Mechanical Engineering

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Browsing Mechanical Engineering by Author "Alugongo, Alfayo, Prof."

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    Identification and analysis of steam temperature maldistribution in superheater tubes via measured and derived parameters
    (Vaal University of Technology, 2019-08) Vilakazi, Lethukuthula Nokwazi; Rousseau, Pieter, Prof.; Alugongo, Alfayo, Prof.
    Superheater and reheater heat exchangers in power plant boilers can experience temperature excursions and gradients significantly above design values due to cyclic operations. This may result in accelerated life consumption of these components. To understand better the influence of different operating conditions, research is ongoing to develop detailed thermo-fluid process models of the various boiler heat exchangers, and real-plant data are required in the validation of these models. In this study, the final superheater of a 620 MW coal-fired power plant unit was analysed based on real plant measurements taken during steady state operation at 100, 80 and 65 percent of the current boiler capacity. Process parameters routinely measured via the plant distributed control system (DCS), such as the steam temperatures, pressures and mass flow rates, were used as input data to derive other unmeasured parameters using the mass and energy balance (MEB) methodology. Thermocouples were installed previously on the inlet and outlet final superheater stub boxes as well as the outlet manifolds. Thermocouple data were collected from a data logger at the corresponding dates and times of the DCS MEB inputs. Measurement uncertainties were determined by considering instrument and statistical uncertainties, which were then propagated through the MEB calculation to the derived parameters. The MEB methodology was applied to determine the flue gas temperature and flow rates at different operating loads (65, 80 and 100 percent). The good comparison obtained between the values calculated with the MEB and those of the C-schedule for the 100 percent boiler maximum continuous rating (BMCR) provided confidence in the validity of the MEB. The MEB was also compared to real plant data of flue gas temperature. The comparison provided a difference that is less than 26℃. Identification of the measurement uncertainties provided a detailed analysis on each instrument and or measurement and how certain I could be about each measurement. Uncertainties of parameters derived using the MEB methodology were determined. This was achieved by uncertainty propagation through the MEB model. Uncertainty propagation also provided a sensitivity percentage relative to the propagated uncertainties. The extent of temperature maldistribution was determined based on the measured outside tube metal temperatures. The results from the thermocouple measurements on the steam pipes connected to the final super heater inlet and outlet manifold headers show that there is temperature maldistribution between the inlet headers of the four legs. There is also significant maldistribution at the outlet headers resulting in noticeable local temperature gradients. It can also be concluded that the low load of 65 percent resulted in the highest temperature maldistribution compared to the higher loads, of 100 and 80 percent. Super heater tube metal temperatures are exposed to high temperatures at low loads which may lead to tube leaks.
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    Structural performance of KAT-7's ball screw raceway in the elastic region subjected to varying loads
    (Vaal University of Technology, 2017-12) Steenekamp, Nicolaas Kruger; Masu, Leonard, Prof.; Alugongo, Alfayo, Prof.
    The structural performance of a KAT-7’s ball screw raceway is not known. No performance data has been generated analytically, numerically nor experimentally. For this study, data was generated in the elastic region of the material. This research was undertaken in three different ways namely analytically, numerically and experimentally. A calibrated load cell was used to validate the analytical solutions. Solid Edge, a parametric software package was used to validate analytically the accumulated rain water mass and structural mass. Abaqus, a finite element analysis software package, was used to model and obtain the ball bearing Hertzian contact stresses numerically. The numerical solution was used to validate the laboratory compression test results on a replica KAT-7 ball screw assembly. The weighted percentage errors between the analytical model data and load cell data were found to be higher for load case scenarios with zero m/s and 10 m/s wind speeds respectively. The parabolic reflector rigid body assumption, exclusion of wind induced hysteresis effects and the quasi-static wind loading site measurement conditions contributed to the weighted percentage error variations. The laboratory and numerical model compression force results revealed a gradual percentage error increase beyond a compression force of 261288 N and up to 572526 N. The percentage error increase had minimum and maximum errors ranging between 6.24 percent and 14.69 percent respectively. The percentage error increase in the numerical model was due to the singular representation of a ball bearing instead of a 212 ball bearings set as experimentally conducted in the laboratory compression test on a replica KAT-7 ball screw assembly. The maximum axial force, F0 , result for load case scenario five was -94469 N with a Hertzian contact stress of 3939 MPa on the raceway surface. The static load rating required to Brinell a deep groove ball bearing raceway was found to be a Hertzian contact stress ranging between 4500 MPa and 4800 MPa. It was evident that the contact stresses incurred under the three considered loads of accumulated rain water, wind loading and structural mass were unable to exceed the 4800 MPa Hertzian contact stress. It was found that a replica KAT-7 ball screw raceway Brinelled under an axial force of 408126 N. The numerical ball screw raceway model Brinelled under an axial force of 380457 N. The Hertzian contact stress at the numerical ball screw raceway surface was determined to be 4898 MPa. Therefore, the Replica KAT-7 ball screw raceway material behaves elastically under an approximate load of up to 38 tonnes.