An Overview of Mountings and Fittings on Steam Boiler
A steam boiler is a closed vessel that boils water to generate saturated or superheated steam. Back in the 19th century, the explosion of steam boilers was a common occurrence. To overcome the challenge, companies conducted independent examinations to reduce the possibility of an explosion and determine its cause. It eventually led to the manufacturing and installation of mountings and fittings on steam boilers that enhanced the safety of the boiler system over time. Mountings on a steam boiler are a set of safety devices installed for the safe operation of a boiler. It safeguards the steam boiler from harm caused due to extreme pressure, low water level, steam backflow, unregulated steam pressure, shell collapse due to vacuum, backflow of feed water to the pump, and dry running.
What is Steam Boiler Mountings?
Steam boiler mountings are fittings, mounted on the steam boiler to ensure its proper functioning. Boiler mountings include water level indicator, safety valve, pressure gauge, etc. It is necessary to note that a boiler cannot function safely without the mountings. These fittings mounted on the steam boiler are required mandatorily for the safe and proper operation of the boiler system.
The primary mountings in the steam boiler are:
- Water level indicator
- Pressure gauge
- Safety valve
- Steam stop valve
- Blow off valve
- Feed check valve
- Fusible plug
- Water Level Indicator:
A water level indicator is installed outside the steam boiler shell to determine the water level in the boiler through a glass tube. It is safety equipment that ensures the safe operation of the steam boiler.
The Water-tube indicator consists of a vertical hard glass tube that is fitted with two gunmetal tubes A and B. Tube A and B join the steam space of the boiler and the water space of the boiler with the glass tube respectively.
Tube A is fitted with a valve, called a steam valve, and tube B is provided with another valve called a water valve. Additionally, a third valve called the drain valve is fitted to the water level indicator. The drain valve helps in periodically draining the water together with condensed steam from the gunmetal tube A.
- Pressure Gauge:
A pressure gauge indicates the steam pressure in the boiler. It is mounted usually on the front top of the steam boiler.
The pressure gauge consists of a circular spring tube A. One end of the tube is closed and joined to a link, while the other end is to a hollow block. The link connects the closed end of the tube to the toothed sector that is hinged. The toothed sector gears with a pinion that carries a pointer. The pointer moves on a dial, indicating the pressure units.
- Safety Valve:
The safety valve is a safety mounting fitted on the steam boiler shell and is crucial on the boiler shell to ensure the safety of the boiler against high pressure. It prevents explosions due to excessive internal steam pressure in the boiler system.
The safety valve is equipment to prevent the increasing steam boiler pressure above its operating pressure. It opens automatically as the pressure in the steam boiler exceeds the normal working pressure. Therefore, it allows excess steam to release into the atmosphere until the pressure returns to its normal valve. A safety valve ensures the safety of the steam boiler from damages due to excessive steam pressure.
The primary types of safety valves include:
Deadweight safety valve
Lever safety valve
Spring-loaded safety valve
- Steam Stop Valve:
A steam stop valve controls the steam flow from within the steam boiler and stops it completely when required. A steam stop valve regulates the steam flow from the boiler system.
The valves mounted on the steam boilers that change the direction of steam flow by 90° are known as junction valves. On the other hand, valves fitted in pipelines that allow the steam in the same direction are called stop valves.
- Blow-Off Valve:
The blow-off valve helps in periodically removing the sediments that are deposited at the bottom of the steam boiler while it is in operation and emptying the steam boiler during its cleaning or inspection.
As the blow-off valve is opened, the water under the pressure of steam rushes out with high velocity and releases the sediments along with it.
- Feed Check Valve:
With the falling level of water in the steam boiler, it is revived to the specified level by supplying excess water. It is known as feed water. The pressure in the boiler will be high, consequently increasing the feed water pressure by a pump before feeding it to the steam boiler. The feed water under high pressure is fed to the boiler system through the feed check valve.
The feed check valve controls the water flow from the feed pump and prevents the backflow of water from the boiler system to the pump. Therefore, the feed check valve is located at the boiler end of the delivery pipe of the feed pump.
- Fusible Plug:
The fusible plug consists of tin or lead alloy of low melting point. The fusible plug puts off the fire in the furnace of the steam boiler as the water level falls below the safe level and thus prevents the explosion that may result from overheating of the tubes and the shell. It is fitted at the apex of the furnace or the combustion chamber.
Conclusion:
Rakhoh Boilers are renowned for their efficient and reliable steam boilers, waste heat recovery boilers, thermic fluid heaters, and boiler accessories since their inception in 1983. We also provide excellent boiler services like energy audit, steam trap assessment, boiler automation, annual boiler maintenance, fuel conversion, etc., to enhance the efficiency and productivity of steam boilers.
Learn more about our products and services at www.rakhoh.com
- Published in Boiler
Causes and Prevention of Boiler Tube Failures in Fireside of Steam Boilers
Steam boilers are an integral asset in the process and manufacturing industries. Boiler tube failures are one of the major concerns of the process plant managers as they can cause severe steam boiler issues if overlooked. In such situations, it is advisable to contact an expert boiler manufacturer to seek guidance and immediate action to prevent boiler tube failure. Usually, a boiler tube failure causes other boiler problems. Therefore, it is important to perform a careful examination to understand the cause of the boiler tube failure, neglecting which may lead to the steam boiler failure and related expenses for the same.
Since it is vital to prevent severe damage in the process plant and the operating personnel, here are the listed causes and solutions of boiler tube failures in fireside of the steam boiler:
- Fuel Ash Corrosion:
Fuel ash corrosion results from the ash characteristics of the fuel and the design of the steam boiler. It occurs usually in coal-fired boilers but also takes place in oil-fired boilers. The ash is considered in the steam boiler design while determining the size and materials to be used in the steam boiler. Additionally, the combustion gas and temperature of the metal in the convection passes are essential considerations. Harmful consequences take place with coal ash constituents remaining in a molten state on the superheater tube surfaces, leading it to be highly corrosive.
It is characterized by loss in external tube walls and increasing boiler tube strain, with an appearance usually seen during the removal of scale and corrosion products. It affects the superheater in a steam boiler. Ash corrosion can be reduced significantly by using materials with higher chromium concentrations. Installing stainless steel boiler tube shields reduces fuel ash corrosion in steam boiler components with very corrosive conditions. Introducing calcium and magnesium to the fuel can also help lower fuel ash corrosion.
- Fireside Corrosion Fatigue:
Fireside corrosion fatigue is caused by corrosion and thermal fatigue. Boiler tube surfaces face thermal fatigue stress cycles that take place due to soot blowing, slag, or cyclic operation of the steam boiler. Additionally, thermal cycling can cause cracking of the less lubricated external boiler tube scales, exposing the base to repeated corrosion.
Boiler tubes develop multiple cracks on the outer surface and spread into the tube wall. It is usually found on furnace wall tubes of coal-fired boilers but also takes place on tubes in drum-type boilers. The solution to prevent fireside corrosion fatigue is to lower the ramp rates during start-up and shut down for reducing the thermal stress. Additionally, optimizing soot-blowing operations can minimize thermal stresses.
- Waterwall Fireside Corrosion:
Waterwall fireside corrosion is found on external surfaces of water wall tubes as the combustion process produces a reduced atmosphere. Coal-fired boilers with maladjusted burners or staged firing are more likely susceptible to localized regions with a reducing atmosphere, resulting in increased corrosion rates. It is indicated by external tube metal loss leading to thinning and increasing boiler tube strain and usually affects water wall tubes.
The ideal method to prevent water wall fireside corrosion is by using a weld of high nickel overlaid on boiler tubes with the worst corrosion. Corrosion-resistant thermal sprays are also an ideal solution for this issue.
- Erosion:
Erosion of boiler tube surfaces occurs from impingement on the external surfaces. Firing fuels with high ash leads to increasing erosion, slagging, and fouling issues in a steam boiler. The erosion occurs along with thermal fatigue in cases of soot blower steam. It is indicated by the boiler tube experiencing metal loss. Ultimately, failure is caused by the rupture due to increasing strain as boiler tube material erodes.
Erosion is commonly found near soot blowers on the economizers, superheaters. It can be prevented by distributing flow evenly in the steam boiler and considering a lower ash fuel.
- Fatigue Failure:
Fatigue failure is the result of cyclical stress in the steam boiler. Contrary to thermal fatigue, damage of fatigue failure is caused by external stress through vibration by flue gas flow, soot blowers, or boiler cycling.
It is characterized by the failures localized to the area of high stress and impact tube penetrations, welds, or supports. Fatigue failure is prevented by detecting and minimizing the source of stress.
General Failure of Boiler Tubes:
- Short-Term Overheat:
Short-term overheat failures occur during boiler start-up and fail with the increasing temperature of tube metal due to lack of cooling steam or water flow and impacts furnace wall tubes and superheaters. It is prevented by avoiding blockages within boiler tubes and bends and following standard procedures for shut-down and start-up.
- Long-Term Overheat:
Long-term overheat takes place over months or years as superheater tubes fail after many years of service due to creep. Furnace water wall tubes also fail due to long-term overheating. It is prevented by cleaning tubes with chemicals for better heat transfer and by balancing the furnace/flue gas temperatures with circulation to reduce the tube temperatures.
Conclusion:
Rakhoh Boilers has been a leading boiler manufacturer since its inception in 1983 with world-class manufactured steam boilers, waste heat recovery boilers, thermic fluid heaters, and boiler accessories. We deliver the best boiler services like annual boiler maintenance, boiler automation, energy audit, steam trap assessment, fuel conversion, etc.
For more details on our products and services, visit www.rakhoh.com
- Published in Boiler
An Overview of Superheated Steam and Its Applications
Steam plays a pivotal role in the process and manufacturing sector for heating purposes. It is an ideal medium for transferring heat due to its high energy holding capacity, safety, and reliability while utilizing it. Most steam boilers use saturated steam for heating and processing in operations. However, in some cases, superheated steam is preferred over saturated steam. A superheated boiler pushes through the saturated steam limit to produce steam that does not include moisture (or wetness). The dryness in superheated steam boilers is an ideal alternative for turbines, along with cleaning, drying, and curing applications.
What is Superheated Steam?
If we analyze the steam closely, we notice that steam varies based on temperature and pressure. To understand steam better, we need to understand the two basic types of steam, i.e., saturated steam and superheated steam. As the saturated steam generated in the steam boiler is exposed to a higher temperature surface, its temperature increases above the evaporating temperature.
Due to its high amount of temperature through its heating above the saturation temperature, it is termed superheated steam.
Interestingly, superheat cannot be passed to the steam in the presence of water, as excess heat would evaporate more water. The saturated steam is passed through an additional heat exchanger, considered a second heat exchange stage in the boiler or a separate superheater unit. The primary heating medium is either the hot flue gas from the steam boiler or a separate fire.
The application of superheated is found in steam turbines that involve steam directed by nozzles onto a rotor that causes the rotor to turn. The energy required to accomplish it is obtained only through steam. Therefore, the steam has less energy after passing through the turbine rotor. If the steam is at saturation temperature, the energy loss causes the steam to condense. Turbines have multiple stages, including the exhaust steam from the first rotor directed to a second rotor on the same shaft. It results in saturated steam getting wetter as it progresses through the successive stages. Such occurrences lead to water hammer and the water particles causing severe erosion within the turbine. Therefore, superheated steam is provided to the turbine at the inlet and the energy is used for the rotor till its temperature/pressure reaches saturation.
Difference between Saturated Steam and Superheated Steam:
Steam is used in almost every process and manufacturing industry due to its heat transfer characteristics. Steam is one of the most efficient, effective, and reliable methods of transferring energy. Let us now understand the difference between both types of steam in detail.
The majority of steam boilers nowadays use saturated steam, generated by the transfer of heat to water that causes it to boil. It is similar to a pressure cooker in its operation. The steam is not released freely as the pressure is controlled for processing and heating purposes. Such steam is a by-product of the saturated steam curve with a relationship between temperature and pressure. Saturated steam is used for efficient and effective heat transfer purposes.
On the other hand, superheated steam is generated as the saturated steam passes out of the boiler drum and into a secondary heating area, termed superheater. It results in the saturated steam reaching beyond the saturated steam curve and into the superheated steam area, leading to the absence of moisture (or wetness) in the steam. Due to this, superheated steam is generated outside the main boiler drum. The lack of moisture in superheated steam is best-suited for turbines, drying, cleaning, and curing purposes.
Applications of Superheated Steam:
Superheated steam is not an ideal medium for transferring heat; it is sometimes utilized for heating in various steam plants, particularly in the HPIs (Hydrocarbon Processing Industries) that deliver oils and petrochemicals. As the superheated steam is available on site for power generation as the energy source for turbines. HPI usually desuperheat steam to about ten degrees of superheat that is removed easily in the first stage of the heating surface.
Superheated steam is not as suitable for process heating as saturated steam due to the following reasons:
- Superheated steam needs to decrease its temperature to saturation before it condenses to release its latent heat. The heat given up by the superheated steam to reach a saturation temperature is lower than its enthalpy of evaporation.
- Steam with a few degrees of superheat is given up promptly before condensation. With steam having a larger amount of superheat takes a longer time for cooling, and the steam releases very little energy.
- The temperature of superheated steam is not uniform, and it needs to cool to give heat. It means that the rising temperature over the heat transfer surface occurs with superheated steam.
- Using superheated steam in a heat exchanger can result in the formation of a drywall boiling zone. It quickly becomes scaled or fouled, and the resulting high temperature of the tube wall causes tube failure.
Advantages of Superheated Steam:
- As steam heats beyond the saturated steam temperature, the lack of condensate is ideal for applications like rotating equipment such as turbines as the blades are damaged from water droplets and condensate.
- The dryness of superheated steam is ideal in the steam engines of locomotives.
- Superheated steam has a higher thermal conductivity for drying and curing applications than air and reduced oxygen content, allowing efficient drying while preventing oxidation.
- Superheated steam is effective for chemical treatment in agricultural soils as the high heat energy helps in the deterioration of organic material as the steam is introduced into the soil.
Conclusion:
Since its inception in 1983, Rakhoh Boilers have emerged as the leading boiler manufacturer in Pune with 3000+ successful boiler installations in over 26 countries worldwide. We have delivered efficient steam boilers, waste heat recovery systems, thermic fluid heaters, boiler accessories, and boiler services to 20 process industries globally.
For more details, visit our website www.rakhoh.com
- Published in Boiler
An Overview of Effects and Prevention of Hard Water in Steam Boiler
Steam Boilers are undoubtedly an essential asset in the process and manufacturing facilities. However, it is imperative to take every measure in ensuring the proper functioning of the steam boiler for optimal productivity. Boiler water and its quality play a vital role in hassle-free operations of the boiler system. Similarly, neglecting the boiler water quality results in damaging the steam boiler components, consequently leading to boiler failure. Hard water can negatively impact the performance of the steam boiler. Hard water consists of a higher concentration of dissolved minerals like calcium and magnesium. Let us learn about hard water in detail to understand it better.
Difference between hard water and soft water:
As mentioned, hard water includes a higher concentration of dissolved minerals, particularly calcium and magnesium. Water from the treatment plant consists of small amounts of dissolved solids such as calcium and magnesium, damaging the tubes in the steam boilers. The minerals can lead to hard scale buildup on the tube surface, resulting in reducing the water heat transfer and overheating the tubes.
On the other hand, soft water has relatively less calcium and magnesium but higher dissolved sodium. Therefore, water softening units utilize sodium pellets for hard water softening by exchanging the calcium for sodium.
Hard Water and Steam Boiler:
Hard water causes several problems in steam boilers. It is due to the dissolved ions left behind as the water is heated to the steam. The leftover ions bond and form calcium carbonate, also known as limescale. The limescale covers pipes and other components of the steam boiler.
- Suspended solids:
Suspended solids are substances present in water as suspended particles, usually minerals. These substances are not a significant issue as they can be filtered out.
- Dissolved solids:
As the name suggests, dissolved solids are substances that dissolve in water, primarily including carbonates and sulfates of calcium and magnesium that form scale when heated. In process operations, it is necessary to examine any salts that form scale in the steam boiler that should be altered chemically to produce suspended solids or sludge instead of scale.
- Dissolved gases:
Dissolved gases include oxygen and carbon dioxide that is dissolved easily by water. The gases are major contributors to corrosion.
- Scum forming substances:
It includes mineral impurities that foam or scum, such as soda in the form of carbonate, sulfate, or chloride.
The impurities present in boiler water are less and generally expressed in water analysis as parts per million (ppm), by weight, or in milligrams per liter (mg/l).
Identifying Hardness in Water:
Water used for steam boilers is either hard water or soft water. It is differentiated by the impact of water on the soap as a higher amount of soap is required for making lather with hard water compared to soft water. Hardness results from the mineral salts of calcium and magnesium that lead to scale formation.
The hardness is further classified into two types:
- Alkaline hardness or temporary hardness:
Alkaline hardness, also termed as temporary hardness, is caused due to the presence of calcium and magnesium bicarbonates. The salts dissolve in the boiler water and form an alkaline solution. As the heat is applied, it decomposes to release carbon dioxide, scale, and soft scale. It is termed temporary hardness because the hardness is removed by boiling.
- Non-alkaline hardness and carbonates or permanent hardness:
Non-alkaline hardness is caused by the salts of calcium and magnesium, although in the form of sulfates and chlorides. As the temperature increases, it precipitates from the solution, due to its reduced solubility and forms a hard scale that is difficult to remove.
Additionally, the silica in the steam boiler water leads to a hard scale that reacts with calcium and magnesium salts and forms silicates that severely cover the heat transfer across the fire tubes, causing it to overheat.
Total hardness:
Total hardness is not a type of hardness, but the total concentrations of calcium and magnesium ions present when both expressed as CaC03. With alkaline water, a proportion of hardness that is equal in magnitude to the total alkalinity, also expressed as CaC03, is considered as alkaline hardness, and the remaining as non-alkaline hardness.
Non-scale forming salts:
Non-hardness salts like sodium salts are relatively more soluble than the salts of calcium or magnesium that do not generally form scale on the surfaces of a steam boiler.
Comparative units:
As salts dissolve in water, it forms electrically charged particles termed ions.
The metallic parts (calcium, sodium, magnesium) are cations, attracted to the cathode, carrying positive electrical charges. On the other hand, anions are non-metallic, carrying negative charges like bicarbonates, carbonate, chloride, sulfate that get attracted to the anode.
pH Value:
pH value is a numerical value indicating the hydrogen content of water that determines the measure of the acidic or alkaline nature of the water. Water, H2O, includes two types of ions – hydrogen ions (H+) and hydroxyl ions (OH-).
With predominant hydrogen ions, the solution is acidic with a pH value between 0 and 6. However, with higher hydroxyl ions, the solution is alkaline with a pH value between 8 and 14. An equal amount of hydroxyl and hydrogen ions results in a neutral solution with a pH value of 7. Acids and alkalis can increase the conductivity of water above the neutral sample.
Prevention for Hard Water:
The ideal solution for the issue of hard water is a water softener in the steam boiler. Water softeners introduce sodium compounds into the steam boiler for the removal of calcium and magnesium and reduce hard scaling in the steam boiler. It improves the water flow, increases efficiency levels, and prevents failures or damages that can reduce boiler lifespan.
It is equally important to schedule regular boiler maintenance to prevent the build-up of the limescale.
Rakhoh Boilers are a trusted name for boiler manufacturing and thermal solutions since 1983. We provide efficient steam boilers, waste heat recovery boilers, thermic fluid heaters, boiler accessories, and boiler services in over 26 countries globally.
For more information, visit www.rakhoh.com
- Published in Boiler
An Overview of Combustion Problems of Biomass Boilers and Its Solutions
Steam Boilers are irreplaceable equipment in the process and manufacturing sector industries. It produces heat through the combustion of fuel that is transferred to the water for converting it to steam at desired temperature and pressure. It is concluded that the efficiency of process industries is dependent on the efficiency of the steam boiler. The increasing fuel prices and concern towards sustainable energy production have led to the rising popularity of biomass boilers for process operations. Biomass is a type of fuel that consists of low density, bulkiness, and releases volatiles. Biomass is gaining preference for various reasons like reducing dependency on fossil fuels, more employment, and reduction in greenhouse gases and acid rain. However, there are various challenges to biomass boilers, particularly with combustion.
Combustion Challenges in Biomass Boilers:
- Agglomeration:
The issue of agglomeration is the ash-related problem, usually found in biomass boilers. Ash is formed from a high-sulfur and low-ash fuel agglomerate, with the sulphation degree varying with time and temperature. Ashes agglomerate by the production of 50–60% or more amount of calcium sulfate and Ca–K-silicates in the deposit. Initially, agglomeration occurs due to carbonation, and later due to sulfation at lower temperatures. Ash agglomeration increases with high iron or alkali metal content as the low-melting alkali chlorides promote the stickiness of fly ash particles. As a result, it increases the ash deposition on the superheater tubes. The deposited alkali chlorides contribute to the increasing corrosion of superheaters.
Solutions for agglomeration:
Agglomeration can be reduced by using additives such as sulfur, kaolin, and ammonia sulfate. Adding kaolin to the bed material before the combustion process can eliminate agglomeration. As alkali sulfates have high melting points compared to corresponding alkali chlorides, it has minimal tendency to stick to the superheaters as deposits. Therefore, it reduces the possibility of deposit formation and corrosion on the superheaters.
- Corrosion:
High-temperature corrosion is termed as the increased oxidation of materials induced by salt film deposition at accelerated temperatures on the fireside of the boiler. The increased temperature ranges from 700 to 1300 °C. High-temperature corrosion includes nitridation, flue gas, carburizing, sulphation, oxidation, chlorination, and corrosion deposit. Fused alkali sulfates are deposited on the hot substrates with the oxidation of metal contaminants such as sulfates and vanadium in the fuel.
Solutions for Corrosion:
- Inhibitors:
Corrosion inhibitors are substances that reduce the rate of corrosion of the metal. The factors for corrosion inhibition are the composition of fluid, flow regime, and quantity of water. Inhibitors are used in processing industries as it is the best defensive agent against corrosion. Corrosion inhibitors are also termed additives to the fluid surrounding the metal.
- Sol-gel coating:
Sol-gel coating is well-known for protection against corrosion as it includes better chemical stability, oxidation control, and corrosion resistance for metallic substrates.
- Varying temperature and pressure:
Findings suggest that the corrosion rate of metal doubles with every 10°C increase in the temperature. Therefore, the corrosion rate of 30 mpy (mils per year) at 30°C would increase to 60 mpy at 40°C. As the temperature increases with passive film remaining intact, it results in the corrosion rate remaining stable. However, with an increase in temperature in the passive film, the corrosion rate increases rapidly. Therefore, the surface temperature of the metal should be determined during process operation. The inner diameter of the tubing gets hotter as the hot wall impacts the re-boiler tubing. As a result, the rate of corrosion is higher than expected.
- Coating:
Coatings protect against erosion and corrosion by shielding the material from several chemical and physical damages that occur due to direct contact of material with the environment. As corrosion results in dilapidation, it causes failure of components in biomass boilers. The coating improves surfaces against corrosion and deposits suitable material on the substrate.
Operational Challenges of Biomass Boilers:
- Availability of biomass and storage problem
- Difficulty with transportation due to moisture content of biomass fuel
- Inefficient fuel conversion, core technology, and equipment
- Industrial chain
Economic Challenges of Biomass Boilers:
- Cost of Feedstock acquisition
- High investment and capital cost
Social Challenges of Biomass Boilers:
- Issues with land use
- Impact on the environment with loss of biodiversity
Policy and Regulatory Challenges of Biomass Boilers:
- Government Policies
- Lack of Special Department for Biomass
Conclusion:
One of the major issues related to biomass fuel is the energy density and moisture content. For instance, 30% moisture in conventional wood means that 1 ton of wood contains 300 kg. of water. Additionally, process industries need to ensure fuel conversion of the steam boiler before utilizing biomass fuel.
Over the last 38+ years, Rakhoh Boilers have emerged as a leading steam boiler manufacturer in Pune by delivering efficient industrial steam boilers in 26 countries worldwide. We also provide excellent boiler services like fuel conversion, energy audit, steam trap assessment, boiler automation, annual boiler maintenance, etc.
To learn more about our products and services, visit www.rakhoh.com
- Published in Boiler