Maintenance program optimization for 600MW units boiler tubing system in the aim of reliability improvement

Cevni sistem kotla predstavlja posebno kritično mesto sa aspekta održavanja termoenergetskih postrojenja, s obzirom da je broj zastoja izazvanih otkazom cevnog sistema značajan. Poboljšanjem sistema održavanja u pravcu razrade i definisanja obima kontrole u eksploataciji, moguće je optimizovati i na ispravan način definisati trajanje svakog remonta postrojenja, ali i smanjiti obim neophodnih ispitivanja uvođenjem periodičnih praćenja samo određenih delova opreme koji su se izdvojili kao kritični. U ovom radu su prikazane aktivnosti iz domena održavanja koje se sprovode na TE Nikola Tesla B sa ciljem podizanja raspoloživosti postrojenja a najvećim delom zasnovane na oceni trenutnog i predviđanju daljeg tehničkog stanja opreme.Boiler tubing system presents particulars critical system having in mind power plant maintenance due to pronounced number of boiler tubes failure provoked forced outages. Maintenance improvement programs should provide exact size of operational control, defined outage duration and reduced number of inspection implementation of periodical investigation of particular critical components. The aim of this paper is to present current state of the art in domain of Nikola Tesla B thermal power plant maintenance program practice which is mainly directed to plant reliability and based on estimation of current and future technical state of equipment

Statistical correlation between vibration characteristics, surface temperatures and service life of rolling bearings - artificially contaminated by open pit coal mine debris particles

Nowadays, two most often used methods for rolling bearings condition monitoring are thermographic inspection and vibrodiagno stic. However, analysis of the relevant literature has shown that so far there is no established correlation between rolling bearings surface temperatures and measured radial vibration intensities. These variables also strongly depend on rolling bearing's service life and it's environmental operating conditions, especially in case of open pit coal mine conveyor idler's rolling bearings; where high concentration of debris particles (such as surface dust, dirt and excavated coal) is present. Taking into account previously listed facts; the main goal of the presented research results was to establish statistically significant correlation between listed variables: rolling bearings surface temperatures, radial vibration intensities, service life and concentration level of debris particles in bearing grease. In order to achieve this goal, specific experimental methodology was developed and implemented. Obtained results were then processed using standardized statistical software and appropriate correlation was generated and later verified in praxis. At this moment, developed experimental methodology is applied only to open pit coal mine conveyor idler's rolling bearings, but its principles are universal, so with minor modifications it could be used for prediction of any of listed variables for different kinds of rolling bearings, operating in different environments

Maintenance program optimization for 600MW units boiler tubing system in the aim of reliability improvement

Cevni sistem kotla predstavlja posebno kritično mesto sa aspekta održavanja termoenergetskih postrojenja, s obzirom da je broj zastoja izazvanih otkazom cevnog sistema značajan. Poboljšanjem sistema održavanja u pravcu razrade i definisanja obima kontrole u eksploataciji, moguće je optimizovati i na ispravan način definisati trajanje svakog remonta postrojenja, ali i smanjiti obim neophodnih ispitivanja uvođenjem periodičnih praćenja samo određenih delova opreme koji su se izdvojili kao kritični. U ovom radu su prikazane aktivnosti iz domena održavanja koje se sprovode na TE Nikola Tesla B sa ciljem podizanja raspoloživosti postrojenja a najvećim delom zasnovane na oceni trenutnog i predviđanju daljeg tehničkog stanja opreme.Boiler tubing system presents particulars critical system having in mind power plant maintenance due to pronounced number of boiler tubes failure provoked forced outages. Maintenance improvement programs should provide exact size of operational control, defined outage duration and reduced number of inspection implementation of periodical investigation of particular critical components. The aim of this paper is to present current state of the art in domain of Nikola Tesla B thermal power plant maintenance program practice which is mainly directed to plant reliability and based on estimation of current and future technical state of equipment

Probability of failure of thermal power plant boiler tubing system due to corrosion

Korozija predstavlja nepovratni proces degradacije materijala. Zbog nepovoljanog uticaja koji ima na radne osobine materijal, posebno materijala cevnog sistema kotla, ona predstavlja predstavlja neizostavni deo rada termoenergetskih postrojenja i glavni uzrok mnogih otkaza. Pored uobičajenih metoda za procenu preostalog radnog veka korisno je primeniti i metode za procenu pouzdanosti kotlovskih cevi termoenergetskih postrojenja koje su oštećene korozijom. Korelacijom rezultata koji se dobijaju primenom ove dve različite metode moguće je dobiti bolja saznanja o trenutnom stanju materijala cevi uz istovremeno izvođenje tačnije procene njihovog ponašanja tokom dalje eksploatacije. Stoga je u ovom radu na primeru cevi ulazne zone paketa naknadnog pregrejača prikazan integralni pristup procene preostalog radnog veka i pouzdanosti. Razmatranja ove vrste su veoma značajna za pristupe u održavanju koji su zasnovani na riziku.Corrosion is irreversible process of material degradation. Due to detrimental effects exerted on the operating material characteristics, especially in the boiler tube system, it is of one the most important issues in the operation of thermal plants and is considered to be the root cause of many outages. Besides common remaining life assessment methods, it is useful to apply a method of reliability evaluation of the thermal power plant boiler tubes with corrosion damages. Correlation of results obtained by these two methods makes possible to obtain a better knowledge of current tubes material state as well as a more accurate assessment of their behavior during future exploitation. Therefore, an integrated approach of remaining life assessment and reliability evaluation, on the first stage reheater tubes system example, is presented in this paper. Considerations of this kind are very important for the risk-based maintenance programs

Probability of failure of thermal power plant boiler tubing system due to corrosion

Korozija predstavlja nepovratni proces degradacije materijala. Zbog nepovoljanog uticaja koji ima na radne osobine materijal, posebno materijala cevnog sistema kotla, ona predstavlja predstavlja neizostavni deo rada termoenergetskih postrojenja i glavni uzrok mnogih otkaza. Pored uobičajenih metoda za procenu preostalog radnog veka korisno je primeniti i metode za procenu pouzdanosti kotlovskih cevi termoenergetskih postrojenja koje su oštećene korozijom. Korelacijom rezultata koji se dobijaju primenom ove dve različite metode moguće je dobiti bolja saznanja o trenutnom stanju materijala cevi uz istovremeno izvođenje tačnije procene njihovog ponašanja tokom dalje eksploatacije. Stoga je u ovom radu na primeru cevi ulazne zone paketa naknadnog pregrejača prikazan integralni pristup procene preostalog radnog veka i pouzdanosti. Razmatranja ove vrste su veoma značajna za pristupe u održavanju koji su zasnovani na riziku.Corrosion is irreversible process of material degradation. Due to detrimental effects exerted on the operating material characteristics, especially in the boiler tube system, it is of one the most important issues in the operation of thermal plants and is considered to be the root cause of many outages. Besides common remaining life assessment methods, it is useful to apply a method of reliability evaluation of the thermal power plant boiler tubes with corrosion damages. Correlation of results obtained by these two methods makes possible to obtain a better knowledge of current tubes material state as well as a more accurate assessment of their behavior during future exploitation. Therefore, an integrated approach of remaining life assessment and reliability evaluation, on the first stage reheater tubes system example, is presented in this paper. Considerations of this kind are very important for the risk-based maintenance programs

Hydrogen embrittlement of low carbon structural steel

Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20- St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials

Life estimation of first stage high pressure gas turbine blades

Posle pojave prevremenih lomova koji su se javili kod lopatica prvog stepena turbine visokog pritiska određenog tipa turboreaktora u jednoj lokalnoj avio kompaniji, usledila su istraživanja u cilju određivanja njihovog sigurnog (pouzdanog) radnog veka. Lopatice prvog stepena turbine visokog pritiska su tokom rada izložene simultanom delovanju pritiska gasa iz komore za sagorevanje, centrifugalne sile u slučaju rotorskih lopatica, velikim temperaturnim promenama, kao i agresivnoj radnoj sredini. Kombinacija svih navedenih činilaca izaziva vrlo kompleksno naponsko stanje lopatica kao i mogućnost pojave višestrukih mehanizama oštećivanja: zamora izazvanog fluktuacijama u mehaničkom naponu, termo-mehaničkog zamora usled temperaturnih promena i korozije naponski opterećenih delova. Da bi se odredio radni vek u navedenim uslovima, neophodno je proceniti napone kojima su lopatice izložene uzimajući u obzir nekoliko promenljivih koje se u radu tretiraju deterministički. Razmatran je prenos toplote između sagorelih gasova i metala lopatica turbine. Izračunat je ukupni napon na dve vrste lopatica imajući u vidu termičke efekte i mehaničko opterećenje. Naponski ciklus je zatim izračunat za različite faze rada turboreaktora uz varijacije termičkih i mehaničkih osobina. Procena sigurnog radnog veka je izvršena primenom dva pristupa: inicijalnog modela i pristupa sa tolerancijom oštećenja uzimajući u obzir mehanički rast oštećenja i rast korozionog pita. Proračun je primenjen i na statorske i na rotorske lopatice turbine visokog pritiska turboreaktora koje su izrađene od čelika NI 738. Nađeno je da su ovo komponente visokog rizika, tako da je procenjen i rizik sa aspekta potencijalnih posledica od loma. Dobijeni rezultati su razmatrani u cilju rešavanja problema i donošenja sigurne odluke sa aspekta procedure održavanja ali i konstrukcijskog rešenja.Based on very early occurring ruptures found in the first stage high pressure turbine blades of a turbo reactor in a local aviation company, this study has the aim to determine their safe life. The first stage blades are subjected to simultaneous action of gas pressure coming from the combustion chamber, centrifugal forces in the case of the rotor blades and to important temperatures transients, which progress in a very aggressive environment due to hot gases. These combined parameters cause a high state of stress involving several complex mechanisms of damage, such as: fatigue caused by mechanical stress fluctuations, thermo-mechanical fatigue caused by temperature variations and corrosion caused on the stressed elements. Life cycle determination asks for stress evaluation of blades regarding several variables which are approached deterministically in the study. Heat exchange between combustion gases and metal blades is considered. The total stress on two kinds of blades is calculated by the addition of the thermal effect and the mechanical loading. The stress cycle is then calculated for different steps of the engine function during the operation by considering the variation of the thermal and the mechanical properties of the system. Safe life determination is done by two different approaches: the safe life approach by the initiation model and the damage tolerance approach considering the defect growth mechanics and considering the pitting corrosion effect. The calculation is applied for stator and rotor blades of an aero engine high pressure turbine made of NI 738. Since these parts are high risk components from the point of view of potential failure consequences, the risk is assessed as well. The results obtained are studied to determine the solution to the problem, and to propose a safe decision to be taken about the design or maintenance procedures.

Life estimation of first stage high pressure gas turbine blades

Posle pojave prevremenih lomova koji su se javili kod lopatica prvog stepena turbine visokog pritiska određenog tipa turboreaktora u jednoj lokalnoj avio kompaniji, usledila su istraživanja u cilju određivanja njihovog sigurnog (pouzdanog) radnog veka. Lopatice prvog stepena turbine visokog pritiska su tokom rada izložene simultanom delovanju pritiska gasa iz komore za sagorevanje, centrifugalne sile u slučaju rotorskih lopatica, velikim temperaturnim promenama, kao i agresivnoj radnoj sredini. Kombinacija svih navedenih činilaca izaziva vrlo kompleksno naponsko stanje lopatica kao i mogućnost pojave višestrukih mehanizama oštećivanja: zamora izazvanog fluktuacijama u mehaničkom naponu, termo-mehaničkog zamora usled temperaturnih promena i korozije naponski opterećenih delova. Da bi se odredio radni vek u navedenim uslovima, neophodno je proceniti napone kojima su lopatice izložene uzimajući u obzir nekoliko promenljivih koje se u radu tretiraju deterministički. Razmatran je prenos toplote između sagorelih gasova i metala lopatica turbine. Izračunat je ukupni napon na dve vrste lopatica imajući u vidu termičke efekte i mehaničko opterećenje. Naponski ciklus je zatim izračunat za različite faze rada turboreaktora uz varijacije termičkih i mehaničkih osobina. Procena sigurnog radnog veka je izvršena primenom dva pristupa: inicijalnog modela i pristupa sa tolerancijom oštećenja uzimajući u obzir mehanički rast oštećenja i rast korozionog pita. Proračun je primenjen i na statorske i na rotorske lopatice turbine visokog pritiska turboreaktora koje su izrađene od čelika NI 738. Nađeno je da su ovo komponente visokog rizika, tako da je procenjen i rizik sa aspekta potencijalnih posledica od loma. Dobijeni rezultati su razmatrani u cilju rešavanja problema i donošenja sigurne odluke sa aspekta procedure održavanja ali i konstrukcijskog rešenja.Based on very early occurring ruptures found in the first stage high pressure turbine blades of a turbo reactor in a local aviation company, this study has the aim to determine their safe life. The first stage blades are subjected to simultaneous action of gas pressure coming from the combustion chamber, centrifugal forces in the case of the rotor blades and to important temperatures transients, which progress in a very aggressive environment due to hot gases. These combined parameters cause a high state of stress involving several complex mechanisms of damage, such as: fatigue caused by mechanical stress fluctuations, thermo-mechanical fatigue caused by temperature variations and corrosion caused on the stressed elements. Life cycle determination asks for stress evaluation of blades regarding several variables which are approached deterministically in the study. Heat exchange between combustion gases and metal blades is considered. The total stress on two kinds of blades is calculated by the addition of the thermal effect and the mechanical loading. The stress cycle is then calculated for different steps of the engine function during the operation by considering the variation of the thermal and the mechanical properties of the system. Safe life determination is done by two different approaches: the safe life approach by the initiation model and the damage tolerance approach considering the defect growth mechanics and considering the pitting corrosion effect. The calculation is applied for stator and rotor blades of an aero engine high pressure turbine made of NI 738. Since these parts are high risk components from the point of view of potential failure consequences, the risk is assessed as well. The results obtained are studied to determine the solution to the problem, and to propose a safe decision to be taken about the design or maintenance procedures.

Risk Assessment of Pressure Vessels by Using Fracture Mechanics and Advanced Ultrasonic Testing

Risk assessment of cracked cylindrical pressure vessels for compressed air by using basic fracture mechanics and advanced methods of non-destructive testing (NDT), such as Phased Array Ultrasound (PAUT) and Time-of-Flight Diffraction (TOFD), is presented. Basic fracture mechanics equations are used to calculate the stress intensity factor, KI, in the case of unacceptable defects found in the pressure vessel 970 in the Reversible Hydro Power Plant "Bajina Basta", and get the ratio KI/KIc, according to the minimum measured values for fracture toughness for welded joints, as the typical zones where crack-like defects are found. The ratio Snet/Sc, where Snet is the net stress in cross-section with a crack, and Sc is the critical stress, is then evaluated to define the operating point in the Failure Assessment Diagramme (FAD), and thus, to estimate the likelihood of failure of the pressure vessel 970. In combination with estimated high consequence for the pressure vessel 970, the risk matrix was used, as a simple tool to assess the risk. In this paper the focus is on one aspect of this procedure, being the NDT role, since it is of utmost importance to use as precise as possible method for detected defects in welded joints. In this paper advanced ultrasound methods, PAUT and TOFD, are used to get precise image of defects in pressure vessel 970 welded joints, which were previously detected by conventional NDT methods. It is shown that the use of PAUT and TOFD is of utmost importance for decision making process in this case

Hydrogen embrittlement of low carbon structural steel

Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20- St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials