Welded steel pipes for low-pressure fluid transportation are made to GB/T 3091 and are used for water, heating, gas, air and similar services where the working pressure is modest but long-term leak tightness is essential. The pipe body is formed from steel strip and welded along the seam, and most output is supplied either black or hot-dip galvanized. The inspection programme is built around the failure mechanisms that actually matter in these services: weld quality, wall thickness, coating integrity and ductility of the finished pipe.
Scope and Standard Framework
GB/T 3091 is the product standard for welded steel pipes used in low-pressure fluid transportation and covers straight-seam welded pipe made from carbon structural steel such as Q235B in black or hot-dip galvanized condition. When the same geometry is used for a designed pressure pipeline instead of a utility service, the governing document changes to GB/T 9711 or API 5L and the inspection plan becomes correspondingly stricter, with full-length non-destructive testing of the weld seam and documented impact testing.
The test methods used for the finished pipe are drawn from the general tube test standards: GB/T 228.1 for tensile testing, GB/T 246 for the flattening test, GB/T 242 for the bend test and GB/T 229 for impact testing where required.
Test Items for Finished Pipe
| Test item | Purpose |
|---|---|
| Surface defects | Confirm that the inner and outer surfaces are free from defects that reduce the effective wall |
| Shape, size and weight | Verify outside diameter, wall thickness and unit mass against the ordered specification |
| Tensile test | Confirm tensile strength, lower yield strength and elongation of the pipe body |
| Flattening test | Confirm the ductility of the pipe wall and the soundness of the longitudinal weld |
| Zinc coating mass | Confirm the total zinc applied to the inner and outer surfaces |
| Coating uniformity | Confirm continuous coverage by copper sulphate immersion |
| Coating surface quality | Confirm a complete coating without uncoated areas, black spots or bubbles |
| Coating adhesion | Confirm the zinc layer remains attached after the specified deformation test |
The test items are selected to match the end use. Plain black pipe for a dry, non-corrosive application does not need coating tests, while galvanized pipe for outdoor water service needs the full coating set.
Technical Requirements and Acceptance Values
Surface quality: inner and outer surfaces shall be smooth, without folds, cracks, lamination, lap welding, arc breakage or burn-through. Where such a defect is removed by grinding, the remaining wall shall not fall below the minimum permitted wall thickness and the ground area shall blend smoothly into the surrounding surface. Local imperfections within the permitted wall thickness deviation may remain.
Tensile strength: not less than 370 MPa.
Lower yield strength: not less than 235 MPa, measured on the pipe specimen during the yield stage, excluding the initial transient effect.
Elongation after fracture: not less than 15 % of the original gauge length.
Flattening: when the plates are closed to two thirds of the outside diameter, no crack may appear in the pipe body or the weld; when they are closed to one third of the outside diameter, no crack or tear may appear in the weld. Flattening continues until the pipe wall meets, with no lamination or metal overburning at any stage.
Galvanizing and Coating Requirements
Coating mass: the total zinc on the inner and outer surfaces shall not be less than 300 g/m2. Using the density of zinc of 7.14 g/cm3, this total corresponds to an average of roughly 21 micrometres of zinc on each surface, which is why the mass is specified rather than a single surface thickness.
Uniformity: a sample immersed in copper sulphate solution for five consecutive dips shall not show red deposits from exposed steel.
Adhesion: after the prescribed deformation the zinc layer shall not flake or peel.
Surface quality: the coating shall cover the whole surface, without uncoated spots or bubbles; slight roughness and isolated zinc nodules are acceptable.
Hot-dip galvanizing itself is performed to GB/T 13912. Threaded or grooved ends and field weld zones are normally left uncoated or coated by a repair system, because welding through zinc produces porosity in the weld and harmful fume; the coating should be removed from at least the heat-affected zone before welding, and adequate ventilation provided.
Common Reject Causes and Inspection Tips
Underweight coating on the inside surface: caused by inadequate fluxing or short immersion time; check by weighing a full-section sample rather than an external gauge.
Weld undercut or incomplete penetration: revealed by the flattening test and by weld-seam inspection; trace back to welding speed and edge condition.
Excess weld flash inside the pipe: reduces the effective bore; check the internal flash height on a sample cut length.
Dimensional drift: outside diameter and wall thickness excursions from strip width variation; verify by gauging both ends of each length.
Handling damage and white rust on galvanized pipe: caused by wet storage; use dry, ventilated stacking with dunnage and avoid contact between galvanized surfaces and water.
FAQ
Q: Which standard governs inspection of welded steel pipes for low-pressure fluid transportation?
GB/T 3091 is the product standard. It fixes the test items, the acceptance values for tensile properties, flattening and coating, and the dimensional tolerances; the individual test methods are taken from the general tube testing standards such as GB/T 228.1, GB/T 242 and GB/T 246.
Q: What tensile properties are required?
The pipe body is required to reach a tensile strength of not less than 370 MPa and a lower yield strength of not less than 235 MPa, with elongation after fracture of not less than 15 % on the pipe test piece.
Q: What does the flattening test prove?
It proves the ductility of the wall and the soundness of the longitudinal weld. Plates closed to two thirds of the outside diameter must not produce cracks in the body or the weld, and closure to one third of the diameter must not crack or tear the weld, with no lamination or overburning throughout.
Q: How is the zinc coating evaluated?
By coating mass, uniformity, surface quality and adhesion. The total coating on both surfaces must be at least 300 g/m2, the copper sulphate dip test must show no red deposit over five consecutive immersions, and the coating must not peel after deformation.
Q: Can galvanized pipe be welded on site?
Yes, but the zinc coating must be removed from the joint area first, low-hydrogen consumables and controlled parameters should be used, and the weld zone coated again afterwards. Ventilation is essential because zinc fume is harmful.
Q: When does a low-pressure fluid pipe require a higher-grade specification?
When it is part of a designed pressure pipeline, carries a hazardous medium, or is required to operate in cold service with documented impact toughness. In those cases GB/T 9711 or API 5L applies, with full-length weld testing and impact testing added to the programme.
