Deaeration is the process of removal of oxygen, Carbon dioxide and other non condensable gases from boiler feed water thereby reducing the risk of corrosion in the pressure parts of the boiler.
Deaeration of two types namely Mechanical Deaeration and Chemical Deaeration.
Mechanical deaeration works on the principle of Henry’s law of physics. 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.
Mechanical deaeration works on the principle of Henry’s law of physics. 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.
The deaerator capacity rating shall exceed the capacity of the steam system it is servicing and have a minimum of 10 minutes storage capacity to the overflow. The deaerator shall be designed for oxygen removal to 0.005 cc/l (7 ppb) or less and carbon dioxide removal to a zero measurable level in the effluent throughout all load conditions between 0% and 100% of rated capacity.
Deaeration is the removal of dissolved or entrained gases from water to be used as boiler feed or for other processes. The gases of concern to steam plant operators are usually oxygen and carbon dioxide which are present in water due to natural cases. Oxygen and carbon dioxide present in untreated water cause corrosion of the usual boiler and steam plant materials. The rate of the corrosive action is proportional to the amount of the gas present in the feed water and is accelerated by high temperature.
Working principle of Deaerator
The primary purpose of deaeration is to remove the dissolved oxygen and carbon dioxide from water to such low levels that their corrosive potential with regard to carbon and low alloy steel is eliminated under the temperature and pressure conditions prevailing in steam generation and transport equipment. The economic value of being able to use steel, rather than higher alloy,
A deaerator is a device that is widely used for the removal of oxygen and other dissolved / non condensable gases from the feed water to steam-generating boilers. Inparticular, dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less.
Methods of Deaeration
Deaeration of water can be achieved by chemical and/or mechanical means. Various chemicals are available which react with the oxygen in water to produce chemical forms that are not harmful to the steam system. Likewise, there are chemicals that can be added to the water to react with carbon dioxide and transform it into neutral forms of the substance.
Non-chemical (mechanical) methods of deaeration remove, rather than transform, the oxygen and carbon dioxide present in the water supply. Mechanical deaeration devices function by reversing the mechanism by which the gases initially go into solution with water.
Mechanical O2 removal:
Removal of dissolved oxygen to very low limits, not exceeding 7 ppb, is possible by mechanical
deaeration only. According to common boiler standards, lower concentrations than 7 ppb are usually not relevant for operation.
Mechanical CO2 removal:
In general, mechanical deaeration can remove all free CO2 to a non-detectable level. Dependent on actual pH value of the condensate, chemically bound CO2, which cannot be completely removed by mechanical deaeration, may be present in the water. However, since the level of CO2 present in treated water seldom exceeds 5 ppm, no further discussion of CO2 removal will be addressed in this bulletin and the remainder of this paper will address only oxygen removal via the deaerator technology.
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 feed water storage tank our deaerator is a tray type deaerator.Tray type deaerators are generally considered the superior choice for most applications.These units use stainless steel for all internal surfaces which come in contact with un-deaerated water. Residence time for non deaerated water inside a tray type deaerator is longer, providing more efficient deaeration, particularly where wide load swings occur. A Large diameter hinged door affords easy access to internal trays and spray tubes for maintenance and replacement. Although tray type deaerators require a larger initial investment, the benefits in efficiency and reduced cost of maintenance tend to pay for themselves quickly
The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feed water storage tank.
Mechanical Deaeration
Mechanical Deaeration for the removal of these dissolved gases is typically utilized prior to the addition of chemical oxygen scavengers. Mechanical Deaeration can be the most economical. They operate at the boiling point of water at the pressure in the deaerator.
They can be of pressure type deaerator or vacuum type deaerator.
The pressure-type deaerator
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.
The vacuum type of deaerator
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.
Theory of Mechanical Oxygen Removal
Oxygen is soluble in water in proportion to the partial pressure of the gas that is acting when it contacts the water (Henry’s Law). The normal source of oxygen in water is the atmosphere which is 21% oxygen and contributes about 3 psi to the normal atmospheric pressure of 14.7 psi. At 60ºF, water in contact with the atmosphere will pick up about 10 ppm of O2. The solubility of oxygen in water decreases as the water temperature increases. As the water temperature increases, the amount of water vapor present in the atmosphere above the liquid also increases. This double effect means that less O2 can be held by water as its temperature is increased, theoretically being ‘zero’ when the water it reaches its boiling point (saturation).
As a consequence of these physical characteristics, oxygen can be removed from water (or better,
the amount of O2 that the water can hold can be reduced) by raising the temperature and reducing
the concentration of O2 in the atmosphere above the water
Why deaerate boiler feedwater?
There are many advantages to deaerating water prior to boiler input, but they all boil down to reduced cost operation.
- Water is heated during deaeration to near the temperature of the boiler water, thus minimizing the risk of thermal shock damage to a high-value boiler system.
- Recycling of steam from vents and flash steam from traps that would otherwise be vented to the atmosphere can result in appreciable energy savings.
- Mechanical deaeration of a feed water deaerator can cut the amount of chemical consumables (hydrazine) used for water conditioning for a continuing operating cost saving
- Removal of oxygen and carbon dioxide reduces corrosion within the boiler and piping,extending the life expectancy of the system and reducing maintenance cost.
- Higher temperature feed water reduces the chance of pressure drop within the boiler which can occur when cold water is added.
- The deaerating process removes dissolved / no condensable gases (oxygen and carbon dioxide) which tend to act as insulators inhibiting the transfer of heat within the boiler.
Corrosion in boiler systems is a major cause of equipment damage or failure, often resulting in expensive, unscheduled downtime and/or excessive maintenance.
Dissolved oxygen is the principal cause of corrosion, so removal of oxygen by deaeration is of prime importance to those who operate and maintain boiler systems. Both mechanical and chemical methods of deaeration are necessary to provide protection against oxygen corrosion in modern steam plants.
The mechanical deaeration process is done in a feedwater heater, which also removes certain amounts of other corrosive gases, such as ammonia and carbon dioxide.
In addition to dissolved oxygen, low pH is also a major contributing factor to corrosion in boiler feed lines, closed heaters, economizers, and boilers.
The main purpose of chemical deaeration is to remove any oxygen remaining after mechanical deaeration. It also provides a means of monitoring the efficiency of mechanical deaeration.
Corrosion of copper and brass equipment can occur when ammonia is present in sufficient concentration in the condensate.
With a properly designed and controlled boiler water treatment program, low pH corrosion does not normally occur in these portions of the system. In the return-condensate portion of the system, however, the combination of oxygen and low pH - due to the carbonic acid that forms when carbon dioxide is present in the steam - is more corrosive than an equivalent concentration of either gas alone.
Effectiveness and Efficiency of Deaerators
The technical efficiency and effectiveness of a deaerator is measured upon:
- 1. Economic efficiency. Additionally, the economic efficiency of a deaerator must be evaluated considering the operating and replacement costs. This is a function of maintenance cost and frequency, equipment lifetime and initial equipment price. A component of the operating costs is the cost of chemicals needed to reach the desired performance. The economic impact of ineffective mechanical deaeration can be evaluated as the incremental operating cost associated with the additional chemical feed requirements.
- Technical effectiveness of a deaerator is measured by the amount of oxygen in the feedwater at the outlet of the storage tank, compared to the amount at the inlet, showing the ability to remove dissolved oxygen. Obviously, a 7 ppb design is more effective than a 20 ppb unit.
- Thermal efficiency of a deaerator can be measured by the amount of vent steam loss. Allmodern deaerators are highly thermally efficient with the only heat loss being that which leaves the vent with the non-condensable gases and losses through the insulation, the latter not being a function of the deaerator design. With regard to the vent, the heat loss may represent a large number of heat units (Btus, kcals) but is usually a small portion of the heat input. The cost of the vent heat loss can be seen as an incremental operating cost.