What are the main inspection items for flanges

Steel pipe flanges are widely used in chemical industry, piping, rail transit and pressure vessels. Inspections include incoming raw material inspection, geometric dimensional inspection, visual surface inspection, non-destructive testing, performance test, special sealing face inspection and third-party type test.

Applicable standards: GB/T 9113, GB/T 9124. For exports to the EU, compliance with EN 1092-1 is required.

I. Raw Material Inspection (Base Material)

• Chemical composition analysis of materials: Spectroscopic analysis to verify elements such as C, Si, Mn, S and P, confirm material grades (Q235, Q355, 304, 316L, etc.) to prevent material substitution.
• Mechanical properties: Tensile test (tensile strength, yield strength), impact test (for flanges under low-temperature service) to guarantee material strength and toughness.

II. Geometric Dimensional Inspection (using calipers, micrometers, protractors, gauges)

• Flange outer diameter, inner diameter and thickness.
• Bolt holes: hole diameter, bolt circle diameter, quantity of bolt holes, positional tolerance / uniform distribution of holes (critical; misaligned holes cause assembly failure).
• Sealing face dimensions: sealing face diameter, raised face height, recess depth; dimensions for RF, MFM and T&G sealing faces.
• Neck height and taper (for weld neck flanges).
• Flatness and parallelism: warped flange end faces will cause sealing leakage.
• Threads (threaded flanges): thread accuracy and thread profile inspection.

III. Special Inspection for Sealing Faces (critical for leakage prevention)

• Sealing face roughness: measured by roughness tester.
• Ra 3.2–12.5 m is typical for RF faces.
• Excessively rough surfaces result in insufficient gasket compression and leakage.
• Sealing face visual check: no scratches, pits, pores or impact damage permitted.

IV. Visual Testing (VT) and Marking Inspection

• No cracks, pores, slag inclusions, blowholes, laps or seams on the surface.
• Cast flanges: check for casting defects; forged flanges: check for forging cracks.
• Marking inspection: stamped marks (material grade, pressure class, heat lot number, size) must be complete and legible.

V. Non-Destructive Testing (NDT) (mandatory for forged and pressure-vessel-grade flanges)

• UT Ultrasonic Testing: Detects internal defects such as internal cracks, shrinkage cavities and laminations inside flanges.
• MT Magnetic Particle Testing (carbon steel / alloy steel): Detects surface and near-surface cracks.
• PT Penetrant Testing (stainless steel flanges): Detects fine surface cracks and open discontinuities.

Only VT visual inspection may be applied for ordinary low-pressure piping flanges. UT plus MT/PT is required for pressure piping, rail transit and pressure vessel flanges.

VI. Pressure Performance Test

• Hydrostatic test: Hold pressure at 1.5 times the nominal pressure; no leakage or visible deformation allowed.
• Pneumatic test: For special service conditions, air tightness test to verify sealing performance.

VII. Coating / Anti-corrosion Layer Inspection (galvanized and anti-corrosion flanges)

• Galvanized layer thickness and adhesion; check for peeling or missing coating.

VIII. Additional Requirements for EU Export under EN 1092-1

• Material certificates 3.1 / 3.2; dimensional tolerances follow European tolerances; third-party type test is required for some cases.

Simplified Guideline: Inspection Priorities for Different Service Conditions

Service condition	Inspection priority
Ordinary low-pressure water supply & drainage flanges	Visual inspection + key dimensions + material certificate
Pressure piping and chemical flanges	Material verification + full dimensional check + NDT + hydrostatic test
Flanges for rail transit projects (Shandong intercity rail application)	Material spectroscopy, dimensional position tolerance, sealing face roughness, MT/PT, anti-corrosion coating inspection.

Full set of EN/CE test reports available.