The purpose of the steam boiler is to produce steam to be used in various stages of the process for purposes such as heating, the cycle of electricity generating turbines, sterilization, and hot water supply. Although boiler systems vary in shape and size, they typically consist of a steam boiler (drama) and auxiliary systems called softening device, deaerator and condensate. For this reason boiler systems conditioning chemicals is important.
The working principle of steam boilers is to obtain heat energy with the effect of a fuel and to evaporate the boiler water. Therefore, the determinants of boiler efficiency and service life are the physical and chemical properties of the water fed to the boiler. Regular maintenance and control ensures that boiler systems operate at high performance. Common problems that need to be addressed are impurities, corrosion, deposits, entrainment and foaming. All of these problems are caused by the characteristics of the boiler feed water. In order to bring the boiler water to the desired properties, a series of physical and chemical processes with boiler systems conditioning chemicals are required. With the in-boiler chemical conditioning following the preconditioning, the boiler feed water is prevented from causing such problems.
A suitable boiler conditioning;
- Saves energy and water
- Reduces the cost of auxiliary units
- Increases the service life of boilers
- Shortens downtime
- Lowers maintenance costs
AquaRedd ® What are the benefits of the boiler systems conditioning program with series chemicals?
- Prevents calcification and corrosion in steam boilers
- It provides efficient heat transfer, less fuel costs, less tube failures and continuous cleaning of the boiler.
- It reduces the corrosion of boiler water to increase boiler reliability and reduce unplanned downtime due to corrosion.
- Provides superior passivation for long-term protection of feed water and boiler surfaces from corrosion and unnecessary repair costs.
- Requires more cycles and less water use
- It ensures that less chemicals and heat energy are evacuated from the blowdown.
- It provides more steam by using less fuel and water.
What is boiler feed water?
It is the water added to the boiler to make up for the water lost by blowdown and evaporation. In many cases, the condensed steam returned from the condensate system to the boiler makes up the bulk of the feedwater. Make-up is the water used to complete the returning condensate. Make-up water is usually natural water, it can be raw or purified before use. Thus, the composition of the feed water changes depending on the quality of the make-up water and the amount of condensate returned.
Feed water purity is related to the amount and nature of impurities. Feedwater purity requirements depend on boiler pressure, boiler design and applications and can vary widely.
Definitions determining the characteristics of boiler feed water
Conductivity:It determines the amount of dissolved ions in the water. As the water is purified, the conductivity decreases. Its unit is the inverse of the resistance unit, µS/cm. Conductivity outside the limit values causes corrosion and entrainment. Feed water conductivity can be brought to the desired range by reverse osmosis and demineralization.
Total dissolved solids:It is a measure of the amount of all solids dissolved in water. There is a direct proportionality between this value and the conductivity value.
Suspended Solids:Substances suspended in water without dissolving give turbidity and undesirable color to the water. This type of water should be fed to the boiler by passing through a physical filter. Otherwise, suspended matter will cause soft deposits, loose sludge and foaming.
PH value:It is a measure of the acidity or basicity of water. pH values measured outside the limit values cause acid or caustic corrosion. The pH can be adjusted by adding acid or caustic.
Alkalinity:The hydroxide, carbonate and bicarbonate contained in the water form the alkalinity of the water. It is expressed in terms of two different values as P alkalinity and M alkalinity. Based on these values, the amounts of hydroxide, carbonate and bicarbonate ions in the water (ppm CaCO3) is calculated in . Too low or too high alkalinity causes foaming in the boiler, caustic cracking and carbon dioxide corrosion in the steam-condensate lines. With the dealkalization process, the alkalinity of the water is brought to the desired range.
Total hardness:The amount of dissolved calcium and magnesium salts in the water is a measure of the hardness of the water. The hardness of water is the CaCO of the hardness-giving substances they commonly contain in practice.3determined by the amount. High hardness causes scale formation in the boiler. The hardness of the water is removed by water softening before entering the boiler.
Preconditioning methods are used to prepare the feed water to the system before it enters the boiler. The most common non-boiler preconditioning process to use is softening. Well waters with very high hardness are used as raw water in many enterprises. It is not possible to completely remove such high hardness and some other impurities with in-boiler chemical conditioning. For this reason, before the water is fed to the boiler, its excess hardness should be removed by passing it through a softening circuit. The most commonly used softening method is to remove the hardness of the water by ion exchange and turn it into soft water.
Filtration:It is the process of separating sand, clay and some organic materials that cannot pass through the filter pores by passing the water through a physical filter.
Reverse Osmosis (RO):To understand reverse osmosis, one must first understand osmosis. Osmosis uses a semipermeable membrane that only allows ions to flow from the concentrated solution to the dilute solution, but not the opposite direction. Reverse osmosis, on the other hand, overcomes the osmotic pressure with a high artificial pressure and operates the osmosis process in reverse, concentrating the dissolved solids on one side of the membrane. Normal operating pressures are 300-900 psi. Reverse osmosis reduces the amount of dissolved solids in the raw water, making the effluent ready for subsequent preconditioning. RO is a cross-flow filtration method with three streams: feed, purified water, and concentrate. This method uses a pressurized feed stream flowing parallel to the membrane surfaces. Water of near purity to pure water passes through the membranes and is called permeate. As the feedwater passes through the membranes, it leaves behind ions and solids remaining in the concentrate. Since there is a continuous flow on the membrane surfaces, the solid particles left do not accumulate on the surface and the membrane does not clog. Instead, it is entrained by the stream of concentrate. While costly at times, this process can be used for any type of water and is becoming increasingly common in industry.
Coagulation-Flocculation:Removal of suspended solids and color from water inputs is called purification. Suspended materials may contain large particles that can settle (sedimentation) under their own weight. In these cases, the treatment consists of settling pools or a filter. But generally, suspended matter in water contains particles so small that they cannot settle on their own and pass through the filter. Coagulants (coagulants) must be used to remove these finely dispersed or colloidal substances. Coagulation is the neutralization of the electrical charges of finely dispersed or colloidal impurities. Colloidal particles have large surface areas that keep them suspended. In addition, the particles have negative electrical charges that attract and hold each other. Flocculation, on the other hand, is the holding of coagulated particles together with the help of electrical attraction force.
Ion exchange:It is the process of removing dissolved solids by passing water through natural or synthetic resins. When minerals dissolve in water, they form electrically charged particles called ions. Certain natural and synthetic substances have the ability to remove mineral ions from water by exchanging them with others. For example, calcium and magnesium ions can be replaced by sodium ions by passing water through a cation exchange softener. Thus, the hardness of the water is removed.
What is Regeneration-Salting?
İyon değiştirici reçinelerin sudaki iyonları gidermek için sınırlı bir kapasiteleri vardır. İyon değiştirme işleminin tersi olan rejenerasyon işlemi reçineyi orjinal formuna dönüştürür. Rejenerasyon çevrimi, geri yıkama, reçine yatağına tuzlu su emişi ve durulamadan oluşur. Geri yıkama ile reçine tanecikleri birbirinden ayrılır ve de tuzlu su ile muameleye hazır hale getirilir. Geri yıkamada su akış hızına dikkat edilmesi gereklidir, reçine yatağının akışkanlaşmasına ve sistemden atılan su ile reçine kaybına izin verilmemelidir. Cihaz 5-10 dakika ters yıkanmalıdır. Rejenerasyonda %15-20’lik tuz çözeltisi kullanılır. Çözelti 45–60 dakika cihazdan geçirilir. Beher litre reçine için 150–250 gr tuz kullanılmalıdır. Cihazın tuzlu su çözeltisi ile teması esnasında, iyon değiştirici reçine sudan uzaklaştırıp tuttuğu iyonları bırakır ve bu iyonlar reçine tankından dışarı atılır. Reçine sonraki kullanım için hazır hale gelmiş olur.
It is possible to reduce the hardness of water passed through water softening systems to zero. Small amounts of hardness leakage may be seen from time to time. Said hardness leakage can be eliminated by increasing the amount of salt during regeneration. However, as the raw water TDS value increases, the hardness leakage at the softening system outlet will also increase.
Deaerator-mechanical deaeration-thermal degassing:Besleme suyu kazana girmeden önce, suyun içinde bulunan çözünmüş oksijen giderilmelidir. Besleme suyunun havasızlaştırılması, degazör ısıtıcısı içinde buhar ısısıyla suyun ısıtılarak çözünmüş oksijenin uzaklaştırılmasıdır. Suyun sıcaklığı, oksijenin su içindeki çözünürlüğü minimum olduğu 102-105 ºC’ye getirilir. Böylece, su içinde çözünemeyen oksijen gazı bir buhar açıklığı ile degazör dışına atılır. Degazörün teorik verimi %85’dir. Bu nedenle, bir miktar çözünmüş gaz su içinde kalır.
In-boiler chemical conditioning
Chemical conditioning of the water in the boiler is mandatory whether or not the water has been pre-treated. In-boiler conditioning is a complementary process to the out-of-boiler pretreatment, which removes impurities entering the boiler with feed water such as hardness, oxygen, silica, iron, regardless of the size of the amount.
Objectives of the internal conditioning program
- Reacting with the hardness of the feed water entering the boiler and preventing it from precipitating on the boiler metal in the form of scale-lime
- Conditioning any suspended matter such as lime sludge in the boiler and making it non-adhesive to the boiler metal
- To control and prevent the causes of boiler water entrainment
- To prevent oxygen corrosion by removing oxygen from the feed water
- To provide sufficient alkalinity to prevent boiler corrosion
In addition, a complete conditioning program should prevent corrosion and scale formation of the feedwater system and protect steam-condensate systems from corrosion.
The efficiency of a boiler directly depends on the quality of the feed water. The feedwater system consists of the deaerator, feedwater pumps and the pipeline to the boiler. The oxygen contained in the feed water must be removed before it enters the boiler. Otherwise, corrosion may occur throughout the entire boiler system, and occasional perforation and decay may be observed. The formation of slits causes swelling in the tube and if this situation continues, it leads to a short-term stop of the plant. The main goal of in-boiler chemical conditioning is to eliminate the scale-lime and corrosion-forming properties of water in the boiler.
Scale, Lime and Deposit Formation in Steam Boilers
Water impurities enter the boiler through condensate leaks and feed water; corrosion products, on the other hand, are formed as a result of corrosion and come from condensate return and feed waters.
Dissolved calcium and magnesium bicarbonate compounds decompose under the influence of heat to form carbon dioxide and insoluble carbonates. These carbonates may precipitate directly on the boiler metal or form a loose sludge in the boiler water that will accumulate on the boiler surfaces. Calcium sulfate and silica usually precipitate directly on the boiler metal and do not form loose sludge. Therefore, these compounds are more difficult to remove. Silica is usually not found in large quantities in water, but under certain conditions it can form excessively hard scale. Suspended or dissolved iron from the feed water also accumulates on the boiler metal. Oil and other contaminants from the process also accumulate on the boiler metal, accelerating the deposit formation of impurities. Under normal conditions, sodium compounds do not accumulate. Sodium deposits occur in unusual situations, such as a dried tube, a stable vapor blanket, or the presence of porous deposits.
Scale formation in steam boilers and cooling water systems occurs when the feed water is not adequately conditioned and the mineral concentration of the system water exceeds the saturation point. As a result of not using chemical additives preventing scale formation, the mineral water layer on the hot boiler pipes, as a result of the removal of water vapor, carbon dioxide, oxygen and similar gases, stores the minerals on it and hardens it. This hardened layer is called scale or limestone.
As a result of scaling and corrosion, a thick layer of limestone is formed. This limestone formed creates a strong insulating layer and prevents heat transfer.
This insulation layer increases the temperature on the heat transfer surfaces by causing excessive fuel consumption and a decrease in efficiency. As a result of the high temperature on the heat transfer surfaces, thermal stresses, burns and material deformation occur in the metals.
Effects of Deposits
Reduction of thermal conductivity: The scales and deposits that form are poor heat conductors and act as insulators, as evidenced by various conductivity values. The resulting scale-lime layer causes the steam boiler to become deaf and the steam output to decrease. In addition, the formed scale-lime layer increases the fuel consumption and increases the unit cost of steam production.
Temperature build-up on the metal wall: Since a wall covered with a scale-lime layer prevents heat transfer, the temperature of the wall rises. This phenomenon is called overheating and the metal may lose some of its mechanical properties (elasticity, etc.). These cause local deformations and cause pipe bursts.
Effect of Lime on Heat Transfer Surfaces on Fuel Consumption
The minerals in the water precipitate on the heat transfer surfaces and form scales. When the crust thickness reaches certain dimensions, first the fuel consumption increases, then the metal deformation reaches dangerous dimensions such as puncture and explosion.
In the examinations made, according to the structure and characteristics of the winter;
1 mm cork thickness, depending on its structure% 8 –10
2 mm kışır kalınlığı, yapısına bağlı olarak %12-16
3 mm kışır kalınlığı, yapısına bağlı olarak%20-26
4 mm kışır kalınlığı, yapısına bağlı olarak %30-35 yakıt kaybına neden olmaktadır.
In steam boilers, after a thickness of 2 mm, the construction is gradually forced by thermal stresses, and loosening occurs between the mirrors and the pipes. Because the thermal conductivity and stretching of the cork layer covering the metal is different from the metal. For this reason, leaks will start in the mirror-pipe connections in the boiler. As the cork thickness increases, the number of leaky pipes will naturally increase.
When the crust thickness reaches 4 mm, the boiler system will become unreliable, as the crystal structure of the metal will deteriorate and hardening will occur. Dangers such as furnace collapse, pipe burst, mirror cracks will be expected at any time.
In addition, problems such as narrowing of the pipe wall, reduction in volume, decrease in efficiency, strain on the discharge pumps will occur due to scaling.
The way to get rid of all these problems is to prevent scale formation by applying chemical water conditioning in steam boilers, heat exchangers and boilers.
The scale-lime layer formed in steam and heating boilers must be cleaned and neutralized without damaging the metal.
Corrosion Formation in Steam Boilers
In its simplest definition, general corrosion is the return of the metal to its ore form. For example, iron turns into iron oxide compounds as a result of corrosion. The corrosion process is a complex electrochemical reaction. Corrosion can cause general damage to a large metal surface or cause the metal to be pierced or pierced in the form of pinholes. Operating load and stress on the system, pH conditions and chemical corrosion have a significant effect and cause different damages.
Where does corrosion usually occur?
Corrosion in the feed water system can occur as a result of the low pH value of the water and the presence of dissolved oxygen and carbon dioxide in the water.
Active boiler corrosion occurs when the boiler water alkalinity is too low or too high. Corrosion occurs when water carrying dissolved oxygen comes into contact with metal, especially when the boiler is out of use. High temperatures and pressures on the boiler metal accelerate the corrosion mechanism. Corrosion in the steam and condensate system is usually the result of carbon dioxide and oxygen pollution. Other contaminants such as ammonia and sulphur-containing gases can also increase damage to copper alloys present in the system.
What are the problems caused by corrosion?
Corrosion causes difficulties in two respects. The first is the decomposition of the metal itself, and the second is the deposition of corrosion products in areas with high heat exposure in the boiler. Identical corrosion on boiler surfaces is very rare in real practice. All boilers suffer a minor amount of general corrosion. There are many insidious forms of corrosions. Deep pitting causing iron loss causes water to penetrate inside the boiler tube walls and split the tubes.
Corrosion at the bottom of the boiler deposits can weaken the metal very much and tube failures can occur. Renewing lines and equipment in steam-condensate systems due to corrosion can be very costly.
Types of Corrosion in Steam Boilers
The various forms of corrosion encountered in boilers are as follows.
Oxygen Corrosion: Oxygen is a very important corrosion factor. It causes deep cavities and pitting corrosion on the metal. An increase in temperature accelerates the corrosion reaction. As the solubility of oxygen decreases as a function of temperature, the oxygen is supersaturated in water and tends to leave the liquid phase and move towards the walls of the boiler. It gives an anodic reaction because it contains excess oxygen in the airless places it comes into contact with. (Differential ventilation)
Carbon dioxide Corrosion: dissolved CO2slightly increases the acidity according to the following equation.
CO2+ H2O ↔ HCO3–+ H+
The acidity resulting from this event is especially important in condensate circuits. The carbon dioxide gas sent to the boiler consists of the dissolution of bicarbonates and dissolves in the condensate water.
2 HCO3–→ CO3-2+ CO2+ H2O CO3-2+ H2O → CO2+2OH–
Caustic Breakdown: Caustic or caustic corrosion is also called caustic cracking. This form of corrosion is an event that occurs between the crystal structure of the substance. There may be a build-up of kalevi in a fracture or crack on the wall. This phenomenon is no longer common in modern boilers. Because almost all of them are sourced, the kalevis are concentrated in a certain place.
Low pH Corrosion (Acid Corrosion): One of the important types of corrosion at low pH levels and caused by hydrogen is hydrogen cracking. The type of corrosion it causes is different from uniform acid corrosion.
No thinning is observed in the pipe wall thickness in pipe bursts caused by hydrogen cracking, which is generally observed in the boiler evaporator and occasionally in the superheater pipes. Hydrogen cracking is usually observed under dense deposits.
Hydrogen formed in a slightly alkaline environment cannot reach the metal. However, hydrogen formed under deposition at low pH and high temperatures easily diffuses into the metal.
Hydrogen Cracking: Unlike acid corrosion in boilers operating under low pH conditions, the corrosion caused by hydrogen is called hydrogen cracking. The hydrogen released as a result of corrosion occurring under the deposit in the boiler diffuses into the metal at high temperature and reacts with the carbon in the steel's structure, realizing the phenomenon called "decarburation".
Hydrogen formed under deposition at low pH and high temperatures diffuses easily within the metal. CH formed by the combination of hydrogen and carbon4that is, methane creates cracks and separations between metal grains with the effect of temperature and pressure, causing the destruction of the metal.
Sub-deposit Corrosion: The bottom parts of the deposits formed in the steam boilers cause local corrosion with the various potential differences they create. In order to prevent the formation of depositional corrosion, attention should be paid to chemical water treatment and the additive concentrations of the boiler water should be controlled.
What measures should be taken to prevent boiler system corrosion?
The main corrosion prevention methods are as follows;
-Dissolved gases in the feed water (O2and CO2etc.) should be removed physically and chemically.
-The pH value and alkalinity of the boiler water should be adjusted.
-The inner surfaces should be kept clean, the accumulation of corrosion accelerating should be prevented and the resulting deposit should be cleaned.
-When it is out of service, the boiler should be protected by wet conservation, the metal surface should be covered with a protective magnetic layer and passivated. -The corrosive gases in the steam and condensate systems should be removed by chemical conditioning.
-Free hydroxide, silica, chloride ions should be controlled by limiting their concentration.
- Corrosion products from condensate and feed water should be removed by preventing corrosion.
For the selection and control of corrosion inhibitor chemicals, corrosion causes and corrective measures must be determined very well. Your SOLECHEM customer representative will offer you this expertise.
Condensate Line Conditioning
The steam used in various processes of the plant is condensed and returned to the boiler. Condensate return water is another component of the feed water. The danger of contamination from operating process materials is quite large. Some pollutants include oil, chemicals, gases and cooling water.
Carbonic acid corrosion occurring in the condensate lines should be prevented with neutralizing and film-forming amines. If the condensate system is not adequately protected, it will cause corrosion cracks and consequent stops. As corrosion occurs, iron and copper compounds go back into the boiler systems and can clog the deaerator and form deposits in the boiler and economizer. With proper conditioning, you can prevent the decrease in boiler efficiency, overheating and boiler cracks. SOLECHEM corrosion inhibitors and AQUAREDDoxygen scavenging, neutralizing and film forming chemicals provide effective and versatile corrosion protection.
Water conditioning program with corrosion and scale inhibitors;
– Provides pure steam
– Extends equipment life
– Increases system reliability
– Minimizes energy, repair and maintenance costs
What is the cause of corrosion in steam condensate systems?
When the pH value of the condensate water is less than 8.3, the carbon dioxide formed in the steam boilers turns into carbonic acid in the condensate lines and causes condensate corrosion.
Carbon dioxide (C0) is the cause of many condensate system corrosion.2) and oxygen (O2) is. Carbon dioxide dissolved in condensed steam, carbonic acid (H2CO3) creates. If there is oxygen together with carbon dioxide, the corrosion rate will increase even more, causing occasional rot and perforation. Condensate corrosion causes abrasions and punctures in the system, as well as the accumulation of corrosion residues at certain points, resulting in pipe blockages and disruptions in the process. If the corrosion residues in the condensate lines are carried to the steam boiler together with the condensate return water, the conductivity of the boiler water increases and contributes to the formation of scurf.
Condensate corrosion can be detected as follows.
- The formation of very thin, pinhole-like holes at certain points of the steam-condensate lines (oxygen corrosion)
- The thinning of especially the lower surfaces of the steam-condensate pipes as if they have been eroded and the formation of water channels (carbon dioxide corrosion)
- Simultaneous formation of carbon dioxide and oxygen corrosion in the condensate system
How to prevent steam-condensate corrosion?
The general approach is to chemically and mechanically remove oxygen from the feedwater and to condition the feedwater to minimize the formation of carbon dioxide and carbonic acid in the boiler. Chemical conditioning reduces the risk of potential future corrosion. Volatile amines neutralize carbonic acid formed by the dissolution of carbon dioxide in condensate. Volatile film-forming inhibitors form a barrier between the metal and the corrosive condensate.
In operating conditions;
- Purification of feed water from carbon dioxide and bicarbonates
-Ensuring uninterrupted operation of the system
-Chemical conditioning should be done in a healthy and effective way.
Oxygen scavenger, neutralizer and film-forming amines are used to prevent corrosion in the condensate system.
Foaming in steam boilers
In steam boilers, when oil, organic substances, silica, salts, total dissolved substances and total alkalinity of boiler water interact with high pressure and temperature inside the boiler, it causes foaming.
In order to prevent foaming and water entrainment in the boilers, the regular blowdown system must be operational and the water treatment chemicals must contain special antifoaming agents. Failure to prevent the foaming of the boiler water will cause the boiler to burn due to the foam.
Foaming in the boiler water causes the water to be dragged into the system. Boiler water entrainment is the contamination of steam with boiler water solids.
Blowdown in steam boilers
Blowdown is the process of removing some of the boiler water from the system in order to reduce the amount of dissolved or suspended solids whose concentration increases as a result of evaporation in the boiler water to the limits determined for the boiler.
Since solid suspensions and dissolved solids coming to the boiler with the feed water cannot pass into the steam, they remain in the boiler water and its concentration increases over time. If the boiler water is not adjusted with the blowdown, the steam quality deteriorates and the boiler becomes inoperable after a short time.
In order to prevent the solid suspended and dissolved substance concentration in the boiler water from damaging the boiler, limit values have been set for some parameters in the boiler water and bluffing is done according to these limit values. Boiler water limit values depend on the type of boiler and system and especially on the working pressure of the boiler. With the blowdown, unwanted impurities (suspended solids, all salts, alkalinity and silica) in the boiler water are thrown out of the boiler and are reduced below the desired limit values.
Benefits of regular bluffing:
If the blowdowns determined by considering the water values used in the enterprises and the type of the boiler and the working pressure are done regularly;
- Purer and cleaner steam is obtained.
- Accumulation at the bottom of the boiler and corrosion and heat loss caused by the accumulation are prevented.
- The foaming of the boiler water and its transport to the steam line are prevented.
- The amount of dissolved solids and suspended matter in the boiler water is controlled.
- In the boiler, especially the area where the level indicator is clogged due to mud, the indicator is disabled and the possibility of the boiler being dehydrated is prevented.
As a result, blowdown is an important and mandatory process that minimizes the tendency of deposit formation, corrosion and entrainment in boiler water. Applying the blowdown program recommended by your SOLECHEM customer representative, who carries out the water treatment program in your steam boiler systems, according to the results of the routine analysis will protect your system.