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A truck-mounted propane tank exploded. The failure occurred through a girth weld, causing a section of the dome to fly from the tank. Initial evaluation of the failed tank showed a crack existed in the girth weld, for several years. The preliminary conclusion of this investigation was that the crack condition was the cause of failure; however, the authors suggested that a fracture mechanics analysis be conducted.
Four compact fracture toughness specimens were fabricated from weld metal in the tank and tested. It was shown that the failure could not have occurred from the existing crack and the tank internal pressure, unless the internal pressure far exceeded the maximum allowable 1.72 MPa (250 psi) pressure.
Further investigation led to the discovery that the relief valve of the tank was corroded so badly that it would not open, thus allowing excessive pressure in the tank. There was evidence that the driver had overfilled the tank. Heat from the sun and from the hot truck tailpipe caused the tank to become liquid full. Then, the pressure exceeded 6.9 MPa (1000 psi), causing the tank to explode. The fracture mechanics analysis thus showed that the explosion was not due only to the existing crack, but also to the stopped-up relief valve and excessive liquid propane in the tank.
fracture mechanics, failure analysis, tank explosion, pressure vessel, weldments, propane tank explosion
President, Pearson Testing Laboratories, Inc., Marietta, GA
(Jesse) Dooman, RG
Versitech, Inc., Norcross, GA