Frangible Roof Weld

Api 650 gives rules for the design of frangible roof joints in fluid storage tanks.

Frangible roof weld.

Frangible is a word that means easily broken. This failure is intended to vent the tank and contain any remaining fluid. The securement of the annular link mechanism to the rim and the roof is such as to permit articulation of the annular link mechanism relative to the rim and roof to a point of failure of the joint. Hi henry you can refer to 3 10 2 5 addendum march 2000 which states roof plates shall be attached to the top angle of the tank with a continuous fillet weld on the top side only you may want to review the tank design if this connection ask for frangible joint not exceeding 5mm continuous weld.

A frangible roof is a roof to shell joint or junction that is weaker than the rest of the tank and will preferentially fail if the tank is over pressurised. 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. 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 concept of frangible roof only applies to flat bottom cone roof tanks with limited roof apex angle.

This video documents the research and testing on frangible joint storage tanks performed at kansas state university and thunderhead engineering. Such joint is practically sacrificial weld which has to collapse in event of excessive internal pressure while shell to bottom joint has to be undamaged to provide structural and safe integrity of tank. One of the safety measures that can be employed for emergency venting is so called frangible roof to shell joint. 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.

A small diameter tank which comprises a shell having a rim at its upper edge a roof and a frangible roof joint the roof joint including annular link means sealingly secured at or adjacent a. The reasoning behind present api design formulas is reviewed.