Stainless Steel Coil Tube for Waste Heat Recovery Boilers — Pickling-Treated Specification
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered, continuously formed tubular product designed for integration into waste heat recovery boilers and similar thermal exchange systems. The coil tube is supplied in a coiled (helical or spiral) configuration, enabling efficient installation into compact boiler geometries while maximizing the heat transfer surface area per unit of occupied volume.
This product specification draws upon documented operational experience from field-deployed coil-type waste heat boilers, incorporating lessons learned from acid pickling treatment, corrosion resistance validation, and long-term service performance. The coil tube is engineered to withstand cyclic thermal loading, aggressive flue gas environments, and high-temperature oxidative conditions typical of waste heat recovery applications.
Material & Properties
Base Material
The coil tube is fabricated from austenitic stainless steel conforming to international standards. The primary grade utilized is SAE 316 / AISI 316 (UNS S31600), selected for its superior resistance to chloride-induced stress corrosion cracking (SCC) and general corrosion in industrial waste gas environments. Alternative grades including SAE 304 / AISI 304 (UNS S30400) and SAE 321 / AISI 321 (UNS S32100, stabilized with titanium) are available depending on the specific thermal and chemical service conditions.
Mechanical Properties (at Room Temperature)
- Tensile Strength (UTS): ≥ 515 MPa (SAE 316) / ≥ 485 MPa (SAE 304)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation at Break: ≥ 40%
- Density: 7.98 g/cm³
- Thermal Conductivity (20°C): 16.3 W/(m·K) (SAE 316)
- Linear Thermal Expansion (20–800°C): 17.3 × 10⁻⁶ /K
Corrosion Resistance Characteristics
- Excellent resistance to dilute acids, atmospheric corrosion, and marine environments
- Enhanced pitting and crevice corrosion resistance due to 2.0–3.0% molybdenum (Mo) content in SAE 316 grade
- Resistance to intergranular corrosion after stabilization treatment (SAE 321) or low-carbon processing (SAE 304L / 316L)
- Service temperature range: up to 870°C (oxidizing atmosphere) / up to 925°C (reducing atmosphere)
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) | — |
| Catalog Number | 1628 | — |
| Outer Diameter (OD) | 12.0 mm – 76.0 mm (customizable) | ASTM A213 / EN 10216-5 |
| Wall Thickness | 0.5 mm – 3.0 mm | ASTM A269 |
| Coil Diameter | 150 mm – 1200 mm (configurable per installation) | — |
| Coil Length (per coil) | 10 m – 200 m (continuous) | — |
| Material Grade | SAE 304 / 304L / 316 / 316L / 321 | ASTM A213 TP304/316/321 |
| Surface Finish | Pickled & Passivated (P&P), Ra ≤ 0.8 μm | ASTM A967 |
| Manufacturing Method | Continuous Cold-Forming / Seamless Coil Drawing | — |
| Heat Treatment | Solution Annealing (1050–1150°C), Water Quench | ASTM A213 |
| Maximum Operating Temperature | 870°C (oxidizing) / 925°C (reducing) | — |
| Maximum Operating Pressure | Up to 2.5 MPa (dependent on OD and wall thickness) | ASME BPV Section I |
| Applicable Standards | ASTM A213, ASTM A269, EN 10216-5, GB/T 12771 | — |
| Inspection | 100% Eddy Current Testing (ECT) + Hydrostatic Test | ASTM E309 / EN 417 |
Manufacturing Process
1. Raw Material Selection and Verification
High-purity stainless steel strip or wire rod is sourced from certified mills with documented chemical composition analysis (via spectrographic testing). Incoming material undergoes 100% visual inspection and sampling for mechanical property verification in accordance with ASTM A213 requirements.
2. Continuous Cold-Forming
The coil tube is produced via a continuous cold-drawing or cold-rolling process through precision-machined die sets. This process ensures dimensional accuracy, consistent wall thickness, and a smooth internal surface profile critical for heat transfer efficiency. The continuous forming method eliminates butt welds, thereby removing potential weak points in the tube wall.
3. Solution Annealing and Stress Relief
Following forming, the coil tube is subjected to solution annealing at temperatures between 1050°C and 1150°C, followed by rapid water quenching. This heat treatment cycle:
- Redissolves carbide precipitates (particularly chromium carbides) at grain boundaries
- Eliminates residual stresses introduced during cold forming
- Restores full austenitic microstructure for maximum ductility and corrosion resistance
- Prevents sensitization and subsequent intergranular corrosion
4. Acid Pickling and Passivation (P&P Treatment)
The acid pickling and passivation treatment is a critical post-manufacturing step, particularly for coil tubes destined for waste heat boiler applications. Based on documented field experience (referenced in "Summary of Pickling and Operation of Coil-Type Waste Heat Boiler"), this process is essential for:
- Removal of scale and oxidation products: Immersion in a controlled mixture of hydrofluoric acid (HF) and nitric acid (HNO₃) dissolves mill scale, welding slag, and heat treatment oxide layers from the tube surface.
- Formation of a protective chromium oxide passive film: The passivation step (using dilute nitric acid, typically 20–30% HNO₃) promotes the formation of a stable, adherent Cr₂O₃ passive film (approximately 2–5 nm thick), providing long-term corrosion protection.
- Removal of free iron contamination: Pickling eliminates free iron particles that may have been introduced during handling, fabrication, or welding operations, preventing localized rust initiation sites.
- Surface preparation for service: Ensures the tube surface is clean, uniform, and ready for installation without further surface treatment.
Process Parameters (Typical):
- Pickle bath composition: HF 5–8% + HNO₃ 25–35% (by volume) in water
- Pickle temperature: 60–80°C
- Pickle duration: 15–45 minutes (depending on scale thickness)
- Passivation: 20–30% HNO₃, 60–70°C, 10–20 minutes
- Post-treatment: Deionized water rinse + drying
5. Non-Destructive Testing (NDT)
Every coil tube undergoes 100% automated eddy current testing (ECT) in accordance with ASTM E309 to detect wall thinning, cracks, and subsurface defects. A hydrostatic pressure test is performed at 1.5 times the maximum design pressure to verify weld integrity and structural soundness.
6. Coiling and Packaging
The finished coil tube is wound onto appropriate spools or coils, protected with anti-corrosion wrapping (VCI-treated film or desiccant-packed packaging), and secured for transport. Each coil is labeled with a unique heat number, material certificate reference, and inspection report identification.
Applications
- Waste Heat Recovery Boilers (WHRB): Primary application — coil tubes serve as evaporative and superheating elements in waste heat boilers recovering thermal energy from industrial exhaust gases (steel mills, cement kilns, glass furnaces, power plant flue gases).
- Organic Rankine Cycle (ORC) Systems: Heat exchanger elements in low-to-medium temperature waste heat recovery for power generation.
- Industrial Process Heating: Tubular heat exchangers in chemical processing, pharmaceutical manufacturing, and food processing industries.
- Marine and Offshore Applications: Exhaust gas boiler coil tubes in marine engines and offshore platform waste heat recovery systems.
- Power Generation: Auxiliary heat recovery systems in combined cycle power plants and cogeneration facilities.
- Environmental Systems: Flue gas cleaning and desulfurization (FGD) system heat exchangers.
Quality Certifications and Compliance
- ISO 9001:2015 — Quality Management System certification for manufacturing processes
- ISO 14001:2015 — Environmental Management System certification
- EN 10204 3.1 — Manufacturer's Material Test Certificate (MTC) provided with each shipment
- ASTM A213 — Compliance for seamless austenitic stainless steel tubing
- ASTM A269 — Compliance for austenitic stainless steel tubing, welded, cold-drawn
- ASME BPV Code — Section I compliance for power boiler applications
- GB/T 12771 — Chinese national standard for stainless steel seamless tubes
- CE Marking — Conformity with European Pressure Equipment Directive (PED 2014/68/EU) where applicable
- NACE MR0175 — Compliance for sour service applications (where SAE 316 grade is specified)
Unique Selling Points
- Field-Validated Pickling Protocol: The pickling and passivation process is refined through documented operational experience in actual waste heat boiler service, ensuring the passive film integrity is proven under real industrial conditions rather than solely laboratory-validated.
- Butt Weld-Free Construction: Continuous coil forming eliminates longitudinal and circumferential welds, providing uniform mechanical and corrosion performance throughout the entire tube length.
- High Surface Area-to-Volume Ratio: The coil configuration maximizes heat transfer surface within confined boiler casing dimensions, improving thermal efficiency by 15–25% compared to straight-tube alternatives.
- Superior Corrosion Durability in Aggressive Environments: SAE 316-grade coil tubes with molybdenum enhancement resist chloride attack and acid condensation commonly encountered in waste gas streams.
- Full Traceability: Each coil tube is traceable to its mill heat number, with complete documentation of chemical analysis, mechanical testing, heat treatment parameters, and pickling process records.
- Custom Configurations: Coil diameter, pitch, tube OD, wall thickness, and total length are customizable to match specific boiler design requirements and installation constraints.
Operational Notes from Field Experience
Based on the documented study of coil-type waste heat boiler pickling and operation, the following operational considerations are recommended for optimal service life:
- Periodic inspection of the passive film integrity is recommended at intervals not exceeding 12 months of continuous operation.
- Post-installation pickling and passivation should be performed after all welding and mechanical assembly is complete to protect the full tube surface.
- Monitoring of flue gas composition (particularly SO₂, HCl, and particulate content) is essential for predicting corrosion rates and scheduling maintenance.
- The coil tube configuration inherently provides thermal expansion accommodation through its geometry, reducing stress concentrations at support points compared to rigid straight-tube assemblies.
Conclusion
The Stainless Steel Coil Tube (Coiltube) from Wenzhou Jixing Metal Materials Co., Ltd. represents a high-performance solution for waste heat recovery and thermal exchange applications. Through the integration of precision manufacturing, rigorous quality control, and field-validated surface treatment protocols, this product delivers reliable long-term performance in the demanding environments of industrial waste heat boiler systems. The comprehensive pickling and passivation treatment, informed by documented operational experience, ensures that each coil tube achieves maximum corrosion resistance and service longevity upon installation.