A STORAGE TANK FOR LIQUIDS Technical Field 5 The present invention relates to a storage tank for liquids. The tank of the present invention has been developed especially for the storage of flammable and/or dangerous liquids, which are required by law to be stored in a double containment system, and therefore is described with particular reference to a double tank system. However, the tank of the present invention also could be used as a single 10 tank, either for applications where a single containment system is permitted or in combination with a bund where a double containment system is required. Background Art The discussion of the prior art below is not an admission that such prior art is 15 widely known or forms part of the common general knowledge in the field. Storage tanks for flammable/dangerous liquids not only have to meet stringent criteria for double containment but also must be capable of meeting specified pressure criteria. For example, fuel storage tanks must be capable of meeting the 20 pressure criteria set by United Laboratories (UL Standard) or by United Laboratories Canada (UL C. Standard). Typically, fuel storage tanks are required to meet a pressure of 15 psig (pounds per square inch gauge pressure). To achieve the required pressure rating, the most common design for storage 25 tanks has been a cylindrical tank with dished ends; a tank of this design is relatively low in its storage capacity compared to the amount of space it occupies. Square cross-section tanks have a much higher storage capacity for the space occupied by the tank, but require a lot of reinforcement to meet the pressure 30 criteria. To overcome this problem, square cross-section tanks with rounded 1 longitudinal corners and dished ends have been designed, but these have proved very costly to manufacture. Disclosure of Invention 5 It would be advantageous if a storage tank could be devised in which the tank design maximises the storage capacity of the tank without compromising the ability of the tank to meet the specified pressure criteria. According to one aspect of this invention there is provided a double containment 10 system which includes a primary tank having a pair of opposed walls, a pair of opposed flat heads, a roof, a floor, and corner portions forming the transition between the upper edge of each wall and the adjacent edge of the roof, and also forming the transition between the lower edge of each wall and the adjacent edge of the floor; each said corner portion being formed as one or more flat portions, the 15 plane of the or each flat portion being inclined relative to the plane of both the corresponding wall and the roof or the floor, respectively, and a secondary tank which surrounds and encloses the primary tank, said secondary tank including: a pair of opposed secondary walls, a pair of opposed secondary flat heads, a secondary roof, a secondary floor, and secondary corner portions forming the 20 transition between the upper edge of each wall and the adjacent edge of the roof, and also forming the transition between the lower edge of each wall and the adjacent edge of the floor; each said corner portion being formed as one or more flat portions, the plane of the or each flat portion being inclined relative to the plane of both the corresponding wall and the roof or the floor, respectively. 25 According to another aspect the present invention provides a storage tank which includes a pair of opposed walls, a pair of opposed flat heads, a roof, a floor, and corner portions forming the transition between the upper edge of each wall and the adjacent edge of the roof, and also forming the transition between the lower edge 30 of each wall and the adjacent edge of the floor; each said corner portion being formed as one or more flat portions, the plane of the or each flat portion being 2 inclined relative to the plane of both the corresponding wall and the roof or the floor, respectively. Preferably, each said corner portion is formed as a single flat portion, but two, 5 three or more corner portions may be used. As used herein, the term 'flat head' means the end wall of the tank which is a plate into which a bracing pattern or shape has been formed. Typically, a 'flat head' is formed with a low-profile pyramidal shape, to provide cross-bracing for the plate. 10 Preferably, the floor of the storage tank is inclined along its length to facilitate drainage. Most preferably, the floor of the storage tank is formed to incorporate a V shape, with the lowest point of the floor at the apex of the V. 15 The present invention further provides a double containment system which includes a primary tank completely contained within a secondary tank, both said primary and secondary tanks being in accordance with the present invention. Alternatively, a double containment system can be formed from a primary tank 20 partially contained within a secondary tank:- the roof of the secondary tank is omitted. Preferably, in said double containment system the secondary tank rests upon and is secured to a supporting base frame and is provided with reinforcing corner posts 25 secured to corner pieces so as to provide a supporting framework for lifting the double containment system. Brief Description of Drawings By way of example only, a preferred embodiment of the present invention is 30 described in detail below with reference to the accompanying drawings, in which: 3 Figure 1 is an isometric view of a primary (inner) tank in accordance with the present invention; Figures 1 a and b show sectional side views of variant corner designs; Figure 2 is an isometric view of a secondary (outer) tank in accordance with the 5 present invention; Figure 3 is a longitudinal section showing a primary tank within a secondary tank; Figure 4 is a section on A - A of Figure 3, on a larger scale; Figure 5 is an isometric view of a tank floor; Figure 6 is an isometric view of a fully-fitted tank in accordance with the present 10 invention; and Figure 7 shows a longitudinal section similar to Figure 4, but showing a different type of secondary tank and with the gap between primary and secondary tanks exaggerated for clarity. 15 Best Mode for Carrying Out the Invention In Figure 1, a primary tank 10 is made of folded sheet metal, typically carbon steel or stainless steel plate, and typically 3-8mm thick. For example, for a diesel tank, the primary tank typically would be made of 6mm carbon steel plate with 8mm carbon steel plate for the ends, to form a tank (typically) 8 foot wide, 81/2 - 91/2 foot 20 high, and 20 foot or 40 foot long. It will be appreciated that the design of the present invention can be scaled up or scaled down, and the materials varied, as required to suit particular applications, and/or the pressures required for those applications. 25 The primary tank 10 is formed with a shaped floor 11, vertical side walls 12, 13, a top 14, and vertical flat heads 19, 20. The top 14 may be flat, but preferably is formed with one or more bracing creases (not shown), in known manner. However, the corners of the tank, i.e. the transitions from the vertical walls 12, 13 to the floor 11 and to the top 14, are not formed as right angled bends or as 30 curves, but are formed as folded corner portions, with the plane of each corner portion 15, 16, 17, 18 at 450 to the plane of the adjacent wall 12, 13. 4 Forming each "corner" as an angled fold in this way greatly increases the strength of the corner, and has the additional advantage of being easy to form in a flat plate. Although each of the folded corner portions is shown as 450 to both the 5 plane of the adjacent wall and the plane of the adjacent top or floor, it will be appreciated that each corner portion may be formed by folds having any combination of angles which adds up to 900 e.g. 400/500, 600/300. Another possibility is to form each corner portion from two or three or more flat portions, so that the 900 transition from wall to roof or wall to floor is spread over a larger 10 number of small angular bends. Figures la and b show a corner portion formed from two and three flat portions respectively. The body of the tank (i.e. the tank minus its flat heads 19, 20) is formed in two sections:- a top section consisting of the top 14, corner portions 15 and 16 and the 15 upper part of each of the walls 12, 13; and a bottom section consisting of the floor 11, corner portions 17 and 18 and the lower part of each of the walls 12, 13. The two sections are secured together by welds 21, 22 which extend down the length of each side of the tank. The welds 21, 22 are indicated by broken lines in Figure 1. The welds 21, 22 do not need to be at the midpoint of the walls. It will be 20 appreciated that the body of the tank may be formed in more than two sections, if preferred. Each flat head 19, 20 of the tank is formed as a plate, mitred at each corner to accommodate the shape of the body of the tank, and welded into place. 25 Preferably, each flat head 19, 20 is recessed slightly into the body to allow an easier weld between each end and the adjacent part of the body, and to comply with construction regulations. Referring in particular to Figure 5, the floor 11 of the tank is not flat, but is inclined 30 and is folded into a tapered V-shape with the distance between the outer ends of the arms of the V equal to the width of the tank floor, and the floor sloping to the 5 low point of the floor at the apex of the V. The portions of the floor 11 a on each side of the apex of the V are higher than the apex of the V. This provides an inclined floor for the tank, which allows water and other contaminants in the liquid contained in the tank to drain to the lower end of the V. The suction pipe (not 5 shown) for extracting liquid from the tank is located a certain distance (typically about 55 mm) above the tank floor. Alternatively, the floor 11 of the tank may be formed as a flat inclined floor; this also allows water and other contaminants to drain to the lower end of the floor. 10 However, a floor formed with a V-shape is preferred, because the water and other contaminants then are concentrated in the small area of the apex of the V, rather than over the whole of the lower end of the floor. The floor 11 is folded integrally with the lower corner portions 17, 18, and the lower 15 portions of the side walls 12, 13. Figure 5 depicts the floor 11 integral with only a small part of the side walls 12, 13, but the proportion of the walls included in the floor pressing can be varied according to the size of the floor and the capacity of the available press equipment. 20 Each wall 12, 13 of the tank is reinforced by a series of spaced vertical braces 30; Figure 1 shows three such braces for each wall, but more or fewer braces could be used, depending upon the required strength of the tank. Each brace 30 is angled at its top and bottom edges to match the shape of the corner portions 15, 16, 17, 18, and is welded in place. 25 Figure 2 shows the secondary tank 40, which forms the outer tank of the double tank system. The secondary tank 40 is formed to the same cross-sectional shape as the primary tank 10, but is slightly larger (typically about 4mm gap between the tanks) to allow the primary tank to be completely contained within the secondary 30 tank. The gap between the primary and secondary tanks may be in the range 0 50mm. 6 The secondary tank may be formed in two or more sections in the same manner as the primary tank, but preferably is formed in four sections:- a first section comprising the secondary floor 43, secondary lower corner portions 46, 47, and 5 the adjacent lower portions of the secondary side walls 41, 42; a second section comprising the remainder of the side wall 41 and the secondary upper corner portion 44, a third section comprising the remainder of the side wall 42 and the secondary upper corner portion 45; and a fourth section comprising a secondary roof panel 48. The sections are connected together by welding. 10 The advantage of forming the roof panel 48 as a separate panel is that the various fittings required in the roof panel (for example the manway, the filler inlet, the dipstick inlet and the emergency vent, none of which are shown in the drawing) can be preformed in the roof panel 48 before it is welded into place in the tank. 15 Corresponding penetrations (not shown) into the roof of the primary tank are formed either during fabrication of the primary tank or before the primary tank is inserted into the secondary tank. The secondary tank 40 is supported from the ground on a base frame 50 formed 20 from two parallel longitudinal members 51 secured together by spaced cross braces 52, and connected to the underside of the secondary tank by supports 59. The corners of the base frame are reinforced by corner pieces 53 which also serve as supports for corner posts 54. The corner posts 54 provide external bracing for the secondary tank. The upper end of each corner post 54 is connected to upper 25 corner pieces 55 which are braced back to the top of the secondary tank by brackets 56. Each of the upper and lower corner pieces 55, 53 is formed with an aperture (not shown) for receiving a twist lock. Figure 2 illustrates the arrangement in which the tanks occupy the whole of the 30 base frame 50. More commonly, the arrangement is as shown in Figure 6, where the base frame 50 is extended to incorporate an access ladder 80, which gives 7 access from ground level to a ladder platform 81, from which the various tank fittings can be inspected. These fittings obviously depend upon the intended use of the tank and the particular regulations for the tank's intended contents, but typically include a man way lid 82 covering a manhole entry in the top of the tanks, 5 interstitial vents 83, for venting the space between the two tanks, and a free-to-air vent and riser 84. The tanks also are fitted with a fill pipe 85, a suction pipe 86, and tank dipsticks 87 (shown covered). All of these fittings are of known type and therefore are not described in detail. 10 In the Figure 6 embodiment, the corner pieces 55 of the base frame 50 are braced back to the top of the secondary tank by brackets 56 at one end and at the opposite end are connected by a crosspiece 89. For applications where a single tank is permitted, the primary tank 10 alone may 15 be mounted on the base frame 50 as shown in either Figures 2 or 6. The combined primary and secondary tanks, complete with the base frame 50, can be lifted using conventional container lifting gear connected to the upper corner pieces 55. The base frame 50 and the corner posts 54 also provide a 20 protective frame for the secondary tank; this helps to avoid paint chafing damage during transport. Referring in particular to Figures 3 and 4, the combined tanks fit closely together, so that the shaped floor 11 of the primary tank is supported by the same-shaped 25 floor 43 of the secondary tank, and the walls and ends of the primary tank also supported by the walls and ends of the secondary tank; in Figure 4, the walls, floor and roof of the two tanks are depicted by a single heavy line. For some applications, it is permissible for the secondary tank to be formed 30 without a roof. This variant is illustrated in Figure 7, in which the spacing between the primary and secondary tanks is exaggerated for clarity. In this variant, the 8 primary tank is as described with reference to Figures 1 - 6. The secondary tank 70 is formed with a secondary floor 71, lower secondary corner portions 72,73, secondary sidewalls, 74,75, and upper secondary corner portions 76,77. However, there is no roof:- the upper edge of each secondary upper corner 5 portion 76,77 is sealed to the primary tank by a weld 78. It will be appreciated that the shape of the above described primary and secondary tanks maximises the storage capacity for the volume of space occupied by the tank, without reducing the strength of the tank. A further advantage is that the 10 shape of the tank lends itself to easy fabrication. 9