Reheater tubes are pressure-containing tubes installed in a boiler reheater. They receive steam exhausted from an earlier turbine stage and add heat before the steam returns to a subsequent turbine stage. Reheating helps maintain the designed steam conditions through the turbine expansion process.
Reheater tubes operate at elevated temperatures and pressures and are exposed to thermal cycling during startup, shutdown and load changes. Material selection requires consideration of tube metal temperature, steam pressure, creep, oxidation, thermal fatigue and the applicable boiler design code.
Product Specifications
| Item | Description |
|---|---|
| Product | Seamless boiler reheater 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 and final reheater 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 reheater design calls for carbon-molybdenum steel properties |
| Chromium-molybdenum steel | 15CrMoG, 12Cr1MoVG, T11, T22, T24 and T91 | Provides alloyed elevated-temperature strength for selected reheater 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 supplied boiler tube handbook identifies these material families for reheater and superheater applications. Actual grade selection must be based on the reheater temperature profile, pressure, gas-side environment and approved material specification.
Reheater Service Conditions
Reheater steam has already expanded through an earlier turbine stage, so its pressure is lower than the main superheater outlet pressure. Its temperature remains high, and the reheater tube metal temperature is influenced by steam flow, gas temperature, tube arrangement and deposits.
Reheater sections may be divided into primary and final banks. The final reheater section generally sees the highest steam temperature within the reheater system, but the hottest tube location must be determined from the boiler thermal design rather than from the section name alone.
Startup, shutdown and load changes can impose repeated thermal stresses. Adequate steam flow, temperature control, support design and expansion allowances are necessary to limit thermal fatigue and local overheating.
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.
Reheater Tubes and Superheater Tubes
| Feature | Reheater tubes | Superheater tubes |
|---|---|---|
| Steam source | Steam returning from an intermediate turbine stage | Steam leaving the evaporation or primary superheater system |
| Main duty | Raise the temperature of returned steam before the next turbine stage | Raise steam above the saturation temperature before turbine or process use |
| Key material factors | Temperature, pressure, thermal cycling, oxidation and creep | Tube metal temperature, steam pressure, 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 temperature profile, pressure and fabrication requirements must be reviewed for each component.
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.
Reheater tubes may be bent or fabricated into tube 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.
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 or final reheater 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.