Explosion Relief for Low-Pressure Tanks

Explosion relief for vapor-air combustion within a closed top low-pressure tank cannot be accomplished by customary tank conservation vents or other light-weight protection. A well-written brief description of safety features of low-pressure tanks can be found in 

In the case of most tanks, explosion venting is accomplished by lifting of part, or all of the roof For this to take place the roof must be attached to the top peripheral angle by the smallest feasible weld. API standards specif that the weld be no greater than 5 mm (3/16 in). Preferably this weld should be of the order of3 mm or 4 mm. 

A tank must have other qualities for successful explosion venting. The cone roof slope must not exceed 1 in 6 and should preferably be no more than 3/4 in 12. If the roof is supported by roof rafters the roof plates must not be attached to the rafters but must simply rest on the rafters. Roof lifting must not be hampered by railings or other structures above the roof. The roof should be able to lift, unhampered, to a height sufficient to give a peripheral open area equal to 1 A times the tank cross section. The roof should be as thin as possible preferably not over 6 mm (1/4 in). Both roof release pressure and roof weight per unit area should be as small as possible to minimize energy development before roof lift. 

The tank must not lift up at the bottom under any conditions.. . . Thus tiedown bolts may be required.. . the primary objective of a weak seam roof is to ensure that the contents remain in the tank.. . .  

There are many reports and photos of low-pressure tanks that experienced overpressure conditions or explosions sufficient enough to tear open the roof and yet the walls and the bottom retained their integrity. [5, 6, and 7] Most of the associated fires were confined to the inside of the tank. 

Tinkering with Pressured Vessei Closure Bolts Ends with a Harmless Bang 

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