A steam separator device is used to remove water vapor from steam. While the term may refer to a specialized set of equipment working in concert, a steam separator device may be anything that eliminates excess water vapor from steam in order to increase the thermal efficiency of a steam-based engine. The end product of a steam moisture separator is referred to as "dry" steam, though it still contains water in a gaseous state. Steam separators are used in everything from classic steam locomotive engines to modern turbine systems driven by nuclear fission.
Regular or "wet" steam that has not passed through a steam separator contains suspended droplets of water that are still in a liquid state. These are created when water is excited enough by the heat of the boiling process to break free of the greater mass of liquid, but energy imparted to the molecules is not great enough to break the surface tension of the droplets. Not only are these droplets still liquid, but they also present nuclei around which gaseous steam may more easily cool and condense.
In the more basic example of the steam-powered locomotive, a steam dome is placed above the boiler to act as a steam separator. Steam on its way from the boiler to the pistons driving the engine must pass up into the steam dome and back down again. This design creates a passive steam separator. Water vapor that is trapped by the steam separator would otherwise pass on to the piston cylinders and condense there. If allowed to condense in great enough quantity, the water would created a non-compressible mass inside the piston cylinder, creating a condition called hydraulic lock, which causes the entire engine to fail.
While some modern steam engines still use a steam dome, this form of steam separator may be augmented by other moisture separators, and the steam may pass through several separators before reaching the turbines. Some passive steam separators may include narrow tubes, valves, or vanes that discourage the passage of liquid droplets and return any harvested water to the boiler. These may be formed in such a manner so as to cause the steam passing through them to swirl, increasing the efficiency of the device.
The dangers are different in a turbine-driven engine, the need for and function of a steam separator are essentially the same. Water droplets in the high-pressure steam running over turbines in modern power generation plants may damage the turbine blades and other parts. Additionally, condensation on the blades and parts of the turbine can lead to erosion over time. While this kind of damage may not be as catastrophic as hydraulic lock, it will still greatly decrease the lifespan of a turbine.
