Inspection of spherical tank support legs

The collapse of a spherical tank for storing liquefied gas was attributed, in part to corrosion of one of the support legs. This led to general concern about the long-term integrity of legs supporting storage spheres worldwide.

Plant Integrity was contacted as there are access problems that make it difficult to inspect sphere legs by conventional NDE techniques. The legs are often encased to give protection either from heat, in the event of a fire, or from low temperatures, should there be a spill of the liquid gas followed by rapid boil off.

Conventional NDE methods would require the legs to be exposed over their full lengths and such methods would call for the installation of scaffolding to provide man access to the full leg height.

Teletest provides a means of overcoming these difficulties as long as access to the leg can be obtained for a sufficient length for the transducer ring to be mounted. It has been used successfully to inspect sphere legs on two sites posing different challenges.