Abstract:
A device and method for conditioning media flowing within a conduit enabling sensors placed within short straight run distances to measure media flow with improved accuracy employing a thermal flow instrument. A flow conditioner downstream of a media flow measuring transducer has walls that diverge in the flow direction to optimize readings of the media flow from the transducer.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to devices that condition flowing media within a conduit, and more particularly, to such devices that condition flowing media to enable more accurate readings of thermal flow sensors used to measure the flowing media. 
     BACKGROUND OF THE INVENTION 
     When using thermal flow meters, the accuracy of mass flow rate measurements of media traveling within a conduit can be adversely affected by random and unpredictable turbulence within the flowing media, as well as by the fact that flow rates are not uniform across the conduit cross section. Flow meters typically employ high performance sensing elements that can be uniquely designed for application requirements with precision signal processing and calibration of sensing elements and transmitting electronics. Random turbulence, or non-uniform flow, within the flowing media introduces inconsistencies in the transducer readings. 
     Thermal dispersion flow meters are well known and are commonly used to measure the flow of media within a conduit. Thermal technology utilizes the relationship between flow rate and cooling effect for direct measurement of mass flow rate. The media flowing in a conduit affects the temperature of sensing elements and this effect is used to create an electrical signal that can be processed to indicate the flow rate or mass flow rate of the media within the conduit. 
     Flow conditioning devices may be used to overcome random turbulence properties that occur in applications where non-ideal upstream flow conditions exist. These turbulence properties may be caused by valves, bends, or elbows, for example, within the conduit, as well as flow rates of the media, and viscosity properties of different types of media. The elements employed by thermal dispersion mass flow meters can suffer accuracy problems due to non-ideal flow conditions in the vicinity of the sensing elements. Such non-ideal flow conditions that exist upstream from the sensing elements can create inaccuracies in the readings obtained from the sensing elements. 
     Within the art of flow meters, numerous flow conditioning devices have been taught. Examples of known flow conditioners are those that use bars, perforated plates, tube bundles, or tab structures to condition media to enhance sensor readings. 
     One turbulence inducing prior art device is disclosed in U.S. Pat. No. 5,780,737. This sensor employs a bar mounted closely upstream from a transducer for the purpose of flow conditioning. The bar generates a predictable vortex stream (turbulence) a short distance upstream of a flow sensing element to counteract the random or unpredictable turbulence that exists within the media flowing through a conduit. The vortex stream generated by the flow conditioning bar is consistent and predictable compared with the non-conditioned turbulence within the flowing media upstream of the bar. Thus, any existing random turbulence within the flowing media is essentially overridden by the turbulence created by the vortex generating bar. 
     A completely different type of flow conditioner is shown in U.S. Pat. No. 4,929,088, which includes several radially, or longitudinally, or both, spaced tabs to create a mixing effect as well as conditioning the flow of the media in the conduit. 
     In order to measure media flowing within a conduit by means of a thermal flow meter, minimum straight runs of the conduit are typically needed for improved accuracy. In order to achieve optimum performance in industrial flow metering systems, upstream and downstream straight run requirements are typically quoted at about 20 conduit diameters upstream and about 10 diameters downstream. These straight run lengths are typically necessary in order to create a consistent flow profile and allow dissipation of the turbulence in the media that may result from elements such as bends, elbows, and valves in the conduits carrying the media. Implementing straight runs of these lengths is not always easy and sometimes impossible to satisfy in any particular installation. Metering systems with insufficient straight run lengths can suffer somewhat degraded meter accuracy if a consistent flow profile is not able to be developed. 
     SUMMARY OF THE INVENTION 
     The embodiments disclosed herein address some shortcomings in the prior art of measuring flowing media within a conduit. The problems associated with the length of straight run requirements in industrial flow metering for about 20 diameters upstream are addressed by flow conditioners disclosed herein. Embodiments are disclosed for flow conditioning that can be accomplished in substantially shorter straight run lengths than required without using flow conditioners. The embodiments disclosed herein provide methods and systems for conditioning media that flows within a conduit to reduce the impact that less than ideal media flow profile has on transducers used to measure the flowing media, thereby enhancing the accuracy of the transducers. 
     An embodiment of the invention provides a flow conditioner that can be used with existing flow measurement systems. 
     Another embodiment provides a flow metering method that conditions media flowing within a conduit to provide flow velocities impinging the transducers which are consistent with the nominal media velocity in the conduit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The purposes, advantages, and features of the invention will be more fully understood from the following detailed description, when read in conjunction with the accompanying drawing wherein: 
         FIG. 1  is an end view of a conventional flow meter mounted to a fluid conduit segment; 
         FIG. 2  is a perspective illustration of an embodiment of a flow conditioner according to the invention; 
         FIG. 3  is an enlarged illustration of an embodiment of the shroud used in the  FIG. 2  flow conditioner; 
         FIG. 4  show the end cap, the thermowells, and the wedge shaped element of the flow conditioner of the present invention; 
         FIG. 5  is an end view of the flow conditioner of  FIG. 2 ; 
         FIG. 6  is a side view of the flow conditioner of  FIG. 2 ; 
         FIG. 7  illustrates a media velocity diagram for a flow metering system similar to the one in  FIG. 1  with an embodiment of a flow conditioner in accordance with the invention; 
         FIG. 8  illustrates a media velocity diagram for a flow metering system similar to the one in  FIG. 1 , without a flow conditioner of the present invention; 
         FIG. 9  is a plot of centerline versus straight run velocity in conduit diameters for media flowing in a conduit without the flow conditioner of the present invention; and 
         FIG. 10  is an end view, similar to  FIG. 5 , showing an alternative embodiment according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to various embodiments for flow conditioning, the media that flows within a conduit can be either a gaseous or a liquid substance, so the term “fluid” may be used to include both. Therefore, the embodiments described herein should not be seen as limiting and the scope of the invention should be measured as defined by the appended claims. These embodiments generally describe gaseous media flowing within a conduit, but it should be understood that liquid embodiments are also envisioned. Also, the metering systems illustrated in these embodiments are generally thermal flow meters to measure flow of media within a conduit using transducer elements and it should be understood that other measuring elements can also be used with the flow conditioners described herein. Any type of thermal flow metering system may be employed, including differential temperature, differential power, and differential current, for example. It is also envisioned that alterations may be made to the embodiments taught herein to condition the flow of liquid media within a conduit. 
     A conventional mass flow meter is illustrated in  FIG. 1 . Conduit  11  has T-connector  12  to which flow meter assembly  13  is mounted. The flow meter has head  14  typically containing the electronics of the instrument and having digital readout  15 . Probe  16  extends from head  14  into conduit  11 . At about the center of the conduit is sensor  17  comprising transducers or sensing elements  21  and  22 , which are protected in the media stream by protector  18 . The actual sensing elements reside within the cylindrical elements shown, which are referred to as thermowells. The thermal flow meter shown operates in a known manner. 
       FIGS. 2-6  illustrate an embodiment of flow conditioner and sensor  25  formed in a cylindrically shaped device having a longitudinal axis as indicated by centerline  30 . Sensors  26  and  27  are the sensing elements of the thermal flow metering system, similar to assembly  13  in  FIG. 1 , which measures flow of a media flowing within a conduit such as conduit  11  in  FIG. 1 . An example of such a metering system is model ST98, sold by Fluid Components International LLC of San Marcos, Calif. The metering system shown in  FIG. 2  includes a flow conditioner comprised preferably of wedge structure or element  31  and shroud  32  that conditions the flow of the media in the vicinity of flow sensing transducers  26  and  27  in order to provide enhanced mass flow readings of the flowing media. The transducers are typically thermowells containing thermal sensing elements. Shroud  32  is mounted on cap  33  at the distal end of probe  16 . The shroud surrounds wedge  31  and sensors  26  and  27 . The distal end of the metering system comprising sensors  26  and  27 , wedge element  31 , and shroud  32 , is inserted into a conduit (not shown in this figure) in a manner that allows media to continue to flow through the conduit. The sensor elements of the flow meter are at the distal end of the cylindrical probe  16  which project through the wall of the conduit in a conventional manner and are generally positioned at approximately the center of the conduit, as shown in  FIG. 1 . 
       FIG. 3  is an enlarged perspective view of shroud  32 . Shroud  32  and cap  33  ( FIG. 2 ) of cylindrical probe  16  may be secured together by any suitable means, such as by welding, or the use of an appropriate adhesive, among others. Either end  35  or  36  of the shroud may be secured to the cap since the shroud is shown here as being symmetrical. However, there is no requirement that the shroud be symmetrical. Cap  33  may be formed with a groove to receive an appropriately shaped annular rim of the shroud, or either or both the cap and the shroud may be shaped in an desired fashion to facilitate them being secured together. Input opening  37  is defined by peripheral rim  41 , while peripheral rim  42  defines output opening  38 . From this view it can be seen that openings  37 ,  38  are diametrically opposed and enable passage of flowing media therethrough, even with wedge element  31  and sensors  26 ,  27  projecting therein. The media flows through openings  37 ,  38  in the shroud in the direction of arrow A, as seen in  FIGS. 2 and 3 . Wedge element  31  is mounted within shroud  32  such that it is downstream of sensors  26 ,  27  in the direction of the flowing media. Alternatives to the shape of the shroud will be discussed below with respect to  FIG. 5 . 
     The sensor and flow conditioner  25  illustrated in  FIG. 2  could be combined with an industry standard ST98 flow meter, previously identified, with the added components of flow conditioner elements  31  and  32 . The ST98 instrument is an insertion style, thermal dispersion mass flow meter that measures the temperature difference between elements, here sensors  26 ,  27 . In the conventional manner for thermal flow instruments, one of the elements is a reference sensor and the other is an active, or heated, sensor. Flowing fluid dissipates heat from the active sensor and the differential temperature, in relation to the current or power applied to the active probe, for example, with the conventional electronics of such a system, converts these data points to a measure of mass flow of the fluid in the conduit. 
     Stated another way, shroud  32  and wedge element  31  are components that can be configured with an industry standard metering system, such as the ST98 thermal dispersion mass flow meter. This metering system is identified for example only and the flow conditioner of the invention can be employed with a variety of thermal flow measurement systems. 
     Wedge element  31  as shown in  FIGS. 2 ,  4 , and  5 , when viewed in the direction of longitudinal axis  30 , has a substantially V-shape with walls  31   a ,  31   b  arranged to diverge from a center or apex  45  in direction A of the flowing media. The apex of V-shaped element  31  is shown essentially equidistant from sensors  26 ,  27  and extends at least partially between the sensors (see  FIG. 5 ). The specific configuration and orientation of the embodiment shown in  FIG. 2  is illustrative of one embodiment and should not be viewed as limiting the invention. For example, the size and/or shape of wedge element  31  can be altered, and this will be addressed below. 
     In differing embodiments, diverging walls  31   a ,  31   b  can be arranged or positioned in various configurations. The diverging walls can be formed to be various heights either relative to the size and height of upstream opening  37 , to shroud  32 , or to the projecting lengths of thermowells  26 ,  27 . Height, as discussed here, relates to a distance in the longitudinal direction from cap  33  of probe  16 , and an embodiment is shown clearly in  FIG. 6 . It may also be referred to as “length.” Diverging walls  31   a ,  31   b  can be formed to have a height nearly as high as opening  37  in the direction of longitudinal axis  30 . In the particular embodiment shown, wedge  31  extends in length about 15% farther from cap  33  than do sensors  26 ,  27 . Thus, the length of wedge  31  may be about 1.91 cm (0.75 inch) and the sensors could be about 1.68 cm (0.66 inch) in a typical installation. However, the length could be shorter or longer than the sensors and still function effectively, and the sensors and wedge element can be longer or shorter, depending upon the size of the conduit, or upon the type of media being measured. 
     The media flowing in the direction of arrow A is forced to impact walls  31   a ,  31   b  and move around wedge element  31 , as shown graphically in  FIG. 7 . 
     One rationale for measuring the length of walls  31   a ,  31   b  in the direction of longitudinal axis  30  to be in relation to the height of sensors  26 ,  27 , is that the sensing or heated areas within the thermowells is normally somewhere near the linear center of the probe in the direction of longitudinal axis  30 . A good deal of variance could then be allowed within embodiments using thermal transducers in forming limits of the height of walls  31   a ,  31   b  in the direction of longitudinal axis  30 . These variances can be employed for embodiments using different media and in different circumstances to enhance the effectiveness of flow conditioner  25  in creating consistent flow velocities in the vicinity of sensors  26 ,  27 . 
     As shown in  FIGS. 5 and 7 , wedge element  31  is mounted downstream of sensors  26 ,  27  such that the flowing media will pass through opening  37  and encounter the sensors before impacting upon walls  31   a ,  31   b  of wedge element  31 . The presence of the diverging walls just downstream from the sensors dramatically reduces the turbulence of the media observed in the vicinity of and surrounding the sensors, compared to similar arrangements for such probes without wedge element  31 . This phenomenon will be further discussed below with respect to  FIG. 7 . 
       FIG. 8  illustrates velocity patterns that have been observed for a prior art flow metering system, such as that shown in  FIG. 1 , having thermal sensors  21 ,  22  but without flow conditioner  25 . The nominal flow at the center of the conduit, in this example, is about 7.62 meters (25 feet) per second. The legend for the velocity pattern is shown in the chart to the left of the velocity diagram, and the following describes the flow velocity with respect to the legend. LR is areas having a flow rate of about 10.51 meters (34.5 feet) per second; O is areas having a flow rate of about 9.36 meters (30.7 feet) per second; Y is areas with a flow rate of about 7.62-9.14 meters (25-30 feet) per second, that is, approximately nominal flow velocity; LG is areas with a flow rate of about 7.01 meters (23 feet) per second, near nominal flow velocity; G is areas with a flow rate of about 5.79 meters (19 feet) per second; LB is about 3.05 meters (10 feet) per second; and B is areas with a flow rate of about 1.22-2.44 meters (4 to about 8 feet) per second. As clearly evident from  FIG. 8 , the flow rates are much higher at a distance from sensors  21 ,  22 , and there is substantially no, or insignificant, 7.62 meters (25 feet) per second flow in contact with the sensor elements. In fact the flow rates in contact with sensors  21 ,  22  are not at all consistent with the nominal flow of 7.62 meters (25 feet) per second, and the average velocity of the media in contact with the sensors is well below the nominal flow rate. Immediately upstream of sensors  21 ,  22 , the velocities are shown as about 5.79 meters (19 feet) per second and immediately downstream of these sensors the velocities of the media are all the way down to 2.44-3.05 meters (8-10 feet) per second. The low rate between the sensors is actually higher than the nominal flow rate, but that flow area is not in contact with the sensors. The flow velocities in close vicinity of sensors  21 ,  22  are not truly representative of the nominal flow rate of the media in the conduit, which is the flow with respect to which the flow meter is intended to measure. Therefore, the mass flow rate detected by sensors  21 ,  22  may not fairly represent what is actually happening within the conduit. 
       FIG. 7  illustrates velocity patterns that have been observed for a flow metering system, such as that shown in  FIG. 2  having thermal sensors  26 ,  27 , except that in  FIG. 7  flow conditioner  25  is included with flow meter assembly  13 . 
     In this case the nominal flow rate is about 12.19 meters (40 feet) per second. The legend for the velocity pattern will now be described, with reference to the chart to the left of the velocity diagram. R is areas having a flow rate of about 12.19 meters (40 feet) per second or higher; LR is areas having a flow rate of about 10.06 meters (33 feet) per second; O is areas having a flow rate of about 8.53 meters (28 feet) per second; Y is areas with a flow rate of about 7.32 meters (24 feet) per second; LG is areas with a flow rate of about 6.10 meters (20 feet) per second; G is areas with a flow rate of about 4.88 meters (16 feet) per second; LB is areas with a flow rate of about 2.44-3.66 meters (8-12 feet) per second; and B is areas with a flow rate of between 0 and about 1.22 meters (4 feet) per second. It can be clearly seen from  FIG. 7  that the flow rates in the vicinity of sensors  26 ,  27  are much more consistent with the nominal flow rate upstream of flow metering system  13  than was observed for the flow in the metering system of  FIG. 8 . The flow rates in contact with sensors  26 ,  27  range from higher than 12.19 meters (40 feet) per second down to about 7.93 meters (26 feet) per second. Since both probes are significantly partially surrounded by media flow at or above about 12.19 meters (40 feet) per second, the readings of the flow meter will be much more accurate than is the meter of  FIG. 8 . These flow rates in the close vicinity of sensors  26 ,  27  are very much consistent with the nominal flow rate in the conduit. 
     In order to obtain the advantageous flow around the sensors that is depicted in  FIG. 7 , some exemplary dimensions are here set forth. The angle of apex  45  between the sides of wedge  31  can range from about 90°-140°, and preferably about 120°. The distance between walls  31   a ,  31   b  and respective sensors  26 ,  27  (gaps  51 ,  52  in  FIG. 5 ) is about 0.04-0.11 cm (0.015-0.045 inch), preferably about 0.08 cm (0.030 inch). From  FIG. 5  it can be seen that apex  45  is shown at about the centerline between sensors  26 ,  27 . For a 120° apex, this is preferred, but not exactly mandatory. For other apex angles, the position of the apex with respect to the centerline between the sensors will vary, it being preferable to maintain the distance from walls  31   a ,  31   b  to the sensors generally in the 0.08 cm (0.030 inch) range. 
     While the foregoing discussion relates to the symmetric arrangement of walls  31   a ,  31   b , apex  45 , and thermowells  26 ,  27 , those relationships are not required for embodiments of the invention to function in a useful manner. The relationships between one of the wedge walls and its adjacent thermowell is primarily relevant only for the active sensor. The wedge wall adjacent to the reference sensor is not nearly as important. Thus, the apex could be moved up or down (with respect to the  FIG. 5  orientation), as long as the venturi-like effect is maintained with respect to the sensor that is the active or heated one. Further, if the apex angle is increased to greater than about 120°, the apex might reside left of a tangent line from thermowell  26  to thermowell  27 . Conversely, with a sharper angle, less than about 120°, the apex could project beyond the right tangent line from the thermowells. 
     As can be seen from  FIG. 7 , a venturi-like effect is created as the media flows around sensors  26 ,  27  and is confined by wedge walls  31   a ,  31   b . This has the effect of at least partially surrounding the sensor element with the media being measured at or near a constant ratio of the nominal flow rate and with a reduced turbulence. By actual measurements, the flow velocity through gaps  51 ,  52  is a constant portion of the nominal flow rate, thus resulting in the average being near the nominal flow rate. 
       FIG. 9  shows how unpredictably varied the centerline velocity of flow is from about 5 to about 18 diameters after a disturbance source, such as a bend, elbow, or valve, for example. It is very easy to understand that a thermal flow transducer that is inserted into the conduit at any point less than about 20 diameters downstream from the cause of unstable flow can provide inaccurate readings. Certainly there is much greater heat dissipation in the active sensor element at three to five diameters, where the flow rate is about 16.46 meters (54 feet) per second while the nominal flow rate is 15.24 meters (50 feet) per second and settles in to that velocity at about 20 diameters. 
     Referring again to  FIGS. 2 and 7 , in conjunction with the conventional sensor in  FIG. 8  the improvements observed using a conventional flow meter with a flow conditioner comprising wedge element  31  and shroud  32  is a result of decreased turbulence of the flowing media in the vicinity of sensors  26 ,  27  after passing through opening  37  in shroud  32 . The diverging walls  31   a ,  31   b  of the wedge element forces flowing media to move around the wedge element and to reduce the media turbulence through gaps  51  and  52 . Flowing media is forced to move either above the wedge element, or through crevices  53 ,  54  between the distal ends of walls  31   a ,  31   b  and the sides of shroud  32 , after impacting walls  31   a ,  31   b , resulting in the average velocity of the flowing media in the vicinity of the sensors being maintained at about the nominal velocity and at a constant ratio of the velocity of the main stream. Because flow conditioner  25  makes the media flow around the thermowells predictable and consistent, turbulence of the flowing media around sensors  26 ,  27  is reduced, thereby enhancing the accuracy of the readings derived from the sensors. 
     It has been found that sides  55 ,  56  of the shroud, coupled with crevices  53 ,  54  ( FIG. 5 ) combine to increase flow around sensors  26 ,  27 . Although they can vary, it has been found that by making crevices  53 ,  54  to be about 0.03 cm (0.0120 inch), the meter has consistent and accurate output. Sides  55 ,  56  are shown to have an included arc of about 55°, and they could range between about 25° and about 75°. 
     In  FIG. 5  the shroud is shown with input opening  37  and output opening  38 . While the spacing between sides  55 ,  56  and the distal ends of the walls  31   a ,  31   b  is, in some embodiments, beneficial to the accuracy of the meter with which the flow conditioner functions, the shape and size of output opening  38  is not in itself significant. Once the media has encountered thermowells  26 ,  27  and walls  31   a ,  31   b , and crevices  53 ,  54 , it is not relevant how the media egresses from the flow conditioner. 
     As an alternative embodiment, the wedge element in combination with the sensors ( FIG. 4 ) provides improved mass flow readings, even without the shroud. This structure is a simplified and effective flow conditioner. The combination with shroud  32  ( FIGS. 2 and 5 ) provides even greater degrees of accuracy for the meter. From the physical standpoint, shroud  32  protects the rather delicate sensor elements from being damaged by handling, and by impurities and debris that may be flowing with the media in the conduit. Additionally, the shroud has a synergistic effect on the accuracy of the mass flow readings for the meter equipped with wedge  31 , because it affects the flow of media through the shroud. It has also been found that the shroud alone, without the wedge, improves the meter accuracy. This is yet another alternative embodiment. The flow conditioner, whether it employs the wedge only with the thermowells, the shroud only with the thermowells, or combines the wedge and the shroud with the thermowells, tends to moderate upstream disturbances and the otherwise resulting random turbulence which thereby caused increased heat transfer resulting in errors in the accuracy of the prior meters. 
     In some instances thermal flow metering systems employ a single sensor, which operates on a time share basis. That is, instead of having one heated, or active sensor, and one non heated, or reference, sensor, the single sensor switches between being the heated sensor and being the reference sensor. In such an embodiment, only a single diagonal wall would replace the wedge element and would be arranged in close proximity to the thermowell containing the sensor. That proximity is discussed in greater detail below. Such an embodiment is shown in  FIG. 10 . In this embodiment, the sensors in thermowells  26 ,  27  are combined into a single, time-shared sensor element. Only a single vane  31  having it wall  31   b  adjacent to the thermowell is required. When the sensor is the active, or heated, sensor, the media flow through gap  58  decreases the turbulence around at least a portion of the thermowell. As stated previously, the flow around the thermowell when it is functioning as the reference sensor is not of particular significance because it represents the ambient temperature of the media and is not a measure of thermal dissipation. Other than operating in a time shared manner, the  FIG. 10  embodiment functions in substantially the same way as the other embodiments presented herein. The angle of wall  31   b  with respect to the media flow direction A is about 45°-700°. 
     The sensors are mounted to and extend through cap  33  in a conventional manner. Wedge  31  may be mounted to cap  33  by any suitable means, such as welding, brazing, or through the use of a suitable adhesive. Similarly, shroud  32  is also secured to cap  33 . It is also possible to mold or machine the cap and shroud together or even the cap, shroud and wedge together. 
     It is contemplated that the media with which the structure of the embodiments of the invention that are shown and suggested here can be any type of fluid, whether a liquid or a gas. Further, while wedge  31  is shown having an open V-shape, it could be a filled in wedge, giving it a delta shape, or it could be arcuate, either convex or concave. In other words, the downstream shape of wedge  31  is generally not significant. It is the shape and position of walls  31   a ,  31   b , interacting primarily with thermowells  26 ,  27 , and shroud  32 , that provides the most advantageous function of the embodiments of the invention. While the wall or walls of wedge  31  are shown to be continuous, they could function as necessary for the embodiments contemplated even if they were formed as a screen, or with a plurality or a multiplicity of holes, or with slots. The media would flow over such non-continuous surfaces sufficiently to be efficacious.