Patent Publication Number: US-11391657-B2

Title: Tubular sensors for inline measurement of the properties of a fluid

Description:
RELATED APPLICATIONS 
     This application is a continuation of application U.S. Ser. No. 15/688,789, filed Aug. 28, 2017, which itself claims benefit of provisional application Ser. No. 62/379,953, filed on Aug. 26, 2016, which is incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND 
     Sensors for measuring the properties of a fluid in a manufacturing process are known. However, sensors that are placed in pipes carrying process fluids are particularly advantageous because they measure the relevant fluid properties—for example viscosity and density—at the point of application, and so better represent these properties at the point of application. Online measurements permit rapid adjustment of process parameters, enabling the operator to maintain process tolerances with minimal waste of material. 
     Among inline sensors, those that produce minimal obstruction to the flowing medium are particularly advantageous, from the standpoint of cleanability, and reduced tendency to trap particulate components of the fluid medium that could cause a blockage and also influence the operation of the sensor. Tubular sensors offer particular advantages in this respect, since they can be placed in series with process piping, without the need of bypass lines or special measurement chambers that introduce unwanted obstructions into the process line. 
     Inline tubular sensors are well known, of which Coriolis mass flow meters are perhaps the most widely employed. Coriolis meters use vibrating tubes to measure both mass flow and density. Of known Coriolis meters, a species thereof uses a straight tube vibrating transversely to make the desired measurements. Of straight-tube Coriolis meters, there are known methods for extracting information about the viscosity of the flowing medium, although this is generally considered a secondary measurement. 
     It is widely known that transverse vibrations in a straight tube are difficult to isolate from the means used to mount the tube in its supporting structure. Such supporting structures must be sufficiently rigid and massive such that the vibrations of the tube are not influenced by forces incurred from installing the sensor in the process pipeline. In the case of viscosity measurement, where it is necessary to measure the mechanical damping of the tubular resonator, any loss of energy through the mounting structure has a negative impact on the measurement of the viscosity of the fluid contained therein. 
     It is known that resonators vibrating torsionally are easier to decouple from their mounting structures because of the absence of the bending forces exerted on such structures by transversely vibrating resonators. Tubular resonators for measuring fluid properties are disclosed in U.S. Pat. Nos. 4,920,787 and 6,112,581. U.S. Pat. No. 6,112,581, in particular, uses a torsionally vibrating tube to measure viscosity, but is also vibrated transversely to measure density, which carries with it the disadvantages described above of transversely vibrating resonators. 
     SUMMARY 
     The present invention consists of a method for measuring fluid properties using a tubular resonator vibrating in torsion, which measures density and viscosity of a fluid contained within it, while providing minimal obstruction to the flow of the fluid. Although the method is described as measuring density and viscosity, it is also capable of measuring other fluid properties, such as flow rate, corrosion effects and tendency of the fluid to deposit materials on solid structures with which they are in contact. The invention therefore has additional applications in monitoring deposition of, for example, scale, hydrates, waxes, and asphaltenes in petroleum flow assurance applications. It is also applicable to measurement of corrosion in pipelines and other fluid conduits subject to corrosion by the media they conduct. 
     The present invention also encompasses a device to perform the method, the device also encompassing a number of species with related approaches to extending the measurement range and field of application of the device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an isometric sectional view of a sensor, according to the present invention. 
         FIG. 1B  is a sectional view of the resonator of  FIG. 1 , taken along line  1 B- 1 B of  FIG. 1A , with a portion of the outer casing not shown. 
         FIG. 2A  is an isometric sectional view of an alternative embodiment of a resonator, according to the present invention. 
         FIG. 2B  is a sectional view of the resonator of  FIG. 2A , taken along line  2 B- 2 B of  FIG. 2A . 
         FIG. 3A  is an isometric sectional view of another alternative embodiment of a resonator, according to the present invention. 
         FIG. 3B  is a sectional view of the resonator of  FIG. 3A , taken along line  3 B- 3 B of  FIG. 3A . 
         FIG. 4  is an isometric sectional view of the resonator of  FIG. 1A , showing further elements of a working resonator. 
         FIG. 5A  is a sectional view of the excitation and sensing transducer of  FIG. 4 , taken along line  5 A- 5 A of  FIG. 4 , but with other elements of the resonator of  FIG. 4  removed, for clarity of presentation. 
         FIG. 5B  is a sectional view of the resonator of  FIG. 4 , taken along a view line horizontally orthogonal to line  5 A- 5 A of  FIG. 4 . 
         FIG. 6  is a block diagram of an embodiment of a fluid sensor according to the present invention. 
         FIG. 7  is a graph showing resonance amplitude as a function of frequency, for both a high density and a low-density liquid with similar viscosities 
         FIG. 8  is a graph showing resonance amplitude as a function of frequency, for both a high viscosity and a low viscosity liquid with similar densities 
         FIG. 9  is a graph showing phase delay between a sinusoidal excitation and a resonant response, as a function of frequency, for both a high viscosity and a low viscosity liquid. 
     
    
    
     DETAILED DESCRIPTION 
     Definition: In the context of this application, a cylindrical volume is round in cross-section. 
     In broad overview, this application discloses several structures for tubular resonators that produce motion of the tube contents perpendicular to the surface of the tube when the tube is driven torsionally, to permit separation of the effects of fluid density and viscosity. In this way, the advantages of a purely torsional resonator can be gained while simultaneously providing an inline sensor that is sensitive to at least density and viscosity of the contained fluid. 
     Referring to  FIGS. 1A and 1B , an Inline resonator  10  consists of a tubular structure  12  mounted in a supporting casing  14  (a portion of the outer portion of casing  14  is not shown, in fact it goes all the way around), with the central section  16  of the tubular structure  12  being flattened in such a manner as to provide a noncylindrical, generally rectangular flow channel  16  with two arcuate sides (as shown in  FIG. 1B . 
     Resonator  10  includes an excitation and sensing transducer assembly (see  FIG. 4 ) for both exciting and sensing torsional vibrations of the tubular structure around its central lengthwise axis. Types of transducers include, but are not limited to, electromagnetic transducers and piezoelectric transducers and other combinations, for example an electromagnetic excitation transducer with an optical pickup. 
     The resonant vibrations of the tubular torsionally resonant structure  12  are modified by the fluid contained within it in two principal ways. As the tube vibrates torsionally, it shears the fluid in a thin boundary layer close to the wall of the tube  12 . The shear stresses produced by this shearing motion are proportional to the viscosity of the fluid and therefore extract energy from the vibrating tube at a rate dependent on the fluid&#39;s viscosity. 
     Furthermore, because the cross-section of tube  12  is flattened, torsional motion about the lengthwise axis produces a motion of the wall perpendicular to its own interior surface, causing apparent additional fluid mass to vibrate along with the tube  12 , the additional fluid mass being proportional to the fluid&#39;s density. The additional mass-loading, combined with the rotational inertia of the tube&#39;s vibrating section, decreases the torsional resonant frequency of the tubular resonator, in proportion to the density of the fluid. 
     In addition to providing means to shear and displace fluid within the resonant structure, resonator  10  includes inertial masses  18 , typically in the form of disks, and mounting fixtures  20 , also typically in the form of disks, affixed to the interior of casing  14  ( FIG. 1 ), which act to vibrationally isolate the resonator  10  from its environment, thereby minimizing the effects of mounting forces on the resonant properties of the resonator  10 . Inertial masses  18  are smaller in diameter than the mounting fixtures  20  so as not to contact the casing  14 . The inertial masses  18  create well defined nodes on the flow tube  12 , minimizing the torsional displacement of the tube  12  on the section of the tube  12  between the mounting fixtures  20  and the inertial masses  18 , resulting in a decoupling of the torsional vibrations of the flow tube  12  from said mounting fixtures  20 . 
     Two further species of resonators meeting the criteria of both shearing and displacing fluid during torsional motion are disclosed as embodiments of this method. It should be understood that these are merely exemplary of possible further embodiments. 
     Referring to  FIGS. 2A and 2B , a second method utilizes a cylindrical tubular resonator  110 , equipped with inertial masses  112  and mounting fixtures  114 , and also provided with fins  116  attached to the inner surface of the tube  118  and projecting substantially radially inwardly toward the rotational axis of the tube  118 . The fins  116  impart perpendicular motion to the contents of tube  118  necessary to produce mass loading by the fluid which modifies the resonant frequency proportionally to the fluid density.  FIG. 2A  shows an embodiment with four such fins  116 , although it is understood that any radially symmetric arrangement would serve an identical function.  FIG. 2B  shows a cross section through the central part of the tube  118  showing the substantially radial disposition of the fins  116 . 
     Radially symmetric fin patterns are used to avoid applying unbalanced transverse forces on the contents of tube  118  that could excite unwanted transverse vibrations. This precludes the use of a single radial fin  116 , although such radially asymmetric fin patterns could be used if such modes were desired. 
     A third embodiment  210  of the resonator, also fitted with inertial masses  212  and mounting fixtures  214 , extends at least two of the radial vanes to create a longitudinal wall or partition  216  through at least a portion of the tube  218 , as shown in  FIGS. 3A and 3B .  FIG. 3A  shows a longitudinal section through a tube  218  with a single longitudinal partition  216 , while  FIG. 3B  shows a cross sectional view through the center of the longitudinally partitioned tube  210 . 
     As shown in  FIG. 4 , the transducers consist of two magnet-coil assemblies  322  symmetrically disposed around the lengthwise plane parallel to the flattened tube surfaces  316 , and corresponding permanent magnets  324  fixed to surfaces  316 . In one embodiment, a first one of the transducers  322  may be used for excitation and a second one of the transducers  322  may be used for sensing. In an alternative preferred embodiment, both transducers  322  may be used for both sensing and excitation by alternately switching the coils between the excitation and the sensing circuit (not shown), as disclosed in U.S. Pat. Nos. 8,291,750 and 8,752,416. This is possible under the condition that the resonator  310  is always operated in such a manner that the vibration induced by the excitation persists long enough after cessation of the excitation signal to permit evaluation of the persistent signal for estimation of the fluid properties of interest. 
     The transducer arrangement shown in  FIG. 4  is one of many possible transducer arrangements, but is shown here as being particularly suitable for exciting and sensing torsional vibrations in the embodiments shown in this application. The operating principle is explained with the help of  FIG. 5A , in which the magnets  324  and coils  326  are shown isolated from the resonator  310  and supporting structures, for clarity of presentation. 
     The two coils  326 , disposed on either side of the lengthwise plane, carry currents I and I′ in opposite directions. The fields of the two magnets  324  bonded to the flattened tube  316  surface are parallel to one another. The resultant Lorentz forces, F and F′, produce matching torsional forces on the tube, as shown in  FIG. 5B , causing rotation. Conversely, a torsional motion of the flattened tube  316 , moves magnets  324 , thereby inducing currents in the two coils  326  proportional to the angular velocity of the tube  316  about its longitudinal axis. 
     Referring, now, to  FIG. 6 , in a preferred embodiment, current from coils  326  caused by movement of magnets  324  drives an analog-to-digital convertor circuit (A/D)  402 , at the input of a signal processing assembly  400 . The output of A/D  402  is fed into a memory  406 , that is at the input of a data processing assembly  405  and analyzed by a CPU  410  that operates in accordance with a computer program stored in a non-transitory program memory  408 . One output from the CPU  410  drives a digital-to-analog convertor circuit (D/A)  418  which drives driving circuitry  420 , which amplifies the signal, and which drives coils  326 . In an alternative embodiment, a first coil  326  drives the A/D convertor  402  and a second coil  326  is driven by the D/A convertor  418  and in turn drives facing magnet  324 . The CPU  410  could also be termed a controller and is part of the signal processing assembly  400 . 
     Referring to  FIGS. 7, 8 and 9 , the resonant properties of a resonator that interacts with a fluid such that it both shears and displaces the fluid are influenced by the resistance of the fluid to the shear and displacement. Fluid properties can be measured by varying the frequency at which the resonator is excited by the transducer and measuring the effect on phase delay and peak resonance. The frequency response of the resonator is influenced by mass loading in that its resonant frequency is lowered as the mass loading by the fluid increases. The displacement of the resonant peak is therefore a measure for the density of the fluid, the resonant peak displacement being roughly linear with the density of the fluid. This is shown by the resonance diagrams in  FIG. 7 . 
     Increasing viscosity of the fluid lowers and broadens the resonant peak, the broadening and lowering being roughly proportional to the square root of the product of the fluid&#39;s viscosity and density. The broadening and lowering of the peak are shown in  FIG. 8 . 
     Electronic means for measuring the damping and resonant frequency are known. A method that is particularly suited to the measurement of the resonant properties is disclosed, for example, in U.S. Pat. No. 8,291,750. In that method, a gated excitation signal excites the resonator at several phase values around its resonant frequency, and a gated phase locked loop measures the frequencies at which the phase values occur. From the frequencies and the phase values, the resonant frequency and width of the resonant peak may be calculated, from which calculated values a viscosity and a density may be derived. 
     The operation of this phase locked loop is shown in  FIG. 9 . The two lines at phases of 45° and 135° intersect the phase response curves at frequencies F1 and F2 for the lower viscosity fluid, and at F1′ and F2′ for the higher viscosity fluid. The phase locked loop alternately locks the resonator to, for example, 45° and 135° and measures the resulting frequency difference for the two-phase angles. The frequency difference F2−F1 is smaller than the frequency difference F2′−F1′, being approximately proportional to the square root of the product of density and viscosity of the fluid. Similarly, the value of the resonant frequency, for which the phase angle is 90°, can be determined by setting the phase locked loop to 90°, and measuring its resultant frequency, the frequency being a measure for the density of the fluid. From these two measurements, F(90°) and F2−F1, both the density-viscosity product and the density may be calculated, from which two quantities the dynamic viscosity can also be calculated. Furthermore, the density and dynamic viscosity may be used to calculate kinematic viscosity.