Columbus IFHX Ammonia Leak Analysis

After the Columbus Moderate Temperature Loop Interface Heat Exchanger (MTL-IFHX) Close-Call Investigation, concerning possible freezing of the IFHX water on-orbit on GMT 345-2013, NASA investigated relevant transient scenarios involving IFHX rupture after freezing and consequent thawing. The result was a recommendation to develop a Fault Detection, Isolation and Recovery (FDIR) procedure that would close the Water On Off Valves (WOOVs) to prevent ammonia from the external Active Thermal Control System (ATCS) from spreading into the cabin. NASA performed preliminary simplified analysis for the reference case of IFHX rupture, but for a deeper understanding TAS developed detailed Sinda-Fluint models built and run through the SINAPS GUI of the Columbus ATCS bus. This allowed simulation of the physical variation of the ammonia due to environmental parameter variations, providing more accurate and specific input to the FDIR under development and in particular finalization in terms of IFHX WOOVs closure sequence, pressure profiles and wait times to contain the ammonia propagation to Columbus and properly identify the leaking IFHX. In addition the analyses results provided reference pressure profiles to be used on console and by engineering support in assessing the telemetry data in case of failure. This paper gives an overview on the issue described and focuses on the analytical aspects of the multi-phase fluid dynamics involved.