Development of a Wet Gas Flowmeter
Authors: Andreussi P. , Ciandri P. 
Published: October 2008

A full-scale prototype (2" nominal size, 120 bar design pressure) of the multiphase flowmeter VEGA, developed by TEA for wet gas metering, was thoroughly tested at Trecate field under conditions similar to those encountered in gas fields. These tests were preceded by a set of laboratory experiments carried out at low pressure with air-water mixtures. Field and laboratory tests covered a wide range of gas (30-120 m3/hr) and liquid (0-6 m3/hr) flowrates, with gas volume fractions (GVF) ranging from 88% to 100%. Only in one experiment the water-cut was appreciably different from zero (3%). Pressure was close to 30 bar in field tests and in the range 1-4 bar in laboratory experiments.

The experiments were conducted in two distinct phases. In the first phase VEGA was operated according to the autocalibration mode. This allowed a calibration equation for the multiphase orifice to be derived without using independent flowrate measurements as a reference. In the second phase a set of data was obtained in stand-alone mode. This mode of operation does not require operator intervention and should be considered the VEGA's standard operating mode. The obtained results show that the maximum difference between VEGA and the input flowrates is less than 6% in autocalibration mode and less than 10% in stand-alone mode. The water cut was determined with an accuracy of better than 0.2% (absolute). This result should be considered fortuitous as the estimated accuracy of water-cut measurements in the present VEGA set-up is 1%.

Considering that the uncertainty of field measurements of the gas and liquid flowrates at Trecate can be assumed to be around 3% after calibration, VEGA's accuracy in the investigated range of variables can be considered excellent. In particular, it was found that VEGA can be installed and calibrated in a gas field without any need for reference field measurements. It was also found that in stand-alone mode, VEGA is able to monitor large changes of the gas and liquid flowrates without resetting the flowmeter to new flow conditions.

Introduction

In recent years a number of multiphase flowmeters have been developed, mainly for oil field applications. These instruments are affected by sizable errors unless they are properly calibrated in situ using standard field measurements of the gas and liquid flowrates as a reference. NEL (National Engineering Laboratory, Aberdeen, UK) tested most of these instruments and reported that typical errors of commercial multiphase flowmeters are often above 20% of the actual flowrates. The errors become absolutely relevant when any of the commercial flowmeters are used for wet gas metering.

The problems encountered in the application of multiphase metering to wet gas have led TEA, with support from AGIP, to develop an entirely new flowmeter, VEGA (W?VEt GAs), whose operating principle is the isokinetic sampling of the gas-liquid mixture, followed by separation and metering of individual phases. The water cut can be determined by different methods (density or dielectric constant measurements) according to the accuracy required.