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XLPE Storage Tank

A 12,000-gallon cross-linked polyethylene (XLPE or PEX) storage tank for 93% sulphuric acid failed after approximately seven months of service. At least 6,000 gallons of acid were released during the incident, requiring environmental remediation. The release of acid occurred through circumferential cracks that developed on the bottom panel of the tank near the corner radius with the sidewall. Examination of the fracture surfaces revealed semi-elliptical cracks that emanated from the inner tank surface.

The fractographic characteristics of the fracture surfaces indicated slow crack growth, as evidenced by a relatively smooth morphology over most of the fracture surfaces and the presence of crack arrest marks. Inspection at higher magnification using a stereomicroscope revealed considerable stress whitening and micro-ductility suggesting elevated stress.

Stress analysis using the Finite Element Method was performed to calculate the stress history of the tank, using the fill history as input. It was concluded that the operating stresses were too low to account for the failure, and that elevated stress from pressurization (which is prohibited by the manufacturer) was the most likely cause of failure. Elevated temperatures from such sources as ambient air, sunlight, and exothermic reactions between the sulphuric acid and water may also have contributed.

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