Boiler steam drum is curved. Why

• Circular hoops can withstand internal pressure by pure tension in the tankmaterial instead of bending. Materials can take more force in tension than inbending. 
• If the shape were not circular, then there would be bending stresses in the tank wall and it would not be able to withstand as much pressure without breaking. 
• Spheres are more efficient than cylinders if you just care about minimizing the mass of the tank to hold a certain amount of pressurized gas. But cylinders are easier to pack together. So it’s a trade-off between efficiency and practicality.