Abstract: The basic characteristics of differential pressure signals and the limitation of flow rate metering methods based on differential pressure are studied analytically and experimentally. Through the investigation into differential pressure signals of orifice plate and V-cone, it is found that the two-phase differential pressure is mainly caused by gas phase and is slightly affected by the liquid phase. In the case of low liquid hold up, the frictional pressure drop can be decreased due to the presence of the thin liquid film on the wall, resulting in that the two-phase differential pressure, pressure loss and pressure loss ratio decrease at first and then increase as the liquid hold up increases. The fluctuation of the differential pressure is stochastic, causing the fluctuation characteristic parameters, such as the standard deviation, having the properties of poor stability and repeatability. This makes it difficult to be applied to industrial wet gas metering. The error propagation analysis demonstrates that the prediction error of liquid phase flow rate is much larger than that of the gas phase, which is a universal phenomenon of metering methods based on over-reading correlations. Finally, we recommend that for wet gas with high gas void fraction, the liquid flow rate should be measured independently. The non-monotonicity of the pressure loss ratio needs to be concerned if it is chosen as the characteristic parameter. Parameters with poor repeatability, such as standard deviation, are not recommended for wet gas metering.

Key words: differential pressure signal, error analysis, flow rate measurement, wet gas