Stainless Steel Coil Tube — Pneumatic Pressure Test (Compressed Air / Nitrogen Pressure Holding)

1. Product Overview

The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered tubular product supplied in continuous coil form, designed for downstream processing including straightening, cutting, and forming into finished pipe sections. As a critical component in industrial piping systems, the coil tube undergoes rigorous quality assurance protocols, among which the Pneumatic Pressure Test (Air/Nitrogen Pressure Holding) serves as a vital non-destructive evaluation method to verify structural integrity, weld soundness, and dimensional stability under operating conditions.

This document details the pneumatic pressure test specification, applicable standards, procedural requirements, and acceptance criteria governing the coil tube product. It is intended for use by quality assurance personnel, engineering consultants, and procurement specialists involved in the specification, verification, and acceptance of stainless steel coil tube products.

2. Material & Properties

2.1 Material Grades

2.2 Mechanical Properties (Typical Values)

Property Unit 304/304L 316/316L 321
Tensile Strength (Minimum) MPa 515 515 515
Yield Strength (0.2% Offset) MPa 205 205 205
Elongation (at Break) % ≥40 ≥40 ≥40
Density g/cm³ 7.93 7.98 7.93
Thermal Conductivity (20°C) W/(m·K) 16.3 14.6 16.3
Maximum Service Temperature °C 870 870 1093

2.3 Surface Condition

3. Pneumatic Pressure Test Specification

3.1 Purpose and Scope

The Compressed Air / Nitrogen Pressure Holding Test is a pneumatic pressure test performed on stainless steel coil tube products to verify the structural integrity, leak-tightness, and dimensional stability of the tube under simulated operating pressure. This test is conducted when hydraulic (water-based) pressure testing is not feasible due to product configuration, material sensitivity, or environmental constraints.

3.2 Applicable Standards

3.3 Test Parameters

Parameter Specification Notes
Test Medium Compressed Dry Air or Nitrogen (N₂) Nitrogen preferred for higher-risk applications
Test Pressure 1.1 × Design Pressure (Pd) Minimum 1.1 times the maximum allowable working pressure
Hold Duration Minimum 30 seconds (standard); 60 seconds for critical applications Pressure drop must not exceed 5% of test pressure
Leak Detection Method Soapy Water Application or Water Immersion Visual inspection for bubble formation
Maximum Allowable Pressure Drop ≤ 5% of applied test pressure during hold period Accounts for temperature variation and elastic deformation
Temperature Range 10°C to 40°C (60°F to 104°F) Record ambient temperature during testing
Pressure Rise Rate Gradual — not exceeding 50% of test pressure in the first 30 seconds Prevents dynamic loading and potential failure

3.4 Acceptance Criteria

  1. No visible leaks — No bubble formation at weld seams, end closures, or tube body when soapy water is applied
  2. No dimensional deformation — Outer diameter, wall thickness, and ovality remain within original manufacturing tolerances after test
  3. Pressure stability — Gauge reading remains stable within ±5% of applied test pressure throughout the hold duration
  4. No audible anomalies — No hissing, cracking, or other abnormal sounds indicating micro-cracking or fatigue failure
  5. No permanent set — Tube dimensions return to pre-test measurements upon depressurization

4. Manufacturing Process

4.1 Coil Tube Production Flow

  1. Strip Preparation — Cold-rolled stainless steel strip is inspected for surface defects, dimensional accuracy, and chemical composition
  2. Coiling / Forming — Strip is wound into coil form using precision coiling equipment with controlled tension parameters
  3. Seam Welding (if applicable) — TIG (Tungsten Inert Gas) or laser welding is performed to close the longitudinal seam
  4. Heat Treatment / Annealing — Solution annealing in a controlled atmosphere furnace to achieve required mechanical properties and corrosion resistance
  5. Surface Finishing — Pickling and passivation to remove scale and restore chromium oxide passive layer
  6. Dimensional Inspection — Ultrasonic testing (UT), optical measurement of OD and wall thickness
  7. Pressure Testing — Pneumatic or hydraulic pressure test per Section 3 specifications
  8. Final Inspection & Packaging — Visual inspection, certification review, and protective packaging

4.2 Pneumatic Test Procedure (Detailed)

  1. Preparation: Seal both ends of the coil tube section using certified pressure-rated end plugs or caps. Verify end closure integrity with a low-pressure pre-test (0.5 bar / 7 psi).
  2. Medium Selection: Introduce dry compressed air (moisture content ≤ 50 ppm) or high-purity nitrogen (≥ 99.99% N₂) into the test chamber. Nitrogen is strongly recommended for high-risk applications due to its inert properties and reduced explosion hazard.
  3. Pressure Application: Increase pressure gradually to 1.1 × Design Pressure using a calibrated pressure regulator. Record the time to reach target pressure.
  4. Hold Period: Maintain test pressure for the specified duration (minimum 30 seconds). Monitor pressure gauge continuously.
  5. Leak Detection: Apply soapy water solution (neutral pH, free of chloride contaminants) to all weld seams, end closures, and tube body. Alternatively, submerge the tube section in water and observe for bubble release.
  6. Depressurization: Slowly release pressure at a controlled rate (not exceeding 10% of test pressure per second) to prevent shock loading.
  7. Post-Test Inspection: Re-measure critical dimensions (OD, wall thickness, ovality) and compare with pre-test values. Document any deviations.

5. Risk Assessment and Safety Controls

5.1 Risk Classification

WARNING: Pneumatic pressure testing is classified as a HIGH-RISK activity. The stored energy in compressed gas is significantly greater than equivalent liquid pressure (approximately 100× the energy of hydraulic pressure at the same pressure level). In the event of catastrophic failure, the release of compressed air or nitrogen can cause severe projectile hazards, acoustic trauma, and asphyxiation risk (nitrogen).

5.2 Mandatory Safety Measures

5.3 Approval Documentation

Document Requirement Responsible Party
Pneumatic Test Authorization Form Signed approval prior to test execution QA Manager + Engineering Lead
Risk Assessment Report Job-specific hazard analysis completed HSE Department
Test Procedure Document Step-by-step test plan with acceptance criteria Quality Control Engineer
Equipment Calibration Records Pressure gauges and instruments within calibration validity Calibration Department
Post-Test Inspection Report Dimensional and visual verification results Inspection Team

6. Product Specifications

6.1 Dimensional Specifications

Parameter Range Tolerance Standard Reference
Outer Diameter (OD) Φ6.0 mm – Φ159.0 mm ±0.05 mm (precision); ±0.1 mm (standard) ASTM A270 / GB/T 12771
Wall Thickness 0.3 mm – 6.0 mm ±10% of nominal ASTM A270 / EN 10216-5
Ovality — ≤ 2% of nominal OD ISO 4427
Coil Length (per coil) Custom (typically 50 m – 500 m) ±1% Customer specification
Coil Weight Up to 2,500 kg per coil — —
Surface Roughness (Ra) 0.2 μm – 1.6 μm Per surface finish grade ASTM A270 Table 4

6.2 Pressure Rating

OD (mm) Wall Thickness (mm) Design Pressure (bar) Test Pressure (bar) [1.1×Pd]
12.7 1.0 225 247.5
19.05 1.5 330 363.0
25.4 2.0 415 456.5
38.1 2.5 430 473.0
50.8 3.0 440 484.0
76.2 4.0 430 473.0

7. Quality Certifications and Traceability

7.1 Certifications