Development of a method for estimating wall thinning tendency due to flow-accelerated corrosion in tee joint piping

Application of laser-generated guided wave for evaluation of corrosion in carbon steel pipe

Flow-accelerated corrosion (FAC) is a phenomenon which causes wall thinning of pipes, fittings, vessels, and other components in the metal based piping systems that carry water or water-steam mixture in power plants and refineries. Currently used nondestructive techniques, such as radiographic testing (RT), ultrasonic testing (UT), and pulsed eddy current (PEC) testing in order to determine the remaining wall thickness, are time consuming and not economical. Hence, in this work, the use of the fundamental torsional mode ultrasonic guided wave to detect FAC was investigated using the finite element method (FEM) simulations and that were validated with experiments. The torsional wave was generated by the magnetostriction principle using surface mounted strips made of magnetostrictive Hyperco (FeCo) material that provided the source for the surface tractions required to generate the ultrasonic guided wave. The transient electric field was provided through a solenoid coil wound over the strips and permanent magnets were employed to provide the bias magnetic field. From this work, it was observed that the pulse-echo method is not suitable for the FAC detection because of the insignificant reflections from FAC defect region that could not be effectively detected. The through-transmission method was found to be more suitable for the FAC detection because the amplitude of transmitted signal decreased with increase in radial depth of FAC in both the simulation and experiment.

Flow accelerated corrosion(FAC) of the carbon steel piping in pressurized water reactors(PWRs) has been major issue in nuclear industry. Severe accident at Surry Unit 2 in 1986 initiated the worldwide interest in this area. Major parameters influencing FAC are material composition, microstructure, water chemistry, and hydrodynamics. Qualitative behaviors of FAC have been well understood but quantitative data about FAC have not been published for proprietary reason. In order to minimize the FAC in PWRs, the optimal method is to control water chemistry factors. Chemistry factors influencing FAC such as pH, corrosion potential, and hydrazine contents were reviewed in this paper. FAC rate decreased with pH up to 10 because magnetite solubility decreased with pH. Corrosion potential is generally controlled dissolved oxygen (DO) and hydrazine in secondary water. DO increased corrosion potential. FAC rate decreased with DO by stabilizing magnetite at low DO concentration or by formation of hematite at high DO concentration. Even though hydrazine is generally used to remove DO, hydrazine itself thermally decomposed to ammonia, nitrogen, and hydrogen raising pH. Hydrazine could react with iron and increased FAC rate. Effect of hydrazine on FAC is rather complex and should be careful in FAC analysis. FAC could be managed by adequate combination of pH, corrosion potential, and hydrazine.

In 1996, Flow Accelerated Corrosion (FAC) was identified as a degradation mechanism affecting carbon steel outlet feeder pipes in CANDU® (CANadian Deuterium Uranium) reactors. The maximum rate of FAC was estimated to be <0.120 mm/year. In response, wall thickness inspection programs have been implemented to identify and measure the minimum wall thickness in outlet feeder pipes. These data are necessary to ensure fitness-for-service of the feeder pipe. These data, together with the thermalhydraulic and geometric parameters for the measured feeders, are also very useful for developing empirical wall thickness models. Such models can be used to enhance the understanding of feeder wall thinning leading to an improved capability to predict future wall thickness minima and their locations. The determined dependency of the wall-thinning rate on thermalhydraulic conditions can be used to quantify the potential benefits of maintenance activities, such as steam generator cleaning. Activities such as steam generator cleaning are generally viewed as beneficial in recovering lost thermal efficiency, thereby reducing the severity of the thermalhydraulic conditions by reducing the amount of quality (steam phase) exiting the reactor core. Finally, when wall thickness models are applied to data from different plants, there is the potential of identifying operating conditions that can lead to lower rates of wall loss. This paper addresses the aforementioned important issues associated with FAC of ASME PVP Class 1 carbon steel piping.

Flow accelerated corrosion (FAC) of the carbon steel piping in nuclear power plants (NPPs) has been major issue in nuclear industry. During the FAC, a protective oxide layer on carbon steel dissolves into flowing water leading to a thinning of the oxide layer and accelerating corrosion of base material. As a result, severe failures may occur in the piping and equipment of NPPs. Effect of alloying elements on FAC of pipe materials was studied with rotating cylinder FAC test facility at <TEX>$150^{\circ}C$</TEX> and at flow velocity of 4m/s. The facility is equipped with on line monitoring of pH, conductivity, dissolved oxygen(DO) and temperature. Test solution was the demineralized water, and DO concentration was less than 1 ppb. Surface appearance of A 106 Gr. B which is used widely in secondary pipe in NPPs showed orange peel appearance, typical appearance of FAC. The materials with Cr content higher than 0.17wt.% showed pit. The pit is thought to early degradation mode of FAC. The corrosion product within the pit was enriched with Cr, Mo, Cu, Ni and S. But S was not detected in SA336 F22V with 2.25wt.% Cr. The enrichment of Cr and Mo seemed to be related with low, solubility of Cr and Mo compared to Fe. Measured FAC rate was compared with Ducreaux's relationship and showed slightly lower FAC rate than Ducreaux's relationship.

Development of wall thinning screening system and its application to a commercial nuclear power plant

Pipeline wall thinning rate prediction model based on machine learning

The objective of this research is to locate and evaluate wall thinning in pipe elbow by a non-contact guided wave technique with laser source as a transmitter and air-bone transducer as a receiver, respectively. Wall thinning of carbon steel pipe is one of the most serious problems in nuclear industry; especially the one in carbon steel pipe elbow caused by FAC (Flow-Accelerated Corrosion). Therefore, development of a robust NDE technique for the pipe elbows is essential for safe operation of nuclear power plants. Specimens used in this study were carbon steel which is widely used in real nuclear power plants. The geometry of wall thinning was given as 120mm extent, 80mm-length and 5mm-depth. The L(0,1) and L(0,2) dominant modes group shows a promising variation in the ultrasound guided wave data analysis based on the response obtained by the laser generation/air-coupled detection system. The trends of these characteristics and subsequent signal processing were used to estimate the size and location of wall thinning.

Flow accelerated corrosion (FAC) of the carbon steel piping in nuclear power plants (NPPs) has been major issue in nuclear industry. Rotating cylinder FAC test facility was designed and fabricated and then performance of the facility was evaluated. The facility is very simple in design and economic in fabrication and can be used in material and chemistry screening test. The facility is equipped with on line monitoring of pH, conductivity, dissolved oxygen(DO), and temperature. Fluid velocity is controlled with rotating speed of the cylinder with a test specimen. FAC test of SA106 Gr. B carbon steel under 4 m/s flow velocity was performed with the rotating cylinder at DO concentration of less than 1 ppb and of 1.3 ppm. Also a corrosion test of the carbon steel at static condition, that is at zero fluid velocity, of test specimen and solution was performed at pH from 8 to 10 for comparison with the FAC data. For corrosion test in static condition, the amount of non adherent corrosion product was almost constant at pH ranging from 8 to 10. But adherent corrosion product decreased with increasing pH. This trend is consistent with decrease of Fe solubility with an increase in pH. For FAC test with rotating cylinder FAC test facility, the amount of non adherent corrosion product was also almost same for both DO concentrations. The rotating cylinder FAC test facility will be further improved by redesigning rotating cylinder and FAC specimen geometry for future work.

Mathematical modeling of orifice downstream flow under flow-accelerated corrosion

Systematic approaches for evaluating flow accelerated corrosion (FAC) are desired before discussing application of countermeasures for FAC. Firstly, future FAC occurrence should be evaluated to identify locations where a higher possibility of FAC occurrence exists, and then, wall thinning rate at the identified FAC occurrence zone is evaluated to obtain the preparation time for applying countermeasures. Wall thinning rates were calculated with the coupled models of static electrochemical analysis and dynamic double oxide layer analysis. Anodic current density and electrochemical corrosion potential (ECP) were calculated with the static electrochemistry model based on an Evans diagram and ferrous ion release rate determined by the anodic current density was applied as input for the dynamic double oxide layer model. Some of the dissolved ferrous ion was removed to the bulk water and others precipitated on the surface as magnetite particles. The thickness of oxide layer was calculated with the dynamic double oxide layer model and then was applied as input for the electrochemistry model. It was confirmed that the calculated results of the coupled models resulted good agreement with the measured ones.

Diagnosing nuclear power plant pipe wall thinning due to flow accelerated corrosion using a passive, thermal non-destructive evaluation method: Feasibility assessment via numerical experiments

Segments of welded small-bore A-106 carbon steel from a piping system that experienced severe flow-accelerated corrosion were characterized for surface damage due to flow-accelerated corrosion (FAC). A computational fluid dynamics (CFD) analysis was done to compare findings of CFD model versus observed surface FAC damage inside the pipes. CFD results expressed in terms of turbulence intensity showed good agreement with actual surface damage due to FAC. It was concluded that the presence of internal grooves would cause turbulent flow regime, and therefore, it would cause pipe material damage.

Flow accelerated corrosion of X65 steel gradual contraction pipe in high CO2 partial pressure environments

Flow accelerated corrosion (FAC) is a combined form of erosion, corrosion and Cavitation. This is prominent in steam condensate lines which results in fast reduction of thickness in piping, piping components and valves. It is estimated that this problem is faced by majority of plants. There has been an increased emphasis on correcting these problems due to fatal accidents that occurred in 1986, 1995, 1996 and 2004 at various locations around the world. [3] After commissioning of the plant, Steam condensate system erosion/corrosion problem started appearing within one year of operation. To ensure uninterrupted plant running on line sealing was done and monitoring was done by proper thickness checking. These on line sealing points were replaced during available opportunity. In some cases plant shutdown was taken to replace leaking piping components &amp; these incidents resulted into revenue loss to company. Aggressive inspection programs were taken up for thickness measurement on condensate lines and as a proactive measure, elbows were encapsulated with higher size elbows, reducers by on line welding/furmaniting with special clamps. Similarly gate and globe valves in condensate service also started failing as a result of erosion of body seat rings. Globe valves installed on bypass lines of control valves were found passing. Once these valves were operated for maintenance of control valves they could not be closed. In some cases valve body developed leak due to high velocity erosion. Various studies conducted for replacing these components by higher schedule fittings &amp; pipes but it did not improve the situation except for slight increase in life of these components. Velocities were calculated at various locations and higher velocity, condensate impingement/cavitation was found as root cause of problems. This problem was solved by various methods like using higher metallurgy P11, P22 material, line size increase with increase in control valve sizing, lay out changes etc. This helped in improving reliability of condensate system and reducing risk associated with failure of piping. This paper presents a variety of cases where single-phase and two-phase steam flows, caused erosion-corrosion damage mainly at turn points of elbows and valves. It was observed that the presence, even of a small amount of the vapor phase can significantly increase the velocity of the condensate. This paper describes the mechanism of failures by study of the failed components, operating conditions &amp; piping lay out. In this study velocity of steam /condensate at reducing section was found to be very high. Other various contributing factors like control valve / piping sizing, metallurgical requirements, effectiveness of steam traps, flow velocity and valve design (globe &amp; gate) were also studied. The main causes of the failures are discussed and recommendations are provided to rectify the root cause of the problems &amp; avoid similar problems in the future.