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BFW Systems - Deaerators
February 27, 2005

Introduction
Corrosion
Mechanical Deaeration

Types of Deaerators
        Tray-Type
        Atomizing-Type
Location
Accessories
Chemical Addition
Additional Links

Introduction

Deaerators serve three functions:

1. Removal of oxygen in the water
2. Heating the water
3. Storage of deaerated and heated water

Corrosion

In water, the presence of dissolved gases, particularly oxygen and carbon dioxide, causes accelerated corrosion.  The corrosion process is especially rapid at elevated temperatures such as are encountered in boilers and heat exchange equipment.  The primary function of the deaerator is to prevent this corrosion by removing the dissolved gases from all sources of water entering the boiler.  

Iron goes into solution in pure water to a slight extent according to the formula:

                Fe + 2 H2O ® Fe(OH)2 + H2

but the ferrous hydrate (Fe(OH)2) formed is alkaline and raises the pH value.  At a definite pH value further dissolving of iron is stopped.  However, if oxygen is present it immediately oxidizes the ferrous hydrate forming ferric hydrate (Fe(OH)3) which is insoluble and precipitates, permitting more iron to go into solution and thus the reaction continues until all oxygen is dissipated.  It is evident, that the removal of oxygen and carbon dioxide from solution is important.

Mechanical Deaeration

Deaeration is the mechanical removal of dissolved gases from a fluid.   The process of deaeration is most frequently applied in boiler feed water heaters to protect piping, boilers and condensate equipment from corrosion.  The three principles of mechanical deaeration are:

1. Heating
2. Mechanical Agitation
3. Gas Removal

Water must be heated to full saturation temperature (i.e. boiling point) corresponding to the steam pressure in the unit.  Theoretically, the solubility of any gas is zero at the boiling point of the liquid, complete gas removal is not possible unless the liquid is kept at the boiling temperature.

The heated water must be mechanically agitated by spraying, cascading over trays or atomization to expose maximum surface area to the scrubbing atmosphere.  This permits complete release of the gases since the distance that the gas bubble must travel for release is decreased.  Thorough agitation also overcomes tendencies of surface tension and viscosity to retain the gas bubbles and increases the rate of gas diffusion from the liquid to the surrounding atmosphere.

Adequate steam must be passed through the water to scrub out and carry away the gases after they are released from the liquid.  Extremely low partial gas pressures must be maintained to minimize the concentration of gases dissolved in the liquid.

Types of Deaerators

There are basically two types of deaerators in common use:

1. Tray-type
2
. Atomizing-type

Tray-Type

Water is heated to full saturation temperature with a minimum pressure drop and minimum vent.  This ensures the best thermal operating efficiency.  Deaeration is accomplished by spreading the heated water over multiple layers of trays designed to provide maximum spilling or weir edge, thereby giving maximum contact of liquid surface and scrubbing steam. 

Atomizing-Type

In the first stage, water is sprayed in direct contact with steam and heated practically to the saturation temperature.  At this stage the bulk of the non-condensible gases are liberated and vented from the unit.

The preheated partially deaerated water then passes to the second stage where it comes in contact with a constant high velocity steam jet for final deaeration.  The energy of the steam jet breaks up the water, producing a fine mist or fog of finely divided particles to assure maximum surface exposure to the scrubbing steam.  Any remaining gas is removed and carried to the first stage by the steam, while the deaerated water falls to the storage section.

Water is atomized by the energy of steam passing through an atomizing nozzle, and the steam both heats and strips the water of its dissolved oxygen.  This type of deaerator requires a temperature difference of at least 50oF between the water and steam.  Because of the pressure drop across the atomizer, this device is less efficient than a tray-type deaerator.

Location

Deaerators are generally elevated sufficiently to ensure that there is sufficient head available (NPSHA) for the BFW pumps to be operated without cavitation.  The height from the minimum water level in the deaerator to the pump centreline should be used in calculations of NPSHA for the BFW pumps.

Piping from the deaerator to the BFW pumps should have a suitable (i.e. large) diameter with as straight a run as possible and minimum fittings.  The piping should run vertically as far as possible before any horizontal runs are taken.

Accessories

A vortex breaker should be installed at the deaerated water outlet of the storage section.  This will help to reduce swirling of the flow in the BFW pump suction line and decrease the chance of cavitation.

An overflow on the storage section of the deaerator is required to prevent overfilling the deaerator.

The deaerator may be equipped with an internal direct contact type vent condenser which will minimize the loss of steam through venting.

A relief valve is required to protect the shell from over pressure due to a failure of the supply steam pressure regulator.

Vacuum protection is required to protect the shell from vacuum due to the sudden condensation of steam during a shutdown of the unit.   The vacuum relief device can be a swing-type check valve installed such that the valve is closed when the deaerator is operating and under pressure but will allow air to be drawn into the unit if a vacuum is created.

Chemical Addition

Chemical addition is sometimes required to the water in the deaerator after deaeration.  In high pressure steam systems (> 600 psig) deaeration to a level of 0.005 mg/l is not adequate.   In this case, an oxygen scavenger may be injected into the storage section to further treat the water and provide an added level of assurance that all oxygen has been removed.

Sodium sulfite is the most common type of chemical scavenger of oxygen.  The amount of sulfite that can be safely carried in the water decreases as the operating pressure of the steam system increases.  At higher system operating pressures, the corresponding high temperatures causes the sulfite to decompose into acidic gases that can cause increased corrosion.  The use of sulfite is limited to a maximum operating pressure of 1800 psig.   About 8 ppm of sulfite is required to remove 1 ppm of oxygen.

An alternate chemical scavenger of oxygen is hydrazine.   The decomposition and oxygen reaction products of hydrazine are volatile so they do not increase the dissolved solids content nor do they cause corrosion where steam is condensed.  The disadvantage of hydrazine is that it is a suspected carcinogen and its use may be restricted in the future.