Experimental studies on the ignition behavior of pure silane released into air

Silane is a well known pyrophoric gas which normally ignites upon contact with air. However, a silane release from a pressure source may not always lead to prompt ignition and frequently the ignition occurs when the release is shutoff. In a confined space, significant quantities of silane can accumulate prior to autoignition leading to an explosion, causing significant damage. To date, the mechanism and condition of pure silane ignition upon release into air has not been completely explained. Thus there is a need for additional experimental investigations covering a wide range of release conditions to enable a full understanding of silane ignition and explosion characteristics.This work presents a series of tests that aims to uncover the precise condition for pure silane ignition upon release into air. Tests were performed for releases at controlled and steady velocities. Steady flow of silane to a burn box and nitrogen to the desired vent stub are first established through mass flow controllers. An electrically controlled four-way switching valve is used to switch the silane and nitrogen flow such that steady silane flow without acceleration to the vent stub can be established. With careful control of vent conditions, we are able to find a reproducible critical exit velocity for prompt ignition of steady silane releases for different vent diameters. If the releases are reduced to below the critical exit velocity, prompt ignition of silane release is ensured. Above this critical exit velocity, silane can be released indefinitely into air without any ignition. The critical exit velocity is found to vary with the vent diameter. This relationship between the critical exit velocity and the vent diameter suggests that the silane release without prompt ignition was most likely caused by quench of the reactive kernel from the scalar dissipation between the release gas and the ambient air. Analysis of locations where prompt ignition occurred from the clips from high speed video camera found that the most reactive ignition kernel occurs in the range when the ratio of volumetric flow rate of entrained air to the silane flow reaches 0.322 ± 0.076, which is equivalent to the most reactive silane concentration of 75.6% in air. The implications from these results are discussed with emphasis on the safety of silane supply systems and operation practices.