Steam Purity Considerations in Geothermal Power Generation

Abstract

Steam purity refers to the amount of contaminants (solids, liquids or gases) that are present in steam, as opposed to steam quality which is a measure of the amount of moisture in steam. Steam purity is determined by a combination of factors including the chemistry of the produced geothermal fluid, the separation pressure(s), and mechanical carry-over rate of the separation process and the effectiveness of passive and active steam washing processes. Steam purity can be a significant factor in maintaining plant reliability, availability and efficiency in geothermal power plants. Poor steam purity can result in steam turbine and steam handling equipment damage from both erosion and a variety of corrosion mechanisms which can lead to unexpected plant outages and expensive repairs. Accurate monitoring of steam purity is essential to understand the risk and mitigation of steam path damage due to poor steam purity in a particular plant. Determining steam purity in geothermal steam plants can be a challenging endeavour. Saturated steam, low pressures and the presence of gases such as carbon dioxide and hydrogen sulphide make representative sampling and analysis of steam samples less than straight forward. These challenges must be overcome regardless of whether sampling is undertaken by manual grab sampling and laboratory analysis or continuous online monitoring. Where steam purity is poor, options are available to improve steam purity such as steam washing systems and secondary moisture separation. This paper presents some examples of damage that can result from poor steam purity in geothermal power plants, while also discussing options to both monitor and improve geothermal steam purity.