Problems concerned with local erosion-corrosion of welded connections in the pipelines of a power unit at a nuclear power station

Abstract

628During the last decades, systematic and pro grammed works have been carried out in leading coun tries around the world for collecting and analyzing data on erosion-corrosion damages inflicted to the metal of equipment and pipelines at nuclear power stations (NPSs), studying the regularities pertinent to the mechanism of erosion-corrosion, developing software tools for estimating and predicting the ero sion-corrosion process, and optimizing measures for monitoring, revealing, and preventing erosion-corro sion thinning of the walls of power equipment and pipelines. In France, for example, works have been carried out for more than 15 years on revealing ele ments belonging to the risk group of intense wear and optimizing the operational monitoring (thickness measurements) of metal thinning in 58 power units at NPSs belonging to the EDF Company with the use of the BRT CICERO computation code [1]. The EDF database has been created using the results of these works, which included 99 534 elements, i.e., around 2000 elements per power unit as of April 2010. As the time for which NPPs have been in operation passed, cases involving inadmissible thinning and fail ures of the metal in welded connections are observed in more and more frequent occasions along with ero sion-corrosion failures of the main metal of pipeline and equipment elements. In 2002, the Electric Power Research Institute (the United States) published a report addressed to selective erosion-corrosion effect on welded connections. Since 2001, EDF has given the status of an individual problem to the wear of welded connections in a program on erosion-corro sion of the metal in NPS power units [1].Since 2007, the OAO Rosenergoatom Concern has been implementing a comprehensive program on the problem concerned with erosion-corrosion of the main metal of pipelines and equipment at Russian NPSs [2]. At present, sufficient grounds have ripen for drawing up a new version of this ongoing comprehen sive departmental program on the problem of damages inflicted to welded connections. The data on damage ability and the results from operational checks of metal that have been acquired and analyzed with due regard of the requirements specified in the above mentioned program made it possible to reveal numerous cases involving the occurrence of inadmissible thinning in welded connections and in near weld zones of NPS pipelines. Cases are also known in which loss of tight ness occurred in welded connections of NPS process loops as a result of their erosion-corrosion thinning.Such incident occurred in November 2004 in a power unit of the Balakovo NPS, at which a leak was found in the zone of welding the bypass pipeline bend (D nom = 108 × 6 mm) to a sleeve (D nom = 120 × 16 mm) of the feedwater pipeline (D nom = 426 × 24 mm). The working parameters of medium in the damaged part are 220°C and 8.0 MPa.Significant inadmissibly thinned places in welded connections of pipelines downstream of the feedwater control valves are observed in power units of Russian NPSs equipped with diffe...

Effect of Local Fluid Disturbance Induced by Weld Reinforcement Height on the Corrosion of a Low Alloy Steel Weld

“…One is the mass transfer [17][18][19][20], and the other one is the wall shear stress (WSS). For the former, the non-streamline profile of the weld can destroy the original fluid boundary layer [21,22] and diffusion layer resulting in different driving forces of the mass transfer [23,24]. It is generally considered that the higher the flow velocity, the faster the mass transfer rate and the higher the corrosion rate [11].…”

“…There is also an argument that the flow velocity does not have a monotonic relationship with corrosion [25,26]. Tomarov et al [23] found that local erosion-corrosion was the dominant mechanism causing thinning of the welded connections in the pipelines of a power unit at a nuclear power station. Hosseini et al [24] investigated the effect of excessive penetration of welding on the sand erosion of elbows and reducers.…”