Liquid Carryover in Geothermal Steam-Water Separators

Abstract

Geothermal steam power plant uses saturated steam to run the turbines. Therefore, it is important to ensure that the steam is high in quality and purity to keep turbines and other equipment working properly. Mineral deposition and moisture damage have been reported in many geothermal turbines around the world, affecting the turbine efficiency and leading to generation losses. Current design models for calculating the separator efficiency reports about 99.995% separation efficiency, though it is often found that scaling and moisture damage continue to take place in newly constructed steam power plants. Recent studies have noted the potential for volatile silica to mainly contribute to this damage for cases of high separation pressures. However, this is not the view of the authors. This work investigates water entrainment due to the formation of a liquid film on the walls of the cyclone separator, resulting in significant amount of entrained liquid droplets in the steam. A new model was proposed for addressing the liquid film entrainment. The model shows that the actual efficiency of separator is expected to be lower than the theoretical/calculated efficiency. Liquid carryover analysis was presented to give better idea on the factors controlling liquid film carryover in vertical-cyclone separators. Liquid film entrainment modeling using field data from the Wairakei geothermal field was reported. The results show that the rate of entrainment increase as the liquid loading fraction increases. It also reveals the impact of liquid film thickness to the rate of entrainment since higher liquid loading leads to thicker liquid film. In addition, inlet velocity is important in determining the rate of liquid entrainment. As the inlet velocity rises, the entrainment rate increase causing more water carryover.