Frangible Roof Requirements

Holes through roof plates shall be repaired or replaced.

Frangible roof requirements.

A frangible roof joint for small diameter tanks wherein the joint forms a flexible gastight connection between the tank shell and the roof and comprises annular link means containing an annular plate having an inner edge corresponding to a first edge thereof and an outer edge corresponding to a second edge thereof sealingly secured at or adjacent the first edge to a rim. 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. What is claimed is. 4 2 api s tandard 653 tank roof to shell joint modification such as reinforcement of the joint attachment of handrail or other frangible joint area change.

4 2 3 floating roofs 4 2 3 1 areas of roof plates and pontoons exhibiting cracks or punctures shall be repaired or the affected sections replaced. Evaluate the strength of the roof joint versus the shell to floor joint. This failure is intended to vent the tank and contain any remaining fluid. The concept of frangible roof only applies to flat bottom cone roof tanks with limited roof apex angle.

This paper presents the results of an investigation into the frangible joint behavior of tanks designed to api 650 rules. See figures 7 and 8. However experience shows that the frangible joint may not always perform as intended. Frangible is a word that means easily broken.

The securement of the annular link mechanism to the rim and the roof is such as to permit. 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. 4 2 3 2 areas that are pitted. Api 650 paragraph 5 10 2 6 c was revised to discourage the use of frangible roof design on small diameter tanks.

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. The reasoning behind present api design formulas is reviewed. 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. That is of course if the tanks are built according to the rules for frangible roofs.

Other means of providing emergency venting are recommended for small tanks. Frangible roof design for tanks under 50 does not work without tank specific design.