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Deaerator A deaerator is a device that is widely used
for the removal of oxygen and other dissolved gases from the feedwater to steam-generating
boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion
damage in steam systems by attaching to the walls of metal piping and other metallic equipment
and forming oxides . Dissolved carbon dioxide combines with water to form carbonic acid
that causes further corrosion. Most deaerators are designed to remove oxygen down to levels
of 7 ppb by weight or less as well as essentially eliminating carbon dioxide.
There are two basic types of deaerators, the tray-type and the spray-type:
The tray-type (also called the cascade-type) includes a vertical domed deaeration section
mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler
feedwater storage tank. The spray-type consists only of a horizontal
(or vertical) cylindrical vessel which serves as both the deaeration section and the boiler
feedwater storage tank. Types of deaerators
There are many different horizontal and vertical deaerators available from a number of manufacturers,
and the actual construction details will vary from one manufacturer to another. Figures
1 and 2 are representative schematic diagrams that depict each of the two major types of
deaerators. Tray-type deaerator
The typical horizontal tray-type deaerator in Figure 1 has a vertical domed deaeration
section mounted above a horizontal boiler feedwater storage vessel. Boiler feedwater
enters the vertical deaeration section above the perforated trays and flows downward through
the perforations. Low-pressure deaeration steam enters below the perforated trays and
flows upward through the perforations. Some designs use various types of packing material,
rather than perforated trays, to provide good contact and mixing between the steam and the
boiler feed water. The steam strips the dissolved gas from the
boiler feedwater and exits via the vent at the top of the domed section. Some designs
may include a vent condenser to trap and recover any water entrained in the vented gas. The
vent line usually includes a valve and just enough steam is allowed to escape with the
vented gases to provide a small and visible telltale plume of steam.
The deaerated water flows down into the horizontal storage vessel from where it is pumped to
the steam generating boiler system. Low-pressure heating steam, which enters the horizontal
vessel through a sparger pipe in the bottom of the vessel, is provided to keep the stored
boiler feedwater warm. External insulation of the vessel is typically provided to minimize
heat loss. Spray-type deaerator
As shown in Figure 2, the typical spray-type deaerator is a horizontal vessel which has
a preheating section (E) and a deaeration section (F). The two sections are separated
by a baffle (C). Low-pressure steam enters the vessel through a sparger in the bottom
of the vessel. The boiler feedwater is sprayed into section
(E) where it is preheated by the rising steam from the sparger. The purpose of the feedwater
spray nozzle (A) and the preheat section is to heat the boiler feedwater to its saturation
temperature to facilitate stripping out the dissolved gases in the following deaeration
section. The preheated feedwater then flows into the
deaeration section (F), where it is deaerated by the steam rising from the sparger system.
The gases stripped out of the water exit via the vent at the top of the vessel. Again,
some designs may include a vent condenser to trap and recover any water entrained in
the vented gas. Also again, the vent line usually includes a valve and just enough steam
is allowed to escape with the vented gases to provide a small and visible telltale plume
of steam. The deaerated boiler feedwater is pumped from
the bottom of the vessel to the steam generating boiler system.
Deaeration steam The deaerators in the steam generating systems
of most thermal power plants use low pressure steam obtained from an extraction point in
their steam turbine system. However, the steam generators in many large industrial facilities
such as petroleum refineries may use whatever low-pressure steam is available.
Oxygen scavengers Oxygen scavenging chemicals are very often
added to the deaerated boiler feedwater to remove any last traces of oxygen that were
not removed by the deaerator. The type of chemical added depends on whether the location
uses a volatile or non-volatile water treatment program. Most lower pressure systems (less
than 650psi) use a non-volatile program. Most higher pressure systems (greater than 650psi)
and all systems where certain highly-alloyed materials are present, are now using volatile
programs as the old phosphate-based programs go away. Volatile programs are further broken
down into oxidizing or reducing programs depending whether the waterside environment requires
an oxidizing or reducing environment to reduce the incidence of flow-accelerated corrosion
(FAC) which is a highly debated topic within the industry today. FAC-related failures have
caused numerous accidents in which significant loss of property and life has occurred.
The most commonly used oxygen scavenger for lower pressure systems is sodium sulfite (Na2SO3).
It is very effective and rapidly reacts with traces of oxygen to form sodium sulfate (Na2SO4)
which is non-scaling. Another widely used oxygen scavenger properly diluted and for
locations using volatile programs is hydrazine (N2H4).
Other scavengers include 1,3-diaminourea (also known as carbohydrazide), diethylhydroxylamine
(DEHA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and hydroquinone.