CGA TR-7-2023
METHODOLOGY TO DETERMINE PIPING FAILURE RATES FOR NONTOXIC CRYOGENIC INDUSTRIAL GAS SUPPLY SYSTEMS WITH PIPE SIZE UP TO 6 INCHES
- Introduction
Background and Technological Evolution of Standard Development
Traditional industrial gas pipeline failure rate data mostly targets large systems, lacking applicable data for small-diameter (≤6 inches) cryogenic systems. CGA TR-7-2023, based on the UK HSE's HCRD database (1992-2015), establishes a universal failure rate formula applicable to components such as pipes, valves, and flanges by filtering small-sized, non-operational leakage events, and introduces the concept of equivalent length to facilitate rapid assessment of overall system risk.
Core Method: Failure Rate Formula Based on HCRD
The standard uses a logarithmic logistic distribution to fit the aperture distribution, as shown in the formula below:
AFR(d₁, d₂, D) = 7.3×10⁻⁵ × D⁻⁰·⁶ × [F(d₂) - F(d₁)], where F(d) = 1/(1+(d/1.5)⁻¹·⁵)
This formula is applicable to stainless steel pipes (300 series austenitic), where the design stress does not exceed 50% of the basic allowable stress. Considering the low-temperature environment (no corrosion, low stress, impact resistance), it is recommended to multiply by a reduction factor of 0.4.
Component Type Equivalent Length (m) Description Pipe or Pipe (per meter) 1 Welded joints included Static mechanical joints (each) 1 Threads, bolts, ferrules, etc. Valve (each) 2 Includes valve body seal and packing; high-frequency action (>96 times/8h) treated as dynamic seal Hose non-bending (per meter) 20 Stainless steel bellows, according to CGA P-82 Maintenance Dynamic Seal (each) 0.5×Cycles⁰·⁵ Cycles is the number of full strokes per year Disassembly and Assembly Connection (each) 0.02×Breaks Breaks is the number of disassembly and assembly connections per year Equivalent Length Method Practice
Total equivalent length of the system = the sum of the equivalent lengths of all components. For example: 10 meters of pipe + 2 valves + 5 flanges, total equivalent length = 10×1 + 2×2 + 5×1 = 19 meters. Then the system leakage frequency can be directly calculated using the failure rate formula, avoiding component-by-component iteration.
**Case Study:** A 5-meter flexible hose, fixed with flanges at both ends and without bends, has an equivalent length of 5 × 20 = 100 meters. If it is bent 10 times per day (3650 times per year), an additional dynamic sealing equivalent length of 0.5 × √3650 ≈ 30.2 meters is required, bringing the total equivalent length to approximately 130.2 meters. **Comparison with Other Methods:** Compared to tabular methods such as IOGP and NFPA 59A, this method continuously distributes orifice diameter and frequency, avoiding tabular step errors. It provides more reasonable predictions for small-diameter pipes (1/2 to 2 inches) and has moderate conservatism. Compared to the HSE method, this method provides richer data for extremely small orifice diameters (<1 mm), making it more suitable for near-retail locations.Implementation Recommendations
1. Scope of Application: 300 series stainless steel, design stress ≤ 50% of basic allowable stress, non-toxic cryogenic gases (such as liquid nitrogen, liquid argon, liquefied natural gas).
2. During risk analysis, after converting the system to an equivalent length, calculate the frequency of each aperture segment using the formula AFR(d₁,d₂,D), and assess individual risk in conjunction with the consequence model.
3. Design Optimization: Prioritize reducing the use of valves, dynamic seals, and hoses, or add protection (such as crash barriers) to reduce the equivalent length.
4. Note: A hose reduction factor of 0.1 requires actual crash protection measures to be applied.
