Frangible Roof Requirements

Evaluate the strength of the roof joint versus the shell to floor joint.

Frangible roof requirements.

Frangible roof joints by zhi lu b s tianjin university 1982 m s tianjin university 1984 a dissertation submitted in partial fulfillment of the requirements for the degree doctor of philosophy department of mechanical engineering college of engineering kansas state university manhattan kansas 66506 1994 approved by. However experience shows that the frangible joint may not always perform as intended. This standard provides for the design of a frangible roof joint but in 1978 the standard was amended to limit applicability of the frangible roof design to tanks having a diameter of fifty feet 15 2m or more. 4 2 3 2 areas that are pitted.

This paper presents the results of an investigation into the frangible joint behavior of tanks designed to api 650 rules. Frangible is a word that means easily broken. The american petroleum institute api has developed a standard for the design and construction of welded steel storage tanks. A frangible roof joint forming a flexible gastight connection between a tank shell and a roof comprising an annular link mechanism sealingly secured at or adjacent a first edge thereof to a rim of the tank and sealingly secured at or adjacent a second edge thereof to the periphery of the roof.

The reasoning behind present api design formulas is reviewed. The concept of frangible roof only applies to flat bottom cone roof tanks with limited roof apex angle. The securement of the annular link mechanism to the rim and the roof is such as to permit. This failure is intended to vent the tank and contain any remaining fluid.

In the event of over pre ssurization the frangible roof to shell joint is designed to fail before the tank shell or the shell to bottom joint. Frangible roof design for tanks under 50 does not work without tank specific design. Api 650 paragraph 5 10 2 6 c was revised to discourage the use of frangible roof design on small diameter tanks. See figures 7 and 8.

Holes through roof plates shall be repaired or replaced. In fact the frangible roof design fulfills the venting requirements due to deflagrations inside tanks for most tank sizes greater than about 35 feet in diameter. In such tanks the roof to shell joint is intended to fail in the event of overpressurization venting the tank and containing any remaining fluid. However in the context of tanks the word has a specific meaning and is defined in the main tank construction standards bsen 14015 and api650.

Api 650 gives rules for the design of frangible roof joints in fluid storage tanks. That is of course if the tanks are built according to the rules for frangible roofs.