Boiler Feedwater Treatment : Why Water Treatment is Necessary
Contents
- Water for boilers
- Impurities in water
- Boiler feedwater
- Purity requirements of feedwater
- Boiler deposits
- Corrosion
- Boiler water carryover
Water for boilers
All natural waters contain varying amounts of suspended and dissolved
matter as well as dissolved gases. The type and amount of impurities in
fresh water vary with the source (lake, river, well) and with the area
of location. Impurities in water become an important consideration when
water is to be used for steam generation. With the trend toward
higher-pressure boilers, pretreatment has become the key to successful
operation of industrial power plants. Feedwater must be pretreated to
remove impurities to control deposition, carryover, and corrosion in the
boiler system. Poor quality water gives poor quality steam. The first
step in any treatment is filtration of suspended solids. On the basis of
proven satisfactory performance, cost, and other considerations,
cartridge filters are a practical solution to most problems of water
clean-up.
Physical properties of water
Pure water is a tasteless, odourless, colorless liquid. Because water
can be converted to steam at a convenient temperature, it is an ideal
medium for generating power or conducting heat.
Chemical composition of water
Pure water,
H2O, is a simple combination of
hydrogen and oxygen. There are, however, several hybrids forms of water
in all supplies. Water often contains about 300 ppm of deuterium oxide,
D2O, or ‘heavy water’. It has no use as
drinking water or in making plants grow, but in pure form has found use
in nuclear reactors. For all practical purposes only ordinary water,
H2O, is considered for use in boilers.
Boiling temperature
The boiling point of water is dependent on pressure. At sea level
atmospheric pressure, water boils at about 212oF. With increasing
pressure, the boiling point also increases. At a pressure of 200 psig,
for example, water boils at a temperature of about 388oF. At the
critical pressure of 3200 psig (where water is converted to steam
without change in volume), the boiling point is 704oF. As the pressure
decreases, the boiling point of water decreases. Under vacuum water will
boil at temperatures as low as 35oF.
Water – an ideal medium for carrying heat energy
It takes one BTu (British Thermal Unit) to raise temperature of one
pound of water 1oF. It takes an additional 970 Btu to change one pound
of water, at boiling point, to steam. This heat energy is stored in the
steam and when it condenses, the energy is given off. Thus much of the
heat from burning fuel can be absorbed by boiler water, transported with
the steam, and released at the points of use.
Impurities in water
All natural waters contain various types and amounts of impurities.
These impurities cause boiler problems and as such consideration must be
given to the quality and treatment required of the water used for
generating steam. For any type of treatment, sediment filtration
(usually with cartridge filters) is the first step.
Natural water
Natural waters contain suspended matter, dissolved solids, and
dissolved gases. Water being a universal solvent dissolves minerals,
rocks and soil that come into contact with it. It dissolves gases from
air and gases that are given off from organics in the soil. It picks up
suspended matter from the earth. Additionally it may also be
contaminated with industrial wastes and process materials.
Dissolved minerals
Dissolved minerals picked up by the water consist mainly of calcium
carbonate (limestone), calcium sulfate (gypsum), magnesium carbonate
(dolomite), magnesium sulfate (epsom salts), silica (sand), sodium
chloride (common salt), hydrated sodium sulfate (Glauber salt), and
smaller quantities of iron, manganese, fluorides, aluminum, and other
substances. The nitrates and phosphates found in water are usually due
to sewage contamination.
Water hardness
Water containing high amounts of calcium and magnesium minerals is
‘hard water’. The amount of hardness in natural water can vary from a
few ppm to 500 ppm. Calcium and magnesium compounds are relatively
insoluble in water and tend to precipitate out. This causes scale and
deposit problems. Such water must be treated to make it suitable for
steam generation.
Dissolved gases in water
Water contains varying amounts of dissolved air (21% oxygen, 78%
nitrogen, 1% other gases including carbon dioxide). Water can contain up
to 9 ppm oxygen at room temperature and atmospheric pressure. As the
temperature increases, the solubility of oxygen decreases, but water
under pressure can hold higher amounts of dissolved oxygen. Nitrogen,
being inert, has little effect on water used in boilers. Water can
contain 10 ppm of carbon dioxide, sometimes much more than that due to
decaying vegetation and organics in soil. Hydrogen sulfide and methane
may be dissolved in water but this is rare. These gases can be
troublesome when they are present in the feed water.
Other impurities in water
Natural waters contain varying levels of soil, sand, turbidity,
colour, precipitated minerals, oil, industrial wastes and other
suspended solid particles. Turbidity is due to very fine organic
materials and microorganisms, as well as suspended clay and silt. Colour
is due to the decaying vegetable matter.
Sources of fresh water
Fresh water can be surface water from rivers, streams, reservoirs or
ground water from wells. Generally ground water supplies are more
consistent in composition than surface water supplies. Surface water
quality is effected by rainfall, soil erosion and industrial wastes, but
ground water is usually harder than surface water. The composition of
fresh water also varies with the location and type and strata of the
earth formations. In limestone areas, for example, water contains large
quantities of dissolved calcium. Apart from the geographic variations,
the local conditions of a particular area may have a great influence in
the composition of the water.
Boiler feedwater
Boiler ‘feedwater’ is the water supplied to the boiler. Often,
steam is condensed and returned to the boiler as part of the feedwater.
The water needed to supplement the returned condensate is termed
‘make-up water’. Make-up water is usually filtered and treated
before use. Feedwater composition therefore depends on the quality of
the make-up water and the amount of condensate returned. Sometimes
people think that there is a great deal of similarity between the
requirements for potable (drinking) water and the requirements for
boiler feedwater. The minerals in drinking water are considered
desirable and are absorbed by the body. On the other hand, minerals in
water cannot be handled as well by boilers. Although a boiler is a big
mass of steel, it is more sensitive to water impurities than the human
stomach. For this reason, a lot of care is needed in filtration and
treatment of the boiler water supply.
Purity requirements of feedwater
Feedwater is a matter both of quantity of impurities and the nature
of impurities. Hardness, iron, and silica, for example, are of more
concern than sodium salts. The purity requirements of feedwater depend
on how much feedwater is used as well as toleration of the particular
boiler design (pressure, heat transfer rate etc.). In today’s
high-pressure boilers practically all impurities must be removed. The
feedwater (make-up water) from outside needs to be treated for the
reduction or removal of impurities by first filtration, and then
followed by softening, evaporation, deariation, ion exchange etc.
Internal treatment is also required for the conditioning of impurities
within the boiler system, to control corrosion, as reactions occur in
the boiler itself and the steam pipelines.
Boiler deposits
Water evaporating in the boiler causes impurities to concentrate.
Boiler scale results from suspended matter settling out on the metal or
dissolved impurities precipitating out on heat transfer surfaces and
becoming hard and adherent.
Impurities that form deposits
Bicarbonates of calcium and magnesium dissolved in water break down
under heat and give off carbon dioxide forming insoluble carbonates.
These carbonates precipitate directly on the boiler metal and or form
sludge in the water that deposits on boiler surfaces. Sulfate and silica
generally precipitate directly on the boiler metal and are much harder
to condition. Silica (sand) if present in water can form exceedingly
hard scale. Suspended or dissolved iron coming in the feedwater will
also deposit on the boiler metal. Oil and other process contaminants can
form deposits as well and promote deposition of other impurities.
Sodium compounds usually do not deposit unless the water is almost
completely evaporated to dryness, as may occur in a starved tube.
Deposits are seldom composed of one constituent alone, but are generally
a mixture of various types of solid sediments, dissolved minerals,
corrosion products like rust, and other water contaminants.
Characteristics of phosphate deposit
Phosphate deposits are usually soft brown or gray deposits that can
be easily removed by normal cleaning methods. They are normally found in
boilers employing a phosphate internal treatment. They are the
‘preferred’ reaction product when using a residual phosphate
treatment on high hardness feed water. Since they are easily conditioned
with organic sludge conditioners, they are relatively nonadherent.
Calcium phosphate is usually the predominant compound in the boiler
deposit.
Characteristics of carbonate deposit
Carbonate deposits are usually granular and sometimes porous. The
crystals are relatively large and often matted together with finely
divided particles of other materials making the scale look dense and
uniform. Carbonate deposit can be easily checked by putting it in an
acid solution. Bubbles of carbon dioxide will effervesce from the scale.
Characteristics of sulfate deposit
Sulfate deposit is brittle, does not pulverize easily, and will not
effervesce when put in an acid solution. It is much harder and denser
than a carbonate deposit due to its smaller crystal structure.
Characteristics of silica deposit
Silica deposits are very hard and resemble porcelain. Their crystals
are very small, forming a dense, impervious scale. This scale is
extremely brittle, very difficult to pulverize, and not soluble in
hydrochloric acid.
Characteristics of iron deposit
Iron deposits are very dark coloured and are due to corrosion
products or iron contamination in feedwater. Iron deposits are usually
magnetic in nature. They are soluble in hot acid, giving a dark-brown
solution.
Problems caused by deposits
The major problem that deposits cause is tube failure from
overheating. This is due to the fact that the deposits act as an
insulator and excessive deposits prevent efficient heat transfer through
the tubes to the water. This causes the metal to become overheated and
over time the metal fails. These deposits can also cause plugging or
partial obstruction of boiler tubes, leading to starvation and
subsequent overheating of the tubes. Underneath the deposit layer
corrosion may also occur. Deposits cause unscheduled outages, increased
cleaning time and expenses. Boiler deposits reduce overall operating
efficiency resulting in higher fuel consumption.
Corrosion
Corrosion is basically the reversion of a metal to its ore form.
Iron, for example, reverts to iron oxide as a result of corrosion. The
process of corrosion is actually not so simple, it is a complex
electro-mechanical reaction. Corrosion may generally be over a large
metal surface but sometimes it results in pinpoint penetration of metal.
Though basic corrosion is usually due to reaction of the metal with
oxygen, other factors including stresses produce different forms of
attack. Corrosion may occur in the feedwater system as a result of low
pH water and the presence of dissolved oxygen and carbon dioxide.
Corrosion in the boiler itself normally occurs when boiler water
alkalinity is too low or too high or when the metal is exposed to
oxygen-bearing water during either operation or idle periods. High
temperatures and stresses tend to accelerate the corrosion. In the steam
& condensate system and pipelines corrosion is generally the result
of contamination with carbon dioxide and oxygen.
Corrosion Fatigue
Cracking in boiler metal may occur due to cyclic stresses created by
rapid heating and cooling. These stresses are concentrated at points
where corrosion has roughened or pitted the metal surface. This is
usually because of improper corrosion prevention. Sometimes even with
properly treated water corrosion fatigue cracking occurs. These cracks
often originate where a dense protective oxide film covers the metal
surfaces, and cracking occurs from the action of applied cyclic
stresses. Corrosion fatigue cracks are often thick, blunt, and across
the metal grains. They start at internal tube surfaces and are most
often circumferential on the tube.
Caustic embrittlement
Caustic embrittlement or cracking is a more serious type of boiler
metal failure showing up as continuous intergranular cracks. This type
of cracking occurs when the metal is stressed, water contains caustic
with a trace of silica, and some mechanism, such as a slight leak, is
present allowing the boiler water to concentrate on the stressed metal.
Caustic embrittlement is more of a problem in older boilers with riveted
drums as they cause stresses and crevices in the areas of rivets and
seams. In the newer welded drum boilers this type of cracking is less
frequent but the rolled tube ends are still vulnerable to attack. The
possibility of caustic cracking should be a consideration in water
treatment.
Other causes of boiler corrosion
Chelate residuals in excess of 20 ppm as CaCO3 or improperly applied
chelate treatment may produce boiler system corrosion. Concentration of
boiler solids at high heat input areas might also produce corrosion. The
recommendations of a water treatment consultant need to be followed to
minimize chances of such corrosion from occurring.
Corrosion problems
Uniform corrosion of boiler metal surfaces is bound to occur and is
not of much concern as all boilers experience a small amount of general
corrosion. Corrosion, however, takes many forms and deep pitting that
causes only a small amount of total iron loss causes penetration and
leakage in boiler tubes. Corrosion beneath certain types of boiler
deposits can weaken the metal and cause tube failure. Likewise corrosion
in steam condensate system can damage pipelines and equipment.
Corrosion measurement
Hydrogen gas sampling of the boiler steam is done to measure the
corrosion potential of the boiler water. This test for corrosion is
based on the release of hydrogen gas when iron corrodes. Measuring the
amount of hydrogen gas released detects boiler water conditions and
indicates if corrosion conditions exist in an operating boiler.
Basic corrosion prevention methods
The common methods for prevention of corrosion include:
- Filtration of solid suspended impurities & particles from water
- Removing dissolved oxygen from the boiler feedwater
- Maintaining alkaline conditions in the boiler water
- Keeping the boiler internal surfaces clean
- Protecting boilers during out of service periods
- Using a chemical treatment programme to counteract corrosive gases in steam and condensate systems
The selection and control of chemicals for preventing corrosion requires an understanding of the causes and corrective measures.
Boiler water carryover
Boiler water carryover is the contamination of steam with boiler water solids. Common causes of boiler water carryover are:
- Bubbles or foam form on the surface of the boiler water and leave
with the steam. This is due to high concentrations of insoluble or
soluble solids in the boiler water. Substances like alkalies, greases,
oils, fats, organic matter and suspended solids are known to cause
foaming.
- Fine droplets of water in the form of spray or mist are thrown up
into the steam space by the bursting of rapidly rising bubbles at the
steam- release surface.
- Priming is a sudden surge of boiler water caused by a rapid change
in load. It may be caused by damaged steam-separating equipment,
operation above the boiler rating, sudden fluctuations in steam demand,
or carrying too high of a water level in the steam- release area.
- Steam contamination may also occur from leakage of water through
improperly designed or installed steam-separating equipment in the
boiler drum.
Boiler carryover measurement
Steam purity can be measured with the use of a sodium ion analyzer.
It measures the sodium ion content in a cooled steam sample that will
correspond to the amount of boiler water solids contaminating the steam.
The sodium ion analyzer can detect carryover down to 1 ppb sodium in
steam.
Affect of oil on carryover
Oil contamination in boiler feedwater is usually from pumps and other
lubricated equipment. Oil can cause serious foaming due to
saponification of oil by boiler water alkalies.
Affect of suspended solids on carryover
Suspended solids tend to collect on the surface film surrounding a
steam bubble, which therefore resists breaking and builds up foam. The
finer suspended particles become the greater is their collection on the
bubble. The type as well as the quantity of suspended solids can affect
carryover. Depending on the type of suspended solids, some boilers
having high suspended solids operate without carryover, while others
have carryover with low suspended solids.
Selective silica carryover
Silica can be present in the steam as the result of general boiler
water carryover, or it can go into the steam in a volatile form. In the
later case, silica acts much like gas and is considered to be
selectively carried over. As the boiler pressures increase above 400
psi, there is an increase in the tendency for silica to be selectively
carried into the steam in amounts proportionate to the amount of silica
present in the boiler water.
Problems caused by carryover
Suspended and dissolved solids in the boiler water tend to deposit in
the steam and condensate system. Impurities carried over with the steam
cause contamination in the many processes for which steam is used,
resulting in overheating, corrosion and reduction of efficiency of the
boiler itself and other equipment.
Preventing carryover
The basic preventive measure is to maintain the concentration of
solids in the boiler water at recommended levels. High water levels,
excessive boiler loads and sudden load changes are to be avoided. Very
often contaminated condensate returned to the boiler system causes
carryover. The return condensate should be filtered to remove suspended
solids before being fed back to the boiler. Efforts should be made to
trace the source of any excessive contamination and the problem
rectified. The use of chemical antifoams is effective in controlling
carryover due to concentration of impurities in the boiler water.
Steam-separating equipment must be inspected for proper installation.
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