Conventional tank containers comprise a round cylindrical tank with domed ends which is mounted within a protective framework having at least a pair of end frames and a base frame. The dimensions of the frame conform with ISO standards, with a typical frame being 6. 058 metres long, 2. 438 metres wide and 2. 591 metres high.

It is naturally desirable to maximize the volumetric capacity of the tank.

Previous efforts to do so include reducing the radius of curvature of knuckles joining the domed ends to the intermediate, round cylindrical shell. Reduction in the knuckle radius tends to create zones of weakness, as a result of which a minimum acceptable knuckle radius of curvature is 10% of the tank diameter according to DIN standards, and 6% of the tank diameter under ASME standards.

It is an object of the present invention to increase the volumetric capacity of a tank within a standard dimension tank container, whilst at the same time not adversely compromising the structural and strength characteristics of the tank.

SUMMARY OF THE INVENTION According to the invention there is provided a tank container including a tank which is mounted within a frame with standard length, width and height dimensions, the tank having a round cylindrical, central shell and a pair of end caps, wherein at least one of the end caps comprises a generally dome- shaped structure which projects from the central shell and which includes a recessed formation or a flattened portion in a central region thereof for reducing the extent to which the dome-shaped end cap projects from the central shell, and wherein the end caps are located at longitudinal extremities of the frame thereby to allow for an effective increase in the length of the central shell relative to the length of the frame.

By increasing the effective length of the central shell relative to the length of the frame, it is possible to increase the overall tank volume. For example, where each of the flattened portions or recessed formations reduce the depth of the dome-shaped end caps by"d", the total length of the shell or belly is increased by a factor of 2d, with the overall effective increase in volume being approximately equal to 2g1R2d less any volume reduction at the end caps due to recessed formations, where : R = the tank end cap main radius of curvature ; and d = the reduction in overall depth of one end cap.

The volume may be increased further by reducing the radius of curvature of knuckles joining the end caps to the central shell to, say, 6% of the end cap radius R.

Preferably, each of the end caps includes a recessed formation which extends across a substantial portion of the cap and which is surrounded by an outer peripheral, torospherical portion.

Conveniently, the tank includes reinforcing means for reinforcing the central flattened portion or recessed formation.

The reinforcing means may include one or more external gussets and/or straps which extend across at least a portion of the central flattened portion or recessed formation.

Alternatively, or in addition, each end cap may be provided with at least one inner brace plate or gusset.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a cross-sectional view of a portion of a conventional tank ; Figure 2 shows a cross-sectional view of a portion of a tank according to a first embodiment of the present invention ; Figure 3 shows, schematically, a cross-sectional view of an entire tank according to the first embodiment of the invention ; Figure 4 shows an end view of the tank illustrated in Figure 3 ; Figure 5 shows a cross-sectional view of one end of a tank according to a second embodiment of the invention ; Figure 6 shows a cross-sectional view of one end of a tank according to a third embodiment of the invention ; Figure 7 shows an end view of a fourth embodiment of the tank according to the present invention ; and Figure 8 shows a cross-sectional view along the line 8-8 in Figure 7.

BRIEF DESCRIPTION OF APPENDIX Appendix A shows detailed volume calculations in respect of a conventional tank and a first embodiment of the tank according to the present invention, indicating an achievable increase in volume.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 of the drawings illustrates a portion of a conventional tank container which includes a stainless steel tank 10. The tank comprises a round cylindrical shell or belly 12 and a pair of dished end caps 14 (only one of which is shown). The shell or belly has an inside diameter D of 2. 355m, and each dished end 14 is part spherical and has an internal radius R of 2. 350m (which is almost identical to the shell diameter). The dished ends each terminate in an annular knuckle 16 having a radius of curvature r of 0. 235m, which corresponds to 10% of the dish radius, and each knuckle terminates in a 25mm long, round cylindrical flange 20 which is welded to the shell 12. Typically, the dished ends 14 are pressed or drawn from a flat sheet of stainless steel, and subsequently the knuckles 16 are formed in a spinning process.

Figure 2 of the accompanying drawings illustrates a portion of a tank forming part of a tank container according to the present invention. The tank is designated generally with the reference numeral 22 and includes a central, round cylindrical shell 12A and a pair of end caps 24 (only one of which is shown) welded to the central shell. Each end cap is generally dome-shaped and includes a centre, reversed domed or concave portion 26 which is press formed by a part spherical press prior to the welding of the end cap to the shell 12A. The concave portion 26 has an arcuate profile, and has a diameter of 1. 2m, just over half the 2. 355m internal diameter of the tank. In order to provide additional reinforcement, a reinforcing gusset plate or fillet 28 is welded diametrically across the concave portion 26, as shown most clearly in Figure 4.

The volumetric advantage of the tank 22 is illustrated more clearly in Figure 3. In this Figure, the tank 22 is shown mounted within a rectangular frame 29 of standard height, width and length dimensions 2. 591m, 2. 438m and 6. 058m respectively. For each end cap 24, the overall loss in volume is represented by the cross-hatched portions 30. However, a resultant increase in the tank volume is achieved by the increase of 2d in the length of the central shell or belly 12A of the tank, as indicated by the cross-hatched portion 31, where"d" is the reduction in the depth of each end cap due to the presence of the concave portion 26. It has been found that for a tank with exactly the same length and diameter as a conventional tank, which can be accommodated in the same frame 29, the volumetric capacity of the tank can be increased by over 700 litres.

The volume of the tank can be increased further by reducing the radius of curvature of knuckles 16A joining the end caps to the central shell to 6% of the tank diameter, as shown at 32 in Figure 2, with the resultant profile being indicated in broken outline at 34.

Appendix A sets out in detail the volume calculations in respect both of a prior art or unmodified end cap or head and of an end cap or head which has been modified along the lines of the first embodiment described above, and indicates an effective increase in volume which can be achieved.

Referring now to Figure 5, an end cap 36 of a tank according to a second embodiment of the invention is formed with a central reverse dished or dimpled portion 38 surrounded by an outer peripheral, torospherical portion 40. A radiused inner knuckle 42 is formed at the interface between the torospherical portion 40 and the central dimpled portion 38. As can be seen, an annular plate 44 spans the knuckle 42 internally, and is joined to the inner surfaces of the torospherical portion 40 and the central dimpled portion 38 by respective circular welds 46 and 48.

An end cap according to a third embodiment of the tank of the invention is illustrated in Figure 6. Here, the end cap is designated with the reference numeral 50 and is provided with alternative reinforcing means in the form of an internal diametrically extending brace plate 52 which is profile to conform with the inner surface of the end cap to which it is welded. In order to supplement the reinforcement of the internal brace plate and to guard against outward bowing of the central dimpled portion 38 in response to internal pressures of up to 600kPa, an external, round cylindrical ring in the form of a short section of piping 54 is welded around the centre of the dimpled portion 38 so as to be coaxial with a central axis 56 of the tank. As is clear from the broken line 60, the ring 54 does not extend beyond the inner knuckle 42 so as to ensure that the overall length of the tank remains unchanged.

Yet a further embodiment of the tank according to the present invention is illustrated in Figures 7 and 8 of the drawings. In this embodiment, end caps 62 (only one of which is shown) include a recessed portion 64 in a central portion of a generally dome-shaped structure 66. The recessed portion includes a round cylindrical side wall 68 and a domed centre 70 extending between the side wall, as shown. External bracing in the form of a plurality of spaced-apart, radially extending steel gussets 72 and steel straps 74 is provided on the outside of the end cap to resist deformation of the cap under pressure. Each gusset is curved along one edge so as to conform with the domed centre of the end cap (see Figure 8), and the straps 74 are joined together in cruciform fashion, as illustrated most clearly in Figure 7.

It should be appreciated that the invention is not limited to the specific shapes and configurations illustrated in the embodiments of the invention described above and that many other alternative shapes and configurations could be used.

It should also be appreciated that the increased capacity of the tank according to the invention allows for the transportation of significantly more fluid in a tank container with standard frame dimensions and that this results in reduced transportation costs per cubic meter of fluid.

APPENDIX A Modified Head details. Volume includes extra skirt length : D = Diameter of central shell = 2355 mm R = Radius of curvature of end cap = 2355 mm r = Knuckle radius = 230 mm Rd = Radius of curvature of recessed formation in end cap = 2355 mm d = Diameter of recessed formation in end cap = 1400 mm Height(y) H = 452.93 mm h = 346. 491 mm H - h - 106. 439mm x<BR> Distance from centreline (x) D F Vmodified : = 2. -x-y (x) dx + 7c-1) (H-h) 0 4 Vmodified = 1577. 4371iter Unmodified head (d = 0) : Now using full formula : Height (y) x Distance from centrreline (x) D 2 Vunmodified : = 2-x-x-y (x) dx o Vunmodified = 1278. 9267 liter Results: Vunmodified = 1278. 9267 liter Vmodified = 1577. 4365 liter Extravolume : = Vmodified - Vunmodified Extravolume = 298. 51 L