Abstract:
A flow meter for measuring flow rate and flow composition of a process fluid includes a microwave source configured to generate a microwave signal. A probe tip is coupled to the microwave source and in near field proximity to the process fluid. The probe tip is configured to apply the microwave signal to the process fluid. A microwave detector coupled to the probe tip is configured to detect a near field microwave signal from the process fluid in response to the applied microwave signal. Flow calculation circuitry determines flow rate and/or composition of the process fluid as a function of the detected microwave signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to industrial process controlled monitoring systems. More particularly, the particular invention relates to measurement of flow of multiphase in industrial processes. 
         [0002]    Industrial processes are used in the manufacturing and refinement of various fluids or components. Examples include oil refining or distribution, paper pulp facilities, and others. In many instances, it is desirable to measure a flow rate of a process fluid. Various techniques are employed to measure flow rates including differential pressure drop across an orifice plate, vortex sensing techniques, magnetic based techniques and others. However, measurement of flow rate of global multiphase process fluids (process fluids which are not homogenous and may contain multiple different materials in more than one phase such as gas, liquid or solid) has been problematic. 
         [0003]    There is an ongoing need to provide measurement techniques for determining flow rate of a multiphase process fluid. 
       SUMMARY 
       [0004]    A flow meter for measuring flow rate of a process fluid includes a microwave source configured to generate a microwave signal. A probe tip is coupled to the microwave source and in near field proximity to the process fluid. The probe tip is configured to apply the microwave signal to the process fluid. A microwave detector coupled to the probe tip is configured to detect a near field microwave signal from the process fluid in response to the applied microwave signal. Flow calculation circuitry determines flow rate and/or composition of the process fluid as a function of the detected microwave signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a simplified diagram showing a system for controlling or monitoring an industrial process including a flow measurement device for measuring the flow rate and composition of a multiphase process fluid. 
           [0006]      FIG. 2  is a simplified diagram showing components of the flow device of  FIG. 1 . 
           [0007]      FIG. 3  is a simplified flow chart showing steps for use in determining the flow rate and composition of a multiphase process fluid. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Many industrial processes operate with multiphase process fluids. As discussed in the Background section, it is difficult to accurately measure flow rate of multiphase process fluids. One example of a process which utilizes a multiphase process fluid is oil production. In particular, production of oil using advanced recovery techniques may utilize a multiphase fluid. The current trend of raising demand and raising difficulties to increase the output from existing wells has led to the use of water or steam injection as a means to recover oil from wells which have a moderate production capacity. 
         [0009]    Oil production from such wells is characterized by a multiphase mixture of water, oil, natural gas and sand. To monitor the flow rate of such wells there are a number of multiphase flow measurement techniques using gamma ray, IR, microwave and ultrasonic sensors, often in combination with a mechanical separation system that creates a bulky and expensive installation. 
         [0010]    Radioactive sources are frequently not attractive due to their handling in production. Further, disposal at end-of-life creates a potential hazard. Optical/IR methods may suffer from the sensitivity of optical parts to a dirty environment. Ultrasonic sensors, while having attractive resolution properties, may suffer similarly from sand and dirt, leading to high losses. or high power requirements. While each of the methods also has their merits, microwaves may be used more effectively to characterize multiphase flows. 
         [0011]    There are existing efforts to characterize a multiphase flow by dielectric measurements using microwaves. The basis of this method is usually taken as the following “inverse” problem: given the (complex) permittivities ∈ 1,2,3  of the constituents and a shape characteristics of the mixture, how can the volume fractions be calculated from a measurement of the permittivity of the mixture ∈ mix . The latter is extracted either from a multi port measurement or the frequency shift or quality factor of a resonant mode (see, for example, http://www.agarcorp.com/, EP0495849B1, and WO2007109772A2). 
         [0012]    However, this approach faces a number of problems. One problem is that the permittivities of the constituents (e.g., water) is dispersive for microwaves and need to be compensated for temperature and minute differences in ionic content (salt, acids) that can be dissolved and which lead to severe losses when frequencies exceed 1 GHz as shown in Table 1: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Attentuation distance for 100 dB path loss in 
                   
                   
                   
               
               
                 water vs. frequency 
                 0.1 GHz 
                 0.1 GHz 
                 0.1 GHz 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Pure water (NTP) 
                 100 m 
                 1 m 
                 1 cm 
               
               
                 Sea water (NTP) 
                  10 cm 
                 5 cm 
                 1 cm 
               
               
                   
               
             
          
         
       
     
         [0013]    Even more challenging is the shape characteristics. There seem to be approximately as many theoretical expressions for the shape dependence (See, for example, Klaus Kupfer (ed.) Electromagnetic Aquametry”, Springer, 2005; J. B. Hasted, “Aqueous Dielectric”, Champman and Hall, 1973; and Ari Sihvola, “Electromagnetic mixing formulas and applications”, IEE, 1999.) as there are experiments. This may indicate the limits of measurement performance of such a system. 
         [0014]    Instead of attempting to find a suitable multiparameter fit of an ill-defined volumetric measurement, the present invention includes the use of a local statistical measurement to characterize the flow. Near field microwave microscopy can be used to determine the structure of dielectric materials at resolutions comparable to lateral extension of a transmission line (TL) tip that scans a surface area. For liquid applications, such instruments can be achieved with micromachined probe tips to cover a length typical for the minimal feature size in multiphase flow, for example, a few millimeters. Mechanical scanning is not necessary because the fluid flow itself moves thereby providing a “scanning” of the mixture as it moves by the probe. Further, the modulation properties of the reflected microwave signal can be used as a measure and discriminator of the local constituents of the flow, i.e. the passage of a certain droplet volume will induce a modulation of the field typical for size of the droplet. By analyzing the signature and statistics of this modulation (frequency, phase amplitude), the concentration and nature of the embedded features can be measured in approximately the same way as shot noise of an electric current can be used to determine the typical size of the charges that are transported. Thus, statistics of the dynamic changes at a local point are used to estimate volumetric information. With the present invention, a statistical sampling technique is used to determine flow rate by extrapolating a local measurement to thereby infer the flow rate and composition of the total volume of fluid. 
         [0015]      FIG. 1  is a simplified diagram of an industrial process  10  including a flowmeter or process device coupled to process piping  14 . Process piping  14  carries a process fluid  16  which flows therethrough in the direction indicated. Flowmeter  12  can be of any particular configuration or design and is configured to measure flow of fluid  16 , such as a multiphase fluid, through process piping  14 . In the example of  FIG. 1 , the flowmeter  12  is shown as being coupled to a remote control room  20  by a process control loop  22 . The process control loop  22  can be in accordance with any configuration such as a two-wire control loop based upon the HART® communication protocol, a 4-20 mA process control loop, a FieldBus based protocol, a wireless communication protocol, or others. 
         [0016]    Flowmeter  12  includes a near field microwave probe  30  arranged to interact in the near field with process fluid  16 . As discussed herein, near field microwave measurements are used to determine the flow rate of process fluid  16  through process piping  14 . 
         [0017]      FIG. 2  is a simplified block diagram of flow measurement device  12  illustrating one example configuration. In  FIG. 2 , flow measurement device  12  includes a microwave source  100  which is configured to generate microwave signals provided to directional coupler  102 . A coaxial cable  104  couples to a microwave probe tip  30  which is immersed in process fluid  16 . Probe  30  interacts with the process fluid  30  in the near field through process fluid  16  in the near field with respect to the frequency of the microwaves generated by microwave source  100 . The near field can be defined as measurements occurring within a distance which is much less than the wavelength of the microwaves. 
         [0018]    The microwave near field interaction with process fluid  16  provides a microwave reflection signal back through probe  30 , and coaxial cable  104  to directional coupler  102 . This reflection signal is related to the size, shape, consistency, permeability, area, volume, and other features of components in a multiphase process fluid such as fluid  16 . The “reflected signal” may be an electrical signal carried by probe  30  or may be an electrical characteristic of probe  30  which is measured with appropriate equipment. Directional coupler  102  provides the reflective signal to feedback circuitry  106 . Feedback circuitry  106  is used to control microwave source  100  in accordance with known techniques. An output  108  from feedback circuitry  106  is provided to an analog to digital converter  110 . The output  108  can comprise, for example, a voltage signal which is related to the near field reflection from the process fluid  16 . A microprocessor  112 , or other processing device, is configured to analyze the output from the analog to digital converter  110 . Microprocessor  112  operates in accordance with instructions stored in a memory  114 . The memory  114  can also be used for temporary or permanent storage of other data or information including configuration data. 
         [0019]    Microprocessor  112  performs a statistical analysis on the output from analog to digital converter  112 . For example, the frequency of peaks in the signal, the width of peaks in the output signal, the duration of peaks or other variations in the signal can be monitored and statistically analyzed. Statistics include average, median, variance, and spatial and temporal correlation coefficients etc. This data can be correlated with the flow rate of the process fluid  16 . 
         [0020]      FIG. 3  is a simplified flowchart showing steps in accordance with the present invention. These steps can be performed, for example, by device  12  shown above. The flowchart  150  begins at start block  152 . At block  154 , near field based flow data is obtained, for example, using the circuitry illustrated in  FIG. 2 . At block  156 , a statistical parameter is calculated from the near field based flow data. This statistical parameter is then correlated to a flow rate at block  158 . As a very simple example, a statistical parameter which identifies the average number of data peaks over a period of time can be used and correlated to a flow rate of the process fluid  16 . This correlation can be determined experimentally and can be performed using known techniques such as through a polynomial curve field. Although only a single statistical parameter is discussed above, multiple statistical parameters can be used in the determination of flow rate. The correlation between the statistical parameter and the flow rate can be determined using modeling or other mathematical techniques, or can be determined empirically by performing tests on a particular type of process fluid. Note that the relationship between a statistical parameter of flow rate may vary depending on the composition of the multiphase process fluid. Control is returned to start block  152  and the measurement procedure is repeated. 
         [0021]    One advantage of a near field probe is the ability to achieve a resolution that is suitable to characterize the features of the multi-phase flow, i.e. the ability to resolve the actual constituents at a given point in time. For some fluids, this cannot be achieved with the centimeter or larger wavelengths using the propagating modes used in current microwave multiphase flow sensors. While the propagating/resonant modes used typically require a certain geometry and/or metallic pipe walls, a near field probe is only dependent on the electromagnetic response in an area comparable to the probe lateral extension. The probe can be shaped to contain the complete circumference of the pipe, or several sensors may be used to obtain more accurate measurements. The local near field can be made significantly larger than the propagating field strength with the power restrictions applicable for industrial sensors. The use of several near field sensors may be used to measure typical relaxation effects that can further be used to characterize the fluid properties (e.g. Doppler of transmitted/reflected field as a function of polarizing field) giving additional information on the fluid at the point of measurement. This idea builds on the polarizability of the natural gas phase (methane), which has a measurable polarizability. An intense, pulsed electric field could therefore induce an electric dipole moment, which then decay and respond/reflect to microwave probing fields in an anisotropic pattern and with a certain Doppler shift, i.e. like a small moving microwave antenna. Electric and magnetic susceptibility is usually quenched in the liquid phase, where the molecular interactions dominate over external electromagnetic field effects. 
         [0022]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. In the discussion above, a microwave detector is used to measure the reflected near field signal. In the example of  FIG. 2 , this detector is formed by the directional coupler  102 , feedback circuit  106 , analog to digital converter  110 . However, the present invention is not limited to this particular type of near field microwave detector and other circuits and configurations may be employed. This embodiment is provided for exemplary purposes only. In the example of  FIG. 2 , the microprocessor provides a flow calculation circuit for use in determining the flow rate. The microwaves discussed herein can be of a frequency range between about 0.3 GHz and about 3 GHz. Although only a single probe is shown, the invention may be implemented with multiple probes or through other techniques.