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TEAM
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FRANGIBLE
ROOF TO SHELL JOINTS Atmospheric and low pressure aboveground, flammable liquid
storage tanks require emergency venting which will relieve excessive pressure
caused by exposure to fire. This
emergency venting requirement can be met by using blow-off manholes, rupture
disks or a roof to shell connection which will fail before the bottom to shell
joint fails. The latter is referred
to as a “frangible roof to shell connection”.
This type failure prevents loss of tank contents and feeding the fire. The codes used to design aboveground flammable liquid storage tanks
give various methods for designing this connection. These codes are: American Petroleum Institute’s API-650 and
API-2000, National Fire Protections NFPA-30 29CFR (OSHA) Para 1910.106 and
Underwriter Laboratories UL-142. The general method of construction for aboveground atmospheric roof to top angle with an outside fillet weld and the shell to bottom with a double fillet weld. Since the roof and upper shell are of equal or thinner thicknesses than the lower shell and bottom; the upper joint is considered the more frangible and should fail first. Internal pressure sufficient to tear the roof loose from the top angle will tend to lift the bottom to shell connection and unless their is sufficient counterbalancing from shell weight or anchorage this uplift will allow the bottom connection to flex and become the weaker connection. API-650 gives procedures for calculating the required counterbalance and unless this is taken into consideration the danger exist that the bottom connection will fail first. Because uplift on the roof caused by fire exposure is pneumatic rather than hydraulic, the failure at the tank bottom would result in a rocket or explosion versus a rounding and cracking of the bottom connection which would have happened if the tank were over filled with liquid. |
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