Double-deck-type external floating roofs API standard 650 flat-bottom oil storage tanks

The mechanical seal of floating roof API standard 650 flat-bottom oil storage tanks relies on mechanical force to press the sealing steel plate tightly against the tank wall, so that it can slide on the tank wall and avoid gap as far as possible. However, mechanical seal has high requirements for the geometry of storage tank and poor adaptability, so mechanical seal is rarely used in newly built floating roof API standard 650 flat-bottom oil storage tanks.

API 620 Tank Standard Post-heat Treatment (Stress Relief)
The post-heat treatment now performed on pressure vessels is of established value, although the mechanism by which the improvement is realized may be open to debate. Post-heat treatment of API standard 650 flat-bottom oil storage tanks tank sections when the plate thickness exceeds 11/4 in. is required as stated in 5.25. In special cases, the possibilities of post-heat treatment after erection should be explored. Post-heating an insulated tank may be possible if an ample source of heat is readily available and if the rigidity of the tank is adequate.
The thermal stress-relief treatment performed on pressure vessels is recognized as a means for reducing the probability of brittle failures.

The welded API 650 tank in various sizes and capacities for internal pressures approximating atmospheric pressure (internal pressures not exceeding the weight of the roof plates), but a higher internal pressure API standard 650 flat-bottom oil storage tanks is permitted when additional requirements are met.