dc.description.abstract |
Globally, geothermal energy utilization for power generation has been ongoing for
decades through conventional technologies. In 2012, Well-head technology was first
utilized in Kenya’s Olkaria geothermal field. However, severe cases of
erosion/corrosion and deposition/scaling were observed within the turbine set after
hardly five years of operation. These defects greatly impacted on the plant’s
performance by reducing its energy conversion efficiency and output. The main
objective of this research was to investigate the factors affecting the performance of the
turbines at Olkaria’s well-head power plants. Its specific objectives were to determine
the composition of geothermal fluid, characterize the solid deposits in comparison with
the turbine blade material, determine the causes of observed defects, determine the
methods of eliminating the defects and to enhance performance of the turbines by
implementing selected methods of eliminating the defects at Olkaria’s well-head power
plants. Analyses of geothermal fluids at various sections of the plant were done using
the potentiometric, AAS and spectrophotometric techniques to determine its
composition. Analyses of solid deposits were done using XRD and XRF techniques.
Blade sample analysis was done using Metal scan spectroscopy and XRF techniques.
Literature review guided an inferential approach used to determine the causes of the
defects. Manufacturing & maintenance standards were used to determine methods of
eliminating the defects. The characteristics of geothermal fluid entering the turbine
were found to be; pH (4.55), TDS (12.40 ppt), Conductivity (24.78 μs/cm), chloride
ions (6.59 ppm), Sulphate ions (6.27 ppm), silica (2.71 ppm), Iron (1.56 mg/L) and
Sodium (1.03 mg/L). The solid deposits on the turbine consisted of silica in form of
SiO 2 (66.20%) iron, Fe (13.78%), K 2 O (9.08%), Sulphur (3.40%), chlorides (2.48%),
P 2 O 5 (1.69%) , Calcium (1.23%), Barium (0.7%), Manganese (0.62%), titanium
(0.39%), and Chromium (0.19%).The turbine blade material was characterized as an
alloy steel with the highest composition being iron (Fe 82.64%), and an average
chromium content of 12.50%. The pH value, chloride and sulphate ions in the fluid
signify acidity and its highly corrosive nature. Significant amounts of oxides in deposits
indicate oxidation reactions as the fluid interacts with the metals at elevated
temperatures leading to deposition/scaling. As observed, the turbine blade was highly
affected by corrosion. This was attributed to the parent material, 12.5%Cr steel alloy,
having low resistance to corrosion under the operating conditions of the turbine. In
conclusion, the results attributed the root cause of the defects to steam quality and blade
material resistance to corrosion. Based on this conclusion, hard facing and machining
techniques were determined as suitable methods of eliminating the defects. These
methods were implemented to refurbish an affected turbine and in the process, a
material composed of 23.9%Cr, 13.0% Ni, 1.8%Mn and 0.15%Mo (AWS
A5.9:ER309L) was selected as a suitable overlay material for the repair in view of its
fusion characteristics, toughness, tensile and creep strengths. The refurbishment
resulted in improving the turbine’s output from 2.75 MW to its design rated output of
3.20 MW. Considering the same quantity of steam being consumed, the energy
conversion efficiency was increased by 16.4%; hence, the turbine performance was
enhanced. However, further research was recommended to investigate the impacts of
hard facing and machining techniques on the life span of the turbine. |
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