Computed Radiography for Quality Control of Hydropower Penstock Pipe in Nepal

Abstract

Non-Destructive Testing (NDT) has been extensively used for the inspection of hydropower components in Nepal. Hydropower component penstock pipes are fabricated by rolling mild steel plate. The edges of the rolled plate are arc welded or submerged arc welded (SAW) along its longitudinal or spiral direction. These pipes are circumferentially welded at hydropower construction site to make a desire length of penstock from headrace to turbine. In Nepal, the quality of such welded penstock pipe is inspected using four different NDT methods such as radiography, ultrasound, magnetic and dye penetrant. These four methods are equally important for the inspection of penstock pipe. But the major part of NDT word is covered by radiography and ultrasound in Nepal. The limitation of radiography is its tedious film developing and processing time required. Therefore, pulse echo ultrasound has been widely used in favor of radiography testing. The pulse echo ultrasound testing results are not reliable because discontinuities results listed by NDT Level 1 personnel depends on their skills and judgments. And the pulse echo ultrasound does not produce visible image or report for record keeping. Thus, modern computed digital radiography which uses imaging plate (IP) has came as an alternative NDT testing of conventional pulse echo ultrasound testing. Computed radiography (CR) being modern digital radiography produces visible radiation images of welding and supports computer, based image processing for image evaluation and discontinuities interpretation. CR technology also supports measurement of wall thickness of the penstock pipe that was remained from corrosion. It also supports double wall double image techniques for the pipes less than 100 mm in outside diameter. Standards and codes for the CR technique have been well developed for its application for pressure vessels such as hydropower, storage tanks, casting products and aluminum products. Finally, this paper illustrates modern CR technology for inspection of penstock pipe used in hydropower for the efficient construction, testing and repairing.