Abstract: Venturi tubes are widely used for wet gas metering. To explore the influence of longer throat Venturi tube pressure tappings position to wet gas measurement model, a theoretical analysis of Venturi pressure drop at various parts is presented at first. A Venturi prototype with longer throat is designed, which inner diameter is 50 mm and beta ratio is 0.4. The distances between pressure tappings and the entrance of throat are 0.5, 3.75 and 7 times the throat diameter, respectively. The experiment verifications are carried out at the wet gas test double closed loop in Tianjin University. The range of operational pressure is from 0.2 MPa to 0.8 MPa and the dryness ranging from 0.5 to 1. Based on the test data, the relationship between over-reading gradient and Lockhart-Martinelli parameter is acquired, as well as the regularity between the K value sensibility and liquid-gas mass ratio. The length of throat has a significant impact on over-reading and K value. Over-reading model and liquid-gas mass ratio model are built based on K value, Froude number and the gas-liquid density ratio, and gas and liquid flow rates are calculated by using iteration method. Through analyzing the influence of different pressure tappings position to the relative error and uncertainty of the gas and liquid flow rates, it is advised that 3.75 times the throat diameter away from the entrance of the throat is the optimum pressure tappings position, where the uncertainty of the gas and liquid flow rates are 5.06% and 2.15%, respectively.

Key words: wet gas metering;long throat Venturi tube;pressure tappings position;over-reading model;mass ratio of liquid and gas;uncertainty