Steam drums are used on recirculating boilers that operate at subcritical pressures. The primary purpose of the steam drum is to separate the saturated steam from the steam-water mixture that leaves the heat transfer surfaces and enters the drum.
The steam-free water is recirculated within the boiler with the incoming feedwater for further steam generation. The saturated steam is removed from the drum through a series of outlet nozzles, where the steam is used as is or flows to a superheater for further heating. (By definition, saturated steam is pure steam that is at the temperature that corresponds to the boiling temperature at a particular pressure. For example, saturated steam at a pressure of 500 psia has a temperature of 467°F.)
The steam drum is also used for the following:
1- To mix the saturated water that remains after steam separation with the incoming feedwater.
2- To mix the chemicals that are put into the drum for the purpose of Corrosion control and water treatment.
3- To purify the steam by removing contaminants and residual moisture.
4- To provide the source for a blowdown system where a portion of the water is rejected as a means of controlling the boiler water chemistry and reducing the solids content.
5- To provide a storage of water to accommodate any rapid changes in the boiler load.
The most important function of the steam drum, however, remains as the separation of steam and water. Separation by natural gravity can be accomplished with a large steam-water surface inside the drum. This is not the economical choice in today’s design because it results in larger steam drums, and therefore the use of mechanical separation devices is the primary choice for separation of steam and water.
- Efficient steam-water separation is of major importance because it produces high-quality steam that is free of moisture.
This leads to the following key factors in efficient boiler operation:
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It prevents the carry-over of water droplets into the superheater, where thermal damage could result.
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It minimizes the carry-under of steam with the water that leaves the drum, where this residual steam would reduce the circulation effectiveness of the boiler.
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It prevents the carry-over of solids. Solids are dissolved in the water droplets that may be entrained in the steam if not separated properly. By proper separation, this prevents the formation of deposits in the superheater and ultimately on the turbine blades.
**(carry-over is the passing of water and impurities to the steam outlet)
**The term critical pressure is the pressure at which there is no difference between the liquid and vapor states of water; i.e., the density is identical. This occurs at 3206 psia.
This information source from ‘’ Steam Plant Operation 8th Ed. - Everett B. Woodruff ‘’ .