What are the main inspection items for flanges?

I. Raw Material Inspection (Base Material)

Chemical composition analysis: 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. Check: flange outer diameter, inner diameter and thickness; bolt holes (hole diameter, bolt circle diameter, quantity, positional tolerance); sealing face dimensions (diameter, raised face height, recess depth for RF, MFM and T&G); neck height and taper (for weld neck flanges); flatness and parallelism; thread accuracy (threaded flanges).

III. Special Inspection for Sealing Faces

Sealing face roughness: Measured by roughness tester. Ra 3.2–12.5μm 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)

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.
• 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 layer thickness and adhesion; check for peeling or missing coating (galvanized and anti-corrosion flanges).

VIII. Additional Requirements for EU Export (EN 1092-1)

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