Heat Transfer in the Components of Power Boilers and Related Technological and Endurance Problems

Issues related to the generation of electricity are currently particularly important in Europe due to the need to ensure basic conditions for the function and development of the economy during crises caused by political conflicts with a global impact. The lack of easy access to fuels, which occurs during a transitional period of energy transformation involving basic energy and raw materials, and the sharp increase in their prices mean that conventional energy based on coal is still important in the overall balance of various sources of energy generation. At the same time, conventional energy faces a new challenge due to the need to ensure greater efficiency of existing and new power units and greater flexibility in the operation of individual units in energy systems, which include conventional power plants and renewable energy sources.

Ensuring the high efficiency of individual power units requires an increase in the operating parameters of their devices and equipment. At the same time, the requirements related to the flexibility of work make it necessary to take into account frequent start-ups and shut-downs as well as changes in the power supplied to the grid in large energy systems. In these conditions, the strength and durability issues of equipment elements subjected to variable mechanical and thermal influences are still important. Long-term exposure to elevated temperatures at constant loads causes processes to creep into them. Simultaneously, changes in working conditions are a potential cause of fatigue phenomena [1,2,3], the impact of which on durability is increasingly being studied.

These studies include analyses of the operating conditions of individual components of energy equipment, the determination of the fatigue characteristics of materials used for operations at elevated temperatures, and analyses of temperature fields and stresses caused by time-varying loads [4,5,6]. These complementary issues are components of the methodology for assessing the durability of equipment elements, in which the variable temperature field is the cause of time-varying thermal stresses.

The production of the elements of power units with high operating parameters usually requires the use of new materials with a higher creep strength and that are fatigue-resistant under variable temperature conditions. In order to ensure the high strength and durability of power units, technologies that join their components are also modified, which mainly involve welding technologies. Striving to increase efficiency is the reason for modifying the structural features of heat exchangers and the interest in related issues of heat transfer; this depends on the type of materials used and the shape of the elements. Thus, the issue of heat transfer in power equipment elements—in this Special Issue of the Energies journal titled “Heat Transfer in the Components of Power Boilers and Related Technological and Endurance Problems”—will be considered both in the context of issues related to the technology of power equipment and their strength and durability, which determine the safety of using these elements. The volume consists of four papers. The articles included in the study cover selected problems in this field.

Issues related to the technological properties of bainitic steel will be discussed, specifically focusing on bainitic steel containing 2.25% Cr and 0.6% Mo with micro-additives V, Ti, and B, which are designated as 7CrMoVTiB10-10 or T24/P24 and commonly used in power equipment operating at elevated temperatures in blocks with supercritical parameters. One of the important technological properties of this steel is its hot weldability. This property is defined as the susceptibility to hot cracking, which depends on phenomena affecting the brittleness of the material at elevated temperatures that fall within the range designated as HTBR (high-temperature brittleness range). This type of brittleness is both determined for the base material to be welded and for the welded joints, and it is dependent on the conditions in which the welding process takes place and takes into account the critical temperature–strain intensity (CST) and critical strain speed (CSS). In one of the papers presented in this Special Issue, an attempt was carried out to determine the HTBR range for typical conditions in which the welding process of 7CrMoVTiB10-10 steel takes place, and CST and CSS values were adopted. The conclusions formulated in the study are of practical importance for developing a method for determining the permissible welding parameters in technological processes.

An important issue in the construction and operation of gas power units, characterized by high efficiency and reliability, is the problem of selecting the structural features of heat exchangers. One of the commonly used solutions in this case is the application of ribbed pipes, which are most often produced in the welding process. The desired properties of this type of element are high thermal efficiency, resistance to high-temperature corrosion in the flue gas atmosphere, and the strength and durability of welded joints that are present in them. Most often, in the process of welding finned pipes, MAG technology is used. Welded joints made using this technology show weld discontinuities, spatters, or non-axial fin locations. The occurrence of such defects affects heat flow in the exchangers under consideration. One can find studies in which it has been shown that such defects can significantly reduce the efficiency of heat flow phenomena in these exchangers. It can be stated that the lack of metallic continuity in the joint results in a drastic reduction in thermal efficiency. Therefore, research on the use of new joining technologies is justified, in the case of which the above-mentioned defects will not occur. The study presents the possibility of using laser welding technologies in order to increase heat flux in the process of its exchange in the considered elements. The thermal properties of laser-welded finned tubes with MAG-welded pipes were compared. It was observed that the thermal efficiency of ribbed welded pipes was three times higher than that of smooth pipes.

Coal-fired units, currently compensating for power shortages in the power system, are characterized by their ability to increase the flexibility of operations. A fluctuating electricity demand requires modifying the operating modes of existing power systems to ensure both an uninterrupted supply of electricity and safety standards. The study presents a method for predicting the fatigue life of a drum, which is one of the most important elements of the boiler in conventional power plants. The procedures for the local analysis of stress–strain fields are described, and the results of low-cycle fatigue tests of the drum material were presented. The assessment of strength and durability was based on the finite element modelling method—FEM. The drum model was developed on the basis of the technical documentation of the selected device. The analysis included calculations of time-dependent temperature distributions, stresses, and strains for different modes of starting the power unit. The boundary conditions adopted in the calculations were defined on the basis of measurements carried out under industrial conditions. Using the results of computer modelling and fatigue properties, the drum life was estimated for different start-up modes. The influence of several start-up procedures and fatigue properties of the material on the fatigue life of a drum working under mechanical and thermal loads was discussed. An approach to fatigue life assessment based on a combination of multiple test methods was also proposed.

Faster start-ups and the possibility of frequent drops and increases in output power cause large temperature gradients in the elements of the power unit and turbine, and lead can increase due to the stress level. The assessment of operational safety in such a case cannot be based solely based on an analysis that only takes into account the start-up characteristics of the power unit, namely time-dependent pressure and temperature, which are defined by designers of power stations. Monitoring the level of stress in areas considered critical in the elements of machinery and equipment then becomes necessary. One of the papers presents a method for assessing the level of stress in selected elements using online monitoring and algorithms based on Green functions and the Duhamel integral. The paper analyzes examples of stress calculations in valves and turbine housings during selected duty cycles. In this case, the influence of the variability of heat transfer coefficients at the level of thermal stresses was taken into account. A comparison of the measurement results with the results obtained from FEM calculations was also presented. Equations were provided to estimate the value of the heat transfer coefficient in turbine valves. The authors predicted that the proposed modification of the calculation algorithm will significantly improve the accuracy of mapping variable stresses in the transient states of turbine operations using modelling methods. According to the authors, such a method of stress monitoring will increase the flexibility of the power unit and facilitate control in operating conditions while simultaneously ensuring safety when starting and shutting down the power unit. Stresses calculated using the applied algorithms can also be used to assess fatigue life and predict the life of individual components.

The studies presented in the Special Issue of Energies cover problems related to the assessment of the durability of power equipment components and, in some cases, their possible solutions. For these problems, the heat flow issue is related to both the manufacture of these components and the methods of their design and operation, ensuring the safe use of power installations. This is important in the process of transforming the modern power industry; although the issue is present in many studies related to power equipment, it has not found a permanent forum where it is discussed and considered, especially in relation to the fatigue of materials. In this sense, the current Special Issue is an attempt to create a forum where the listed problems can be discussed. The exchange of thoughts should also take into account technological experiences in the field of materials engineering and the mechanics of materials including the production of elements with desired properties and the possibility of predicting their functional properties [7,8,9]. The latter should be based on the thermodynamic analysis of heat flow along with a mechanical description of stress and strain fields.