Plastic-lined pipe

Plastic-lined steel pipe is a steel–plastic composite pipe that uses a steel pipe as the base and features an inner lining of thermoplastic. The base pipe may be seamless steel, straight-seam welded steel, or hot‑dip galvanized steel. Through processes such as diameter‑reduction compounding, thermal expansion compounding, and rotomolding, the plastic liner is tightly bonded to the inner wall of the steel pipe. The outer steel layer provides mechanical pressure resistance and structural strength, while the inner plastic lining isolates the conveyed medium, delivering corrosion resistance and anti‑scaling performance. These pipes are widely used in domestic water supply and drainage, chemical fluid handling, environmental wastewater treatment, and process‑medium transport applications.

1. Core Features

The outer steel pipe ensures the pipe’s mechanical strength, impact resistance, and pressure‑bearing performance; the inner plastic lining completely isolates the conveyed medium from the metal substrate, offering resistance to acid and alkali corrosion, a smooth inner surface, low fluid friction, reduced scaling, and inhibition of microbial growth. The base pipe can be selected as a seamless tube to enhance its capability under high‑pressure conditions and may be equipped with external anti‑corrosion treatment. On-site welding is prohibited; after cutting, the port must be sealed and protected. 。

2. Main Categories

• By base pipe type     ‑ Seamless base‑pipe lined pipe: The base pipe is made of seamless steel, offering high pressure resistance and suitable for high‑pressure, corrosive service conditions; ‑ Welded‑base‑pipe lined pipe: The base pipe is a straight‑seam or hot‑dip galvanized welded steel pipe, commonly used for low‑pressure water supply and drainage applications, with excellent cost‑effectiveness.
• According to the inner lining plastic material     ‑PE (polyethylene): Suitable for cold-water applications, with a temperature range of ‑25°C to 70°C; widely used for water supply and drainage. ‑PE‑RT/PE‑X (heat‑resistant/cross‑linked polyethylene): Designed for hot‑water service. ‑PP (polypropylene): With a temperature resistance of ‑15°C to 110°C and excellent chemical resistance. ‑PVC‑U, PVC‑C: Resistant to corrosive chemical media. ‑PTFE (polytetrafluoroethylene): Ideal for highly aggressive environments involving strong acids, alkalis, and other severe corrosive conditions.
• According to the medium temperature of use     ‑Cold-water lined plastic pipe: for conveying cold water and ambient-temperature media; ‑Hot-water lined plastic pipe: for conveying domestic hot water and medium-temperature process media.
• By forming process : Cold-drawing with diameter reduction, thermal expansion bonding, and rotomolding lining.

3. Common Specification Notation

The specifications follow the base pipe size system and are indicated in two ways: 1) Base pipe outer diameter × base pipe wall thickness (in mm), with the thickness of the internal plastic liner specified separately; for example: 108×6 mm (base pipe: seamless steel pipe), with an internal PE liner thickness of 2.5 mm; 2) Nominal diameter DN: DN15–DN500, with a standard industry‑specified length of 6 m and an allowable deviation of ±20 mm.

4. Common Materials

Base tube substrate : 20#, Q235B, Q355B seamless steel pipes; Q235 series hot-dip galvanized welded steel pipes;   Plastic lining : Engineering plastics such as PE, PE‑RT, PE‑X, PP‑R, PVC‑C, and PTFE.

Different applications correspond to specific implementation standards; procurement should prioritize verifying these standards.

• GB/T 28897‑2021 “Steel–Plastic Composite Pipes and Fittings for Fluid Transport”: a national standard for plastic-lined steel pipes, specifying requirements for dimensions, bonding strength, flattening, bending, and other performance characteristics. Shenyang Water Affairs Group...
• CJ/T 136‑2007 “Plastic-Lined Composite Steel Pipes for Water Supply”: A national industry standard for plastic-lined pipes used in municipal cold and hot water supply systems; recommended as the preferred choice for potable water systems.
• GB/T 17219: Standard for the Hygienic and Safety Evaluation of Drinking Water Linings; must be met by water supply projects.
• Base pipe standards: For seamless steel pipes, the standard is GB/T 8163‑2008 “Seamless Steel Pipes for Fluid Transportation”; for welded steel pipes, the standard is GB/T 3091‑2015 “Welded Steel Pipes for Low-Pressure Fluid Transportation”.

Key Dimension Datum and Deviations

1. Base standard for pipe dimensions: GB/T 17395‑2024 “Steel Pipe Dimensions, Shape, Weight, and Permissible Deviations,” which continues to adopt the steel pipe outer diameter series and tolerances.
2. Outer diameter deviation of the base tube: Grades D1–D4 (with D2 at ±1.0%, minimum ±0.5 mm); conventional wall thickness deviation of the base tube: ±12.5%.
3. Plastic lining: In accordance with GB/T 28897, the lining thickness is 1.5–2.5 mm for DN15–DN200; 3.0 mm for DN300–DN500. The steel–plastic bonding strength shall be ≥0.3 MPa in cold water and ≥1.0 MPa in hot water.
4. Performance requirements: For DN ≤ 50, perform a bending test; for DN > 50, perform a flattening test. After testing, no cracks or delamination between the steel and plastic layers shall be observed.

Connection method

• DN≤100: threaded connection;
• DN＞100: Groove clamp connection, flanged connection;