Page 302 - Boiler plant and distribution system optimization manual

Boiler Water Treatment 287

• Alters boiling characteristic in the mud drum “stirring up” the sludge level.
• Dirty boilers; the high cost of cleaning With the sludge in suspension from this action, it
and extra down time. can be swept up the generating tubes where it can
• Absorption of chemicals (especially P04) bake onto the tube surfaces, resulting in deposits.
leading to scale and loss of efficiency. TDS are controlled by continuous blowdown,
which is typically removed from the steam drum.
Hardness and Salt Levels Too High Guidelines published for government plants are
• Scale and loss of efficiency shown in Table 15.5.

Alkalinity Too High Table 15.5—Total dissolved solids (TDS) and conduc-
• Scale and loss of efficiency tivity limits for steam boilers.

Energy Conservation through Optimum
Blowdown Control
While blowdown is a key to safe, clean boiler
operation, it must be remembered that blowdown
water leaving the boiler carries a high level of Btus
with it. Energy conservation requires maintaining
the highest permissible cycles of concentration in
the boiler water. To do this a margin of safety of-
ten must be sacrificed and good controls installed
to insure that no damage occurs to the boiler as a
result.
Many plants are able to increase their cycles
of concentration and therefore reduce blowdown
by reducing total solids concentration in the feed-
water or altering the boiler water treatment pro- Boiler Blowdown Calculations
gram. The rate of blowdown from a boiler is a crit-
Reducing the solids content of the makeup in- ical operating control for TDS.
volves a change in the plant makeup water source
or altering the external water treatment program. a. The water added to a boiler must equal wa-
The makeup water source can be changed or pre- ter loss from the boiler.
treatment equipment such as filters, deionizers or
other equipment can be installed. F = E+B

Efficient Bottom Blowdowns F = feedwater lb/hr
Often plants will adhere to a strict schedule E = Steam generated, 1b/hr
of bottom blowdown and still develop problems B = Blowdown, lb/hr
with excessive sludge buildup in the mud drum.
This can be due to improper timing of blowdown b. The blowdown can be related to the feedwa-
periods. Experience has shown that frequent ter using cycles of concentration (COC).
blowdowns of short duration (10-20 seconds) are
more effective in removing sludge than occasion- C = FIB
al blowdowns of longer duration.
The blowdown is only effective for the first C = Cycles of Concentration
few seconds of the blowdown. Blowdowns of F = Feedwater, lb/hr
long duration create a great deal of turbulence B = Blowdown, lb/hr