DESCRIPTION

The invention relates to a multi-phase flow meter according to the preamble of claim 1.

To determine an exact mass flow rate of multiphasic fluids such as oil/water/gas mixtures in pipelines, volumetric flow as well as phase composition of the fluid needs to be determined. Volumetric flow can be detected by conventional flow rate detectors, e.g. by recording the pressure drop in a Venturi tube.

To measure phase composition, it is known to employ X-ray absorption, utilizing the fact that gas and liquid phases usually exhibit different absorption coefficients. By measuring X-ray or gamma absorption at at least two different wavelengths, the ratio of the individual phases can therefore be determined. One example for a flow meter based on this technique is known from the US 6 265 713 B 1.

One problem hampering exact flow rate measurements lies in the flow properties of multi-phase mixtures within pipes. In particular, the liquid phase tends to form a film along the pipe wall which moves with a speed different from the main flow. The presence of such liquid film can impede the determination of phase ratios and thereby lead to imprecise flow measurements.

It is therefore the objective of the present invention to provide a multi-phase flow meter according to the preamble of claim 1 which allows for improved measurement accuracy.

This objective is reached by a flow meter according to claim 1.

A multi-phase X-ray flow meter according to the invention comprises a first detection means for measuring a volumetric flow rate of a multi-phase fluid within a tubing section and a second detection means for determining an X-ray absorption of the fluid within the tubing section at at least two distinct wavelengths.

In order to avoid liquid film formation, a tubing section wall comprises a circumferential undercut located upstream of the first and second detection means. Liquid films formed upstream of the tubing section can be stripped from the wall at the undercut, so that the liquid phase rejoins the main fluid stream in the area of the tubing used for detection, thereby reducing inaccuracies due to incorrectly determined phase ratios within the flow. The fluid dead zone created by the undercut helps to prevent the reformation of the liquid film for a considerable section of the tubing, so that a film-free state over the whole detection area can be assured.

In a preferred embodiment of the invention, the undercut forms an acute-angled edge with a first tubing wall part upstream of the undercut. The presence of such a sharp edge facilitates film disruption and the formation of liquid droplets, which can be carried away by the main fluid stream.

It is further advantageous to provide a second tubing wall part upstream of the first tubing wall section which is inclined outwardly from the tubing center in a direction opposite to the flow direction. In other words, the tubing constricts itself upstream from the undercut, directing and concentrating the liquid film towards the edge and assisting the breakdown of the film.

In a further embodiment of the invention, the second tubing wall part is substantially parallel to a undercut wall part immediately extending from the edge. Such a geometry aids in film breakdown and assures a sufficiently long film-free tubing section for proper detection of the phase ratio.

In an alternative embodiment of the invention, the first tubing wall part is inclined outwardly from the tubing center at an angle less than the second tubing wall section. Droplets being freed from the film at the edge of the first tubing wall part in the direction of the flow are thereby directed towards the tubing center, prolonging the reattachment of droplets to the tubing wall and making the section usable for flow rate measurement longer.

In the following part, the invention and its preferred embodiments will be explained with reference to the drawings, which show in:

Fig. 1 a schematic perspective representation of tubing usable in an exemplary embodiment of a flow meter according to the invention;

Fig. 2 a close-up view of a tubing wall edge delimiting a detection section of the tubing according to Fig. 1 ;

Fig. 3 a cross-section view of the tubing wall edge according to Fig. 2;

Fig. 4 a cross-section view of a tubing wall edge with an alternative geometry.

To allow for the precise determination of mass flow within a pipeline transporting an oil/water/gas-mixture, tubing 10 is provided, consisting of a wide section 12 and a narrower section 14 downstream of the wide section 12 in the direction 16 of the fluid flow.

At the boundary between the sections 12, 14, the tubing wall 18 forms an undercut 20 and a sharp edge 22. Liquid film may occur at the walls of the section 12. Wall 18, wall 24, edge 22 and undercut 20 are aimed to detach the liquid film and the further break down. On reaching first the edge between wall 18 and 24 and then edge 22, this liquid film is disrupted and forms droplets which are carried away within the main fluid stream. The tubing wall 18 in the narrow section downstream of the edge 22 is therefore largely free of adhering liquids. This allows for a precise determination of fluid phase composition within the narrow section by means of X-ray absorption spectroscopy at two different wavelengths.

Fig., 3 shows the geometry of the tubing wall 18 surrounding the edge 22 in greater detail. The part of the tubing wall 18 forming the undercut 20 encloses an acute angle β with a first tubing wall part 24 running parallel to the main axis of the tubing 10. Upstream, the tubing wall part 24 is followed by a second tubing wall part 26, which runs roughly parallel to the undercut wall 28 immediately connected to the edge 22. and encloses an angle a with the first tubing wall part 24, which roughly equals β.

The inclination of the second tubing wall part 26 helps to direct the liquid film adhering to the tubing wall towards the center of the tubing 10. After said film breaks down at the edge 22, the undercut 20 prevents reformation of the film over a considerable length of the narrow section 14.

Fig..4 shows an alternative geometry of the edge part of the tubing 10. It differs from the embodiment shown in Fig. 3 in that the first tubing wall part 24 is not parallel to the tubing main axis, but rather inclined outwardly against the flow direction 16 by an angle γ, which is smaller than the angle a between the first 24 and the second tubing wall part 26. This imparts an additional momentum towards the tubing center on the droplets forming at the edge 22, thereby prolonging reattachment of said droplets to the tubing wall. LIST OF REFERENCE SIGNS

10 tubing

12 wide section

14 narrow section

16 flow direction

18 tubing wall

20 undercut

22 edge

24 tubing wall part

26 tubing wall part

28 undercut wall part