1.3 Deaeration

 

 Deaeration

Why Deaeration

Corrosion of iron or steel in boilers or boilers feed water piping is caused by three fundamental factors:

1. Feed water temperature

2. Feed water ph value

3. Feed water oxygen content

Temperature and ph value influence the aggressiveness of corrosion. The higher the temperature, and the lower the pH value the increased aggressiveness of the feed water. The dissolved oxygen content of the feed water is a large factor in determining the amount of corrosion that will take place. The presence of oxygen, and other non-condensable gases, in the feed water is a major cause of corrosion in the feed water piping, boiler, and condensate handling equipment.

In de-aeration, dissolved gases, such as oxygen and carbon dioxide, are expelled by preheating the feed water before it enters the boiler.

All natural waters contain dissolved gases in solution. Certain gases, such as carbon dioxide and oxygen, greatly increase corrosion. When heated in boiler systems, carbon dioxide (CO₂) and oxygen (O₂) are released as gases and combine with water (H₂O) to form carbonic acid, (H₂CO₃).

Removal of oxygen, carbon dioxide and other non-condensable gases from boiler feed- water is vital to boiler equipment longevity as well as safety of operation. Carbonic acid corrodes metal reducing the life of equipment and piping. It also dissolves iron (Fe) which when returned to the boiler precipitates and causes scaling on the boiler and tubes. This scale not only contributes to reducing the life of the equipment but also increases the amount of energy needed to achieve heat transfer.

De-aeration can be done by mechanical de-aeration, by chemical de-aeration or by both together.

 

Mechanical de-aeration

Mechanical de-aeration for the removal of these dissolved gases is typically utilized prior to the addition of chemical oxygen scavengers. Mechanical de-aeration is based on Charles’ and Henry’s laws of physics. Simplified, these laws state that removal of oxygen and carbon dioxide can be accomplished by heating the boiler feed water, which reduces the concentration of oxy- gen and carbon dioxide in the atmosphere surrounding the feed water. Mechanical de-aeration can be the most economical. They operate at the boiling point of water at the pressure in the de- aerator. They can be of vacuum or pressure type.

The vacuum type of de-aerator operates below atmospheric pressure, at about 82 °C, can reduce the oxygen content in water to less than 0.02 mg/liter. Vacuum pumps or steam ejectors are required to maintain the vacuum.

The pressure-type de-aerators operates by allowing steam into the feed water through a pressure control valve to maintain the desired operating pressure, and hence temperature at a minimum of 105 °C. The steam raises the water temperature causing the release of O₂ and CO₂ gases that are then vented from the system. This type can reduce the oxygen content to 0.005 mg/liter.

Where excess low-pressure steam is available, the operating pressure can be selected to make use of this steam and hence improve fuel economy. In boiler systems, steam is preferred for de-aeration because:

–                      Steam is essentially free from O₂ and CO₂,

–                      Steam is readily available

–                      Steam adds the heat required to complete the reaction.

 Chemical de-aeration

While the most efficient mechanical deaerators reduce oxygen to very   low   levels (0.005 mg/liter), even trace amounts of oxygen may cause corrosion damage to a system. Consequently, good operating practice requires removal of that trace oxygen with a chemical oxygen scavenger such as sodium sulfite or hydrazine. Sodium sulphite reacts with oxygen to form sodium sulphate, which increases the TDS in the boiler water and hence increases the blow down requirements and make-up water quality. Hydrazine reacts with oxygen to form nitrogen and water. It is invariably used in high pressures boilers when low boiler water solids are necessary, as it does not increase the TDS of the boiler water.

Deaerator Principles

Deaeration is the mechanical removal of dissolved gases from the boiler feed water. There are three principles that must be met in the design of any deaerator.

1.     The incoming feed water must be heated to the full saturation temperature, corresponding to the steam pressure maintained inside the deaerator. This will lower the solubility of the dissolved gases to zero.

2.     The heated feed water must be mechanically agitated. This is accomplished in a tray deaerator by first spraying the water in a thin film into a steam atmosphere. Creating a thin film reduces the distance the gas bubble has to travel to be released from the water. Next, the water is cascaded over a bank of slotted trays, further reducing the surface tension of the water. This allows for the removal of any gases not liberated by the initial spraying.

3.     Adequate steam supply must be passed through the water, in both the spray section and the tray section to sweep out the gases from the water.

Deaerator Classification:

1. According to working pressure -

i) Vacuum deaerator - 0.116 MN/m2 - 1.18 Kg/cm2

ii) Atmospheric deaerator  - 0.12 to 0.17 MN/m2 - 1.22 to 1.73 kg/cm2.

iii) High pressure deaerator - 0.17 to 0.7 MN/m2 - 1.73 to 7.13 Kg/cm2.

2. According to mode of steam water distribution  -

i) Atomising deaerator 

ii) Tray type or jet tray deaerator ( parallel flow or counter flow tray deaerator)

iii) Film type deaerator 

Type of Atmospheric deaerator - 

I) Direct contact (mixing) deaerator 

II) Overheated water deaerator 

Deaerator Designs

Over the years, various types of deaerators have been developed. They are the counter-flow tray type, the atomizer type, the packed tower type, and the parallel down flow type, to name a few.

Advantages	Disadvantages	Conclusion
Parallel down flow deaerators
1. Time proven design 2. Thousands of installations worldwide 3. Design suitable for small to medium   size plants 4. Can meet outlet guarantees at varying plant conditions. 5. High tray loading, resulting in higher outlet capacity for any given diameter. 6. Large tray spilling edge, resulting in high deaerating efficiency 7. Low vent rate, resulting in increased operating efficiency.	1. More complicated design, resulting in slightly higher cost.
Counterflow deaerators
1. The counter flow deaerator is cheaper to manufacture	1. Inability to deliver 0.007 ppb outlet quality in applications with a low inlet water temperature, or when 100% make-up is required. 2. Low tray loading. This reduces the flow rating for a given diameter deaerator vs. a parallel down flow unit. 3. High vent rate. This reduces operating efficiency
Atomizer deaerators
1. Low cost 2. Low overall height	1. Inability to deliver 7 ppb outlet quality when plant conditions vary from design specifications. Requires constant plant conditions. 2. Failure rate of the atomizer valve, and maintenance required to keep it operating properly.
Packed tower
1. Low cost 2. Low maintenance 3. Ability to handle varying plant conditions	1. Height requirement 2. Typically considered for small size plants.

 

Description of  Deaerator Operation

• Parallel downflow

In this design, the inlet water is sprayed into a steam atmosphere through variable orifice, spring loaded spray nozzle(s). This action heats the water to within 2 to 3 degrees of the steam temperature, while liberating 90% to 95% of the dissolved gases.

This pre-heated, partially departed water then flows down through a water seal(s) for distribution over the tray bank. The water seal(s) serve two functions. First they prevent gases liberated in the initiate heating, from entering the tray bank. Second they direct the steam to flow down through the trays, before entering the upper heating section.

The main function of the tray bank is to remove the remaining amounts of dissolved gases, not liberated in the initial heating. Since very little, or no heating takes place in the trays, the entire volume of steam is used to scrub out the remaining gases. The trays are slotted, and provide a great amount of spilling edge. This allows for a great amount of water surface area to be exposed to the steam. Water flows downward through the trays. Steam flows downward through the trays. Thus the name parallel downflow.

The steam, after exiting the tray bank, then flows upward into the top of the deaerator where it is used to heat the incoming water being discharged by the spray nozzles. The steam is condensed by the colder inlet water, and a small amount is vented to atmosphere, along with the dissolved gases.

The deaerated water flows from the deaerator down into the storage tank. The stored water is covered by a steam blanket, to maintain heat, pressure, and prevent recontamination of the deaerated water.

 

 • Packed tower

In this proprietary design, the inlet water is sprayed into a steam atmosphere through a variable orifice, spring loaded spray nozzle. This action heats the water to within 2 to 3 degrees of the steam temperature, while liberating 90% to 95% of the dissolved gases.

The heated water flows down onto a distribution plate. This plate evenly distributes the water over the entire cross-sectional area of the tower packing.

As the water flows down through the distribution plate it enters a steam chest area, where the water is further heated by up flowing steam, and more of the dissolved gases are liberated.

Any remaining dissolved gases are removed when the water flows down from the steam chest and then down through the packing tower. The packing tower function is to expose a greater surface area of the water; while up flowing steam completes the deaeration process