[ oxygen ] through the kindling chain, which describes the progression of fire from easily ignited materials to those that are more resistant to ignition or have higher ignition temperatures.
Fig. 4. Impingement mechanism on a pipe bend
liquid separators. Conventional filter materials such as polypropylene, with an AIT of 174 ° C under the conditions outlined in ASTM G72,¹¹ and polyester, with an AIT of 181 ° C,¹¹ are found in the form of very thin fibres. Given that this filter media can have a diameter of 80 microns, the susceptibility of these to be the start of a kindling chain is significant. Therefore, alternative materials must be considered to esnure that a system is designed safely for an oxygen-enriched environment.
Ignition sources
The range of ignition sources that must be considered for an oxygen-enriched system is broader than in many other systems due to the increased potential for catastrophic events and the ease of ignition and combustion. An ignition source can trigger catastrophic combustion
In addition to conventional ignition sources such as open flames, the following must also be considered ¹:
• Heating by adiabatic compression, such as from rapidly opening a valve to pressurise a downstream system.
• Friction, either mechanical or caused of high gas velocities.
• Mechanical impact.
• Electrical sparks, from static or current electricity.
• High gas velocity with the presence of particles.
High gas velocities combined with particles can create an ignition source when the flow stream changes direction abruptly or when the presence of eddies leads to particles impinging on the system’ s walls( see Figure 4).
Material choices
When selecting materials for use in oxygen systems, it is important to choose those that are compatible with enriched oxygen and resistant to oxidation, corrosion, and potential combustion.
Table 4. Increase in burn rate at elevated temperatures ² Flammability of Metals by Temperature in Oxygen a, b, c
Material Identification
Test Pressure Test Temperature Burn Length
Burn Rate( in / s) [ mm / s ]
304L Stainless Steel |
500 psi( 3447 kPa) 500 psi( 3447 kPa) |
75 ° F( 297 K) 1000 ° F( 811 K) |
4”( 102 mm) 12”( 305 mm) |
0.35( 8.9) 0.43( 10.9) |
15-5 PH Stainless Steel |
500 psi( 3447 kPa) 500 psi( 3447 kPa) |
75 ° F( 297 K) 1000 ° F( 811 K) |
2.9”( 74 mm) 12”( 305 mm) |
0.33( 8.4) 0.35( 8.9) |
Inconel™ 718 |
750 psi( 5171 kPa) 750 psi( 5171 kPa) |
75 ° F( 297 K) 1700 ° F( 1200 K) |
4.2”( 107 mm) 12”( 305 mm) |
0.35( 8.9) 0.60( 15.2) |
316 Stainless Steel |
500 psi( 3447 kPa) 500 psi( 3447 kPa) |
75 ° F( 297 K) 1700 ° F( 1200 K) |
5.6”( 142 mm) 12”( 305 mm) |
0.34( 8.6) 0.37( 9.4) |
a
Data for temperature effect comparison only; not to be considered standard values for the listed materials. b
Data based on rods tested per ASTM G 124, with rod dimensions of 12” in length and 0.125” in diameter. c
Data derived from testing at NASA’ s George C. Marshall Space Flight Centre.
46 Hydrogen Tech World | Issue 21 | April 2025