Maintenance of internal and external surfaces of pressure part tubes/pipes in healthy conditions is essential to achieve high load factor and long life of any boiler. Deposition of insoluble material on internal surfaces of water and steam tubes/pipes influence heat transfer adversely and sometimes results in tube failure. Treatment of water should therefore be in its entirety that is from raw water to quality of steam in terms of impurities.
Working pressure of water and steam at different stages also significantly affect rate of reaction and solubility of chemicals used in water treatment. Higher operating pressure and temperature of boiler call for further stringent requirementand close limits of tolerance. Invariably water treatment system to ensure quality of water at feed water entry point as agreed and also take care of dosig appropriate chemicals at low pressure and high pressure in steam drum to ensure that impurities are within specified limits given in VGB or similar internationally accepted code.
Present day practice in water treatment technology suitable for the circofluid boilers generating steam at about 90 to 120 kg/cm2 G pressure and 500 deg C plus temperature is generally discussed in the following pages to serve as guidelines only.
Oxygen Corrosion Of Internal Surfaces
Carbon steel is protected by a fundamental iron water chemical reaction that forms a protective layer of magnetic iron oxide on the internal metal surfaces. A small amount of iron reacts with water unit to protective oxide film is formed. Once the film is established the film the reaction virtually stops and does not resume until the film is disturbed or removed. If the film is removed chemically or mechanically , the iron water reaction starts again to build up the protective oxide. This of course removes more iron the surface of the boiler parts and if allowed to continue will definitely thin the metal parts. It is therefore very important to maintain boiler water chemistry within close limits to prevent the chemical removal of the initial protective magnetic iron oxide film on the internal metal surfaces
The most common from of corrosion is that caused by the presence of oxygen in the water steam cycle. The most logical approach to the prevention of corrosion due to dissolved oxygen is to eliminate the entrance of oxygen to the cycle as far as possible and by expelling at the first opportunity the oxygen which has unavoidably entered the cycle.
The most common method of expelling oxygen is by de aerating the feed water from heaters to condensers.
HYDRAZINE HYDRATE AND SODIUM SULPHITE
Reation of Hydrazine with dissolved oxygen produces nitrogen and water. Even products of decomposition of hydrazine are volatile and tend to form , alkaline solutionswith water. This therefore donot increase dissolve solids content in boiler water. The reaction is of course dependant on hydrazine concentration , temperature and time. Due to the volatility of hydrazine and its decomposition at elevated temperature , only a small residual can be maintained I the boiler water. It is apparent that with this small quantity of hydrazine available only minute amounts of oxygen can be allowed to enter the boiler system.
Products of thermal decomposition at higher operating pressure of boiler are hydrogen sulphide and sulphur dioxide.. This will re dissolve at a point where condensation of steam occurs leading to acidic condition in water circuit. Hydrazine is therefore preferred as oxygen scavenger in the boiler operating in the region of 100 bars pressure.
pH Control of condensate feed water
in accordance with table 1 feed water pH should be controlled in the range of 8.5 to 9.5 to reduce iron and copper pickup in the condensate,feed water.
The most commonly used neutralizing chemicals used for controlling the pH of condensate water are Ammonia, Morpholine, Cyclohexamine, and hydrazine.
This chemicals are volatile alkalizers, which dissolved and distills with the steam and neutralize trace acids formed in the condensate. Hydrazine is included with the volatile alkalizers besides being an oxygen scavangers.It decomposes at the operating temperature of the boiler ito ammonia in accordance with the following reaction
2N2H4----------N2+H2+2NH3
Experience has shown that condensate pH, when using Hydrazine will stabilize in the range of 8.5-9.5 due to ammonia formation depending upon the residual of hydrazine maintained at the economizer inlet.
Proper pH control and selection of neutralizing chemical can only be determined by a critical study of the materials making up the condensate feed water system and on the basis of iron and copper concentration in the feed water system which would be indicative of the attackon these cycle materials.
I general a high pH due to ammonia concentration is considered more aggressive to copper bearing alloys but is more protective to carbo steel surfaces.
INTERNAL TREATMENT OF BOILER WATER
There are various methods for the internal treatment of boiler water. A blanket recommendation of any one method is not realistic. The final decision as to the type of treatment to be used in a particular boiler should be used on the raw water supply, history of condenser leakage, the percent of makeup required, the nature of the condensate returns, and other unique factors, A short summary of the recommended internal water treatment as follows
This type of treatment ivolves the addition of phosphates and caustic through the chemical feed ine to the steam drum. The caustic is added to the boiler water to maintain the pH in the range of 10.2- 11.2
The primary purpose of phosphate addition to boiler water is to precipitate the hardness constituents under the proper pH conditions. The calcium reacts with phosphates to precipitate calcium phosphate as hydroyapatite Ca10 (PO4)(OH)2. This is a flocculant tending to less adherent to boiler surfaces than simple tricalcium phosphate, which is precipitate below a pH of 10.2. also caustic reacts with magnesium to form magnesium hydroxide This precipitate is forme in presence to magnesium phosphate at a pH above 10.5 and is considered less adherent than magnesium phosphate.
At a higher pressure comparatively low phosphate residuals must be maintained in order to avoid appreciable phosphate hideout. Hideouts the term used to express the phenomenon of the partial disappearance of phosphate in the boiler wter upon increase in load and its reappearance upon load reduction. A change in phosphate concentration greater than 5 ppm as PO4 between high load and Low load is considered Hideout.
Working pressure of water and steam at different stages also significantly affect rate of reaction and solubility of chemicals used in water treatment. Higher operating pressure and temperature of boiler call for further stringent requirementand close limits of tolerance. Invariably water treatment system to ensure quality of water at feed water entry point as agreed and also take care of dosig appropriate chemicals at low pressure and high pressure in steam drum to ensure that impurities are within specified limits given in VGB or similar internationally accepted code.
Present day practice in water treatment technology suitable for the circofluid boilers generating steam at about 90 to 120 kg/cm2 G pressure and 500 deg C plus temperature is generally discussed in the following pages to serve as guidelines only.
Oxygen Corrosion Of Internal Surfaces
Carbon steel is protected by a fundamental iron water chemical reaction that forms a protective layer of magnetic iron oxide on the internal metal surfaces. A small amount of iron reacts with water unit to protective oxide film is formed. Once the film is established the film the reaction virtually stops and does not resume until the film is disturbed or removed. If the film is removed chemically or mechanically , the iron water reaction starts again to build up the protective oxide. This of course removes more iron the surface of the boiler parts and if allowed to continue will definitely thin the metal parts. It is therefore very important to maintain boiler water chemistry within close limits to prevent the chemical removal of the initial protective magnetic iron oxide film on the internal metal surfaces
The most common from of corrosion is that caused by the presence of oxygen in the water steam cycle. The most logical approach to the prevention of corrosion due to dissolved oxygen is to eliminate the entrance of oxygen to the cycle as far as possible and by expelling at the first opportunity the oxygen which has unavoidably entered the cycle.
The most common method of expelling oxygen is by de aerating the feed water from heaters to condensers.
HYDRAZINE HYDRATE AND SODIUM SULPHITE
Reation of Hydrazine with dissolved oxygen produces nitrogen and water. Even products of decomposition of hydrazine are volatile and tend to form , alkaline solutionswith water. This therefore donot increase dissolve solids content in boiler water. The reaction is of course dependant on hydrazine concentration , temperature and time. Due to the volatility of hydrazine and its decomposition at elevated temperature , only a small residual can be maintained I the boiler water. It is apparent that with this small quantity of hydrazine available only minute amounts of oxygen can be allowed to enter the boiler system.
Products of thermal decomposition at higher operating pressure of boiler are hydrogen sulphide and sulphur dioxide.. This will re dissolve at a point where condensation of steam occurs leading to acidic condition in water circuit. Hydrazine is therefore preferred as oxygen scavenger in the boiler operating in the region of 100 bars pressure.
pH Control of condensate feed water
in accordance with table 1 feed water pH should be controlled in the range of 8.5 to 9.5 to reduce iron and copper pickup in the condensate,feed water.
The most commonly used neutralizing chemicals used for controlling the pH of condensate water are Ammonia, Morpholine, Cyclohexamine, and hydrazine.
This chemicals are volatile alkalizers, which dissolved and distills with the steam and neutralize trace acids formed in the condensate. Hydrazine is included with the volatile alkalizers besides being an oxygen scavangers.It decomposes at the operating temperature of the boiler ito ammonia in accordance with the following reaction
2N2H4----------N2+H2+2NH3
Experience has shown that condensate pH, when using Hydrazine will stabilize in the range of 8.5-9.5 due to ammonia formation depending upon the residual of hydrazine maintained at the economizer inlet.
Proper pH control and selection of neutralizing chemical can only be determined by a critical study of the materials making up the condensate feed water system and on the basis of iron and copper concentration in the feed water system which would be indicative of the attackon these cycle materials.
I general a high pH due to ammonia concentration is considered more aggressive to copper bearing alloys but is more protective to carbo steel surfaces.
INTERNAL TREATMENT OF BOILER WATER
There are various methods for the internal treatment of boiler water. A blanket recommendation of any one method is not realistic. The final decision as to the type of treatment to be used in a particular boiler should be used on the raw water supply, history of condenser leakage, the percent of makeup required, the nature of the condensate returns, and other unique factors, A short summary of the recommended internal water treatment as follows
This type of treatment ivolves the addition of phosphates and caustic through the chemical feed ine to the steam drum. The caustic is added to the boiler water to maintain the pH in the range of 10.2- 11.2
The primary purpose of phosphate addition to boiler water is to precipitate the hardness constituents under the proper pH conditions. The calcium reacts with phosphates to precipitate calcium phosphate as hydroyapatite Ca10 (PO4)(OH)2. This is a flocculant tending to less adherent to boiler surfaces than simple tricalcium phosphate, which is precipitate below a pH of 10.2. also caustic reacts with magnesium to form magnesium hydroxide This precipitate is forme in presence to magnesium phosphate at a pH above 10.5 and is considered less adherent than magnesium phosphate.
At a higher pressure comparatively low phosphate residuals must be maintained in order to avoid appreciable phosphate hideout. Hideouts the term used to express the phenomenon of the partial disappearance of phosphate in the boiler wter upon increase in load and its reappearance upon load reduction. A change in phosphate concentration greater than 5 ppm as PO4 between high load and Low load is considered Hideout.
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