The construction of a boiler involves a diverse range of materials and components, broadly categorized into boiler steels, structural steels, and bought-outs (BOs). These materials are chosen based on factors like weldability, strength, and compliance with safety standards to ensure integrity and reliability.
1. Parts of Boilers
The construction of a boiler plant requires a diverse range of raw materials, components, and auxiliary equipment. These materials are broadly classified into three main categories, each contributing approximately 30 to 40% to the overall cost of the boiler plant. The categories are outlined in below table.
- Boiler Steels: These are materials primarily used for the construction of the boiler itself.
- Structural Steels: Employed for the structural components supporting the boiler.
- Bought-Outs (BOs): Components purchased externally, contributing significantly to the overall boiler construction.
a. Common Bought-Outs (BOs): Often included in the scope of smaller industrial plant boilers.
b. Excluded in Larger Boilers: In larger boilers, some bought-outs, particularly item 3c, may be excluded from the scope.
Photo 1: Boiler Steel Plates
In a typical industrial plant, the common bought-outs play a crucial role. However, in larger boilers, item 3c might be excluded (See below Table 1). Nonferrous materials are generally not required, except for aluminium sheets used for cladding the boiler. The distribution of these materials depends on the boiler type and its specific requirements.
Photo 1: Boiler Steel Plates
2. Boiler Steels
Boiler steels are characterized by low carbon content, high weldability, consistency, and well-defined high-temperature properties. To meet stringent quality standards, boiler steels are manufactured and tested according to specified codes. The critical issues regarding boiler steels are;
- Code-Approved Manufacturer: Boiler steels must be procured exclusively from manufacturers approved by the relevant codes. Code recognition is contingent upon rigorous and periodic verification of compliance with manufacturing standards, including processes, inspections, and documentation.
- Qualification at Every Stage: Machines, work procedures, inspection stages, and welders, must be individually qualified. This ensures that all pressure part (PP) materials in a boiler adhere to defined quality standards.
- Material Test Certificates (TCs): All boiler PP materials come with material test certificates detailing the material source and relevant tests at various manufacturing stages, as specified in the code. These certificates are often certified by independent third-party agencies to enhance reliability.
- Documentation: The code-mandated record-keeping includes recording all steels used in various parts of a boiler, along with stage inspections. This comprehensive documentation, including manufacturing and material TCs, serves as the complete history of a PP.
- Safety Measures: The stringent quality measures implemented throughout the manufacturing process, from raw steels to components, contribute to the safety of pressure parts in boilers. These measures ensure that boilers can withstand high pressures and temperatures without leakage or rupture.
- Mandatory Registration: The compiled documentation for each PP is submitted to code-approved authorities or insurance companies for mandatory boiler registration before commercial operation. This process ensures regulatory compliance and safety.
- Traceability: The meticulous documentation enables the traceability of any failed part, allowing for a thorough investigation in case of any failure during boiler operation.
The safety of pressure parts in boilers is a result of the robust quality measures and documentation practices implemented across all stages of manufacture. These measures provide confidence in the integrity and reliability of boiler components and contribute to the overall safety of boiler operations.
2.1. Boiler Quality (BQ) Plates
Boiler quality plates are crucial components in boilers, tasked with withstanding high pressure and temperature conditions.
Key Requirements for Boiler Quality Plates:
- Consistency and Uniformity:
- Properties must be consistent and uniform across the length, width, and thickness of the plate.
- Consistency ensures structural integrity and reliable performance under varying conditions.
- High Purity of Steel:
- Boiler quality plates require a high degree of steel purity.
- The high steel purity minimizes impurities, enhancing the overall strength and durability of the plates.
- No Inclusions or Air Pockets:
- Strict adherence to quality standards requires the absence of inclusions or air pockets in the plates.
Characteristics of Boiler Quality Plates:
- Low Carbon Content:
- Low carbon content is essential for good weldability.
- It also facilitates efficient welding processes during the fabrication of boiler components.
- High Tensile Strength:
- Boiler quality plates exhibit high tensile strength.
- This characteristic is critical for limiting drum thicknesses and ensuring structural stability under high-pressure conditions.
- High Thicknesses:
- Boiler quality plates are designed to be effective in higher thicknesses.
- Suitable for applications where robust and thick plates are required to withstand extreme conditions.
Categories of Boiler Quality Plates Based on Tensile Strength:
- Carbon Steel:
- Subdivided into low, medium, and high carbon categories.
- Offers a range of tensile strengths suitable for diverse boiler applications.
- Low-Alloy Steel:
- Incorporates alloying elements to enhance specific properties.
- Provides improved performance compared to standard carbon steel.
- High-Alloy Steel:
- Contains higher proportions of alloying elements.
- Offers the highest tensile strength among the three categories, catering to specialized requirements.
2.2 Boiler Tubes
Tubes and pipes play distinct roles, each subjected to different challenges based on their location within the boiler system. The key considerations for boiler tubes are;
- Seamless Tubes:
- Produced by piercing a red-hot billet.
- ERW Tubes:
- Produced by folding and welding steel strip.
- Preferred for lower thicknesses, providing benefits like smoother inside surfaces, concentric bores, and cost efficiency.
- Hot-Finished and Cold-Drawn Seamless Tubes (HFS and CDS):
- HFS tubes have higher pressure drop and tolerance levels.
- CDS tubes, produced by reheating and rerolling HFS tubes, offer closer tolerance.
- Selection depends on the boiler section and specific duties.
- Tube Materials and Comparison:
- Carbon, alloy, and Stainless Steel (ss) tubes are used.
- Selection of Tube Materials:
- Temperature limits and material strength at operating temperatures govern selection.
- A good boiler design optimizes the use of low-carbon steels and minimizes the use of expensive materials like Stainless Steel.
- Tube Ordering:
- Tubes are received in bundles, marked with specifications, and color-coded for identification.
- Punching tubes is discouraged to avoid outer layer damage.
- Tubes can be ordered in exact lengths, and hydraulic testing (HT) is an ideal QA measure.
Photo 2: Boiler Tubes
2.3 Boiler Structural Steels
Structural Steel is used for the Non-Pressure Parts (NPPs) of boilers, which include essential components like flues, ducts, casing, hoppers, penthouse, and bunkers. Unlike Pressure Part (PP) steels, the specifications for structural steels are outlined in the codes, certain considerations must be addressed, particularly as structures become larger.
- No stringent requirements similar to PP steels, but challenges arise with larger sections becoming excessively heavy.
- Fabricated sections from plates become competitive when welding and handling costs are lower, despite limited suppliers, protracted deliveries, and higher specific costs.
Basic Requirements of Structural Steel:
- Good weldability and strength are fundamental requirements.
- Cold structure members need adequate notch toughness, especially in very cold climates.
- Components for hot structures, like buckstays and sling rods, are often designed for temperatures <400°C and do not require specific high-temperature properties like those for Pressure Parts (PPs).
Structural Steel Codes:
- Below Table 2 provides an overview of structural steels required in boilers, detailing various types used in NPPs.
- Table 3 offers the structural material codes for American standards, along with their nearest BS and DIN equivalents.
- Specifications within codes cover basic material grades and their properties, while individual standards detail dimensions and other related mechanical aspects of each rolled section.
Table 2 Structural Steels in Boilers
Table 3: Boiler Structural Steel Grades
3. Novelty Steel Boiler Fabrications
Novelty Steel supplies various types of boilers for a variety of applications.