Performance of expellers in evacuating gas pipelines—Part I: Measurements, models and field verification

Abstract

This is a two-part paper. Part (I) presents results of measurements conducted on two different types of expellers of various sizes mounted on matching or mismatching size stacks, with equivalent flow resistance imposed by the gas pipeline section. Expeller performance in terms of its induction ratio has been found to be correlated to the system equivalent flow resistance coefficient in the form of a power-law. It was also found that the induction ratio deteriorates significantly as flow resistance is increased, and that manufacturer's specifications are typically given for full-size free-access inlet (i.e. with no flow resistance). In the field this is never the case, and hence the methodology adopted here in quantifying the performance characteristics is useful in practical applications. A model based on these characteristics has been developed for any asymmetric expelling system. Model results indicate that there is a stonewall limiting characteristic of the system beyond which, a larger size expeller does not induce higher suction flows. Two practical aspects are highlighted: (1) requirements for equal suction flows on both sides of the work-site and (2) time to complete one full sweep of pipeline gas with ambient fresh air. Effects of plug valves installed on blowdown stacks and effects of driving the expellers with natural gas are discussed in a companion paper (Part II).