Previous promoted combustion tests have demonstrated that the threshold pressure (the minimum oxygen pressure required to self-sustain combustion) of various engineering alloys is dependent on the configuration of the sample. Monel 400, when configured as a 3.2-mm (0.13-in.) -diameter rod, has a threshold pressu re 500 times greater than when configured as a 60 × 60 wire mesh constructed of 0.19-mm (0.0075-in.) -diameter wire. Sintered filter elements, often used in oxygen systems, have finely divided construction (a high surface to volume ratio) similar to a wire mesh configuration. This similarity raises the question whether the threshold pressure data for the 3.2-mm (0.13-in.) -diameter rod, the wire mesh, or neither should be used for various alloys configured as sintered filter elements.
This study was conducted on three engineering alloys configured similar to sintered filter elements. Monel 400 powder, 316L stainless steel (SS316L) powder, and three types of tin-bronze powder were sintered into rods ranging in diameter from 3.2 to 6.3 mm (0.13 to .25 in.), with pore sizes representative of 3- to 150-μm-rated filters. Each sample was mounted vertically in a 12.4 liter (757 in.3) vessel and ignited at the bottom using an aluminum-palladium igniter wire. For each sample a threshold pressure was determined.
The test results indicated that the sintered Monel 400 and the sintered SS316L have threshold pressures closer to the wire mesh than 3.2-mm (0.13-in.) -diameter solid rods. As tested, the Monel had a threshold pressure of 0.69 MPa (100 psig) and the SS316L had a threshold pressure of White Sands Test Facility ambient, 82 kPa (12 psia). Except for two grades of A type sintered tin-bronze, all the sintered tinbronzes had threshold pressures above 68.9 MPa (10000 psig). Grade 153A and grade 103A had threshold pressures of 68.9 MPa (10000 psig) and 37.9 MPa (5500 psig), respectively. The results indicate that P or HP types of sintered tin-bronze filter elements are preferred for use in oxygen systems.