Effectiveness of evacuating combustible gases by two parallel expellers closely coupled at one end of a gas pipeline

Expeller performance has been formulated in terms of its capability to create suction pressure at the throat. This formulation has been used to assess the effectiveness of evacuating combustible gases from a pipeline section from one end using dual expellers mounted in parallel on two adjacent blow-down stacks. A general formulation was derived to address any situation of asymmetry in the stack resistance, asymmetry in the expellers' power as well overall pipeline resistance to suction flow. Solutions of the closed-form equations were obtained and presented on performance graphs showing the ratio of the suction flow using dual expellers to that using either one in a single mode. It was found that there are conditions at which expelling with dual expellers exceed that of either expeller operating alone. It was also shown that when asymmetric expellers are used, where one expeller is more powerful than the other, the benefits of using two expellers is realized up to a limiting degree of asymmetry, beyond which the weaker expeller could be stalled and then reverse flow.