Page 98 - Boiler plant and distribution system optimization manual
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The Control of Boilers 83
steam bubbles in the drum water which the drum Boiler blowdown water is often passed
level controller interprets as a need for more through a heat exchanger to recover the energy
water, which is colder than the boiler saturation which would otherwise be wasted. Heat from
temperature causing further shrinking. the boiler blowdown is usually used to heat the
The three element control loop is very pre- incoming boiler makeup feedwater.
cise and stable because of the closed loop control
of the feedwater valve. It measures steam flow
and water flow as well as drum water level. This FURNACE PRESSURE CONTROL
system can adjust for actual steam and water flow
conditions balancing this information against Furnace pressure conditions must be closely
measured water level. controlled to establish safe and efficient firing
Feedwater controllers are available in pneu- conditions. Air fuel mixing for the burner systems
matic, analog electric and digital electronic mod- depends to a great degree on the differential pres-
els. sure across the burner.
Different challenges apply to different siz-
es of boilers. Figure 8.11 shows a natural draft,
BLOWDOWN CONTROL balanced draft, induced draft and forced draft
systems. Each configuration will require special
Blowdown is done either intermittently or control considerations.
continuously. On many smaller boilers blow- Damper openings, forced draft fans, induced
down is performed manually after an analysis draft fans and stack effect all play an important
of the boiler water indicates that the water is ex- role in various furnace pressure control systems.
ceeding impurity limits.
If too much hot boiler water is blown down Steam Temperature Control
from the boiler an excessive amount of energy is Superheaters are used to raise the steam
wasted. If the concentration of impurities in the temperature above the saturation point for super-
boiler is allowed to get to high, carry-over and heated steam. In doing so, the temperature of the
scale formation can occur. superheated steam must be regulated. The main
Control systems are available which will au- control loop is to regulate the superheated steam
tomatically perform the operations necessary to temperature by a desuperheater steam valve.
blow down the boiler. This control loop is enhanced by a feed forward,
Automatic blowdown controllers use electro- usually air flow, signal into the loop plus an in-
chemical cells to measure the concentration of ner loop cascade signal for spray water flow to
minerals by measuring electrical conductivity eliminate the effects of spray valve performance
and acid (PH) levels. variations.
The signal corresponding to the impurity
level is compared to the setpoint and when ex- Cold End Temperature Control
ceeded, blowdown is initiated. Cold end corrosion occurs when metal tem-
Boilers operated at a steady load may be peratures fall below the dew point for sulfurous
equipped with a continuous blowdown system. and sulfuric acids. The most critical point for the
A continuous blowdown system removes a con- economizer is the feedwater entrance point to
tinuous, usually small amount, of water from the the heat exchanger where metal surfaces can be
steam drum to keep the concentration of undesir- cooled below the acid dew point.
able chemicals at a safe level. In air preheaters a steam or glycol heating
Manual blowdown to remove sludge which system is used to regulate the cold end tempera-
has settled in the mud (lower) drum is still neces- ture. A single element control of the heating con-
sary. trol valve may be adequate. However, the three
