Modelling of the deaerator system in Flownex

Banda, Richard Bobby

Modelling of the deaerator system in Flownex

Master Thesis

2015

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

The study of the steady-state and dynamic behaviour of thermal power plants is of interest and significant benefit in different engineering fields ranging from research and design, to the assistance of operator training, plant optimization, fault finding and failure analysis. In light of the these benefits, and the increasing electrical energy demand in South Africa, the Eskom Power Plant Engineering Institute Centre for Energy Efficiency intends to build a transient simulation model of a coal fired power plant. The software identified for this task is Flownex SE. Flownex is a one-dimensional thermal-hydraulic solver that solves user defined networks by obtaining a numerical solution of the governing equations of fluid dynamics and heat transfer. The software contains a vast library of low level standard industrial components such as valves and pipes that can be linked together to form networks. Due to the overall size and complexity of the intended plant model, it was suggested that individual plant components be modelled separately and then integrated together to form the complete model. The primary objective of this study was to develop one such model, of a deaerator, in Flownex. In addition to being a building block for the complete plant model, the deaerator model will also be used as a standalone model to predict the steady state, transient and non-condensable gas extraction characteristics of the equipment. The first activity performed was to establish the types and operating principles of the deaerators used in industry, particularly in Eskom power stations. This was achieved through a literature survey complemented by six power station visits and a review of some assets owned by Eskom. It was established that the tray and spray type deaerators were the most commonly used deaerator types, and that their operating principle was based on the temperature-solubility relationship of gases in water and Henry's Law. Based on this knowledge, an analytical model of a deaerator was developed. The purpose of this analytical model was to serve as a verification tool for the final Flownex Model. The analytical model was developed by writing a Mathcad algorithm that solved the steady state one-dimensional mass and energy conservation equations around the deaerator boundary together with the oxygen component continuity equation. The model was successfully validated by comparing its predictions to acceptance tests data from an Eskom's Plant 1 power station. The final step was the development, verification and validation of the Flown ex model. The Flownex model was developed and successfully verified by comparing its predictions to that of the analytical model. Three case studies were then performed as a validation exercise in order to demonstrate the integrity of the model in simulating both steady state and transient scenarios. In all three studies the model predicted the unknown values satisfactorily and within acceptable error margins. It was therefore concluded that the primary objective of the study had been met.

Keywords

MEchanical Engineering

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