Inspection and acceptance of pipe fittings are critical to ensuring the safe operation of piping systems. Strict control must be carried out in accordance with the technical standards specified in the contract (such as GB/T, ASME, ASTM, etc.). The entire acceptance process generally consists of six main parts: documentation review, visual inspection, dimensional inspection, non-destructive testing, physical and chemical property re-inspection, and pressure testing. The detailed inspection and acceptance standards are as follows:
I. Documentation Review
Before physical inspection, the accompanying technical documents shall be verified to ensure consistency between the goods and certificates.
- Mill Certificate: Must include chemical composition analysis report, mechanical property test report (tensile strength, yield strength, elongation, hardness), heat treatment record, non-destructive testing results, etc.
- Specification and Model Verification: Check whether the material grade, dimensions, wall thickness schedule (such as Sch40, Sch80), and applicable standard are consistent with the purchase order.
- Heat Lot Control: Confirm that the heat number stamped on the fittings corresponds to the number on the mill certificate to ensure full traceability.
II. Visual Inspection (VT)
This is the most intuitive inspection, usually performed with naked eyes or a magnifier not exceeding 5x magnification.
- Surface Defects: The inner and outer surfaces of fittings shall be free from cracks, folds, laps, delaminations, hairline cracks, and scabs. Any such defects must be removed; the removal depth shall not exceed 10% of the nominal wall thickness, and the remaining wall thickness after removal shall not be less than the minimum required wall thickness.
- Corrosion and Contamination: Surfaces shall be smooth and clean, without excessive scale, rust, or oil stains. Stainless steel fittings shall be free from localized ferrite contamination (shall be pickled and passivated).
- Weld Quality: For welded fittings (such as plate-type fittings), welds shall be full and well-formed, without incomplete penetration, burn-through, porosity, or slag inclusions. Weld reinforcement is normally restricted (e.g., not exceeding 3mm).
- Roundness: Fitting ends shall be round, without obvious oval deformation.
III. Dimensional Inspection
Measurements are performed using calipers, micrometers, templates, protractors, and other gauges.
- Outer Diameter and Inner Diameter: Verify that the OD and ID at both ends are within the allowable tolerances specified in the standard.
- Wall Thickness: Measure actual wall thickness, focusing on the extrados (thinnest point) and intrados (thickest point) of elbows, to ensure wall thinning complies with standards (generally, wall thinning of formed elbows shall not exceed 12.5% or 15%).
- Length: Check the overall length of fittings (such as center-to-end distance of elbows).
- Angle and Perpendiculararity: Verify angular deviation of elbows (e.g., for 90° elbows) and perpendicularity between end faces and the centerline (end squareness).
- Radius of Curvature: Confirm that the bending radius (R) meets the requirements for long radius (1.5D) or short radius (1.0D).
IV. Non-Destructive Testing (NDT)
Depending on project importance, testing may be done on a sampling or 100% basis.
- Radiographic Testing (RT): Mainly used to detect internal defects in the fitting body and welds (such as porosity, slag inclusions, incomplete penetration). High-pressure or critical fittings usually require 100% RT, with an acceptance level generally not lower than Level II or III.
- Ultrasonic Testing (UT): Used to detect uneven wall thickness, lamination, or large internal defects.
- Surface Testing:
- Magnetic Particle Testing (MT): For carbon steel and low-alloy steel fittings, to detect surface cracks.
- Liquid Penetrant Testing (PT): For stainless steel or non-magnetic fittings, to detect open surface cracks.
- Hardness Testing: Verify the effectiveness of heat treatment by ensuring hardness values fall within the standard range (e.g., stainless steel typically requires HB ≤ 201).
V. Physical & Chemical Re‑testing
When material authenticity is in doubt or the project has high requirements, destructive tests shall be performed on samples taken from fittings.
- Spectral Analysis: On-site rapid chemical composition verification using a handheld spectrometer to prevent material mixing (e.g., mixing 304 and 316).
- Mechanical Property Tests: Cut specimens for tensile or impact tests to verify material strength and toughness.
- Metallographic Analysis: Examine microstructure to check for excessive grain size, over-thick decarburization layer, or other structural defects.
VI. Hydrostatic / Pressure Testing
Used to verify the strength and tightness of fittings.
- Hydrostatic Test: Most fittings require hydrostatic testing before delivery. The test pressure is generally 1.5 times the design pressure (or a specific value specified in the standard).
- Pressure Holding Time: After reaching the test pressure, maintain for a specified period (e.g., 10–30 seconds) with no leakage, weeping, or visible plastic deformation permitted.
- Alternative Testing: For stainless steel or special materials, pneumatic testing using dry, clean air may be allowed instead of hydrostatic testing to avoid water stain corrosion, but requires extremely high safety protection.
VII. Other Special Requirements
- Intergranular Corrosion Test: For austenitic stainless steel fittings (such as 304, 316) used in corrosive environments, intergranular corrosion susceptibility tests (e.g., Method A, Method E) may be required.
- Surface Treatment Acceptance: Inspect the thickness and adhesion of anti-corrosion coatings, or the quality of pickling and passivation films on stainless steel (verifiable by blue spot test).
Summary
Acceptance of pipe fittings must ensure complete documentation, defect-free appearance, accurate dimensions, and qualified performance. Failure in any stage may lead to unsuccessful pipeline welding or leakage during operation.
