Failure Mechanisms in Pressure Vessels: Understanding Risks and Inspection Strategies

Introduction to Pressure Vessel Inspection

Pressure vessel inspection is vital for maintaining the mechanical integrity of equipment in refining, petrochemical, and power industries. These vessels operate under high temperatures and pressures, often insulated, which exposes them to aggressive degradation mechanisms. Carbon steel and stainless steel are the two most common materials used in pressure vessel construction. Each material has unique failure modes under these harsh conditions.

This article explores the dominant failure mechanisms affecting both carbon steel and stainless steel. We will focus on corrosion under insulation (CUI), high-temperature hydrogen attack (HTHA), sulfidation, chloride stress corrosion cracking (Cl-SCC), sensitization, creep, and thermal fatigue. Additionally, we will discuss best practices for inspection and risk-based approaches as outlined in API 510 and API RP 571. We will also explain how acoustic emission (AE) testing and fitness-for-service evaluations per API 579 can help detect, monitor, and mitigate these damage mechanisms.

Understanding Failure Mechanisms in Carbon Steel

Corrosion Under Insulation (CUI)

CUI is one of the most common failure mechanisms in carbon steel pressure vessels. When water seeps beneath the insulation, it introduces oxygen and salts, which accelerate corrosion. This typically occurs at temperatures between 25–120 °C, with potential damage extending up to 175 °C. Areas most affected include supports, nozzles, and low points where water tends to accumulate.

High-Temperature Sulfidation and Oxidation

In refinery and hydrocarbon applications, sulfur species and oxygen react with steel at temperatures above 260 °C. This reaction forms iron sulfides and oxides, leading to rapid wall thinning. To mitigate this risk, alloy upgrades or protective coatings are recommended.

High-Temperature Hydrogen Attack (HTHA)

HTHA occurs when hydrogen diffuses into steel at elevated pressures and temperatures (greater than 205 °C). This hydrogen reacts with carbides, producing methane and resulting in micro-fissures and decarburization. The embrittlement caused by HTHA can be severe, and assessments should adhere to API 941 guidelines.

Creep and Stress Rupture

Long-term exposure to temperatures above 400 °C can lead to time-dependent deformation, known as creep. Creep cracking often occurs near welds and stress concentrators. Evaluating creep life is essential in API 579-1/ASME FFS-1 fitness-for-service assessments.

Thermal Fatigue

Cyclic thermal stresses from startup and shutdown processes can cause fatigue cracking, particularly at weld toes and nozzle intersections.

Figure 2: Stress corrosion cracking in stainless steel pressure vessel.

Failure Mechanisms in Stainless Steel

Chloride Stress Corrosion Cracking (Cl-SCC)

Cl-SCC is the most significant damage mechanism for insulated stainless steel pressure vessels. Chlorides from insulation materials or the environment, combined with tensile stress and temperatures between 60–200 °C, can lead to cracking. This cracking often remains undetected until leakage occurs.

Localized Corrosion Under Insulation

Even without visible cracking, stainless steels can suffer from deep pitting or crevice corrosion beneath chloride-laden insulation. These localized attacks can quickly compromise wall integrity.

Sensitization and Intergranular Corrosion

Exposure to temperatures between 500–800 °C can cause chromium carbide precipitation at grain boundaries. This phenomenon leads to sensitization and intergranular attack. Using low-carbon or stabilized grades (L, Ti, Nb) can help mitigate this risk.

Sigma-Phase Embrittlement

Long-term exposure to temperatures between 565–980 °C can promote sigma-phase formation, which reduces toughness and ductility, especially in duplex grades.

Creep and Thermal Fatigue

Sustained service above 540 °C can lead to creep in stainless steels. Additionally, cyclic operations contribute to fatigue cracking at welds and geometric transitions.

Shared Degradation Challenges

Both carbon and stainless steels face challenges related to thermal fatigue and corrosion under insulation. Since insulation hides external surfaces, degradation may remain undetected until significant wall loss occurs. This underscores the importance of on-stream inspection (OSI) and risk-based inspection (RBI) planning in line with API 510 guidelines.

Inspection and Mitigation Strategies

Carbon Steel Inspection Strategies

1. Perform ultrasonic thickness mapping and monitor corrosion rates.

2. Selectively remove insulation for visual external inspections.

3. Use guided-wave ultrasonics (GWUT) or profile radiography for hard-to-reach areas.

4. Conduct HTHA assessments per API 941 and evaluate life reduction using API 579 Level 2 FFS.

   
   

Stainless Steel Inspection Strategies

1. Apply penetrant testing (PT) and radiography (RT) to detect Cl-SCC.

2. Utilize eddy current or phased-array ultrasonic testing (PAUT) to identify pitting and wall loss.

3. Inspect insulation supports and terminations where water ingress and chloride buildup are common.

   
   

General Recommendations

• Implement risk-based inspection (RBI) to determine inspection intervals based on the likelihood of damage and its consequences.

• Conduct fitness-for-service (FFS) evaluations per API 579-1/ASME FFS-1 when wall loss or cracking exceeds code limits.

• Maintain insulation systems to prevent moisture ingress and avoid using chloride-bearing insulation on stainless equipment.

  
  

Acoustic Emission Testing in Pressure Vessel Inspection

Acoustic emission (AE) testing is an advanced, on-stream inspection method used to detect active damage in operating pressure vessels. AE is a passive nondestructive testing (NDT) technique that listens for high-frequency stress waves (20 kHz–1 MHz) generated by the material during crack propagation, hydrogen blistering, corrosion pitting, or leak development. Piezoelectric sensors mounted on the vessel surface detect these signals, which are then analyzed to locate and evaluate active damage zones.

AE testing is particularly valuable for API 510 inspection programs because it can be performed while the vessel is in service or during controlled pressurization. This eliminates the need for internal access or insulation removal. It is most effective for detecting HTHA, stress corrosion cracking, and incipient leaks. AE provides real-time, whole-vessel coverage and early detection of flaws at initiation, long before they become critical.

However, AE testing does not measure wall thickness or flaw dimensions, and background process noise must be filtered carefully. Therefore, it is primarily used as a screening tool to identify suspect areas for follow-up examination using ultrasonic, radiographic, or phased-array methods. When integrated with API 579 fitness-for-service evaluations, AE testing significantly enhances a facility’s capability to monitor vessel integrity under live operating conditions.

For high-risk equipment in hydrogen, high-pressure, or cyclic service, AE serves as a predictive maintenance tool within risk-based inspection programs. It provides early warnings of active degradation and supports proactive repair planning.

Conclusion

Pressure vessels operating at high temperatures and pressures under insulation face multiple interacting damage mechanisms. For carbon steel, the major threats include CUI, sulfidation, HTHA, creep, and thermal fatigue. For stainless steel, Cl-SCC, pitting, sensitization, sigma-phase embrittlement, and creep are predominant. Both materials are susceptible to thermal fatigue and hidden corrosion.

By integrating acoustic emission testing, risk-based inspection (RBI), and fitness-for-service (API 579) evaluations into the API 510 inspection program, operators can detect early signs of degradation, extend service life, and reduce unplanned shutdowns. A holistic approach that combines material knowledge, inspection technology, and data-driven risk assessment represents the most effective strategy for ensuring long-term reliability of pressure vessels.

References

1. American Petroleum Institute (API). API 510: Pressure Vessel Inspection Code – In-Service Inspection, Rating, Repair, and Alteration. 11th Edition, 2022.

2. American Petroleum Institute (API). API RP 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. 3rd Edition, 2020.

3. American Petroleum Institute (API). API 579-1 / ASME FFS-1: Fitness-for-Service. 2021.

4. American Petroleum Institute (API). API 941: Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants. 8th Edition, 2016.

5. NACE International. Corrosion Under Insulation and Fireproofing (CUI), SP0198-2017.

6. T. T. Allen, “Application of Acoustic Emission Testing in Pressure Vessel Integrity Assessment,” Journal of Pressure Vessel Technology, ASME, 2021.

7. TÜV SÜD, “On-Stream Inspection and Acoustic Emission Testing of Pressure Equipment,” Technical White Paper, 2020.

8. Inspectioneering Journal. “Corrosion Under Insulation and Risk-Based Inspection Planning.” Vol. 27, No. 4, 2022.