25 May 2005
feedwater heaters and pumps. The pre-boiler section may include a storage tank for the
treated makeup water. The boiler feedwater is usually composed of the makeup water
combined with the condensed steam (condensate) that is returned from the distribution
system (called the condensate return). The feedwater, pre-heated or not, then enters
the deaerator, which is used to remove (deaerate) oxygen and other volatile gases.
(See paragraphs 3-2.6.1 and 3-22.214.171.124 for more information on deaerators.)
After-Boiler. The after-boiler, or post-boiler, section of the boiler system
encompasses all structural components of the boiler system that hold, move, and
process the steam and water downstream from the actual boiler. The after-boiler
includes steam piping, heat exchangers, steam traps, condensate piping, turbines,
process equipment, and superheaters.
. A fire tube boiler passes the hot combustion gases
through the tubes, which are surrounded by the water to be converted to steam. A water
tube boiler passes the hot combustion gases around the tubes, which contain the water
to be converted to steam. In either type of boiler, the steam passes into a steam drum
and then into the steam lines. The circulation of water in the water tube boiler may be
accomplished by heating only, through a process of "natural circulation," which requires
no external force or pumping. Alternatively, the water can be pumped through the
heating circuit of the boiler by a process referred to as "forced circulation." Usually, a
mud drum is provided at the lowest point in the water circulation section to allow the
removal of any water-formed sludge. There will be at least two locations for the removal
of boiler water blowdown: 1) for surface blowdown, sometimes referred to as a skimmer,
located just below the operating water level of the steam drum; and 2) one or more
bottom blowdown locations at the mud drum.
Steam Drum. The feedwater is added to the steam drum, where a mixture
of steam and water is produced and where the steam is separated from the water. This
separation process usually includes using mechanical devices to assist in removing any
entrained boiler water from the steam. Chemicals used for internal boiler water
treatment may be added (fed) to the water in the steam drum. A process of either
continuous or intermittent surface blowdown is used to maintain the TDS of the boiler
water and achieve the optimal operating conditions in the boiler. In some boiler
systems, the blowdown water will be discharged to a flash tank, where a lower pressure
steam is produced (possibly for use in the deaerator). Also, water from continuous
blowdown may be used to preheat the makeup water by means of a heat exchanger.
Steam from the drum may be discharged directly to the steam header or, in some
boilers, heated further in a superheater to generate superheated steam.
Steam Header. The steam generated by the boiler is discharged from the
steam drum to a header. The steam header feeds the steam to the steam distribution
system. The steam is consumed by process equipment, lost through leaks, lost from
valves, fittings, or steam traps, or condensed for return to the deaerator and
subsequently reused for additional steam production.