During a routine inspection of a spherical 2,000-m³ propane tank (previous test date: 2002) located in an LPG depot, the facility manager heard a whistling sound caused by a leak. The origin was a 1″ tube positioned 2.50 m from the top of the tank and used to feed a gauge device that at the time had been dismounted. Another tube connected with the first one was also serving no function. The gas detectors did not show any unordinary readings; moreover, the tank level indicators did not reveal any visible variations. A walk-through inspection the day before at the top of the tank did not suggest anything abnormal. The tank was holding 830 tonnes of propane when the event took place.

The leak, which occurred less than 22 months after the most recent ten-year test, was due to an external 3-mm perforation resulting from a water retention point opening that triggered a differential aeration mechanism with local acidification, otherwise known under the name “EVANS drop”. The repair work, which entailed removal of the 2 redundant tubes, required draining the spherical tank, installing scaffoldings and then inspecting welds once the tubes had been dismounted and the surfaces smoothed and levelled. While waiting for this sequence of repairs, the operator plugged the crack with a resin and inserted reinforcement collars. Two tests were conducted daily on this temporary repair job using a leak detection product. The tank was subsequently drained and degassed at the end of July. A subcontractor executed these repairs during the second half of August. The suspected faulty tube was submitted for expert appraisal.

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