Most geothermal systems currently producing power are called hydrothermal systems. They are characterized by the presence of very hot water in the geothermal reservoir. Because these fluids are thousands of feet underground, they are subject to very high pressures. As these fluids ascend from depth to the generating plant, they will flash to steam. To imagine how this might occur, think about a can of soda. When sealed, the soda is under pressure and not yet bubbly. When you open the can the pressure is released and the gases that are dissolved in the liquid bubble out. Tapping the geothermal reservoir is analogous to opening a can of soda - as the water starts ascending the bore hole the liquid water flashes to steam as it expands with decreased pressure. This steam is then used to run a turbine and make electricity.
Types of Flash Systems:
Although all flash plants use basically the same technology, different designs are used to assure that the efficiency of the energy extraction process is maximized. The basic types of flash plants are single-flash and dual-flash. Flash plants have a flash (expansion) tank to ensure that the flashing process is as complete as possible. This design keeps the pressure that drives the turbine in the generator as high as possible. Single-flash plants are essentially a single cycle process in which the flashed steam drives the turbine, the pressure drop across the turbine is kept as large as possible, and the spent steam (i.e., condensate) is then collected and re-injected. Dry steam plants, which are discussed here, commonly utilize a generating system that is similar to the single flash plant. The primary difference between these systems, as you can see in the pictures below, is that dry steam systems do not have a flash (expansion) tank as the resource comes in as a forceful & dry (unsaturated) steam. The single flash plant however does.
Dual-flash plants differ from single-flash plants in that they capture the earliest and hottest spent steam (i.e., condensate) and then allow it to flash in a different part of the generator. This process allows extra energy to be extracted from the steam/condensate that would otherwise have been lost. Dual-flash systems require a series of turbines specifically designed to operate within relatively narrow pressure and temperature intervals, and they are highly efficient. Whether one uses single- or dual-flash plants depends upon the temperature of the resource and other characteristics of the reservoir and local conditions.
-Diagrams courtesy of U.S. Department of Energy