A METHOD OF PRODUCING BIDIMENSIONALLY CURVED, THIN-WALLED TANK END WALLS

Abstract: A method of producing bidimensionally curved, thin-walled tank end walls (4, 5, 9) on a disc-shaped tool (1), involving the following consecutive operations: placing a reinforcing ring (4) of angled section around the periphery of the tool; welding two flat plates (5) into position on the ring to either side of the tool; creating positive pressure between the plates for the purpose of forming them simultaneously into domes; splitting the reinforcing ring peripherally so as to release the two end elements; and welding a cylindrical tank casing ring (9) to the splitted reinforcing ring.

Description: 86/0343 A METHOD OF PRODUCING BIDIMENSIONALLY CURVED, THIN- WALLED TANK END WALLS Technical Field The present invention relates to a method of producing bidimensionally curved, thin-walled tank end walls, in which two end walls - arranged to either side of a disc-shaped tool - are formed simultaneously by means of positive pressure between the walls. Background of the Invention The manufacture of tank end walls from metal plate presents problems of various kinds. Smoothing hammers can be used in the more simple cases, although this calls for considerable skill. It is more usual to make use of special tank end forming tools or large, costly special machines, although these are less common. Consequently, long delivery times and high costs are unavoidable in practice. In the case of cylindrical tanks the bidimensionally curved tank end wall forms part of a sphere, or a dome, which is ' ideal from the stress point of view, since only membrane stresses - of identical value in all directions - occur. Hemispherical tank end walls are ideal from the stress point of view, although they are unfavourable from the point of view of volume utilization, for which reason the spherical component is normally very much smaller than a hemisphere. In this case the tank end element must be curved additionally by a considerable amount - the so called deflection - at the transition to the cylindrical tank casing. Under operating conditions - usually positive internal pressure - the stresses at this point are very much greater than in the other parts of the tank. The process currently used in the manufacture of thin tank end walls imposes a lower limit of about 2 mm on the plate thickness. In the case of thinner materials, especially stainless materials, the rim can easily exhibit rippling, which impedes or even rules out further processing altogether. Smaller plate thicknesses, of 1 mm or even 0.3-0.5 mm, are used in one particular design of tank, although this does not in itself constitute any part of the present invention. In view of the problems referred to above, however, it has been usual until now to accept the situation in which sufficiently thin plate can be used only in the tank casing, with thicker plate being used in the ends. A previously disclosed method involves starting with a flat plate blank and imparting the desired, bidimensionally curved, domed form to the tank end by subjecting it on one side to a uniform pressure of such magnitude as to cause it to bulge permanently outwards (the cold-stretching process). Examples of this previously disclosed method are to be found in US 1 013 0 6 and DE 635 179. However, the equipment used in this process is large and heavy, because only a single tank end element at a time is formed and because the considerable forces which are generated during the forming process must be absorbed by the equipment. One method by which this problem can be avoided is described in GB 1 421 938, in which two tank end walls to either side of a central, disc¬ shaped tool are formed simultaneously by means of positive internal pressure, whereby the forming forces (except the radial forces) balance out one another. The tank end elements which can be manufactured in accordance with the method described here are of essentially uniform thickness, also at the transition to the tank casing. In order to be able to meet the functional specifications, however, the especially thin-walled tank end walls required or the tanks referred to above must be provided with considerable reinforce¬ ment at the transition to the tank casing. The Invention The problem associated with the method . indicated above for the manufacture of thin-walled, bidimensionally curved tank end walls is solved by the method in accordance with the invention through the following consecutive operations: placing a reinforcing ring of angled section common to both end walls around the periphery of the tool; welding two flat plates into position on the reinforcing ring around their entire periphery to either side of the tool; creating a positive pressure between the plates in a way known per se for the purpose of forming them simultaneously to the desired curvature; and splitting the reinforcing ring peripherally so as to release the two tank end walls produced in this way from the tool. For this reason the angled section of the reinforcing ring should be such that the ring and the tank end plate lie on the same plane after the forming of the latter, i.e. so that there is a smooth transition from one to the other. The forming of the plates by positive pressure can be achieved by means of gas pressure or fluid pressure depending on the circumstances, but in the method according to the invention use is made of water, which is supplied via a connection in one of the plates, a vent hole in the other plate being closed when the space between the plates has been filled with water. The plates are cold-stretched by the method in accordance with the invention, for which reason the plates need a certain period - perhaps an hour or thereabout - after forming in order to stabilize; sufficient positive pressure to maintain the form which has been achieved must be maintained during this period. No connection to the tank casing has been made by the method in accordance with the invention as described until now. For this purpose a cylindrical casing ring - preferably of the same thickness as the reinforcing ring - can be welded in position on the last-mentioned ring. The fact that the transition between the tank end wall and the tank casing has been divided up into two rings makes the method as a whole applicable from the financial, quality and production engineering point of views. Without dividing the reinforcement into two in this way, it is difficult to achieve the necessary dimensional accuracy of the tank end walls. Brief Description of the Drawing The invention is described below in greater detail with reference to the accompanying drawing, in which Fig. 1 is a perspective view of a tool used for the method in accordance with the invention, Figs. 2 and 3 in cross- section illustrate two operations in the manufacturing method according to the invention, Fig. 4 illustrates a subsequent operation in cross-section and on an enlarged scale, Fig. 5 shows the manufactured tank end element, Fig. 6 shows the production of a casing ring, and Fig. 7 shows said ring welded to the tank end element so as to produce a finished tank end wall. Description of a Preferred Embodiment A disc-shaped tool 1 for use in the method in accordance with the invention consists of an upright ring 2 and a flange 3 (comprising two circular rings) welded to it. Since the tool only requires to be dimensioned for forces acting in its plane, it can be made small, light and cheap; a practical tool with an external diameter of 240 cm has a weight of only 450 kg. The method in accordance with the invention for the manufacture of a tank end wall by the use of this tool starts by forming a reinforcing ring identified in the drawing by the numeral 4. A very long strip of sheet metal (which may have a width of 20 cm for the manufacture of a tank end with a diameter of 240 cm) is provided at its centre with a longitudinal ground groove, along which the strip is sub¬ sequently folded to the angle (of about 60°) which may be appreciated from Figs. 2-4. The folded strip of sheet metal is introduced into a coil bending machine (with the back facing outwards) and is given a radius which corresponds to the radius of the tool 1, such that the strip assumes a helical form. A 'circle' of this strip is then cut off, is laid around the tool 1, and is welded together; it now forms the reinforcing ring 4 for two tank ends. A thin, circular, flat plate 5 is welded in position around the entire edge of the reinforcing ring 4 to either side of the tool (see Fig. 2) in such a way that an entirely enclosed space is formed between the plates. A water hose 6 for pressurized water is connected to the lower plate, whereas a vent hole 7 is made in the upper plate. The space between the plates 5 is completely filled with water, whereupon the vent hole 7 is plugged permanently. The water pressure is increased until the plates 5 begin to 'creep' and until the desired dome shape in accordance with Fig. 3 is finally achieved. It is obvious that the reinforcing ring 4 must exhibit the inclination indicated by the height of arch in the finished tank end element, i.e. in such a way that the ring 4 and the plate 5 form a smooth transition from one to the other after forming. Once the forming operation is complete, the two elements must be allowed to 'mature' for an hour or thereabout in order for permanent deformation or cold-stretching to be achieved. A rotating grinding wheel 8 or similar (see Fig. 4) is used to divide the double element into two identical components in accordance with Fig. 5. A flat casing ring 9 is now rolled in a fashion similar to the reinforcing ring 4 into a circle with the same radius as a cylindrical tank casing 10 (see Fig. 6). The ends are welded together, and the casing ring 9 is welded in position along its entire periphery to the reinforcing ring 4 so as to form a tight seal. The remaining 'wing' 11 on the now complete tank end wall 4, 5, 9 may be left intact, since a thin-walled tank of this kind is intended to be embedded in insulation. By the inexpensive method described above a tank end wall is manufactured which is thin and light-weight, is dimensionally accurate, and yet is strong at the critical transition between the tank casing and the tank end wall. The tool used is cheap and light-weight.

Claims: CLAIMS 1. A method of producing bidimensionally curved, thin-walled tank end walls, in which two end walls - arranged to either side of a disc-shaped tool (1) - are formed simultaneously by means of positive pressure between the end walls, characterized by the following consecutive operations: placing a reinforcing ring (4) of angled section common to both end walls around the periphery of the tool (1); welding two flat plates (5) into position on the reinforcing ring around their entire periphery to either side of the tool; creating a positive pressure between the plates in a way known per se for the purpose of forming them simultaneously to the desired curvature; and splitting the reinforcing ring peripherally so as to release the two tank end walls produced in this way from the tool. 2. A method according to claim 1, characterized in that the angled section of the reinforcing ring (4) is such that the ring and the end plate (5) lie in the same plane after the forming of the latter. 3. A method according to claim 1, characterized in that the forming of the plates (5) by means of positive pressure is effected in a previously known way by means of water supplied via a hose (6) connected to one of the plates, a vent hole (7) in the other plate being closed when the space between the plates has been filled with water. 4. A method according to any of the preceding claims, characterized in that a sufficient positive pressure to maintain the form which has been achieved is maintained for a sufficient period to stabilize the plates (5). 5. A method according to any of the preceding claims, characterized in that a cylindrical casing ring (9) for connecting the tank end wall to a cylindrical tank casing (10) is welded in position on the reinforcing ring (4).

Inventors: BJURLING ANDERS, BJURLING, ANDERS
Applicants: DYNATRANS TECHNOLOGY LTD, DYNATRANS TECHNOLOGY LTD.
Application year: 1985
Main classification: B21D26
IPC codes: B21D51