Combustion is pivotal for heating in steam boilers. Air is crucial for combustion and plays an important role in fuel burning and must be supplied with precision, in different ratios, depending on the type of fuel used. Steam boiler systems in Thermal Power plants use different types of centrifugal fans. It provides air for combustion, fuel to the burners, circulates the gasses for enhanced heat transfer, and releases exhaust gasses. Depending on the boiler size and airflow requirement, various types of fans are used with varying capabilities. Draught fans are essential in thermal power plants by providing tremendous support to the various process activities related to power generation.
Two types of fans used in a boiler power plant are– Draft Fans and Other Process Fans.
- Draft Fans:
Draft fans are important for thermal power plants as it maintains the flow of gasses through the steam boiler system. Draft fans are classified further as Forced Draft fans and Induced Draft Fans.
Forced Draft fans in the power plant are centrifugal fans, utilizing radial airflow blading. It provides the necessary air for fuel combustion by pushing air through the combustion air supply system to the furnace. Located at the inlet of the boiler to push high-pressure fresh air into the combustion chamber, it mixes with the fuel to produce positive pressure.
Compared to Induced Draft fans, Forced Draft fans are more efficient and easy to maintain with clean operating conditions. Backward curved and airfoil centrifugal fans are the most common centrifugal fans used as FD fans.
Induced Draft Fans exhausts all flue gasses from the boiler through the dust collector, passing it to the chimney and into the open atmosphere. Therefore, ID fans are located at the outlet of the steam boiler system. Induced Draft fan creates a negative pressure to discharge the gasses after combustion from the furnace.
As Induced Draft fans can capably handle hot flue gasses, it faces more corrosion and erosion issues. Radial fans and backward inclined blade centrifugal fans are the most common types of Induced Draft fans used in a boiler power plant.
Other Process Fans:
Other Process fans used in a boiler power plant are,
- Primary Air Fan
- Secondary Air Fan
- Flue Gas Recirculation Fan
- Primary Air Fan:
Primary air fans or PA fans are high-pressure fans that supply the air for transportation of coal in the boiler power plants directly from the pulverizer to the furnace. PA fans supply positive pressure upstream of the coal pulverizer.
In power plants, Primary Air fans provide air in conveying the pulverized coal to the furnace area. PA fans produce pressure upstream of the pulverizer for pushing the coal and air mixture through the pulverizer into the furnace.
The most common centrifugal fan used as a PA fan in a boiler power plant is Backward Curved Centrifugal Fan.
- Secondary Air Fan:
The secondary fans or SA fans play a vital role in power plants. The secondary fan helps with the complete combustion of the fuel in the furnace. SA fans increase the airflow as required, to improve efficiency and avoid fuel wastage. Coal-fired thermal plants account for the highest share of electricity generation compared to other methods like hydroelectric, wind, gas, or solar-based plants. Furthermore, SA fans can run continuously. Considering the transportation expenses and high costs of coal mining, it is not ideal to do inefficient burning of the coal in the furnace. Due to all the mentioned factors, secondary fans play a key role in maintaining the efficiency of the thermal power plant.
- Flue Gas Recirculation Fan:
Flue gas recirculation fan reduces the flue gas pollution content in the exhaust gasses from thermal combustion processes. Flue gas recirculation fans are used in a power plant boiler for controlling steam temperature and furnace heat absorption.
Flue gas recirculation fans extract gas from the economizer outlet and the inlet of the pre-heater. It later discharges the gas either to the bottom of the furnace to control the steam temperature or to different locations in the furnace.
Backward curved centrifugal fans are the most common type of gas recirculation fans used in a boiler power plant.
Role of Primary Air Fan and Secondary Air Fan in Thermal Plants:
Air supplied in thermal plants boilers for combustion reacts with the fuel as per its stoichiometric ratio. However, some percentage of air provided for combustion remains unreacted.
It usually occurs due to the fuel quantity fed at once and variation in the fuel particles size. Therefore, a certain amount of extra air requires to be provided to compensate for the air that went unreacted.
Fans of various types and ratings are used as process fans in power plants. Draught fans are typically heavy-duty centrifugal fans that are an essential and irreplaceable component of a thermal power plant.
Rakhoh Boilers is a leading boiler manufacturer in Pune since 1983 with a successful record of installing 3000+ boilers in over 26 countries worldwide. We provide superior quality steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories.
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Steam Boilers are one of the most important equipment in any power plant and process industry that require constant monitoring and inspection at frequent intervals. In a steam boiler, various sections are controlled at the same time with primary sections, such as the boiler drum, a boiling section producing high-temperature water for steam generation. Excessive water level leads to improper steam generation. Steam & Drum Level is critical and difficult to measure and maintain to automate the system and reduce human errors. Precise control of the water level in the drum is an important factor that is performed by developing a PLC and SCADA system. It helps to reduce the manual operation and provide enhanced control and monitoring of the plant and process operations through a SCADA system, a centralized system for supervising the process by preparing the required data respective to process changes.
Programmable Logic Controller (PLC):
Programmable Logic Controller (PLC) is a digital computer for automation that can re-program the logic as per the users’ requirements in industries. PLC is also used for the internal storage of instruction for the function implementation such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog input/ output modules.
Working of Programmable Logic Controller (PLC):
At the start of each operational cycle, the CPU brings in all the field input signals from the input signals from the module to store them into internal memory as a process of the input signal. This internal memory of the CPU is known as the process input image (PII).
The user program is available in the CPU program memory. The CPU takes the status of input from PII and processes it in the user program, and its result is stored in the internal memory of the CPU. This internal memory is called process output image. At the end of the program run, the CPU transfers the signal states in the process image output to the output module and resultantly, to the field control.
- Input-Output in PLC
- Input Layout:
- Feed Pump
- RTD Transmitter
- Level Switch
- Pressure Transmitter
- Output Layout:
- Solenoid Valve for the water inlet
- Solenoid Valve for Steam Outlet
Supervisory Control and Data Acquisition (SCADA):
As the name suggests, SCADA stands for Supervisory Control and Data Acquisition that helps with the supervisory level as it is not a complete control system. SCADA is used for monitoring and controlling plants or equipment. The control may be automatic or managed by operator commands. The data acquisition is completed by the PLC’s scanning the field inputs connected to the PLC that is usually at a fast rate. The central host scans the PTU’s at a slow rate and the data is processed to detect alarm conditions. If an alarm is present, it is displayed on special alarm lists.
PLC and SCADA in Industries:
PLC and SCADA are used to develop and implement boiler automation. Various sensors measure the temperature, pressure, and water level. The parameters are assessed through SCADA, and the operation is controlled by using PLC. The entire arrangement shuts down if the temperature and pressure reach a predetermined value and the automatic check valve opens to discharge the steam and pressure. An emergency alert is activated, and automatic check valves are opened to avoid any hazardous failure. Boiler control is a crucial part of any process plant.
The steam boiler in a process plant or power generation project is controlled through a variety of methods. The method to be used is determined by several objectives such as rising profit, increased efficiency, superior quality, and other factors that are dependent on the industry. Boiler automation has become vital for meeting the primary goal of providing easy operation to these facilities and the needs of the industries. The changes that are consistently taking place in the industrial sector are partially the result of boiler automation. The automation process, which is gradually assuming its position in all process plants throughout the world, has been given priority. It has prepared itself to examine the critical components of the entire process operation, along with their implementation and any difficulties that may arise.
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
Steam boilers are integral for the processing operations in the manufacturing industries. Plant managers follow several guidelines and practices to ensure that the boiler system is operating without any issue. Boiler problems such as corrosion, carryover, scaling, etc., are usually inevitable and require immediate action when detected. However, in case of boiler issues like overheating, prevention is better than cure. Boiler overheating is a serious boiler problem that may cause severe damage to the process plant and the operating personnel by putting them at the risk of explosion. Although advanced boilers are equipped with safety fittings to ensure locking the component before the boiler overheats, it is prudent to be aware of boiler overheating and its signs to prevent it.
What is Boiler Overheating?
Boiler overheating is caused by the failure of the safety equipment. Steam boilers overheat when hot water transported to the central heating system is hindered from circulating. It is usually due to pump failure or blockage in the system. The failure of safety controls maintaining the temperature, pressure, gas, and water supply lead to boiler overheating. In some cases, it may result in a condition termed ‘runaway boiler’ that is hazardous to both the plant and the operating team.
As the boiler system overheats, the internal components of the boiler face immense stress. It causes irreparable damage or, in severe cases, boiler explosion. With the boiler overheating, the components may melt and release toxic smoke in the vicinity. Nowadays, advanced steam boilers with automation control lock the units to prevent overheating and display an error message to notify the plant managers.
Causes of Boiler Overheating:
If the boiler overheating is not prevented at early detection, it may cause tremendous harm to the surroundings and the people. Common causes of boiler overheating are as follows,
- Limescale Build-up
- Blockage in the system
- Faulty pump
- Faulty thermistor
Limescale build-up on the heat exchangers hinders the water flow that leads to boiler overheating. The purpose of a heat exchanger is to heat the water before circulating it for the combustion process. Limescale build-up is also known as kittling as it produces a kettle’s whistle sound.
Blockage in the system:
Blockage in the steam boiler is indicated by strange noises emitting from the system. Radiator not heating properly is another sign of a blockage in the boiler system. It is generally cleaned by a power flush process that eliminates the limescale, rust, and other impurities to allow the seamless circulation of hot water.
The purpose of the pump is to circulate the water in the steam boiler system. If the pump fails to operate, the hot water fails to leave the steam boiler system, resulting in boiler overheating. The pumps are repaired by an engineer, but usually, they need to be replaced.
The thermistor monitors the water temperature to determine the requirement to increase or decrease the temperature as per the demand. As the thermistor fails, it leads to water temperature getting too high. It consequently results in boiler overheating.
Prevention for Boiler Overheating:
It is prudent to prevent boilers overheating rather than repairing the aftermath. Boiler overheating can be prevented by,
- Fitting a magnetic filter to collect debris and rust from the heating system before it builds-up and forms a blockage. The build-up of debris is one of the common causes of various boiler issues that should be avoided.
- Installing a scale reducer decreases the amount of limescale formation in hard water. Reduced limescale ensures that the steam boiler system is free of hindrance in water flow that may cause boiler overheating.
- Do not supply the steam boiler with cool water when the system is overheated. It results in the boiler flashing into steam as it is introduced to the hot components of the steam boiler, causing a tremendous and immediate rise in pressure. It can lead to boiler explosion or rupture of components.
- Avoid constant flow of gases in the steam boiler as it causes rapid increase of temperature and pressure, leading to a runaway boiler.
- Reparation of steam boilers by an inexperienced engineer leads to issues like wiring errors, manual opening of gas valves that leads to a constant flow of gas, removing pressure relief valves while repairing boiler leakage and neglecting to fit it back that prevents the boiler system from facing excess pressure. It consequently leads to boiler overheating.
Rakhoh Boilers, with their 38+ years of expertise in steam boiler manufacturing and thermal solutions, have been serving over 20 process industries globally. We manufacture an efficient and reliable range of industrial steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories. We provide the best boiler services like steam trap assessment, boiler automation, energy audit, annual maintenance contract, etc., to boost the efficiency and productivity of the steam boilers.
To know more about our products and services, visit www.rakhoh.com
Heat is an integral part of steam boilers for the process operations in manufacturing units. Steam is ideal for heating, drying, and sterilizing purposes. Steam boilers require heat from the combustion process to heat the water and generate steam. However, due to various causes, plant managers observe heat loss in the steam boiler system. Heat loss in boilers results in further loss of fuel and productivity. Although it is essential to prevent heat loss in its initial stage, it is challenging to detect its exact cause. Steam boilers do not operate with 100% efficiency, but their actual efficiency ranges between 75% and 80%, while the remaining 20% to 25% is considered a loss. The losses reduce the performance of the steam boiler and increase the operating costs.
Causes of Heat Loss in Steam Boilers:
Heat loss is caused by various factors. Two of the major causes of heat loss in boiler systems are stack losses and radiation and convention losses.
Stock loss refers to the heat in the flue gas lost to the atmosphere on entering the stack. Stack loss in steam boilers depends on the fuel composition, flue gas temperature, and firing temperature. Flue gas loss is further classified into two types:
Dry Flue Gas Loss is the sensible heat energy loss in flue gas due to the flue gas temperature.
Flue Gas Loss Due to Moisture is the latent energy in the steam of the flue gas stream due to the water produced by the combustion reaction being vaporized from the high temperature of flue gas.
The losses are determined by inferring fuel analyses and other assumptions that help in calculations.
Radiation and Convention Losses:
Radiation and convention loss are the heat lost to the surroundings from the warm surfaces of the steam boiler or high-temperature water generator. Radiation and convention loss depend on the equipment size and the actual output based on the maximum design output.
Some of the other factors that account for the heat loss in steam boilers are as follows,
- Loss due to presence of hydrogen in fuel
- Loss due to moisture in the fuel
- Loss due to moisture in the air
- Loss because of carbon monoxide
- Unburned losses in fly ash
- Unburned losses in bottom ash
- Losses by blowdown, leakage, etc.
Prevention for Heat Loss in Steam Boilers:
Radiation and Convention Loss:
The outer surface of the shell boiler is hotter than its surrounding. Depending on the surface area and the temperature difference between the surface and the surrounding, the surface loses heat to the surrounding. Repairing or insulation can reduce heat loss by walls and piping of steam boilers.
It is essential to keep stack temperature at a minimum but not too low so that water vapor in the exhaust condenses on the stack walls. It is vital for fuels containing sulfur due to low temperature leading to sulfur dew point corrosion. Stack temperature exceeding 200°C requires early shutdown for water or flue side cleaning.
Incomplete combustion is caused due to shortage of air, excess fuel, or improper distribution of fuel. Incomplete combustion is indicated by color or smoke and should be examined promptly. With coal as fuel, unburned carbon causes great loss as carbon in the ash can account for more than 2% of the heat supplied to the boiler system. Non-uniformity of fuel size is one of the causes of incomplete combustion. Chain grate stokers face difficulty in combustion as large lumps of coal do not burn completely and finer pieces block the air passage, resulting in poor air distribution. It is necessary to ensure fuel composition and uniform distribution for proper combustion.
Excess Air Control:
Excess air is required for complete combustion. Optimal excess air levels take place with minimal loss by incomplete combustion and loss by heat in flue gases. Excess air control varies with furnace design type.
Combustion Air Preheater:
Combustion air preheating is an ideal alternative for feed water heating. Increasing air temperature by 20°C can raise the thermal efficiency by 1%
Feedwater preheating by Economizer:
Usually, the flue gases leaving the steam boilers are of 200°C to 300°C in temperature. The heat from the gases is recovered by using an economizer and utilized for other process operations.
Uncontrolled blowdown leads to tremendous heat loss. Therefore, automatic blowdown control is essential to prevent heat loss in steam boilers.
Reduction of scaling and soot losses:
In steam boilers, scale and soot formation on tubes acts as an insulator and hinders the heat transfer. It is important to remove the deposits regularly.
Heat Loss is one of the significant issues of steam boilers that need to be addressed immediately to prevent excessive production costs.
Rakhoh Boilers provides efficient industrial steam boilers and the best thermal solutions since their inception in 1983 as boiler manufacturers in Pune. We have successfully installed 3000+ boilers in over 26 countries worldwide for 20 process industries.
For more details on our products and services, visit www.rakhoh.com
Steam Boilers are irreplaceable for operational purposes in manufacturing facilities. Plant managers ensure that the boilers are functioning seamlessly by taking every precaution. However, some issues in the steam boiler system are inevitable that need immediate action. One of such issues is corrosion and erosion that occurs in every component of the steam boiler, including piping. Corrosion can lead to overheating or failure of boilers, resulting in severe losses or injuries as well as extensive plant downtime. Poor quality of feedwater is a major cause of the corrosion, along with the temperature of water or steam, presence of oxygen, pH, quality of steam, flow velocity, the chemical composition of steel pipe, and oxide layer on the inside of pipe.
Corrosion in Steam Boilers:
The feed water used for the processing operations should not be of poor quality with any impurities, impacting the boiler performance.
Utilizing water for heat transfer and steam generating purposes requires attention to prevent problems arising due to corrosive gases like dissolved oxygen and carbon dioxide. The intensity of the gases attacking the steam boiler depends on the quantity of dissolved substances, water pH, and temperature.
Corrosion mechanisms in steam boilers are caused by collective depositions of ferric oxides and dissolved salts of calcium, magnesium, iron, copper, and aluminum. The deposit acts as thermal insulation in high heat transfer areas, leading to local overheating and extreme corrosion in steam boilers.
Causes of Corrosion in Steam Boilers:
Corrosion in steam boilers are caused due by various factors such as,
- Negligence in maintenance or/and reparation that halts the system from operating, resulting in production loss
- Increased fuel utilization and reduced efficiency in steam boiler
- Exceeding maximum metal resistance
- Extensive time period in reaching system parameters
Corrosion in Piping:
Corrosion in pipes, if overlooked, can eventually advance to the internal part of the metal, leading to pipe thinning and pipe failure. Additionally, corrosion by-products are transported through pipes that contaminate the fluid and cause erosion and further corrosion of piping and valves.
Corrosion of Steel:
Steel piping in steam boilers includes a considerable amount of steel and is exposed to rust in specific conditions. The oxidation of the metal surface by contact with air and water results in iron rust. Steel condensate recovery pipes are particularly at risk due to ample reagents for rust formation.
In properly maintained steam pipes, there is considerably less air and water present after startup. It consequently decelerates the spread of rust. It must be noted that less rust is formed in a closed condensate recovery system compared to an open recovery system. After shutdown, contaminated air in the steam boiler system results in extensive corrosion. Frequent shutdowns in boiler systems cause increased rusting of pipes with condensate remaining in the system during the shutdown.
Steam trap helps to eliminate condensate and keep the steam dry to prevent rust. Air vents release excess air from the steam boiler to decrease the possibility of rust formation. It is vital to manually drain condensate during shutdown that steam traps fail to drain.
Corrosion of Copper:
Because of its low installation cost and easy bending around the equipment and flanges, copper is used in tracing lines. However, high temperature and low pH value in condensate destroy the copper ions that dissolve in the condensate. Copper-filled condensate reaching the steam trap causes the lower pressure on the outlet side of the trap to flash into steam. It results in some copper ions precipitating and accumulating as solid build-up around the valve, causing the reduced temperature in the tracing line and orifice blockage. It is important to treat feed water and monitor pH levels to prevent corrosion of copper. Neutral pH between 7 and 9 and low dissolved oxygen content is ideal for copper.
Preventing Corrosion in Piping:
- Stainless steel is considered a corrosion-resistant metal due to a process termed passivation. Passivation is the formation of a thin oxide layer on the metal surface when introduced to the air. The oxide layer safeguards the stainless steel by naturally forming a resistant layer for corrosion and rust.
- Properly implementing steam traps and air vents during shutdown to avoid air and water from reacting.
- Adequate water treatment and monitoring of condensate pH level
- Draining the system of condensate during shutdown
- Selecting pipes and traps based on the needs and requirements of the system
Rakhoh Boilers is a leading thermal solution provider and steam boiler manufacturer in Pune since 1983. With our dedication and expertise, we have delivered over 3000+ efficient steam boilers in 26 countries worldwide. We manufacture a range of industrial steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories. We provide the best boiler services like steam trap assessment, energy audit, boiler automation, annual maintenance contract, etc.
Visit our website www.rakhoh.com to know more about our products and services.