Why they are spherical in large scale and cylindrical in small scale

• Containers experience two types of stresses due to applications of inside pressure.
• They are ' Hoop stress ' and ' Longitudinal Stress '
• Hoop stress is greater in magnitude so it's the cause of failure of any container.
• We know that Hoop stress in case of cylinder is Pd/2t and Hoop stress for Sphere is Pd/4t . 
• So it is clear that in care of sphere less Hoop stress is developed as compared to cylinder.
• Hence spherical tanks are preferred in large scale where internal pressure is very high 
• Though spherical tank is the best option from strength point of view, cylindrical tanks are used in small scale for convince in use, as inside pressure is less in small scale use.