A detailed fracture mechanics evaluation is the most accurate and reliable prediction of process equipment susceptibility to brittle fracture. This article provides an overview and discussion on brittle fracture. The discussion covers the reasons to evaluate brittle fracture, provides a brief summary of historical failures that were found to be a result of brittle fracture, and describes key components that drive susceptibility to a brittle fracture failure, namely stress, material toughness, and cracklike defect. It also presents industry codes and standards that assess susceptibility to brittle fracture. Additionally, a series of case study examples are presented that demonstrate assessment procedures used to mitigate the risk of brittle fracture in process equipment.

A fire in a storage yard engulfed several propane delivery trucks, causing one of them to explode. A series of elevated-temperature stress-rupture tears developed along the top of the truck-mounted tank as it was heated by the fire. Unstable fracture then occurred suddenly along the length of the tank and around both end caps, following the girth welds that connect them to the center portion of the tank. The remaining contents of the tank were suddenly released, aerosolized, and combusted, creating a powerful boiling liquid expanding vapor explosion (BLEVE). Based on the metallography of the tank pieces, the approximate tank temperature at the onset of explosion was determined. Metallurgical analysis provided additional insights as well as a framework for making tanks less susceptible to this destructive failure mechanism.

A failure of an aboveground storage tank occurred due to external corrosion of the tank floor. The liquid asphalt tank operated at elevated temperatures (approximately 177 deg C, or 350 deg F) and had been in service for six years. Cathodic protection (rectifiers) had been installed since start-up of the tank operation. It was noted, however, that some operational problems with the rectifier may have interrupted its protection. Investigation (visual inspection, on-site examination and testing, EDS analysis of scale deposits, and MIC testing of the soil) supported the conclusion that corrosion may have been caused by an interruption in cathodic protection. The effectiveness of cathodic protection on established microbial deposits is questionable. Recommendations included ultrasonically testing the tank floor and replacing portions based on the remaining wall thickness. Doubling the wall thickness of the floor plates was also recommended.

When a large LPG low-carbon steel storage tank was put into service for the first time and filled beyond the proof testing level, a brittle fracture crack initiated at a fillet weld between a stiffener ring and the wall. The crack propagated to a length of 5.5 m and arrested. Analysis showed that the plates satisfied the criteria of BS 4741. It was concluded that the cause of crack initiation was the lack of a mouse hole at the junction between the stiffening ring and the wall of the tank. The tank was repaired and put back in service. When it was filled beyond the proof test level, again a brittle crack was initiated at a horizontal weld defect and propagated vertically, destroying the tank and the liquefaction plant. The initiation site was a thumbnail elliptical crack in a horizontal weld, having a depth of 1.5 mm, and a length of 4.5 mm. This showed that as late the mid-1970s, misunderstanding of brittle fracture led to the wrong design and construction of an LPG storage tank. The best design specification is to use a correlation between LAST, the Lowest Anticipated Service Temperature, and the DBTT measured by either Charpy tests or DTT.

U-shaped leaf springs, intended to serve as spacers between oil tank floats and the inner walls of the containers, broke while being fitted, or after a short time in use, in the bend of the U. The springs were made of tempered strip steel of type C 88 with 0.84 % C, bent at room temperature, and electroplated with cadmium for protection against corrosion. Each fracture showed seven or eight kidney-shaped cracks. At the origins of these cracks on the concave inner surface of the springs, crater-like depressions and beads of melted and resolidified material were found. Fracture of the springs was caused by stress cracks as a consequence of local hardening. The hardening caused by melting and resolidification, and therefore the cracks in the springs, was the result of a faulty procedure during cadmium electroplating.

A four-million gallon capacity (15,142 cu m) oil storage tank ruptured upon filling after re-erection near West Elizabeth, PA on 2 Jan 1988. The tank shell split vertically with failure originating at a flaw existing prior to the reconstruction. Brittle fracture occurred both up and down from the defect when the stress induced by filling reached a critical value for the steel, which had poor toughness properties. This steel had been used in the original construction of a tank in Ohio more than 40 years previously. The defect at which brittle fracture originated in the tank shell showed evidence of burning from a torch. This tank failure was the catalyst for the introduction of new rules concerning the inspection and assessment of older storage tanks.

A 100,000 barrel crude oil storage tank rupture caused extensive property damage in Dec 1980, in Moose Jaw, Saskatchewan. Failure was attributed to a brittle fracture that originated at a weld between a reinforcing pad and a manway nozzle. Factors that contributed to the brittle fracture included incomplete penetration in a single-bevel groove weld, poor impact properties of the hot rolled ASTM A283 low-carbon steel base material, and air temperature down to 27 C on the day of failure. Details of the analysis and results of impact testing are discussed.