The article considers the specific features related to operation of the power valve and orifice lines within turbine plant piping systems whose inlets receive water medium with saturation parameters (separated moisture or condensate). It is shown that transportation of working medium in these piping systems is accompanied by pressure drop, boiling, and formation of various two-phase flow patterns from bubble in the initial segment to dispersed-annual in the end segment. Under certain conditions, a slug flow pattern can occur, which behaves as a source of piping vibration. Practical experience has shown that the use of homogenizing inserts for suppressing vibration load in the heating steam condensate (HSC) discharge lines downstream of the moisture separator reheaters (MSRs) of nuclear power plant (NPP) turbines often leads to intensified local flow-accelerated corrosion and pipeline failures. The article considers examples illustrating failures of piping segments downstream of homogenizing inserts and presents statistical data on damageability of heating steam and separated moisture piping of NPP turbines. The steam–water flow patterns in the MSR HSC transportation line of an NPP turbine are determined. The results from hydrodynamic modeling of working medium flow under the conditions of an abrupt expansion at the outlet from the homogenizing insert channel are presented. It is shown that the location of zones characterized by the maximum wear of piping downstream of homogenizing inserts in the MSR HSC discharge lines is determined by the flow pattern and specific features of the working medium flow hydrodynamics. It has been established that droplet impingement erosion is the dominating mechanism causing destruction of stainless-steel piping segments downstream of the homogenizing inserts in the MSR HSC lines of NPP turbines. It is important to note that, if the pipeline is made of carbon or low alloy steel, its metal experiences a combined effect of droplet impingement erosion and flow-accelerated corrosion. The obtained study results can be used in elaborating measures aimed to prevent wear of piping components in the heating steam condensate and separated moisture discharge lines of NPP turbines.

— Flow-accelerated corrosion of metal caused by single- or two-phase flow of working fluid leads to thinning and destruction of welds in NPP power unit pipelines. Field experience gained from the operation of Russian and foreign nuclear power plants testifies that there have been many cases of damages inflicted to welds due to local flow-accelerated corrosion, the number of which tends to grow with the time for which the power units have been in operation. The article gives examples of the most typical places and characteristic features pertinent to the way in which the flow-accelerated corrosion of weld elements' metal takes place. The role and influence of the chemical composition of the weld metal and heat-affected zone metal on the location and rate of local flow-accelerated corrosion thinning in welds are determined. The flow hydrodynamic characteristics having an essential effect on the rate of flow-accelerated corrosion processes and the local thinning occurrence places are determined. Results from numerical modeling of the flow-accelerated corrosion weld thinning development process for different contents of chromium in the weld and heat-affected zone metal are given. The possibility of increasing the flow-accelerated corrosion rate in pipelines downstream of a weld with a preceding extended straight pipeline segment made of metal not prone to flow-accelerated corrosion is analyzed. Methods aimed at preventing the occurrence of damage to pipelines of NPP power units due to flow-accelerated corrosion are discussed. The article gives information on the results of activities for modifying the standard programs for in-service inspection of pipelines at VVER-based NPPs and also on the use, at Russian NPPs, of software systems providing support to the personnel on optimizing the arrangement and scheduling of in-service monitoring of flow-accelerated corrosion wear of heat-affected zones in welds of the pipelines used at NPP power units equipped with VVER-440, VVER-1000, and BN-600 reactors. It can be expected that the application of regularly updated software systems for supporting the personnel will make it possible to timely prevent inadmissible thinning and abrupt flow-accelerated corrosion-induced failures of pipeline welds in the power units of Russian nuclear power plants.

The second part of this review considers physicochemical models and computer codes used for predicting flow-accelerated corrosion wear of power generating equipment. Approaches used to prevent the occurrence of general and local flow-accelerated corrosion that are based on selecting metals resistant to flow-accelerated corrosion and adjusting the water chemistry of power units are also discussed. The existing computer codes use physicochemical models of flow-accelerated corrosion and statistical data on damages inflicted to power units due to flow-accelerated corrosion processes. Advantages and drawbacks of different analytical physicochemical models describing the flow-accelerated corrosion process are pointed out together with the specific features of using them in elaborating flow-accelerated corrosion computing codes. It is shown that the processes lying at the heart of the flow-accelerated corrosion mechanism include, on the one hand, the occurrence of a protective oxide layer on the metal surface and, on the other hand, the dissolution of this layer and carryover of dissolution products in the flow. Differences between the processes through which metal undergoes flow-accelerated corrosion in a single-phase water flow and in a two-phase wet steam flow are analyzed. Thus, the redistribution of admixtures and gases between the phases that takes place in two-phase media may cause a change in the pH values, thereby significantly influencing the flow-accelerated corrosion rate. In addition, the rate with which flow-accelerated corrosion products are carried over into a two-phase stream depends on the liquid film flow mode on the streamlined surface. The flow-accelerated corrosion rate computing codes most widely known around the world, including the COMSY code (Germany), CHECWORKS SFA code (United States), BRT-CICERO TM code (France), and RAMEK code (Russia) are considered. Their specific features and application limits are pointed out. Information on the effect the content of chromium, molybdenum, and copper has on the flow-accelerated corrosion rate is given. It is shown that the choice of metals resistant to flow-accelerated corrosion is a combined technical and economic problem, and the way in which it is solved has an effect on the safety and reliability of power unit operation. It is pointed out that the liquid phase pH value is essentially affected by the steam wetness degree if the latter exceeds 20%.