Page 300 - Boiler plant and distribution system optimization manual
P. 300
Boiler Water Treatment 285
What is the basis for choosing between The makeup water is combined with the
neutralizing and filming inhibitors? condensed steam returned from the distribution
The proper choice depends on the boiler sys- system (called condensate return) to become boil-
tem, plant layout, operating conditions and feed- er feedwater. The feedwater is deareated to strip
water composition. In general, volatile amines out noncondensible gases and treated with oxy-
are best suited to systems with low makeup, low gen scavengers.
feedwater alkalinity and good oxygen control.
Filming inhibitors usually give more eco- Internal Water Treatment
nomical protection in systems with high make The removal of scale-forming materials from
up, air in-leakage and high feedwater alkalinity the boiler makeup by reducing the hardness to
or where the system is operated intermittently. In near zero is the best control method.
most cases a combination of these treatments may Internal treatment of boiler water refers to
be best to combat condensate system corrosion. chemical additions required to prevent scale for-
mation from materials not removed by makeup
treatment and to prevent sludge deposits from
ExTERNAL WATER TREATMENT forming due to the precipitation of these materials.
Makeup Water
Deposit Formation
Makeup water is water added to the boiler
There are two basic causes of boiler deposit
system from an external source to replace water
formation:
lost in the boiler room and in the distribution sys-
tem. This includes blowdown water, steam leaks,
• Scale; the high temperatures found in boil-
condensate losses and steam used directly in pro-
ers cause precipitation of compounds whose
cess applications.
solubilities are inversely proportional to the
The usual source of makeup water is the po-
solution temperature.
table water supply or what has been referred to in
many cases as city water. This represents a treated
• Sludge; the concentration of boiler water
water that has a predictable and uniform quality
causes certain compounds to exceed their
on a day-to day basis. Other sources of makeup
maximum solubility at a given temperature,
water include well water, surface water or hold-
forcing precipitation in areas of highest con-
ing ponds that are not treated to the extent that
centration.
the potable water source is treated.
The uniformity of makeup water quality is
While these represent a somewhat simplified
important if the boiler water system is to be oper-
view of the mechanisms involved, they do sum-
ated reliably.
marize the factors essential to boiler deposits.
Makeup water treatment varies on the needs
of a particular installation. Various processes are
Scale
used to improve makeup water quality including:
The build up of boiler scale constitutes a
a. Lime-Soda Softening growth of boiler crystals on waterside heat trans-
b. Ion Exchange Process-General fer surfaces and is most severe in those areas of
c. Sodium Ion Exchange the boiler where maximum heat transfer occurs.
d. Hydrogen Ion Exchange Figure 15.3 shows the relationship between heat
e. Deionization transfer efficiency and scale deposit thickness.
f. Dealkalization
g. Distillation Problems caused by scaling
h. Reverse Osmosis The steam boiler normally uses an external
i. Electrodialysis heat source at a much higher temperature than
