Abstract:
A device for determining a mass flow rate of a multiphase fluid within a pipe includes an X-ray source for providing X-rays at at least 2 different wavelengths and a corresponding X-ray detector arranged such 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. A calibration chamber is located parallel to the detection section within the optical path of the X-rays.

Description:
[0001]    This application is the National Stage of International Application No. PCT/RU2012/000317, filed Apr. 25, 2012. The entire contents of this document are hereby incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present embodiments relate to a device for calibrating an X-ray based multiphase flow meter and a method for calibrating an X-ray based multiphase flow meter. 
         [0003]    To determine the volumetric flow rate of a fluid through a pipe, Venturi tubes may be employed. Such a flow meter includes 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 may be immediately derived from the volumetric flow rate. 
         [0004]    In many technical applications (e.g., in petroleum and natural gas production), mass flow rates of ill-defined multiphase fluid mixtures (e.g., mixtures of natural gas and condensates or mixtures of crude oil, natural gas and water) are to be determined. Since the density of such mixtures is not known and may 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. 
         [0005]    In such cases, the density of composition of the fluid and a volumetric flow rate of the fluid are to be measured to accurately determine mass flow. This may be accomplished by 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 may therefore be used to quantify fluid composition and thereby density. A multiphase flow meter of this type is described in EP 1 286 140 B1. 
         [0006]    To reach the desired accuracy of measurement, such devices are to be calibrated in regular intervals. This may be done manually, which is a labor intensive process and may result in disruptions of the fluid flow. For these reasons, manual calibration incurs high costs. 
       SUMMARY AND DESCRIPTION 
       [0007]    The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary. 
         [0008]    The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, a device and a method that allow for a fast and non-disruptive calibration of X-ray based flow meters are provided. 
         [0009]    Such a device for determining a mass flow rate of a multiphase fluid within a pipe includes an X-ray source for providing X-rays at at least 2 different wavelengths and a corresponding X-ray detector arranged such that a detection section of the pipe is placed within the optical or projection path of the X-rays between the X-ray source and the X-ray detector. 
         [0010]    According to one or more of the present embodiments, a calibration chamber is located parallel to the detection section within the optical path of the X-rays. 
         [0011]    This placement of the calibration chamber allows for easy on-line calibration of the device without interruption in the flow rate measurements. 
         [0012]    The calibration chamber may be connected to the pipe via a first duct opening into an aperture of the pipe wall and a second duct opening into a sampling probe within the an inner volume of the pipe. The first duct includes a first shut-off valve, and the second duct includes a second shut-off valve. 
         [0013]    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. 
         [0014]    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 may 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 may be separated by gravitational settling, so that the device may be separately calibrated with regard to all liquid phases present. 
         [0015]    To achieve the desired separation, the opening of the second duct into the calibration chamber may be located above the opening of the first duct. 
         [0016]    To purge the sample chamber, the sample chamber may be connected to the pipe by a third duct that opens into a Venturi section of the pipe and may be closed by 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-valves, thereby replacing all liquid contents of the chamber by the gaseous fraction. To provide complete removal of the liquid phase from the chamber, the third duct may open into the bottom part of the chamber. 
         [0017]    Connecting the chamber to the atmospheric environment via a fourth shutoff-valve within the second duct allows for another measurement. If the fourth 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. 
         [0018]    In order to acquire meaningful calibration results, the calibration chamber may have the same cross-sectional shape and/or the same wall thickness as the detection section of the pipe. 
         [0019]    One or more of the present embodiments relate 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 may be calculated from known absorption coefficients of the pure phases. In order to calibrate such a flow meter, according to one or more of the present embodiments, a portion of the fluid is diverted to a calibration chamber located within the X-ray optical path such that at least one pure phase is accumulated within the sample chamber. Subsequently, the X-ray absorption of the pure phase is measured for calibration purposes. 
         [0020]    This allows for a quick, on-line determination and correction of measurement accuracy, as detailed above. The process may be automated and performed in regular intervals, so that no intervention is necessary to provide a constant quality of flow measurements. 
         [0021]    To determine the X-ray absorption of the pure gaseous phase of the multiphase liquid within the scope of one or more of the present embodiments, the calibration chamber is connected to the pipe via a first duct opening into an aperture of the wall of the pipe. This allows for diffusion of the gaseous phase into the chamber without any sampling of the liquid phase. 
         [0022]    The liquid phase may be collected in the calibration chamber by connecting the chamber to a sampling probe located within the inner volume of the pipe. The sampling probe diverts part of the flow to the sampling chamber, where the liquid phase is retained, while the gaseous phase may flow back to the pipe via the first duct. After filling the calibration chamber, X-ray absorption of the pure liquid phase may be determined. 
         [0023]    In case of the presence of multiple mutually insoluble liquid phases, such as in an oil-water-gas-mixture, the liquid phases may be separated within the calibration chamber by gravitational settling. Subsequently, the X-ray absorption of the liquid phases may be determined independently (e.g., by using a matrix-type X-ray detector). 
         [0024]    If the calibration chamber is connected to the surrounding atmosphere while filled with liquid, volatile components of the liquid evaporate. This may be used to determine the ration of stable and unstable condensates in the liquid phase. 
         [0025]    After determining the X-ray absorption of the at least one liquid phase, the calibration chamber may be purged by connecting calibration chamber 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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows a front view of an embodiment of a device; 
           [0027]      FIG. 2  shows a top view of an embodiment of a device; 
           [0028]      FIG. 3  shows a left side view of an embodiment of a device; 
           [0029]      FIG. 4  shows a right side view of an embodiment of a device; 
           [0030]      FIG. 5  shows thermal insulation of one embodiment of a calibration chamber; and 
           [0031]      FIG. 6  shows an alternative design for thermally coupling the calibration chamber to a pipe. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    A device  10  to determine a mass flow of a multiphase fluid within a pipe  12  is provided, as shown in  FIGS. 1-4 . A volumetric flow is determined by a constriction  14  in the pipe  12  acting as a Venturi device. By measuring a difference in static pressure between a constricted part  14  and an unconstructed part of the pipe  12 , flow speed may be determined. 
         [0033]    In order to calculate the mass flow from the volumetric flow, a density of the multiphase fluid is determined. For known densities of the individual phases, this may be achieved by measuring the phase composition of the fluid. Since in many applications (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. 
         [0034]    An X-ray source  16  provides X-rays at at least two different energies. The X-rays 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 . 
         [0035]    To provide a constant and high accuracy of measurements, the device  10  is to be calibrated in regular intervals. This is best achieved by measuring the X-ray absorption of pure phases of the multiphase mixture. For this purpose, a calibration chamber  22  is located parallel to the pipe  12  within the optical path  24  of the X-rays. 
         [0036]    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 may be closed by a first shut-off valve  32 . 
         [0037]    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 . 
         [0038]    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 . 
         [0039]    The second duct  34  is connectable to the surrounding atmosphere via a fourth shut-off valve  48 . 
         [0040]    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 an X-ray absorption of the multiphase fluid may be measured via the detector  20 . 
         [0041]    After the measurement is performed, shut-off valve  36  is opened. A 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 may be analyzed by the X-ray detector  20 . 
         [0042]    In order to determine the ratio between stable and unstable condensates, the pressure within the calibration chamber  22  may 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 may be spectroscopically analyzed. 
         [0043]    The fourth shut-off valve  48  is closed, and the second and third shut-off valves  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. 
         [0044]    In case of fluids with multiple liquid phases (e.g., crude oil/natural gas/water—mixtures), the calibration process is slightly different. 
         [0045]    In a first act, 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 a top part of the calibration chamber, and the third duct is connected to a 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. 
         [0046]    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. 
         [0047]    If a matrix sensor  22  is used, the X-ray absorption for all three phases may be measured simultaneously, thereby achieving the desired calibration. 
         [0048]    To provide meaningful calibration data, the calibration chamber  22  and contents of the calibration chamber  22  are 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 . 
         [0049]    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  may be heated by heating elements  54 . Further, heat transfer between the pipe  12  and the calibration chamber  22  is facilitated by a direct thermal contact  56 . 
         [0050]      FIG. 6  shows an alternative design for providing 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  including a thermal contact portion  56 . 
         [0051]    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. 
         [0052]    It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims can, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification. 
         [0053]    While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.