How to determine if a pressure vessel repair is qualified?

I. Appearance and Geometric Dimensions Inspection Qualified The appearance quality of the repaired area is the first inspection checkpoint and must meet the following requirements:

1. Good Weld Formation: The weld should smoothly transition to the base material, without sharp corners, abrupt changes, or stress concentration structures.

2. No Surface Defects: The repaired weld and heat-affected zone must be free of visible defects such as cracks, lack of fusion, undercut (depth > 0.5mm), arc craters, and porosity.

3. Fillet Weld Shape: It should be a concave, smooth transition to the base material, avoiding right-angle connections.

4. Geometric Dimensions Compliant:
Shell Straightness ≤ 1H/1000 (H is the shell length);
Shell Roundness (difference between maximum and minimum inner diameter) ≤ 1%Di and ≤ 25mm (Di is the inner diameter);
Flange Face Perpendicularity ≤ 1% of flange outer diameter and ≤ 2mm.

II. Non-Destructive Testing (NDT) Results Compliance NDT is the core method for verifying repair quality and must cover the repaired weld and its surrounding area.

1. Surface Inspection:

Magnetic particle testing (MT) is used for ferromagnetic materials; penetrant testing (PT) is used for non-ferromagnetic materials. The testing standard is based on NB/T 47013-2015. Surface cracks are not allowed (Level I acceptance).

2. Internal Inspection:

Ultrasonic testing (UT) or radiographic testing (RT) is used to inspect for internal defects.

When using radiographic testing, the radiographic quality should be no lower than Grade AB, and the acceptance level should be Grade II or III (depending on the vessel type). For repaired welds, the inspection length should be no less than 20% of the total weld length. If defects exceeding the standard are found, supplementary testing is required to reach 100%.

✅ Special Note: For steel with a standard tensile strength ≥540MPa, Cr-Mo steel, stainless steel, and vessels prone to stress corrosion, the weld surface must not have undercut.

III. Pressure Test Passed – Verifying Overall Strength
The pressure test is the most critical verification step after repair, used to check the overall strength and sealing performance of the container.

1. Situations requiring a repeat pressure test:

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

2. Test Type:

Hydraulic testing (e.g., hydrostatic testing) is preferred due to its high safety; If liquid filling is not possible, pneumatic testing can be used, but it must be ensured that Class A and B welds have passed 100% UT or RT inspection.

3. Acceptance Criteria:

No leakage;
No visible deformation;
No abnormal noise;
Stable pressure during pressure holding.

IV. Safety Accessories and External Condition Inspection Completed Before putting the repaired container into use, the following auxiliary systems must be confirmed to be in normal condition:

1. Safety Accessories Validated:

Safety valves are within their valid calibration period and open and close sensitively;
Pressure gauges meet the required range and accuracy, and readings are consistent within the same system;
Level gauges are clearly visible and have high and low limit markings.

2. External Structure Intact:

Supports are stable, and the foundation has not settled;
Insulation and anti-corrosion layers have been restored to full integrity;
Connecting pipelines are free from additional stress or vibration.

3. Airtightness Test (if necessary):

For containers holding flammable, explosive, or toxic media, an airtightness test is required;
Using soapy water or other leak detection solutions, no bubbles should be generated.

V. Material Performance and Hardness Testing (Under Specific Conditions) For containers operating under high temperature, high pressure, or special service conditions, further verification of the material condition is required:

1. Material Verification: Confirm through spectral analysis that the material in the repaired area has not been confused;

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

3. Metallographic Examination: Assess whether the microstructure has deteriorated due to welding thermal cycling (e.g., grain coarsening, precipitation, etc.).

VI. Complete and Traceable Repair Records All repair processes must be documented in complete technical files, including:

Defect description and removal record;
Welding process parameters and welder qualifications;
Non-destructive testing reports (MT, PT, UT, RT);
Heat treatment records (if any);
Pressure test data;
Final acceptance signed document.