BFW Systems - BFW Treatment
February 27, 2005
Introduction
Deposits
Corrosion
Carryover
Treatment
External Treatment
Suspended Solids
Hardness
Alkalinity
Silica
Total Dissolved Solids (TDS)
Organics
Dissolved Gases
Internal Treatment
Coagulation Programs
Phosphate Programs
Chelant Programs
Coordinated Phosphate Programs
Blowdown
Miscellaneous Treatment Methods
Antifoaming Agents
Dispersants
Oxygen Scavengers
Condensate Treatment
Introduction
The
pressure and design of a boiler determine the quality of water it requires for
steam generation. Municipal or plant water of good enough quality for
domestic use is seldom good enough for boiler feed water. Makeup water to
a boiler is nearly always treated to reduce contaminants to acceptable levels.
In addition, corrective chemicals are added to treated water to counteract any
adverse effects of the remaining trace contaminants. The sequence of
treatment depends on the type and concentration of contaminants found in the
water supply and the desired quality of the finished water to avoid the three
major boiler system problems - deposit, corrosion and carryover.
The design
of a water treatment system is left to the experts, however, a basic
understanding of the methods and processes is required to properly evaluate bids
and evaluate existing water treatment programs.
Deposits
Deposits, particularly scale, can form on water tubes as the equilibrium
conditions in the water are upset by external forces such as heat. If
water is is contact with a hot surface and the solubility of the contaminant is
lower at higher temperatures, the contaminant will precipitate from the water to
form scale on the hot surface. The most common components of boiler
deposits are calcium phosphate, calcium carbonate, magnesium hydroxide,
magnesium silicate, various forms of iron oxide, silica absorbed on the previous
mentioned precipitates and alumina.
Deposits are a serious problem because they reduce heat transfer which can lead
to boiler tube failure. The maximum temperature for the boiler tubes in
generally in the range of 480oC to 540oC. Water
circulating through the tubes normally conducts heat away from the tubes
preventing the metal temperature from exceeding the maximum allowable
temperature. The insulating effect of the layer of deposit reduces heat
transfer and raises the metal temperature above the point at which failure can
occur.
Corrosion
The
most common form of corrosion is due to attack of steel by oxygen.
Oxygen attack is accelerated by high temperature and by low pH. Another
form of corrosion is alkali attack which can occur where caustic is
concentrated. Direct attack by steam on the boiler metal can occur at
elevated temperatures according to the following reaction:
4H2O + 3 Fe -> Fe3O4 + 4 H2
This
type of corrosion can occur in areas of restricted water flow where the entire
metal surface is blanketed with steam. Monitoring the levels of hydrogen
in the steam is a simple method of detecting this form of corrosion.
Carryover
Carryover is generally a mechanical problem caused by an upset in boiler
operation, broken baffle or mist eliminator. Carryover may also be caused
by the volatility of certain boiler water salts, such as silica and sodium
compounds. Foaming may also result in carryover from the boiler to
the steam system.
Treatment
There are 3 basic means
of treatment for the control of deposits, corrosion and carryover.
a) External Treatment - Treatment of water - makeup, condensate or both before
it enters the boiler, to reduce or eliminate chemicals (such as hardness or
silica), gases or solids.
b) Internal Treatment - Treatment of the boiler feed water, boiler water, steam
or condensate with corrective chemicals.
c) Blowdown - Control of the concentration of chemicals in the boiler water by
bleeding off a portion of the water from the boiler.
External Treatment
The
following unit operations are used alone or in combinations with others to
treat water supplies:
a)
Direct Addition
b) Coagulation/Flocculation
c) Solids/Liquid Separation
d) Precipitation
e) Adsorption
f) Ion Exchange
g) Evaporation
h) Degasification
i) Membrane Separation
These
9 unit operations are used to treat water in 7 broad categories of impurities:
i)
Suspended Solids
ii) Hardness
iii) Alkalinity
iv) Silica
v) Total Dissolved Solids (TDS)
vi) Organic Matter
vii) Gases
Suspended Solids
Suspended solids are generally treated by filtration, precipitation or
coagulation/flocculation. Removal of suspended solids is usually the
first step required because other treatment methods require low levels of
suspended solids. For example, water to be processed by ion exchange
requires less than 10 mg/l of suspended solids to prevent fouling of the
exchanger beds.
Hardness
The
treatment of hardness also known as water softening is the removal of
calcium and magnesium from water. The removal of hardness is
accomplished by precipitation (partial removal), ion exchange, and to a
lesser degree by evaporation, degasification and membrane separation.
Some
water softening processes are sodium ion exchange, split-stream, partial
lime (cold) and hot lime-zeolite. Some methods like sodium ion
exchange only treat hardness while others like hot lime-zeolite also are
effective in reducing alkalinity, silica and TDS.
Alkalinity
It
is desirable to have some alkalinity in boiler water, so complete removal of
alkalinity is not required except in demineralization. Some alkalinity
is also needed to provide optimum pH in the feed water to prevent corrosion
of piping and equipment. Alkalinity may be present in the
bicarbonate (HCO3-), carbonate (CO3-) or hydroxide
(OH-) forms. The bicarbonate form will be prevalent if the water is
zeolite softened and in the carbonate form if lime softened.
The
control of alkalinity is accomplished by direct addition,
coagulation/flocculation, precipitation (partial removal), ion exchange,
evaporation and membrane separation.
Through a series of reactions, bicarbonate and carbonates breakdown at
boiler operating temperatures to form CO2. The CO2
leaves the boiler with the steam and redissolves when the steam condenses to
produce carbonic acid. This can be controlled by chemical addition to
the boiler or directly into the steam to control the condensate pH in the
range of 8.5 to 9.0.
Silica
Coagulation/flocculation, solids/liquid separation, precipitation and
adsorption all provide partial removal of silica. Ion exchange,
evaporation and membrane separation and also used to remove silica.
The degree to which silica is reduced depends on the end use of the steam.
If there is a condensing turbine, the low levels of silica (<0.05 mg/l)
achieved by demineralization is required.
Total Dissolved Solids (TDS)
The
methods used to control TDS are: precipitation, ion exchange, evaporation
and membrane separation. Direct addition methods may actually increase
the level of TDS in the water
Sodium zeolite softening increases TDS by exchanging sodium ions for the
lower equivalent weight calcium or magnesium ions.
Organics
Partial removal of organics is achieved by coagulation/flocculation.
precipitation, adsorption, ion exchange, evaporation and membrane
separation. Problems related to organic matter are generally the
result of the formation of organic acids.
Dissolved Gases
Removal of gases such as H2S, CO2 and O2 is
accomplished by direct addition, precipitation, adsorption, degasification
and membrane separation. The most common method of reducing gas
concentrations in boiler feed water is the use of a deaerator which produces
a CO2 free effluent and O2
concentrations in the range of 0.005 to 0.01 mg/l. If further
reduction of O2
is required, an oxygen scavenger (volatile oxygen-reducing compounds) such
as sulfite, hydrazine or hydrazine substitutes can be added to completely
eliminate O2.
Internal Treatment
Scale formation within a boiler is controlled by one of four chemical
programs: coagulation (carbonate), phosphate residual, chelation or
coordinated phosphate.
Coagulation Programs
In
this process, sodium carbonate, sodium hydroxide or both are added to the
boiler water to supplement the alkalinity supplied by the makeup, which is
not softened. the carbonate causes precipitation of calcium carbonate,
magnesium hydroxide and magnesium silicate. This method is limited to
boilers (usually firetube) using high-hardness feed water and operating
below 250 psi. In addition, some form of sludge conditioner must be
used and blowdown rates are high due to high suspended solids.
Coagulation programs are slowly becoming obsolete because of high cost.
Phosphate Programs
When
operating pressures exceed 250 psi, the high concentration of sludge
associated with coagulation programs is undesirable. For a phosphate
program to be effective, the feed water should contain less than 60 mg/L of
hardness.
In this process, sodium phosphate is fed into the BFW or directly into the
steam drum. Insoluble precipitates are formed, primarily
hydroxyapatite (Ca10(PO4)6(OH)2),
magnesium hydroxide, magnesium silicate and calcium silicate.
These programs produce high suspended solids and often the addition of
sludge conditioners/dispersants are required.
Chelant Programs
A
chelate is a low molecular weight molecule, soluble in water and similar to
an ion exchanger. The most common chelating agents are sodium salts of
ethylene diamine tetraacetic acid (EDTA) and nitrilotriacetic acid (NTA).
These chelating agents form complex ions with magnesium and calcium
which are soluble. Since less sludge is produced, blowdown rates
are reduced. Because of their high cost, chelate programs are limited
to feed water containing low hardness. They are also limited to boiler
operating pressures below 1500 psi.
Chelates can also react with oxygen in the water resulting in higher dosages
to control hardness and hence higher operating costs.
Coordinated Phosphate Programs
When
solids and free caustic alkalinity must be minimized, a coordinated
phosphate program is used. This program differs from the regular
phosphate program in that the phosphate is added to provide a controlled pH
range in the boiler water as well as to react with calcium should hardness
enter the boiler. Combinations of disodium phosphate with
trisodium or monosodium phosphate are added to control pH without the
presence of free OH-. To achieve this, the feed water must be
extremely pure and of consistent quality. A dispersant must be added
to treat deposits so they don't interfere with heat transfer. This
program is ideal for high pressure, high heat transfer rate boilers.
Blowdown
Boiler feed water, regardless of the type of treatment program used still
contains measurable concentrations of impurities. To maintain reliable
boiler operation the concentration of each component of the boiler water must be
limited to certain maximums. This is accomplished by blowing down water
from the boiler.
Miscellaneous Treatment Methods
Anitfoaming
agents, dispersants and oxygen scavengers are also added as part of an internal
treatment process to condition boiler water.
Antifoaming Agents
A high percentage of carryover is caused by foaming in the steam drum.
Antifoaming agents reduce foaming and help maintain steam purity.
Dispersants
Chemicals are used to condition boiler water particles so they do not form
larger particles which can settle out and form scale and deposits.
Dispersants keep the particles small so they remain suspended at the liquid
velocities encountered in a boiler.
Oxygen Scavengers
Deaeration reduces oxygen in the water to low levels but does not completely
eliminate it. Application of sulfite, hydrazine or hydrazine-like
compounds after deaeration , scavenges the remaining oxygen and maintains a
reducing condition in the boiler water. Hydrazine has the advantage of
leaving with the steam and is then available in the condensate to protect
against oxygen corrosion in the condensate system.
Condensate Treatment
The
recovery of condensate saves costs in terms of energy, fresh makeup water and
water treatment chemicals. The kind of treatment process required
depends on what the steam has been used for and how the condensate has been
handled. In acid plants, the condensate will generally be contaminated
with corrosion products and in-leakage of hard water. High flow rate
condensate 'polishers' (sodium ion exchangers) are often used to treat
condensate prior to reusing the water.
