Yukio Suginohara

The properties of scales and methods to prevent them

Abstract The productive wells of Otake geothermal power plant together produce a total of 400 t/h of hot water mixture. This mixture is transported to the water reservoir of a hydro power plant through an underground pipe line, in order to avoid thermal and chemical pollution of the Kusu river. The rate of deposition was so high that the pipe line was blocked by the deposit, which consists of siliceous matter. The output of the power plant was thus reduced. The chief ingredient of the scale at well No. 6 was calcium carbonate, and so three sectional valves were installed for chemical cleaning of the pipeline, but unfortunately the scales from wells No. 7 to No. 10 were siliceous deposits. Various chemical and mechanical methods of scale removal were tried, but without success. The power production at Otake was restricted from the summer of 1967. Finally the open duct with a cross sectional area of 1.1 m 2 was installed in the middle of September of 1969. Therefore, Otake is now generating 12 MW. To prevent the adhesion of scales to hot water pipeline, physical and chemical analyses of scales (siliceous deposits) and hot water were conducted, and at the same time, there was an attempt to widen the knowledge about scale deposition in hot water under various conditions, using test equipment. After examining the results collectively, the authors have discovered that the deposit of scales is fastest immediately after the spout of hot water, and slows down as time passes. As a preventive measure utilizing this result, a means was invented by which hot water was retained for about an hour and then flowed into the pipeline: for the purpose of this experiment a retaining tank was built. Furthermore, by agitating the hot water flowing into the tank, the effect could be improved. Based on these results, a practical plant was erected to retain all of Otakes hot water for an hour, and satisfactory results have been obtained since its use in September, 1969.