The double-flash steam plant is an improvement on the single-flash design in that it can produce 15—25% more power output for the same geothermal fluid conditions. The plant is more complex, more costly and requires more maintenance, but the extra power output often justifies the installation of such plants. Double-flash plants are fairly numerous and are in operation in ten countries. As of August 2011, there are 59 units of this kind in operation, 10`% of all geothermal plants. The power capacity ranges from 4.7 to 110 MW, and the average power is about 31 MW per unit.
Since many aspects of a double-flash plant are similar to a single-flash plant, we will generally follow the same format as in Chap. 5 but will focus on the differences between the two concepts. The fundamental new feature is a second flash process imposed on the separated liquid leaving the primary separator in order to generate additional steam, albeit at a lower pressure than the primary steam.

 

Gathering system design considerations of Double-Flash Steam Power Plants

For double-flash plants, the addition of the second flash process of possible arrangements. Practically, the Separators and flashers at each wellhead, with high- and low-pressure steam lines to the powerhouse, and hot water pipelines from each production well to the injection wells (Fig. 6.1). Two-phase pipelines from each well to the powerhouse, with separators) and flashers) at the powerhouse, short high- and low-pressure steam lines to the turbine(s), and hot water pipelines from the powerhouse to the injection wells. Two-phase pipelines from several wells to satellite separator/flasher stations in the field, with high- and low-pressure steam lines from the satellites to the powerhouse, and hot water pipelines from the satellites to the injection wells.Two-phase pipelines from several wells to satellite separators in the field, with high-pressure steam lines and hot water pipelines to the powerhouse, flashers) at the powerhouse, short low-pressure steam lines to the turbine, and hot water pipelines from the powerhouse to the injection wells. An example of the type of system shown in Fig is presented in Fig. 6.5. The list of arrangements is not exhaustive. For example, there may be a set of conditions that favor wellhead separators with satellite flashers, or some combination of the above arrangements. The best choice will be determined by thermodynamic and economic analysis, taking into accountsite-specific conditions including the tempera- true, pressure, and chemical nature of the geofluid, the location of production and injection wells relative to the powerhouse, topography of the site, and method of fluid disposal, including any required scale-control techniques. The latter are now routinely used at many plants and involve both down-well treatment to prevent calcite scaling in the production wells, and/or post-plant treatment to prevent silica deposition in the injection piping and wells. These potential problems are common to both single- and double-flash plants.