Page 89 - Boiler plant and distribution system optimization manual

74 Boiler Plant and Distribution System Optimization Manual

water level under very dynamic conditions when COLD END TEMPERATURE CONTROL
the heat rate changes in the boiler causes the water
level to shrink and swell, due to steam bubble vol- When economizers and air heaters are used
ume changes in response to firing rate changes. to remove heat from flue gases, sulfur oxides
The boiler feedwater control system must can form sulfuric and sulfurous acids which can
also respond to momentary changes in steam de- cause damage to boiler and exhaust system com-
mand replacing steam that has left the boiler with ponents.
feedwater.
If the feedwater control system fails, there
may be serious problems. A high water level can SOOT BLOWER CONTROL
cause severe damage to the distribution system
and machinery like turbines. A low water level can The accumulation of soot, fly ash and other
allow the high flame temperatures to weaken and deposits on heat exchange surfaces lowers the
melt the boiler steel causing a catastrophic high heat transfer and efficiency of a boiler raising the
energy release of steam from inside the boiler. stack temperature (Figure 8.1). It is an economic
consideration to keep boilers as clean as possible.
On many boilers soot blowing operations are
BLOWDOWN CONTROL automated and since the accumulation of soot is
roughly proportional to the number of hours the
As water in the boiler is evaporated to pro- boiler has been operated, automatic sootblow-
duce steam, impurities are concentrated. Unless ers are usually activated on a simple time clock
the concentration of these impurities is kept un- mechanism.
der control, severe scaling of the heat transfer
surfaces and tube failure can occur.
Impurities can also be carried over into the
distribution system and depending on conditions
at the steam water interface, can cause surges of
water into the steam system.
Blowdown is also controlled to eliminate the
waste of energy by preventing more hot water
than is necessary from being dumped from the
operating boiler.

FURNACE PRESSURE CONTROL

Some larger boilers have been designed
for balanced or negative draft and requiring a
furnace pressure slightly below atmospheric for
proper combustion and safe operation.

STEAM TEMPERATURE CONTROL

In boilers where superheaters are used to
raise steam temperature above the saturation
point, the temperature of this superheated steam
must be regulated. Figure 8.1 —Effect of soot on fuel consumption