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Steam Turbines And
Silica
The Effects of
Silica on Steam Turbines
By Maria Kielmas
Steam turbines are machines
that convert the heat energy of steam from a water boiler into rotary motion.
Their interior consists of a
series of blades that capture the steam and provide a rotational force. As it
rotates within a magnetic field, the turbine generates electric power.
This principle constitutes
80 percent of the means of electric power generation worldwide.
The purity of steam that
passes through the turbine is crucial to its function and efficiency.
Mineral and organic
contaminants are present in the reservoir and river water that provides the
steam source. These can be silica, detergents from urban waste or salts such as
sodium chloride and sodium sulfate.
Silica
Silicon is the world’s most
abundant element after oxygen.
It does not occur as a sole
element but in compounds with oxygen, forming silicon dioxide or silica, and
iron, potassium, aluminum, magnesium and calcium.
The natural waters used in
power stations contain large amounts of dissolved silicates.
Carryover
Carryover is any contaminant
that leaves the power station boiler within the steam that flows into the
turbine.
Silica is the most common
contaminant. It volatilizes -- becomes a gas -- at the high pressures and
temperatures within the boiler.
It also forms a colloidal
solution -- a stable suspension of silica particles -- with water.
Deposits
Steam cools as it moves
through the turbine. At these lower temperatures, silica precipitates onto the
turbine blades where it accumulates as a glassy deposit. Its removal requires
chemical treatment.
Pressure Decrease
As the silica deposits
accumulate on the turbine blades, they cause a pressure drop within the turbine
itself. The deposits are of random thickness and cause balance and vibration
problems inside the turbine.
Corrosion
Corrosion is a chemical
attack that causes a loss of metal in the turbine blade. Most turbine blades
are steel.
Even high grade steels
oxidize partly at the turbine temperatures and react with silica. If left
untreated, such corrosion will rupture the turbine.
Capacity Reduction
Silica deposits on the
blades and other elements of turbine restrict steam flow to it from the boiler.
This results in a loss of output from the turbine and a reduction in the
turbine’s electricity generation capacity.
Based
in London, Maria Kielmas worked in earthquake engineering and
international petroleum exploration before entering journalism in 1986. She has
written for the "Financial Times," "Barron's,"
"Christian Science Monitor," and "Rheinischer Merkur" as
well as specialist publications on the energy and financial industries and the
European, Middle Eastern, African, Asian and Latin American regions. She has a
Bachelor of Science in physics and geology from Manchester University and a
Master of Science in marine geotechnics from the University of Wales School of
Ocean Sciences.
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