S Chaudhuri

Analysis of failed reformer tubes

Reformer tubes from a fertilizer plant made of modified HK 40 steel which failed after 4 years service were investigated for failure mechanism and life evaluation. The investigation included hot tensile tests, hardness measurement, dimensional measurement, microscopy and a few accelerated creep tests. Analysis revealed that longitudinal cracks found in the tubes were caused by overheating because of inadequate feed flow caused by the choking of damaged catalyst. To avoid such choking, precautions should be taken while charging the catalyst that no broken piece of catalyst or any external material goes along. Nitrogen which is used for cooling down the tubes and which was retained inside during idle periods should be dry enough. Overheating during service is primarily responsible for significant degradation in mechanical properties and microstructures in the bottom portion of the reformer tubes.

Residual life prediction of service exposed main steam pipe of boilers in a thermal power plant

This paper deals with residual life prediction methodology for more than 12.5 years service exposed main steam pipes of various boilers in a thermal power plant. Health assessment was made using destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service exposed main steam pipes of all the boilers. So far as the remaining life at 550°C is concerned, it is possible to obtain a life of greater than 100,000 h at the hoop stress level of the service exposed pipes, provided no localised damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 50,000 h of service exposure at 550°C.

Evaluation of mechanical properties and assessment of residual life of a service-exposed water wall tube

This paper is aimed at investigating the residual life, spanning 32 years, of a service-exposed water wall tube from one of the steam generating boilers of a thermal power station. The remaining life prediction and safety assessments were made using destructive accelerated stress rupture and tensile tests at different temperatures, as well as some non-destructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service-exposed water wall tube. So far as the remaining life at 425°C is concerned, it is possible to obtain a life greater than 100,000 h at the hoop stress level of the service-exposed tube, providing that no localised damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 50,000 h of service exposure.

Simulation of bond coat properties in thermal barrier coatings during bending

Analytical models are presented for predicting bond coat properties of thermal barrier coatings (TBCs) during crack propagation studies induced by bending. Studies on crack propagation behaviour in TBCs were performed with plasma spray coated zirconia, bonded by a MCrAlY layer to Ni-base superalloys (Inconel 617 and CMSX-4). Such thermal barrier composites are currently considered as candidate materials for advanced gas turbine stationary components. Coating as a protective layer improves the surface properties of the substrate. At a temperature of 1073 K, the crack propagation was found to be confined to the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. The stress-strain behaviour is a function of the elastic modulus of coating, bond coat as well as that of substrate. Thus, from a knowledge of the elastic modulus of TBC and that of substrate, the elastic modulus of the bond coat needs to be evaluated which is a basic parameter for characterizing coating performance. In this paper, the elastic modulus of the bond coat has been determined by modifying the existing model for a two-layered composite beam to a three-layered composite beam.

Investigation and modeling of mechanical properties for thermal barrier coatings under bending

This paper is aimed at developing analytical models for predicting bond coat properties during crack propagation studies under bending on thermal barrier coatings (TBCs) and to reveal the stress distribution pattern along the directions of global axes and the equivalent stress for all types of specimens used in the present investigation. Crack propagation studies were performed with plasma spray coated zirconia, bonded by a MCrAlY layer to Ni-base superalloys (Inconel 617 and CMSX-4). Such thermal barrier composites are currently considered as candidate materials for advanced stationary gas turbine components. Coating as a protective layer improves the surface properties of the substrate. At a temperature of 800 degreesC, the crack propagation was found to be confined to the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. The stress-strain behaviour is a function of the elastic modulus of coating, bond coat as well as that of substrate. Thus, having known the elastic modulus of TBC and that of substrate, the elastic modulus of the bond coat needs to be evaluated which is a basic parameter for characterising coating performance. In this paper, the elastic modulus of the bond coat has been determined by modifying the existing model for two-layered composite beam specimen to a three layered composite beam specimen. FEM (finite element) analysis for all types of tested specimens was conducted with ANSYS software using a 2-D isoparametric solid element, defined by four nodal points having two degrees of freedom at each node. Simulations were carried out all specimens with selected load conditions in the elastic region of the stress-strain curves.. The outputs of the stresses along the direction of global axes i.e. sigma (x), sigma (y) (component stresses), the shear stress sigma (xy), the principal stresses sigma (1), sigma (2), sigma (3) and the equivalent stress, sigma (eq) have been presented and discussed.

Remnant life assessment of service-exposed pendent superheater tubes

Remnant life assessment and life extension has become an integral part of plant maintenance activities, Cr–Mo steels are widely used in thermal power plants at a temperature of about 550°C. This paper deals with the high temperature tensile and creep rupture properties of 1Cr–0.5Mo steels for pendent superheater tubes in a boiler of a thermal power plant after five years service exposure for 43,000 h. Based on stress vs Larson Miller Parameter plot and at the operating hoop stress level, the pendent superheater tubes in general are found to be in a good state of health and can continue to remain in service for a length of 10 years, provided the temperature of the tube is brought down below 500°C. This observation may not be true for those selected tubes where already expansion in the diameter has taken place. Such tubes should be identified and replaced. In the case that it is not possible to bring down the temperature below 500°C, it may be desirable to use a superior grade of material like 2.25Cr–1Mo or 9Cr–1Mo steel. A thicker tube of the same grade may also be considered provided the overall load of the panel does not exceed the limit. It is, however, recommended that a similar health check be carried out after five years.

Modelling high temperature creep of CrMo steel

Abstract A computer based design aid called CRISPEN using established physical models of creep deformation has been developed jointly by NPL Teddington and Cambridge University for creep strain prediction of engineering alloys. This has been used to successfully predict the creep behaviour of a range of superalloys where strain softening is the most dominant mechanism of creep deformation. The present work examines how this approach could be modified to include the effect of softening due to time dependent particle coarsening, which is the most dominant mechanism of creep deformation in CrMo steel. A computer program has been developed to analyse creep curves to extract the relevant constants. A large volume of existing database on 2.25 Cr1Mo steel has been used to validate the approach. The physical significance of the parameters used has been analysed to identify the nature of stress dependence of particle coarsening behaviour in these steels.

Mechanical Property Evaluation and Remaining Life Assessment Of Service-Exposed Steam Pipe of Boilers in a Thermal Power Plant

This paper is aimed at investigating the residual life of more than eleven years service - exposed main steam pipes of various boilers in a thermal power plant. The remaining life assessment for safety were made using destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service exposed main steam pipes of all the boilers. So far as the remaining life at 823 K is concerned, it is possible to obtain a life of greater than 100000 hours, both at the allowable as well as operating hoop stress levels of the service - exposed pipes, provided no localised damage in the form of cracks or dents has been developed. It is recommended that a health check should be carried out after 50,000 hours of service exposure at 823 K.

Life prediction of service exposed reactor and distillation column materials used in a petrochemical industry

Determination of mechanical properties is a powerful method for investigating and assessing the performance of service exposed materials. The present paper highlights the trend recently observed in service exposed (38 years) plain carbon steels used as reactor and distillation column materials in a petrochemical industry. Health assessment of both base and welded joints was made using destructive accelerated stress rupture and tensile tests at different temperatures, and some nondestructive tests. So far as the remaining life at 520°C is concerned, it is possible to obtain a life of about 1.00 lakh h at the respective hoop stress level of both materials, provided no localized damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 5 years of service exposure at 520°C.

Creep life extension of high-temperature components under wall thinning conditions

Current methods of life extension of high temperature components have been examined. Importance of changing section size as a result of corrosion and erosion has been emphasized. A simple method based on the influence of service exposure on rupture strength of the material and the data collected on loss of section size during routine shut-down of plants has been suggested. Highlights of the computer software developed to assist such an exercise have been discussed. The outputs have been shown to compare fairly well with the results reported in literature.