I fight this battle every year! It’s usually a learning experience for people unfamiliar with chemical processing and ASME B31.3. But I do my best to ensure they understand how the 10-minute Pressure Test is done, and then how the Leak Testing is done. So today I decided I need to “test” my practice that I have been doing for 30+ years; as I began to wonder “maybe I was taught wrong and I’m reading the RAGAGEP wrong. Although I hate Ai, I asked it…
Does ASME B31.3 leak test permit “pressure decay” over the 10-minute pressure test? This is what Google Ai (Gemini) stated:
ASME B31.3 requires leak testing by visual inspection during hydrostatic or pneumatic tests, not solely pressure decay, though decay methods (like helium sniffer) can supplement sensitive tests for Category M fluids or when water/pneumatic tests are impractical due to lining damage, contamination, or safety risks. The core test involves holding pressure (1.1x design for pneumatic, higher for hydro) for 10+ mins, reducing to design pressure, then visually checking joints for leaks; pressure decay monitoring is an alternative or supplementary tool, not a replacement for the primary code-mandated visual exam, especially for standard Category D/Normal fluid service where decay isn’t the main criteria.
Key ASME B31.3 Leak Test Principles
- Primary Method: 100% visual examination of joints after holding test pressure.
- Test Pressures:
- Hydrostatic: Higher than 1.5x design, adjusted for temperature/stress.
- Pneumatic: At least 1.1x design pressure, with lower limits.
- Hold Time: Minimum 10 minutes, extended for full examination.
- Pressure Decay Role: Used in Alternative Leak Tests (for difficult situations) or Sensitive Leak Tests (Category M).
Pressure Decay as an Alternative/Supplement
- When Used: When hydro/pneumatic tests are risky (brittle fracture, lining damage, contamination) or for high-integrity systems (Category M).
- Sensitive Leak Test (Category M): Pneumatic test at ≥ 105 kPa (15 psi) or 25% design pressure (lesser of) using gas bubble/sniffer methods.
- Decay Method: Calculates leak rate (QL) using formula
𝑄𝐿=(Δ𝑃⋅𝑉)/𝑡, where ΔP is pressure change, V is volume, t is time.
- Not a Substitute: Pressure decay monitoring alone doesn’t replace the required 100% visual inspection for standard tests.
Summary of Application
For most ASME B31.3 piping, you pressurize, hold, reduce pressure to design, then look for leaks. If water or high-risk pneumatic testing is impossible, or for Category M, you can use methods like gas pressure decay (sniffer) as allowed by the code, but it’s part of the alternative test, not the default replacement for the visual exam