Stainless Steel Coil Tube for Tank Heating Systems — Technical Specification
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product designed primarily for immersion heating applications within industrial storage tanks, reactors, and process vessels. Unlike straight seamless tubes, coil tubes are formed continuously in a helical configuration, offering superior flexibility, enhanced thermal exchange surface area, and improved resistance to thermal fatigue. This product category is extensively utilized in the petrochemical, pharmaceutical, food processing, and chemical engineering industries where reliable heat transfer under cyclic thermal loading is critical.
Based on comprehensive failure analysis research — particularly the study documented in "Failure Analysis and Protective Measures for Tank Heating Coil Tubes" — this product has been specifically engineered to address common degradation mechanisms including corrosion fatigue, intergranular corrosion, stress corrosion cracking (SCC), and thermal cycling-induced cracking. The design incorporates advanced material selection, optimized manufacturing tolerances, and protective surface treatments to maximize service life under aggressive operating conditions.
Material & Properties
Material Grades
The coil tube is available in a range of austenitic and duplex stainless steel grades selected to meet specific service environment requirements:
- Austenitic Grades: SUS304 (UNS S30400), SUS304L (UNS S30403), SUS316 (UNS S31600), SUS316L (UNS S31603), SUS316Ti (UNS S31635), SUS321 (UNS S32100)
- Duplex Grades: SUS31803 (UNS S31803), SUS32205 (UNS S32205) — offering superior resistance to chloride-induced SCC and pitting corrosion
- High-Nickel Alloys (Special Order): Inconel 625, Hastelloy C-276, Alloy 825 — for extreme chemical environments
Mechanical Properties (Typical — Annealed Condition)
| Property | SUS304/304L | SUS316/316L | SUS321 | SUS31803 (Duplex) |
|---|---|---|---|---|
| Tensile Strength (MPa) | ≥515 | ≥515 | ≥515 | ≥550 |
| Yield Strength (MPa) | ≥205 | ≥205 | ≥205 | ≥450 |
| Elongation (%) | ≥40 | ≥40 | ≥40 | ≥25 |
| Thermal Conductivity (W/m·K, 20°C) | 16.3 | 14.6 | 16.3 | 13.6 |
| Maximum Service Temperature (°C) | 870 | 870 | 900 | 350 |
| Pitting Resistance (PREN) | 18–20 | 24–26 | 18–20 | 36–38 |
Chemical Composition (Typical, wt%)
| Element | SUS304 | SUS316 | SUS321 | SUS31803 |
|---|---|---|---|---|
| C | ≤0.08 | ≤0.08 | ≤0.08 | ≤0.030 |
| Cr | 18.0–20.0 | 16.0–18.0 | 17.0–19.0 | 22.0–23.0 |
| Ni | 8.0–10.5 | 10.0–14.0 | 9.0–12.0 | 6.0–8.0 |
| Mo | — | 2.0–3.0 | — | 3.0–3.5 |
| Ti | — | — | 5×C–0.70 | — |
| N | — | — | — | 0.14–0.20 |
| Si | ≤1.00 | ≤1.00 | ≤1.00 | ≤1.00 |
| Mn | ≤2.00 | ≤2.00 | ≤2.00 | ≤2.00 |
| P | ≤0.045 | ≤0.045 | ≤0.045 | ≤0.030 |
| S | ≤0.030 | ≤0.030 | ≤0.030 | ≤0.015 |
Specifications
| Parameter | Specification / Range |
|---|---|
| Outer Diameter (OD) | 6.0 mm – 159.0 mm |
| Wall Thickness | 0.5 mm – 12.0 mm |
| Coil Diameter (CD) | 50 mm – 2000 mm (customizable) |
| Tube Length (per coil) | 10 m – 500 m (continuous) |
| Surface Finish | 2B, BA (Bright Annealed), Pickled & Passivated, Mirror (Ra ≤ 0.4 μm) |
| End Condition | Plain end, Beveled end, Threaded end (per customer requirement) |
| Manufacturing Standard | ASTM A269, ASTM A270, GB/T 14976, EN 10216-5 |
| Dimensional Tolerance | OD: ±0.15 mm; Wall: ±0.05 mm (precision grade) |
| Operating Pressure | Up to 4.0 MPa (design dependent) |
| Operating Temperature Range | −196 °C to +870 °C (grade dependent) |
| Heat Medium | Steam, Hot Oil, Thermal Fluid, Electric Trace Heating |
| Delivery Condition | Annealed (Solution Treated), Cold Worked, or Annealed & Cold Worked |
| Packaging | Wooden reel with moisture-proof wrapping; export-standard packaging available |
Manufacturing Process
Process Flow
- Raw Material Inspection: Ingot or strip material undergoes spectrographic analysis (OES) to verify chemical composition within specified limits. Heat number traceability is established at this stage.
- Hot Rolling / Hot Extrusion: Billet or strip is heated to recrystallization temperature and formed into hollow tube stock through controlled rolling or extrusion operations.
- Piercing & Elongation: Initial tube blank is pierced to achieve preliminary dimensions, followed by elongation to reduce wall thickness and refine microstructure.
- Cold Drawing / Cold Pilgering: Multi-pass cold reduction achieves precise final OD and wall thickness dimensions with tight tolerances. Intermediate annealing cycles prevent work hardening.
- Solution Annealing (Stress Relief): Tubes are heated to 1050–1150 °C (grade-dependent) and rapidly quenched to dissolve carbides and restore full austenitic/duplex microstructure, ensuring maximum corrosion resistance.
- Coil Forming: Annealed tubes are fed through a coil forming machine with controlled bending radius, tension, and speed to produce the helical coil configuration without inducing residual stresses or ovality.
- Pickling & Passivation: Surface is treated with mixed acid solution (HNO₃ + HF) to remove oxide scale, followed by passivation treatment to form a stable chromium oxide protective layer (minimum 200 Å thickness).
- Final Inspection & Testing: Includes hydrostatic pressure testing, eddy current (ET) inspection, dimensional verification, surface roughness measurement, and optional intergranular corrosion (IGC) testing per ASTM A269 Appendix.
Key Process Controls Addressing Failure Modes
Based on failure analysis findings from tank heating coil service, the following process controls are specifically implemented:
- Carbide Precipitation Prevention: Low-carbon (L-grade) compositions and rapid quench rates during solution annealing minimize chromium carbide precipitation at grain boundaries, preventing intergranular corrosion (IGC).
- Residual Stress Elimination: Post-coil-forming stress relief annealing (850–900 °C, 1 hour) eliminates cold-working residual stresses that could initiate stress corrosion cracking (SCC).
- Ovality Control: Coil forming parameters are optimized to maintain cross-sectional ovality below 2% OD, preventing localized flow stagnation and accelerated corrosion at flat spots.
- Weld Seam Integrity (if applicable): For welded coil tubes, seam welds are ground flush and undergo full-length eddy current inspection to detect lack of fusion or porosity.
Applications
Primary Applications
- Industrial Storage Tank Heating: Immersion coil heating systems for crude oil, refined petroleum products, chemical intermediates, and polymer storage tanks requiring temperature maintenance or viscosity control.
- Reactor Jacket & Internal Coil Heating: Heat transfer coils installed inside chemical reactors for exothermic reaction temperature control and endothermic process heating.
- Pharmaceutical Processing: Sanitary-grade coil heaters (BA finish, Ra ≤ 0.4 μm) for bioreactors, fermentation tanks, and CIP/SIP-compatible heating loops.
- Food & Beverage Industry: Heating coils for milk processing tanks, juice pasteurization vessels, and beer fermentation tanks (316L, food-grade compliant).
- Heat Exchanger Coils: Shell-and-coil heat exchangers for HVAC systems, district heating networks, and industrial process cooling/heating.
- Boiler Tubing: Fire-tube and water-tube boiler heating elements in marine and land-based applications.
- Heat Tracing Systems: Steam tracing coils for pipeline insulation in cryogenic and sub-ambient environments.
Typical Service Environments
| Application Sector | Recommended Grade | Key Challenge Addressed |
|---|---|---|
| Petrochemical Crude Storage | 304L / 316L | H₂S corrosion, thermal cycling fatigue |
| Marine / Seawater Systems | 31803 (Duplex) / 316Ti | Chloride-induced pitting and SCC |
| Pharmaceutical / Biotech | 316L (BA Finish) | Sanitary requirements, CIP/SIP compatibility |
| High-Temperature Heat Treatment | 321 (Ti-stabilized) | Intergranular corrosion at elevated temperatures |
| Aggressive Chemical Processing | Hastelloy C-276 / Inconel 625 | Concentrated acids, halide-containing media |