Stainless Steel Coil Tube for Molecular Sieve Dehydration Regeneration Gas Heater
Product Overview
The Stainless Steel Coil Tube (Coiltube) designed by Wenzhou Jixing Metal Materials Co., Ltd. is a specialized heat-exchange element engineered for use in molecular sieve dehydration regeneration gas heaters within natural gas processing systems. These coil tubes operate under demanding thermomechanical conditions characterized by sustained high temperatures in the range of 250–300 °C (482–572 °F) and the presence of trace acid gases, including hydrogen sulfide (H₂S), carbon dioxide (CO₂), and sulfur dioxide (SO₂). The product is fabricated from austenitic stainless steel seamless tubes of Grade 321 (UNS S32100) and Grade 347 (UNS S34700), selected specifically for their superior resistance to high-temperature oxidation, thermal fatigue cracking, and intergranular corrosion under cyclic thermal loading.
In molecular sieve dehydration units, regeneration gas—typically a mixture of dried natural gas or nitrogen—is superheated and routed through the desiccant bed to desorb adsorbed water molecules. The coil tubes serve as the primary heat-transfer medium, absorbing thermal energy from the regeneration furnace or waste-heat recovery system and delivering it to the process gas stream. The continuous cyclic nature of adsorption–desorption operations subjects these components to repeated thermal expansion and contraction, making thermal fatigue resistance a critical design parameter.
Material Selection & Properties
Grade 321 (UNS S32100) — Titanium-Stabilized Austenitic Stainless Steel
Grade 321 is a titanium-stabilized variant of the 300-series austenitic stainless steels. The addition of titanium (Ti) forms titanium carbides (TiC), effectively depleting the matrix of free carbon and thereby preventing chromium carbide (Cr₂₃C₆) precipitation at grain boundaries during prolonged exposure to elevated temperatures. This stabilization mechanism is essential for resisting intergranular corrosion (IGC) in the temperature range of 450–850 °C, which encompasses the operating window of molecular sieve regeneration heaters.
- Chemical Composition (Typical, wt%): C ≤ 0.08, Si ≤ 1.00, Mn ≤ 2.00, P ≤ 0.045, S ≤ 0.030, Cr 17.00–19.00, Ni 9.00–12.00, Ti ≥ 5×C (minimum 0.40)
- Operating Temperature Range: -196 °C to 870 °C (non-oxidizing atmosphere); 870 °C to 1000 °C (oxidizing atmosphere)
- Tensile Strength: ≥ 515 MPa (minimum, at room temperature)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation: ≥ 40% (in 50 mm gauge length)
- Thermal Conductivity (at 300 °C): Approximately 22 W/(m·K)
Grade 347 (UNS S34700) — Niobium-Stabilized Austenitic Stainless Steel
Grade 347 employs niobium (Nb) as the primary carbide stabilizer, forming NbC precipitates that similarly sequester carbon and prevent sensitization. Compared to Grade 321, Grade 347 exhibits marginally higher creep strength and improved resistance to thermal fatigue cracking, making it particularly advantageous for applications involving frequent thermal cycling, such as regeneration heater coil tubes that undergo repeated heating and cooling cycles during the adsorption–desorption cycle.
- Chemical Composition (Typical, wt%): C ≤ 0.08, Si ≤ 1.00, Mn ≤ 2.00, P ≤ 0.045, S ≤ 0.030, Cr 17.00–19.00, Ni 9.00–13.00, Nb 8–10×C (minimum 1.00)
- Operating Temperature Range: -196 °C to 900 °C (non-oxidizing); up to 1050 °C (oxidizing)
- Tensile Strength: ≥ 515 MPa (minimum, at room temperature)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation: ≥ 40% (in 50 mm gauge length)
- Creep Strength (at 800 °C, 10,000 hours): Approximately 35–45 MPa
Comparative Summary: Grade 321 vs. Grade 347
| Property | Grade 321 (UNS S32100) | Grade 347 (UNS S34700) |
|---|---|---|
| Stabilizing Element | Titanium (Ti) | Niobium (Nb) |
| Maximum Service Temperature (Oxidizing) | 1000 °C | 1050 °C |
| Intergranular Corrosion Resistance | Excellent | Excellent |
| Thermal Fatigue Resistance | Good | Superior |
| Creep Strength (High-Temp.) | Good | Excellent |
| Weldability | Excellent | Excellent |
| Cost (Relative) | Baseline | +15–25% |
Technical Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Type | Seamless Stainless Steel Coil Tube (Coiltube) | — |
| Material Grades | ASTM A312 / A213 Type 321 & Type 347 | ASTM A312, ASTM A213 |
| Tube Outer Diameter (OD) | 19.05 mm – 168.28 mm (3/4" – 6.5") | ASTM A53, ASME B36.10M |
| Tube Wall Thickness | 1.65 mm – 9.53 mm (Schedule 10S – Schedule 80S) | ASTM A312 |
| Coil Bend Diameter | Custom (typically 0.5 m – 3.0 m) | Project-Specific |
| Coil Length per Unit | Custom (typically 50 m – 300 m) | Project-Specific |
| Operating Temperature | 250 °C – 300 °C (500 °F – 572 °F) | — |
| Design Temperature | ≤ 870 °C (Grade 321); ≤ 900 °C (Grade 347) | ASME VIII Div. 1 |
| Operating Pressure | Up to 10.0 MPa (1,450 psi) (typical) | ASME VIII Div. 1 |
| Design Pressure | Up to 15.0 MPa (2,175 psi) (typical) | ASME VIII Div. 1 |
| Corrosion Environment | Trace H₂S, CO₂, SO₂ in natural gas matrix | NACE MR0175 / ISO 15156 |
| Surface Finish | Pickled & Passivated (Ra ≤ 1.6 µm) | ASTM A380 |
| Heat Treatment | Solution Annealed (1050–1150 °C, water quench) | ASTM A312 |
| Welding Method | GTAW (Gas Tungsten Arc Welding) / TIG | ASME IX, AWS D10.9 |
| Coil Forming Method | Hot Coiling or Warm Bending (post-anneal) | — |
| Non-Destructive Testing | Eddy Current (ET), Hydrostatic Test | ASTM A312, ASME BPV VIII |
| End Condition | Plain End, Beveled End, or Threaded (per specification) | ASME B36.10M |
| Applicable Codes | ASME Boiler & Pressure Vessel Code, Section VIII | ASME BPV VIII Div. 1 |
Manufacturing Process
The production of high-performance stainless steel coil tubes for regeneration heater applications follows a rigorous multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical integrity, and resistance to thermal and corrosive degradation.
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Raw Material Procurement & Mill Certification:
Hot-rolled stainless steel seamless tubes conforming to ASTM A312 Type 321 or Type 347 are procured from certified mills. Each heat lot is accompanied by a Material Test Report (MTR) verifying chemical composition, mechanical properties, and heat treatment history.
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Incoming Inspection & Material Verification:
Upon receipt, tubes undergo spectrographic analysis (OES – Optical Emission Spectroscopy) to confirm chemical composition, dimensional measurement for OD and wall thickness tolerance (±10% per ASTM A312), and visual inspection for surface defects, laps, and cracks.
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Cutting & End Preparation:
Tubes are cut to required lengths using precision saws or plasma cutting. Ends are deburred, chamfered, or beveled per the engineering specification to facilitate field welding and flange connection.
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Solution Annealing:
Tubes are solution annealed at 1050–1150 °C followed by rapid water quenching. This process dissolves any precipitated carbides, restores full austenitic microstructure, and maximizes corrosion resistance and ductility. The resulting microstructure exhibits a single-phase austenite with stabilized TiC or NbC precipitates at grain boundaries.
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Pickling & Passivation:
Following annealing, tubes are subjected to pickling (nitric-hydrofluoric acid blend) to remove oxide scale and contamination, followed by passivation treatment (dilute nitric acid or citric acid) to form a protective chromium oxide (Cr₂O₃) passive film on the surface. This is critical for resisting trace acid gas attack in the operating environment.
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Coil Forming (Bending):
Annealed tubes are formed into coils using either hot coiling (while the tube is heated to approximately 600–700 °C to reduce bending stresses) or cold bending with intermediate annealing cycles. The minimum bend radius is maintained at a minimum of 3× the tube OD (typically 4× OD for high-cycle fatigue applications) to prevent wall thinning, ovality distortion, and stress concentration. Post-bending stress relief annealing at 850–950 °C is performed to eliminate residual bending stresses.
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Welding of Coil Ends & Header Connections:
Coil ends are joined to header pipes or inlet/outlet manifolds using Gas Tungsten Arc Welding (GTAW/TIG) with matching or compatible filler metals (ER321 for Grade 321; ER347 for Grade 347). Weld parameters are qualified per ASME Section IX, and welders are certified to AWS D10.9 or equivalent standards.
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Non-Destructive Testing (NDT):
- Eddy Current Testing (ET): Automated ultrasonic or eddy current inspection per ASTM A312 to detect internal laminations, cracks, and wall thickness variations along the full tube length.
- Visual Inspection (VT): 100% visual examination of welds and formed coil sections for geometric irregularities, surface defects, and weld quality.
- Hydrostatic Pressure Testing: Each coil assembly is hydrostatically tested at 1.5× the design pressure (minimum 1.5× MAWP) for a sustained hold time of 30 minutes, with zero leakage permitted.
- Helium Leak Testing: For critical applications, helium mass spectrometer leak testing is performed at 10⁻⁶ atm·L/s sensitivity to ensure hermetic integrity.
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Final Passivation & Protection:
Post-welding passivation is applied to restore the passive film disrupted by welding heat-affected zones. Protective coatings or wrap-around insulation may be applied for shipping and storage.
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Documentation & Traceability:
A comprehensive Quality Dossier is compiled for each production lot, including MTRs, NDT reports, pressure test certificates, dimensional records, weld procedure qualifications (WPS/PQR), and final inspection reports.
Applications
Primary Application: Molecular Sieve Dehydration Regeneration Gas Heater
In natural gas processing, molecular sieve (typically 3A or 4A zeolite) dehydration units remove water vapor from natural gas to prevent hydrate formation and downstream equipment damage. The adsorption–desorption cycle involves:
- Adsorption Phase: Wet natural gas flows through the molecular sieve bed, where water is adsorbed onto the zeolite surface. The bed operates at ambient to moderate temperatures (30–80 °C).
- Regeneration (Desorption) Phase: Heated regeneration gas (250–300 °C) is circulated through the bed in a counter-current or co-current fashion, driving off adsorbed water as steam. This is where the coil tubes function as the heat-transfer element.
The coil tubes absorb thermal energy from a fired heater, waste-heat boiler, or electric heating system and transfer it to the regeneration gas stream. The cyclic nature of the process—alternating between hot regeneration and cooler adsorption phases—subjects the coil tubes to repeated thermal cycling, making thermal fatigue resistance a paramount design requirement.