Reassessing the Purge Valve Architecture in the Exploration Extravehicular Mobility Unit
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"Earlier designs of the xEMU purge valve architecture mimicked the design of the ISS/Shuttle EMU architecture with the use of two purge valves. When opened, both purge valves will allow suit gas to flow from the pressurized suit volume to the ambient environment. One valve provides a short duration, high flow rate emergency purge, as well as a pre EVA nitrogen purge, while the other provides a long duration, low flow rate emergency purge. The nitrogen present in the suit must be purged to increase the suit oxygen concentration to greater than 95% prior to initiating the pre-breathe protocol. Removing most of the nitrogen in the suit minimizes the onset of decompression sickness, while maintaining the critical partial pressure of oxygen in the suit necessary to support life at the EVA suit pressure of 4.3 psia. It is preferential for the nitrogen purging process to occur efficiently to minimize the duration of pre-EVA operations. The purge valves additionally provide emergency oxygen flow in the event of a failure. Either purge valve is used during emergency EVA operations when the ventilation loop is not providing conditioned oxygen or sufficiently removing carbon dioxide from the helmet. In the event of this scenario, the purge valve is activated, thus causing reduction in the suit pressure. The regulators respond by flowing oxygen to maintain the pressure set point. One potential goal of this assessment is to evaluate the possibility of eliminating the high flow purge valve if the low flow purge valve can adequately provide nitrogen and emergency purge functions. The architecture will be significantly simplified with a single purge valve. This paper will detail the purge routing design concepts considered and the methods used to analyze the suitability of potential designs, to ensure that the CO2 washout and nitrogen purge requirements are met."