After the acid cleaning the generating bank tubes, the tubes were inspected using the Internal Rotary Inspection System (IRIS) IRIS is an ultrasonic inspection technique used to measure wall thicknesses in tubes. The probe is made up of a centering device, an ultrasound transducer and a rotating mirror (turbine). It is performed by immersion in the tubes. An ultrasound pulse is generated by the transducer and guided towards the wall tube through a mirror that is inclined at 45º. The ultrasound reflection (echo)captured are from the inner and outer walls of the tube. The remaining thickness is calculated using the velocity of the inspected material and the difference between the inner and outer echoes. The generating bank tubes were found at or above original wall thickness.
In-situ metallography was performed at four locations within the boiler, including (a) an externally accessed generating bank tube, (b) an internally accessed generating bank tube, (c) the superheater supply tube and (d) the internal steam drum wall above the generating tubes.
These sites were prepared by superficial grinding, polishing to a 1μm polish and then etching using 3%
nital (diluted nitric acid). The prepared regions were replicated using acetate tape and further examined off-site. Optical examination was performed at magnifications up to 1000x.
At the four locations evaluated, no life-limiting degradation was observed. No creep damage/voids or any other life-limiting damage were observed at any of the sites evaluated.