A very simple, low-cost gas-liquid flow meter that only employs conventional field instrumentation has been used to monitor severe slugging occurring at the exit of a vertical pipe. This meter was originally developed for conventional oil field applications [1] and is based on the readings of a multiphase orifice and the pressure drops of the gas-liquid mixture flowing in a vertical section of the pipe. Liquid and gas flow rates have been determined by means of semi-empirical equations developed for the specific set of flow parameters (geometry, flow rates, physical properties) adopted in a series of laboratory tests conducted in the Multiphase Flow Laboratory of TEA Sistemi. The transient behavior of the flow system, including the orifice, has also been predicted by means of a 1-D flow simulator [2]. The results of these simulations agree well with the experimental readings, thus providing a powerful method to monitor severe slugging by means of low cost instrumentation, in particular, by replacing a cumbersome instrument such as a gamma-densitometer with a differential pressure transmitter. In field operation, the multiphase orifice used in these experiments can be replaced by a calibrated control valve.
The accuracy of multi-phase flow meters (MPFMs) has been and still is disappointing. To some extent, this is due to the complexity of the flow system and to several mechanical or chemical effects, such as solids deposition, erosion and corrosion, that alter the flow conditions inside the meter. On the other hand, the complexity of the flow system would suggest the use of "as simple as possible" MPFMs, but often this is not the case. To give an example, a radioactive source is often deployed to measure the mixture density, but this requires a careful analysis of gamma ray attenuation through the pipe wall and the multiphase stream, introducing several potential errors, and definitely represents a cumbersome device [3]. At the same time, it only provides the measurement of the liquid hold-up, which is related to the non-slip liquid volume fraction by some type of empirical equation.
