The most common shape of a storage vessel is a Bullet with two heads which are either, hemispherical, elliptical or tori-spherical. Spherical vessels have larger surface area per unit volume. The advantage of a sphere over a horizontal bullet with the same volume is increased by the static head because it is a practice to let the thickness of the sphere vary from top to bottom as the pressure increases. Spheres are preferred for large capacity storages. Spherical vessel is good choice above 10 m diameter.
A tank-type will usually be selected considering the cost or the size for transportation. The spherical type is usually employed for sizes greater than 500 m3. The horizontal Bullet type is usually used for sizes smaller than 100 m3. An important aspect in the design of spheres for LPG is the considerable weight involved. Such spheres are normally supported upon vertical legs attached to the vessel at its equator. In order to withstand the compressive stresses the total cross sectional area of the legs must increase linearly with the weight of the vessel under ASME Section Ⅷ Division I : Rules for Construction of Pressure Vessels