A CFD study of low pressure wet gas metering using slotted orifice meters

Wet gas metering is becoming an increasingly important problem to many industries, in particular the oil and gas industry. Extensive studies have been done in the past on Venturi and standard orifice differential pressure (DP) flow meters to tackle wet gas flow problems. However in recent years, the slotted orifice flow meter has been developed in the attempt to improve the performance of the standard orifice meter. The novel flow meter is shown to be insensitive to the upstream flow profile with lower head loss and faster pressure recovery. This paper describes the numerical studies to establish the effect of different geometrical perforations on the performance of the slotted orifice. Three sets of slotted orifices with varying aspect ratios (1.5≤l/w≤3.01.5≤l/w≤3.0), of rectangular perforations and one slotted orifice with a circular perforation and a ββ ratio of 0.40 are simulated in a 1.6 m horizontal pipe using the k–εk–ε turbulence model over a range of parameters, i.e.   gas volume fraction (GVF) and gas mass flow rate. The commercial CFD code, FLUENT 6.3 was used to model the wet gas flow. Simulation results revealed that the shape of the perforation has no effect on the differential pressure, However, a marginally better pressure recovery was observed with rectangular perforations of l/w=3.0l/w=3.0. The relatively higher over-reading values obtained in this work are consistent with the results of Geng et al. (2006) [1] that for a slotted orifice, a low ββ ratio is more sensitive to the liquid presence in the stream and hence is preferable for wet gas metering. Mass flow prediction by wet gas correlations showed that the homogeneous model, Steven’s and De Leeuw’s correlations had the best performance, with a calculated mean error of 4%–5%.

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► Pressure drop and recovery is better with the slotted orifice than the standard orifice.
► Rectangular slots improve static pressure recovery as the aspect ratio increases.
► The slotted orifice is more sensitive to wet gas flow compared to the standard orifice.
► A low ββ ratio is more sensitive to liquid presence, so is preferable for wet gas metering.
► The geometry of perforations does not seem to have any effect on meter over-reading.