Everywhere in the world shape of LPG cylinder is mostly cylindrical due to the fact that corners are
considered weak points for holding such high pressure, ( 6 to 8 kg/cm2).
Cylinder are designed and manufactured by having less weld joint (again a weak point) and also the pressure inside will be equally(almost) spread throughout the cylinder having no single weak point.
Another best design can also be obtained by using *spheres* but the construction of sphere is very complicated and not cost-effective at all for such huge quantities. Hence such a shape is accepted worldwide.