Stainless Steel Coil Tube for PET Process Column Applications

Product Overview

The Stainless Steel Coil Tube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchanger component specifically designed for use in Polyethylene Terephthalate (PET) process columns and distillation systems. These coil tubes serve as critical heat exchange elements within the PET polymerization and purification stages, where they facilitate efficient thermal transfer under demanding process conditions involving high temperatures, corrosive chemical environments, and sustained mechanical stress.

Coil tubes in PET process columns are helically wound stainless steel tubes that integrate directly into the column internals, providing high surface-area-to-volume ratios for optimal heat exchange performance. The product addresses one of the most significant engineering challenges in PET manufacturing — the prevention of Stress Corrosion Cracking (SCC), Intergranular Corrosion (IGC), and thermal fatigue cracking that commonly affect coil tubes operating in glycol-rich, high-temperature environments.

Material & Properties

Coil tubes for PET process column service are fabricated from high-performance austenitic stainless steels selected specifically for their resistance to the aggressive chemical and thermal conditions encountered during PET production.

Grade 316L (UNS S31603) — The preferred material for PET process column coil tubes, offering superior resistance to chloride-induced Stress Corrosion Cracking (SCC) due to its molybdenum (Mo) content of 2.0–3.0%. The low-carbon variant (≤0.03% C) minimizes sensitization risk during thermal cycling.
Grade 321 (UNS S32100) — Titanium-stabilized austenitic stainless steel providing enhanced resistance to intergranular corrosion in high-temperature glycol service. Titanium carbide precipitation prevents chromium depletion at grain boundaries.
Grade 304L (UNS S30403) — Utilized in less aggressive zones of the process column where chloride concentrations are lower, offering good general corrosion resistance at reduced cost.
Grade 316Ti (UNS S31635) — A titanium-stabilized variant of 316L, providing dual protection against both chloride SCC and intergranular sensitization.

Key Mechanical and Physical Properties

Tensile Strength: 515–750 MPa (Grade 316L), suitable for sustained load-bearing under thermal cycling
Yield Strength: 205–310 MPa, providing adequate ductility for helical forming
Elongation: ≥40% (minimum), ensuring resistance to crack propagation

Operating Temperature Range:

Corrosion Resistance: Excellent resistance to ethylene glycol (EG), methanol, acetic acid, and trace chloride ions at process temperatures

Technical Specifications

Parameter	Specification	Standard / Reference
Material Grade	316L, 321, 304L, 316Ti (austenitic stainless steel)	AISI / ASTM / GB
Outer Diameter (OD)	10 mm – 50 mm (customizable)	—
Wall Thickness (WT)	0.8 mm – 3.0 mm	—
Coil Pitch (Helical Pitch)	15 mm – 80 mm (process-dependent)	—
Coil Diameter	50 mm – 500 mm	—
Tube Length per Coil	3 m – 30 m	—
Surface Finish	Electropolished (Ra ≤ 0.4 μm) or Pickled & Passivated	ASTM A478
End Preparation	Beveled (ASME B16.25) or Flat-cut with deburr	ASME B16.25
Maximum Operating Pressure	Up to 2.5 MPa (gauge)	ASME Section VIII
Maximum Operating Temperature	285°C (continuous) / 320°C (intermittent)	PET Process Spec
Manufacturing Standard	ASTM A213 / A269 / GB/T 12771 / GB/T 21832	—
Heat Treatment	Solution annealed at 1050–1150°C, water quenched	ASTM A213
Non-Destructive Testing	100% Eddy Current (ET), Visual Inspection (VT), Hydrostatic Test	ASTM A269

Manufacturing Process

The production of high-integrity coil tubes for PET process columns follows a rigorous multi-stage manufacturing workflow designed to eliminate residual defects and ensure metallurgical homogeneity:

Raw Material Selection & Inspection: Virgin stainless steel strip or seamless tube blanks are sourced from certified mills. Incoming material undergoes 100% chemical composition analysis (per ASTM E415), spectroscopic verification, and mechanical property testing to confirm conformance with specified grades.
Strip Preparation & Cold Rolling: For welded coil tube production, stainless steel strip is cold-rolled to precise thickness tolerances (±0.02 mm) and edge quality requirements. Surface defects are removed through leveling and edge trimming.
Forming & Welding: The strip is formed into a circular cross-section and welded using Automatic Argon Arc Welding (TIG) with high-purity shielding gas (99.999% Ar) to achieve a clean, oxide-free weld bead. Weld penetration is verified by internal inspection.
Hippering & Reshaping: The welded tube is passed through a series of concentric mandrels (hippers) to achieve precise dimensional accuracy, uniform wall thickness, and improved surface finish. This step also relieves residual welding stresses.
Solution Annealing: Tubes are annealed in a controlled atmosphere furnace at 1050–1150°C followed by rapid water quenching. This restores full austenitic microstructure, eliminates sensitization, and ensures maximum corrosion resistance. For Grade 321 and 316Ti, stabilizing heat treatment may be applied.
Pickling & Passivation: Annealed tubes are pickled in a mixed acid solution (HNO₃/HF) to remove oxide scale, followed by passivation in dilute nitric acid to establish a stable chromium oxide passive film. This is critical for preventing intergranular corrosion initiation in chloride-containing PET process fluids.
Helical Coiling: The straight tube is wound into the specified helical geometry using precision coiling equipment with controlled bending radii to prevent plastic deformation, ovality, or work-hardening. The coil pitch and diameter are controlled to within ±1 mm tolerance.
Post-Coiling Stress Relief: Coiled tubes undergo low-temperature stress relief (≤450°C) to minimize residual stresses introduced during helical forming, which is critical for preventing Stress Corrosion Cracking (SCC) during service.
Final Testing & Quality Assurance:
- 100% Eddy Current Testing (ET) per ASTM A269 to detect internal and external surface defects
- Hydrostatic Pressure Testing at 1.5× maximum working pressure
- Visual Inspection (VT) of weld seams and coil geometry
- Penetrant Testing (PT) or Magnetic Particle Testing (MT) on weld areas (where applicable)
- Crystallographic Orientation Analysis to verify absence of detrimental precipitates
- Corrosion Testing including Intergranular Corrosion Test (IGCT) per ASTM A262 Practice E and Potential-Step Test (PST) for SCC susceptibility

Applications

Stainless steel coil tubes manufactured by Wenzhou Jixing Metal Materials Co., Ltd. are deployed in the following critical applications:

PET Process Column Heat Exchangers: Primary application — coil tubes serve as internal heat exchange elements in PET purification and polymerization columns, facilitating thermal management of the polymer melt and glycol solvent.
PET Distillation Columns: Coil tubes used in the recovery and purification of ethylene glycol (EG) and dimethyl terephthalate (DMT) within distillation systems operating at elevated temperatures and under vacuum.
Chemical Process Columns: General-purpose applications in organic chemical processing, pharmaceutical intermediates, and fine chemical distillation columns requiring corrosion-resistant heat exchange elements.
Food and Beverage Processing: Coil tubes in pasteurization columns, fermentation vessels, and CIP (Clean-in-Place) compatible systems where hygienic surface finish is required.
Pharmaceutical Manufacturing: Heat exchange coils in crystallization columns, solvent recovery systems, and reactive distillation columns.
Textile and Fiber Industry: Coil tubes in polymer extrusion and spinning bath temperature control systems for polyester filament production.

Crack Prevention & Engineering Considerations

Based on extensive field experience and technical analysis of failure modes in PET process column coil tubes, the following engineering measures are incorporated into the product design and manufacturing process:

Material Selection: Mandatory use of low-carbon (L-grade) or stabilized (Ti-grade) austenitic stainless steels to prevent chromium carbide precipitation at grain boundaries, which is the primary mechanism for intergranular corrosion (IGC) initiation.
Chloride Ion Control: Specification of maximum permissible chloride concentration (≤50 ppm for 316L, ≤20 ppm for 304L) in process fluids to prevent Chloride Stress Corrosion Cracking (Cl-SCC).
Residual Stress Elimination: Post-coiling stress relief annealing to reduce residual tensile stresses below the SCC threshold (typically <100 MPa).
Weld Quality Control: 100% eddy current inspection of weld seams to detect porosity, incomplete fusion, and lack of penetration that serve as crack initiation sites.
Surface Integrity: Electropolishing or mechanical polishing to achieve surface roughness Ra ≤ 0.4 μm, eliminating surface defects that act as corrosion nucleation sites.
Thermal Gradient Management: Coil geometry design optimized to minimize thermal stress concentration at support points and bends.

Quality Certifications & Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management and product certification systems ensuring full compliance with international standards:

ISO 9001:2015 — Quality Management System certification covering the entire manufacturing and supply chain
ISO 14001:2015 — Environmental Management System certification
ISO 45001:2018 — Occupational Health and Safety Management System certification
ASME Certificate of Compliance (CoC) — For pressure-containing components manufactured to ASME Section VIII Division 1
EN 10204 3.1 Test Certificates — Third-party verified material test reports for every production batch
ASTM A213 / A269 Conformance — Product specifications meet American Society for Testing and Materials standards for seamless and welded stainless steel tubes
GB/T 12771 / GB/T 21832 Compliance — Chinese national standards for welded stainless steel tubes and seamless stainless steel tubes
CE Marking — Conformity with European Pressure Equipment Directive (PED 2014/68/EU) for pressure applications
Material Traceability: Full heat number traceability from mill certificate through final product delivery, enabling root cause analysis in the event of field performance issues

Unique Selling Points

PET-Specific Engineering Expertise: Deep technical knowledge of crack mechanisms in PET process column environments, enabling proactive design and material selection to prevent premature failure.
Comprehensive Failure Analysis Support: Technical team experienced in root cause analysis of coil tube cracking (SCC, IGC, thermal fatigue), providing diagnostic support and corrective action recommendations.
Custom Helical Geometry: Precision coiling capability accommodating project-specific pitch, diameter, and length requirements with ±1 mm dimensional tolerance.
End-to-End Quality Assurance: 100% non-destructive testing coverage including eddy current, hydrostatic, and penetrant testing at every production stage.
Stabilized Material Options: Availability of Ti-stabilized grades (321, 316Ti) that provide dual protection against both sensitization and chloride SCC, extending service life in aggressive PET environments.
Post-Coiling Stress Relief: Proprietary low-temperature stress relief process specifically optimized for helical coil geometries to minimize residual stresses and prevent in-service cracking.

Conclusion

The Stainless Steel Coil Tube product line from Wenzhou Jixing Metal Materials Co., Ltd. represents a technically advanced solution for PET process column heat exchange applications. By integrating premium austenitic stainless steel materials, precision manufacturing processes, rigorous quality assurance protocols, and deep domain expertise in crack prevention engineering, these coil tubes deliver reliable, long-duration performance in the most demanding chemical processing environments. The product is engineered to meet or exceed the requirements of ASTM, ASME, and GB standards, providing customers with confidence in both material integrity and operational longevity.