Gas Pipe

Natural gas seamless steel pipes are the core pressure-bearing pipes for natural gas transmission. With their characteristics of no welds, high strength, and corrosion resistance, they are widely used in long-distance natural gas pipelines, urban gas networks, and industrial gas transmission systems. They are key materials for ensuring the safe and efficient transportation of natural gas.

I. Core Features and Technical Requirements

1. high strength and high toughness

Natural gas transmission pressure typically ranges from 0.4MPa to 10MPa (higher for long-distance pipelines). Seamless steel pipes must withstand prolonged exposure to high-pressure media, requiring stringent specifications for yield strength, tensile strength, and impact toughness. Common steel grades include **L245, L290, L360** (Chinese Standard GB/T 9711) and **X42X80** (International Standard API 5L). Higher steel grades provide greater pressure-bearing capacity.

The seamless structure eliminates stress concentration at welds, significantly reducing cracking risks under high pressure—a key advantage over welded steel pipes.

2. corrosion resistance

Natural gas may contain corrosive substances such as hydrogen sulfide (H₂S), carbon dioxide (CO₂), and moisture, which can easily cause hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC) in pipelines. Therefore, seamless steel pipes for natural gas must meet the following corrosion resistance requirements:

• using low sulfur and low phosphorus pure steel smelting;

• Corrosion-resistant coatings are required for partial high-corrosion environments, including epoxy powder coatings and 3PE (three-layer polyethylene) anti-corrosion layers.

3. High Precision Dimension and Good Sealing

The tolerances for outer diameter and wall thickness must be strictly controlled (typically within ±1% to ±2% according to national standards) to ensure the sealing performance of pipe butt welds. Pipe ends should be machined into bevel forms (e.g., V-bay bevel) to accommodate on-site welding procedures and prevent gas leakage.

II. Common Materials and Implementation Standards

III. Production Process

The production of natural gas seamless steel pipes mainly adopts hot rolling and cold drawing (cold rolling) processes, with the core procedures as follows:

1. Hot-rolled seamless steel pipe (mainstream process, suitable for large diameter and thick wall pipes)

Cylindrical tube blank →Heating (1200~1300℃)→Pitting (to form hollow capillary tubes) →Continuous rolling/diameter adjustment (to regulate outer diameter and wall thickness) →Cooling→Straightening→Non-destructive testing (ultrasonic, eddy current testing) →Anti-corrosion treatment →Finished product.

Features: Excellent mechanical properties, suitable for high-pressure transportation scenarios.

2. Cold-drawn (cold-rolled) seamless steel pipe (suitable for small-diameter, thin-walled, high-precision pipes)

Hot rolled tube →pickling (removing oxide scale) →lubrication→cold drawing (reducing diameter and wall thickness through die drawing) →annealing (eliminating internal stress) →finishing→inspection→finished product.

Characteristics: High dimensional accuracy and excellent surface finish, suitable for fine-diameter pipeline transportation in urban networks.

IV. Key Points of Quality Inspection

To ensure the safety of natural gas transmission, seamless steel pipes must undergo multiple mandatory inspections prior to leaving the factory.

1. Non-destructive testing: Ultrasonic testing (UT) for internal defects, eddy current testing (ET) for surface defects, and radiographic testing (RT) for random weld sampling (if applicable).

2. Mechanical property testing: Tensile test (yield strength and tensile strength measurement), impact test (low-temperature toughness measurement; impact tests at-20°C/-40°C required for low-temperature environments), hardness test;

3. Corrosion resistance testing: Hydrogen-induced cracking (HIC) test and Sulfide stress corrosion (SSCC) test.

4. Sealing test: Hydrostatic test (pressurize to 1.5 times the rated pressure and maintain pressure without leakage).

V. Precautions for Application

1. Scenario matching: For long-distance pipelines, high-grade (X70, X80) hot-rolled seamless steel pipes are preferred; for urban networks, medium and small-diameter cold-drawn pipes can be selected to reduce costs.

2. Installation requirements: Pipe laying must account for thermal expansion and contraction, with expansion joints installed; butt welds must undergo 100% non-destructive testing to ensure airtightness.

3. Regular maintenance: During operation, periodic inspection of pipeline wall thickness (using ultrasonic thickness measurement) and integrity of the anti-corrosion coating is required to prevent corrosion and perforation.