Stainless Steel Coil Tube for Power Plant High-Pressure Feedwater Heaters
Product Overview
The Stainless Steel Coil Tube (盘管, Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product designed specifically for use in high-pressure feedwater heaters (High-Pressure Feedwater Heater, HPF) within thermal power generation systems. Coil tubes are integral components of surface heat exchangers, where they facilitate efficient heat transfer between feedwater and extraction steam. This product category is critical to the thermodynamic efficiency and operational reliability of coal-fired and nuclear power plants worldwide.
Based on extensive field performance analysis—including documented investigations into coil tube cracking mechanisms in operating power plants (e.g., Unit 2, HPF #1 coil tube failure analysis)—Wenzhou Jixing Metal Materials Co., Ltd. has developed a comprehensive product specification that addresses the root causes of degradation, ensuring extended service life and superior metallurgical integrity under severe thermal and mechanical cycling conditions.
Material & Properties
Coil tubes for high-pressure feedwater heater applications are typically fabricated from austenitic stainless steels selected for their exceptional resistance to corrosion, stress corrosion cracking (Stress Corrosion Cracking, SCC), and thermal fatigue. The primary material grades include:
- SUS304 / AISI 304 / UNS S30408 — General-purpose austenitic stainless steel with balanced mechanical and corrosion properties.
- SUS316L / AISI 316L / UNS S31603 — Low-carbon variant with added molybdenum (Mo) for enhanced pitting and crevice corrosion resistance in chloride-containing environments.
- SUS316Ti / AISI 316Ti / UNS S31635 — Titanium-stabilized grade offering superior resistance to intergranular corrosion during welding and high-temperature service.
Key material properties for coil tube applications include:
- Tensile Strength (Ultimate Tensile Strength, UTS): ≥ 520 MPa (SUS304); ≥ 485 MPa (SUS316L)
- Yield Strength (0.2% Offset): ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Hardness (Rockwell B, HRB): 80–100
- Thermal Conductivity: 16.3 W/(m·K) at 100°C (SUS304); 14.6 W/(m·K) at 100°C (SUS316L)
- Operating Temperature Range: −196°C to +870°C
- Corrosion Resistance: Excellent resistance to atmospheric corrosion, dilute acids, and chloride solutions
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 14.0 mm – 25.4 mm (typical range) | GB/T 14976, ASTM A213 |
| Wall Thickness | 0.8 mm – 2.0 mm | GB/T 14976, EN 10216 |
| Bend Radius (Minimum) | ≥ 2.5D (D = outer diameter) | ISO 4427 |
| Coil Diameter (As-Supplied) | 500 mm – 1,500 mm | Custom per application |
| Length per Coil | Up to 300 m (continuous coil) | — |
| Surface Finish | 2B / No.4 Pickled & Passivated | ASTM A403 |
| End Condition | Plain ends / Beveled ends | Per customer drawing |
| Pressure Rating (Design) | Up to 2.5 MPa (feedwater side) | ASME Section VIII Div.1 |
| Tolerance (OD) | ±0.05 mm | GB/T 14976 |
| Tolerance (Wall Thickness) | ±10% of nominal | GB/T 14976 |
| Material Grade | SUS304 / SUS316L / SUS316Ti | GB/T 12771, ASTM A213 |
| Heat Treatment | Solution Annealed + Water Quenched | ASTM A213 |
Manufacturing Process
The production of stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage manufacturing sequence to ensure dimensional accuracy, metallurgical homogeneity, and surface integrity:
- Hot Rolling (Hot-Finishing Mill): Continuous casting billets are reheated to 1,100–1,200°C and hot-rolled into seamless tube blanks with controlled wall thickness and outer diameter.
- Piercing & Elongation: The solid ingots are pierced and elongated through rolling mills to produce hollow tubes of preliminary dimensions.
- Cold Drawing / Cold Rolling: Tubes are cold-worked to achieve precise final dimensions (OD and wall thickness), with surface finish improvement and enhanced mechanical properties through work hardening.
- Solution Annealing (Heat Treatment): Tubes are heated to 1,050–1,150°C and rapidly quenched (water quench or controlled atmosphere) to dissolve carbide precipitates, restore full austenitic structure, and achieve maximum corrosion resistance.
- Pickling & Passivation: The tube surface is chemically cleaned using mixed acid solutions (HNO₃/HF) to remove scale and oxide films, followed by passivation to form a protective chromium oxide layer.
- Cold Bending / Coiling: Annealed tubes are cold-bent into the required coil geometry using precision mandrel-equipped bending machinery. Bend radii are strictly controlled to prevent wall thinning, ovality distortion, or micro-cracking at the bend apex.
- End Welding & Forming: Tube ends are prepared for welding to inlet/outlet headers (manifolds), with controlled bevel geometry and interpass temperature monitoring.
- Hydrostatic Pressure Testing: Each coil tube assembly undergoes 100% hydrostatic testing at 1.5× design pressure to verify weld integrity and absence of through-wall defects.
Applications
Stainless steel coil tubes from Wenzhou Jixing Metal Materials Co., Ltd. are deployed in demanding industrial applications where thermal efficiency, corrosion resistance, and long-term reliability are paramount:
- Thermal Power Generation: High-pressure and low-pressure feedwater heaters in coal-fired, oil-fired, and combined-cycle power plants. Coil tubes serve as the primary heat exchange surface for regenerative feedwater heating, directly impacting the Rankine cycle efficiency of the plant.
- Nuclear Power Plants: Feedwater heating systems and auxiliary heat exchangers in pressurized water reactors (Pressurized Water Reactors, PWR) and boiling water reactors (Boiling Water Reactors, BWR).
- Petrochemical & Refining: Heat exchangers in crude oil distillation, hydrocracking, and catalytic reforming units where chloride stress corrosion resistance is essential.
- Pulp & Paper Industry: High-temperature heat recovery systems and condensate recovery units.
- Marine & Offshore Platforms: Heat exchangers in marine power generation systems exposed to saltwater corrosion environments.
Quality Certifications & Standards Compliance
All stainless steel coil tube products manufactured by Wenzhou Jixing Metal Materials Co., Ltd. comply with the following international and national standards:
- GB/T 14976 — Stainless steel seamless tubes for general engineering purposes (Chinese National Standard)
- GB/T 12771 — Welded stainless steel tubes for general engineering purposes
- ASTM A213 / A213M — Standard specification for seamless austenitic stainless steel tubes for high-temperature service
- ASME BPV Section VIII, Division 1 — Rules for construction of pressure vessels (applicable to heat exchanger design)
- EN 10216-5 — Technical delivery conditions for wrought seamless and welded stainless steel tubes
- ISO 4427 — Thermoplastic tubes (referenced for bend radius methodology)
Manufacturing facilities maintain certifications including:
- ISO 9001:2015 — Quality Management Systems
- ISO 14001 — Environmental Management Systems
- NACE MR0175 / ISO 15156 — Materials for use in H₂S-containing environments in oil and gas production (where applicable)
Unique Selling Points & Technical Differentiators
Wenzhou Jixing Metal Materials Co., Ltd. distinguishes its stainless steel coil tube offerings through the following technical advantages, informed by extensive field failure analysis and metallurgical research:
- Cracking-Resistant Metallurgical Design: Drawing upon documented case studies of coil tube failure in power plant high-pressure heaters (including stress corrosion cracking, thermal fatigue cracking, and flow-accelerated corrosion mechanisms), the product incorporates optimized carbon content control (C ≤ 0.03% for SUS316L), stabilized titanium addition, and refined grain structure to mitigate intergranular and transgranular cracking susceptibility.
- Precision Cold Bending Technology: Proprietary bending processes with controlled strain rates and adequate inter-pass annealing ensure uniform wall thickness distribution at bend apexes, preventing premature fatigue failure at stress-concentration zones.
- Full Traceability System: Each coil tube lot is traceable to raw material heat numbers, with comprehensive documentation including chemical composition analysis, mechanical test certificates, and non-destructive examination (Non-Destructive Examination, NDE) reports (including eddy current testing and hydrostatic testing).
- Extended Service Life Engineering: Through rigorous solution annealing and passivation protocols, the product achieves a protective passive film thickness of 2–5 nm, providing long-term corrosion protection in high-temperature feedwater environments (150–280°C) with dissolved oxygen and chloride ion content.
- Customization Capability: Flexible manufacturing capacity accommodates non-standard dimensions, special alloy grades (including duplex stainless steels such as UNS S31803/S32205 for chloride-rich environments), and application-specific coil geometries tailored to OEM heat exchanger designs.
Failure Analysis Insights & Design Considerations
Based on post-mortem analysis of coil tube cracking in a power plant Unit 2, HPF #1 (as documented in the reference case study), the following degradation mechanisms were identified as contributing factors to premature failure:
- Stress Corrosion Cracking (SCC): Chloride-induced SCC initiated at weld heat-affected zones (Heat-Affected Zone, HAZ) where localized sensitization occurred due to inadequate post-weld annealing.
- Thermal Fatigue: Cyclic thermal loading during plant start-up and shutdown operations generated thermomechanical fatigue cracks at tube-to-header weld junctions and bend apex regions.
- Flow-Accelerated Corrosion (FAC): High feedwater velocity (>1.0 m/s) in narrow coil tube passages accelerated oxide layer removal, exposing bare metal to continued corrosion attack.
- Crevice Corrosion: Accumulated debris and oxide scale within coil tube bends created occluded environments conducive to localized crevice corrosion.
These insights directly inform the enhanced manufacturing specifications and quality control protocols employed in Wenzhou Jixing Metal Materials Co., Ltd.'s coil tube production, ensuring that material selection, welding procedures, surface preparation, and geometric design collectively mitigate the identified failure modes.