(98h) Numerical Simulation of Wet Gas Flow At Orifice Plate Meters

Orifice plate meters are one of the most widely used technologies in industry for gas flow metering, due to their simplicity. The measurement principle of an orifice plate meter is based on the pressure differential across the orifice plate due to the passage restriction. Their performances in single phase flow metering have been studied for many years, leading to extensive data sets and are well documented in the standards.

Wet gas metering is an important problem to many industries, in particular the oil and gas industry, and it is common for gas flow meters to be installed in applications where some liquid entrainment exist in a predominantly gas flow. The effect of wet gas flow on differential pressure meters is complicated and there are several on going research programs worldwide aiming to improve the understanding of the response of these meters to wet gas flow, as the accuracy of such meter systems is crucial in applications like production process control and fiscal metering.

Assuming this pressure differential to be caused only by the gaseous phase, orifice plate meters tend to over-read the true gas flow due the presence of the liquid. The uncorrected gas mass flow rate prediction is often called the apparent gas mass flow, and the over-reading is defined as the ratio of the apparent to the actual gas mass flow. The Lockhart-Martinelli parameter (X_LM) is commonly used to relate the liquid mass flow to the gas mass flow and several researchers concluded, from experimental data, that the over-reading is a function of the X_LM parameter and gas to liquid density ratio. Therefore, some correlations that have been proposed to correct the over-reading are based on it. 

The use of computational fluid dynamics (CFD) as a design tool is increasing as the computational power and available CFD codes efficiency rises. It allows obtaining details of hydrodynamics within the device which would be impossible to be obtained by experimental methods, giving a deeper insight into underlying physical mechanisms.

This work aims to develop a methodology to investigate the flow of wet gas across a standard orifice plate by using CFD and compare the obtained results with correlations available in literature. The two-fluid model, which is based on an Eulerian-Eulerian approach, is used to model the two-phase flow inside the device. It solves a set momentum and continuity equations for each phase and coupling is achieved through the pressure and interphase exchange coefficients. The gas is considered the continuous phase, following the ideal gas law, whereas the liquid is treated as the secondary phase, Newtonian and incompressible fluid. To deal with the turbulence influence, the RANS modeling is used, in which the Navier-Stokes equations are time averaged and the arising terms are modeled using the turbulent viscosity concept, according to Boussinesq hypothesis. The realizable κ-ε model, which is based on the modeling of the turbulent kinetic energy and turbulent dissipation rate, is used, with the enhanced version of the wall function treatment.

The pressure is measured 1 in upstream and downstream the plate. The predicted pressure drop is then compared to experimental data from NEL (National Engineering Laboratory) for a 4 inch diameter pipe for different liquid amounts (X_LM ranging from 0 to 0.3) at 15 bar. The over-reading is then compared and several correlations, as the Chisholm equation, are evaluated.