Abstract:
A multimode flow meter can use both the time-of-transit of upstream and downstream ultrasonic signals and time for transmission of downstream-only signals to determine a flow velocity of a medium flowing through a conduit. Based on factors, such as previously computed flow velocity and signal-to-noise ratio of the upstream signal, a mode of operation may be switched and only the time for transmission of the downstream signals may be used to determine flow velocity. The multimode flow meter can computer cross-flow to reduce its effect on the determination of flow velocity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to the determination of flow velocity, including flow velocity of gas, liquid, or a multiphase medium flowing through a conduit. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ultrasonic flow meters are used to measure the flow velocity of a medium, such as gas, flowing through a conduit. A transit-time or time-of-flight ultrasonic flow meter uses the time of travel for both an ultrasonic upstream signal (defined to be substantially against the flow of the medium) and downstream ultrasonic signal (in the opposite direction as upstream) between the two transceivers to determine the flow velocity of the medium in the conduit. 
         [0003]    Downstream signals normally produce better signal-to-noise ratio (SNR) than upstream signals, especially at high flow velocities. That is, the upstream ultrasonic signal has lower signal-to-noise ratio (SNR) than the downstream ultrasonic signal, especially as the velocity of the flow of the medium increases. A downstream only flow meter uses downstream signals transmitted by two ultrasonic emitters to two receivers to measure the flow velocity of the medium. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to one aspect of the invention, a flow meter determines a flow velocity of a medium. The ultrasonic flow meter includes a first ultrasonic transceiver configured to transmit a first signal along a first transmission path, the first transmission path being downstream relative to a flow direction of the medium, and the first transmission path extending at a first angle from the flow direction of the medium; a second ultrasonic transceiver configured to receive the first signal transmitted by the first ultrasonic transceiver and to transmit a second signal along a second transmission path, the second transmission path being upstream relative to the flow direction of the medium, the second signal being transmitted to the first ultrasonic transceiver, and the second transmission path extending at the first angle from the flow direction of the medium; an ultrasonic emitter configured to transmit a third signal along a third transmission path, downstream of the flow direction of the medium, the third transmission path forming a second angle with the flow direction of the medium, wherein the first angle and the second angle are different; and an ultrasonic receiver configured to receive the third signal from the ultrasonic emitter. The flow velocity of the medium is calculated according to at least one of a first set of a first time of arrival of the first signal from the first ultrasonic transceiver to the second ultrasonic transceiver, a second time of arrival of the second signal from the second ultrasonic transceiver to the first ultrasonic transceiver, and a third time of arrival of the third signal from the ultrasonic emitter to the ultrasonic receiver, and a second set of the first time of arrival and the third time of arrival based on a selection of a mode of operation. 
         [0005]    According to another aspect of the invention, a system determines flow velocity of a medium. The system includes a first ultrasonic transceiver configured to transmit a first signal along a first transmission path, the first transmission path being downstream relative to a flow direction of the medium, and the first transmission path extending at a first angle from the flow direction of the medium; a second ultrasonic transceiver configured to receive the first signal transmitted by the first ultrasonic transceiver and to transmit a second signal along a second transmission path, the second transmission path being upstream relative to the flow direction of the medium, the second signal being transmitted to the first ultrasonic transceiver, and the second transmission path extending at the first angle from the flow direction of the medium; an ultrasonic emitter configured to transmit a third signal along a third transmission path, the third transmission path being downstream relative to the flow direction of the medium, and the third transmission path extending at a second angle from the flow direction of the medium, wherein the first angle and the second angle are different; an ultrasonic receiver configured to receive the third signal from the ultrasonic emitter; a calculator configured to determine the flow velocity of the medium according to at least one of a first mode of operation by using a first time of arrival of the first signal from the first ultrasonic transceiver to the second ultrasonic transceiver, a second time of arrival of the second signal from the second ultrasonic transceiver to the first ultrasonic transceiver, and a third time of arrival of the third signal from the ultrasonic emitter to the ultrasonic receiver, and a second mode of operation using the first time of arrival and the third time of arrival based on a selection of a mode of operation; and a mode selector configured to select the mode of operation. 
         [0006]    According to yet another aspect of the invention, a method determines flow velocity of a medium. The method includes transmitting a first signal along a first transmission path, the first transmission path being downstream relative to a flow direction of the medium, and the first transmission path extending at a first angle from the flow direction of the medium; transmitting a second signal along a second transmission path, the second transmission path being upstream relative to the flow direction of the medium, and the second transmission path extending at the first angle from the flow direction of the medium; transmitting a third signal along a third transmission path, the third transmission path being downstream relative to the flow direction of the medium, and the third transmission path extending at a second angle from the flow direction of the medium, wherein the first angle and the second angle are different; selecting a first mode of operation or a second mode of operation, the first mode of operation being a default mode of operation; and determining the flow velocity of the medium according to a first time of arrival of the first signal, a second time of arrival of the second signal, and a third time of arrival of the third signal when the first mode of operation is selected, and according to the first time of arrival and the third time of arrival when the second mode of operation is selected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]      FIG. 1  schematically illustrates an ultrasonic flow meter with two paths, according to an embodiment of the invention; and 
           [0008]      FIG. 2  is a block diagram of a multimode flow meter system according to an embodiment of the invention. 
       
    
    
       [0009]    The drawings are not necessarily to scale, emphasis instead generally being placed on illustrating the principles of the invention. Like numerals are used to indicate like parts throughout the various views. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0010]      FIG. 1  schematically illustrates an ultrasonic flow meter with two paths  110 ,  120  according to an embodiment of the invention. The path  110  is formed by a transceiver pair  111 ,  112  that transmit an upstream signal  113  and a downstream signal  114  to each other. The path of travel of the upstream and downstream signals  113 ,  114  forms an angle  115  with the direction of flow of the medium  105 . That is, the path of travel of the signals  113 ,  114  extends at an angle  115  from the direction of flow of the medium  105 . The path  120  is formed by an emitter  121  that transmits a downstream signal  124  to a receiver  122 . The path of travel of the downstream signal  124  forms an angle  125  with the direction of flow of the medium  105 . That is, the path of travel of the downstream signal  124  extends at an angle  125  from the direction of flow of the medium  105 . Based on the angles  115 ,  125  corresponding to the two paths  110 ,  120 , the path lengths of the signals  113 ,  114  associated with path  110  are shorter than the path length of the signal  124  associated with path  120 . Accordingly, path  110  is chosen as the path in which to include the upstream signal  113 , because the shorter path length of path  110  results in a higher SNR for the upstream signal  113  than if an upstream signal were part of path  120 . 
         [0011]    The paths  110 ,  120 , as showing  FIG. 1 , are formed by transceivers  111 ,  112 , an emitter  121 , and a receiver  122  that are wetted transducers, which penetrate the conduit  101 , as indicated by the dashed lines denoting paths  110  and  120 , unlike clamp-on transducers. While clamp-on transducers are also contemplated to form the paths  110 ,  120 , wetted transducers can provide a more accurate indication of the flow velocity of the medium  105  by eliminating relative movement between the paths  110 ,  120  (changes in the angles  115 ,  125 ) during installation. The exemplary transceivers  111 ,  112  and emitter  121  may be driven by a 4-cycle square wave centered at, for example, 100 kHz with an amplitude of 200V peak-to-peak. 
         [0012]    Flow velocity of the medium  105  can be determined from the time (t 313 ) of transit of the upstream signal  113  and time of transit (t 114 ) of the downstream signal  114  in the following way: 
         [0000]        V =( P 1 2 /2* L 1)*(( t   113   −t   114 )/( t   113   −t   114 ))   [EQ 1]
 
         [0000]    where 
         [0013]    V=flow velocity of the medium  105 , 
         [0014]    P 1 =path length of the upstream and downstream signals  113 ,  114   
         [0015]    L 1 =path length, P 1 , projected along the axial direction of the conduit  101   
         [0016]    Flow velocity of the medium  105  can also be determined from the time (t 114 , t 124 ) of transit of each of the downstream signals  114 ,  124  in the following way: 
         [0000]        V= (( P 1/ t   114 )−( P 2/ t   124 ))/(cos(115)−cos(125))   [EQ 2]
 
         [0000]    and 
         [0000]        c =((( P 1/ t   114 )*cos(125))−( P 2/ t   124 )*cos(115)))/(cos(125)−cos(115))   [EQ 3]
 
         [0000]    where 
         [0017]    V=flow velocity of the medium  105 , 
         [0018]    c=speed of sound through the medium  105 , 
         [0019]    P 1 , P 2 =path length of the downstream signals  114 ,  124 , respectively 
         [0020]    L 1 , L 2  =path length, P 1  and P 2 , respectively, projected along the axial direction of the conduit  101   
         [0021]    As indicated by the denominators of EQ 2 and EQ 3 above, the angles  115  and  125  of the two paths  110 ,  120  with the cross-sectional line of the conduit  101  cannot be the same (denominator of EQ 2 and EQ 3 would be 0). 
         [0022]    When used together, the two paths  110 ,  120  allow both transit-time and downstream-only determination of flow velocity. As such, the combination can increase turn down ratio (range of measurement) and accuracy of the computed flow velocity value. When both paths  110 ,  120  are fully used (upstream signal  113  and downstream signals  114 ,  124 ), the velocity values determined by each path  110 ,  120  are averaged to increase accuracy of the flow velocity output. When both paths  110 ,  120  are fully used but the upstream signal  113  is diminishing (SNR decreasing), then the velocity values determined by using the upstream  113  and downstream  114  signals and by using the downstream only signals  114 ,  124  act as a cross-check. The combination of the paths  110 ,  120  also allows computation and mitigation of cross-flow, which cannot be computed by a transit-time flow meter or downstream-only flow meters alone. 
         [0023]    Cross-flow is circulating flow (rather than strictly axial flow) of the medium  105 . Cross-flow may be caused by a thermal effect, for example, which causes stratification of the medium  105 . That is, one side (the bottom, for example) of the conduit  101  may be hotter than other parts of the conduit  101 , thereby creating a thermal effect that causes circulating flow of the medium  105  in addition to axial flow. This circulating flow can interfere with the time of transit of an ultrasonic signal through the medium  105  (t 113 , t 114 , t 124 ) and thereby reduce the accuracy of the computed flow velocity (V). Because using a combination of the flow meters  110 ,  120  provides three different time measurements (t 113 , t 114 , t 124 ) and three equations with two unknowns (V, c), the combined flow meters  110 ,  120  can together be used to compute cross-flow (W) as a third unknown. 
         [0024]    Specifically, by employing both the upstream  113  and downstream  114  signals of the path  110  and also the downstream signal  124  of the path  120 , the following three equations could be used to solve for flow velocity (V), speed of sound (c) in the medium  105 , and cross-flow (W) upon measuring transit times (t 113 , t 114 , t 124 ) of the signals  113 ,  114 ,  124 : 
         [0000]        t   113   =P 1/( c−V* cos(115)+ W* sin(115))   [EQ 4]
 
         [0000]        t   114   =P 1/( c+V* cos(115)− W* sin(115))   [EQ 5]
 
         [0000]        t   124   =P 2/( c+V* cos(125)− W* sin(125))   [EQ 6]
 
         [0025]    By using EQ 4 through EQ 6, above, the cross-flow element (W) can be accounted for in the determination of the flow velocity (V), according to the following: 
         [0000]        W={ 2* P 2*cos(115)/ t   124   −P 1*[cos(125)+cos(115)]/ t   114   +P 1*[cos(125)−cos(115)]/ t   113 }/{2*sin(115)*[cos(125)−cos(115)]}  [EQ 7]
 
         [0026]      FIG. 2  is a block diagram of the multimode flow meter system  200  according to an embodiment of the invention. The multimode flow meter system  200  of  FIG. 2  includes a controller  210  in communication with the paths  110 ,  120  shown at  FIG. 1 . The exemplary controller  210  includes a mode selector  220 , a calculator  230 , a user interface  240 , and a display  250 . Although shown together, the elements of the controller  210  may be housed separately and in communication with each other. In addition, one or more memory devices and the one or more processors that are understood to be part of the controller  210  are not shown. The calculator  230  computes flow velocity of the medium  105  based on a mode of operation determined by the mode selector  220 . The mode selector  220  may select the mode of operation based on user input through the user interface  240  or based on an interaction with the calculator  230  and predetermined rules. As a default, the calculator  230  may use EQ 1 and EQ 2 to determine flow velocity of the medium  105  on a continual, periodic, or user-selected basis. Exemplary bases by which the mode selector  420  may change the default mode of operation are discussed below. 
         [0027]    The calculated flow velocity may be indicated to a user through the display  250 . If the calculated flow velocity exceeds either a user-input or predetermined limit, such as, for example, 230 ft/sec, the mode selector  220  may switch the mode of operation by instructing the calculator  230  to use only EQ 2 in the calculation of the flow velocity. If a subsequent calculation indicates that the flow velocity has dropped below 230 ft/sec, the mode selector  220  may switch the mode of operation back to the default mode of using both EQ 1 and EQ 2. 
         [0028]    In one embodiment, the SNR is indicated to a user through the display  250 . If the SNR of the upstream signal  113  drops to or below a user-input or predetermined limit, such as, for example,  55 , the mode selector  220  switches the mode of operation by instructing the calculator  230  to use only EQ 2 in the calculation of the flow velocity. If a subsequent determination indicates that the SNR has increased above  55 , the mode selector  220  switches the mode of operation back to the default mode. 
         [0029]    In another embodiment, the user input through the user interface  240  may be used to directly change the mode of operation rather than indirectly through the selection of limits of flow velocity or SNR as a basis for a change of the mode of operation by the mode selector  220 . 
         [0030]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.