An outflow valve is disclosed for use in a pressure vessel. The outflow valve includes a dual wall valve housing having a central bore, a butterfly plate assembly and an angled drive shaft. The butterfly plate assembly is designed to fit and rotate within the central bore. The angled drive shaft has first and second ends rotateably coupled to the dual wall valve housing, and the angled drive shaft is attached to the butterfly plate. The drive shaft is designed to rotate the butterfly plate assembly between a closed position and an open position within the central bore.