How to detect cracks in a pressure vessel?

I. Preliminary Inspection: Visual Inspection (Applicable to visible surface cracks)

Before professional testing, a quick screening can be performed using the naked eye or auxiliary tools to identify obvious abnormalities.

1. Visual Inspection: Use the naked eye or a 5-10x magnifying glass to observe stress concentration areas such as welds, nozzles, and end cap transition zones. Cracks often appear as thin, elongated lines and may be accompanied by rust, leakage, or discoloration.

2. Light Illumination Method: Shine a flashlight parallel to the vessel surface, using the contrast of light and shadow to highlight tiny pits or cracks.

3. Hammer Test: Gently tap the vessel wall with a 0.5kg hammer. A dull sound or an abnormally bouncing feel suggests possible internal delamination or cracks.

✅ Applicable Scenarios: Routine inspections, shutdown inspections, or as a preliminary assessment before non-destructive testing.

II. Precise Detection: Non-Destructive Testing (NDT) Technology (Confirming Crack Presence and Characteristics)

When suspicious areas are discovered or during periodic inspections according to procedures, NDT technology must be used for precise assessment.

1. Magnetic Particle Testing (MT) – Preferred for Ferromagnetic Materials

Principle: After magnetization, a leakage magnetic field is formed at the crack, attracting magnetic particles for development.

Advantages: High sensitivity to surface and near-surface cracks, especially suitable for carbon steel and low-alloy steel containers.

Applications: Commonly used for stress corrosion crack detection in high-strength steel and highly crack-sensitive materials; fluorescent magnetic particles are easier to identify under black light.

2. Penetrant Testing (PT) – General-Purpose Surface Open Crack Detection

Principle: Penetrant enters the crack; after cleaning, a developer is applied to reveal the defect.

Advantages: Applicable to various metals and non-magnetic materials; simple operation.

Limitations: Cannot detect closed cracks or internal defects.

3. Ultrasonic Testing (UT) – A "X-ray Vision" for Internally Buried Cracks

Principle: High-frequency sound waves reflect off cracks, and the location and depth are determined by the echo signal.

Advantages: Strong penetrating power; capable of detecting internal cracks in thick-walled containers; high efficiency.

Recommendation: For internal cracks, ultrasonic testing is the preferred method.

4. Radiographic Testing (RT) – Direct Imaging of Internal Weld Defects

Principle: X-rays or gamma rays penetrate the container; the absorption rate varies in the crack area, forming an image on film.

Advantages: Intuitive results; permanently stored; suitable for weld quality assessment.

Limitations: High equipment cost; strict radiation protection requirements.

5. New Technologies Improve Detection Efficiency

Ultrasonic Guided Wave Testing: Can achieve propagation over distances of tens of meters, suitable for rapid screening of large containers; has successfully detected pitting defects deeper than 2mm.

Phaseed Array Ultrasonic Testing: Electronic scanning + 3D imaging; accurately locates fatigue cracks down to 0.5mm level with an error of less than 0.1mm.

Circumferential Guide Wave Positioning Model: Specifically designed for non-destructive positioning of cracks in pressure vessel pipelines, enabling cyclic inspection.

III. Key Inspection Areas: Enhancing Targeted Inspection
The following areas are high-risk for cracks and should be included as mandatory inspection points:

Welds and heat-affected zones (especially areas requiring multiple repairs)

Around nozzles and openings

Transition zone between end caps and cylinder

Stress concentration areas such as supports and flanges

Areas in long-term contact with corrosive media or undergoing thermal cycling
IV. Abnormal Operational Signals: Indirectly Identifying Potential Cracks
Even if cracks are not directly detected, the following phenomena indicate potential latent defects:

Frequent safety valve tripping or abnormal pressure fluctuations

Periodic leakage at flanges and welds

Abnormal localized temperature increases (detectable with infrared thermal imagers)

Increased vibration or abnormal noises