Stainless Steel Coil Tube for Ammonia Synthesis Startup Heating Furnace
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product specifically designed for high-temperature furnace applications in ammonia synthesis units. These coil tubes serve as critical heat-transfer elements within startup heating furnaces (Ignition Furnaces / Preheating Furnaces) that condition process gases prior to catalytic reaction stages. The product represents a significant advancement in coil tube manufacturing technology, incorporating innovative fabrication techniques that address the unique thermal, mechanical, and chemical demands encountered during ammonia plant commissioning and steady-state operation.
Coil tubes in ammonia synthesis startup furnaces must withstand extreme thermal cycling, elevated internal pressures, and exposure to corrosive process media including hydrogen, nitrogen, ammonia, and trace hydrocarbons. The manufacturing technology improvements and innovations embodied in this product line ensure superior dimensional accuracy, consistent metallurgical properties, and extended service life compared to conventional coil tube solutions.
Material & Properties
Base Material
The coil tube is fabricated from high-grade austenitic stainless steel conforming to internationally recognized standards. The primary material grades include:
- SAE 321 (UNS S32100) — Chromium-Nickel-Titanium stabilized austenitic stainless steel, providing excellent resistance to intergranular corrosion and superior high-temperature strength up to 900°C.
- SAE 347 (UNS S34700) — Chromium-Nickel-Niobium stabilized austenitic stainless steel, offering enhanced resistance to pitting and stress corrosion cracking.
- SAE 310S (UNS S31008) — High-chromium austenitic stainless steel with exceptional oxidation resistance for temperatures exceeding 1100°C.
Mechanical Properties (Typical Values at Room Temperature)
| Property | SAE 321 | SAE 347 | SAE 310S |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 520 | ≥ 520 |
| Yield Strength (0.2% offset, MPa) | ≥ 205 | ≥ 205 | ≥ 205 |
| Elongation (%) | ≥ 40 | ≥ 40 | ≥ 40 |
| Modulus of Elasticity (GPa) | 193 | 193 | 193 |
| Thermal Conductivity at 100°C (W/m·K) | 16.3 | 16.3 | 15.1 |
| Thermal Conductivity at 500°C (W/m·K) | 21.7 | 21.7 | 20.1 |
| Maximum Service Temperature (°C) | 900 | 925 | 1150 |
Chemical Composition (wt%)
| Element | SAE 321 | SAE 347 | SAE 310S |
|---|---|---|---|
| C (Carbon) | ≤ 0.08 | ≤ 0.08 | ≤ 0.08 |
| Cr (Chromium) | 17.0–19.0 | 17.0–19.0 | 24.0–26.0 |
| Ni (Nickel) | 9.0–12.0 | 9.0–12.0 | 19.0–22.0 |
| Stabilizer | Ti ≥ 5×C | Nb ≥ 10×C | — |
| Si (Silicon) | ≤ 0.75 | ≤ 0.75 | ≤ 1.50 |
| Mn (Manganese) | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 |
| P (Phosphorus) | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| S (Sulfur) | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) |
| Product Catalog No. | 952 |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd. |
| Outer Diameter (OD) | Φ25 mm – Φ114.3 mm (configurable) |
| Wall Thickness | 1.5 mm – 8.0 mm (configurable) |
| Coil Bending Radius | ≥ 5× OD (minimum); typically 8× – 15× OD |
| Coil Length (Total Developed Length) | Up to 500 m per coil (continuous) |
| Dimensional Tolerance (OD) | ±0.10 mm (precision grade); ±0.15 mm (standard grade) |
| Dimensional Tolerance (Wall Thickness) | ±10% of nominal |
| Surface Finish | 2B / BA (Bright Annealed) / Custom ground finish |
| Surface Roughness (Ra) | ≤ 0.8 μm (BA finish); ≤ 1.6 μm (2B finish) |
| End Treatment | Plain end / Beveled end / Flanged connection (as specified) |
| Heat Treatment | Solution annealing at 1050°C – 1150°C followed by water quench |
| Operating Pressure (Design) | Up to 25.0 MPa (gauge) |
| Operating Temperature Range | −196°C to +900°C (SAE 321/347); up to +1150°C (SAE 310S) |
| Coating / Passivation | Acid passivation treatment; optional PTFE or ceramic internal coating |
| Packaging | Industrial-grade coil packaging with moisture-proof wrapping and wooden crate |
Manufacturing Process
The manufacturing technology for these coil tubes incorporates significant innovations that address the challenges identified in ammonia synthesis startup furnace applications. The process flow is as follows:
- Raw Material Selection & Incoming Inspection — Cold-rolled stainless steel strip or seamless tube blanks are sourced from certified mills. Each batch undergoes spectroscopic analysis (OES), mechanical testing, and ultrasonic inspection to verify compliance with material specifications.
- Dimensional Pre-Processing — Tube blanks are precisely cut to required lengths using CNC plasma or laser cutting systems. Surface preparation includes wire brushing or chemical cleaning to remove oxide scale and contaminants.
- Coil Forming (Cold Bending) — A proprietary multi-axis coil bending system with computerized roll control is employed. The innovation lies in the use of programmable counter-rotation rolls that distribute bending stress uniformly, minimizing residual strain and preventing wall-thickness reduction at the inner bend radius. This technique achieves bending radius tolerances within ±1% of nominal.
- Intermediate Annealing — After forming, coils undergo controlled atmosphere annealing (reducing atmosphere, dew point ≤ −60°C) at temperatures between 900°C and 1050°C to relieve residual stresses introduced during cold bending while preserving grain structure integrity.
- Heat Exchanger Assembly & Brazing — Where required, coil tubes are welded or brazed into heat exchanger assemblies using orbital TIG (Tungsten Inert Gas) welding with argon shielding. Inert gas backing ensures full penetration and eliminates oxidation on the inner surface.
- Final Solution Annealing & Pickling — A final solution anneal at 1050°C – 1150°C followed by rapid water quench homogenizes the microstructure. Subsequent acid pickling (HNO₃/HF mixture) removes all surface oxides, restoring the passive chromium oxide layer.
- Passivation Treatment — Citric acid or nitric acid passivation ensures a stable, protective chromium-rich oxide film on the surface, critical for resistance to hydrogen and ammonia corrosion.
- Non-Destructive Testing (NDT) — 100% hydrostatic pressure testing at 1.5× design pressure; helical ultrasonic testing for internal defects; eddy current testing for surface discontinuities; and visual inspection of weld seams.
- Dimensional Verification & Final Inspection — Laser-based diameter measurement, wall thickness verification using ultrasonic gauges, and coil geometry inspection using 3D laser scanning to confirm bending radius accuracy.
Key Manufacturing Innovations
- Variable-Radius Bending Technology — Allows transition sections with continuously varying coil radii, reducing stress concentrations at geometric discontinuities that are common in furnace tube layouts.
- Protective Atmosphere Forming — Cold bending performed under controlled nitrogen atmosphere to prevent surface oxidation and contamination during the forming process.
- Computer-Aided Coil Layout Optimization — Finite element analysis (FEA) software models the thermal expansion and mechanical stress distribution throughout the operating cycle, enabling optimization of coil geometry for uniform heat absorption and minimized differential expansion.
- Microstructure Control Through Controlled Cooling — Post-anneal cooling rate is precisely controlled to achieve a fine-grained, uniform austenitic microstructure with controlled grain size (ASTM Grain Size 7–8), enhancing both creep resistance and fatigue life.
Applications
The primary application domain for this coil tube product is the ammonia synthesis industry, specifically within startup heating furnaces (also referred to as ignition furnaces or preheating furnaces) that condition synthesis gas (syngas) prior to introduction into the ammonia converter. Additional applications include:
- Ammonia Synthesis Startup Furnaces — Preheating of nitrogen-hydrogen mixtures to reaction temperature (300°C – 500°C) prior to catalytic conversion. The coil tubes absorb radiant and convective heat from furnace burners.
- Hydrogen Production Units — Steam reforming furnaces and shift converters in hydrogen production facilities for ammonia and methanol plants.
- Urea Production — CO₂ stripping and absorption column reboilers requiring high-temperature heat transfer tubing.
- Petrochemical Processing — Crude oil fractionation furnaces, reforming coils, and coking furnace tubes in petroleum refining.
- Power Generation — Supercritical and ultra-supercritical boiler waterwall tubes and reheater coils in thermal power stations.
- Chemical Processing — Polymerization reactors, polymer melt processing equipment, and specialty chemical reactors requiring corrosion-resistant heat transfer elements.
- Hydrogen Energy Systems — Emerging applications in green hydrogen production via electrolysis, where high-purity hydrogen loops require ultra-clean, corrosion-resistant tubing.
Quality Certifications & Standards Compliance
All coil tube products are manufactured in accordance with the following international standards and carry the requisite certifications:
- ASTM A213 — Standard Specification for Ferritic, Austenitic, and Ferritic-Austenitic (Duplex) Stainless Steel Tubing for Heat-Transfer and Similar Applications
- ASTM A269 — Standard Specification for Cold-Drawn (or Cold-Rolled) Seamless Austenitic Stainless Steel Tubing
- ASTM A312 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severities of Service
- ASME SB-312 / SB-313 — Boiler and Pressure Vessel Code, Section II, Part D
- EN 10216-5 — Technical Delivery Conditions for Stainless Steel Tubes for Particular Purposes — Part 5: Tubes of Austenitic Steels for General Purposes
- ISO 3506 — Stainless Steel — Chemical Composition and Designation System
- GB/T 12771 — Seamless Stainless Steel Tubes for Fluid Transport (Chinese National Standard)
- GB/T 5310 — Seamless Steel Tubes for High Temperature Boilers (Chinese National Standard)
- NACE MR0175 / ISO 15156 — Materials for Use in H₂S-Containing Environments in Oil and Gas Production
The manufacturing facility holds the following quality management certifications:
- ISO 9001:2015 — Quality Management Systems
- ISO 14001:2015 — Environmental Management Systems
- ASME "U" Stamp — Authorization for the design and fabrication of pressure vessels and pressure-retaining components
- TÜV Certification — Third-party inspection and certification for pressure equipment
- CCS (China Classification Society) Approval — Where applicable for marine and offshore applications
Unique Selling Points
- Extended Service Life — Innovative manufacturing processes yield coil tubes with