AE: Acoustic emission testing of an HF acid sphere

The paper describes how an Acoustic Emission (AE) testing programme was used to detect environmental cracking in an HF storage sphere at an early stage, without the need for frequent internal inspections.

The first AE test (in 1985) revealed a number of welding flaws in the top circumferential weld. These were verified by ultrasonic testing but repairs were not necessary. AE testing continued at approximate three-year intervals and recorded minimal levels of AE until the fourth (1995) test. This produced increased AE activity, giving the first indication of environmental damage.

Follow-up examinations found fissuring and small blisters in the affected areas. This was not serious enough for immediate remedial action, so a follow-up AE test was scheduled to see if the condition changed after a further year of operation.

This test, carried out in 1996, produced a further increase in AE activity and more areas were affected. The sphere was shut down for inspection and repair. Extensive cracking was found in the areas of AE activity, most of these were removed by grinding but some weld repairs were needed. AE testing six-months after recommissioning showed AE activity had returned to the previous low levels.