4. Steam Boilers - Water and Steam Drums .... Page 1/1

Steam & water drums are fusion welded, radiographed and stress relieved.
They shall be complete with all necessary connections, a 12" x 16" manway opening in each head with cover and yokes, internal piping for feedwater, chemical feed, and continuous blow-down.
The upper drum shall include adequate steam separating equipment to assure the specified steam quality.
The lower drum shall include a slotted inverted channel to assure uniform blow-off. All internal fittings and pipe must be adequately supported and removable for maintenance purposes.

The water drum is larger than the header, but both are smaller than the steam drum. The water drum equalizes the distribution of water to the generating tubes. Both the water drum and the header collect the deposits of loose scale and other solid matter present in the boiler water. Both the drum and the header have bottom blowdown valves. When these valves are opened, some of the water is forced out of the drum or header and carries any loose particles with it. DO NOT OPEN THE BOTTOM BLOWDOWN VALVES ON A STEAMING BOILER. Opening these valves will interrupt the circulation of the steam cycle.

A Vertical Boiler whose major design features are the gastight waterwall furnace and the vertical in-line inverted U-loop superheater. The boiler shown is top-fired with resulting improved gas distribution over the entire superheater furnace. Both the main-bank tubes and superheater elements are in-line for improved tube cleaning. Normally, soot-blowing equipment includes retractable blowers in the superheater and rotary blowers in the main bank and economizer. The combustion air preheater, on the top of the boiler, heats the inlet combustion air to the burners by means of the flue cases and improves the efficiency of the boiler.

4.1 Water & Steam Drums Arrangement

Water and steam drum connected by a bank of generating tubes

Present day marine propulsion boilers are usually composed of a steam drum and a water drum connected by a bank of generating tubes and downcomers (downcomers are not shown in the diagram). Natural circulation in these boilers is obtained by rising of warmer water through generating tubes and lowering of cooler water through the downcomers. When the load is reduced, some of the generating tubes may allow water to flow down (thereby behaving like downcomers instead of generating tubes) to the water drum automatically.
The water drum is located at the bottom of the boiler below the main generating bank and acts as a lower reservoir of water for distribution to the main generating bank. Also, this large drum serves as a collection point for solids (sludge) that precipitate to the bottom that are removed by bottom blowdown.

Water and steam flow diagram

A Vertical Boiler (Combustion Engineering Boiler example)whose major design features are the gastight waterwall furnace and the vertical in-line inverted U-loop superheater. The boiler shown is top-fired with resulting improved gas distribution over the entire superheater furnace. Both the main-bank tubes and superheater elements are in-line for improved tube cleaning. Normally, soot-blowing equipment includes retractable blowers in the superheater and rotary blowers in the main bank and economizer. The combustion air preheater, on the top of the boiler, heats the inlet combustion air to the burners by means of the flue cases and improves the efficiency of the boiler.

Blue: the downcomers lead the water from the steam drum to the water drum and the waterwalls headers.
Red and blue: the evaporation tubes and the water walls lead the water and steam emulsion back to the steam drum.
Red: the steam passes through the superheater before it leaves the boiler for the consumers.

4.2 Steam Drum Internals

A good place to begin in classifying boilers is to consider their intended service. By this method of classification, boilers are divided into two classes,

Firetube Boiler Schematic
[Will not be covered in this course]

STEAM DRUM INTERNALS The steam drum contains steam separating equipment and internal piping for distribution of chemicals to the water, for distribution of feedwater and for blowdown of the water to reduce solids concentration. Typical drum internal components are noted below: Steam Horizontal Separators - Standard horizontal, in-line. Driers - Integral Chevron, non-drainable. Feed Pipe - Perforated, tee'd inlets. Continuous Blowdown Pipe - A single outlet perforated, tee'd from centerline of drum. Chemical Feed Pipe - A single inlet perforated, tee'd from centerline of drum. Downcomer Pipes - Pipes, equidistant across the drum. An internal (girth) baffle extending almost the full length of the shell forms an annulus along the bottom half of the steam drum. The steam-water mixture from the furnace waterwall risers enters the drum in this annulus and then passes through the horizontal steam separators. The first stage of separation of steam from the water is accomplished here. As the mixture follows the curved contour of the separator, the heavier water particles are forced to the outside, discharging through the primary drain and then through wire mesh into the bottom of the drum. The wire mesh dissipates the discharging water velocity and allows any entrained steam to escape. The separated steam flows out of the openings on the outboard sides of the separators and into the Chevron driers. The final separation of moisture in the steam is accomplished as the steam comes into contact with the W-shaped Chevron elements forming the drier assemblies. Steam enters the driers at a low velocity and makes several abrupt changes in flow direction. This causes the entrained moisture to adhere to the large surface area presented by the Chevrons. The water film then drains by gravity to the lower part of the drum. The separated steam flows into the dry box and leaves the drum through the steam tubes at the top. These tubes supply steam to the superheater sections. The water separated from the steam falls to the water space of the drum and through the radial vortex eliminators to downcomer pipes. The downcomer pipes carry the water to the bottom of the unit for distribution to the furnace waterwalls. Since suspended solids may accumulate on the surface of the water in the steam drum, there must be means of removing them. The surface blow pipe is used to remove these light suspended solids from the surface of the water and to reduce the total dissolved solid content of the boiler water. Suspended solids usually consist of oil, salt contaminants, or excessive treatment chemicals which can cause foaming on the water surface. Dissolved solids usually consist of salt contaminants and treatment chemicals that are in solution.

4.3 Steam & Water Drums Arrangement in a Marine Roof-fired Watertube Boiler