What is “Deaerator”? What does it do? How does it work? Where do we commonly use deaerators? what is the function of “Deaerator” in steam generation systems?
In the following article I am going to bring a brief and clear answer to all above questions and provide a short review on deaerators.
What is Deaeration Process?
The process through which the dissolved gases are removed from water is called “Deaeration”. The equipment, which is used in this method, is called “Deaerator”.
Why do we use deaerators?
Deaeration process is generally used to control corrosion in specific systems. The presence of dissolved gases, particularly oxygen and carbon dioxide, causes accelerated corrosion. The dissolved carbon dioxide in water forms a weak carbonic acid which attacks the metal in feed systems, boiler or condensate system. One of the most serious aspects of oxygen corrosion is that it occurs as pitting. The degree of oxygen attack depends on the concentration of dissolved oxygen, the pH and the temperature of the water. Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen can cause serious corrosion.
” The primary function of the deaerator is to prevent corrosion by removing the dissolved corrosive gases from all sources of water entering the downstream system such as steam generating boilers, condensate lines, etc.”
Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can also enter the feed water system from the condensate return system. Possible return line sources are direct air-leakage on the suction side of pumps, systems under vacuum, the breathing action of closed condensate receiving tanks, open condensate receiving tanks and leakage of non-deaerated water used for condensate pump seal and/or quench water. With all of these sources, good housekeeping is an essential part of the preventive program.
One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type of corrosion can produce failures even though only a relatively small amount of metal has been lost and the overall corrosion rate is relatively low. The degree of oxygen attack depends on the concentration of dissolved oxygen, the pH and the temperature of the water.
The corrosion process is especially rapid at elevated temperatures such as are encountered in boilers and heat exchange equipment. The influence of temperature on the corrosivity of dissolved oxygen is particularly important in closed heaters and economizers where the water temperature increases rapidly. Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen can cause serious corrosion.
The primary function of the deaeration and deaerator is to prevent this corrosion by removing the dissolved gases from all sources of water entering the downstream system such as lines, piping, especially boilers, condensate lines, etc…
Because boiler systems are constructed primarily of carbon steel and the heat transfer medium is water, the potential for corrosion is high. Iron is carried into the boiler in various forms of chemical composition and physical state. Most of the iron found in the boiler enters as iron oxide or hydroxide. Any soluble iron in the feed water is converted to the insoluble hydroxide when exposed to the high alkalinity and temperature in 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.
How effective a gas can be removed from liquid depends upon these factors:
Following are the major factors which govern a gas removal from a liquid.
1. Ionization
2. Relative partial pressure of the gas on the surface of the liquid and the inside the water.
3. The water temperature.
4. Agitation.
5. Gas removal (Venting).
Ionization:-
Oxygen does not ionize and is easily removed. Carbon dioxide and ammonia do ionize. Only that portion that remain in molecular form, non ionized, can be easily removed.
Therefore, Carbon Dioxide can more easily be removed at low pHs and ammonia at high alkaline pH’s.
Relative Partial Pressure:-
Liquid will dissolve the free gas only to the point at which pressure equilibrium is reached between the partial pressure of the gas in the water and the partial pressure of the gas in the atmosphere in contact.
Since air contains 21% oxygen and 79% nitrogen. The partial pressure of the oxygen in the air is about 1/5 atmosphere. Water surrounded by air will dissolve oxygen until the oxygen in the water exerts a partial pressure of 1/5 atmosphere.
In degasification, the equilibrium between oxygen in the water and the oxygen in the atmosphere is deliberately upset.
Temperature:
As the temperature of the water raised to its oiling point, all the free gases theoretically become insoluble. Practically, total removal of gases does not occur. This is because, if water in the vapor phase is in contact with water in the liquid state, the liquid will be slightly cooler than the vapor and some gas will remain dissolved in the water phase.
Water must be heated to an optimum temperature for minimizing the solubility of the undesirable gases and providing the highest temperature water for injection to the boiler. The higher the water temperature, the lower the gas content will be in the water.
In the deaerator, steam is used for scrubbing gases from the water and for the water heating purpose. 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.
Water must be heated close to 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. But this must be kept in mind that increasing the water to its saturation point will increase the water losses which is uneconomical.
Typical Performance Levels:
Following are the performance levels of the miscellaneous Degasifying Equipments.
Operating Ranges mg/lit Remaining
Type Pressure (psig) Temp (C) O2 CO2
Blower –type Atmosphere 99o 0.5-1.0 5-10
(Open heater)
Deaerating Heaters 20-25 125 0.04 ——
Deaerators 20-25 125 0.007 0
Ammonia removal is the function of both pH and temperature. For example,
pH 9.0 @ 25o C = 35 % ammonia
pH 9.0 @ 38o C = 18 % ammonia
pH 8.5 @ 25o C = 18 % ammonia
pH 8.5 @ 38o C = 7.5 % ammonia
Agitation:
The heated water must be mechanically agitated 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.
Some methods of agitation are:
1. Spraying.
2. Cascading over trays.
3. Atomization.
etc…
Deaerator Functions:
Deaerators serve following basic functions.
- Heating the water, Closed to the saturated steam temperature.
2. Mechanical Agitation to vigorously scrub the heated water with fresh oxygen free steam.
3. By maintaining the partial pressure of oxygen and carbon dioxide at the lowest possible level at the point where steam contacts water for Removal of oxygen and other non-condensable gases in the water.
4. Continuously vent these noncondensible gases from the deaerator.
5. Storage of deaerated and heated water.
However, after the efficient deaeration chemical treatment is still needed to remove last traces of oxygen.
Types of Deaerator:
Deaerator equipment is designed to reduce oxygen in the boiler feed water. This is done at times by heating the water to the temperature closed to the saturated temperature at the pressure within the deaerating equipment. A deaerator, generally, reduces the oxygen level to 7ppb. There are basically two types of deaerators in common use:
1. Tray-type
2. Atomizing-type
Tray-type Deaerator:
Water is heated close to 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.
Tray-type deaerating heaters release dissolved gases in the incoming water by reducing it to a fine spray as it cascades over several rows of trays. The steam that makes intimate contact with the water then scrubs the dissolved gases by its counter-current flow. The steam heats the water to within 3-5 º F of the steam saturation temperature and it should remove all but the very last traces of oxygen. The deaerated water then falls to the storage space below, where a steam blanket protects it from recontamination.
Atomizing-type Deaerator:
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.
Spray-type deaerating heaters work on the same general philosophy as the tray-type, but differ in their operation. 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-condensable gases are liberated and vented from the unit. Spring-loaded nozzles located in the top of the unit spray the water into a steam atmosphere that heats it. Simply stated, the steam heats the water, and at the elevated temperature the solubility of oxygen is extremely low and most of the dissolved gases are removed from the system by venting. The spray will reduce the dissolved oxygen content to 20-50 ppb.
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. A tray section may also be provided at the end for the final removal of the gases. Trays further reduce the oxygen content to approximately 7 ppb or less.
During normal operation, the vent valve must be open to maintain a continuous plume of vented vapors and steam at least 18 inches long. If this valve is throttled too much, air and non-condensable gases will accumulate in the deaerator. This is known as air blanketing and can be remedied by increasing the vent rate.
For optimum oxygen removal, the water in the storage section must be heated to within 5 º F of the temperature of the steam at saturation conditions. From inlet to outlet, the water is deaerated in less than 10 seconds.
Storage Section:
The storage section is usually designed to hold enough water for 10 minutes of boiler operation at full load.
Limitations:
Inlet water should be virtually free of suspended solids that could clog spray valves and ports of the inlet distributor and the deaerator trays. In addition, spray valves, ports and deaerator trays may become plugged with scale that forms when the water being deaerated has high hardness and alkalinity levels. In this case, routine cleaning and inspection of the deaerator is very important.
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 typical deaerator with instrumentation is shown in Figure 4.2.
- 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:
Remember the deaerator reduces the oxygen content of the water to 7ppb. If the boiler is rated at 50, 000 PPH then it mean it will produce 1.2 million lbs. per day of steam. At oxygen concentration of 7ppb, which is in turn will be the 0.0084 lbs. of oxygen per day or roughly 03 lbs. per year.
The formula for the reaction of iron and oxygen is
4Fe + 3O2 → 2Fe2 O3
Based on the molecular weights, 1 lb of oxygen will react 2.4 lbs. of iron. Therefore, 3 lbs. of oxygen will react with 7 lbs. of iron. This is a volume of approximately 25 cubic inch of iron. If the average pit is 3/16” in diameter and 3/16” deep, the volume of pit would be 0.002 cu. in. Theoretically 25 cu. in. divided by 0.002 equals 12,626 pits per year from this small amount of oxygen. In fact, the pits would be large enough to result in failure of boiler tubes long before the end of the year.
The foregoing discussion shows the importance of proper deaeration of boiler feed water in order to prevent oxygen corrosion. Complete oxygen removal cannot be attained by mechanical deaeration alone. Equipment manufacturers’ state that a properly operated deaerating heater can mechanically reduce the dissolved oxygen concentrations in the feed water to 0.005 cc per liter (7 ppb) and ‘zero’ free carbon dioxide. Typically, plant oxygen levels vary from 3 to 50 ppb. Traces of dissolved oxygen remaining in the feed water can then be chemically removed with the oxygen scavenger.
So, Chemical addition is 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.
Oxygen scavengers:
The chemical oxygen scavenging operation is the last opportunity to remove oxygen. Any oxygen not removed in this operation will enter the boiler system and may cause corrosion problems.
The oxygen scavenger should be fed into system as early as possible to protect the system as much of the system as possible. The typical point of addition for oxygen scavenger is the storage section of the deaerator.
To prevent chemical loss, the chemical feed line to a deaerator storage tank should be into an area of good mixing. Oxygen scavenger should be fed continuously.
Types of Oxygen Scavenger:
The most common oxygen scavenger used in boiler feed water treatment are Sodium Sulfite and Hydrazine. Each has its own set of advantages and disadvantages and must be considered.
Sodium Sulfite (Na2SO3):
Sodium sulfite is the most common type of chemical scavenger of oxygen in low pressure systems. The amount of sulfite that can be safely carried in the water decreases as the operating pressure of the steam system increases. The oxygen scavenging property of sodium sulfite is shown by the following reaction.
Na2SO3 + O2 → 2Na2SO4
At higher system operating pressures, the corresponding high temperature 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.
Here are some advantages and disadvantages of sodium sulfite.
Advantages Disadvantages
- Ease of handling. 1. Adds solids to the boiler water
2. Ease of application. 2. Can decompose to form acidic gases.
3. Simplicity of testing 3. Contribute sodium ions that are undesirable.
4. Relatively low cost. 4. Can not be used in steam desuperheating.
5. Rapid reaction at low pressure. 5. Speed of reaction pH dependant.
6. Can be used in food plants. 6. Reduce oxide deposits only above 221o C.
Hydrazine:
An alternate chemical scavenger of oxygen is hydrazine. Hydrazine removes dissolved oxygen by the following reaction.
N2H4 + O2 → 2H2O + N2 ↑
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 hydrazine decomposition starts at about 200o C according to the reaction.
3N2H4 → 4NH3 + N2
More than 90% hydrazine is decomposed at about 316o C within one minute.
A residual concentration of 0.125 – 0.150 ppm is maintained in boiler feed water. The disadvantage of hydrazine is that it is a suspected carcinogen and its use is restricted.
Here are some advantages and disadvantages of Hydrazine.
Advantages Disadvantages
- Does not add solids to the boiler water. 1. Strong alkaline solution requiring
protective clothing and handling precautions.
2. Promotes formation of passive oxide films. 2. Decomposes to ammonia above 260o C.
3. Reduce oxide deposits above 120o C 3. Slow direct scavenging of oxygen at
low temperature.
4. Can be used in steam desuperheating. 4. Can not be used in food plants.
5. Does not add sodium ions. 5. Carcinogenic
Magnetite Layer:
Metal passivation has traditionally been considered to be the reduction of hematite (Fe2O3) to magnetite (Fe3O4) in iron based boiler. Actually, it is a process by which bare metal surfaces form protective oxide film preventing further corrosion, the passive film is very thin and dense. It is distinguishable from the base metal by the coloration. In case of carbon steel, this protective layer is magnetite (Fe3O4) and black in color. Before developing magnetite it is recommended that surface should be cleaned with respect to any deposit, oil and grease.
If the oxide layer is not uniform, porous and disturbed by deposits of any kind, the corrosion process will continue with dynamic process of hot water corrosion in areas of high heat transfer, with subsequent failure due to leakage.
At the boiling point of water, steel react with water/oxygen in a 2-stage process to form magnetite. The presence of any dissolved oxygen in the water affects the quality of magnetite, and leads to the formation of hematite. This causes an uneven coating of various iron oxides and allows increase corrosion of the steel surface.
When starting up, the H2 content may be 20-30 ppb and drops down to approx. 2-5 ppb when the protective magnetite layer is formed.
3Fe + 4H2O → Fe3O4 + 8H+ + 8e- (4H2)
Hydrazine and eleminox are also react with ferric oxide (Fe2O3) to form a passive magnetite film on the boiler surface, preventing further corrosion.
Fe2O3 + N2H4 → Fe3O4 + N2 + H2O
12 Fe2O3 + (N2H3)2CO → 8Fe3O4 + 3H2O + 2N2 + CO2
To maintain stable protection iron oxide film, the pH of the boiler water should be maintained between 9-10.
Corrosion of Iron:
Fe + OH- → (FeOH) + e
(FeOH) → (FeOH)+ + e
(FeOH) + → Fe++ + OH-
____________________________
Fe + H2O → (FeOH) + H+ + e
(FeOH) → (FeOH)+ + e
(FeOH)+ + H+ → Fe++ + H2O
Fe + 2H2O → Fe++ + H2 + 2OH-
____________________________________
Fe + 2H+ → Fe++ + H2
Corrosion of iron in aerated water:
Fe + O2 + 2H2O → Fe++ + 4 OH-
2 Fe++ + 4 OH- → 2Fe (OH)2
2Fe (OH)2 + H2O + ½ O2 → 2Fe(OH)3 ↓
_________________________________________________
Fe++ + 2H2O → HFeO2- + 3H+
HFeO2- + H+ → Fe(OH)2+ + e-
Fe(OH)2+ + 2H2O → FeO4– + 6H+ + 3e-
HFeO2- + 2H2O → FeO4– + 5H+ + 4e-
________________________________________________
Fe++ → Fe+++ + e-
Fe+++ + H2O → FeOH++ + H+
FeOH++ + 3H2O → FeO4– + 7H+ + 3e-
Fe+++ + 4H2O → FeO4– + 8H+ + 3e-
___________________________________________________
Fe++ + H2O → FeOH++ + H+ + e-
FeOH++ + 3H2O → FeO4– + 7H+ + 3e-
_____________________________________________________
Fe++ + 2H2O → Fe(OH)2+ + 2H+ + e-
Fe(OH)2+ + 2H2O → FeO4– + 6H+ + 3e-
___________________________________________________________
Fe + H2O → FeO + 2H+ + 2e-
3Fe + 4H2O → Fe3O4 + 8H+ + 8e-
2Fe + 3H2O → Fe2O3 + 6H+ + 6e-
___________________________________________________
3FeO + H2O → Fe3O4 + 2H+ + 2e-
2FeO + H2O → Fe2O3 + 2H+ + 2e-
________________________________________________
2Fe3O4 + H2O → 3Fe2O3 + 2H+ + 2e-
________________________________________________
Fe++ + H2O → FeO + 2H+
FeO + H2O → HFeO2- + H+
3HFeO2- + H+ → Fe3O4 + 2H2O + 2e-
2HFeO2- → Fe2O3 + H2O + 2e-
____________________________________________________
2 Fe+++ + 3H2O → Fe2O3 + 6H+
__________________________________________________
FeOH++ + H2O → Fe2O3 + 4H+
__________________________________________________
2Fe(OH)2+ → Fe2O3 + H2O + 2H+
___________________________________________________
Fe → Fe++ + 2e-
3Fe++ + 4H2O → Fe3O4 + 8H+ + 2e-
___________________________________________________
Fe + 2H2O → HFeO2- + 3H+ + 2e-
3HFeO2- + H+ → Fe3O4 + 2H2O + 2e-
_____________________________________________________
Fe → Fe+++ + 3e-
2 Fe+++ + 3H2O → Fe2O3 + 6H+
_____________________________________________________
2Fe++ + 3H2O → Fe2O3 + 6H+ + 2e-
Thus iron will corrode in presence of water and a large number of reactions will take place and which will be vigorous in acidic pHs. At pHs 10-13 the iron will be covered with a film of iron oxide (magnetite). The passivation will be relatively difficult or even impossible at pHs bellow 8 and easier at pHs between 10-13.
Protection by alkalinization is a special case of protection by passivation. It involves the adjustment of the pHs of the corrosive water or medium by the addition alkaline substances so as the protection by passivation is achieved particularly easy e.g. for pH 10-13. this process is used in the treatment of water through phosphate treatment program or others.
Deaerators and Boilers:
Deaerators are commonly used to protect boilers from corrosion in steam generating systems. Feed water which is used in high-pressure boilers must be very pure, otherwise severe corrosion in boiler tubes can rapidly destroy and damage materials in boilers.
** Simply speaking, the purposes of deaerators in steam generating systems are:
1. To remove oxygen, carbon dioxide and other non-condensable gases from feed water
2. To heat the incoming makeup water and return condensate to an optimum temperature for:
>> Minimizing solubility of the undesirable gases
>> Providing the highest temperature water for injection to the boiler
Deaerators are typically elevated in boiler rooms to help create head pressure on pumps located lower. This allows hotter water to be pumped without vapor locking should some steam get into the pump.
The Deaeration principle:
Deaeration is based on two scientific principles. The first principle can be described by Henry’s Law. Henry’s Law asserts that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. The second scientific principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide and other non-condensable gases from boiler feedwater. The feedwater is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation. Spraying feedwater in thin films increases the surface area of the liquid in contact with the steam, which results in more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes them from the feedwater. The liberated gases are then vented from the deaerator.
As illustrated in above figure, deaeration is done by providing proper contact between boiler feed water(condensate) and steam. The steam is fed from the bottom of the deaerator. As the steam contacts with BFW, heats up the BFW to its saturation temperature and eventually dissolved corrosive gases are released from feed water with some vapors from the vent valve. Then deaerated water falls to the storage tank below the deaerator.
Types of deaerators
Deaerators can be classified under two major categories:
- Tray-type
- Spray-type
In both cases, the major portion of gas removal is accomplished by spraying cold makeup water into a steam environment. In the spray-type deaerator a jet of steam mixes intimately with the feed water being sprayed into the unit. In the tray-type the incoming water is allowed to fall over a series of trays causing the water to be broken up into small droplets to permit intimate contact with incoming steam.
Figure 2: Tray-Type Deaerator
The tray-type deaerator features a vertical or horizontal section that is domed and mounted to the top of a horizontal tank that stores the water for the boiler. The incoming water is sprayed into steam atmosphere, where it is heated up to a few degrees to the saturation temperature of the steam. Most of the non-condensable gases (principally oxygen and free carbon dioxide) are released to the steam as the water is sprayed into the unit. Seals prevent the re-contamination of tray stack water by gases from the spray section. Water falls from tray to tray, breaking into fine droplets of film, which intimately contact the incoming steam.
Figure 3: Spray-Type Deaerator
The Spray-type deaerator work on the same general philosophy as the tray-type, but differ in their operation. The Spray-Type Deaerator consists of a horizontal or vertical cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank. The typical spray-type deaerator is a horizontal vessel which has a preheating section and a deaeration section. The two sections are separated by a baffle. Low-pressure steam enters the vessel through a sparger in the bottom of the vessel.The boiler feedwater is sprayed into a section where it is preheated by the rising steam from the sparger. The purpose of the feedwater spray nozzle 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, 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. The deaerated boiler feedwater is pumped from the bottom of the vessel to the steam generating boiler system.
“Oxygen Scavengers” Role in Boiler Deaeration Operation
According to CleanBoiler.org, mechanical deaeration alone cannot ensure complete oxygen removal. Deaerator manufacturers state that properly operating deaerators can reduce dissolved oxygen concentrations in feed water to 0.005 cc per liter, or 7 ppb, and 0 free carbon dioxide. However, plant oxygen levels vary from 3 to 50 ppb, which makes chemical removal of oxygen in feed water with oxygen scavengers necessary. Best practices with oxygen scavengers include placing them in the deaerator’s storage tank so they have the most time possible to react with remaining dissolved oxygen. However, some conditions warrant placing oxygen scavengers in other locations.
One commonly used oxygen scavenger is sodium sulfite. Some higher pressure boilers, however, cause the sulfite to decompose and enter the steam, which may pose a hazard in condensate systems and condensing steam turbines. It’s worth noting that a change in demand for chemical oxygen scavengers could point to a failing deaerator.
Credit:
Elham Ghorbanzadeh
Piping Design Lead Engineer at Sazeh Consultants
References:
Deaerating Principle, Sterling deaerator
Deaerator Working Principle, Boilers info
Deaeration in boilers, Lenntech
Boiler Feed Water Treatment, thermidaire industrial water treatment
What is boiler deaerator and how does it operate? – MPC
Deaerator introduction, CleanBoiler.org, ESC
Spray-Type Deaerator, STS Canada
What is deaeration process? – deaerator.it
2nd Article by another Author for more Information
Deaeration:
The process through which the dissolved Gases are removed from the water is called deaeration. The equipment which is used in this process is called Deaerator.
Necessity of deaeration:
Deaeration is needed to control the corrosion processes in the downstream system. In water, the presence of dissolved gases, particularly oxygen and carbon dioxide, causes accelerated corrosion. Of these, oxygen is the most aggressive. The importance of eliminating oxygen as a source of pitting and iron deposition cannot be over-emphasized. Even small concentrations of this gas can cause serious corrosion problems.
Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can also enter the feed water system from the condensate return system. Possible return line sources are direct air-leakage on the suction side of pumps, systems under vacuum, the breathing action of closed condensate receiving tanks, open condensate receiving tanks and leakage of non-deaerated water used for condensate pump seal and/or quench water. With all of these sources, good housekeeping is an essential part of the preventive program.
One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type of corrosion can produce failures even though only a relatively small amount of metal has been lost and the overall corrosion rate is relatively low. The degree of oxygen attack depends on the concentration of dissolved oxygen, the pH and the temperature of the water.
The corrosion process is especially rapid at elevated temperatures such as are encountered in boilers and heat exchange equipment. The influence of temperature on the corrosivity of dissolved oxygen is particularly important in closed heaters and economizers where the water temperature increases rapidly. Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen can cause serious corrosion.
The primary function of the deaeration and deaerator is to prevent this corrosion by removing the dissolved gases from all sources of water entering the downstream system such as lines, piping, especially boilers, condensate lines, etc…
Because boiler systems are constructed primarily of carbon steel and the heat transfer medium is water, the potential for corrosion is high. Iron is carried into the boiler in various forms of chemical composition and physical state. Most of the iron found in the boiler enters as iron oxide or hydroxide. Any soluble iron in the feed water is converted to the insoluble hydroxide when exposed to the high alkalinity and temperature in 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.
How effective a gas can be removed from liquid depends upon these factors:
Following are the major factors which govern a gas removal from a liquid.
- Ionization
- Relative partial pressure of the gas on the surface of the liquid and the inside the water.
- The water temperature.
- Agitation.
- Gas removal (Venting).
Ionization:-
Oxygen does not ionize and is easily removed. Carbon dioxide and ammonia do ionize. Only that portion that remain in molecular form, non ionized, can be easily removed.
Therefore, Carbon Dioxide can more easily be removed at low pHs and ammonia at high alkaline pH’s.
Relative Partial Pressure:-
Liquid will dissolve the free gas only to the point at which pressure equilibrium is reached between the partial pressure of the gas in the water and the partial pressure of the gas in the atmosphere in contact.
Since air contains 21% oxygen and 79% nitrogen. The partial pressure of the oxygen in the air is about 1/5 atmosphere. Water surrounded by air will dissolve oxygen until the oxygen in the water exerts a partial pressure of 1/5 atmosphere.
In degasification, the equilibrium between oxygen in the water and the oxygen in the atmosphere is deliberately upset.
Temperature:
As the temperature of the water raised to its oiling point, all the free gases theoretically become insoluble. Practically, total removal of gases does not occur. This is because, if water in the vapor phase is in contact with water in the liquid state, the liquid will be slightly cooler than the vapor and some gas will remain dissolved in the water phase.
Water must be heated to an optimum temperature for minimizing the solubility of the undesirable gases and providing the highest temperature water for injection to the boiler. The higher the water temperature, the lower the gas content will be in the water.
In the deaerator, steam is used for scrubbing gases from the water and for the water heating purpose. 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.
Water must be heated close to 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. But this must be kept in mind that increasing the water to its saturation point will increase the water losses which is uneconomical.
Typical Performance Levels:
Following are the performance levels of the miscellaneous Degasifying Equipments.
Operating Ranges mg/lit Remaining
Type Pressure (psig) Temp (C) O2CO2
Blower –type Atmosphere 99o 0.5-1.0 5-10
(Open heater)
Deaerating Heaters 20-25 125 0.04 ——
Deaerators 20-25 125 0.007 0
Ammonia removal is the function of both pH and temperature. For example,
pH 9.0 @ 25o C = 35 % ammonia
pH 9.0 @ 38o C = 18 % ammonia
pH 8.5 @ 25o C = 18 % ammonia
pH 8.5 @ 38o C = 7.5 % ammonia
Agitation:
The heated water must be mechanically agitated 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.
Some methods of agitation are:
- Cascading over trays.
etc…
Deaerator Functions:
Deaerators serve following basic functions.
- Heating the water, Closed to the saturated steam temperature.
- Mechanical Agitation to vigorously scrub the heated water with fresh oxygen free steam.
- By maintaining the partial pressure of oxygen and carbon dioxide at the lowest possible level at the point where steam contacts water for Removal of oxygen and other non-condensable gases in the water.
- Continuously vent these noncondensible gases from the deaerator.
- Storage of deaerated and heated water.
However, after the efficient deaeration chemical treatment is still needed to remove last traces of oxygen.
Types of Deaerator:
Deaerator equipment is designed to reduce oxygen in the boiler feed water. This is done at times by heating the water to the temperature closed to the saturated temperature at the pressure within the deaerating equipment. A deaerator, generally, reduces the oxygen level to 7ppb. There are basically two types of deaerators in common use:
- Tray-type
2. Atomizing-type
Tray-type Deaerator:
Water is heated close to 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.
Tray-type deaerating heaters release dissolved gases in the incoming water by reducing it to a fine spray as it cascades over several rows of trays. The steam that makes intimate contact with the water then scrubs the dissolved gases by its counter-current flow. The steam heats the water to within 3-5 º F of the steam saturation temperature and it should remove all but the very last traces of oxygen. The deaerated water then falls to the storage space below, where a steam blanket protects it from recontamination.
Atomizing-type Deaerator:
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.
Spray-type deaerating heaters work on the same general philosophy as the tray-type, but differ in their operation. 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-condensable gases are liberated and vented from the unit. Spring-loaded nozzles located in the top of the unit spray the water into a steam atmosphere that heats it. Simply stated, the steam heats the water, and at the elevated temperature the solubility of oxygen is extremely low and most of the dissolved gases are removed from the system by venting. The spray will reduce the dissolved oxygen content to 20-50 ppb.
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. A tray section may also be provided at the end for the final removal of the gases. Trays further reduce the oxygen content to approximately 7 ppb or less.
During normal operation, the vent valve must be open to maintain a continuous plume of vented vapors and steam at least 18 inches long. If this valve is throttled too much, air and non-condensable gases will accumulate in the deaerator. This is known as air blanketing and can be remedied by increasing the vent rate.
For optimum oxygen removal, the water in the storage section must be heated to within 5 º F of the temperature of the steam at saturation conditions. From inlet to outlet, the water is deaerated in less than 10 seconds.
Storage Section:
The storage section is usually designed to hold enough water for 10 minutes of boiler operation at full load.
Limitations:
Inlet water should be virtually free of suspended solids that could clog spray valves and ports of the inlet distributor and the deaerator trays. In addition, spray valves, ports and deaerator trays may become plugged with scale that forms when the water being deaerated has high hardness and alkalinity levels. In this case, routine cleaning and inspection of the deaerator is very important.
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 typical deaerator with instrumentation is shown in Figure 4.2.
- 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:
Remember the deaerator reduces the oxygen content of the water to 7ppb. If the boiler is rated at 50, 000 PPH then it mean it will produce 1.2 million lbs. per day of steam. At oxygen concentration of 7ppb, which is in turn will be the 0.0084 lbs. of oxygen per day or roughly 03 lbs. per year.
The formula for the reaction of iron and oxygen is
4Fe + 3O2 → 2Fe2 O3
Based on the molecular weights, 1 lb of oxygen will react 2.4 lbs. of iron. Therefore, 3 lbs. of oxygen will react with 7 lbs. of iron. This is a volume of approximately 25 cubic inch of iron. If the average pit is 3/16” in diameter and 3/16” deep, the volume of pit would be 0.002 cu. in. Theoretically 25 cu. in. divided by 0.002 equals 12,626 pits per year from this small amount of oxygen. In fact, the pits would be large enough to result in failure of boiler tubes long before the end of the year.
The foregoing discussion shows the importance of proper deaeration of boiler feed water in order to prevent oxygen corrosion. Complete oxygen removal cannot be attained by mechanical deaeration alone. Equipment manufacturers’ state that a properly operated deaerating heater can mechanically reduce the dissolved oxygen concentrations in the feed water to 0.005 cc per liter (7 ppb) and ‘zero’ free carbon dioxide. Typically, plant oxygen levels vary from 3 to 50 ppb. Traces of dissolved oxygen remaining in the feed water can then be chemically removed with the oxygen scavenger.
So, Chemical addition is 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.
Oxygen scavengers:
The chemical oxygen scavenging operation is the last opportunity to remove oxygen. Any oxygen not removed in this operation will enter the boiler system and may cause corrosion problems.
The oxygen scavenger should be fed into system as early as possible to protect the system as much of the system as possible. The typical point of addition for oxygen scavenger is the storage section of the deaerator.
To prevent chemical loss, the chemical feed line to a deaerator storage tank should be into an area of good mixing. Oxygen scavenger should be fed continuously.
Types of Oxygen Scavenger:
The most common oxygen scavenger used in boiler feed water treatment are Sodium Sulfite and Hydrazine. Each has its own set of advantages and disadvantages and must be considered.
Sodium Sulfite (Na2SO3):
Sodium sulfite is the most common type of chemical scavenger of oxygen in low pressure systems. The amount of sulfite that can be safely carried in the water decreases as the operating pressure of the steam system increases. The oxygen scavenging property of sodium sulfite is shown by the following reaction.
Na2SO3 + O2 → 2Na2SO4
At higher system operating pressures, the corresponding high temperature 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.
Here are some advantages and disadvantages of sodium sulfite.
Advantages Disadvantages
- Ease of handling. Adds solids to the boiler water
- Ease of application. Can decompose to form acidic gases.
- Simplicity of testing Contribute sodium ions that are undesirable.
- Relatively low cost. Can not be used in steam desuperheating.
- Rapid reaction at low pressure. Speed of reaction pH dependant.
- Can be used in food plants. Reduce oxide deposits only above 221o C.
Hydrazine:
An alternate chemical scavenger of oxygen is hydrazine. Hydrazine removes dissolved oxygen by the following reaction.
N2H4 + O2 → 2H2O + N2 ↑
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 hydrazine decomposition starts at about 200o C according to the reaction.
3N2H4 → 4NH3 + N2
More than 90% hydrazine is decomposed at about 316o C within one minute.
A residual concentration of 0.125 – 0.150 ppm is maintained in boiler feed water. The disadvantage of hydrazine is that it is a suspected carcinogen and its use is restricted.
Here are some advantages and disadvantages of Hydrazine.
Advantages Disadvantages
- Does not add solids to the boiler water. Strong alkaline solution requiring
protective clothing and handling precautions.
- Promotes formation of passive oxide films. Decomposes to ammonia above 260o C.
- Reduce oxide deposits above 120o C Slow direct scavenging of oxygen at
low temperature.
- Can be used in steam desuperheating. 4. Can not be used in food plants.
- Does not add sodium ions. 5. Carcinogenic
Magnetite Layer:
Metal passivation has traditionally been considered to be the reduction of hematite (Fe2O3) to magnetite (Fe3O4) in iron based boiler. Actually, it is a process by which bare metal surfaces form protective oxide film preventing further corrosion, the passive film is very thin and dense. It is distinguishable from the base metal by the coloration. In case of carbon steel, this protective layer is magnetite (Fe3O4) and black in color. Before developing magnetite it is recommended that surface should be cleaned with respect to any deposit, oil and grease.
If the oxide layer is not uniform, porous and disturbed by deposits of any kind, the corrosion process will continue with dynamic process of hot water corrosion in areas of high heat transfer, with subsequent failure due to leakage.
At the boiling point of water, steel react with water/oxygen in a 2-stage process to form magnetite. The presence of any dissolved oxygen in the water affects the quality of magnetite, and leads to the formation of hematite. This causes an uneven coating of various iron oxides and allows increase corrosion of the steel surface.
When starting up, the H2 content may be 20-30 ppb and drops down to approx. 2-5 ppb when the protective magnetite layer is formed.
3Fe + 4H2O → Fe3O4 + 8H+ + 8e– (4H2)
Hydrazine and eleminox are also react with ferric oxide (Fe2O3) to form a passive magnetite film on the boiler surface, preventing further corrosion.
Fe2O3 + N2H4 → Fe3O4 + N2 + H2O
12 Fe2O3 + (N2H3)2CO → 8Fe3O4 + 3H2O + 2N2 + CO2
To maintain stable protection iron oxide film, the pH of the boiler water should be maintained between 9-10.
Corrosion of Iron:
Fe + OH– → (FeOH) + e
(FeOH) → (FeOH)+ + e
(FeOH) + → Fe++ + OH–
____________________________
Fe + H2O → (FeOH) + H+ + e
(FeOH) → (FeOH)+ + e
(FeOH)+ + H+ → Fe++ + H2O
Fe + 2H2O → Fe++ + H2 + 2OH–
____________________________________
Fe + 2H+ → Fe++ + H2
Corrosion of iron in aerated water:
Fe + O2 + 2H2O → Fe++ + 4 OH–
2 Fe++ + 4 OH– → 2Fe (OH)2
2Fe (OH)2 + H2O + ½ O2 → 2Fe(OH)3 ↓
_________________________________________________
Fe++ + 2H2O → HFeO2– + 3H+
HFeO2– + H+ → Fe(OH)2+ + e–
Fe(OH)2+ + 2H2O → FeO4— + 6H+ + 3e–
HFeO2– + 2H2O → FeO4— + 5H+ + 4e–
________________________________________________
Fe++ → Fe+++ + e–
Fe+++ + H2O → FeOH++ + H+
FeOH++ + 3H2O → FeO4— + 7H+ + 3e–
Fe+++ + 4H2O → FeO4— + 8H+ + 3e–
___________________________________________________
Fe++ + H2O → FeOH++ + H+ + e–
FeOH++ + 3H2O → FeO4— + 7H+ + 3e–
_____________________________________________________
Fe++ + 2H2O → Fe(OH)2+ + 2H+ + e–
Fe(OH)2+ + 2H2O → FeO4— + 6H+ + 3e–
___________________________________________________________
Fe + H2O → FeO + 2H+ + 2e–
3Fe + 4H2O → Fe3O4 + 8H+ + 8e–
2Fe + 3H2O → Fe2O3 + 6H+ + 6e–
___________________________________________________
3FeO + H2O → Fe3O4 + 2H+ + 2e–
2FeO + H2O → Fe2O3 + 2H+ + 2e–
________________________________________________
2Fe3O4 + H2O → 3Fe2O3 + 2H+ + 2e–
________________________________________________
Fe++ + H2O → FeO + 2H+
FeO + H2O → HFeO2– + H+
3HFeO2– + H+ → Fe3O4 + 2H2O + 2e–
2HFeO2– → Fe2O3 + H2O + 2e–
____________________________________________________
2 Fe+++ + 3H2O → Fe2O3 + 6H+
__________________________________________________
FeOH++ + H2O → Fe2O3 + 4H+
__________________________________________________
2Fe(OH)2+ → Fe2O3 + H2O + 2H+
___________________________________________________
Fe → Fe++ + 2e–
3Fe++ + 4H2O → Fe3O4 + 8H+ + 2e–
___________________________________________________
Fe + 2H2O → HFeO2– + 3H+ + 2e–
3HFeO2– + H+ → Fe3O4 + 2H2O + 2e–
_____________________________________________________
Fe → Fe+++ + 3e–
2 Fe+++ + 3H2O → Fe2O3 + 6H+
_____________________________________________________
2Fe++ + 3H2O → Fe2O3 + 6H+ + 2e–
Thus iron will corrode in presence of water and a large number of reactions will take place and which will be vigorous in acidic pHs. At pHs 10-13 the iron will be covered with a film of iron oxide (magnetite). The passivation will be relatively difficult or even impossible at pHs bellow 8 and easier at pHs between 10-13.
Protection by alkalinization is a special case of protection by passivation. It involves the adjustment of the pHs of the corrosive water or medium by the addition alkaline substances so as the protection by passivation is achieved particularly easy e.g. for pH 10-13. this process is used in the treatment of water through phosphate treatment program or others.
