New Perspective on the Behavior of Silane Leaks in Ventilated
Enclosures - Implications for the Design of Protection Measures
Antonio C. Braga - Factory Mutual Research Corp., Francesco Tamanini - Factory Mutual Research Corp. (SSA Journal Volume 11 Number 4 - Winter 1997 pp. 21 - 36 )
The design of safety measure for silane handling systems has been largely based on incomplete and, at times, inaccurate understanding of the behavior of this pyrophoric material. This has led, for example, to prescriptions requiring a ventilation velocity of 200 lfpm (1 m/s) across potential leakage points and access ports in gas cabinets and exhausted enclosures handling silane. Recent results obtained at Factory Mutual Research Corporation (FMRC), under partial support from SEMATECH, have made a significant contribution to the clarification of the behavior of silane releases. Through carefully controlled experiments in laboratory and intermediate-scale facilities, issues of ignition and rate of reaction (following ignition) have been addressed independently. With regard to ignition behavior, the main finding is that, contrary to prevailing current opinion, the ventilation velocity has no measurable effect on prompt ignition of the release. Ventilation flow, however, is important as it affects the concentration field developed by the leak and the rate of reaction following an ignition. The reactivity testing has determined that quiescent silane/air mixtures of concentrations between 1.4 and 4.1% (by volume) are explosive but stable. Mixtures above 4.5% on the other hand, are metastable and will undergo violent spontaneous ignition after delays ranging form 15 to 120 seconds. These research findings have been applied to the development of new loss prevention guidelines for use by FM consultants that are aimed at limiting equipment damage under all possible ignition scenarios. The new guidelines represent a departure from prescriptive design recommendations in favor of a performance-based approach. In practice, this means that ventilation requirements and the sizing of restrictive flow orifices (RFOs) are no longer set to fixed values but are dependent on system parameters.