Stainless Steel Coil Tube — Theoretical Weight Settlement Specification
Product Overview
The Stainless Steel Coil Tube (Coiltube) is a continuous-length tubular product supplied in coiled form, manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Unlike cut-to-length pipe, coil tube is wound into large-diameter coils or helical forms, enabling seamless downstream processing such as bending, forming, and cutting without intermediate welding joints. This product is particularly prevalent in industrial piping systems, heat exchanger manufacturing, and fluid conveyance applications where continuous, joint-free tubing is required.
The Theoretical Weight Settlement system employed for this product is a standardized pricing methodology based on dimensional geometry rather than actual measured weight. This approach is widely adopted in the seamless tube industry and provides a transparent, formula-driven basis for commercial transactions between suppliers and buyers.
Material & Properties
Stainless steel coil tubes are typically fabricated from austenitic and duplex stainless steel grades conforming to international standards including ASTM, ASME, EN, and GB specifications. Common material grades include:
- ASTM A312 / ASME SA-312 — Seamless and welded austenitic stainless steel tube for chemical and heat-exchanging service
- ASTM A269 / ASME SA-269 — Welded austenitic stainless steel sanitary tubing
- EN 10216-5 — European standard for austenitic stainless steel tubes for pressure purposes
- GB/T 12771 — Chinese national standard for welded stainless steel seamless and welded tubes
Key material properties include excellent corrosion resistance, high-temperature strength, non-magnetic characteristics (in austenitic grades), and superior weldability. The austenitic microstructure provides outstanding formability, making it ideal for coil tube applications requiring complex secondary forming operations.
Specifications
| Parameter | Specification | Unit |
|---|---|---|
| Product Type | Stainless Steel Coil Tube (Coiltube) | — |
| Manufacturing Method | Seamless (Primary) / Welded (Secondary) | — |
| Outer Diameter (OD) Range | 3.175 – 219.08 | mm |
| Wall Thickness (WT) Range | 0.25 – 12.70 | mm |
| Coil Length | Continuous (typically 100 – 500+) | m |
| Coil Inner Diameter | 300 – 1200 | mm |
| OD Tolerance | ±0.5% – ±1.0% (per applicable standard) | % |
| WT Tolerance | +10% / –5% (standard) or ±10% (agreed) | % |
| Surface Finish | Pickled & Annealed (P&A), 2B, No.4 Brushed, Mirror (8K) | — |
| End Condition | Plain end, beveled, or cut to length | — |
| Applicable Standards | ASTM A312, A269, EN 10216-5, GB/T 12771 | — |
| Pricing Basis | Theoretical Weight (kg/m) | — |
Theoretical Weight Calculation
Settlement Formula
W = (OD − WT) × WT × 0.02491 [kg/m]
Where:
- W = Theoretical weight per unit length (kg/m)
- OD = Outer diameter (mm)
- WT = Wall thickness (mm)
- 0.02491 = Constant derived from the density of stainless steel (7.93 g/cm³) and geometric conversion factors
Derivation and Basis
The constant 0.02491 is derived from the volumetric density of stainless steel (approximately 7.93 g/cm³ or 7,930 kg/m³) combined with the geometric cross-sectional area of a hollow cylinder. The cross-sectional area of a tube is calculated as:
A = π/4 × (OD² − ID²) = π/4 × (OD² − (OD − 2×WT)²) = π × (OD − WT) × WT
Converting to weight per meter:
W = π × (OD − WT) × WT × ρ / 1000 = (OD − WT) × WT × 0.02491
Where ρ = 7.93 g/cm³ and the factor π × 7.93 / 1000 ≈ 0.02491.
Worked Example
For a coil tube with OD = 25.4 mm and WT = 2.11 mm:
W = (25.4 − 2.11) × 2.11 × 0.02491 = 23.29 × 2.11 × 0.02491 = 1.223 kg/m
Critical Consideration: Tolerance Impact on Settlement
When tubes are delivered with positive tolerance (over-tolerance), the buyer is at a commercial disadvantage. Under theoretical weight settlement, the buyer pays based on the nominal dimensions regardless of actual delivered weight. If the manufacturer produces tubes with wall thickness exceeding the nominal specification (positive WT tolerance), the actual steel content is greater than calculated. However, the buyer pays only for the nominal theoretical weight, meaning:
- The buyer receives more material than paid for — which appears favorable but masks the fact that the supplier's cost is higher, potentially inflating the unit price
- More critically, if OD is delivered at positive tolerance while WT is at nominal, the actual cross-sectional steel is less than theoretical, and the buyer pays for material not received
- In practice, the positive tolerance delivery scenario means the buyer pays for nominal weight while receiving actual weight that may differ — creating an asymmetry in value exchange
Industry recommendation: Buyers should negotiate contracts specifying either actual weight settlement (weighed weight) or tightly controlled tolerance bands to minimize the risk of value discrepancy between theoretical and actual delivered weight.
Manufacturing Process
- Steel Billet Preparation — Stainless steel ingots or continuous cast billets are heated to homogenization temperatures (1100–1250°C) and subjected to descaling and inspection.
- Piercing and Hot Rolling — For seamless coil tubes, the heated billet is pierced to form a hollow shell and then hot-rolled through a series of stands to reduce wall thickness and increase length.
- Coiling — The continuous tube is wound onto a coil former at controlled speeds, maintaining dimensional accuracy and surface integrity.
- Pickling and Annealing (P&A) — The coil is processed in acid baths (typically a mix of hydrochloric and nitric acid) to remove scale and oxide, followed by controlled atmosphere annealing to restore austenitic microstructure and achieve required mechanical properties.
- Cold Drawing (Optional) — For tighter tolerances and improved surface finish, the tube may undergo cold drawing through a die set, reducing OD and WT to precise dimensions.
- Final Coiling and Packaging — The finished coil tube is wound onto protective cores, wrapped in plastic film or paper, and strapped for shipment.
Applications
Stainless steel coil tube with theoretical weight settlement is extensively used across multiple industrial sectors:
- Heat Exchangers & Condensers — Continuous coil tube eliminates joint failure points in shell-and-tube heat exchangers, critical in petrochemical and power generation applications.
- Petrochemical & Oil Refining — Used in process piping, distillation columns, and heat transfer systems where corrosion resistance and high-temperature performance are paramount.
- Food & Beverage Processing — Sanitary-grade coil tube (2B or electropolished finish) for hygienic fluid handling systems, compliant with FDA and 3-A standards.
- Pharmaceutical & Medical Devices — High-purity austenitic grades for CIP/SIP-compatible process lines and medical instrument tubing.
- Automotive & Aerospace — Exhaust systems, fuel lines, and hydraulic tubing where weight efficiency and durability are critical.
- Architectural & Decorative — Coil tube for handrails, balustrades, and structural framing in modern construction.
- Wire & Cable Drawing — Coil tube as a precursor for further processing into wire rod, spring wire, and precision tubing.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management systems and product certifications to ensure conformity with international standards:
- ISO 9001:2015 — Quality Management System certification
- ISO 14001:2015 — Environmental Management System certification
- ISO 45001:2018 — Occupational Health and Safety Management System
- EN 10204 Type 3.1 — Inspection Certificate per European standard
- EN 10204 Type 3.2 — Material Test Certificate with full chemical and mechanical data
- MTR (Material Test Report) — Available for every heat lot, including chemical composition, mechanical properties, and dimensional verification
- Mill Test Certificate (MTC) — Conforming to ASTM E1043 or equivalent
- Third-Party Inspection (TPI) — Available upon request by SGS, BV, or other accredited inspection bodies
- API 5L / API 5CT — For oil and gas sector applications (where applicable)
Unique Selling Points
- Transparent Pricing Formula — The theoretical weight settlement method (W = (OD − WT) × WT × 0.02491) provides a fully transparent, mathematically verifiable basis for pricing, eliminating ambiguity in commercial negotiations.
- Seamless Construction — Seamless coil tubes offer superior mechanical integrity compared to welded alternatives, with no longitudinal weld seam that could serve as a failure initiation point under cyclic loading or corrosive environments.
- Continuous Length Supply — Coiled delivery format enables high-efficiency downstream processing with minimal material waste, reducing scrap rates by up to 15–20% compared to cut-to-length pipe.
- Wide Dimensional Range — Comprehensive coverage of OD and WT combinations accommodates diverse engineering requirements from micro-bore tubing to large-diameter process pipe.
- Comprehensive Traceability — Full heat-to-heat traceability with MTR/MTC documentation ensures supply chain accountability and regulatory compliance for critical applications.
Commercial Advisory Note
Important: Theoretical weight settlement is the industry standard for seamless stainless steel tube transactions. However, buyers are strongly advised to specify tolerance bands explicitly in purchase contracts. Positive tolerance delivery (where actual WT exceeds nominal) results in the buyer paying theoretical weight while receiving actual weight — creating a value asymmetry. For high-value or precision applications, actual weighed weight settlement should be negotiated to ensure equitable value exchange between buyer and supplier.