Page 24 - Boiler plant and distribution system optimization manual

Boiler Plant Orientation 9

Figure 1.12—Many operating variables affect steam temperature such as load,
excess air, feedwater temperature, heating surface cleanliness, fuel and burner
settings etc. Attemperator control is necessary to regulate steam temperature
within required limits to correct fluctuations caused by these operating vari-
ables. This is an attemperator which sprays high purity water into steam to
control its temperature. It can be located at the superheater outlet or between
superheater sections. There is the possibility that water could carry over into
the turbine if control problems develop with the attemperator, however.

tending into the center of the header. valve to open further, injecting more cool water
Attemperator water, often supplied by the into the steam causing a reduction in the final
boiler feed pumps, is sprayed into the hot steam steam temperature leaving the boiler. Similarly,
by a nozzle. Since the water is evaporated by the if the final temperature falls below the design
steam, the water must be free of solids if steam value, the control will reduce the attemperator
purity is to be maintained. The rate of water injec- spray flow to allow the final temperature to in-
tion is regulated by an automatic control system crease.
to maintain the final steam temperature. From the attemperator, the steam flows to the
Water enters the attemperator and passes inlet of the secondary superheater which, like the
through a spray nozzle located at the entrance primary superheater, is a bundle of tubes located
to a restricted venturi section. The steam passing in a higher gas temperature zone of the furnace.
the nozzle picks up the water spray, and flows The primary superheater is located in a zone of
through the venturi. This action evaporates the lower temperature than the secondary super-
water and thoroughly mixes it with the steam. heater. As in the primary superheater, the steam
The temperature control system measures flowing through the inside of the tubes is heated
the final superheated steam temperature going by the hot combustion gases flowing around the
to the turbine system, and adjusts the attemper- outside of the tubes leaving the secondary su-
ator spray flow in order to keep this temperature perheater at the design final temperature. At this
constant. Suppose that the final temperature is to point, the steam in the individual tubes is piped
be kept at 1,000F but that it gradually increases to large manifolds or headers, which tie into one
above this value. When the control senses this or more large diameter pipes which then run to
increase, it causes the attemperator spray control the turbine system.