Abstract
Several Chromel-Alumel thermocouple assemblies from the IGT HYGAS Pilot Plant failed after about 316 hours of service in the steam-oxygen gasifier (SOG) section of the plant. The failures consisted of the rupture of the Type 310 stainless steel sheath material along considerable portions of its length and the corrosive attack of the sheath inner surface, thermocouple wires, and MgO insulating material. Failure occurred due to the penetration of the SOG environment to the interior of the thermocouple assembly through preexisting flaws in the sheath. The limited oxygen supply present at the inner surface of the sheath prevented the formation of a protective oxide over this surface, and significant sulfidation occurred. Water vapor present in the SOG environment caused the MgO insulation to transform to Mg(OH)/sub 2/, and the volume expansion associated with this transformation caused the sheath, which had already been embrittled at the inner surface due to sulfidation, to rupture.
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