Development of NDT techniques and inspection for detection and sizing of thermal fatigue cracking in steam-pipework bores of flexibly operated coal-fired power stations

During the 2001 programme to inspect welds in high pressure-temperature steam pipework for in-service flaws, a number of thermal fatigue cracks were detected associated with changes of section at the pipe bores such as weld roots or counter-bores. Metallurgical examinations, including oxide dating showed that the cracks had been in existence for a considerable length of time and, in many cases, welds in close proximity to the bore cracks had been subject to repeat inspections in the past. This raised questions about the adequacy of the inspections, particularly as sectioning showed some of the cracks to be significant in through-wall dimension.

As more information became available through sectioning and NDT of a larger population of cracks, it become clear that the cracks were, in fact, not easy to detect and especially to size using manual pulse-echo ultrasonics. The cracks are orientated vertically, are smooth-sided and are very tight and oxide-filled, all factors which work against satisfactory inspection by traditional ultrasonic inspection techniques. Furthermore, more close attention is given to the near surface areas of welds because this is where life-limiting creep cracks usually initiate due to stress concentrations in these positions. This is a human factor, but understandable under the circumstances.

The first NDT developments that were carried out after the discovery of the cracks on a number of coal-fired units around the country centred on detection, rather than sizing, as it was important to find out the scale of the problem. Latterly, sizing has been the focus of Innogy NDT work, and detection has been addressed by introduction of an operator familiarisation-training module and some modifications to existing technical procedures. This paper covers both the technical and managerial aspects of developing effective strategies to meet commercial, safety and legislative requirements in relation to these issues.