Superheater tubes are pressure-containing tubes installed in a boiler superheater. They carry steam after the evaporation stage and absorb additional heat from the boiler gas path, raising the steam temperature above saturation before it reaches the turbine or process system.
Superheater tubes operate at high temperature and pressure for long periods. Material selection therefore requires consideration of tube metal temperature, steam pressure, oxidation, creep, thermal cycling and the applicable boiler design code.
Product Specifications
| Item | Description |
|---|---|
| Product | Seamless boiler superheater tubes |
| Material families | Molybdenum steel, chromium-molybdenum steel, chromium-molybdenum-tungsten steel and austenitic stainless steel |
| Representative grades | 15MoG, 20MoG, T11, T22, T24, T91, T23, T92, T122, TP304H, TP321H and TP347H |
| Product standards | GB/T 5310; ASTM A209/A213 or corresponding ASME SA specifications, as applicable to the grade |
| Manufacturing route | Hot-finished or cold-finished seamless tubes |
| Delivery condition | Heat treatment and surface condition according to the selected grade and standard |
| Dimensions | Outside diameter × wall thickness × length |
| Principal applications | Primary, secondary and final superheater sections, as specified by the design |
The representative grades belong to different standards and are not interchangeable. The purchase specification should identify the exact grade, product standard and standard edition.
Material Families and Grade Selection
| Material family | Representative grades | Selection considerations |
|---|---|---|
| Molybdenum steel | 15MoG, 20MoG | Used where the design calls for carbon-molybdenum steel properties |
| Chromium-molybdenum steel | 15CrMoG, 12Cr1MoVG, T11, T22, T24 and T91 | Provides alloyed elevated-temperature strength for selected superheater zones |
| Chromium-molybdenum-tungsten steel | T23, T92, T911 and T122 | Used where the specified alloy design and heat treatment suit demanding service conditions |
| Austenitic stainless steel | TP304H, TP321H, TP347H, S30432 and TP310HCbN | Used where the design requires austenitic high-temperature strength and oxidation resistance |
The handbook identifies these material families for superheater and reheater applications. Actual grade selection must be based on the design data and approved material specification.
Representative Mechanical Properties
The following values are historical reference values listed in the supplied boiler tube handbook.
| Grade | Minimum tensile strength, MPa | Minimum yield strength, MPa | Minimum elongation, % |
|---|---|---|---|
| T91 | 585 | 415 | 20 |
| T92 | 620 | 440 | 20 |
| T23 | 510 | 400 | 20 |
| T122 | 620 | 400 | 20 |
| TP304H | 515 | 205 | 35 |
| S30432 | 590 | 235 | 35 |
| TP310HCbN | 655 | 295 | 30 |
Data basis: Baosteel Boiler Tube & Pipe handbook, printed in June 2007. The table is a historical comparison and is not a complete acceptance specification. Final mechanical properties, hardness, test conditions and heat-treatment requirements follow the standard edition and purchase specification stated in the order.
Room-temperature tensile properties do not establish an allowable service temperature. High-temperature design requires the applicable allowable stresses, creep data, oxidation behavior and expected operating life.
Superheater Sections and Heat Transfer
Superheaters are commonly arranged in multiple banks or sections. Steam conditions and gas temperature change along the gas path, so tube metal temperature is not necessarily the same in every section.
The design may use primary, platen or final superheater sections. Tube spacing, gas velocity, soot deposits, steam-side flow and heat flux all affect the thermal duty. The selected material and wall thickness must suit the hottest and most highly stressed locations identified by the boiler design.
Fouling and ash deposits can increase tube metal temperature and alter gas-side heat transfer. Operation, soot blowing, water chemistry and inspection are therefore part of the tube service requirements.
Heat Treatment and Fabrication
Heat treatment establishes the microstructure and mechanical properties required by the selected grade. The required process varies between molybdenum, chromium-molybdenum, chromium-molybdenum-tungsten and austenitic stainless steels.
Superheater tubes may be bent or fabricated into assemblies. Bending, welding and post-weld heat treatment must follow qualified procedures and the applicable construction code. Procedure qualification should address preheat, interpass temperature, consumables, heat input and any required post-weld treatment.
T91, T92 and other creep-resistant ferritic grades require controlled fabrication and heat-treatment practices. Austenitic stainless steel grades have different thermal expansion and thermal conductivity from ferritic alloy steels; mixed-material connections should be designed accordingly.
Superheater Tubes and Reheater Tubes
| Feature | Superheater tubes | Reheater tubes |
|---|---|---|
| Steam source | Steam leaving the evaporation or primary superheater system | Steam returning from an intermediate turbine stage |
| Main duty | Raise steam above the saturation temperature | Reheat returned steam before the next turbine stage |
| Key material factors | Tube metal temperature, steam pressure, oxidation and creep | Temperature, pressure, thermal cycling, oxidation and creep |
| Typical material families | Mo steel, Cr-Mo steel, Cr-Mo-W steel and austenitic stainless steel | Mo steel, Cr-Mo steel, Cr-Mo-W steel and austenitic stainless steel |
The same grade is not automatically suitable for both locations. The superheater or reheater design, temperature profile and pressure conditions determine the required product.
Manufacturing and Inspection
Seamless tubes are produced by piercing solid billets followed by hot working or cold finishing. Required heat treatment, straightening, surface finishing and final inspection depend on the product standard and order.
Depending on the specification, inspection may include:
- Heat and product chemical analysis.
- Tensile, yield and elongation testing.
- Hardness testing and heat-treatment records.
- Outside diameter, wall thickness, length and straightness checks.
- Internal and external surface inspection.
- Flattening, flaring or other specified technological tests.
- Hydrostatic testing and/or specified nondestructive examination.
- Additional impact, metallographic or grain-size tests where required by the grade or project.
Inspection documents should identify the product standard, grade, heat number, dimensions, delivery condition and applicable test results. Traceability should be maintained through bending, welding and final assembly.
Ordering Information
For a quotation, specify:
- Grade: required Mo, Cr-Mo, Cr-Mo-W or austenitic stainless steel grade.
- Standard: GB/T 5310, ASTM A209/A213, ASME SA-209/SA-213 or another specified standard and edition.
- Dimensions: outside diameter, wall thickness basis and length.
- Quantity: number of tubes, total length or weight.
- Delivery condition: heat treatment and surface condition.
- Application: primary, secondary or final superheater service.
- Fabrication: bend drawings, weld requirements and post-weld heat treatment where applicable.
- Inspection: certificate requirements, nondestructive examination and additional tests.
If an ASTM or ASME pipe grade such as P91 or P92 is required for a steam line, specify it as a pipe product under its applicable standard. T91 or T92 tube designations do not automatically establish equivalence with P91 or P92 pipes.