Multidiscipline Analysis of a Fatal Lpg Tank Rupture

An LPG tank fabncated by a major manufacturer in the late 1940s with rolled skid plate, commonly used in steel floor construction, burst catastrophcally under direct sunlight. The rupture caused extensive property damage and fatal injury. Metallurgxal analysis revealed that at time of fabrication the full penetration longitudinal seam weld of the tank had been torch cut and then reconnected with only an external partial penetration weld. In consequence, a full-length axial crack was introduced in the tank shell. Moreover, the rewelding was poor with rampant porosity slag deposits and lack of homogeneity. Prominent weld flaws were identified as singular origins of the burst fracture. A variety of analytical techniques were undertaken to develop the etiology of failure. First, a maximum equivalent stress failure criterion was applied to determine the nominal rupture pressure for the tank. Discontinuity stresses at the elliptical heads were acknowledged in the stress analysis. Next, an extensive fracture mechanics study which accounted for the combined effects of lack of penetration and discrete flaws in the longitudinal weld seam was performed. The presence of a flaw embedded in a cylinder with a semi-infinite axial crack presented a unique geometry not easily modeled by normal solutions. A series of burst tests was carried out under hydrostatic loadmg using exemplar propane tanks fabricated with an analogous partial penetration seam weld. Results of those tests including burst pressures, depth of weld penetration, fracture mode, and material hardness were extrapolated to determine the critical pressure. A minimum rupture pressure of 535 psi was calculated for the accident tank which under proper construction should have manifested a burst pressure of 1600 psi. A thermodynamics study of the two-phase liquid-vapor propane system was performed. This work established circumstantially that the LPG tank was necessarily overfilled to the 93% level, vis-a-vis the standard 80% requirement. Under this condition, the requisite burst pressures could be achieved at prevailing tank surface temperatures because the LPG contents were in the saturated liquid phase. Concomitantly, it was established through bench testing that the 40-year-old pressure relief valve (safety valve) was frozen due to severe environmental rusting.