Jan Taler

Analysis of Thermal Stresses in a Boiler Drum During Start-Up

The paper presents an analytical way of calculating thermal stress distributions in cylindrical vessels, nonuniformly heated on their circumference. In thick-walled vessel elements, simplified analytical formulas do not give satisfactory results. A new method for determining thermal stresses has been developed. On the basis of temperature history measurements at several points on the drum outer surface, a time-space temperature distribution in the component cross section is determined, and next, thermal stresses are calculated using the finite element method (FEM). The new method, proposed for the solution of the inverse heat conduction problem, is sufficiently accurate. Knowledge of the boundary conditions on the inner surface of the drum, i.e., fluid temperature and heat transfer coefficient, is not necessary because the transient temperature distribution in the component is obtained from the solution of the inverse heat conduction problem. Comparison of the thermal distributions from FEM versus the new method demonstrate the accuracy of the new method. An example application of the new method demonstrates its benefits over the solution of the boundary-initial problem obtained by FEM.

Simple method for monitoring transient thermal stresses in pipelines

ABSTRACT This paper presents a seminumerical method for solving inverse heat conduction problems (IHCP) encountered in the monitoring of thermal stresses in pressurized thick-walled elements of steam boilers. The objective is to give a simple and quick method of determining transient temperature histories in thick-walled components based on temperature measurements on the outer thermally insulated surface. The method is suitable for solving one-dimensional problems. However, it can be extended to multidimensional temperature fields. The IHCP will be solved using the control volume approach. The accuracy of the method is demonstrated by comparing computational and experimental results. Gram orthogonal polynomials are used to smooth the measured time-dependent temperature and for evaluating time derivatives of noisy data with high accuracy. Due to the simplicity of the final formulations, the developed method is very useful for estimating the thermal stresses and controlling the fatigue damage of boiler components.

Simplified Analysis of Radiation Heat Exchange in Boiler Superheaters

This article describes the determination of the radiation heat transfer coefficient in radiant platen superheaters and on convective heating surfaces. A new simple formula for determination of the heat transfer coefficient is derived on the basis of a diffusivity model of radiation heat exchange. The radiation heat transfer coefficients are determined on the tube surface in a convective evaporator, in a second stage convective heat superheater, and in a platen superheater of a pulverized coal-fired boiler. The calculations were carried out applying the method presented in this article, the Central Institute for Boilers and Turbines method, and formulas resulting from the analysis of heat exchange in an enclosure containing a gas of a constant temperature. In order to assess the accuracy of the achieved results, the flow of flue gas and the heat exchange were modeled using a commercial computational fluid dynamics program.

Mathematical Modeling of Cross-Flow Tube Heat Exchangers With a Complex Flow Arrangement

The general principles of mathematical modeling of heat transfer in cross-flow tube heat exchangers with complex flow arrangements that allow the simulation of multipass heat exchangers with many tube rows are presented. The finite-volume method is used to solve the system of differential equations for temperature of the both fluids and the tube wall with appropriate boundary conditions. A numerical model of a multipass steam superheater with 12 passes is presented. The convection and radiation heat transfer on the flue gas side are accounted for. In addition, the deposit layer is assumed to cover the outer surface of the tubes. Comparing the computed and measured steam temperature increase over the entire superheater allows for determining the thermal resistance of the deposits layer on the outer surface of the superheater. The developed modeling technique can especially be used for modeling tube heat exchangers when detailed information on the tube wall temperature distribution is needed.

Numerical Modeling of Cross-Flow Tube Heat Exchangers with Complex Flow Arrangements

Cross-flow tube heat exchangers find many practical applications. An example of such an exchanger is a steam superheater, where steam flows inside the tubes while heating flue gas flows across the tube bundles. The mathematical derivation of an expression for the mean temperature difference becomes quite complex for multi-pass cross-flow heat exchangers with many tube rows and complex flow arrangement (Hewitt et al., 1994; Kroger, 2004; Rayaprolu, 2009; Stultz et al., 1992 ; Taler, 2009a). When calculating the heat transfer rate, the usual procedure is to modify the simple counter-flow LMTD (Logarithmic Mean Temperature Difference) method by a correction FT determined for a particular arrangement. The heat flow rate Q

Tubular Type Heat Flux Meter for Monitoring Internal Scale Deposits in Large Steam Boilers

transferred from the hot to cool fluid is the product of the overall heat transfer coefficient UA, heat transfer area A, correction factor FT and logarithmic mean temperature difference ΔTlm. The heat transfer equation then takes the form:

Numerical method for the solution of non‐linear two‐dimensional inverse heat conduction problem using unstructured meshes

The deposition of scale on the inner surfaces of the water-wall tubes in the high heat flux regions in a steam boiler furnace can cause serious operation problems. In this paper, a numerical technique for determining the heat flux absorbed by the water-wall tubes, water-steam temperature, and thermal resistance on the inner tube surface from a temperature measured at several interior locations of the tube wall is developed. The scale deposition tube is capable of monitoring changes in the flow of heat transfer caused by scale depositions and changes due to varying furnace conditions. It can work for a long time in the destructive high-temperature atmosphere of a coal-fired boiler. The scale deposition monitor is an online plant monitoring system designed to improve the operation of steam boilers and enhance tube life.

Inverse Space Marching Method for Determining Temperature and Stress Distributions in Pressure Components

A new method of solving multidimensional heat conduction problems is formulated. The developed space marching method allows to determine quickly and exactly unsteady temperature distributions in the construction elements of irregular geometry. The method which is based on temperature measurements at the outer surface, is especially appropriate for determining transient temperature distribution in thick-wall pressure components. Two examples are included to demonstrate the capabilities of the new approach. Copyright

Slag Monitoring System for Combustion Chambers of Steam Boilers

Thermal stresses can limit the heating and cooling rates of temperature changes. The largest absolute value of thermal stresses appears at the inner surface. Direct measurements of these stresses are very difficult to take, since the inner surface is in contact with water or steam under high pressure. For that reason, thermal stresses are calculated in an indirect way based on measured temperatures at selected points, located on an outer thermally insulated surface of a pressure element. First, time-space temperature distribution in pressure element is determined using the inverse space marching method. High thermal stresses often occur in partially filled horizontal vessels. During operation under transient conditions, for example, during power plant start-up and shut-down, there are significant temperature differences over the circumference of the horizontal pressure vessels (Fetkoter et al., 2001; Rop, 2010). This phenomenon is caused by the different heat transfer coefficients in the water and steam spaces. This takes place in large steam generator drums, superheater headers and steam pipelines. High thermal stresses caused by nonuniform temperature distribution on vessel circumference also occur in emergency situations such as fire of partially filled fuel tanks. The upper part of the horizontal vessel is heated much faster than the lower part filled with liquid. Similar phenomenon occurs in inlet nozzles in PWR nuclear reactor, at which high temperature differences on the circumference of the feed water nozzles are observed. The study presents an analysis of transient temperature and stress distribution in a cylindrical pressure component during start-up of the steam boiler and shut-down operations. Thermal stresses are determined indirectly on the basis of measured temperature values at selected points on the outer surface of a pressure element. Having determined transient temperature distribution in the entire component, thermal stresses are determined using the finite element method. Measured pressure changes are used to calculate pressure caused stresses. The calculated temperature histories were compared with the experimental data at selected interior points. The presented method of thermal stress control was applied in a few large conventional power plants. It can also be used successfully in nuclear power plants. The developed method for monitoring thermal stresses and pressure-caused stresses is also suitable for

Ein numerisches Verfahren zur experimentellen Ermittlung des Wärmeübergangskoeffizienten in zylindrischen Bauteilen

The computer-based boiler performance system presented in this article has been developed to provide a direct and quantitative assessment of furnace and convective surface cleanliness. Temperature, pressure, and flow measurements and gas analysis data are used to perform heat transfer analysis in the boiler furnace and evaporator. Power boiler efficiency is calculated using an indirect method. The on-line calculation of the exit flue gas temperature in a combustion chamber allows for an on-line heat flow rate determination, which is transferred to the boiler evaporator. Based on the energy balance for the boiler evaporator, the superheated steam mass flow rate is calculated taking into the account water flow rate in attemperators. Comparing the calculated and the measured superheated steam mass flow rate, the effectiveness of the combustion chamber water walls is determined in an on-line mode. Soot-blower sequencing can be optimized based on actual cleaning requirements rather than on fixed time cycles contributing to lowering of the medium usage in soot blowers and increasing of the water-wall lifetime.