What inspections are required after pressure vessel repair?

I. Non-destructive Testing (NDT) – Confirming the internal and surface quality of the weld
When repair involves welding operations, non-destructive testing (NDT) of the repaired area is essential; this is the most crucial inspection step.

1. Surface NDT: Magnetic particle testing (MT) or penetrant testing (PT) is used to check for defects such as surface cracks, undercut, and lack of fusion in the repaired weld and heat-affected zone. This is particularly suitable for ferromagnetic materials (MT) or non-porous, non-ferromagnetic materials (PT).

If the repaired area is Cr-Mo steel, a cryogenic vessel, or low-alloy steel with a standard tensile strength ≥540MPa, a second surface inspection is required after the pressure test.

For materials prone to reheat cracking (such as certain alloy steels), an additional surface inspection should be performed after heat treatment.

2. Internal Non-Destructive Testing: Ultrasonic testing (UT) or radiographic testing (RT) is used to detect buried defects such as incomplete penetration, slag inclusions, porosity, or cracks within the weld.

Ultrasonic testing is the most commonly used method in the field due to its portability, efficiency, and sensitivity to area-type defects.

Radiographic testing is suitable for situations requiring direct imaging and is often used for re-inspection after anomalies are found in UT.

✅ Scope of Inspection: Not limited to the weld itself, but also includes the heat-affected zone, adjacent base material, and other potentially affected connection areas.

II. Pressure Testing – Verifying Overall Strength and Sealing Performance Whether a pressure test is required after repair depends on the depth and extent of the repair:

1. Situations requiring a pressure test:

Repair depth exceeding half the wall thickness; Replacement of major pressure-bearing components (such as cylinder sections, heads); Multiple repairs or significant impact on the original structural strength.

2. Test Type Selection:

Hydraulic testing (such as hydrostatic testing) is preferred due to its high safety. When filling with liquid is not possible or operating conditions do not allow for residual liquid, pneumatic testing or a combined pneumatic-hydraulic test can be used, but it must meet the prerequisite of 100% UT or RT for Class A and B welds.

3. Acceptance Criteria:

Hydraulic Test: No leakage, no visible deformation, no abnormal noise;
Pneumatic Test: In addition to the above, a leak test using soapy water or other leak detection liquid is required to check for leaks.

⚠️ Note: The pressurization process should be carried out slowly in stages. Initially, pressurize to 10% of the test pressure and hold for a check. Only continue pressurizing after confirming no leaks.

III. Appearance and Geometric Dimension Inspection – Ensuring Compliance with Morphological Standards

After repair, a systematic inspection of the appearance quality is required:

The weld seam and base material should have a smooth transition, without sharp corners, undercut, or abrupt changes;
The fillet weld seam should have a concave, smooth transition;
After grinding, the weld reinforcement should have a slope angle ≤15°, and the transition curvature radius ≥3 times the plate thickness;
Undercut depth >0.5mm or continuous length >100mm requires re-welding and re-inspection.

In addition, the overall geometric dimensions of the container must be checked, such as the straightness of the cylinder, roundness deviation (not exceeding 1% of the design diameter and ≤25mm), and flange flatness, to ensure that installation and sealing are not affected.

IV. External Inspection and Safety Accessory Verification – Restoring Operating Conditions Before being put back into service, the repaired container must undergo the following routine inspections:

1. External Structure Inspection:

Insulation and anti-corrosion layers: Are they intact?

Supports and foundations: Are they stable, without subsidence or tilting?

Connecting pipes: Are there any abnormal vibrations or added stress?

The outer wall of the container: Is there any corrosion, leakage, or localized overheating?

2. Safety Accessory Verification:

Safety Valves: Confirm they are within their calibration validity period, open and close sensitively, and have intact seals.

Pressure Gauges: Readings are consistent throughout the system, and the range and accuracy meet regulations.

Water Level Gauges: Indicate clear indications, with high and low level markings, and no leaks.

These inspections are typically performed annually at least once a year, but should be conducted earlier after repair.

V. Material and Hardness Testing (if necessary) – Preventing Material Deterioration

For high-temperature, high-pressure, or special media containers, the following may also be necessary:

Material Verification: Confirm through spectral analysis that the material in the repaired area has not become confused or deteriorated;

Hardness Testing: Check whether the hardness of the weld and heat-affected zone exceeds the standard to prevent hydrogen-induced cracking or brittle fracture;

Metallographic Examination: Assess whether the microstructure has become abnormal due to the welding thermal cycle (e.g., grain coarsening, precipitation, etc.).