Stainless Steel Coil Tube for High-Pressure Feedwater Heaters (HPFHE)
1. Product Overview
The stainless steel coil tube is a precision-engineered heat exchange element designed for use in High-Pressure Feedwater Heaters (HPFHE) within thermal power generation systems. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, these coil tubes serve as critical components in regenerative heating systems, where they facilitate efficient heat transfer between the feedwater and extraction steam. The product is engineered to withstand the extreme thermal cycling, high-pressure differentials, and erosive flow conditions inherent in modern power plant operations.
Based on extensive failure analysis studies — including a documented investigation into a crack failure event in Unit 2, HPFHE #1 coil tube of a commercial power plant — the design and material selection of these coil tubes have been rigorously optimized to address root causes such as Stress Corrosion Cracking (SCC), Flow-Induced Vibration (FIV), thermal fatigue, and chloride-induced degradation. This empirical knowledge base ensures that the product specifications incorporate validated mitigation strategies against the most prevalent failure modes observed in real-world service conditions.
2. Material & Properties
2.1 Base Alloy Composition
The coil tubes are fabricated from austenitic stainless steel grades selected for superior resistance to stress corrosion cracking and thermal fatigue under high-temperature water environments:
- Grade 316L (UNS S31603) — Primary grade, with added Molybdenum (Mo) for enhanced chloride resistance
- Grade 321 (UNS S32100) — Titanium-stabilized variant for elevated temperature applications
- Grade 304L (UNS S30403) — Available for moderate service conditions
- Grade 316Ti (UNS S31635) — Titanium-stabilized 316 for weld joint integrity
2.2 Mechanical Properties (Typical, Grade 316L)
| Property | Specification | Standard Reference |
|---|---|---|
| Tensile Strength (Rm) | ≥ 485 MPa (≥ 70 ksi) | ASTM A213 / ASME SA-213 |
| Yield Strength (Rp0.2) | ≥ 170 MPa (≥ 25 ksi) | ASTM A213 / ASME SA-213 |
| Elongation (A5) | ≥ 40 % | ASTM A213 / ASME SA-213 |
| Impact Energy (Charpy V-Notch, −40 °C) | ≥ 47 J | ASTM A370 |
| Hardness (HBW) | ≤ 217 | ASTM E10 |
2.3 Chemical Composition (Grade 316L, wt%)
| Element | Specification |
|---|---|
| Carbon (C) | ≤ 0.03 % |
| Chromium (Cr) | 16.0 – 18.0 % |
| Nickel (Ni) | 10.0 – 14.0 % |
| Molybdenum (Mo) | 2.0 – 3.0 % |
| Nitrogen (N) | ≤ 0.10 % |
| Sulfur (S) | ≤ 0.03 % |
| Phosphorus (P) | ≤ 0.035 % |
| Iron (Fe) | Balance |
2.4 Corrosion Resistance Characteristics
- Stress Corrosion Cracking (SCC) Resistance: Enhanced by low-carbon content (≤ 0.03 % C) and Molybdenum addition, mitigating chloride-induced SCC in hot feedwater environments
- Pitting Resistance: PREN (Pitting Resistance Equivalent Number) ≥ 24, providing robust protection against localized corrosion
- Intergranular Corrosion Resistance: Low carbon grade eliminates sensitization risk during welding and heat treatment operations
- Thermal Fatigue Resistance: Maintained ductility and toughness across thermal cycling ranges up to 350 °C
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Coil Tube for HPFHE | — |
| Product Code | No. 644 | Internal Catalog |
| Outer Diameter (OD) | 12.7 mm (1/2") – 31.75 mm (1-1/4") | ASME B36.19M |
| Wall Thickness (WT) | 1.24 mm – 3.18 mm (0.049" – 0.125") | ASME B36.19M |
| Bend Radius (Minimum) | ≥ 3D (three times outer diameter) | ASME SA-213 |
| Coil Diameter | As per OEM drawing / 600 mm – 2,000 mm | Customer Specification |
| Operating Temperature | −20 °C to 350 °C (−4 °F to 662 °F) | — |
| Operating Pressure | Up to 25 MPa (3,626 psi) | ASME BPV Section I |
| Surface Finish | Pickled & Passivated, Ra ≤ 0.8 μm | ASTM A967 |
| Heat Treatment | Solution Annealed (1050 °C – 1150 °C, Water Quenched) | ASTM A999 |
| Welding Method | TIG (GTAW) / Argon Shielded | ASME BPV Section IX |
| End Preparation | Beveled per ASME B1.20.1 / Flared per OEM Spec | ASME B1.20.1 |
| Applicable Standards | ASTM A213, ASME SA-213, EN 10216-5, GB/T 14976 | — |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement & Incoming Inspection: Stainless steel seamless tubes sourced from certified mills, subjected to spectroscopic analysis (OES), chemical verification, and dimensional inspection
- Material Cutting & Marking: Precision cutting to required coil length with laser or plasma cutting; each coil assigned unique heat traceability marking
- Hydroforming / Cold Bending: Tubes are formed into the required coil geometry using CNC-controlled hydraulic bending machines. Bending is performed at controlled speed with internal mandrel support to prevent wall thinning and ovality exceeding ±5 %
- Welding: Coil-to-coil and coil-to-end connections performed via Gas Tungsten Arc Welding (GTAW) with inert gas shielding (Argon ≥ 99.999 % purity) to ensure oxide-free weld beads
- Weld Heat-Affected Zone (HAZ) Treatment: Post-weld annealing to restore grain structure and eliminate residual stresses in the HAZ
- Pickling & Passivation: Immersion in citric acid or nitric acid solution followed by passivation treatment per ASTM A967 to remove oxide scale and establish a protective chromium oxide film
- Non-Destructive Testing (NDT): 100 % liquid penetrant inspection (PT) of all welds per ASTM E165; ultrasonic testing (UT) per ASTM E213 for internal defects; dimensional verification per ASME SA-213
- Pressure Testing: Hydrostatic pressure test at 1.5× maximum operating pressure for minimum 30 seconds with zero leakage
- Final Inspection & Documentation: Comprehensive quality dossier including material certificates (EN 10204 3.1), test reports, dimensional records, and traceability matrix
4.2 Critical Process Controls (Lessons from Failure Analysis)
Based on documented crack failure investigation of a power plant HPFHE #1 coil tube: The root cause was identified as a combination of stress corrosion cracking (SCC) initiated at weld HAZ and exacerbated by flow-induced vibration (FIV). The following process controls have been implemented as direct countermeasures:
- Weld HAZ Optimization: Tight control of heat input (≤ 1.2 kJ/mm) during welding to minimize grain coarsening and sensitization in the HAZ
- Vibration Mitigation Design: Coil geometry optimized with proper support spacing (≤ 1.2 m) and vibration dampers to reduce FIV amplitudes below 0.1 g
- Chloride Control: Specification of feedwater quality limits with chloride concentration ≤ 10 ppb to prevent chloride SCC initiation
- Residual Stress Management: Stress-relief annealing (800 °C – 850 °C, 1 hour) applied to fully welded coil assemblies to reduce residual stresses below 50 MPa
5. Applications
5.1 Primary Applications
- High-Pressure Feedwater Heaters (HPFHE): Primary application — coil tubes immersed in the feedwater shell side, receiving heat from extraction steam on the tube side
- Low-Pressure Feedwater Heaters (LPFHE): Adapted coil configurations for lower pressure regimes
- Regenerative Heating Systems: Integral component of multi-stage regenerative cycles in subcritical, supercritical, and ultra-supercritical (USC) power plants
- Boiler Feedwater Preheating: Improves overall thermodynamic efficiency of the Rankine cycle
5.2 Industry Segments
- Thermal Power Generation (Coal-Fired, Gas-Fired)
- Nuclear Power Plants (BWR, PWR — secondary loop components)
- Petrochemical and Refinery Process Heaters
- Marine Power Systems
- Industrial Steam Generation Facilities
6. Quality Certifications
6.1 Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing operations |
| ISO 14001:2015 | Environmental Management System certification |
| EN 10204 Type 3.1 | Material test certificates with full traceability documentation |
| ASME "S" Stamp | Pressure vessel and piping component certification |
| ASTM A213 / ASME SA-213 | Seamless austenitic stainless steel boiler, heat exchanger, and heat transfer tubing |
| EN 10216-5 | Heat exchangers and heat transfer equipment — austenitic stainless steel tubes |
| NACE MR0175 / ISO 15156 | Materials for use in H2S-containing environments (where applicable) |
| NORSOK M-501 | Materials for subsea applications (where applicable) |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes |