At a refinery, works were initiated to install a series of bottom valves on spherical liquefied petroleum gas (LPG) storage tanks, which had been operating for several tens of years and deliberately emptied for these works. The valves were positioned upstream of the point where gas is drawn from the tank into the supply pipelines (DN 250, Pbutane = 5 bar, Ppropane = 13.7 bar), which for this occasion were disconnected. The pipe cut on one of the spherical tanks, downstream of the designated valve installation location, made it possible to detect a zone of very limited residual thickness. In this particular zone, pipe wall thickness was found to be roughly 2 to 3 mm for a nominal thickness of 12.7 mm, i.e. equal to a thickness loss of over 70%. The cut in the off-take pipe of a second tank allowed identifying the same type of defect. These two defects, both 13 cm long by 3 cm wide, were located immediately downstream of the elbow along the tank base and upstream of the first stop valve. The defect had been overlooked by the company’s in-house inspection unit. The operator proceeded by controlling all of the site’s off-take pipes and initiated a five-year inspection interval for these lines.
The implementation of bottom valves by the operator (featuring hydraulically-controlled safety valves) served to satisfy requests repeatedly submitted over the past few years by the Classified Facilities Inspectorate (installation of a positive safety valve or tap, in accordance with Article 8 of the May 10th, 1993 Ministerial decree).
The defects detected did not seem to be of a metallurgical origin, as no modification appeared at the bottom of the opening. Corrosion due to cavitation was also eliminated as a cause given both the pipe diameter and flow velocity (between 0.3 and 1 m/s). Moreover, defects were observed in an elbow section on tubes with different metallurgical compositions. The operator oriented his search towards erosion problems tied to the in-house operation of tank sand blasting, since the technician would have been capable of temporarily securing the sand-blast lance in the tank bottom when carrying out this task.
Due to the potential seriousness of the defects recorded, a national inspection campaign was launched on all installations of this type (see March 19th, 1999 Ministerial order); this same defect turned up on an LPG spherical tank at another French refinery.