Upheaval buckling of pipelines

UPHEAVAL BUCKLING OF OFFSHORE pipelines occurs as a result of axial compression induced along the pipelines due to large temperature differences and high internal pressures. This paper aims to research the causes of upheaval buckling, give an overview of the analytical methods, and develops an Excel spreadsheet for initial assessment. Several models of upheaval buckling have been identified and discussed, such as those based on idealized or perfect pipelines, which are related to the railway track analysis and those based on imperfections. The buckle temperatures of the perfect pipelines are proportional to the buckle lengths and axial forces. With the consideration of imperfections, buckle temperatures become inversely proportional to the imperfection heights, therefore larger imperfections would require smaller temperatures to propagate upheaval buckling. Increasing the downward load on the pipelines aids the prevention of upheaval buckling. Also, relevant methods to mitigate against the occurrence of upheaval buckling have been discussed. The use of finite-element analysis which considers the seabed profile and plastic deformation of pipe wall would be suitable for precise analysis

Assessment of Coating Performance on Waterwalls and Superheaters in a Pulverised Fuel-Fired Power Station

Protective coatings offer one route to increase the lives of heat exchangers in pulverised fuel power plants. A range of candidate coatings have been exposed on the waterwall and superheaters of a 500 MWe UK power station unit for periods of up to ~4 years (24,880 operating hours), during which time this unit was fired on a mixture of UK and world-traded coals. Both nickel- and iron-based candidate coatings were included, applied using high velocity oxy-fuel or arc-wire process; a selection of these also had a surface sealant applied to investigate its effectiveness. Dimensional metrology was used to evaluate coating performances, with SEM/EDX examinations used to investigate the various degradation mechanisms found. Both the waterwall and superheater environments generated their characteristic corrosion damage morphologies which depended on the radial positions around the tube. Coating performances were found to depend on the initial coating quality rather than composition, and were not improved by the use of a sealant

Impact specimen geometry on T23 and TP347HFG steels behaviour during steam oxidation at harsh conditions

Ferritic T23 steel and austenitic TP347HFG steel have been studied with an emphasis on understanding the impact of specimen geometry on their steam oxidation behaviour. The selected materials were tested over a wide range of temperatures from 600 to 750°C. The tests were carried out in 100% steam conditions for 1000 hours. The tests indicated that the ‘curved-shaped’ specimens show slower mass gain, scale ticking and void nucleation rates than ‘bridge-shaped’ specimens (with flat and convex surfaces combined). Furthermore, a bridge TP347HFG sample showed the formation of lower amount of flaky oxide at 750°C.We would like to acknowledge the support of The Energy Programme, which is a Research Councils UK cross council initiative led by EPSRC and contributed to by ESRC, NERC, BBSRC and STFC, and specifically the Supergen initiative (Grants GRyS86334y01 and EPyF029748) and the following companies; Alstom Power Ltd., Doosan Babcock, E.ON, National Physical Laboratory, Praxair Surface Technologies Ltd, QinetiQ, Rolls-Royce plc, RWE npower, Siemens Industrial Turbomachinery Ltd. and Tata Steel, for their valuable contributions to the project

Analysis of high temperature steam oxidation of superheater steels used in coal fired boilers

The present work compares the behaviour of four steels: (T23, T92, T347HFG, Super304H) in the temperature range 600–750 °C. This study focuses on the analysis of the oxidation kinetics in terms of mass change, metal loss and thickness change of the selected materials. In order to understand the differences in oxidation rates between the selected steels, the impact of chromium and the alloying elements were considered in this work. The obtained results show that the impact of alloying elements differs with exposure conditions and importance of the synergy effect

Fireside corrosion degradation of ferritic alloys at 600°C in oxy-fired conditions

This paper reports the results of a study carried out to investigate the effects of simulated coal/biomass combustion conditions on the fireside corrosion. The 1000 h deposit recoat exposure (5 × 200 h cycles) was carried out at 600 °C. In these tests ferritic alloys were used 15Mo3, T22, T23 and T91. Kinetics data were generated for the alloys exposed using both traditional weight change methods and metal loss measurements. The highest rate of corrosion based on EDX results occurred under D1 deposit where provoke mainly by the formation of alkali iron tri-sulphate phase

Development of on-line FTIR spectroscopy for siloxane detection in biogas to enhance carbon contactor management

Activated carbon filters are used to limit engine damage by siloxanes when biogas is utilised to provide electricity. However, carbon filter siloxane removal performance is poorly understood as until recently, it had not been possible to measure siloxanes on-line. In this study, on-line Fourier Transform Infrared (FTIR) spectroscopy was developed to measure siloxane concentration in real biogas both upstream (86.1–157.5 mg m−3) and downstream (2.2–4.3 mg m−3) of activated carbon filters. The FTIR provided reasonable precision upstream of the carbon vessel with a root mean square error of 10% using partial least squares analysis. However, positive interference from volatile organic carbons was observed in downstream gas measurements limiting precision at the outlet to an RMSE of 1.5 mg m−3 (47.8%). Importantly, a limit of detection of 3.2 mg m−3 was identified which is below the recommended siloxane limit and evidences the applicability of on-line FTIR for this application

Simulating novel gas turbine conditions for materials assessment: cascade design and operation

Integrated gasification combined cycles can incorporate pre-combustion carbon capture. High-H2 syngas produces high H2O levels after combustion, potentially accelerating gas turbine component damage. Determining materials systems’ suitability for this novel environment requires exposures in representative environments. Thus, an existing 0.7 MW burner rig was modified to generate the combustion environment and incorporate a cascade of 15 air-cooled turbine blades. Computational fluid dynamic calculations using blade dimensions and flow requirements supported the cascade design and determined blade placement within the gas flow. Trials of the modified unit have shown that a simulated combusted H2-rich syngas composition was generated at gas temperatures ≤1440°C. A 1000 h exposure has been carried out with thermal barrier coated blades to demonstrate the operation of the unit

Fireside corrosion study of superheater materials in advanced power plants

Conventional power plants are major emitters of CO2 gases, which are believed to be contributing to global warming. An efficient, co-firing biomass-coal power plant with oxy-firing combustion system (running at high steam temperature and pressure), can play a vital role in CO2 emission reduction. However, these techniques will further worsen the issue of fireside corrosion of heat exchangers. An increase in fireside corrosion rates can cause short component lives and unexpected failures if not dealt with appropriately. The aim of this PhD study was to use laboratory-based testing to assess the performance of alloy materials under superheater conditions in simulated co-fired (biomass and coal) air and oxy-fired combustion. In this PhD project five different alloys were used. Synthetic deposits were also prepared to simulate superheater deposit compositions. Tests were carried out at temperatures appropriate for metal temperatures in superheaters/reheaters of future power plants. The performance of samples was determined using: mass change data, advanced microscopy techniques, x-ray diffraction and dimensional metrology. Additional tests were carried out to investigate deposit stability and the effect of high concentrations of salts. The results achieved have confirmed the hypothesis that increased fireside corrosion rates are due to the combined effect of extreme environment: high temperatures, SO2 and HCl gases, aggressive deposits. Corrosion damage follows trends that resembles ‘bell-shaped’ curve in both air and oxy-fired conditions. Alloy corrosion damage in novel oxy-firing compared to air-firing conditions was significantly higher at 700C. The peak of the curve shifts from 650 to 700C in oxy-fired conditions. The alloys with higher chromium content clearly showed better corrosion resistance. The work on deposit chemistry and exposure to high salt concentrations has improved the understanding of corrosion reaction mechanisms. Corrosion damage data have been used to produce basic fireside corrosion mathematical model; which can be used as a stepping stone towards further development of fireside corrosion models.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Co-firing fossil fuels and biomass : combustion, deposition and modelling

The application of advanced technologies employing combustion/co-firing of coal and biomass is seen as a promising approach to minimising the environmental impact and reducing CO2 emissions of heat/power production. The existing uncertainties in the combustion behaviour of such fuel mixes and the release of alkali metals with other elements during the combustion (or co-firing) of many bio-fuels are some of the main issues that are hindering its application. The potential presence of high levels of alkali chlorides and low levels of sulfates in the deposits formed on heat exchanger can cause enhanced corrosion and/or limit the heat transfer between the hot combustion gases and the water/steam system within the process plant. This work has investigated the detailed gas compositions and deposition characteristics of the combusted gas streams produced from fossil and biomass fuels pure and/or blend in a pilot-scale combustors (PF and FBC) at Cranfield University. Combustion gas analysis were obtained on-line by a high resolution multi-component Fourier Transform Infra-Red (FTIR) gas analyser and deposits samples were collected from the flue gas using air-cooled probes with surface temperatures of about 500, 600, 700 o C and analysed using SEM-EDX and XRD techniques. Fuels included several biomass fuels (cereal co-product (CCP) straw, miscanthus (pulverised), oil seed rape straw (against stored pellets), miscanthus (pellets), willow, fast pyrolysis bio-oil) and two commercially-used coals (El-cerrejon and Daw Mill). The results of the experimental studies have been compared with thermodynamic equilibrium predictions. High combustion efficiency was maintained throughout the range of fuel mixes. The SO2 and HCl levels were low in pure biomass combustion and increased as the biomass fraction of the fuel decreased when co-fired with these coals. However, the NOx output remained stable except for Miscanthus:Daw Mill mixtures and OSR stored pellet combustion. The deposition flux was highest on the coolest probes for each fuel. The lowest deposition fluxes were found for the combustion of either fast pyrolysis bio-oil or coppiced willow. There is evidence of significant differences deposition fluxes between El-cerrejon coal and Daw Mill coal mixed with CCP and/or miscanthus. The presence of chlorine was identified in deposits produced from combustion of pure biomass or high biomass mixes. The lowest levels found here in fast pyrolysis bio-oil combustion and only detected at higher shares (≥ 80 %) of biomass co-fired with Daw Mill coal, whereas, mixed biomass with El-cerrejon coal produced Cl in deposits at a low % biomass share. The application of thermodynamic equilibrium modelling has been found to be useful tool for providing a qualitative understanding of elements present and/or control by hot gas in modern combustion processes.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Effect of stress state and simultaneous hot corrosion on the crack propagation and fatigue life of single crystal superalloy CMSX-4

Operating conditions within industrial gas turbines are changing in response to pressures to reduce environmental impact and enable use of renewable sources. This is driving an increase in the operational temperatures and pressures of combustion in turbine systems. Additionally, diverse operating environments can result in higher sulphur and trace metal contaminant levels, exacerbating hot corrosion in GT systems. Low cycle fatigue (LCF) cycling can also be intensified as a result of increased start/stop shutdowns. The combined effects of hot corrosion and stress are experimentally studied on CMSX-4 single crystal (SC) γ/γ' system under both fatigue and static stress conditions, with either a multi-axial bending or uniaxial stress state. The associated stress intensity thresholds (KTH) under the various stress conditions were evaluated using finite element analysis (FEA). Cracking was observed both under static and fatigue stress conditions in a hot corrosion environment. Crack morphologies were analysed using SEM techniques. Bending stresses and fatigue cycles demonstrated increased crack propagation in the presence of hot corrosion with static uniaxial stresses showing the longest nucleation times and lowest propagation rates