Case Studies - Structural Integrity of Storage Spheres

Storage Spheres are in widespread use in the chemical and petrochemical industries for the containment of hazardous materials, usually under pressure. These vessels, typically 15m-20m in diameter, are made by welding together a number of steel plates. They are expensive to produce and it is therefore understandable that operators seek to extend their operating lives.

In order to assess whether the operating life can be safely extended, an engineering critical assessment is necessary. This requires information on operating conditions, vessel geometry, defect population and material properties. Much of this information is readily available. However, one crucial material property, the fracture toughness, presents considerable difficulty. This is because the fracture toughness of the parent material is not routinely determined during manufacture and because welding causes local changes (usually a reduction) in toughness. Hence there is a requirement to establish the fracture toughness of the sphere material and its weldments, without causing significant damage to the vessel, before an assessment can be performed.

HSL was asked by HSE to investigate this problem in order that authoritative advice could be provided to industry. Various test methods, using specimens that could be manufactured from small amounts of material cut from the vessel wall, were investigated. Initially the use of small-scale punch tests, as used in the nuclear industry, was considered and a technique using 3mm diameter disc specimens (0.25mm in thickness) was developed. However, for the carbon-manganese steels used in storage spheres, research showed that relevant fracture toughness data could not be obtained under practicable experimental conditions. The use of small-scale compact tension fracture toughness specimens was then investigated in a joint industry project and this has yielded more promising results.

In addition, HSL has participated in the HSE investigation of fabrication-induced hydrogen cracking in LPG storage spheres. This work included examination of metallurgical samples, engineering critical assessments and advice on inspection.

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