“Zero leakage” does not exist in engineering practice; what exists is “leakage below an acceptable rate.” Understanding leakage class standards for flanged connections helps set reasonable acceptance criteria based on service risk, avoiding both over-specification and safety gaps.
ISO 5208: Leakage Classes for Industrial Valves
Although primarily for valves, ISO 5208 leakage classes are often referenced for flange connections:
- Class A: No visible leakage under test pressure, but micro bubbles are allowed. Suitable for air or gas media.
- Class B: Allows a small amount of liquid leakage, typically expressed in mL/min.
- Class C: Allows higher leakage rates.
For gas service flanges, Class A is typically required.
TA-Luft (VDI 2440): German Clean Air Standard
For flanges handling volatile organic compounds or toxic gases, TA-Luft is one of the strictest fugitive emission standards. It specifies:
- Leakage rate: Under 1 bar differential pressure, leakage per meter of flange circumference ≤ 1×10⁻⁵ mbar·L/(s·m). This is an extremely微量 leakage, detectable only by helium mass spectrometry.
- Sealing system: Requires spring-energized bolts, high-quality gaskets (e.g., flexible graphite spiral wound with inner ring), and strict tightening procedures.
- Periodic testing: Leak detection required at specified intervals after commissioning.
EPA Method 21: US EPA Volatile Organic Compound Leak Detection
Method 21 defines procedures for detecting leaks from components such as flanges using portable flame ionization detectors or photoionization detectors:
- Detection position: Slowly move the probe around the entire circumference of the seal, at the gasket-to-flange interface.
- Concentration threshold: Depending on media type and facility type (e.g., refinery, chemical plant), a leak is defined as a detected concentration exceeding 500 ppmv or 10,000 ppmv, respectively.
- Repair timeline: Leaks must be repaired within a specified number of days.
Helium Mass Spectrometry Leak Testing: The Most Accurate Quantitative Method
For vacuum systems, nuclear facilities, or high-purity gas lines, helium mass spectrometry is the most common quantitative method:
- Vacuum method: Evacuate the tested flange assembly, spray helium externally, and measure the amount of helium entering the system.
- Sniffer method: Pressurize the system with helium, then scan externally with a sniffer probe.
- Detectable leakage rate: As low as 1×10⁻⁹ mbar·L/s.
Specifying Leakage Class in Procurement Documents
When procuring a flange assembly (flange + gasket + bolts), explicitly specify:
- Test medium: Air, nitrogen, helium, or water.
- Test pressure: Typically 1.1 times the maximum allowable working pressure.
- Allowable leakage rate: e.g., “≤ 10⁻⁵ mbar·L/s at 1 bar air pressure.”
- Detection method: e.g., “Helium mass spectrometry, vacuum method.”
For projects with environmental regulatory requirements (e.g., EU BREF documents or US LDAR programs), the supplier must provide performance proof of compliance with the corresponding fugitive emission leakage class.
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