PRESSURE VESSEL MADE OF ALUMINIUM AND A METHOD FOR MANUFACTURING SUCH VESSEL

Abstract: Pressure vessel made of aluminium for transport and storage of fluids. The vessel comprises two mainly semi-spherical, forged parts (2), that are joined together along their periphery by means of plasma welding. Each semi-spherical part is provided with a boss (4) that is formed under the forging operation, one or both of the bosses may be provided with passing bores (5) having internal threads for attaching a valve/valves thereto. Further, one or both of the bosses may be equipped with external threads for the attachment of protective caps (8), or one protective cap and a support leg.

Description: Pressure vessel made of aluminium and a method for manufacturing such vessel The present invention relates to a pressure vessel made of aluminium, and a method for manufacturing such a vessel. It has previously been known to produce pressure vessels of aluminium, involving extruded, substantially circular profiles, having lids at the ends. The lids may be joined to the profiles by means of welds or threaded connections. Extruded profiles may, caused by the extrusion process, be encumbered with longitudinal structural defects in the material. It is therefore necessary that the profiles are examined by comprehensive and expensive material tests to detect possible defects. An another disadvantage is that vessels of extended shape have relatively low strength and little volume as to the shape. Further, spherical high pressure vessels made of steel is known in the art. Such vessels may be manufactured when joining spherical segments by riveting operations (rivet connections), or, as most common today, by welding operations. Spherical vessels do have the best properties as to strength, and do at the same time have the largest volume as to total weight and surface area. As known to the inventors, such spherical vessels have not yet been manufactured of aluminium. The main reasons for this is that aluminium is not well suited for welding as heat cracks may easily occur in the welding zone, followed by a reduced material strength in this area. As the material costs in relation to aluminium are high, it is required to apply a low-cost manufacturing method, to compensate for the total manufacturing costs when manufacturing pressure vessels in aluminium. On the other hand, aluminium is a light-weight material compared to steel, and is therefore advantageous as to manual handling and transport. According to the invention it is provided a pressure vessel of aluminium and a manufacturing method for such a vessel where the aforesaid known problems are solved, and where the above mentioned advantages are achieved. More precisely, it is obtained a pressure vessel of aluminium having a good strength which is capable to withstand high pressures as compared to its weight. The vessel is simple, cheap to manufacture, and easy to handle. According to the invention, the vessel is characterised in that it consists of two mainly semi-spherical, forged parts that are joined along their periphery by means of plasma-welding, as described by constructional features in the accompanying independent claim 1, or as expressed by measures or actions in the accompanying independent claim 5. The dependent claims 2-4 and 5-9 describes advantageous features of the invention. In the following, the invention is described in detail with reference to drawings that illustrate embodiments thereof: Fig. 1 is a cross section of a manufactured vessel according to the invention, Fig. 2 is the right half-part of a pre-forming tool Fig. 3 is the right half-part of a finishing forming tool. As mentioned above, Fig. 1 shows a vessel 1 according to the invention where the manufacture has been fulfilled. The vessel consists of two forged, semi-spherical parts 2, made of aluminium and welded (at 3) together at their periphery. Each semi-spherical part is provided with a boss 4 having passing bores 5. In relation to each bore there is arranged a filling valve 6 and a bleeding valve 7, respectively. To protect the valves, there are arranged protection caps 8 in relation to each boss, the caps being adapted to be attached to the bosses 4 by fastening means. The purpose of using two valves as shown here, is that the vessel may be used in collecting gas samples offshore. The vessel may be completely filled, when filling gas through the filling valve 6 while air is evacuated through bleeding valve 7. The invention as it is defined in the claims is not limited to the embodiment including two valves. The vessel may have other applications than mentioned above, e. g. it may be used for storing propane gas on shore, and then having only one valve for filling and evacuating gas. With such application, the lower boss may be used to support a support leg or the like, to support the vessel under storage and transport. The two semi-spheres 2 is, as previously mentioned, manufactured by forging (die forging). The blank used may advantageously be cut off a aluminium stud. The manufacturing process for the semi-spheres is as follows: The blanks to be forged are heat-treated in a heat treatment oven at 510 ° C (± 5°C) for at least 4 hours. When the heat treatment has been fulfilled, a pre-forging of the blanks is performed in a press comprising a pre-forming tool with an upper 9 and a lower 10 tool part as shown in Fig. 2. At this stage, the tool is at an temperature of 300 - 325 °C and is coated with a lubricant. The forming operation may comprise 1 - 3 forging strokes, depending on the size (capacity) of the press and the dimensions of the blanks. The pre-forming operation forms the blanks into pre-fabricated, non-rounded semi-products, with its external periphery formed to a final thickness, while the inner boss section still have to be subject to further forming operations. A grading//burring operation is then performed to the semi-product, followed by a heat treatment in an oven, as mentioned above, at 510 °C for at least 4 hours. The final forging step is performed at 300 - 325 °C with a final forming tool, comprising an upper 11 and a lower tool part as shown in Fig. 3. To obtain a good floating ability of the material in the boss area in the final forging step, the semi-product is placed with its projecting section pointing downwards in the final-forming tool. The material is thereby being twisted into the upper tool part 11, said part comprising a cavity that receives the boss of the semi-sphere. The final forming operation rounds the semi-sphere and forms said boss 4 in the central part of the half-part. The final forming operation may advantageously comprise two strokes, applying lubricant at each forging stroke. When the final forging operation is fulfilled, the semi-spheres are rapidly cooled to room temperature in a water bath. The parts are then machined to their correct dimensions prior to the welding operation. As mentioned previously, the vessel according to the invention is manufactured by welding the two semi-spheres to each other at their periphery by means of plasma welding. See Fig. 1. It was observed that the use of plasma welding in combination with a correct flux material as to the material in the vessel (base material), results in a welded connection with mechanical characteristics as good as that of the rest of the vessel. Advantageously the vessel may be constituted of an alloy of the AIMgSM (AA6082) type, and as flux material in the welding operation an alloy of the AISi5 (AA4043) type may be used. AISi5 has a melting point that is lower than that of AIMgSil, and this feature is of substantial importance to avoid heat cracks in the zone close up to the solidification front (the PMZ-zone - partially melting zone). More particularly, when developing the invention, it was discovered that the aluminium alloy used as base material in the vessel should not contain less than 0,95 weight % Si, and not less than 0,9 weight % Mg. Further, it was discovered that the content of hydrogen should not exceed 0,1 ppm, either in the base material or in the flux material (the welding wire). When the welding operation is fulfilled, the vessels (spheres) are hardened at approx. 540 °C for approx. 4 hours, followed by a sudden cooling in a water bath. Further, the vessels are hardened at approx. 130 °C for approx. 24 hours, followed by final work up and coating operations.

Claims: Claims 1. Pressure vessel made of aluminium for transport and storage of fluids, characterized in that it comprises two essentially semi-spherical forged parts (2) joined together at their periphery by means of plasma welding. 2. Pressure vessel according to claim 1 , characterized in that each semi-spherical part is provided with a boss (4) formed by the forging operation. 3. Pressure vessel according to claim 2, characterized in that one or both of the bosses (2) are provided with passing bores (5), said bores being threaded for the attachment of a valve/valves. 4. Pressure vessel according to claim 2, characterized in that one or both of the bosses are provided with extemal threads for the attachment of protection caps (8), or one protection cap and a support leg. 5. Method for manufacturing a aluminium vessel, characterized in that the vessel is manufactured by joining two semi-spherical parts (2) that are forged out of circular blanks, the joining of said parts comprising plasma welding (at 3) along the periphery of the two parts. 6. Method according to claim 5, characterized in that the forging operation of each of the parts comprises two steps, a first pre-forming step where the periphery of the blank achieves a final thickness, and further comprising forming of a central projecting part that serves as a basis for a boss, and a second step comprising that the periphery is formed into a final shape as to the final curvature, and that the central part is forged to have final thickness and geometry under the formation of a boss (4). 7. Method according to claims 5 and 6, characterized in that the vessel is made out of a AIMgSM -alloy. 8. Method according to claim 5, characterized in that a flux material having a lower melting point than that of the base material is added when performing the welding operation. 9. Method according to claim 8, characterized in that the flux material consists of an AIMgSiδ-alloy.

Inventors: BOEE EINAR, CARLSEN HANS-PAUL, HAUGERUD OLAV SVEINUNG, HOLGERSEN STIG, BOEE, EINAR, CARLSEN, HANS-PAUL, HAUGERUD, OLAV, SVEINUNG, HOLGERSEN, STIG
Applicants: BOEE EINAR, CARLSEN HANS PAUL, HAUGERUD OLAV SVEINUNG, HOLGERSEN STIG, NORSK HYDRO AS, BOEE, EINAR, CARLSEN, HANS-PAUL, HAUGERUD, OLAV, SVEINUNG, HOLGERSEN, STIG, NORSK HYDRO A.S.
Application year: 1995
Main classification: F17C1
IPC codes: B23K10, F17C1