the two inner shells and the discontinuous support are made of a metal or alloy which is not subject to corrosion by the materials to be processed in the vessel.

6. A pressure vessel according to claim 1 wherein the inner 'shell and meridional members are formed with mutually cooperating corrugations whereby to carry the meridional loads on the inner shell.

7. A pressure vessel according to claim 1 wherein the meridional members are formed with abutments which cooperate to form a seating maintaining the end members in position.

8. A pressure vessel comprising an inner shell having a strength insufficient to withstand the circumferential and meridional stresses at the intended working pressure of the vessel and formed with mainly circumferential corrugations in the region of its axial extremities, a set of reinforcing rings supporting the inner shell and capable together of maintaining it against the-circumferential stresses at the intended working pressure, a set of meridional members capable of withstanding the meridional loads on the vessel and each formed with corrugations corresponding to the corrugations in the inner shell and an abutment at each end, a pa r of inwardly convex shell end members, the meridional members being in contact with the inner shell, the corrugations of both mutually cooperating, the abutments of the meridional members forming a seating maintaining the inwardly convex end members in position and the meridional members being surrounded by spirally wound reinforcing rings.

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