MULTIPHASE FLOW METER

DESCRIPTION

The invention relates to a device for calibrating an X-ray based multiphase flow meter according to the preamble of claim 1 and to a method for calibrating an X-ray based multiphase flow meter according to the preamble of claim 8.

To determine the volumetric flow rate of a fluid through a pipe, Venturi tubes are commonly employed. Such a flow meter consists of a constriction within the pipe, which leads to a decrease of fluid pressure in the constricted part. The pressure differential between the constricted and open part is directly dependent on the volumetric flow rate. For well-defined fluids of known density, the mass flow rate can be immediately derived from the volumetric flow rate.

In many technical applications, particularly in petroleum and natural gas production, it is necessary to determine mass flow rates of ill-defined multiphase fluid mixtures, such as mixtures of natural gas and condensates or mixtures of crude oil, natural gas and water. Since the density of such mixtures is not known a priori and can furthermore fluctuate on short timescales as a function of mixture composition, a straightforward derivation of mass flow rates from measured volumetric flow rates is not possible.

In such cases, it is necessary to measure the density of composition of the fluid as well as its volumetric flow rate to accurately determine mass flow. This is usually accomplished by means of X-ray absorption, since, for example crude oil, natural gas and water have significantly different X-ray absorption spectra. Measuring the absorption of X-rays at at least two different wavelengths can therefore be used to quantify fluid composition and thereby density. A multiphase flow meter of this type is described in EP 1 286 140 B l .

To reach the desired accuracy of measurement, such devices need to be calibrated in regular intervals. This is usually done manually, which is a labor intensive process and often necessitates disruptions of the fluid flow and for these reasons incurs high costs.

It is therefore the object of the present invention to provide a device according to the preamble of claim 1 and a method according to the preamble of claim 8, which allow for a fast and non-disruptive calibration of X-ray based flow meters.

This object is achieved by a device according to claim 1 and a method according to claim 8.

Such a device for determining a mass flow rate of a multiphase fluid within a pipe comprises an X-ray source for providing X-rays at at least 2 different wavelengths and a corresponding X-ray detector arranged in such a way that a detection section of the pipe is placed within the optical path of the X-rays between the X-ray source and the X-ray detector.

According to the invention, a calibration chamber is located parallel to the detection section within the optical path of the X-rays.

This placement of the calibration chamber allows for easy on-line calibration of the device without interruption in the flow rate measurements.

It is particularly advantageous, if the calibration chamber is connected to the pipe via a first duct opening into an aperture of the pipe wall and comprising a first shut-off valve and a second duct opening into a sampling probe within the pipe's inner volume and comprising a second shut-off valve.

Opening the first shut-off valve while the second shut-off valve is closed allows gas exchange between the pipe and the sample calibration chamber, while no significant amount of liquids is transferred from the pipe to the chamber. After an equilibration period, the calibration chamber is therefore filled with the gaseous fraction of the multiphase fluid, allowing for an easy calibration of the detector with regard to the X- ray absorption coefficient of the gaseous fraction.

To collect the liquid fraction of the multiphase fluid, the first and second valve are both opened. The liquid fraction is collected by the sampling probe and streams into the calibration chamber via the second duct. Gas still contained within the chamber is replaced by the liquid fraction and flows back into the pipe via the first duct. As soon as the chamber is filled, the device can be calibrated with regard to the X-ray absorption coefficient of the pure liquid phase. In case of multiphase fluids with multiple unmixable liquid phases, e.g. oil-water-mixtures, the liquid phases can be separated by gravitational settling, so that the device can be separately calibrated with regard to all liquid phases present.

To achieve the desired separation, it is particularly advantageous to locate the opening of the second duct into the calibration chamber above the opening of the first duct.

To purge the sample chamber, it can be connected to the pipe by means of a third duct, which opens into a venturi section of the pipe and can be closed by means of a third shut-off valve. The lower static pressure in the venturi section creates suction towards the third duct. Purging is accomplished by opening the first and third shutoff- valve, thereby replacing all liquid contents of the chamber by the gaseous fraction. To ensure complete removal of the liquid phase from the chamber, the third duct preferentially opens into the bottom part of the chamber.

Connecting the chamber to the atmospheric environment via a 4 ^th shutoff-valve within the second duct allows for another measurement: If the 4 ^th shut-off valve is opened while the chamber is filled with liquid, the pressure drop will cause unstable condensates to evaporate, making it possible to determine the ratio of stable to unstable condensates in the liquid phase.

In order to acquire meaningful calibration results, it is furthermore of advantage to design the calibration chamber with the same cross-sectional shape and/or same wall thickness as the detection section of the pipe.

The invention further relates to a method for measuring a mass flow rate of a multiphase fluid. To determine the phase composition, the X-ray absorption of the fluid is measured, so that the composition can be calculated from known absorption coefficients of the pure phases. In order to calibrate such a flow meter, according to the invention a portion of the fluid is diverted to a calibration chamber located within the X- ray optical path in such a way that at least one pure phase is accumulated within the sample chamber. Subsequently, the X-ray absorption of said pure phase is measured for calibration purposes.

This allows for a quick, on-line determination and correction of measurement accuracy, as detailed above. The process can easily be automated and performed in regular intervals, so that no intervention is necessary to assure a constant quality of flow measurements.

To determine the X-ray absorption of the pure gaseous phase of the multiphase liquid within the scope of the invention, the calibration chamber is connected to the pipe via a first duct opening into an aperture of the pipe's wall. This allows for diffusion of the gaseous phase into the chamber without any sampling of the liquid phase.

The liquid phase can be collected to the calibration chamber by connecting the chamber to a sampling probe located within the pipe's inner volume. The sampling probe diverts part of the flow to the sampling chamber, where the liquid phase is retained, while the gaseous phase can flow back to the pipe via the first duct. After filling the calibration chamber, X-ray absorption of the pure liquid phase can be determined.

In case of the presence of multiple mutually insoluble liquid phases, such as in an oil-water-gas-mixture, the liquid phases can be separated within the calibration chamber by gravitational settling. Subsequently, the X-ray absorption of the liquid phases can be determined independently, preferably by using a matrix-type X-ray detector.

If the calibration chamber is connected to the surrounding atmosphere while filled with liquid, volatile components of the liquid evaporate. This can be used to determine the ration of stable and unstable condensates in the liquid phase.

After determining the X-ray absorption of the at least one liquid phase, the calibration chamber can be purged by connecting it to a venturi part of the pipe, so that the liquid is sucked back into the pipe due to the lower static pressure of the venturi part.

Further advantages, features and details of the invention appear from the following description of an embodiment as well as based on the drawings, which show in:

FIG 1 a schematic representation of an embodiment of a device according to the invention viewed from the front,

FIG 2 a schematic representation of an embodiment of a device according to the invention viewed from the top,

FIG 3 a schematic representation of an embodiment of a device according to the invention viewed from the left,

FIG 4 a schematic representation of an embodiment of a device according to the invention viewed from the right,

FIG 5 a schematic representation of thermal insulation of the calibration chamber and FIG 6 a schematic representation of an alternative design for thermally coupling the calibration chamber to the pipe.

A device 10 to determine the mass flow of a multiphase fluid within a pipe 12, the volumetric flow is determined by means of a constriction 14 in the pipe 12 acting as a Venturi device. By measuring the difference in static pressure between the constricted part 14 and an unconstructed part of the pipe 12, flow speed can be determined.

In order to calculate the mass flow from the volumetric flow, one needs to determine the density of the multiphase fluid. For known densities of the individual phases, this can be achieved by measuring the phase composition of the fluid. Since in many applications, as e.g. for crude oil/water/natural gas - mixtures, the X-ray absorption coefficients of the individual phases differ strongly, X-ray spectroscopy is a straightforward method to achieve this goal. .

To this end, an X-ray source 16 provides X-rays at at least two different energies, which permeate a detection section 18 of the pipe 12 and are detected by a corresponding X-ray detector 20 located opposite to the X-ray source 16.

To ensure a constant and high accuracy of measurements, the device 10 has to be calibrated in regular intervals. This is best achieved by measuring the X-ray absorption of pure phases of the multiphase mixture. To this purpose, a calibration chamber 22 is located parallel to the pipe 12 within the optical path 24 of the X-rays.

The calibration chamber 22 is connected to the pipe 12 by a first duct 26 opening into an aperture 28 of the pipe wall 30. The first duct can be closed by a first shut-off valve 32.

A second duct 34 with a second shutoff- valve 36 connects the calibration chamber 22 to a sampling probe 38 within the inner volume 40 of the pipe 12.

A third duct 42 with a third shut-off valve 44 further connects the bottom part 46 of the calibration chamber with the constricted section 14 of the pipe 12.

The second duct 34 is finally connectable to the surrounding atmosphere via a fourth shut-off valve 48.

To calibrate the device 10 when used for measuring the flow of a natural gas/condensate mixture, the first shut-off valve 32 is opened, while all other valves 36, 44, 48 stay closed. This allows for diffusion of the gaseous phase into the calibration chamber 22. After a certain amount of time, the chamber 22 is completely filled with the gaseous phase of the multiphase fluid, so that it is possible to measure it's X-ray absorption via the detector 20.

After the measurement is performed, shut-off valve 36 is opened. Now the liquid portion of the multiphase flow is collected via the sampling probe 38. Liquid entering the calibration chamber 22 forces the gaseous phase out via the first duct 26, so that the condensates accumulate within the calibration chamber 22 and can be analyzed by the X-ray detector 20.

In order to determine the ratio between stable and unstable condensates, the pressure within the calibration chamber 22 can be lowered by opening shut-off valve 48 and closing all other valves 32, 36, 44. The pressure drop causes the unstable condensates to evaporate so that only the stable condensates remain and can be spectroscopically analyzed.

Finally, the 4 ^th shut-off valve 48 is closed and the second and third shut-off valve 32, 44 are opened. The pressure differential between the aperture 28 and the constricted part 14 of the pipe 12 causes the liquid to be expelled from the chamber 22 via the third duct 42. The device is now ready to commence normal measurements and/or for another calibration run.

In case of fluids with multiple liquid phases, such as crude oil/natural gas/water - mixtures, the calibration process is slightly different,

In a first step, the calibration chamber 22 is filled with a sample of the fluid flowing through the pipe 12. To achieve this, shut-off valves 36 and 44 are opened, while valves 32 and 48 stay closed. Due to the pressure differential between the sampling probe 38 and the constricted part 14 of the pipe 12, a mixture of all phases of the fluid is sucked into the calibration chamber 22. Since the second duct 34 is connected to the top part and the third duct is connected to the bottom part of the calibration chamber, the gas content of the mixture in the calibration chamber 22 will be somewhat higher than the actual gas content in the multiphase fluid.

After filling the calibration chamber 22, valves 36 and 34 are closed and valve 32 is opened. During this phase, gravitational stratification of the multiphase mixture within the calibration chamber 22 occurs. The water phase collects at the bottom of chamber 22, followed by the oil and the gas phase.

If a matrix sensor 22 is used, the X-ray absorption for all three phases can now be measured simultaneously, thereby achieving the desired calibration.

To ensure meaningful calibration data, the calibration chamber 22 and its contents need to be held at approximately the same temperature as the multiphase fluid within the pipe 12. The detection section 18 and the calibration chamber 22 are therefore encased in a thermal insulation 50.

As shown in FIG 5, thermal sensors are in thermal contact with the pipe 12 and the calibration chamber 22 at multiple points 52. In case of a temperature difference, which would not only hamper the accuracy of calibration but also be conducive to precipitation of wax from the liquid phase, the calibration chamber 22 can be heated by means of heating elements 54. Further, heat transfer between the pipe 12 and the calibration chamber 22 is facilitated by a direct thermal contact 56.

FIG 6 shows an alternative design for ensuring thermal equilibrium between the fluid in the pipe 12 and the calibration chamber 22. In this embodiment, the wall of the calibration chamber 22 is thermally isolated from the pipe 12 by the thermal insulation 50. Equilibration of temperature is reached by connecting the top and bottom portions of the calibration chamber 22 by a duct loop 58 comprising a thermal contact portion 56.

After completely filling the calibration chamber 22, a piston 60 is retracted, thereby increasing the volume of the calibration chamber 22, which leads to evaporation of a small amount of hydrocarbons. The saturated vapor condenses in the thermal contact portion and flows back to the bottom part of the calibration vessel 22 at about the temperature of the fluid in pipe 12. Additional heating may be applied to prevent wax precipitation.

List of reference signs

10 device

12 pipe

14 constriction

16 X-ray source

18 detection section

20 X-ray detector

22 calibration chamber

24 optical path

26 first duct

28 aperture

30 wall

32 first cut-off valve

34 second duct

36 second cut-off valve

38 sampling probe

40 inner volume

42 third duct

44 third cut-off valve

46 bottom part

48 fourth cut-off valve

50 thermal insulation

52 points

54 heating elements

56 thermal contact portion

58 duct loop

60 piston