Comparison of the two-phase mass flow coefficient and the over-reading for wet gas flow metering with DP meters

A DP meter with a wet gas correlation is a widely applied technology for metering wet gas flow. The over-reading (OR) is historically the most used parameter for establishing the wet gas correlation. Recently, we proposed a new parameter termed the two-phase mass flow coefficient (K) and whereby new wet gas correlations were developed. In the present study, the comparison of K and OR is conducted. By applying the separated flow model, the basic form of the wet gas correlation based on K is derived, and meanwhile the effect of influential parameters on K is theoretically analyzed. Experiments with an orifice plate meter indicate that, compared to OR, K increases with better linearity as the Lockhart-Martinelli parameter (X_LM) increases within the whole operating range and is more sensitive to the change of X_LM. Experimental results also show that increasing the gas-to-liquid density ratio (ρ_g/ρ_l) results in a decrease of K, while the gas densiometric Froude number (Fr_g) has a minor effect. These findings coincide with our previous studies on V-cone meters and Venturi meters. The function relationship between K and its influential parameters (including X_LM, ρ_g/ρ_l and Fr_g) is obtained through data fitting and then the specific form of the new wet gas correlation is derived. The relative error of gas mass flow rate predicted by the new correlation is within ±3.0% at the confidence level of 98.6%, superior to the existing correlations based on OR. This result is attributed to the good linearity and sensitivity of K to X_LM. Note that in our previous studies on V-cone meters and Venturi meters, correlations derived from K also perform better than those derived from OR. We can therefore conclude that the two-phase mass flow coefficient provides an alternative as well as better approach to correct the liquid-induced prediction error of gas flow rate in wet gas with DP meters.