UBC Theses and Dissertations
New design for pressure washer drums Fok, Sunny Chi Man
In the kraft pulp and paper industry, pressure washer drums are used to separate bleaching liquor and chemicals from pulp. These drums have experienced serious structural damage in terms of cracks in critical areas that have led to excessive production down time of the washers. In this thesis, a new mechanical design for these washers that eliminates problems in the original design and greatly extends the life of the equipment is presented. The first part of this thesis discusses finite element simulation and experimental testing of the current and the proposed new design whereas the second part of the thesis discusses the design optimization and fatigue life prediction of the new design using finite element sub-modeling technique. The final part of the thesis investigates special start-up procedures that minimize the thermal shock effect on the new design. The development of the new design starts by numerically simulating the existing washer drum design in an attempt to predict real life observations for such drums. This process involves experimental testing of a full scale drum in order to assess the load carrying capacity of various parts of the drum and to identify key boundary conditions for use in the finite element computer model. Various design modifications are then studied and numerically assessed. Shape optimization is then carried out for a specific area in the final design. A prototype was built based on the final model and experimental testing was performed in actual operating conditions. In order to numerically assess the fatigue life of the new design, the growth rate of a fatigue crack located at the critical area is investigated and presented in the second part of the thesis. The procedure involves putting a crack in the finite element model of the new design rather than using a standard or 'can-model'. The stress intensity factors are then calculated for the crack under actual loading conditions and with the actual geometry of the structure. Various crack lengths are considered and a plot of the variation of the stress intensity factor (SIF) with the crack length is obtained. Displacement extrapolation method is used to calculate the SIF at the crack tip. The fatigue life cycle of the structure is then estimated based on the calculated SIF and Paris law. Comparison of this detailed and more accurate procedure with the standard 'can model' procedure is given. In the final part of the thesis, a detailed transient heat transfer analysis is performed to predict the temperature distribution in the structure during start-up and shut-down stages. Based on the predicted temperature distribution and the corresponding thermal stresses, a start up procedure is proposed with the objective of minimizing both the impact of thermal shock as well as the time required for start up.
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