The development of modern, high-efficiency steam turbines has
led to an increase in deposition, erosion, and corrosion problems. Close
tolerances in the turbines, the use of high-strength steels, and impure steam
all contribute to these conditions.
Although several factors influence the formation of deposits
on turbine components, the general effect is the same no matter what the cause.
Adherent deposits form in the steam passage, distorting the original shape of
turbine nozzles and blades. These deposits, often rough or uneven at the
surface, increase resistance to the flow of steam. Distortion of steam passages
alters steam velocities and pressure drops, reducing the capacity and efficiency
of the turbine. Where conditions are severe, deposits can cause excessive rotor
thrust. Uneven deposition can unbalance the turbine rotor, causing vibration
As deposits accumulate on turbine blades, stage pressures
increase. Figure 18-1 shows the effect of gradual deposit buildup on stage
pressure. The deposits were caused by the use of contaminated water to
attemperate the steam. In a fouled condition, this 30-MW turbine lost over 5% of
its generating capacity.
Turbine deposits can accumulate in a very short time when steam purity
is poor. The turbine shown in Figure 18-2 was forced
off-line by deposition only 3 months after it was placed in operation.
Carryover of boiler water, resulting from inadequate steam-water separation
equipment in the boiler, caused this turbine deposit problem.
The nature of silica deposits found on turbine blades varies
greatly. Table 18-1 lists a number of silica compounds that have been identified
in various studies of turbine blade deposition. Of these, amorphous silica (SiO2)
is the most prevalent.
Table 18-1. Silicate deposits found in steam turbines.
||sodium metasilicate pentahydrate
||sodium metasilicate nonahydrate
||sodium aluminum silicate
||sodium aluminum silicate hydroxide
||sodium aluminum sulfate silicate
||sodium iron silicate
||sodium chlorohexaaluminum silicate
||potassium aluminum silicate
||potassium trisodium aluminum silicate
||magnesium octahydride silicate
||magnesium silicate hydrate
||calcium aluminum silicate hydroxide
|3Al2O3 4Na2O 6SiO2
||iron magnesium hydroxide silicate
||sodium aluminum molybdenum oxide silicate
Causes of Turbine Deposition
Some mechanical entrainment of minute drops of boiler water in
the steam always occurs. When this boiler water carryover is excessive,
steam-carried solids produce turbine blade deposits. The accumulations have a
composition similar to that of the dissolved solids in the boiler water. Priming
and foaming are common causes of high levels of boiler water carryover. Because
of the high levels of carryover often encountered, these conditions often lead
to superheater tube failures as well.
Attemperating Water Impurity.
Turbine deposits are also caused by the use of impure water for steam
attemperation and by leakage in closed heat exchangers used for attemperation.
If a boiler produces pure steam and turbine deposits still occur, the
attemperating system should be investigated as a possible source of
contamination. Attemperating water should be of the same purity as the steam.
Any chemical treatment in the attemperating water should be volatile.
Vaporization of Boiler Water Salts.
source of turbine deposition is the vaporization of salts present in boiler
water. With the exception of silica, vaporization of boiler water salts is
usually not significant at pressures below 2400 psig. Silica can vaporize into
the steam at operating pressures as low as 400 psig. This has caused deposition
problems in numerous turbines. The solubility of silica in steam increases with
increased temperature; therefore, silica becomes more soluble as steam is
superheated. As steam is cooled by expansion through the turbine, silica
solubility is reduced and deposits are formed, usually where the steam
temperature is below that of the boiler water. To minimize this problem, the
quantity of silica in the steam must be controlled. Silica deposits are not a
problem in most turbines where the silica content in the steam is below 0.02 ppm.
Therefore, it has become customary to limit silica to less than 0.02 ppm in the
steam. Sometimes, because of the more stringent operating conditions of certain
turbines, vendors specify that steam silica be maintained at less than 0.01 ppm.
The conditions under which
vaporous silica carryover occurs
have been thoroughly investigated and documented. Researchers have found that
for any given set of boiler conditions using demineralized or evaporated quality
makeup water, silica is distributed between the boiler water and the steam in a
definite ratio. This ratio, called the distribution ratio, depends on two
factors: boiler pressure and boiler water pH. The value of the ratio increases
almost logarithmically with increasing pressure and decreases with increasing
pH. The effect of boiler water pH on the silica distribution ratio becomes
greater at higher pH values. A pH increase from 11.3 to 12.1 reduces the ratio
by 50%, while a pH increase from 7.8 to 9.0 has no measurable effect. For any
boiler pressure and pH, the distribution ratio for silica can be determined from
Figure 18-3. The amount of silica vaporized with the steam can be determined by
measurement of boiler water silica. The proper boiler water silica level
necessary to maintain less than 0.02 ppm silica in the steam is shown in Figure
When soluble, the silica present in boiler feedwater does not
influence the amount of silica present in the steam. When added to boiler water
in separate experiments, equivalent quantities of silicic acid and sodium
silicate produced the same amount of silica in the steam. Because the amount of
silica in the steam is greatly affected by pH, it is likely that silicic acids
are involved in the vaporization mechanism.
Silica has a higher solubility in superheated steam than in
saturated steam for any given pressure. If mechanical carryover contributes to
the silica content of the saturated steam, the silica will be dissolved during
superheating, provided that the total silica present does not exceed the
solubility of silica in the superheated steam. Therefore, silica deposits are
seldom found in superheater sections of a boiler.
After steam reaches a turbine it expands, losing pressure and temperature.
As a result, the solubility of the silica decreases. Studies have shown
that with a maximum of 0.02 ppm of silica in the steam, a pressure of
less than 200 psig is reached in the turbine before silica starts to
condense from the steam. Therefore, silica preferentially deposits in
the intermediate-pressure and low-pressure sections of the turbine where
the specific volume of the steam varies from approximately 1 to 10 ft3/lb.
Solubility data shown
in Figure 18-5 helps to explain the distribution of silica deposits
in the turbine.
Localized Silica Saturation.
Turbine deposits are also formed where localized silica saturation occurs and
silica condenses from the steam in those areas of the turbine. Partial
evaporation of the precipitated silica can then occur with only a portion of the
silica being dissolved by the continuous steam flow. Deposits remain as a
Turbine Velocity. Another factor affecting the location of turbine deposits is
the velocity in the turbine. Steam flows from the inlet to the outlet of the
turbine in only a fraction of a second. Consequently, deposition is shifted
downstream from the saturation point by the high steam velocities.
Prevention of Silica Deposits
The most significant factor in minimizing turbine silica
deposits is the maintenance of low silica concentrations in the boiler water.
External treatment equipment must be operated carefully to limit the amount of
silica entering with the makeup water, and the condensate must be monitored to
minimize contamination. After silica enters the boiler water, the usual
corrective action is to increase boiler blowdown (to decrease the boiler water
silica to acceptable levels) and then to correct the condition that caused the
Removal of Deposits
When a turbine becomes fouled with water-soluble salts as a
result of boiler water carryover or attemperating water contamination, turbine
capacity can often be restored by water washing. Because it can cause severe
turbine damage, water washing should be supervised carefully and the
recommendations of the turbine vendor should be followed.
When the turbine becomes fouled with compounds that are not
water-soluble (including silica), water washing rarely restores capacity.
Out-of-service cleaning by blasting with aluminum oxide or other soft grit
material is required to remove these deposits.
Erosion of turbine blades results in rough, uneven surfaces
that alter steam flow paths. This reduces turbine efficiency and can also limit
capacity. Erosion at the high-pressure end of a turbine is usually caused by
solid particles (usually iron oxide) present in the steam. Iron oxide particles
are present if they were not removed by steam blows during system start-up. They
can also result from exfoliation of superheater or main steam header oxides or
can be introduced into the steam by contaminated attemperating water.
Erosion of intermediate and low-pressure blades is usually
caused by water in the steam. Operation below design inlet steam temperature or
at low load can cause condensation in these stages, leading to erosion problems.
Carbon dioxide or other acidic species present in the
condensate can accelerate the damage. Some protection against erosion-corrosion
can be provided by low distribution ratio amines, which neutralize the acidity
and elevate the pH of the condensate.
Pitting, corrosion fatigue, and stress corrosion cracking
problems all occur in steam turbines. The major corrodents are sodium hydroxide,
chloride, sulfate, and sulfides. Usually, the level of contaminants present in
steam is not high enough to corrode the system components. As steam expands
through a turbine, the solubility of contaminants in the steam decreases. They
condense onto surfaces at solution concentrations much higher than the original
contaminant concentration in the steam. These concentrated solutions promote
Pitting is commonly associated with chloride deposits and
occurs on rotors, disks, and buckets. Pitting attack often occurs when a moist,
oxygen-laden atmosphere develops in out-of-service turbines. Damage is most
severe when chloride deposits are also present. An oxygen-free or
condensate-free atmosphere should be maintained to protect out-of-service
turbines from corrosion.
Corrosion fatigue and stress corrosion
cracking of blades and disks are commonly associated with sulfides (see
Figure 18-6), chlorides, and caustic. The problems are most
common in low-pressure sections of large power plant turbines, which
are characterized by high stresses, crevices, and operating temperatures
conducive to the condensation of concentrated solutions of steam contaminants.
Problems also occur in high-pressure sections and smaller industrial-sized
turbines, usually when substantial levels of steam contamination occur.
These problems can be mitigated by designs that prevent crevices, lower
stresses, and/or employ lower-strength materials. It is also important
to avoid unnecessary stresses and to maintain high-purity steam during