Boiler contaminants: jeopardising power plant operation
Optimum positioning of analytical instruments at key points
throughout the water and steam cycle and water treatment plant on modern
power stations can provide operators with a valuable insight for
improving plant efficiency and cost effectiveness. Les Slocombe, of ABB
Measurement Products UK, explains the benefits of balanced boiler
chemistry and how key contaminants can be kept in check to ensure a safe
and efficient process.
Vast
quantities of water are needed to produce steam for power generation.
Achieving well-balanced water chemistry can optimise the efficiency of
steam raising and distribution.
quantities of water are needed to produce steam for power generation.
Achieving well-balanced water chemistry can optimise the efficiency of
steam raising and distribution.
The elevated
temperatures and pressures inherent in power generation applications
increase the speed of chemical reactions taking place in a boiler. The
American Society of Mechanical Engineers (ASME) advises that to control
deposition and corrosion in the boiler, plant operators should ensure
effective monitoring of makeup water, condensate, feed water and boiler
water qualities.
temperatures and pressures inherent in power generation applications
increase the speed of chemical reactions taking place in a boiler. The
American Society of Mechanical Engineers (ASME) advises that to control
deposition and corrosion in the boiler, plant operators should ensure
effective monitoring of makeup water, condensate, feed water and boiler
water qualities.
The absence of adequate monitoring and control is likely to lead to both increased costs and more frequent component failures.
However,
by measuring and monitoring the water in the boiler and the steam
distribution loop and other areas around a power plant, it is possible
to obtain a better overview of current conditions. When incorporated
into a planned preventative maintenance programme, this information can
help to reduce the risk of unplanned outages.
by measuring and monitoring the water in the boiler and the steam
distribution loop and other areas around a power plant, it is possible
to obtain a better overview of current conditions. When incorporated
into a planned preventative maintenance programme, this information can
help to reduce the risk of unplanned outages.
Why measure boiler chemistry?
A
culprit behind many boiler failures is the accumulation of scale and
corrosion brought about by contaminated water entering the boiler. Even
in a well-controlled regime, it is not possible to eliminate the
presence of potential contaminants present in boiler feedwater. For
example, in a 500 megawatt boiler, around 1,500 tons of water is boiled
off per hour. Most of the resulting contaminants present in the water
will remain in the boiler. Close monitoring and control can determine
the optimum time for boiler blow down operations to bleed off a measure
of the contaminated water. This helps to prevent precipitated scale
deposits from thermally insulating the heat surfaces, which can decrease
the rate of steam generation and reduce operating efficiency.
culprit behind many boiler failures is the accumulation of scale and
corrosion brought about by contaminated water entering the boiler. Even
in a well-controlled regime, it is not possible to eliminate the
presence of potential contaminants present in boiler feedwater. For
example, in a 500 megawatt boiler, around 1,500 tons of water is boiled
off per hour. Most of the resulting contaminants present in the water
will remain in the boiler. Close monitoring and control can determine
the optimum time for boiler blow down operations to bleed off a measure
of the contaminated water. This helps to prevent precipitated scale
deposits from thermally insulating the heat surfaces, which can decrease
the rate of steam generation and reduce operating efficiency.
Extensive
on-line chemical monitoring is now established practice in the power
industry. Online monitoring enables careful control of the water
chemistry to achieve peak efficiency and minimise down time caused by
excessive boiler corrosion or scaling.
on-line chemical monitoring is now established practice in the power
industry. Online monitoring enables careful control of the water
chemistry to achieve peak efficiency and minimise down time caused by
excessive boiler corrosion or scaling.
Controlling contamination
Online
analysis of the key parameters that can affect boiler water and steam
quality enables operators to achieve a continuous picture of conditions
in and around the steam raising and distribution loop. The following is a
breakdown of some of the key parameters that should be covered by
online monitoring.
analysis of the key parameters that can affect boiler water and steam
quality enables operators to achieve a continuous picture of conditions
in and around the steam raising and distribution loop. The following is a
breakdown of some of the key parameters that should be covered by
online monitoring.
Dissolved oxygen
Dissolved
oxygen is a major cause of corrosion in steam systems. Oxygen
contamination of steam condensate can lead to inefficient or improper
feedwater aeration, air leakage at pump seals, receivers and flanges,
leaking heat exchangers and ingress into systems that are under vacuum.
It can also promote localised pitting corrosion, which can cause rapid
failure of critical equipment in the steam system.
oxygen is a major cause of corrosion in steam systems. Oxygen
contamination of steam condensate can lead to inefficient or improper
feedwater aeration, air leakage at pump seals, receivers and flanges,
leaking heat exchangers and ingress into systems that are under vacuum.
It can also promote localised pitting corrosion, which can cause rapid
failure of critical equipment in the steam system.
One
way to control dissolved oxygen levels is by dosing boiler feedwater
with oxygen scavenging chemicals, such as hydrazine. When these
chemicals are used, operators can assess the efficiency of their dosing
regime by measuring for dissolved oxygen at the economiser or boiler
inlet, with any fluctuations able to be addressed by increasing or
reducing the dose quantities.
way to control dissolved oxygen levels is by dosing boiler feedwater
with oxygen scavenging chemicals, such as hydrazine. When these
chemicals are used, operators can assess the efficiency of their dosing
regime by measuring for dissolved oxygen at the economiser or boiler
inlet, with any fluctuations able to be addressed by increasing or
reducing the dose quantities.
The dramatic
variations in oxygen levels during the load cycle of a plant, combined
with the different levels required for different boiler chemistry
regimes, require an analyser that offers a fast response across both
high and low dissolved oxygen concentrations.
variations in oxygen levels during the load cycle of a plant, combined
with the different levels required for different boiler chemistry
regimes, require an analyser that offers a fast response across both
high and low dissolved oxygen concentrations.
Hydrazine
Hydrazine
is used to remove trace levels of dissolved oxygen in boiler feedwater,
forming nitrogen and water. At high temperatures and pressures, it will
also form ammonia, which increases the feedwater pH level, reducing the
risk of acidic corrosion. It also reacts with soft haematite layers on
the boiler tubes to create a hard protective magnetite layer, that
protects the tubes from further corrosion.
is used to remove trace levels of dissolved oxygen in boiler feedwater,
forming nitrogen and water. At high temperatures and pressures, it will
also form ammonia, which increases the feedwater pH level, reducing the
risk of acidic corrosion. It also reacts with soft haematite layers on
the boiler tubes to create a hard protective magnetite layer, that
protects the tubes from further corrosion.
Placing
a hydrazine monitor at the feedwater inlet will help check that
feedwater is being dosed with the correct amount of hydrazine.
Typically, the most effective dosage of hydrazine is 3:1 parts hydrazine
to the expected level of dissolved oxygen, which should result in a
dissolved oxygen concentration level of five parts per billion.
a hydrazine monitor at the feedwater inlet will help check that
feedwater is being dosed with the correct amount of hydrazine.
Typically, the most effective dosage of hydrazine is 3:1 parts hydrazine
to the expected level of dissolved oxygen, which should result in a
dissolved oxygen concentration level of five parts per billion.
Sodium
Sodium
is the root cause of many types of corrosion in boilers. Traditionally,
conductivity measurement was used to indicate the total dissolved
solids. However, it lacks the sensitivity to measure sodium at low
levels.
is the root cause of many types of corrosion in boilers. Traditionally,
conductivity measurement was used to indicate the total dissolved
solids. However, it lacks the sensitivity to measure sodium at low
levels.
A problem with sodium is the cycle it
undergoes during hydrolysis. During this process, sodium carbonate is
turned into sodium hydroxide, which then attacks iron in the boiler. As
iron dissolves, it forms sodium ferroate, which under hydrolysis,
regenerates into sodium hydroxide.
undergoes during hydrolysis. During this process, sodium carbonate is
turned into sodium hydroxide, which then attacks iron in the boiler. As
iron dissolves, it forms sodium ferroate, which under hydrolysis,
regenerates into sodium hydroxide.
Prolonged
exposure to this cycle will put boiler components such as bends and
joints under constant attack, causing them to become embrittled and
increasing the risk of leaks and cracks. If carried over in the steam,
sodium can also build up on critical components as the steam condenses.
exposure to this cycle will put boiler components such as bends and
joints under constant attack, causing them to become embrittled and
increasing the risk of leaks and cracks. If carried over in the steam,
sodium can also build up on critical components as the steam condenses.
To
safeguard against sodium operators should measure levels at key points
in the steam generation and distribution loops. Sample points should
include the water treatment plant, the condenser extractor pump, the
polishing plant outlet and the saturated and superheated steam
distribution loops. At the water treatment plant, monitoring for sodium
can help identify any sodium breakthrough from the cation exchange and
mixed bed outlets caused by exhaustion of the ion exchange beds.
safeguard against sodium operators should measure levels at key points
in the steam generation and distribution loops. Sample points should
include the water treatment plant, the condenser extractor pump, the
polishing plant outlet and the saturated and superheated steam
distribution loops. At the water treatment plant, monitoring for sodium
can help identify any sodium breakthrough from the cation exchange and
mixed bed outlets caused by exhaustion of the ion exchange beds.
Monitoring
for sodium also acts as a useful measure of bed efficiency as well as a
precursor measurement for potential sodium contamination further down
the line.
for sodium also acts as a useful measure of bed efficiency as well as a
precursor measurement for potential sodium contamination further down
the line.
On-line measurement of sodium after
the extraction pump provides a useful indicator of condenser leaks.
Operated under high vacuum, the condenser is prone to leaks that cause
cooling water to become mixed with the condensate.
the extraction pump provides a useful indicator of condenser leaks.
Operated under high vacuum, the condenser is prone to leaks that cause
cooling water to become mixed with the condensate.
A
key concern is the ingress of chloride and sulphate. As sodium monitors
have 10 to 100 times the sensitivity of on-line chloride measurement
techniques, measuring sodium levels provides a good way of detecting for
the presence of chloride and sulphate.
key concern is the ingress of chloride and sulphate. As sodium monitors
have 10 to 100 times the sensitivity of on-line chloride measurement
techniques, measuring sodium levels provides a good way of detecting for
the presence of chloride and sulphate.
Polishing
plants can also use sodium monitors to detect ion exchange bed
exhaustion as well as for monitoring water quality. In some power
stations, the polishing plant is incorporated into the main water
treatment plant. In high pressure boilers, any chemical contaminants
present in the steam can quickly build up in the boiler drum and can be
carried over in the steam to the turbine.
plants can also use sodium monitors to detect ion exchange bed
exhaustion as well as for monitoring water quality. In some power
stations, the polishing plant is incorporated into the main water
treatment plant. In high pressure boilers, any chemical contaminants
present in the steam can quickly build up in the boiler drum and can be
carried over in the steam to the turbine.
Monitoring
for sodium in the saturated and superheated steam distribution loops
helps to protect against corrosion and the formation of sodium salts on
the superheater or turbines caused by steam carryover. Measuring the
purity of the steam and comparing it to the measurements taken from the
saturated steam before the superheater and condensate stages, operators
can assess whether quality is being affected by issues such as
deposition of sodium salts or condenser leaks. The same measurement can
also be performed for Once-Through boilers. However, as these have no
separate superheaters, the sample is taken from the superheated steam
before the turbine.
for sodium in the saturated and superheated steam distribution loops
helps to protect against corrosion and the formation of sodium salts on
the superheater or turbines caused by steam carryover. Measuring the
purity of the steam and comparing it to the measurements taken from the
saturated steam before the superheater and condensate stages, operators
can assess whether quality is being affected by issues such as
deposition of sodium salts or condenser leaks. The same measurement can
also be performed for Once-Through boilers. However, as these have no
separate superheaters, the sample is taken from the superheated steam
before the turbine.
Silica
Silica
is the main culprit behind the build-up of dense scale inside the
boilers and turbines of power generation plants. It has a low thermal
conductivity and forms a dense scaling that cannot be removed even with
acid. A 0.5mm build-up of silica reduces thermal transfer by 28%.
is the main culprit behind the build-up of dense scale inside the
boilers and turbines of power generation plants. It has a low thermal
conductivity and forms a dense scaling that cannot be removed even with
acid. A 0.5mm build-up of silica reduces thermal transfer by 28%.
The
only way to control silica build-up is through an effective monitoring
regime. Silica should be measured at multiple points around the steam
system, including the demineralisation plant, boiler feedwater, boiler
drums, superheater and condenser outlets.
only way to control silica build-up is through an effective monitoring
regime. Silica should be measured at multiple points around the steam
system, including the demineralisation plant, boiler feedwater, boiler
drums, superheater and condenser outlets.
Measuring
silica in the steam from the boiler, either at the superheater or at the
entrance to the turbine, gives a good indicator of overall steam
purity. Provided that the silica concentration remains below 20 parts
per billion, the level of scale deposition should be minimal.
silica in the steam from the boiler, either at the superheater or at the
entrance to the turbine, gives a good indicator of overall steam
purity. Provided that the silica concentration remains below 20 parts
per billion, the level of scale deposition should be minimal.
Dissolved
silica is only very weakly ionised, so it cannot be detected using a
simple conductivity measurement but instead requires a dedicated
monitor.
silica is only very weakly ionised, so it cannot be detected using a
simple conductivity measurement but instead requires a dedicated
monitor.
Other parameters that operators may
also wish to monitor for include phosphate, ammonia and chloride, using
sensors that offer quick response times, are temperature tolerant and
require minimal maintenance.
also wish to monitor for include phosphate, ammonia and chloride, using
sensors that offer quick response times, are temperature tolerant and
require minimal maintenance.
Online monitoring
To cut the costs and maintenance effort, modern analysers for power plants should include:
• Carefully designed wet sections
• Remote management
• Automatic calibration and cleaning
• Diagnostic messaging
Any
programme aiming to maximise the efficiency of online monitoring
systems should include using instruments that can respond quickly to
changes in boiler chemistry and offer self-diagnostic capabilities where
possible.
programme aiming to maximise the efficiency of online monitoring
systems should include using instruments that can respond quickly to
changes in boiler chemistry and offer self-diagnostic capabilities where
possible.
The location of monitoring
equipment is a vital factor in ensuring the best return on investment in
a power plant. Ideally, monitoring equipment should be situated in an
environment that has less potential for damage, has easy access for
maintenance and allows for enhanced measurement accuracy.
equipment is a vital factor in ensuring the best return on investment in
a power plant. Ideally, monitoring equipment should be situated in an
environment that has less potential for damage, has easy access for
maintenance and allows for enhanced measurement accuracy.
Incorporating
digital communications technology, such as Ethernet, enables data to be
relayed to a central control room, opening up the accessibility of the
measurement data beyond the local operator.
digital communications technology, such as Ethernet, enables data to be
relayed to a central control room, opening up the accessibility of the
measurement data beyond the local operator.
Having
the ability to gauge maintenance frequency, coupled with enhanced life
cycle costs, offers a good opportunity to improve reliability of supply
and minimise unscheduled disruptions.
the ability to gauge maintenance frequency, coupled with enhanced life
cycle costs, offers a good opportunity to improve reliability of supply
and minimise unscheduled disruptions.
