How to Repair Manufacturing Defects in Pressure Vessels?

I. Grinding Repair – Suitable for Minor Surface Defects

For surface defects such as shallow cracks, undercut, craters, scratches, or minor corrosion, if they do not affect structural strength and the remaining wall thickness after grinding meets strength verification requirements, grinding can be used to eliminate them.

1. Use an angle grinder or finger grinder to grind the defective area. The grinding contour should have a smooth transition, with an angle control of 1:3 or higher to avoid forming sharp corners.

2. After grinding, penetrant testing (PT) or magnetic particle testing (MT) must be performed to confirm that cracks and other defects have been completely removed.

3. The grinding depth generally should not exceed 5% of the base material thickness, and the continuity of the weld shape and connection to the base material should not be damaged.

4. This method does not require hot work, has low construction risk, is suitable for rapid on-site treatment, and is the preferred non-destructive repair method.

II. Repair Welding and Overlay Welding – For Deeper or Penetrating Defects
When the defect depth is significant, such as incomplete penetration, lack of fusion, porosity, slag inclusions, or deep cracks, repair welding or overlay welding is necessary to restore material integrity.

1. First, thoroughly remove the defect using carbon arc gouging or machining, ensuring a U-shaped bevel at the bottom. After removal, PT/MT testing confirms the absence of residual cracks.

2. Preheating is required before welding. The preheating temperature depends on the material and thickness, typically 150–300℃. The interpass temperature should not be lower than the preheating temperature to prevent cold cracking.

3. Use welding materials that are the same as or compatible with the original weld. The electrode diameter should not exceed Ø3.2mm to ensure weld quality.

4. After repair welding, perform the same non-destructive testing (RT/UT/MT/PT) as the original weld. Post-weld heat treatment may be necessary to eliminate residual stress.

Special Note: For equipment containing extremely or highly hazardous media, cryogenic containers, Cr-Mo steel containers, and equipment prone to stress corrosion, welding repair requirements are more stringent, and welding procedure qualification must be strictly followed.

III. Patching Repair – Addressing Severe Localized Damage When a large area exhibits corrosion, bulging, material deterioration, or repeated repair failures, patching can be used to replace locally pressure-bearing components.

1. The defective area should be completely removed. The patch plate should be circular, elliptical, or rectangular with rounded corners, with a corner radius of not less than 100mm to avoid stress concentration.

2. The material, thickness, and performance of the patch plate must be consistent with the base material. Free expansion and contraction should be allowed during welding to prevent additional stress.

3. The patch length should generally be not less than 300mm, and the distance between the patch and adjacent welds should be greater than three times the nominal wall thickness or more than 100mm.

4. When the repair depth exceeds half the wall thickness, a pressure test should be repeated according to standards such as GB/T150.

Because patching involves extensive welding work, it easily introduces new welding defects and residual stress, and is now used cautiously, only when absolutely necessary.

IV. Component Replacement – ​​For Irreversible Severe Defects
When critical pressure-bearing components such as cylinders, heads, and nozzles exhibit irreparable crack propagation, severe corrosion, or repeated repair failures, they should be replaced decisively.

1. Replacement components must meet the original design requirements, including material, specifications, and heat treatment status.

During installation, care must be taken to protect sealing surfaces and connection points to avoid secondary damage.

2. After replacement, non-destructive testing, pressure testing, and functional testing must be performed again to ensure overall safety.

This method is more expensive, but it can fundamentally eliminate hidden dangers and is suitable for situations with high safety requirements.

V. Composite Materials and Mechanical Reinforcement – ​​Emerging and Emergency Repair Technologies

1. Composite Material Repair
Suitable for non-pressure-bearing areas or as a temporary emergency measure, such as using carbon fiber cloth + epoxy resin for surface bonding reinforcement.

Before repair, the bonding surfaces of the tank must be treated with paint removal, rust removal, and degreasing to ensure bonding strength.

After applying epoxy adhesive, press and cure. It can be put into use after 48 hours at room temperature or 4 hours at 80℃ accelerated curing.

This method does not require open flame and is suitable for rapid leak sealing in flammable and explosive environments, but should only be used as a transitional repair method.

2. Mechanical Reinforcement Used in emergency situations where shutdown or open flame is not possible, such as using segmented clamps in conjunction with sealing adhesive to achieve pressurized leak sealing.

T-bolts are inserted into the crack and rotated to fix it, and then tightened with a steel plate and nut to achieve rapid sealing.

The operation time should be controlled within 30 minutes. Workers must wear respirators to ensure safety.

VI. Inspection and Acceptance After Repair After all repair work is completed, a strict inspection procedure must be performed:

1. Visual Inspection: Confirm that the weld formation is good and there are no surface defects such as undercut, cracks, or porosity.

2. Non-destructive testing: Perform RT, UT, MT, or PT testing on the repaired area according to the original standards to ensure there are no internal defects exceeding the standards.

3. Pressure testing: Especially for repairs with a depth exceeding half the wall thickness, a hydrostatic or pneumatic pressure test must be performed again to verify the pressure-bearing capacity.

4. Functional testing: Check the sealing performance, working pressure, and temperature response to ensure they are normal.

Final acceptance should be recorded and signed for confirmation, and included in the equipment technical file management.