Causes and Prevention of Scale Formation in Boilers
Industrial boilers are paramount for generating steam that is used for processing operations. Steam generation occurs by transferring heat energy to the water. As boilers are made of metal that are good conductors of heat, heat transfer efficiency is a major factor that influences boiler performance. Depending on the amount of impurities in feed water quality, it causes a significant impact on the formation of deposits in the boiler. During continuous boiler operation, it is challenging to maintain clean metal surfaces and prevent any layer formation. Scale formation is one of the effects of low-quality feed water that reduces the heat transfer in the boiler.
What is Scale Formation in Boilers?
The precipitation of impurities from the feed water on the heat transfer or metal surface on the boiler causes scale formation. During evaporation, the deposits become hard and concentrated, which hinders the heat transfer and causes hot spots, eventually leading to local overheating. Feedwater contaminants such as calcium, magnesium, iron, silica, and aluminum are the major causes of deposit formation. Scale formation occurs due to salts that are not completely insoluble in the boiler water. Hence the salts reach the surface in a soluble form and precipitate.
The presence of calcium and magnesium salts is the main cause of scaling in boilers, along with high concentration of silica to the water alkalinity in the boiler. Scale salts generally consist of carbonates, bicarbonates, and sulfates that form slowly in a concentrated pattern, reducing the heat transfer. If overlooked, scales can be extremely difficult to remove even by using chemical treatment.
The carbonate deposits are generally granular or porous. The calcium carbonate crystals are large and matted, which leads to dense scale formation in boilers. Carbonate deposits can be identified by suspending them in an acid solution as the carbon dioxide bubbles will effervesce from the scale.
The sulfate deposit is denser compared to the carbonate deposit as the crystals are smaller and bond together. The sulfate deposit is brittle that does neither easily pulverize nor effervesce when introduced to acid.
Deposits with high silica are hard with extremely small crystals that lead to a dense scale formation in boilers. The scale is light in color and brittle that causes difficulty in pulverizing. It is not soluble in hydrochloric acid.
Causes of Scale Formation in Boiler:
Let us look at the factors that contribute to the scaling in steam boilers:
Feedwater quality:
One of the most impactful factors causing scales in the boiler is the low quality of feed water. Boiler manufacturers suggest pretreatment of feedwater that involves analyzing the water source, its quality, and the removal of suspended or dissolved solids through the process of filtration, softening, or demineralization. Feedwater of low quality can result in scaling and deposition because of hardness and iron.
Contaminated condensate:
When the steam vapor is returned to its liquid state, it forms condensate taking place during utilizing steam in process operations. It retains about 13% of the steam energy that is used in boilers to reduce make-up water and fuel costs. Contaminated condensate, if not treated properly, causes scaling in boilers.
Boiler Pressure:
The amount of scale formation in the boiler depends on the pressure at which the steam boiler operates. Depending on industry and system, boiler manufacturers suggest the ideal chemical properties and pressure to use for optimal functioning of the boiler.
Boiler Design:
Some boiler designs are prone to scaling on a greater scale than others. Few steam boilers have varying heat flow rates at the tube surface with the same pressure rating. Boiler design and efficiency can be major factors causing scaling in boilers.
Irregular Boiler Maintenance:
Irregular and poor maintenance of the boiler system or negligence with feed water treatment causes scaling that may cause harm to the steam boiler and the process plant.
Prevention of Scale Formation in Boiler:
If scaling is neglected, it may cause a reduction in heat efficiency by acting as an insulator that would slow down the heat transfer. Scaling, if left unchecked, may lead to overheating and rupture of the tube in the steam boiler. Some of the ways to prevent scale formation in boilers are:
- Boiler Water Treatment: Scaling is caused by the presence of insoluble salts, calcium, and magnesium in the feed water. It is crucial to ensure proper boiler water treatment before it is utilized for operations.
- Water Softeners: Installing water softeners can help in preventing limescale in the steam boiler.
- Removal of Scales: Scales that are formed loosely on a boiler surface can be removed with a wire brush or a scraper.
- Cooling Water Inhibitors: Cooling water inhibitors are suitable for extending the solubility of the salts and prevent scaling in boilers.
- Thermal Shocks: Thermal shocks are conducted for scale that is brittle in nature.
- Chemical Cleaning: If the scale formation is extensive in the boiler, then boiler manufacturers can perform chemical cleaning for the removal of scale and corrosion in boilers.
- Blowdown Operations: Frequent blowdown can help in removing the scale that is formed loosely on the surface of the boiler.
- Regular Maintenance: Regular monitoring and maintenance of the steam boiler can help in preventing scale formation before it gets denser.
Conclusion:
Scaling can be harmful to the steam boiler as it increases fuel requirement and cost and reduces heat transfer and boiler efficiency. Rakhoh Boilers is one of the leading boiler manufacturers since its inception in 1983 that delivers high-quality steam boilers and the best boiler services to more than 20 process industries.
Know more about us by visiting www.rakhoh.com
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Steam Quality in Boilers | Importance, Effects and Ways to Measure It
Boilers are the heart of any manufacturing industry, and therefore, the boiler used in process industries must be efficient. Boilers require steam generation for heating purposes in the industry. Boiler efficiency calculates the amount of heat energy converted into steam. Compared to other heat sources, steam is the most preferred one as it is cost-effective for operations and reusable. Yet, despite the many advantages of using steam for heating, boiler operators face many challenges such as corrosion, erosion, steam trap overload, and even boiler shut down due to low water levels. The cause behind most of these boiler problems is poor steam quality.
Steam Quality in Boilers:
Steam Quality can be termed as the amount of liquid contained in the steam. For instance, 100% steam quality refers to steam, without water present in it, and if the steam quality is 80%, then it contains 80% of steam and 20% of water. The water in steam can cause erosion in pipes and valves of the boiler. If water gathers in the steam pipes, it would be carried by high-velocity steam that may lead to vibration, erosion, and water hammer that, in turn, may cause loose pipe fittings, causing boiler shutdown.
Importance of Steam Quality:
It is important to understand that steam is essential for quality manufacturing and hassle-free process operations. Granted, that the steam boiler and its equipment must be regularly checked and maintained. However, in most cases, the steam quality is overlooked. Thus, it is necessary to keep a tab over steam quality to prevent boiler problems and ensure the high quality of the products manufactured through steam.
Effects of Steam Quality on Boiler:
Poor steam quality may lead to harmful consequences in the boiler operation. Here are some of the most commonly found effects of steam quality on boiler:
Failure of the Valves:
The passing of the poor steam quality containing liquid from the steam control valves causes erosion inside the valves that fail the steam valves in boilers.
Failure of the Turbine Components:
Steam consisting of liquid when generated in the turbine operation may reduce the total lifespan of the internal components of the turbine.
Heat Transfer Efficiency:
One of the most occurring problems caused by low steam quality is the reduced heat transfer efficiency in the operational process. It must be noted that poor steam quality can reduce as much as 65% of the heat transfer efficiency.
The liquid contained in the steam has around 16% of sensible energy. However, the latent energy in steam is about 94% which is significantly higher in comparison to sensible energy. It results in less amount of reusable energy being supplied to the steam process equipment. Secondly, the excess liquid in the steam accumulates on the surface of the heat exchanger, causing the formation of a water layer that hinders the transferring of the latent energy to the equipment.
Water Hammer:
Steam tends to condense due to the heat loss in the pipe, forming droplets in the internal walls. As they pass with the steam, they merge as a layer, gravitating towards the bottom of the pipe as the layer begins to thicken. It results in the formation of a slug of water that is dense, incompressible, and contains a high amount of kinetic energy.
Obstruction by the bend in the pipe compels the kinetic energy to convert into pressure energy. It results in pressure shock coming in contact with the obstruction. The effect caused by the slug water and obstruction is termed as water hammer. Water Hammer may considerably reduce the life cycle of the pipes and, in extreme cases, may cause an explosion.
Factors that Determine the Steam Quality:
Various factors determine the steam quality for the boiler operation. Some of them are as follows:
Steam Temperature and Pressure:
The steam temperature and pressure must be appropriate depending on the process industry. Failing that may affect the performance of the boiler.
Quantity:
The quantity of steam used in processing affects the flow rate of heat. The flow rate depends on the sizing of the pipes. Therefore, an improper flow rate may be harmful to the boiler and reduce productivity.
Cleanliness of Steam:
Steam is a crucial element in the process operation and hence, it is necessary to ensure it is clean and prevent the formation of scaling on pipes that leads to increased erosion, reducing the quality of steam.
Dryness of Steam:
Improper chemical treatment of feed water causes priming and carryover to the steam mains that gets settled on the heat transfer surfaces, eventually reducing the boiler efficiency. Also, the presence of liquid in steam results in the formation of scale on the pipes. The high amount of droplets in steam hinders the heat transfer process in the boiler. Therefore it is necessary to ensure the dryness of steam.
How to Measure the Steam Quality in Boilers?
Rakhoh Boilers, with their extensive knowledge and expertise in thermal solutions, advises that steam quality assessment is one of the important factors to consider in ensuring the efficiency and lifespan of the boiler. Here are the ways to measure steam quality:
- Testing the steam quality through Throttling Calorimeter or Ganapathy’s Steam Plant Calculations
- Assessing the steam by opening the steam valve that releases the steam into the atmosphere to provide an estimated steam quality result
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Importance, Types, Effect, and Treatment of Feed Water Quality in Steam Boilers
One of the major concerns faced by process plant managers is to maintain the functioning of the steam boilers by ensuring its wear and tear to prevent boiler failure, breakdown, or worse, explosion. Process plants generally face challenges such as corrosion, scaling, and foaming in boilers, caused by the poor quality of feed water. Feed water quality is a crucial factor to ensure boiler safety and quality of processing, and therefore, it is necessary to monitor the quality of feed water and treat any impurities before utilizing it in the steam boiler.
Why is Feed Water Important?
Water is one of the excellent raw elements for the heating and power generation process. That is because it extracts more heat when the temperature increases that eventually leads to steam generation. However, water contains some amount of impurities such as dissolved solids and gaseous substances that might prove harmful for the boiler operation.
Feed water quality actually depends on the amount, nature of impurities contained in it, and the capability of the steam boiler to handle it. For instance, low-pressure fire-tube boilers can tolerate feed water hardness if it is treated beforehand.
Types of Impurities that Affects the Boiler Operation:
Here are the different types of impurities found in water that may damage the steam boilers depending on their amount:
Total Alkalinity:
Total alkalinity includes bicarbonate and a slight amount of carbonate in water. Raw water with total alkalinity, if used in the boiler may cause scaling.
Bicarbonate and Carbonate:
As mentioned, total alkalinity contains bicarbonate. If heated, it decomposes to carbonate and carbon dioxide that, on further heating produces caustic (OH) that may or may not be beneficial for the boiler based on its operation. However, carbon dioxide may cause corrosion in boilers.
Calcium:
Calcium is the primary source of hardness in feed water. Calcium salts lead to corrosion in the steam boiler system.
Iron:
Iron is found in a soluble ferrous form that forms insoluble hydroxides or oxides if contacted with air. The iron amount must be reduced if it exceeds 0.3 ppm.
Magnesium:
Magnesium produces hydroxides or silicates in the feed water that forms sludge in the boilers. Magnesium phosphate is a sticky substance that forms scaling.
Chloride:
The concentration of chloride must be avoided in the feed water, as it causes crevice corrosion and pitting in the boiler system.
Silica:
Controlling the amount of silica in feed water is essential, particularly in high-pressure boilers, as it causes scaling problems.
Sulfate:
Sulfate is unavoidable in the feed water, but excessive sulfate may cause corrosion and scaling due to calcium sulfate.
pH:
The pH of the feed water must range between 5.5 and 8.0. If it exceeds the limit, it causes scale formation, and if it reduces the limit, it causes corrosion.
Total Dissolved Solids (TDS):
Total Dissolved Solids must be controlled, as it is the major cause of corrosion in the boiler system. TDS levels must be regularly monitored and maintained.
Hardness:
Hardness in feed water can be determined by the amount of calcium carbonate in water. Excess hardness in feed water may cause deposits and scaling.
Effects of Poor Feed Water Quality in Boilers:
Some of the major problems caused due to the poor quality of the feed water are as follows:
Foaming and Carry Over:
Foaming and carry-over are the most occured problems in steam boilers that are caused due to high levels of solid concentration in feed water.
Corrosion:
Corrosion causes harm to boilers by an electrochemical reaction or by pitting dissolved oxygen in boiler water. In other words, feed water containing high levels of oxygen or carbon dioxide may react with the metal of the boiler and result in corrosion. It causes reduced efficiency, high maintenance costs, pitting, rusting of ferrous metal, and shorter life cycle of boilers.
Treatment for Improving Feed Water Quality:
Water is pivotal for steam generation in boilers. Therefore, treating the feed water quality results in safe process operation, increased boiler efficiency, and a long boiler life cycle. Boiler feed water needs treatment to prevent corrosion and scaling in the steam boiler system. Some of the commonly adopted feed water treatment systems are:
- Filtration and Ultra-filtration
- Softening of Ion Exchange
- Coagulation and Chemical Precipitation
- Reverse Osmosis
- Degasification
- Dealkalization
Rakhoh Boilers, with their 38+ years of expertise and experience in boiler manufacturing and thermal solutions, have noted that feed water quality is one of the imperative factors to consider for ensuring safe and reliable boiler operation. Overlooking and neglecting plant maintenance might be hazardous. Rakhoh provides the best boiler-related services for upgrading the process plant operations and productivity.
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The Stages in Sugar Processing and the Role of Steam Boilers
Sugar is a staple commodity in most countries worldwide. It is believed that the production of sugar dates back to 8000 B.C., produced from two primary sources namely, sugar cane and sugar beets. Presently, around 120 countries produce sugar, with 80% of sugar extracted from sugar cane and the remaining 20% from the sugar beet. In India, sugar cane is one of the dominant plantations, taking place in October, March, and July annually, depending on the location. India is the world’s largest consumer and producer of sugar, with sugar mills producing more than 305 lakh tons of sugar annually. Boilers in sugar industry play an imperative role in various process operations.
Sugar is available in various forms, and the most widely consumed among them is granulated sugar, also known as table sugar. Table sugar with large crystals is known as coarse sugar, and the one with smaller crystals is called superfine sugar. Other types of sugar consumed are brown sugar, powdered sugar, pearl sugar, muscovado sugar, turbinado-style sugar, and demerara-style sugar.
Process Stages in a Sugar Mill:
Sugar cane or sugar beet undergoes several stages of processing before sending it to markets and shops. The process stages in the sugar industry include:
Washing:
Once the sugar cane or sugar beet is transported from farms to sugar mills, it is washed before further process. Washing takes place on belts sprayed with water or flue gases containing water. The products are washed by rotating and removing dirt in a rotating drum sprayed with water. Sugar cane is crushed through rollers or swing-hammer shredders and then sprayed with hot water. On the other hand, sugar beet is sliced as small stripes called cossettes that are soaked in hot water to swell plant cells for the extraction process.
Extraction:
Extraction of juice from sugar cane takes place through milling, in which a series of mills crush the sugar cane fiber to separate the juice from the bagasse that can be used as a fuel source. The juice collected is dark green in color and acidic with the sugar concentration that is measured. In the case of sugar beets, the cossettes are filled in tanks of 10 to 20 meters in length that transport sugar beet upwards through a rotating shaft as the sugar is extracted.
Purifying Juice:
In this stage, the sugar cane juice extracted is further purified in a tall tower-like structure. The sulfur dioxide vapor at the bottom starts rising in a process termed sulfitation. Soluble non-sugar material is further separated from sugar juice by the process of carbonation that consists of calcium carbonate or calcium sulfite that assists in precipitation. The juice is heated to denature the protein properties and then mixed with calcium hydroxide. Carbon dioxide bubbles are administered to reduce the alkalinity and the sludge formation that requires filtration to purify the juice.
This process may take several hours, and the sludge is further filtered to remove the remaining sugar. The purified juice is later boiled in a series of evaporators till it reaches 50% to 65% of sugar concentration.
Crystallization:
Crystallization is a major process stage that relies on a steam boiler. In the crystallization process, a vacuum pan evaporates the syrup to saturate with sugar crystals through a process, termed seeding. This seed is pure sucrose suspended in alcohol and glycerin that is added to the syrup. The minute grains of sugar in the solution helps in extracting the sugar in the solution and forming it into crystals. With the boiling of the mixture in the vacuum pan, the crystals convert into a paste known as ‘massecuite’ that is a mixture of sugar crystal and syrup. The mixture is further processed in a large container named ‘crystallizer’ to continue crystallization by stirring and cooling the massecuite.
Centrifugation:
The massecuite is transferred to a high-speed centrifuge to separate sugar crystals and molasses. The centrifuge rotates at 1000 to 2800 revolutions per minute to remove the molasses and retain the sugar in the centrifuge basket. After the process of centrifugation, the sugar is washed with water.
Drying:
Large hot air dryers are used to dry damp sugar crystals and reduce their moisture content to as low as 0.02% and then pass it through hot air in a granulator. The dried crystals are later segregated as per their sizes and packed to transfer to the market.
Role of Steam Boilers in Sugar Processing:
Steam boilers are pivotal in the processing operations of crystallization and drying in sugar mills. Boilers in sugar industry primarily use bagasse, coal, and biomass as fuel. Boilers with traveling grates can ensure proper combustion with fuels like coal and biomass, thus saving excess fuel costs. Additionally, boilers in sugar industry also generate electricity through cogeneration plants.
For optimal performance of the steam boiler in sugar processing, it is advisable to reduce the excess air for the combustion of solid fuels. It helps in improving energy efficiency and reducing emissions. The excess air that reduces the boiler efficiency leads to carry over. Carryover causes erosion in equipment such as economizer, Induced Draft (ID) fan, etc., resulting in unexpected downtime.
It is also essential to monitor the fuel quality and modify the amount of air required for complete combustion and preventing unburned fuels and stack losses.
Rakhoh Boilers have manufactured and delivered efficient steam boilers for the sugar industry that are ideal for sugar processing and generating electricity. Our models such as Bi-Drum/D Type/Power X, Solid Fuel Bi-Drum, Combo X are preferred by our sugar process industry clients.
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Combustion, Furnace, and Types of Grates in Solid Fuel Boiler
Combustion in Steam Boiler:
Combustion is the process of burning fuels in the steam boiler that requires air, heat, and fuel. It can be defined as a chemical process that blends oxygen with the fuel elements. It produces a certain amount of heat, depending on the element that was combined with oxygen. Other elements present in the combustion process are the particles of iron and silica that are known as the impurities or pollutants from fuels that are transferred to the ash pit after the combustion process.
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The following three steps are necessary to ensure complete combustion in solid fuel boilers:
- Extraction of heat that increases the fuel temperature to the point of combustion
- Burning of volatile gases
- Fixed carbon combustion
Furnace in Steam Boiler:
Usually, an average of 12 pound of air is required to ensure the complete combustion of one pound of fuel. In reality, the forced draft fan provides approximately double the amount of air to achieve complete combustion. That is because it gets challenging to supply air uniformly to every part of the furnace. On the other hand, too much air supplied to the furnace results in the gases blowing off before the combustion process is complete. It is vital that the air provided in the furnace is optimum and not beyond the maximum amount required.
The primary objective of the furnace in boilers is to achieve optimal combustion with minimal smoke.
- Excess release of smoke is avoided due to,
- Smoke is one of the main contributors to air pollution
- Excess smoke is an indication of incomplete combustion as it carries unburned gases along with it
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For optimal combustion, a boiler furnace consists of three main components that are,
- Combustion Chamber wherein the combustion process takes place
- Ash Pit for collecting and disposing the waste particulates from the fuel during combustion
- Grates that assist in the ignition of the solid fuels
Role of Grates in Solid Fuel Boilers:
Grates are provided in solid fuel boilers to help with the fuel-burning such as biomass or coal. It is designed to enable the passing of air for combustion. Due attention is given to the opening of the grates to assure that it is neither too broad to pass the unburned fuel particles that may fall nor too confined that it hinders the air passage for the combustion process.
Types of Grates Used in Solid Fuel Boiler:
Among various types of grates used in boilers, three of the most commonly used are stationary grates, traveling grates, and reciprocating grates.
Stationary Grates:
Stationary grates are ideal for burning fuels with less ash content. It is constructed with high-quality, heat-resisting cast iron alloy that is uniformly spaced, tapered, and generally located at the floor tubes of the furnace.
For complete combustion, hot air is spread evenly over the grate surface. The fine particles of the fuel are burned quickly with the suspension. However, the coarse particles remain and spread over the grate area, forming a thin fuel bed. The ash formed is removed by the nozzles in the grate surface, and later on, transferred to the ash pit.
Travelling Grate:
Traveling grates in solid fuel boilers need to ensure constant tension for smooth and hassle-free operation and avoid obstruction of the grate and the grate chain. The tension in the traveling grate is controlled by gravity. Gravity pulls the weight of the grate and maintains the tension.
As the traveling grate operates continuously, it discharges ash frequently. It is ideal for granulated fuels of moderate size with high ash content and is preferred by processing plants that operate extensively. It facilitates a high level of automated cleaning and fuel loading, low emission, and enhanced boiler performance.
Reciprocating Grate:
The reciprocating grate comes in a staircase structure that consists of a series of fixed grates and moving grates arranged in step-like sections from the fuel inlet to the discharge section at the bottom. The rows of grate bars are moved through the hydraulic drive, and the zoned partition beneath the grate ensures separate zones for primary air. Ash pit or ash conveying system is located at the end of the grate.
Reciprocating grate in solid fuel boilers is most suitable for a wide range of fuels. It is constructed with high alloy steel that ensures effective combustion by enhanced air distribution. It uses high-pressure secondary/over-fire air injections to reduce NOX emissions.
Ash Pit:
Ash pit is placed below the grate in a solid fuel boiler that collects the ash particulates from the combustion chamber. Sufficient space is necessary between the bottom of the ash pit and the grates to ensure ample air space.
Rakhoh’s Range of Solid Fuel Boilers:
Rakhoh, with its extensive knowledge and expertise on thermal solutions, offers a range of efficient and reliable solid fuel boilers for more than 20 process industries worldwide. We manufacture world-class boilers that operate effectively with high-quality grates to assist with complete combustion, depending on the fuels used.
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