PE and ETFE wire insulation flame morphologies and spread rates under subatmospheric pressures

Understanding the effects of pressure and oxygen concentration on the flame spread behaviors of electrical wires is crucial for establishing fire safety strategies for plateau cities, aircraft, and spacecraft. During the combustion, the polymeric coating first liquefies to form molten agglomerations, after which these droplets boil to release combustible gases that propagate the combustion process, which brings a potential fire threat. To better evaluate this fire risk, the flame spread of polyethylene (PE) and ethylene tetrafluoroethylene (ETFE)-insulated nichrome wires was investigated in a subatmospheric cabin. During the combustion of the insulation, the PE flame always presents a closed teardrop-like structure, while at the limiting oxygen concentration condition, the ETFE flame presents an asymmetric pentagram-like structure due to uneven heat losses. In this experiment, dripping was exhibited only by the PE insulation mainly because of the difference in surface tension. When the oxygen concentration was raised from 21% to 30%, the critical pressure for dripping shifts from 60 kPa to 38 kPa. Air pressure has a slight negative effect on flame spread rates, because, at lower pressure, the burning rate is lower, but the heat loss and convective and radiant heats of the flame decrease.

Effect of inert agents on the flammability parameters of selected gases and organic liquid vapours

Flammable substances may form explosive atmospheres when mixed with air. To prevent their formation or minimise the risk of their occurrence, it is necessary to understand the properties of the mixtures of flammable substances and to apprehend the properties characterising the course of a potential explosion. To minimise the risk of a fire or an explosion, a process called inerting is used in which, e.g. nitrogen plays the role of an inert agent. The article discusses the method for testing the flammability limits, the “bomb” method, in accordance with the European standard PN-EN 1839 and the limiting oxygen concentration (LOC) according to the European standard PN-EN 14756. The study shows the influence of inert gas on the flammability range of selected substances: hydrogen, methane, and hexane, which in practice allows the assessment of the explosion hazard of closed and open spaces, the establishment of safe working conditions, and the selection of equipment operating in given explosion hazard zones. The tests were carried out at 25 °C for hydrogen and methane and at 40 °C for hexane, at ambient pressure.