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Title: Materials selection, stress analysis and CFD modelling of flare tips
Author: Abolghasemi, Sobhan
ISNI:       0000 0004 2716 7556
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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Oil and gas platforms, refineries and chemical plants need to burn off the excess gas resulting from pressure variations during production. The failure of flare tips, sometimes with short lifetimes, has been a major cause for concern in the oil and gas industry for many years. The aim of this study was to evaluate and improve the performance of flare tips. The study has been approached from two perspectives: (i) material requirements, identifying the most suitable alloys for use in flare tips, and (ii) design optimisation, aimed at the development of a flare tip that minimises interaction with flame, therefore giving lower operating temperatures and longer lifetimes. The thesis also includes an Infra-Red (IR) thermal imaging study to establish flare tip temperature profiles during flaring. Examination of failed flare tips has provided evidence of intergranular oxidation and stress corrosion cracking as possible failure mechanisms. A study of the effect of thermal shock on the oxidation resistance of alloys 800H and 625, currently used in flare tips, is presented. The embrittlement of alloy 625 in the range 650 °C to 800 °C has also been investigated. Thermal imaging of three flares in operation has indicated metal temperatures of up to 1000 °C, above levels that can be sustained by alloys currently in use. A Finite Element model of stress distributions based on the temperature profiles has been developed. It was concluded that flare tip lifetimes would be limited by a combination of creep and fatigue of the support brackets, and by plastic deformation at the top of the windshield. The model successfully predicted the failure of two flare tips and lead to a timely replacement, resulting in significant financial savings and the prevention of catastrophic failure. Commercial Computational Fluid Dynamics software, that solves the Navier-Stokes equation combined with a combustion model, has been used to assess the effect of gas flow rates and wind conditions on combustion behaviour and the resulting operating temperatures of flare tips. The model has been validated with data obtained from thermal imaging studies and shows reasonable agreement, especially at low gas flow rates. As a result, a procedure has been developed to calculate flare tip temperature profiles (via CFD) and mechanical integrity (via FE stress analysis) of flare tips, and thus assess suitability of any flare tip design prior to manufacture and installation.
Supervisor: Lee, Peter ; Lindley, Trevor Sponsor: EPSRC ; DTA ; Shell
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral