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Most cone roof tanks are designed with roof-supporting structures inside the tank. If the vapor space of the tank is corrosive, then the roof structure will corrode, causing tank integrity issues. Coatings for corrosion mitigation are ineffective for internal roof rafters because the coating is prone to peeling due to flexing of the roof plates that rest on the rafters. For this reason, tanks with corrosive vapor space are sometimes designed with external rafters. In such designs, the bottom flange of the rafters is welded to the roof plates.
Tanks with external rafters. Since these tanks are not common, the American Petroleum Institute's tank design code API 650 is not written with this type of construction in mind. API 650 Appendix F provides the compression area requirement for the roof-to-shell junction. The roof plates in a tank with internal rafters are not welded to the rafter, and the roof will bulge out under internal pressure, which causes hoop compression at the shell-to-roof joint. This does not occur when the rafters are above the roof, and will provide rigidity along the rafter, thereby restricting the bulging of the roof under internal pressure. API 650 Appendix F would overestimate the compression area requirement for a tank with external rafters because such a design has an inherent rigidity against hoop compression.
For aboveground steel tanks, it is important that the tank meets the frangibility criteria. In a frangible design, the weakest joint is between the shell and the roof plates. This joint will give way in the event of overpressure inside the tank, thereby preventing the failure of other joints, particularly those within the liquid space of the tank.
The roofs of tanks with external rafters...