On the performance of expellers to evacuate combustible gases from confined spaces

Effective and complete evacuation of any combustible substance is critical prior to commencing any cuts or repair work on any part of the containment system. This is normally performed by expellers driven by other higher pressure gases, typically air. Measurements and numerical CFD simulations were conducted on a typical commercial 8″ expeller mounted on a stack with a throttling orifice to control the suction resistance. The expeller performance was investigated in terms of its mass induction ratio, correlated to the system equivalent flow resistance coefficient and drive air pressure. The effect of jet-induced swirl at the throat of the expeller, the drive air pressure, and number and size of holes were also examined. The outcome is a general performance curve for the expeller in terms of its induction ratio as function of a dimensionless drive number (Nd) involving the number and size of holes, drive pressure to ambient pressure ratio and expeller throat area. It was shown that the induction ratio increases as Nd decreases. The developed performance curve helps in the design of an optimum expeller in terms of the number and size of holes and the drive air pressure to achieve best results.