Design and Successful Operation of a New pH Measurement System for Geothermal Brine

Abstract

New generation geothermal flash plants generally make use of pH-modification through acid addition to the geothermal brine thereby allowing extraction of energy beyond the brine’s silica solubility. The accurate and reliable measurement of the brine pH is important as this is used as a part of the control logic for the acid injection. If too much acid is injected in the geothermal brine this increases risks of corrosion in the brine handling system, whilst if too little acid is injected this heightens the risk of silica polymerization and subsequent deposition either within the injection pipe, injection well or the reservoir formation. Therefore the pH measurement system must provide accurate and reliable pH readings, have a high availability factor and a fast response time.

The previous system installed at one of Mighty River Power’s plants resulted in slow response times and provided pH values which at times caused control issues for the acid dosing system. New pH meters were procured, with the ability to make use of ion-sensitive field-effect transistors (ISFET) rather than only glass electrodes. Glass electrodes were found to be prone to deposition from species within the cooled geothermal brines resulting in a slowed response time and drift. Significant design work was conducted to ensure sample conditioning was improved, alongside control and instrumentation work to allow the power plant to communicate more effectively with the pH meters allowing provision of reliable high-availability brine pH values.

Two years of successful operation of the new pH monitoring system has now been achieved. During this time minor refinement of the system design has been made to improve the availability factor. Key learnings are discussed in this paper.