Flammability Evaluation of Metals in Piping Configurations Under Flowing Conditions: Hollow-Vessel Promoted Ignition Test

摘要

The industry experienced burn-through and failure of large oxygen piping systems corresponding to geometry and flow pattern conditions very different from the ASTM G124 Promoted Ignition Test standard. In Air Liquide's hollow-vessel promoted ignition test, puncturing ignition is effected by a pyrotechnic promoter inserted into the wall of a hollow structure confining high-pressure gaseous oxygen. This is in strong contrast to a rod burning vertically in static atmosphere. The critical flow of oxygen through the breach both supplies efficiently oxygen to the peripheral burning front and blows out the formed oxides, so that combustion propagates with the breach diameter expanding. The facility can supply approximately 9000 Nm~3/h oxygen, maintaining the internal pressure at the breach, similar to the case of a real-life piping accident. The final extension of the breach is studied as a function of pressure from 2.8 to 40 barg (40 to 580 psi). Even at the lowest pressure, self-sustained radial propagation is unambiguously evidenced from initial diameter close to promoter size. However, the breach extension indicative of the severity of damages, is, on average, scaled to pressure, and, moreover, substantially larger for carbon steel. Also at highest pressure (30-40 barg), local flow patterns at the breach-like turbulence, recycling, and swirling of strongly heated oxygen tend to enhance metal combustion. It is further observed that increasing the burning metal wall thickness (e.g., from 5.5 mm to 12.5 mm) enhances the burning intensity and aggravates damages. Results give a different vision and understanding regarding the flammability of stainless steel compared to ASTM G124 standard where larger rod section is found to have a mitigating effect.