Venturi tubes are one of the most common types of flow measurement devices used worldwide for wet-gas conditions as they are simple, well-established technology, robust, and cost-effective flowmeters. They also form the main component in most of the commercial multiphase flowmeters.
An extensive research effort has been spent over the past 60 years to develop robust correlations for the use of Venturi flowmeters to correct the over-reading due to the unexpected or not the presence of liquid in the gas phase; this led to the writing of the ISO 2012. The uncertainty quoted for using this correlation is claimed to be between ±2.5% to ±3% and was based on extremely low uncertainties input parameters; in other words, the correlation was developed using data from flow laboratories with well-characterized fluids. However, in the field conditions, the input parameters uncertainty, which contributes to overall gas and liquid flow rate measurements, could be substantially higher.
When using Venturi flowmeters for wet-gas flows and looking at the actual performance based on the uncertainty of all input parameters, the uncertainties quoted are often from testing in flow measurement facilities with well-defined fluid properties, and they are always at line conditions or without any additional fluid properties uncertainties, however, in the field, the uncertainties from the meters can be substantially different and much higher.
The purpose of this paper is to highlight this statement to compensate for well-defined over/under-readings, and not forgetting that the uncertainty of the input parameters must be considered to state the correct performance. This is often overlooked.
Furthermore, the uncertainty from the correlation equations is used as the total uncertainty on the wet-gas flowmeter performance, but this over-simplifies other significant uncertainty contributions and can result in considerable measurement discrepancies. These discrepancies can critically affect the allocation and lead to potential disputes between different production platforms or more critical in custody transfer applications.
Wet-gas business is growing around the world, particularly in the Asia Pacific and the Middle East, where many installations are using such Venturi equipment and proper performance, or uncertainties should be stated. This paper will provide practical information and recommendations about meter performance.