A vapor separator device removes water vapor from steam to increase thermal efficiency. Wet steam contains liquid droplets that can cause damage to engines. Steam separators are used in everything from steam locomotives to modern nuclear fission-driven turbine systems.
A vapor separator device is used to remove water vapor from steam. While the term can refer to a specialized set of equipment that works in concert, a steam separator device can be anything that purges excess water vapor from steam in order to increase the thermal efficiency of a steam engine. The end product of a steam moisture separator is referred to as “dry” steam, although it still contains water in a gaseous state. Steam separators are used in everything from classic steam locomotive engines to modern nuclear fission-driven turbine systems.
Normal or “wet” steam that has not passed through a steam separator contains suspended water droplets that are still in a liquid state. These are created when the water is excited enough by the heat of the boiling process to get rid of the greater mass of liquid, but the 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 have nuclei around which the gaseous vapor can more easily cool and condense.
In the most basic example of the steam locomotive, a steam dome is placed over the boiler to act as a steam separator. The steam on its way from the boiler to the pistons driving the engine must pass in the steam dome and back. This design creates a passive vapor separator. The water vapor trapped by the vapor separator would otherwise pass to the piston cylinders and condense there. If allowed to condense in sufficient quantity, the water would create an incompressible mass within 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 can be augmented by other moisture separators and the steam can pass through several separators before reaching the turbines. Some passive steam separators may include narrow pipes, valves or vanes that discourage the passage of liquid droplets and return the collected water back to the boiler. These can be formed in such a way that the vapor passing through them swirls, increasing the efficiency of the device.
The dangers are different in a turbine engine, the need and function of a vapor separator are essentially the same. Water droplets in the high-pressure steam flowing over turbines in modern power plants can damage turbine blades and other parts. Additionally, condensation on turbine blades and parts can lead to erosion over time. While this type of damage may not be as catastrophic as hydraulic lockout, it will still significantly shorten the life of a turbine.
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