Steam drum

A steam drum is a standard feature of a water-tube boiler. It is a reservoir of water/steam at the top end of the water tubes. The drum stores the steam generated in the water tubes and acts as a phase-separator for the steam/water mixture. The difference in densities between hot and cold water helps in the accumulation of the "hotter"-water/and saturated-steam into the steam-drum.

Drum-type boiler:A water-tube steam boiler in which the tubes terminate in upper and lower steam-and-water drums. Also known as drum-type boiler.

header:A pipe that serves as a central connection for two or more smaller pipes.

downcomer:A tube in a boiler waterwall system wherein the fluid flows downward.

There are two regimes of boiling mechanisms, namely, the nucleate boiling and the film boiling.Nucleate boiling is formation and release of steam bubbles at the tube surface, with water still wettingthe surface immediately. Since the tube surface temperature is closer to saturation temperature the tubeis always safe against failure.Film boiling is the formation of steam film at the tube surface,sharply. This leads to instantaneous or long term overheating of due to high heat flux or low velocity or inclined tubes.in which the metal temperature risestubes & failures.

a marine-type water tube boiler

A watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire.

working

Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.

Water tube boiler

The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a superheater to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades.

Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it pre-heats the feedwater supply. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such watertube boilers in thermal power station are also called steam generating units.

Steam drum workingThe separated steam is drawn out from the top section of the drum and distributed for process. Further heating of the saturated steam will make superheated steam normally used to drive a steam turbine. Saturated steam is drawn off the top of the drum and re-enters the furnace in through a superheater. The steam and water mixture enters the steam drum through riser tubes, drum internals consisting of demister separate the water droplets from the steam producing dry steam.

The saturated water at the bottom of the steam drum flows down through the downcomer pipe, normally unheated, to headers and water drum. Its accessories include a safety valve, water-level indicator and level controller. Feed-water of boiler is also fed to the steam drum through a feed pipe extending inside the drum, along the length of the steam drum.

A steam drum is used without or in the company of a mud-drum/feed water drum which is located at a lower level. A boiler with both steam drum and mud/water drum is called a bi-drum boiler and a boiler with only a steam drum is called a mono-drum boiler. The bi-drum boiler construction is normally intended for low pressure-rating boiler while the mono-drum is mostly designed for higher pressure-rating.

A steam drum of the boiler is meant for performing the following.1. Steam is purified by the steam drum internals which may have the baffle separators, cycloneseparators, Chevron separators, demisters / Screen driers.2. Acts as reservoir and provides the necessary head for the circulation of water through evaporativesurfaces.3. Supplies water for a brief period in the event of stoppage of water supply.

use

Bi drum boiler - are used for power generation and steam generation both. For power generation they are used now seldom and are replaced by single drum boilers as the bi drum boilers are non-reheat units.

Single drum boiler- used mainly for the power plants for power generation.

Circulation It is the motion of working fluid in the evaporating tubes.

The circulation may be natural or forced.

In natural circulation,working fluid circulates due to density difference.In forced circulation,working fluid is forced byan external pump.

circulation ratio:The ratio of mass flow rate of circulating water to the rate of steam generation.

Once through boiler: A boiler in which water flows, without recirculation, sequentially through the economizer, furnace wall, and evaporating and superheating tubes.

Utility boiler/power boiler is boiler meant for supplying steam to a steam turbine for electricity generation.

BlowdownRemoval of a portion of water from the boiler drum is called blow down.

Reason:To adjust TDS level of boiler.

Types:1.Continuous 2.Intermittent

1.Continuous blowdown:To maintain a definite concentration of dissolved non-volatile substances.2.Intermittent blowdown : To remove sludge

Drum internalsDrum contains some internal fittings to carryout various functions.Drum internals include1.Feed water distributor2.Surface blowdown header3.Chemical dosing header4.Steam separators and purifiers.

Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with its environment.

Galvanic corrosion (also called ' dissimilar metal corrosion' or wrongly 'electrolysis') refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte.

ElectrolyteA substance whose aqueous solutions conduct electricity.Aqueous solution:An aqueous solution is a solution in which the solvent is water.

Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the creation of small holes in the metal.