Deaerators
Explained
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What are deaerators?
Deaerators are pressurised feedwater tanks also referred to as open feedwater
heaters. Deaerators are critical components of many steam systems and have several
functions:
•
Remove non-condensable gases from boiler feedwater.
•
Increase the temperature of makeup water as it enters the system.
•
Increase the temperature of condensate prior to it entering the boiler.
•
Provide a storage capacity of treated feedwater.
There are two common deaerator designs, the spray type and tray type (also known as
the spray-tray type). A variation of the spray type is the spray-scrubber type.
Deaerator
All medium to large steam systems require a deaerator to reduce the levels of dissolved
oxygen (O2) and carbon dioxide (CO2) in the boiler feedwater, both of which will cause
corrosion to boiler system components if not removed.
Deaerators achieve deaeration by raising the temperature of the feedwater, which reduces
the solubility of the non-condensable gases i.e. the gases are released from the water.
Once the dissolved gases have been released, the likelihood of corrosion is drastically
reduced and the feedwater can be fed to the boiler.
The Deaeration Process
The process of deaeration may occur mechanically or chemically. Deaerators provide
the mechanical solution whilst chemicals provide the chemical solution.
A typical deaerator will remove almost all dissolved oxygen and CO2, with the remainder
being removed by oxygen scavengers (sodium sulphite, hydrazine etc.) and CO2
scavengers (neutralising amines, bicarbonate etc.).
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Most deaerators are designed to reduce dissolved oxygen levels to 0.05 cc/l (7 ppb), with
oxygen scavengers removing the remainder.
Why remove oxygen and carbon dioxide?
Corrosion of boiler components exposed to water will occur if dissolved oxygen is present,
or, the water pH is low.
Boilers and their ancillary systems are mostly constructed of carbon steel. As steel is iron
based and oxygen reacts with iron to form red iron oxide (rust), the potential for
corrosion is high. For this reason, it is imperative that the boiler feedwater dissolved
oxygen content is as low as possible.
The amount of dissolved carbon dioxide in water dictates how acidic the water is. The
greater the dissolved CO2 in the water, the lower the water’s pH i.e. the more acidic the
water is. Low pH values will cause corrosion of boiler parts and consequently must be
avoided. A typical boiler will operate with a pH value of between 8 to 11 (approx.), but this
depends heavily upon the boiler system.
The corrosion rate is not only dependent upon the dissolved oxygen and dissolved carbon
dioxide levels, it is also dependent upon temperature. High temperatures cause high
corrosion rates, even with low amounts of dissolved gases. For this reason, low
temperature steam systems can tolerate much higher levels of dissolved oxygen and
carbon dioxide than high temperature systems.
What is corrosion?
Corrosion can be classed as general, localised, or stress.
General corrosion occurs within a single system component, or throughout the entire
system, and is usually considered light corrosion. A thin red oxide layer covering the water
side heat transfer surfaces of a boiler is an example of general corrosion. General
corrosion is often red (iron oxide) or black (magnetite oxide) in colour. If waterside metal
surfaces are red, the metal is corroding, and corrective action must be taken. Black
surfaces are desired as magnetite oxide deters further corrosion.
Localised corrosion relates to corrosion within a specific area; this type of corrosion is
usually moderate to extensive. Oxygen pitting (small holes in a metal surface caused by
corrosion) is an example of localised corrosion. Oxygen pitting often occurs wherever the
water and steam phases meet (waterline in boiler or deaerator), or under sediment that
has settled somewhere in the system.
Stress corrosion occurs in high stress point areas. High chloride levels, thermal shock and
high pH, can all cause stress corrosion. Stress corrosion caused by high pH levels is
referred to as caustic embrittlement. Stress corrosion caused by thermal shock is
referred to as fatigue corrosion.
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Deaerator System
Boilers generate steam which is delivered to the process. Some of the steam transfers its
energy to the process and condenses; the resultant water is termed condensate.
Condensate is gathered throughout the steam system and is returned to a central storage
tank, this is either an atmospheric feedwater tank, or a pressurised feedwater tank
(deaerator).
Spray Deaerator
Makeup water replaces water losses within the system. Water losses may be an
unavoidable part of the process e.g. steam cleaning of glass bottles in the edible food
industry, or, may be due to leaks or evaporation losses etc.
When condensate reaches the feedwater tank, it is termed feedwater, as it is then fed to
the boiler. Similarly, when makeup water enters the feedwater tank, it is thereafter termed
feedwater.
Systems with low condensate returns must continually add large amounts of makeup
water. Continuously adding makeup water introduces more untreated water to the system
compared to when reusing condensate that has already been treated. For these reasons, a
deaerator is much more likely to be installed on a system with low condensate returns than
one with high condensate returns. It should be noted that systems with low condensate
returns will have larger operating costs due to higher water consumption, higher
heat/energy consumption (water has to be heated before entering the boiler) and higher
chemical treatment consumption.
The solubility of dissolved gases in water reduces as the temperature of the water
increases. In order to raise the boiler feedwater temperature, low pressure steam is
supplied. The steam transfers its heat to the feedwater until the feedwater approaches its
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saturation point (boiling point). As the water approaches its saturation point, the dissolved
oxygen level approaches zero. In power plant, the steam supplied is often waste steam
from the steam turbine(s).
Gas Solubility Reduces as Temperature Increases
Dissolved gases released by the deaeration process are vented to atmosphere along with
trace amounts of steam. A typical vent will use a plate with an orifice (hole) to control the
rate at which gas is vented. If the orifice is too large, steam will be vented, which reduces
the overall plant efficiency (because of the reduction in steam cycle efficiency) and drives
up costs. If the orifice is too small, some gases may return to the feedwater, which is also
undesirable.
Chemical dosing occurs on the makeup inlet line, within the deaerator, or between the
deaerator and boiler. The chemicals needed, their quantity, and where dosing should
occur, depend upon the system design. For example, makeup water supplied from a
reverse osmosis (RO) plant will have a low pH and should be treated before it enters the
deaerator.
Deaerator Components
A deaerator is an unfired pressure vessel. Deaerators are typically manufactured from
carbon steel, although some industries -such as the pharmaceutical industry- use stainless
steel. The pressure vessel is cylindrical in shape with as few welds and penetrations as
possible.
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Attached to the deaerator are connections to the various systems it serves and other
appendages required to operate the deaerator. Common deaerator system connections
and appendages include:
•
Low pressure steam inlet.
•
Makeup water inlet.
•
Condensate inlet.
•
Feedwater outlet.
•
Safety relief valve (SRV).
•
Water column or siphon.
•
Level sight glass.
•
Level control sensor.
•
Drain line (for maintenance).
•
Overflow pipe (non-return design).
•
Chemical injection point.
•
Flanges for gauges (pressure and temperature gauges etc.).
Tray Deaerator Connections
A pressurized deaerator will operate at approximately 5 psi at 230°F (imperial), or 0.4 bar
at 105°C (metric). Deaerator feedwater will be maintained as close to the saturation
temperature as possible in order to reduce the level of dissolved gases as much as
possible, but without the water changing phase to steam. If the feedwater exceeds its
saturation temperature, it will begin to form steam and will either condense, or be vented
to atmosphere, both of which are not desired.
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How Deaerators Work
There are two common deaerator designs, spray and tray (spray-tray). Each design has
its own operating characteristics. Regardless of the design employed, both deaerators
designs:
•
Maximize the contact surface area between the water and steam to obtain a high
heat transfer rate.
•
Rely upon direct contact between the steam and water (usually tray, spray,
bubbling, or a combination of these).
•
Employ spray nozzles.
•
Utilize steam as the heat source.
•
Agitate the water using steam.
•
Can be mounted onto the top of a feedwater storage tank.
•
Are often manufactured from the same materials.
•
Are open/vented to atmosphere.
Although spray deaerators are often mounted to the top of feedwater tanks, they can also
be installed within the feedwater tank. A deaerator mounted above a feedwater tank will
appear either as a small tank connected by a pipe to the feedwater tank, or, as a dome or
vertical column, mounted directly to the feedwater tank.
Makeup water will pass through the deaerator when it enters the system. Condensate may
or may not pass through the deaerator depending upon its condition when returned to the
feedwater tank. Deaerator designs vary because each steam system has unique
requirements.
How Tray Deaerators Work
Water enters the deaerator and fills the water box. The water box is a temporary holding
area that ensures water is fed evenly through a series of spray nozzles, then into the
deaerator.
Each spray nozzle acts as a non-return valve and will close if the water box has insufficient
water pressure. To ensure long life, spray nozzles, the surrounding spray area and the
trays, are all constructed of stainless steel.
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Spray Tray Deaerator
Once the water has passed through the spray nozzles, it comes into direct contact with the
steam. The steam flows in a counter direction to the water. As the steam heats the water,
the dissolved gases are liberated. The quantity of dissolved gases present reduces as the
water cascades down through each successive tray. The upper trays are referred to as
heating trays, or first stage trays. The lower trays are referred to as deaeration trays, or
second stage trays. The water then exits the tray area and is discharged to the feedwater
tank.
Dissolved gases and some steam are constantly discharged through the vent. A typical
deaerator will vent between 5% to 15% of the steam that passes through the deaerator. As
steam costs money to generate, it is beneficial to vent as little steam as possible.
Approximately 90% to 95% of deaeration occurs within the spray area with much of the
remainder occurring in the tray area. Mechanically deaerated water is usually designed to
lower the oxygen content to 7 parts per billion (ppb). Any remaining oxygen in the
feedwater is stripped using oxygen scavenging chemicals (sodium sulphite, hydrazine
etc.).
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How Spray Deaerators Work
Condensate and makeup water enter the water box directly above the spray nozzles; the
spray nozzles are spring loaded. Water pressure causes the nozzles to open and water is
sprayed into the deaerator. Spraying the water into the deaerator ensures a large contact
surface area between the water and steam, which ensures good heat transfer between the
two fluids (fluids are defined as either liquid or gas).
Spray Scrubber Deaerator
Water exits the nozzles and passes through a large perforated circular tray before joining
the water in the feedwater tank. A submerged steam sparge/sparger pipe distributes
steam to the preheating and deaeration sections of the deaerator. The steam heats the
water to within 2°C (approx. 4°F) of its saturation temperature to ensure as many
condensable gases are liberated from the water as possible.
The heated water then passes around a baffle plate to reach the deaeration section, and
is discharged as heated, deaerated, feedwater.
How Spray Scrubber Deaerators Work
Spray scrubbers function in a similar manner to spray deaerators, but they have a
scrubber installed. Water enters a water box, is sprayed through spray nozzles, then drains
through a tray and is directed to a scrubber.
Scrubbers utilise steam to agitate (using steam bubbles) and heat water after it leaves the
spray area of the deaerator. Close contact with the steam ensures good heat transfer and
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rapid liberation of dissolved gases. The deaerated water then collects in the feedwater tank
and is ready for use as boiler feedwater.
Spray Scrubber Deaerator
Design Considerations
The type of deaerator chosen for a steam system is heavily dependent upon the system.
Some systems are effectively closed-loop systems which require very low amounts of
makeup water (1-3%); other systems require large amounts of makeup water. The
temperature of the returned condensate is also a factor that must be accounted for.
Steam System Comparison
A power plant steam system providing steam to steam turbines has the following
characteristics:
•
Operates within a closed system and so requires little makeup water.
•
Is not exposed to atmosphere, so there is little chance of gas entrainment.
•
Returns condensate at a temperature near the saturation point of the water and
thus contains low amounts of oxygen and carbon dioxide.
A paper pulp plant steam system has the following characteristics:
•
Operates within an open system with condensate returns typically less than 50%.
Consequently, makeup water requirements for the system are typically 50% or
more.
•
Is exposed to atmosphere, so gases will become entrained in the condensate.
•
The reduction in pressure and temperature is large, which increases the solubility of
gases in the water.
Deaerator Entry Point