Development of HOIS guidance for ultrasonic NDT for non-intrusive inspection at elevated temperatures

Abstract

The non-intrusive inspection (NII) of pressure vessels using external in-service ultrasonic NDT has multiple advantages compared to internal visual inspection, including the significant reduction of down-time related costs and safety benefits from avoiding manentry into vessels. The HOIS joint industry project, with support from the Net Zero Technology Centre (NZTC), has recently been focussing on NII of components with operating temperatures of up to 250°C (typical of upstream applications). These inspections are challenging because they require probes to be in contact with the hot surface much longer than for spot thickness measurements. The aim of the work has been to develop recommendations for in-service inspection of components at relatively high operating temperatures, thereby extending the benefits of the NII process, including avoiding or deferring internal visual inspection, to a wider range of vessels. Experimental trials have been performed on test components at up to ~250°C, with an emphasis on scanning ultrasonic systems used for corrosion mapping and weld inspection using TOFD and angled-beam phased-array (PA). The most successful trials gave corrosion mapping and TOFD weld examination results that had similar differences from benchmark values to those achieved in earlier ambient temperature trials. For angled-beam PA, it was found that some trials appeared to have issues with accurate correction of the data for the effects of the elevated component temperature on both the refracted beam angle and signal arrival times. Other trials were more successful in this respect. The lessons learned from trials on both ex-service and manufactured samples are leading to the development of specific guidance for NII at elevated temperatures. The paper highlights the technical challenges associated with ultrasonic scanning systems at elevated temperatures, share the lessons learned from the experimental trials and summarise the guidance developed.