CAUSTIC CLEANING OF GEOTHERMAL PREHEATER HEAT EXCHANGERS AT NGATAMARIKI POWER STATION

Abstract

The Ngatamariki power station uses OEC units to extract heat from geothermal fluid and drive pentane turbines for power generation. The preheater stage of heat exchangers at this plant experience scaling from antimony and arsenic sulphides as a function of both the temperature and pH of the mixed brine and condensate that supplies them. This type of scaling is well documented in the geothermal industry and caustic cleaning is standard practice at many plants. This paper describes a case study of a recent set of caustic cleans of the preheaters at Ngatamariki and the results from the cleaning.
During each unit’s cleaning operation a series of samples of the cleaning fluid were taken and analysed for dissolved sodium, antimony, arsenic, sulphide, total alkalinity and solids. Sodium concentration was used to calculated the total system volume and enable the total mass of each dissolved species to be determined. This data was analysed against the improvements in heat transfer in order to better understand cleaning efficacy.