Helium permeability in rubber rings during container leakage testing
WANG Pengyi, ZHUANG Dajie, CHEN Lei, SUN Hongchao, LI Guoqiang
RADIATION PROTECTION. 2023, 43(S1):
78-83.
Abstract
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The O-ring sealing rubber is usually used in container for radioactive material transport to achieve the containment of the contents and air-tightness requirements. Among the quantitative gas-leakage testing methods, the most commonly used one is gas filled envelope or evacuated envelope method by using helium as the tracer gas and helium mass spectrometer as the gas detector. During leakage testing, in addition to helium leakage caused by cracks or defects between the cap and the O-ring, helium will also permeate through the O-ring to the side with smaller pressure, thus interfering with the true leakage rate. The helium permeation process goes through three processes: dissolution, diffusion and desorption. The permeability of rubber is composed of dissolution and diffusion coefficient. Both of them can be affected by the type of gas, rubber and environmental temperature. Actually, the leakage rate caused by helium gas permeation is also affected by the pressure difference, exposed area, thickness, compression rate of rubber and the existence time of pressure difference, etc. This paper proposes a gas filled envelope method using helium mass spectrometer as gas detector, hoping to decrease the existence time of pressure difference, which can be used in quantitatively testing for sealed containers. Testing is divided into six steps: qualitative leakage detection of pipeline connection systems, background characterization before measurement, measuring instrument calibration before measurement, the actual leakage rate measurement, measuring instrument calibration after measurement and background characterization after measurement. Obvious helium permeation has been observed after helium is pumped into the envelope 8 minutes later during the delivery inspection of a container, and the leakage rate reaches 1.2×10-7 Pa·m3/s. After replacing the contaminated O-ring, this method is used to test the same container again. As a result, helium permeation is largely reduced and the leakage rate goes down to 7×10-10 Pa·m3/s.
