A steam drum is a pressure chamber of a steam boiler located at the upper extremity of steam boiler circulatory system. It is a reservoir of water/steam at the top end of the water tubes. The steam drum stores the steam generated in the water tubes and acts as a phase-separator for the steam/water mixture. Steam is separated from the water and then discharged at a position above the water level maintained there.
|Fig.1: Steam Drum of Boiler|
In natural circulation of steam boilers, the steam drum occupies a position of importance
because of: it is the heaviest PP and often the heaviest component, the layout and erection of the steam boiler revolve around it, It takes the maximum time for manufacturing and needs the costliest machinery, moving it demands a high level of logistics, it needs the heaviest crane or lifting tackle during both fabrication and erection.
Thermodynamically, however, a steam drum is merely a surge tank in the circulation
system to which the following parts are connected: Economizer outlet and Superheater inlet pipes on one side, downcomers and risers on the other side.
Under steady operating conditions when the flows are balanced, the steam drum produces
a steady water level. Drum water level is one of the most important parameters, of a steam boiler control system. The other important function of the steam drum is to house the drum internals, which perform the following functions: saturated water from evaporator circuits, which contain varying amounts of steam, is separated by the steam separators. Water-free steam to the SH and steam-free water to the downcomers are required of steam separators and purifiers, chemical dosing to maintain the chemistry of the circulating water, continuous blowdown of drum water to keep the carryover under specified limits.