NASA-LLIS-0571

Abstract

<strong>Description of Driving Event:</strong> <br>There was an inadvertent release of nitrogen tetroxide (N2O4) at JSC on April 21, 1994 at Approximately 11:20 am. The release occurred in the Thermochemical Test Area (TTA) during the preparation phase of a simulated altitude test of a small bi-propellant rocket engine. The immediate trigger event was the failure of a three-way solenoid valve. This failure enabled liquid oxidizer (N2O4) to flow in a vent line and the oxidizer burner stack (which was used as a control device to burn off small amounts of N2O4 which might leak or be released from the system). As the system was being charged to the desired operating pressure of 800 psig, the leak rate increased and the burner stack was filled with liquid oxidizer and saturated oxidizer vapor. The system was initially charged to 850 psig, instead of the desired pressure of 800 psig, and was vented to reduce the pressure. Venting resulted in the release of a small cloud of oxidizer from the burner stack. Although this was an indication of anomalous system operation it was mistakenly attributed to an improper setting on the methane supply valve for the N2O4 burner, and test was continued. Other indications that the system was not operating properly were available in the control room, but were not noticed or were misinterpreted. A subsequent attempt to raise system pressure to 800 psig for the test caused the release of a large oxidizer cloud and percolated liquid oxidizer from the burner stack onto the ground area surrounding the burner, resulting in a spill. <br>Subsequent to this incident, several fittings started leaking. Two fittings were successfully tightened, but attempts to tighten the third led to an increase in the leak rate. This fitting ( a mechanical Parkertype flareless, single ferrule fitting) was removed and sent to the materials laboratory for analysis. Leakage was attributed to the collapse of the ferrule component. The ferrule had clearly lost all of its functional integrity and separated into small fragments. All surfaces showed evidence of severe intergranular attack. Chemical analysis identified the ferrule as a 316-type stainless steel. Metallographic examination of a sample piece confirmed that gross intergranular corrosion attack of the ferrule material occurred. In addition, the examination suggested that the grain boundary <br>sensitization may have occurred in the material, increasing the susceptibility of the ferrule to intergranular corrosion. Examination of other fittings removed from the system also revealed at least slight amounts of intergranular corrosion. Discussions with the manufacturer indicated that at least some of the ferrules had been nitrided, a process which may require extended times at temperatures of from 480 degrees C to 590 degrees C. Since austenitic stainless steels becomes susceptible when subjected to temperatures from 480 degrees C to 815 degrees C, the subject ferrules may have inadvertently become susceecptible during the nitriding process.