Significance of Pressure Let Down Station In Mitigating Effects of Silica Precipitation In Steam Pipelines. A Case Study of Olkaria

In 2012, KenGen began construction of two geothermal power plants; 140MWe Olkaria I Additional Units 4 & 5 and 140MWe Olkaria IV thereby increasing power generation in Kenya by 280MWe. The two power plants have turbine inlet pressures of 6.0 bara and 5.0 bara in Olkaria IV and Olkaria IAU respectively. This meant the two distinct steam gathering systems would also operate at the respective pressures. While the project was in progress, a new optimization study by Manvit of Iceland reported that the deeper wells in Domes, East and North East field which serve the power plants with steam were drilled through reservoirs rich in silica. The issue was compounded by high enthalpies of the wells averaging 1790kj/kg to 2300kj/kg thereby increasing silica solubility and chances of precipitation in steam pipelines. The concern raised made KenGen to incorporate Pressure Let down Stations (PLDS) in the two power plants that would maintain the steamfield pressure at the maximum but within the constraints of the already selected equipment, and then expand the steam to drop its pressure to required turbine inlet pressure values. This paper will use the case study of these power plants to highlight the significance of the Pressure Let down Stations in scaling down effects of silica deposition in steam gathering systems.