- Types of Boilers in the Power Industry and Top Causes of Boiler Corrosion
- Why is Boiler Tube Coating Critical? Introduction to HVTS
- The Benefits of Boiler Tube Coating
- HVTS Boiler Tube Coating Process and Application
Types of Boilers in the Power Industry and Top Causes of Boiler Corrosion
There are several reasons why boilers may suffer from corrosion. The types of boilers that IGS works within the Waste-to-Energy (WtE) and wider power industry, include:
- Circulating Fluidized Bed (CFB) Boilers
- Pulverized Coal (PC) Fired Boilers
- Bubbling Fluidized Bed (BFB) Boilers
- Grate Boilers
The main causes of boiler tube corrosion, include:
- Corrosive Flue Gases
Power plants produce flue gases which contain corrosive elements such as sulfur dioxide (SO2), sulfur trioxide (SO3), chlorine (Cl2), and hydrogen chloride (HCl). The flue gases react with the exposed surfaces of the boiler tubes which can cause corrosion. Boilers that burn sulfur-containing fuels such as coal or petroleum coke are more likely to have high concentrations of corrosive gases which can accelerate the corrosion process, making it critical to take mitigative action.
- Fireside Corrosion
According to EPRI, fireside corrosion, also known as boiler fireside corrosion, is the most common cause of tube failure. It is a type of corrosion that occurs on the external surfaces of boiler components exposed to high-temperature combustion gases. It refers to the corrosion phenomena happening on the side of the boiler that faces the heat source, such as the furnace or flame.
Fireside corrosion can affect various boiler components, including boiler tubes, walls, refractory materials, and other exposed surfaces. It is typically caused by the combination of factors present in the combustion environment and fuel, such as high temperatures, corrosive gases, ash particles, and chemical reactions.
- High Temperatures
Power plant boilers operate at high temperatures and pressures. For example, CFB boilers operate between 850oC – 950oC. These high temperatures accelerate the corrosion process and can increase the reactivity between the metal surface and corrosive elements, leading to faster corrosion rates.
- Stress Corrosion Cracking
Stress corrosion cracking occurs when a combination of a corrosive environment, tensile stress, and vulnerable material leads to crack initiation and propagation. It can be a significant concern in power plant boilers, particularly in areas where high stresses are present, such as welds or bends.
- Deposit Build-ups and Fouling
The accumulation of deposits, such as ash or soot, on the exposed surfaces of boiler tubes acts as a barrier and reduces heat transfer efficiency. The deposits create a localized environment on the tube surface, which increases the chance of corrosion.
- Water Chemistry and Flow Issues
Impurities in the water used in a boiler such as dissolved oxygen, chlorides, and sulfates can cause corrosion when they interact with the inner surface of boiler tubes. High alkalinity or acidity levels in the water can also contribute to corrosion.
Furthermore, uneven flow of water and steam within the boiler can cause localized areas of high temperature or stagnant water. This can encourage corrosion and thermal degradation, especially in areas where protective oxide layers have been disrupted.
- Poor Tube Material Selection
In some cases, the material chosen for boiler tubes may not be suitable for the operating environment, especially if the conditions are inherently corrosive or if there is a change to the operation, for example, due to fuel availability or diversification. This can lead to premature failure of the tubes. It is important to work with the OEM to ensure the correct materials are chosen for your requirements, taking into consideration fuel composition, operating conditions, and boiler design.
Why is Boiler Tube Coating Critical?
Boiler tube coatings are important for several reasons. They are frequently used by asset owners and managers to prolong the life of boiler tubes and to protect the tubes from high-temperature environments.
Firstly, coatings act as a protective barrier between the tube material and the corrosive boiler operating environment including flue gases, fuel impurities and ash. By preventing corrosive interactions, coatings can significantly extend the life of boiler tubes and upgrading the surface metallurgy of the tubes enables fuel diversification.
Boiler tubes, especially those in CFB boilers, can also experience erosion due to high-velocity fluid flow or the impact of solid particles. A tube coating acts as an additional, hard-wearing layer of base material protection against erosion and helps to maintain the structural integrity of the tubes.
Boiler tube coatings or claddings help to prevent the build-up of fouling. The smooth coating surface prevents fouling adhesion, making it easier to clean during planned maintenance. This also ensures heat transfer efficiency is maintained and overall performance is improved.
Introduction to HVTS
High Velocity Thermal Spray (HVTS) is a proprietary boiler tube cladding solution developed by Integrated Global Services (IGS). Due to its superior corrosion protection properties, it is referred to as a cladding, rather than a coating. HVTS involves the on-site application of a high-alloy corrosion-resistant cladding which produces a coating that consists of flat and tightly packed micro-sized metallic particles.
HVTS provides excellent value and results for improving boiler system reliability and longevity at a fraction of the cost and time of weld overlay.
The Benefits of Boiler Tube Coating
A boiler tube cladding solution, specifically HVTS, has many benefits for power plants. The benefits include:
HVTS cladding provides excellent protection against corrosion. The dense and tightly bonded coating acts as a protective barrier. This helps extend the lifespan of the boiler tubes, reduces maintenance requirements, and enhances overall boiler reliability.
- No Dilution or Heat Affected Zone (HAZ)
Unlike Weld Metal Overlay (WMO), HVTS does not generate a Heat Affected Zone (HAZ) on the pressure parts and does not place residual stresses on the base metal as the temperature of the base metal remains low even without water in the boiler tubes.
- Refurbished or Replaced In-situ
HVTS can be reliably refurbished or even replaced in situ without replacing the underlying component. This contrasts with 625 weld-overlaid components which must be replaced at the end of life.
HVTS cladding can be applied relatively quickly, making the process efficient for large-scale industrial applications. The high-velocity spraying technique enables fast coverage of large surface areas, reducing downtime and production interruptions.
HVTS and other boiler tube coatings offer a cost-effective solution for extending the service life of components and equipment. By providing corrosion and wear protection, these coatings/ cladding can reduce the frequency of component replacements and maintenance requirements, resulting in cost savings over time.
HVTS coatings can contribute to achieving environmental targets. By improving the durability and longevity of components, they help reduce the consumption of resources and minimize waste generated from frequent replacements. Additionally, HVTS coatings can enhance the energy efficiency of equipment, leading to reduced energy consumption and lower CO2 and NOx emissions.
HVTS Boiler Tube Coating Process and Application
The application of High Velocity Thermal Spray (HVTS) to boiler tubes is always undertaken by trained IGS boiler tube cladding technicians as training and expertise in HVTS application techniques are crucial to achieving high-quality and reliable coatings on boiler tubes.
The application of HVTS involves the following steps:
- Surface Preparation
The surfaces of the boiler tubes need to be properly prepared before the application of HVTS cladding. This typically involves cleaning the surfaces to remove contaminants such as rust, scale, oil, or any other substances that could affect the adhesion of the coating. Mechanical methods like grit blasting or wire brushing are sometimes used for surface preparation if required.
- Coating Selection
The appropriate HVTS coating material is selected based on the specific requirements of the boiler tubes, such as corrosion resistance, wear resistance, or thermal insulation.
- HVTS Equipment Setup
The HVTS equipment, including the spray gun, gas supply system, and control unit, is set up according to stringent IGS guidelines and specifications. The system is calibrated to ensure accurate and consistent application of the coating.
- Application of HVTS Coating
The HVTS cladding is applied using a high-velocity thermal spray gun. The gun atomizes a proprietary wire feedstock in a supersonic gas stream, producing a cladding that consists of flat and tightly packed micro-sized particles.
- Post-application and Quality Assurance
Quality control measures, such as visual inspection, thickness measurement, and adhesion testing, are conducted to ensure the integrity and effectiveness of the HVTS cladding. Any necessary touch-ups or rework can be performed if required.
As the frequency of operations and fuel variability increases, boiler tube coatings and claddings are vital to improving boiler reliability and performance. The cost and performance benefits of protecting boiler tubes from a range of corrosion mechanisms far outweigh the initial outlay.
Whilst material selection and technology are important, selecting a reliable service provider with a proven track record of operational excellence will ensure that the project runs smoothly and safely.
At IGS, we offer a full range of surface protection solutions for a range of applications and industries. Our team will work with yours to understand every aspect that affects your boiler’s performance and provide a turnkey service that delivers an effective, economical, and safe boiler tube coating solution.
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Read More: Boiler Tube Coating Case Studies