Expeller performance has been evaluated in terms of the capability to create suction pressure at the throat. This formulation has been used to assess the effectiveness of evacuating combustible gases from an isolated, depressurized, pipeline section involving mainline block valves up to two times normal spacing with an intermediate vent stack. Additionally, the effects of elevation changes that promote buoyancy driven flows are accounted for in time as the interface between air and gas travels along the pipeline section during expelling. Two expelling strategies were introduced and assessed. These are simultaneous expelling, in which gas is expelled from the pipeline section from both ends, and sequential expelling, in which an intermediate vent stack is used to expel gas from the upstream and downstream segments. The effects of elevation changes and the location of the intermediate vent stack determine the best strategy for expelling so as to maximize the purge velocity in the section of a pipeline to be purged, while maintaining the mean flow velocity in the pipe above the minimum purge velocity to prevent air-gas stratification. It was found that for a ‘Flat-’ or a ‘Cusp-type’ elevation profile it is advantageous to follow a sequential expelling procedure using one expeller at the intermediate vent stack location. In the case of a ‘Vee-type’ elevation profile, a simultaneous expelling procedure is a better option in terms of expelling time, at the cost of needing to deploy two expellers to different sites quite far apart. Air ingress location depends on the expelling strategy and elevation profile.

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