Search

Allowable rates of fluid temperature changes in critical elements of supercritical steam boilers were determined assuming a quasi-steady state of temperature distribution in pressure components. A method was proposed for monitoring the thermal stresses in the critical pressure components. [ABSTRACT FROM AUTHOR]

Two methods for monitoring the thermal stresses in pressure components of thermal power plants are presented. In the first method, the transient temperature distribution in the pressure component is determined by measuring the transient wall temperature at several points located on the outer insulated surface of the component. The transient temperature distribution in the pressure component, including the temperature of the inner surface is determined from the solution of the inverse heat conduction problem (IHCP). In the first method, there is no need to know the temperature of the fluid and the heat transfer coefficient. In the second method, thermal stresses in a pressure component with a complicated shape are computed using the finite element method (FEM) based on experimentally estimated fluid temperature and known heat transfer coefficient. A new thermometer with good dynamic properties has been developed and applied in practice, providing a much more accurate measurement of the temperature of the flowing fluid in comparison with standard thermometers. The heat transfer coefficient on the inner surface of a pressure element can be determined from the empirical relationships available in the literature. A numerical-experimental method of determination of the transient heat transfer coefficient based on the solution of the 3D-inverse heat conduction problem has also been proposed. The heat transfer coefficient on the internal surface of a pressure element is determined based on an experimentally determined local transient temperature distribution on the external surface of the element or the basis of wall temperature measurement at six points located near the internal surface if fluid temperature changes are fast. Examples of determining thermal and pressure stresses in the thick-walled horizontal superheater header and the horizontal header of the steam cooler in a power boiler with the use of real measurement data are presented. [ABSTRACT FROM AUTHOR]

Thick-walled components of the thermal power unit limit the maximum heating and cooling rates during startup or shutdown of the unit. A method of monitoring the thermal stresses in thick-walled components of thermal power plants is presented. The time variations of the local heat transfer coefficient on the inner surface of the pressure component are determined based on the measurement of the wall temperature at one or six points, respectively, for one- and three-dimensional unsteady temperature fields in the component. The temperature sensors are located close to the internal surface of the component. A technique for measuring the fast-changing fluid temperature was developed. Thermal stresses in pressure components with complicated shapes can be computed using finite element method (FEM) based on experimentally estimated fluid temperature and heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The aim of this study is to develop a numerical model of transient pipeline operations. The finite volume method (FVM) will be used to solved the energy conservation equations for the pipeline wall and steam. The steam temperature, the temperature of the pipeline wall and thermal stresses can be calculated using the numerical model developed in the paper. This model will be used to simulate the operation of the pipeline during start-up, load changes or shut-down of the boiler. [ABSTRACT FROM AUTHOR]

Copyright of Technical Transactions / Czasopismo Techniczne is the property of Sciendo and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Thick-wall boiler components limit maximum heating and cooling rates during start-up or shut-down of the boiler. First, the allowable heating rates of the critical pressure components of the boiler were determined, and the temperature of the fluid was determined. The rate of change of the wall temperature of the pressure element and the thermal stress on the inner surface are controlled online and compared with the permissible values. Boiler manufacturers designate thermal stresses on the inner surface of the pressure component on the edge of the hole based on the measurement of the wall temperature at two points located inside it. Due to the low accuracy of the method used by boiler manufacturers, a new stress-determination method has been proposed in this paper in which only the internal temperature measurement point is used to determine the stresses on the inner surface of the component. In the method proposed in the paper, first, the internal surface temperature is determined from the inverse heat conduction solution, and then the stresses are calculated. Numerous computational tests were performed for cylindrical and spherical elements. Thermal stresses on the inner surface were also determined based on actual measurement data. Thermal stresses can be monitored at small time intervals. The advantage of the method is its high accuracy even at rapid changes in the fluid temperature. [ABSTRACT FROM AUTHOR]

The paper presents a new method of determining optimum temperature variations of the fluid during heating and cooling of cylindrical thick-walled components weakened by holes. The temperature of the fluid located or flowing through the pressure component is equal to the saturation temperature. The stress concentration factors at the edges of the holes were determined using the Finite Element Method (FEM) to represent the real construction of the junction between the element and the spigot. Optimum fluid temperature variations were determined so that the maximum equivalent stress at the edge of the opening is equal to the permissible stress determined concerning thermal fatigue. The equivalent stress at the hole edge at the point of its concentration includes thermal stress and stress due to pressure. Optimum temperature changes of hot water and steam-water mixture were determined during heating up and cooling down of the boiler drum, i.e., during boiler start-up and shutdown. Three-dimensional computations of the transient field of temperature and stress were carried out for the junction of the drum and downcomer to show that the equivalent stress at the edge of the hole does not exceed the allowable stress if the optimum temperature of the fluid was estimated by the proposed method. • A new method of determining optimum fluid temperature variations was developed. • The circumferential stress at the hole edge should not exceed the allowable stress. • Optimum fluid temperature during heating and cooling of the drum were determined. • The start-up and shutdown time of a steam drum boiler can be greatly reduced. • The flexibility of a power unit can be improved by using the prosed method. [ABSTRACT FROM AUTHOR]

Abstract Two methods for monitoring the thermal stresses in pressure components of thermal power plants are presented. In the first method, the transient temperature distribution in the pressure component is determined by measuring the transient wall temperature at several points located on the outer insulated surface of the component. The transient temperature distribution in the pressure component, including the temperature of the inner surface is determined from the solution of the inverse heat conduction problem (IHCP). In the first method, there is no need to know the temperature of the fluid and the heat transfer coefficient. In the second method, thermal stresses in a pressure component with a complicated shape are computed using the finite element method (FEM) based on experimentally estimated fluid temperature and known heat transfer coefficient. A new thermometer with good dynamic properties can be used for providing a much more accurate temperature of the flowing fluid in comparison with standard thermometers. A numerical-experimental method of determination of the transient heat transfer coefficient based on the solution of the 3D-inverse heat conduction problem was proposed. The heat transfer coefficient on the internal surface of a pressure element can be estimated based on an experimentally determined local transient temperature distribution on the external surface of the element or on the basis of wall temperature measurement at six points located near the internal surface if fluid temperature changes are fast. Examples of determining thermal and pressure stresses in the thick-walled horizontal steam header with the use of real measurement data are presented. Highlights • Two methods for monitoring the thermal stresses in pressure components are presented. • A new thermometer with good dynamic properties are used. • Examples of determining thermal and pressure stresses are presented. [ABSTRACT FROM AUTHOR]

Thermal steam units cooperating in one power system with wind and photovoltaic farms must have high flexibility, i.e., operate over a wide range of load variations, typically between 40 and 100%. In addition, the start-up and shutdown time of the unit should be short. Thick-walled pressure components that limit the fast start-up of the units are turbines, steam boilers, and pipeline branches due to the occurrence of high thermal stresses in them. This paper determines the allowable heating rates of thick-walled components of a 97.22 kg/s natural circulation steam boiler. The boiler drum is the critical element, with a heating time of about 6000 s from the cold state. Computer simulations of the boiler combustion chamber were also performed at three different loads, i.e., 100, 60 and 40%. The calculations show that the maximum emissions of harmful substances do not exceed the permissible values given in European regulations: CO ≤ 140 mg / m n 3 and NO x ≤ 100 mg / m n 3. The maximum content of CO = 136.8 mg / m n 3 occurs at 60% boiler load. The maximum content of NO x = 217.3 mg / m n 3 in the exhaust gas upstream of the SCR (Selective Catalytic Reduction) system occurs at 40% load, but downstream of the SCR system is reduced below the limit value 100 mg / m n 3. • Measures to improve the flexibility of a steam boiler were investigated. • Allowable heating rates for boiler pressure components were determined. • The boiler drum is a critical component for boiler start-up time. • CFD simulations of the boiler combustion chamber for various loads were conducted. • Pollutant emissions are acceptable with boiler loads from 100% to 40%. [ABSTRACT FROM AUTHOR]