Helium contamination through polymeric walls

The concentration of impurities in helium gas is an important parameter for a recovery and liquefaction plant. A low level of impurities is necessary to maintain an optimum liquefaction rate in any kind of liquefier. The main origin of the impurities is the air contamination that enters into the helium mainstream at some point in the recovery cycle. In this work we have: i) identified the main sources for impurities in an experimental helium recovery plant, ii) quantified the contamination rate and iii) proposed a mitigation strategy. An analysis of the He impurities composition reveals a nitrogen/oxygen ratio different to the one existing in air. This observation is in accordance with the permeability values for nitrogen and oxygen through the polymer materials used in the plant. Experimental on line measurements for oxygen content in the He mainstream with sensitivity below 1 ppm, have been performed after recirculation through metal and polymeric pipelines, respectively, to validate our hypothesis. In addition, the dependence of the impurities concentration with the He retention time in the recovery gas bag has been evaluated. Finally some operational recommendations are given for practical applications