Sphere is for pressurized storage. Spherical shape is an advantage to reduce stress in shell. However keeping allowable stress (and looking to market availability of thickness) means to limit the diameter of sphere, hence the capacity is limited.
Tank cannot be designed for large pressure, but for moderate pressures- let's say around 200 mbarg.
For example you can deposit LPG in spheres- 3000 or 4000cum, under pressure at atmospheric temperatures, or in you have to refrigerate LPG and keep it in 50000 cum tanks under low pressure.
Tank's roof of dome shape respond positively to internal pressure, especially such shape assures a reasonable thickness for "compression ring/compression bar" - in fact these words describe the junction "roof to shell". Larger tangent-to roof angle at periphery is an advantage to keep there the thickness at 20-50mm; for dome the peripheral angle is around 30 degree, for cone roof type cannot be so large.
External floating roof means "free space" above floating roof. Internal floating roof means there is space over the floating roof and beneath the roof (dome, umbrella or cone) that can be pressurized (let's say blanketing for limiting vaporization).
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