Serious pitting hazard in the raft river 5MW(e) Geothermal Power Plant isobutane cooling loop

摘要

The 5MW(e) Dual Boiling Cycle Geothermal Power Plant, hence referred to as the Raft River plant, is being developed for DOE by EG and G, Inc., Idaho Falls, Idaho. This pilot power plant is of the binary concept and utilizes isobutane as the working second fluid. The plant will demonstrate the feasibility of power generation from an intermediate temperature ({approx} 290 F) resource. The plant is schematically diagrammed in Figure 1. During the final design phase and after the major components were specified to be made of carbon steel, and ordered, various conditions forced the power plant design to switch from surface water to geothermal fluid for the condenser cooling loop make-up water. Because the geothermal fluid contains significant concentrations of chlorides and sulfates, about 1000 ppm and 65 ppm respectively, aeration in the cooling tower causes this water to become extremely aggressive, especially in the pitting of carbon steel components. Although essentially all of the condenser cooling loop materials are carbon steel, the isobutane condenser and turbine lube oil cooler are the most vulnerable. These components are tubed with carbon steel tubes of 0.085 and 0.075 inch wall thickness. These two components are extremely leak critical heat exchangers. For example, even a single pit perforation in the isobutane condenser can cause plant shutdown through loss of isobutane. Such a leak also poses an explosion or fire hazard. As isobutane pressure falls, the incursion of cooling water into the isobutane loop could occur, causing damage to anhydrous service seals. Under a DOE contract for geothermal failure analysis, Radian Corporation has made a preliminary investigation of the pitting hazard presented by the aggressive cooling fluid and the corrosion inhibition treatment that has thus far been proposed. This report documents Radian's understanding of the present situation and the results of its investigation on possible mitigation of this hazard. Finally, various conclusions and recommendations are made that may, if pursued, lead to a satisfactory solution that will avert a certain early prolonged plant shutdown due to failure of the thin walled isobutane and turbine lube oil cooler tubes.