Frangible Tank Roof Design

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

Frangible tank roof design.

1 evaluation of the area inequality as a method to predict the buckling response of the compression ring 2 effect of roof slope tank diameter and weld size on the frangible joint and 3 effect of the relative strength. This video documents the research and testing on frangible joint storage tanks performed at kansas state university and thunderhead engineering. 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.

The american petroleum institute api has developed a standard for the design and construction of welded steel storage tanks. 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. At the api committee meeting in november i brought up the subject of the frangible roof design requirements. 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.

That is of course if the tanks are built according to the rules for frangible roofs. On fixed roof tanks where the roof shell attachment is constructed according to api standard 650 1 the roof to shell joint may be considered frangible so that excessive internal pressure may cause it to fail before failure occurs in either the tank shell joints or in the shell to bottom joint. In particular i asked about the requirement if 5 10 2 6 d which says for anchored tanks of any diameter the tank shall meet the requirements of 5 10 2 6 a and the anchorage and counterweight shall be designed for 3 times the failure pressure calculated by f 6 as specified in 5 12. In the event of over pressurization the frangible roof to shell joint is designed to fail before the tank shell or the shell to bottom joint.

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. Frangible is a word that means easily broken. The api 650.