Frédéric Crayssac

Ignition of Contaminated Aluminum by Impact in Liquid Oxygen—Influence of Oxygen Purity

Impact tests in liquid oxygen and oxygen enriched liquids have been conducted on aluminum foils (0.2 mm thick) used for heat exchangers of air separation units (ASUs). Some adaptations to the ASTM Standard D2512, “Standard Test Method for Compatibility of Materials with Liquid Oxygen,” have been done to ensure the good control of oxygen content in the cryogenic liquid and to get reproducible results in aluminum ignition with hydrocarbon coating. Modified striker pins with a cavity on the contact surface have been used. The vertical guiding and the centering of the striker pins have been ensured by using two machined guides. Test samples were three layers of aluminum foil, two corrugated and one flat, contaminated by various quantities of hexadecane. The criteria for the identification of aluminum ignition (positive tests) have been validated by performing Scanning electron microscope surface analysis on residuals of a few impact tests performed in copper cups. Criteria have also been defined to check that hexadecane combustion took place in negative tests. A sufficient number of tests have been performed to plot the probability of ignition of the aluminum samples as a function of the average thickness of hexadecane coating and as a function of the oxygen content in the cryogenic liquid varying from 50 % to above 99.95 %. The remaining part in the cryogenic liquid was nitrogen and 2.3 % argon except for one series of tests where the influence of the absence of argon could be identified. The results of these tests can be used to assess the risk of ignition of some aluminum exchangers or structures used in ASU.