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Last updated: 06.12.2018
  

Contents Issue 04 (2013)

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English Abstracts

Monitoring Crevice Corrosion via the Coupling Current Part I: Detecting Crevice Activation, Inversion, and Inhibition
Sang-Kwon Lee, Wenjun Kuang, James A. Mathews, and Digby D. Macdonald

A simple crevice corrosion monitor was developed to monitor crevice corrosion in 1018 mild steel, Type 304 SS, and Type 410 SS in NaCl solutions with and without the addition of a chemical corrosion inhibitor. The monitor, which measures the electron coupling current, accurately followed the evolution of crevice activity in a manner that can be understood in terms of the cathodic process that occurs on the external surface and the partial anodic process that develops within the crevice, due to the accumulation of H+ and Cl. The crevice initiation time is typically very short, but appears to depend upon the chloride concentration and possibly on the inhibitor concentration. The coupling current increases with time after initiation, passes through a maximum and then decreases, eventually changing sign to mark crevice inversion. The inversion is attributed to the gradual build-up of H+ in the crevice to the extent that proton reduction within the crevice becomes the principal cathodic reaction in the system, while the anodic reaction moves to the external surfaces. In addition, amines are effective corrosion inhibitors of crevice corrosion of mild steel and stainless steels in NaCl solutions, provided that they are present in sufficiently high concentrations.

PowerPlant Chemistry 2013, 15 (4)
Determination of High-Risk Zones for Local Wall Thinning due to Flow-Accelerated Corrosion
Shunsuke Uchida, Masanori Naitoh, Hidetoshi Okada, Hiroaki Suzuki, Seiichi Koshizuka, and Derek H. Lister

A 6-step evaluation procedure based on the 3D computational fluid dynamics (CFD) code has been developed to evaluate local wall thinning due to flow-accelerated corrosion (FAC). As a result of verification and validation (V&V) evaluation of the 3D FAC code, it was confirmed that a wall thinning rate could be predicted with an accuracy of a factor of 2. One of the disadvantages of the 3D FAC code was that it required a lot of computational time and memory. In order to minimize computational time, a speedy and easy-to-handle FAC code based on 1D CFD analysis was developed. As a result of a V&V evaluation based on a comparison of calculated and measured wall thinning, it was confirmed that regional maximum wall thinning rates could be predicted with an accuracy within a factor of 2.

Then it was applied for FAC risk evaluation for entire plant systems to point out the locations where future problems might occur and to prepare for continuous pipe inspections and early implementation of suitable counter measures. For this purpose, not only the probability of serious wall thinning occurrence in the future but also a hazard scale ofpipe rupture due to the serious wall thinning was analyzed. FAC risk was defined as the mathematical product of the possibility of serious wall thinning occurrence and its hazard scale.

PowerPlant Chemistry 2013, 15 (4)
Evaluating Contributions of Flow-Accelerated Corrosion and Liquid Droplet Impingement to Pipe Thinning in HRSG Evaporator Tubes
James Malloy, Mark Taylor, Andreas Fabricius, Marc Graham, and David Moelling

Recent research indicates that liquid droplet impingement (LDI) is a potentially significant contributing factor, alongside flow-accelerated corrosion (FAC), in the incidence of boiler tube and pipe wall thinning. The difficulty remains in predicting the relative contribution of each phenomenon in a given set of operating conditions. Results from several field investigations into tube thinning and failures in the two-phase regions of low-pressure evaporator circuits of heat recovery steam generators (HRSGs) are presented. The primary mechanism for the observed wear is attributed based on the observed wear morphology, rate of wall thinning, water chemistry and location in the HRSG circuit. The attributed wear mechanism is compared with predictions obtained using well-known semi-empirical formulations for material loss due to FAC and LDI. Process conditions used in the formulations are generated using a boiler simulation program. Using the results, basic rules are proposed for evaluating the relative potential of LDI and FAC to contribute to flow-induced wall thinning in HRSG components.

PowerPlant Chemistry 2013, 15 (4)
Considerations on Conductivity and pH in Water/Steam Cycles Using Organic Cycle Chemistry
Wolfgang Hater and André de Bache

Conductivity and pH are important parameters for monitoring and controlling water/steam cycles. For all-volatile treatment (AVT) with ammonia, pH is frequently calculated from conductivity measured on-line. The impact on the calculated pH is discussed for systems treated with volatile organic amines instead of ammonia. By means of an equation describing the dependence of the pH on thermodynamic data, the difference in pH between ammonia and various amines is analyzed. The necessary data for film-forming amines have been determined with a specific measuring unit minimizing the adsorption. From the obtained data the conductivity and pH of solutions of amine mixtures with and without film-forming amines are calculated and compared to data from steam generators. The acid conductivity measured in water/steam systems treated with film-forming amines is analyzed in dependence on operation data from the plant as well as analytical data from liquid chromatography – organic carbon detection and ion chromatography. The data are discussed with regard to water/steam quality specifications for organic cycle chemistry based on filming amines, and recommendations for monitoring are given.

PowerPlant Chemistry 2013, 15 (4)
Instrumentation for Monitoring and Control of Cycle Chemistry for the Steam-Water Circuits of Fossil-Fired and Combined Cycle Power Plants
Albert Bursik

A guidance document on the instrumentation for monitoring and control of cycle chemistry for the steam-water circuits of fossil-fired and combined cycle power plants was developed within the IAPWS Power Cycle Chemistry Working Group. This technical guidance document was first issued in 2009. In response to comments received, the 2012 revision includes a small number of minor updates and clarifications. These do not constitute significant changes to the scope of the document or to the guidance contained in it. This technical guidance document was authorized by the International Association for the Properties of Water and Steam (IAPWS) at its meeting in Boulder, Colorado, USA, 30 September to 5 October, 2012, for issue by its Secretariat. The members of the IAPWS are: Britain and Ireland, Canada, the Czech Republic, Germany, Japan, Russia, Scandinavia (Denmark, Finland, Norway, Sweden), and the United States of America. Associate Members are Argentina and Brazil, Australia, France, Greece, Italy, New Zealand, and Switzerland. The document represents the accumulated experience of the IAPWS Power Cycle Chemistry (PCC) Working Group with representation from 15 countries.

In order to achieve suitable chemical conditions in steam-water circuits it is essential to establish reliable monitoring of key parameters on every plant. This enables the demonstration of operation within cycle chemistry targets, and alerts the operators to the need to take corrective action when the target conditions are compromised.

This technical guidance document considers conventional fossil and combined cycle/HRSG plants and identifies the key instrumentation and monitoring techniques required for each plant type and cycle chemistry treatment. It is emphasized that this is an IAPWS guidance document and that, depending on local requirements, the use of simpler instrumentation may be adequate, whereas more complex techniques and instrumentation may be necessary when specific issues arise.

PowerPlant Chemistry 2013, 15 (4)
  
  
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