Browsing by Author "Wu, S."

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    Improving water asset management when data are sparse
    (Cranfield University, 2013) Dlamini, Delly; Pollard, Simon; Wu, S.
    Ensuring the high of assets in water utilities is critically important and requires continuous improvement. This is due to the need to minimise risk of harm to human health and the environment from contaminated drinking water. Continuous improvement and innovation in water asset management are therefore, necessary and are driven by (i) increased regulatory requirements on serviceability; (ii) high maintenance costs, (iii) higher customer expectations, and (iv) enhanced environmental and health/safety requirements. High quality data on asset failures, maintenance, and operations are key requirements for developing reliability models. However, a literature search revealed that, in practice, there is sometimes limited data in water utilities - particularly for over-ground assets. Perhaps surprisingly, there is often a mismatch between the ambitions of sophisticated reliability tools and the availability of asset data water utilities are able to draw upon to implement them in practice. This research provides models to support decision-making in water utility asset management when there is limited data. Three approaches for assessing asset condition, maintenance effectiveness and selecting maintenance regimes for specific asset groups were developed. Expert elicitation was used to test and apply the developed decision-support tools. A major regional water utility in England was used as a case study to investigate and test the developed approaches. The new approach achieved improved precision in asset condition assessment (Figure 3–3a) - supporting the requirements of the UK Capital Maintenance Planning Common Framework. Critically, the thesis demonstrated that, on occasion, assets were sometimes misallocated by more than 50% between condition grades when using current approaches. Expert opinions were also sought for assessing maintenance effectiveness, and a new approach was tested with over-ground assets. The new approach’s value was demonstrated by the capability to account for finer measurements (as low as 10%) of maintenance effectiveness (Table 4-4). An asset maintenance regime selection approach was developed to support decision-making when data are sparse. The value of the approach is its versatility in selecting different regimes for different asset groups, and specifically accounting for the assets unique performance variables.
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    Modelling deposition environments around superheater tubes
    (Cranfield University, 2011-11) Davies, Martyn W.; Simms, Nigel J.; Wu, S.
    In an important initiative to reduce carbon dioxide emission from pulverized fuel boilers, coal is now co-fired with biomass. During the combustion process, however, chlorine and sulphur in chemical compounds associated with sodium and potassium are released in a form which can deposit onto and then corrode the steel heat exchanger tubes. The deposition and corrosion can have serious implications for the power generation industry because the corrosive damage on heat exchange tubes can shorten the operational life of the boilers and lead to significant economic penalties. The deposition and corrosion processes have been widely studied but eliminating the deposits and corrosive materials is still a challenging problem. Computational fluid dynamics (CFD) has been used to model the deposition processes and it mainly focuses on experimentally deriving constants in the models that capture some aspects of the problem such as the motion behaviour of aerosols. However, using CFD has a limitation that results in periodic instability when solving the models with numerical computation. Modelling deposition environments around superheater tubes is a complex problem as many aspects, such as particle motion, condensation of matter and continuous combustion of particles, should be considered.This thesis has: (1) developed a new mathematical approach that uses mesh-free methods to solve Hamilton’s equations with a consideration of the total energy of the system, where the Hamilton’s equation is scale independent; (2) developed a model that can simulate the mass accumulation process based on graph and combinatorics theory; (3) developed a model depicting the continuous combustion of particles in motion; (4) developed a model depicting the behaviour of changing matter states; (5) developed a model depicting the vapour phase deposition on particles; and (6) verified the developed models with case studies. This work shows the importance of homogenous and heterogeneous vapour depositions on binding particles onto superheater tubes.
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    Prediciton of the remaining service life of superheater and reheater tubes in coal-biomass fired power plants
    (Cranfield University, 2013-04) Asgaryan, Mohammad; Simms, Nigel J.; Wu, S.
    As a result of concern about the effects of CO2 emssions on the global warming, there is increasing pressure to reduce such emissions from power generation systems. The use of biomass co-firing with coal in conventional pulverised fuel power plants has provided the most immediate route to introduce a class of fuel that is regarded as both sustainable and carbon neutral as it produces less net CO2 emissions. In the future it is anticipated that increased levels of biomass will be required to use in such systems to accomplish the desired CO2 emissions targets. The use of biomass, however, is believed to result in severe fireside corrosion of superheater and reheater tubing and cause unexpected early failures of tubes, which can lead to significant economic penalties. Moreover, future pulverised fuel power systems will need to use much higher steam temeptures and pressures to increase the boiler efficiency. Higher operating temperatures and pressures will also increase the risk of fireside corrosion damage to the boiler tubing and lead to shorter component life. Predicting the remaining service life of superheater and reheater tubes in coal-biomass fired power plants is therefore an important aspect of managing such power plants. The path to this type of failure of heat exchangers involves five processes: combustion, deposition, fireside corrosion, steam-side oxidation, and creep. Various models or partial models each of these processes are available from existing research, but to fully understand the impact of new fuel mixtures (i.e. biomass and coal) and changing operating conditions on such failures, an integrated model of all of these processes is required. This work has produced an integrated set of models and so predicted the remaining service life of superheater/reheater tubes based on the three frameworks which have been developed by analysing those models used in depicting the five processes: one was conceptual and the other two were based on mathematical model. In addition, the outputs of the integrated mathematical models were compared with the laboratory generated data from Cranfield University as well as historical data from Central Electricity Research Laboratories. Furthermore, alternative models for each process were applied in the model and the results were compared with other models results as well as with the experimental data. Based on these comparisons and the availability of models constants the best models were chosen in the integrated model. Finally, a sensitivity analysis was performed to assess the effect of different model input values on the residual life superheater and reheater tubing. Mid-wall metal temperature of tubes was found to be the most important factor affecting the remaining service life of boiler tubing. Tubing wall thickness and outer diameter were another critical input in the model. Significant differences were observed between the residual life of thin-walled and thick-walled tubes.