Never mind found it with a different search
LPG tanks are only supplied as Cylinders or Toroidal (Polo Mint Shaped).
All LPG tanks are pressure vessels. They have to be designed to withstand the pressure held within. That means no corners, and, consequently no square or rectangular tanks. To understand why this is so, consider any type of Hot Water tank or Boiler. As pressure inside a square or rectangular shape is increased it will try and reform itself into a rounded shape, just like a Balloon does. This would result in failure at any corner. The design solution is to have no corners.
*The cylinder is the most popular choice as it is the cheapest to produce, which is reflected in its purchase price. The cylinder will take up quite a lot of load or boot space .
*The high pressure inside of the tank executes bending stress on the welds if the bottoms are flat. ( Bending stress and bending moment on the welded corner). For that reason the ends can not be made flat like a kerosene tank.
-If the bottoms of a cylindrical tank have a spherical or elliptical shape, the welding line is subjected to pulling stress only.
*Theoretically, a sphere would be the optimal shape of a pressure vessel. Unfortunately, a spherical shape is difficult to manufacture, therefore more expensive, so most pressure vessels are cylindrical with 2:1 semi-elliptical heads or end caps on each end. Smaller pressure vessels are assembled from a pipe and two covers.
A disadvantage of these vessels is that larger diameters make them relatively more expensive, so that for example the most economic shape of a 1,000 litres (35 cu ft), 250 bars (3,600 psi) pressure vessel might be a diameter of 914.4 millimetres (36 in) and a length of 1,701.8 millimetres (67 in) including the 2:1 semi-elliptical domed end caps.
Standards referred:
" ASME Boiler and Pressure Vessel Code"
EEC Directive R67/01
