Helium Contamination Through Polymeric Walls

The concentration of impurities in helium gas is an important parameter for a recovery andliquefaction plant. A low level of impurities is necessary to maintain an optimum liquefaction rate inany 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) identifiedthe main sources forimpurities in anexperimental helium recovery plant, ii) quantified the contamination rate and iii) proposeda mitigation strategy.An analysis of the He impurities composition revealsa nitrogen/oxygenratio different to the one existing in air. This observation is in accordance with the permeability values for nitrogenand 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 polymericpipelines, respectively, to validate our hypothesis. In addition, the dependence of the impurities concentration with the Heretention time in the recovery gasbag has beenevaluated. Finally some operational recommendations are given for practical applications.