Coil Tube for Thermocouple & RTD Protective Sheaths — High-Temperature Instrumentation Grade
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
Product Category: Stainless Steel Coil Tubes — Instrumentation & Temperature Measurement Applications
Knowledge Point Reference No.: 168
1. Product Overview
Thermocouple (TC) and Resistance Temperature Detector (RTD) protective sheaths represent one of the most critical yet demanding applications in industrial instrumentation. These components serve as the primary barrier between the temperature-sensing element and the process environment, which may include extreme temperatures, aggressive chemical media, and high-pressure conditions simultaneously. The protective sheath must fulfill a dual mandate: mechanical integrity to withstand process pressures and thermal cycling, and thermal conductivity to minimize the thermal response lag of the sensing element.
This coil tube product is specifically engineered for the fabrication of TC/RTD thermowell and protective sheath assemblies used in continuous industrial temperature monitoring. The material selection, wall thickness optimization, and manufacturing tolerances are all governed by the fundamental engineering trade-off between response time (thermal response speed) and structural strength (mechanical robustness). Thinner walls deliver faster thermal response but reduce pressure resistance; thicker walls provide robustness but introduce measurement lag. Wenzhou Jixing Metal Materials Co., Ltd addresses this trade-off through precise alloy selection and controlled manufacturing of seamless coil tubes in grades such as 310S (UNS S31008) and Inconel 600 (Alloy 600, UNS N06600).
2. Material Selection & Properties
2.1 Primary Alloy Grades
The two principal material grades for TC/RTD protective sheath coil tubes are:
- 310S / UNS S31008: An austenitic stainless steel with enhanced carbon control (≤0.08% C) relative to 310. It offers superior resistance to oxidation and scaling at elevated temperatures, making it the preferred choice for dry-oxidizing atmospheres and high-temperature service up to approximately 1150 °C (2100 °F).
- Inconel 600 / Alloy 600 (UNS N06600): A nickel-chromium-iron superalloy providing outstanding resistance to a wide range of corrosive media, including halides, sulfides, and organic acids, at temperatures up to approximately 1100 °C (2012 °F). It is the material of choice when the process environment combines high temperature with aggressive chemical attack.
2.2 Comparative Material Properties
| Property | 310S (UNS S31008) | Inconel 600 (UNS N06600) |
|---|---|---|
| Composition (typical) | 19–25% Cr, 24–30% Ni, ≤0.08% C, balance Fe | 14–17% Cr, 58–65% Ni, 5–7% Fe, balance others |
| Maximum Service Temperature | 1150 °C (2100 °F) | 1100 °C (2012 °F) |
| Oxidation Resistance | Excellent (dry oxidizing atmospheres) | Outstanding (broad spectrum) |
| Corrosion Resistance | Good (limited in halide environments) | Exceptional (halides, acids, sulfides) |
| Thermal Conductivity (20 °C) | ~22 W/(m·K) | ~16 W/(m·K) |
| Yield Strength (RT) | ~205 MPa (annealed) | ~205 MPa (annealed) |
| Creep Resistance | Good up to 1000 °C | Excellent up to 1000 °C |
| Cost Index (relative) | 1.0 – 1.5 | 3.0 – 5.0 |
2.3 Material Selection Guidelines
- Select 310S when the primary challenge is high-temperature oxidation in dry or mildly corrosive atmospheres (e.g., furnace walls, steam generators, calcination processes).
- Select Inconel 600 when the environment involves aggressive chemical species such as H2S, HCl, molten sulfides, or concentrated organic acids at elevated temperatures (e.g., petrochemical cracking, acid manufacturing, waste incineration).
3. Technical Specifications
3.1 Coil Tube Dimensions & Tolerances
| Parameter | Specification | Notes |
|---|---|---|
| Outside Diameter (OD) | Φ1.6 mm – Φ25.4 mm (0.063" – 1.0") | Custom sizes available upon request |
| Wall Thickness (WT) | 0.10 mm – 1.50 mm | Optimized per application for response time vs. strength |
| OD Tolerance | ±0.05 mm (tight tolerance grade) | Critical for thermowell insertion fit |
| Wall Thickness Tolerance | ±10% of nominal | Uniformity ensures consistent thermal response |
| Coil Form | Continuous coil, annealed condition | Ready for downstream cutting, forming, and welding |
| Surface Finish | Bright annealed / polished (Ra ≤ 0.4 μm available) | Low roughness minimizes fouling in corrosive media |
| Manufacturing Method | Seamless (tubular drawing / cold drawing) | No weld seam — eliminates leak path and stress concentrator |
| Standard References | ASTM A213, ASTM A313, ASTM B167, AMS 5510 | Grade-specific standards apply |
3.2 Wall Thickness vs. Thermal Response Time Optimization
The thermal response time of a TC/RTD assembly is directly governed by the wall thickness and thermal conductivity of the protective sheath material. For a given thermocouple junction configuration, the time constant (τ) can be approximated by the lumped capacitance model:
τ = (ρ · cp · V) / (h · A)
Where: ρ = density, cp = specific heat capacity, V = sheath volume, h = heat transfer coefficient, A = surface area
For cylindrical sheaths, reducing wall thickness proportionally reduces the volume-to-area ratio, thereby decreasing the time constant and improving response speed. However, wall thickness must remain sufficient to withstand the process pressure (governed by the Barlow formula for hoop stress):
P = (2 · σ · t) / D
Where: P = allowable internal pressure, σ = allowable stress at operating temperature, t = wall thickness, D = outside diameter
| Application Severity | Recommended Wall Thickness | Approximate Response Time (τ90) | Pressure Rating (approx.) |
|---|---|---|---|
| Low pressure, high response needed (e.g., air ducts, HVAC) | 0.10 – 0.25 mm | < 2 seconds | < 0.5 MPa |
| Medium pressure, moderate response (e.g., process piping) | 0.30 – 0.50 mm | 2 – 10 seconds | 0.5 – 2.5 MPa |
| High pressure, corrosion-resistant (e.g., petrochemical, acid service) | 0.60 – 1.50 mm | 10 – 60 seconds | > 2.5 MPa |
4. Manufacturing Process
4.1 Seamless Tube Production
The coil tubes for TC/RTD sheaths are manufactured via a seamless process, ensuring the absence of weld seams that could serve as preferential corrosion initiation sites or leak paths in harsh environments. The manufacturing sequence is as follows:
- Billet Preparation: High-purity ingots of 310S or Inconel 600 are produced via vacuum arc remelting (VAR) or vacuum induction melting (VIM) to ensure tight inclusion control and homogeneity.
- Hot Piercing / Extrusion: The billet is heated to forging temperature and pierced or extruded to produce a seamless hollow tube (preform).
- Hot Rolling / Stretching: The preform is hot-worked to approximate dimensions, followed by cold drawing through progressively smaller dies to achieve precise OD and wall thickness tolerances.
- Solution Annealing: The drawn tube is heated to 1050–1150 °C (for 310S) or 1050–1150 °C (for Inconel 600), held for sufficient time for carbide dissolution and grain homogenization, then rapidly cooled (water quench or air quench) to achieve a fully solution-treated, austenitic microstructure.
- Coiling & Straightening: The annealed tube is wound into coils at controlled diameters suitable for downstream processing. For precision applications, the tube may be cold-straightened to ensure dimensional straightness.
- Surface Finishing: Optional pickling, passivation, and mechanical or electrolytic polishing to achieve specified surface roughness (Ra ≤ 0.4 μm).
- Quality Inspection: Dimensional verification (OD, wall thickness, coil diameter), visual inspection, and destructive testing (tensile, hardness, PMI) per applicable standards.
4.2 Why Seamless is Critical
In TC/RTD sheath applications, the absence of a longitudinal weld seam is paramount for three reasons:
- Corrosion Resistance: Weld seams, even after post-weld heat treatment, exhibit microstructural heterogeneity and residual stress that accelerate localized corrosion (intergranular, pitting, crevice).
- Pressure Integrity: Seamless tubes provide uniform hoop strength, whereas welded tubes are limited by the weld joint efficiency factor (typically 0.85 per ASME Section VIII).
- Thermal Uniformity: A seamless sheath provides consistent thermal conductivity around the circumference, ensuring the sensing element responds uniformly to process temperature changes.
5. Applications
These coil tubes are the foundational material for manufacturing thermocouple and RTD protective sheaths (thermowells, immersion probes, surface-mount sensors) across a wide spectrum of industrial applications:
5.1 Petrochemical & Refining
- Catalytic cracking units (FCC) — high-temperature, sulfur-containing environments
- Hydrogenation reactors — high pressure, hydrogen embrittlement risk
- Distillation columns — temperature monitoring at multiple elevation points
5.2 Power Generation
- Boiler and furnace temperature monitoring (310S preferred for dry oxidation)
- Gas turbine exhaust temperature measurement
- Steam generator and superheater monitoring
5.3 Chemical Processing
- Acid manufacturing (H2SO4, HNO3) — Inconel 600 essential
- Polymerization reactors — high-temperature, monomer vapors
- Waste incineration and flue gas monitoring
5.4 Food & Pharmaceutical
- Sterilization processes (autoclaves) — high purity requirements
- Pasteurization and UHT processing — hygienic surface finish
5.5 Semiconductor & Vacuum
- CVD/PVD chamber temperature monitoring — ultra-clean, low outgassing
- Crystal growth furnaces — extreme temperature stability
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains rigorous quality management systems and product certifications to ensure compliance with international standards:
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent product quality and process control |
| ASTM A213 | Standard Specification for Seamless Ferritic Alloy Steel Tubing for High Temperature Service (applies to 310S) |
| ASTM B167 | Standard Specification for Nickel-Chromium-Iron Alloy (Alloy 600) Seamless Tubing |
| AMS 5510 | Aerospace Material Specification for Inconel 600 seamless tubing |
| EN 10216 | European standard for seamless tubes for general engineering purposes |
| Mill Test Reports (MTR) | Full chemical analysis, mechanical properties, and heat number traceability provided with each shipment |
| Third-Party Inspection (TPI) | Available upon request — SGS, BV, or equivalent inspection agencies |
7. Unique Selling Points
- Precision Wall Thickness Optimization: Customizable wall thickness ranging from 0.10 mm to 1.50 mm, enabling OEMs and integrators to fine-tune the response time vs. pressure