Repair of spherical storage tanks for liquified hydrocarbon gases

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Repair of spherical storage tanks for liquified hydrocarbon gases

Hrivnak, I.

In this work the causes of damage of huge storage tanks for liquefied hydrocarbon gases as well as their repair method and revalidation were described. Hard areas in the zone of welds as the result of weld root grooving by carbon electrodes and subsequent insufficient regrinding of grooved surfaces were the cause of damage. Presence of martensitic microstructure with high hardness exceeding even 450 HV was revealed in these areas. Presence of local lacks of penetration or chains of welded-in slag particles was also detected in some welds. The above-mentioned areas were ground-out and repaired by welding with electrode which assures sufficient plastic properties of weld metal. Precise NDT inspection based on colour defectoscopy, magnetic spraying method and mainly ultrasonic defectoscopy was a part of repairs. Also hardness measurement in the heat affected zone was a part of inspection. Hardness did not have to be higher than 240 HV, exceptionally 260 HV. After repair finish pressure test combined with stress test followed and its aim was to redistribute residual stresses and to correct geometry (spherical shape) of storage tanks. Evaluation of tests including residual service life computation led to revalidation of storage tanks to original parameters with their possible use for other minimum 15 years.