Applying acoustic pulse reflectometry in a geothermal energy plant: A case study

One major type of heat exchanger in geothermal powerplants acts to condense an organic vapor driving the generating turbines. Routine inspection carried out at such a plant in the spring of 2008 detected several leaking tubes in one of the condensers. The condenser consists of 11 banks, each with 284 tubes. The tubes in this condenser are 1" in diameter and 18m in length. A sample of tubes was investigated by a metallurgical laboratory which determined that the failure mechanism was corrosion. This was eventually traced to a faulty maintenance procedure which left water traces after pressure hydro testing. Several NDT methods were considered in order to screen the tubes for leaks, so the unit could be brought up to standard performance levels. Eventually AcousticEye's APR equipment was selected, based on the very short cycle time it offered, which could dramatically shorten plant downtime. After an initial feasibility test of one of the tube banks, the remaining 2840 tubes were inspected in 2 days. Four percent of the tubes were found either leaking or deteriorated above a predefined maximum acceptable damage level (pinholes and tube wall thickness reduction) and plugged. Benefits were prevention of a continued loss of 70 Kg/day of organic fluid (which is not only wasteful but also infringes on environmental legislation) and an increase of 1.5% in plant performance. Based on this experience, the operator decided to adopt APR based inspection as a standard procedure in its plants.