Steam Drum

Steam Drum •A steam drum is a pressure chamber of 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.

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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.

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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.

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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.

The steam drum is a key component in natural, forced and combined circulation boilers.

The functions of a steam drum in a subcritical boiler are: • Mix fresh feed water with the circulating boiler water. • Supply circulating water to the evaporator through the down-comers. • Receive water/steam mixture from risers. • Separate water and steam. • Remove impurities. • Control water chemical balance by chemical feed and continuous blow down. • Supply saturated steam • Store water for load changes (usually not a significant water storage) • Act as a reference point for feed water control

• The Diameter of the drum is lager for better separation of moisture. There are different kinds of devices for water separation such as • Plate baffles for changing the flow direction • Separators based on centrifugal forces (cyclones) • Steam purifiers like screen dryers (banks of screens) • and washers. • The separation is usually carried out in several stages. Common separation stages are primary separation, secondary separation and drying.

• Impurity damages • Impurities in steam causes deposits on the inside surface of the tubes. • This impurity deposit changes the heat transfer rate of the tubes and causes the superheater to overheat (CO3 and SO4 are most harmful). • The turbine blades are also sensitive for impurities (Na+ and K are most harmful). • The most important properties of steam regarding impurities are : • Steam quality, Water content: percent by weight of dry steam or moisture in the mixture , Solid contents, Steam purity: parts per million of solids impurity in the steam

• Steam quality • There are salts dissolved in feedwater that need to be prevented from entering the superheater and thereby into the turbine. • Depending on the amount of dissolved salt, some impurity deposition can occur on the inner surfaces of the turbine or on the inner surface of superheater tubes as well. • Steam cannot contain solids (due to its gaseous form), and therefore the water content of steam defines the possible level of impurities. • The water content after the evaporator (before superheaters) should be << 0.01 %-wt (percents by weight) to avoid impurity deposition on the inner tube surfaces. • If the boiler is a high subcritical-pressure or supercritical boiler, the requirements of the steam purity are higher (measured in parts per billion).

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Steam drum placement

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In natural circulation boilers the steam drum should be placed as high as possible in the boiler room because the height difference between the water level in the steam drum and the point where water begins its evaporation in the boiler tubes, defines the driving force of the circuit.

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The steam drum is normally placed above the boiler. Controlled circulation and once-through boilers.

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For controlled circulation and once-through boilers the steam drum can be placed more freely, because their circulation is not depending on the place of the steam drum (pumpbased circulation).

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This is a reason why controlled circulation and once-through boiler have been preferred in e.g. boiler modernizations, when the biggest problem is usually lack of space.

• Downcomers- In the down comer also no vapor bubble should move with water. • B.coz it decreases the density difference hence pressure head for natural circulation.

• Water should be treated prior to feeding • but then also certain amount of impurities will be present in the makeup water and feed water. • In the drum continuously the total dissolved solid increases. As water gets converted into vapor but impurities remains in the drum. • To maintain a certain impurity blow down is necessary.