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

Abstract

A pressure let down station is a set of equipment that allows a geothermal system to change the operating pressure of the steam gathering system to a higher point and still maintain the power plant inlet pressure at the designed point. In 2012, KenGen began construction of two geothermal power plants; 140MWe Olkaria IV and 2 additional units 4 & 5 (140MWe Olkaria IAU) thereby increasing power generation in Kenya by 280MWe. The two power plants have turbine inlet pressures of 6.0bara and 5.0bara for Olkaria IV and Olkaria IAU respectively. This meant the steam gathering systems would also operate at that pressure. While construction was in progress, a new optimization study reported that the deeper wells in Domes, East and North East fields which serve the power plants with steam would tap fluids from reservoirs that are 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 (Mannvit, 2012) This concern resulted in KenGen incorporating Pressure Let down Stations 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 values. It was also not feasible to incorporate a topping plant because the project was already at an advanced stage and most of the equipment was already designed and even manufacture started for some. Venting was ruled out since it was not good practice bearing in mind wastage of steam. 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.