Fatigue testing of boiler steels under an asymmetric stress range

Proceedings of the Institution of Mechanical Engineers, Part C:

2012

Steam fraction in riser tubes of boilers is a critical process variable which impacts the life of the tubes and could lead to tube rupture, long boiler down time, and expensive repairs. Unfortunately this parameter is difficult to measure by hardware sensors. This article presents a new neural network softsensor for estimation and monitoring steam mass and volume fractions in riser tubes. First, conventional data were collected from a target industrial boiler. The data are then used to develop a detailed nonlinear simulation model for the two phase flow in the riser tubes and risers and downcomers water circulation. The model output is verified against the collected field data. Next, the boiler nonlinear model is used to generate data covering a wide rage of operating conditions for training and testing the neural network. The input of the neural network includes the heating power, the steam flow rate, the water feed rate, the drum level, and the drum pressure. The neural networks predict the mass steam quality and the steam volume fractions. The softsensor achieves a root mean square error on the test data less than 1.5%. The predicted steam quality is then compared with the critical limits to guide the operators for safe and healthy operation of the boilers. The developed softsensor for estimation of the steam quality has simple structure and can be implemented easily at the operator stations or the application servers.

Section: Thermal Stresses Problemmentioning

confidence: 99%

Softsensor for estimation of steam quality in riser tubes of boilers

Elshafei

Proceedings of the Institution of Mechanical Engineers, Part C:

2012

“…The latter is caused by repeated thermal expansions and contractions of the boiler metal. The impact type ''shock failure" is usually preceded by fatigue of the metal but it has been observed to happen in short-term overheating, Wolf and Neill [6], and Kudryavtsev et al [7]. Boiler startup is discussed in Kruger et al [8].…”

Section: Introductionmentioning

confidence: 97%

Prediction of risers’ tubes temperature in water tube boilers

Emara-Shabaik

Applied Mathematical Modelling

Al-Zaharna

2009

A dynamic model is developed which enables the prediction of risers' tubes temperature of water tube boilers under various operating conditions. The model is composed of fluid dynamics model representing the fluid flow in the drumdowncomer-riser loop and a dynamic thermal model of the riser's temperature. The model gives a detailed account of the two-phase heat transfer process which takes place between the risers' inner walls and the water-steam mixture flow inside the tubes. The model is used to simulate various operational scenarios of water tube boilers. Results of the simulation provide insight into the dynamic interactions of the boiler's main variables including the drum pressure, water volume, steam quality and risers' temperature. Such a model is useful in checking operational scenarios before their actual plant implementation, can be a basis for developing boiler start up procedures and online temperature predictions to prevent eminent tube overheating.

Influence of Heat Flux and Friction Coefficient on Thermal Stresses in Risers of Drum Boilers under Dynamic Conditions of Steam Demand

Advances in Mechanical Engineering

Al-Zaharnah

Elshafei

et al. 2013

Boiler swing rate, which is the rate at which the boiler load is changed, has significant influence on the parameters of the boiler operating conditions such as drum water pressure and level, steam quality in the riser tubes, wall temperatures of riser tubes, and the associated thermal stresses. In this paper, the thermal stresses developed in boiler tubes due to elevated rates of heat transfer and friction are presented versus thermal stresses developed in tubes operated under normal conditions. The differential equations comprising the nonlinear model and governing the flow inside the boiler tubes were formulated to study different operational scenarios in terms of resulting dynamic response of critical variables. The experimental results and field data were obtained to validate the present nonlinear dynamic model. The calculations of the heat flux and the allowable steam quality were used to determine the maximum boiler swing rates at different conditions of riser tube of friction factor and heat flux. Diagrams for the influence of friction factor of the boiler tubes and the heat flux, that the tube is subjected to, on the maximum swing rate were examined.