Industrial boiler damage is a common phenomenon encountered in boiler operation which usually lasts several decades. Since boiler shutdown may be required because of localized failures, it is crucial to predict the most vulnerable parts. If damage occurs, it is necessary to perform root cause analysis and devise corrective measures (repairs, design modifications, etc.). Boiler tube bundles, such as those in superheaters, preheaters and reheaters, are the most exposed and often the most damaged boiler parts. Both short-term and long-term overheating are common causes of tube failures. In these cases, the design temperatures are exceeded, which often results in decrease of remaining creep life. Advanced models for damage evaluation require temperature history, which is available only in rare cases when it has been measured and recorded for the whole service life. However, in most cases it is necessary to estimate the temperature history from available operation history data (inlet and outlet pressures and temperatures etc.). The task may be very challenging because of the combination of complex flow behaviour in the flue gas domain and heat transfer phenomena. This paper focuses on estimating thermal load non-uniformity on superheater tubes via Computational Fluid Dynamics (CFD) simulation of flue gas flow including heat transfer within the domain consisting of a furnace and a part of the first stage of the boiler.
Superheater-tube failure is listed among the major causes of a fossil-fuel-fired boiler outage. Therefore, it is necessary not only to identify and repair it in the case of failure but also to eliminate the root cause of this problem. As there may be multiple reasons of failure in exposed equipment such as a superheater, a thorough investigation of more than one probable cause is usually required. This article focuses on a failure analysis of a boiler located in a chemical plant. After a leak was discovered, several cracks on the superheater tubes were identified as its main cause. It was necessary to assess the extent of the damage, detect the root cause and propose corrective actions. Two problematic locations with cracks were identified during the visual inspection: the first was on the superheater-tube bends and the other was the weld joint between the superheater and the transition pipe. As the first step, the material-microstructure and composition analyses of the tubes in these critical locations were carried out. Even though small weaknesses were found in the microstructure, the main cause of the tube failure was not identified. As the next probable cause, thermal-dilatation stresses were investigated using the finite-element analysis (angl. FEA). The support system, consisting of fixed and spring supports, as well as the compensator were included in the analysis that confirmed the thermal-dilatation stresses as the major cause of the failure. Based on the results, a new technical solution for the supports was suggested and verified with the FEA. Keywords: corrosion, weldment cracks, superheater supports, thermal dilatation Napaka cevi pregrevalnika je znana kot ena najpogostej{ih napak pri kotlu na fosilna goriva. Zato jo je treba, ne le prepoznati in v primeru okvare popraviti, pa~pa tudi v splo{nem odpraviti vzrok za njen nastanek. V tako izpostavljenem elementu kot je pregrevalnik, je za tovrstno napako lahko ve~vzrokov, zato je za ugotovitev le-teh, potrebna temeljita preiskava.^lanek je osredoto~en na analize okvar kotla, ki se nahaja v kemi~ni tovarni. Potem, ko so odkrili pu{~anje, je bilo na pregrevalnih ceveh ve~razpok, ki so bile opredeljene kot glavni vzrok. Treba je bilo oceniti obseg {kode, odkriti vzrok in predlagati ukrepe za popravilo. Med vizualnim pregledom sta bili ugotovljeni dve problemati~ni lokaciji z razpokami. Prva na zavojih cevi pregrevalnika in druga v spoju zvara med pregrevalnikom in prehodno cevjo. Najprej je bila izvedena analiza mikrostrukture materiala in analiza sestave cevi na kriti~nih mestih.^eprav so bile ugotovljene pomanjkljivosti v mikrostrukturi, glavni vzrok napake cevi ni bil ugotovljen. Naslednji mo`ni vzrok bi lahko bila termodilatacijska napetost, ki je bila raziskovana z uporabo analize kon~nih elementov (FEA). Sistem za podporo, ki je sestavljen iz fiksnih in podpornih vzmeti pa tudi kompenzator, so bili vklju~eni v analizo, ki je potrdila, da je toplotna dilatacijska napetost glavni vzrok napake. Na podlagi rezultatov je bila predlagana nova tehni~na re...
A new strategy for fast, approximate analyses of fluid flow and heat transfer is presented. It is based on Finite Element Analysis (FEA) and is intended for large yet structurally fairly simple heat transfer equipment commonly used in process and power industries (e.g., cross-flow tube bundle heat exchangers), which can be described using sets of interconnected 1-D meshes. The underlying steady-state model couples an FEA-based (linear) predictor step with a nonlinear corrector step, which results in the ability to handle both laminar and turbulent flows. There are no limitations in terms of the allowed temperature range other than those potentially stemming from the usage of fluid physical property computer libraries. Since the fluid flow submodel has already been discussed in the referenced conference paper, the present article focuses on the prediction of the tube side and the shell side temperature fields. A simple cross-flow tube bundle heat exchanger from the literature and a heat recovery hot water boiler in an existing combined heat and power plant, for which stream data are available from its operator, are evaluated to assess the performance of the model. To gain further insight, the results obtained using the model for the heat recovery hot water boiler are also compared to the values yielded by an industry-standard heat transfer equipment design software package. Although the presented strategy is still a “work in progress” and requires thorough validation, the results obtained thus far suggest it may be a promising research direction.
Flange joints and their sealing play an important role in many industries. The gasket performance and its behaviour are influenced by a number of factors, such as non-linear material properties with permanent deformations, assembly procedures and the preparation of sealing surfaces. Additionally, a proper seal function is also affected by the design and strength design of the flanges. Determination of the respective tightening torque needed to achieve a suitable contact pressure between the seal and the flange face is equally important. This paper deals with finite element method (FEM) analyses of a flange joint designed in accordance with the modern standard EN 13445-3 Annex G and examines the influence of operating conditions on the gasket contact pressure. The article also discusses the effects of assembly on the tightness of the joint and the reason for the leakage of the operating medium that took place. The analyses show the effects of operating states on the contact pressures of gaskets and the pre-stressing of bolts. They demonstrate the contact pressure after the application of the pre-stressing of the bolts and its reduction after the temperature-field stabilization due to the start-up of the device. The results of the analyses show that only a relatively small surface of the seal achieves the compression required by the manufacturer to maintain the seal integrity during the application of the tightening forces determined in accordance with EN 13445-3 Annex G. The force of the pre-stressing of the bolts is reduced by approximately 6 % when the normal operation condition is reached. The analyses were performed due to a suspicion of a significant influence of the temperature distribution on flange joints. The main cause of the flange leakage was subsequently revealed by a physical inspection that demonstrated assembly failures when installing gasket 2. The description of these deficiencies is not a subject of this article. Keywords: flange joint, sealing, gasket, FEM, contact pressure Spoji prirobnic in njihovo tesnenje igrajo pomembno vlogo v mnogih industrijskih vejah. Lastnosti tesnila in njegovo vedenje je odvisno od mnogih faktorjev, kot so: nelinearne materialne lastnosti s stalno deformacijo, kakor tudi postopek namestitve in tesnilna povr{ina. Dodatno je pravilno tesnenje odvisno tudi od oblike in trdnosti prirobnice. Prav tako je pomembna dolo~itev tesnilnega navora za doseganje primernega kontaktnega tlaka med tesnilom in povr{ino prirobnice. V~lanku avtorji opisujejo analizo spoja prirobnice s pomo~jo metode kon~nih elementov (FEM; angl.: Finite Element Method), ki je oblikovana v skladu z modernim standardom EN 13445-3 Annex G in preiskuje vpliv delovnih pogojev na kontaktni tlak tesnila. V~lanku avtorji prav tako razpravljajo o vplivih monta`e na tesnost spoja ter podajajo mo`ne razloge za pu{~anje delovnega medija. V analizi so pokazali, kak{en je vpliv obratovalnih pogojev na kontaktni tlak tesnila in prednapetost pritrdilnih vijakov. Analiza je pokazala, kak{en je kontaktni tlak po predob...
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