A research on rigid polyurethane foam resistibility to ignition from an electric arc welding spark

Introduction. A fire investigation assumes that electric arc welding sparks (hereinafter referred to as “sparks”) are effective sources of ignition. However, the spark ignition of a combustible material depends on the contact time. This work has experimentally proven that a smooth vertical wall, made of combustible rigid polyurethane foam (PUF), is not subjected to spark ignition. To explain this fact, the author calculated the time of contact between the spark and the wall τint, and compared it with the minimal estimated contact time τmin required to ignite the wall. The sample and the testing procedure. Sparks (the arc power up to 6 kW, the current up to 160 A) reached the PUF wall located at a distance of 0.1–0.15 m from the arc. An everyone experiment was continued until the electrode was burned down. Research results and discussion. Neither attempt to inflame the PUF sample by a spark was successful. Sparks bounced off the PUF sample and fell down. The characteristic dimensions of cooled iron droplets ranged from 0.2 to 3 mm. An evaluation of τint. The authors applied a model of elastic interaction between a drop of molten iron and a rigid wall, assuming that the sum of the potential energy of the drop, associated with its surface, and the kine­tic energy of a spreading drop, if flattened, is conserved. The longest contact time is achieved for drops, featuring the maximum diameter of 3 mm: τint ≈ 0.004 s. Estimation τmin. The authors applied the experimental modeling of the process, whereby the effect of an iron drop on PUF was replaced by the time-controlled effect, produced by a wooden cylinder (6 mm in diameter), inflamed at one edge. τmin ≈ 0.3 s. The ratio τint τmin explains the inability of electric arc welding sparks to ignite the PUF wall. Conclusions. Electric arc welding sparks, having a diameter of up to 3 mm, cannot ignite a vertical wall, made of rigid combustible polyurethane foam, in case of a side impact. The lack of ignition is explained by the short-term contact between the spark and the surface of the polyurethane foam.