Mechanical thermal stresses and creep analysis of boiler tubes

Advanced Materials Research, 2012

The boiler tubes are operated continuously at high temperature and pressure. During operation, scales are formed in boiler tube due to tube geometries, flue gas and steam temperature. The remaining wall thickness decreases due to the formation of scale which eventually causes failure of the boiler tubes. In this investigation an iterative technique was used to determine the temperature distribution across the tube with the operating time. The operating time was considered up to 160,000 hours. The remaining life of the steam generator tube was found by finding hoop stress and Larson Miller Parameter from the Larson Miller Parameter curve for SA213-T22 material. By utilizing finite element modelling software, ANSYS 9/ANSYS 11 the temperature distribution across the steam generator tube was evaluated. The increase of heat transfer rate across the wall caused the oxide scale thickness to grow more rapidly than normal condition. It was also observed that due to formation of scale the thermal conductivity in the boiler tubes was affected and the remaining life of boiler tubes was decreased and accelerated creep damage. The ANSYS result was analyzed by Minitab 16 to determine the main and interactive effects of operating conditions. Steam temperature was influencing most the wall thinning and creep damage in comparison to the flue gas temperature. The interactive effects of both the parameters were also prominent. Moreover, the optimum operating condition was identified in order to maximizing the remnant life of the tubes while minimizing the creep rupture damage.

In the current scenario of power shortage in India, the main objective is to ensure availability of power plant and increasing its reliability. During assessment ,testing and inspection a simple question has to be asked again and again‖ How long the particular power plants can be operated safely and cost-effectively with satisfying increased requirements and operational availability with reduced pollutant emissions, even after their designed life. So to answer this important question regarding the operational capability of the existing plant the remaining life analysis (RLA) has to be done. The condition of the plant equipments can be assessed only by way of a RLA methodology. On the basis of RLA proper decision can be made about the plants safety and availability. There are many methods to carry out the RLA of the critical components out of which -microstructure study‖ is a method. In this paper we have tried to outline the RLA procedures and review the various damage mechanisms based on microstructure study. It is also presents the microstructure changes and properties of 106720 service hour exposed boiler tube in a 120 MW boiler of a thermal power plant.

A major portion of the total electricity generated in our country is through thermal power plants using direct combustion of pulverized coal. The majority of forced outages of these thermal power stations are due to premature failure of vital components such as boiler tubes. Case studies pertaining to the failure analysis of various kinds of boiler tubes such as super heater tubes, reheater tubes, and water wall tubes that have failed involving creep deformation and damage have been studied. In the present study the metallurgical investigations revealed microstructural degradations through the formation of creep voids at the grain boundaries and intercrystalline cracks due to continued exposure to higher temperatures. The microstructure of the lip portion of the burst has been found to change depending upon the temperature of the rupture. Rupture taking place between Ac and Ac 3 has revealed a mixed structure consisting of bainite due to the quenching effects of the steel. Similarly rupture taking place below temperature Ac 1 have been marked by divorced /degenerated pearlite and or spheroidised carbides in the ferrite matrix. Analysis made regarding the overheating (creep) failure of pendant reheater tubes indicates that surrounding temperature of the tube exceed several degrees higher than the components are designed for and also due to factors like erosion of tube surface by Fly ash, short supply of water through the boiler tubes caused by internal deposits.

2011

Site inspections of Hot Reheat Piping system have indentifed cracking at a number locations at butt welds between pipe runs and elbows.Furthermore,it was discovered that pipe runs are instaled with wall thickness of SCH60(ANSI/ASME code[3]),and elbows with wall thickness of SCH30 what is not according to design documentation.This wall thickness discrepancy creates a stress enhancement at the connections between elbows and piping runs.Also,because the elbows are more flexible,there could be a propensity for a creep strains to be concentrated in the elbows as the piping system stresses will be relaxed at these lower strength elbows rather than being distributed throughout the main piping runs.So,a detailed finite element analysis,including elastic and creep analysis, is performed to determine if the secondary stresses during relaxation,or if the stress enhacement of the wall thickness discrepancy,play significant roles in the cracking detected in the piping system.

2014

Site inspections of Hot Reheat Piping system have indentifed cracking at a number locations at butt welds between pipe runs and elbows.Furthermore,it was discovered that pipe runs are instaled with wall thickness of SCH60(ANSI/ASME code[3]),and elbows with wall thickness of SCH30 what is not according to design documentation.This wall thickness discrepancy creates a stress enhancement at the connections between elbows and piping runs.Also,because the elbows are more flexible,there could be a propensity for a creep strains to be concentrated in the elbows as the piping system stresses will be relaxed at these lower strength elbows rather than being distributed throughout the main piping runs.So,a detailed finite element analysis,including elastic and creep analysis, is performed to determine if the secondary stresses during relaxation,or if the stress enhacement of the wall thickness discrepancy,play significant roles in the cracking detected in the piping system.

A boiler or steam generator is a closed vessel used to generate steam by applying heat energy to water.During the process of generating steam, the steam boiler is subjected to huge thermal and structural loads.To obtain efficient operation of the power plant, it is necessary to design a structure to withstand these thermal and structural loads. Using CAD and CAE softwares is the advanced methodology of designing these structures before constructing a prototype.In this project finite element analysis of the steam boiler was carried out to validate the design for actual working conditions. The main tasks involved in the project are performing the 3D modelling of the boiler and finite element analysis. In this project, design optimization of the boiler is also done based on the results obtained from the thermal and structural analysis. NX-CAD software is used for design and 3D modelling. ANSYS software is used for doing finite element analysis. PROBLEM DEFINITION AND METHODOLOGY The objective of this project is to make a 3D model of the steam boilerand study the structural and thermal behaviour of the steam boilerby performing the finite element analysis.3D modelling software (UNIGRAPHICS NX) was used for designing and analysis software (ANSYS) was used for structural and thermal analysis. The methodology followed in the project is as follows:  Create a 3D model of the steam boilerassembly using NX-CAD software.  Convert the surface model into parasolid file and import the model into ANSYS to do analysis.  Perform thermal analysis on the steam boilerassemblyfor thermal loads.  Perform static analysis on the existing model of the steam boilerassemblyfor pressure loads and thermal loads to find deflections and stress, optimized if enquired.

Procedia Engineering, 2013

Creep-rupture properties of T91 steam generator (SG condition in the stress range 55-150 MPa. At all stres creep rate in the transient creep followed by a minim tertiary creep stage. A systematic decrease in creep tertiary creep. Stress dependence of minimum creep r and rupture life exhibited deviations in terms of lowe at high stresses. A decrease in creep ductility was transgranular at all test conditions. Creep-rupture s reported in literature as well as specified in French Nu

Materiali in Tehnologije, 2018

Creep life is a limiting factor in the case of boiler tubes operating above the creep temperature and, for a given material, it is generally determined by the actual operating temperature and stress. Standard approaches to the calculation of a tube creep life mainly take into account the stress caused by pressure loading and the mean temperature in the tube wall. However, the estimation of the temperature and stress may be difficult for water-tube boilers because oxide scales tend to form on the inner surfaces of the tubes, exposed to steam and indirectly affecting the resulting creep life. As the scales increase the thermal resistance of a tube wall and, consequently, the wall temperature, the creep life is reduced. Moreover, the presence of oxide scales leads to a higher hydraulic resistance of a tube, which can cause further increase in the temperature in some tubes of the bundle if the oxide-scale growth rate is not uniform. Additionally, the elements increasing the creep streng...

Boiler tube material plays an important role in efficient power generation from a fossil fuel power plant. In order to meet out the gap between fluids to increase heat available per unit mass flow of steam. Waste heat utilization phenomenon is a big challenge on fossil fuel power plants as after use of high grade coal in thermal power plants the efficiency of power plants is not at the level of required value. Clean and efficient power generation with economical aspects is the basic need of growing power generation plants to justify the quality of power and clean power generation. Life analysis technique to calculate remaining life of boiler tubes at critical zones of high temperature requires much attention and is an important hypothesis in research field. Generation of repetitive and fluctuating stress during flow of high temperature and pressure fluid require proper attention on the methodology to be used to calculate the efficiency of system and absorption efficiency of tube material. In this paper complete mathematical analysis of boiler tubes is conducted for calculation of remaining life of boiler tubes, Hoop stress values are calculated and used with mathematical tool to calculate the efficiency. Hoop stress based calculation of efficiency is more reliable and may give more accurate and practical aspects based results.