Frangible is a word that means easily broken. 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. The concept of frangible roof only applies to flat bottom, cone roof tanks with limited roof apex angle. 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. Since this junction will fail before any other part of the tank, such as the shell, the bottom, or the shell to bottom joint, the bottom and shell can be relied on to be intact. Since failure at the roof is in the least damaging mode of failure for a liquid storage tank, liquid chemicals or stored products will probably not be released.
The failure mechanisim when a tank with a frangible roof to shell joint is over pressurised this area buckles into a wave shaped pattern and the roof tears away from the top angle rim allowing gases to escape.
Frangible roofs are normally specified as a means of providing emergency relief venting in the situation when a tank is engulfed in fire, or the adjacent tank is on fire, causing rapid vaporisation of the stored product such that the normal venting system cannot cope and the pressure in the tank rises to a point where the roof to shell joint fails. Consequently although the tank is damaged and the vapours escaping may well ignite, the liquid is retained within the tank shell avoiding burning liquid in the bund area and further hazard to tanks, equipment and personnel.
For a tank to be considered as having a frangible roof to shell junction the following criteria must be met:-
- The strength of the roof to shell junction must be weak enough to meet the criteria of the tank design code in question.
- The roof plates must not be welded to the tank roof structure, which normally rules out externally stiffened tank roofs from being considered as being frangible.
- The roof plate to curb angle fillet weld must be greater than 5mm.
- The roof slope must not be greater than 1:5 for BSEN 14015 and 1:6 for API 650 tanks.
For small tanks of less than 15 metres it is not normally possible to make the strength of the compression ring to shell junction weak enough to provide failure before the base to shell or shell its self fails. Consequently for smaller tanks emergency-venting capacity has to be provided by the use of emergency relief man-ways situated on the tank roof.
AS PER API 650 If a roof is said to be frangible the conditions are given below