Design service life of Class II pressure vessels

Analysis of the Code for Design Service Life of Class II Pressure Vessels

In the field of special equipment safety management, pressure vessels are pressure-bearing special equipment, and their design service life must be set strictly in accordance with technical specifications and safety standards. In accordance with the relevant provisions of the “Safety Technical Supervision Regulations for Fixed Pressure Vessels”(TSG21), combined with industry practice, the following professional instructions are given on the design service life of Class II pressure vessels:

1. Equipment classification and definition
According to the “Special Equipment Catalog”, pressure vessels are divided into three categories according to the degree of danger. Among them, the second-class pressure vessels mainly include:

1. Medium pressure vessel (design pressure 1.6MPa≤P<10.0MPa)
2. Low pressure vessel (design pressure 0.1MPa≤P<1.6MPa) and meets one of the following conditions:
   • The medium is flammable and the toxicity level is moderate
   • Contains mixed gas and the concentration of a component exceeds the limit
   • Design temperature below-40℃ or above 400℃

2. Specifications for design service life

1. Basic provisions
   The design service life of Class II pressure vessels is generally set to 10-15 years, and the specific value is comprehensively determined by the design unit based on the following elements:
   • Material property attenuation curve (e.g. creep strength of carbon steel and low-alloy steel)
   • Medium corrosion rate (average annual corrosion amount ≤0.1mm is the safety threshold)
   • Number of alternating load cycles (verified based on 10 design life)
2. influencing factors
   • Material selection: The use of corrosion-resistant materials such as chromium-molybdenum alloy steel can extend the design life by 30-50%
   • Manufacturing process: Heat treatment quality and welded joint coefficient (≥0.85) directly affect fatigue life
   • Use conditions: For every 20℃ increase in operating temperature, the allowable stress of the material will decrease by 10-15%
3. Adjustment under special operating conditions
   • Frequent start-up and shutdown of equipment: design life needs to be shortened by 20-30%
   • Strong corrosiveness of medium: Calculated based on actual corrosion rate, the minimum design life is ≥5 years
   • High temperature and high pressure conditions: Design life is determined using damage mode analysis (DMA) method

3. Service life management requirements

1. Regular inspection system
   • Comprehensive inspection cycle: Safety status level 1-2, once every 6 years; level 3, once every 3 years
   • Core inspection content:
     • Wall thickness measurement (the original wall thickness should be scrapped if the thinning amount is ≥30%)
     • Hardness test (HB value drops by more than 20% to be evaluated)
     • Metallographic analysis (if the grain size exceeds the standard, the inspection cycle should be shortened)
2. Security assessment process
   Containers that exceed the design service life must undergo a safety assessment by a professional agency, which includes:
   • Residual life prediction (using fracture mechanics analysis)
   • Risk level assessment (divided by failure probability and consequence severity)
   • Conditions for continued use (develop downgraded use plans or strengthen inspection measures)
3. Scrap disposal standards
   Any of the following circumstances shall be scrapped immediately:
   • Cracks in major compression components are found and cannot be repaired
   • The wall thickness corrosion reduction exceeds the design reserved corrosion allowance
   • The deformation exceeds the allowable value of GB/T 150 standard
   • Safety attachment fails and cannot be replaced

4. Life extension management specifications
Containers that have passed the safety assessment can apply for life extension and must meet:

1. Extension period: A single application shall not exceed 3 years, and the cumulative extension period shall not exceed 2 cycles
2. Technical requirements:
   • Add online monitoring devices (such as strain gauges, corrosion probes)
   • Implement a risk-based inspection (RBI) strategy
   • Formulate special operation and maintenance procedures (such as heating and cooling rate control)

Understanding the design service life specifications of Class II pressure vessels will help enterprises establish a scientific equipment management system. During use, the regular inspection system should be strictly observed, equipment technical files should be established, and full life cycle management should be implemented. For containers approaching the design service life, a safety assessment needs to be carried out in advance, and a life extension plan or update plan should be formulated based on the assessment results to ensure that the pressure-bearing equipment is always in a safe and controllable state.