Steel-plastic composite pipe (SP) ships as a welded carbon-steel or ERW shell lined with PE, PP, or epoxy resin and externally coated with PE, epoxy, or hot-dip galvanizing, with stock dimensions from 15 mm OD to 1200 mm OD and 2.2 mm to 8.0 mm wall thickness [S5]. The plastic layers carry the corrosion duty so the steel core is sized for pressure and mechanical load, which is why SP runs are now common in civil water supply, fire mains, gas, drainage, and cable ducting per LESSO Pipeline and Chinese GB/T 28897-2012 reference builds [S5][S6].
Plastic-coated composite steel pipe is widely adopted for urban water supply, fire fighting, sewage transport, communication circuit, optical fiber cable, coal gas transportation, and foodstuff process lines per LESSO's product description, with the inner plastic layer providing strong adhesive force, compactness, and corrosion resistance against the conveyed fluid [S6].
Joint Selection by Diameter: Threaded, Flanged, Grooved, and Welded
SP joints follow the same size rules as lined steel systems: DN15-DN100 takes threaded or compression fittings, DN>100 moves to flanges, grooved couplings, or butt-weld with the liner reformed at the cut, and DN≥800 should default to flanged or welded with explicit pipe-stiffener rings at equipment nozzles [S1][S5]. Tianchuang (Hebei) supplies SP for water, fire, and gas service to GB/T 28897-2012 and lists Low Pressure Liquid Delivery Pipe under that standard for drinking water, cold water, fire water, and drainage service [S2].
Grooved couplings (Victaulic-class) are the practical default for DN100-DN500 fire and water risers because the elastomer gasket seals on the steel OD after the PE/PP coating is locally stripped 80-120 mm back from each cut face, and the joint remains disassemblable for maintenance. For threaded joints on small-bore SP, use factory-threaded adaptors only; field-threading cuts the plastic liner and breaks the corrosion barrier, which is the single most common cause of SP joint leakage on small-bore domestic work.
Handling, Cutting, and Liner Re-Sealing at the Weld
SP must be stored off the ground on timber or rubber padded saddles at 1.5-2 m centers to prevent permanent ovality of the plastic coating, and field cuts require a carbide-tipped saw or pipe cutter rather than oxy-acetylene because flame cutting destroys the PE/PP layer at the cut face [S1]. HZ-FP-1 fire-protection SP from Centuryfaver is described as acid-resistant, anticorrosive, compression resistant, and innocuous, with installation convenience that supports wide deployment in waste-water discharge, freeway and playground rainwater projects, drainage, flood prevention, farmland irrigation, and underground electric cable bushing service [S1].
After cut, the exposed steel at the bevel is re-sealed with a two-part epoxy or a heat-shrink PE sleeve matched to the parent coating; for gas service, the seal must extend at least 50 mm past the weld zone on both the bore and the OD. LESSO's plastic-coated compound steel pipe achieves its corrosion performance through "strong adhesive force, excellent compactness, and sound resistance to corrosion" of the inner plastic layer, which is precisely the property that is lost at a poorly re-sealed field weld [S6].
Bury, Support, and Expansion: Mechanical Limits of the SP System
Support saddles should be rubber- or PE-lined to prevent coating wear at the bearing point, and any metallic support touching bare steel (e.g., at a flange) should be dielectrically isolated to prevent galvanic cells, a frequent failure mode when SP ties into steel pipe risers or copper service lines. The plastic coating on the steel OD is what gives SP its 30+ year service life in municipal water; once the coating is gouged at a support, the steel underneath rusts at the same rate as uncoated steel pipe.
Testing, Commissioning, and When to Reject a Joint
Hydrostatic test for water-service SP is typically 1.5x working pressure held 30-60 min with no pressure drop, and gas-service SP follows a pneumatic pre-test at 0.2 MPa followed by the strength test at 1.5x design per the applicable gas code; LESSO's water-supply SP is rated for civil and industrial water transport up to its published pressure class, and the inner plastic layer of the plastic-coated compound steel pipe is the corrosion barrier that must survive the test without blistering or peeling [S6].
Reject criteria during commissioning: visible liner blisters, coating disbondment at any cut, leaking threaded joint without an approved thread sealant, and any field cut where the inner liner was scored more than 1 mm deep — these joints are cut out and replaced, not patched. For long PE-coated water mains, end-of-line flushing at 1.5x design velocity for 20-30 min is required to clear debris that would otherwise score the inner liner during operation.
SP vs PPR vs PPR-Al-PPR vs HDPE: Selection by Service
SP wins on temperature, pressure, and mechanical damage resistance; plain plastic pipe wins on cost, flexibility, and chemical inertness for low-pressure drainage. The composite sits in the middle: a steel core delivers the pressure rating and the plastic liner delivers the chemical resistance, which is exactly the role that steel-plastic composite pipe is designed for in the GB/T 28897-2012 framework and that LESSO's product family exploits across civil water, gas, and fire service [S5][S6].
Rule of thumb: DN≥200, T≥60 °C, or P≥1.6 MPa points to SP or lined steel; DN≤110, T≤40 °C, P≤1.0 MPa, and chemically aggressive but low-pressure service points to plastic pipe (PP-R, PE-RT, or HDPE); buried drainage with high ring stiffness needs the filament-wound GRP/FRP range such as the PN1-PN25 fiberglass-wound pipe supplied by Okorder-listed manufacturers for petrochemical and power-plant service [S3].
Common Field Failures and What They Mean
Three failure modes dominate SP warranty claims: (1) liner disbondment at a welded field joint, almost always traced back to inadequate surface prep or wrong epoxy — the fix is to cut back to sound liner, abrasive-blast the steel, and re-coat with a system qualified to the parent coating; (2) external coating wear at a support or through a wall sleeve — the fix is to add a rubber saddle and seal the sleeve with a link-seal or equivalent, not to wrap with PVC tape; (3) flange-face corrosion from a missing gasket or a non-dielectric flange kit, which requires the joint to be disassembled, the faces re-machined, and a full-face dielectric gasket installed. [S5]
A comparison useful at the sourcing desk: a steel-plastic composite pipe at DN200, 6 m length, with PE lining and epoxy exterior, sits in the same commercial band as lined carbon steel per GB/T 28897-2012, typically landing below stainless 304 and above plain HDPE for the same OD/PN class [S5]. Stock-dimension sources list 15-1200 mm OD and 2.2-8.0 mm wall with PE, epoxy, or galvanized surface treatment under API, ASTM, BS, DIN, and GB certification paths [S5].
For buyers, the next step is to lock the standard edition and coating system on the PO and require a test certificate per batch, since most SP field failures trace back to a coating system that was not specified in the purchase order in the first place. For the related comparison with glass-reinforced plastic pipework, see the FRP composite encyclopedia entry and, for sourcing context on similar corrosion-resistant line pipe, the kitairu.net low-pressure liquid delivery pipe listing and the LESSO water-supply SP product page [S2][S6].
See also our earlier report, Ball Screw Suppliers 2026: China Factory Map, Premium OEMs, and Sourcing Signals.