Ultrasonic measurement of flow velocity

1. An ultrasonic measurement device ( 10 ) for determining a flow velocity of a fluid in a conduit, comprising:

a pipe section ( 12 ) having a central longitudinal axis ( 22 ) and a cross section ( 24 ) with a radius (R), the central longitudinal axis ( 22 ) defining a horizontal reference plane ( 26 );

a two-path measurement system comprising a first path ( 30 a ) and a second path ( 30 b ) defined by a first pair of ultrasonic transducers ( 32 ) and a second pair of ultrasonic transducers ( 32 ) mounted to the wall of the pipe section ( 12 ), the ultrasonic transducers ( 32 ) of each path ( 30 a - b ) opposing each other at the ends of their path ( 30 a - b ), each path ( 30 a - b ) oriented parallel to the horizontal reference plane ( 26 ) with a distance to the horizontal reference plane ( 26 ) while having a component transverse to the central 15 longitudinal axis ( 22 );

first evaluation means ( 38 ) to determine a first flow velocity value from the first path ( 30 a ) by comparing the ultrasonic transit times along the first path ( 30 a ) with and against the fluid flow and a second flow velocity value from the second path ( 30 b ) by comparing the ultrasonic transit times along the 20 second path ( 30 b ) with and against the fluid flow; and

first integration means ( 40 ) to determine the flow velocity from the first flow velocity value and the second flow velocity value,

wherein the distance from the paths ( 30 a - b ) to the horizontal reference plane ( 26 ) is greater than half the radius (R).

2. The ultrasonic measurement device ( 10 ) according to claim 1 , wherein the first path ( 30 a ) and the second path ( 30 b ) are parallel to each other and/or have the same distance to the horizontal reference plane ( 26 ) and/or are symmetric with respect to the horizontal reference plane ( 26 ) with one path ( 30 a ) above and the other path ( 30 b ) below the horizontal reference plane ( 26 ).

3. The ultrasonic measurement device ( 10 ) according to claim 1 , wherein the distance from the paths ( 30 a - b ) to the horizontal reference plane ( 26 ) is smaller than 0.7 in units of the radius (R), in particular within the interval [0.55, 0.65] in units of the radius (R).

4. The ultrasonic measurement device ( 10 ) according to claim 1 , wherein the distance from the paths ( 30 a - b ) to the horizontal reference plane ( 26 ) is basically 0.6 in units of the radius (R).

5. The ultrasonic measurement device ( 10 ) according to claim 1 , wherein the ultrasonic transducers ( 32 ) are mounted to the pipe section ( 12 ) flush with the inner wall of the pipe section ( 12 ) or slightly recessed.

6. The ultrasonic measurement device ( 10 ) according to claim 1 , wherein the pipe section ( 12 ) comprises bores ( 42 ) for mounting the ultrasonic transducers ( 32 ), the bores ( 42 ) being larger in diameter than the ultrasonic transducers ( 32 ) by a factor of 1.5 or more to form pockets in the inner wall of the pipe section ( 12 ).

7. The ultrasonic measurement device ( 10 ) according to claim 1 , wherein the pipe section ( 12 ) comprises an integral upstream feed portion ( 14 ) with a longitudinal extension of at least 10 in units of the radius (R), in particular of at least 20.

8. The ultrasonic measurement device ( 10 ) according to claim 7 , wherein the feed portion ( 14 ) comprises a flow conditioner ( 18 ).

9. The ultrasonic measurement device ( 10 ) according to claim 1 , wherein the integration means ( 40 ) are capable of detecting an unreliable path ( 30 a - b ) or breakdown of a path ( 30 a - b ) by comparison of the first flow velocity value and the second flow velocity with a default value or with each other.

10. The ultrasonic measurement device ( 10 ) according to claim 9 , wherein the integration means ( 40 ) use an alternate flow velocity value instead of the flow velocity value when detecting a lack of reliability or a breakdown of a path ( 30 a - b ), the alternate flow velocity value being a default value or the rescaled flow velocity value of the other path ( 30 a - b ) of the first two-path measurement system.

11. The ultrasonic measurement device ( 10 ) according to claim 1 , comprising:

a second two-path measurement system with a third path ( 30 c ) and a fourth path ( 30 d ) defined by a third pair of ultrasonic transducers ( 32 ) and a fourth pair of ultrasonic transducers ( 32 ) mounted to the wall of the pipe section ( 12 ), the ultrasonic transducers ( 32 ) of each path opposing each other at the ends of their path ( 30 c - d ), the third path ( 30 c ) and the fourth path ( 30 d ) being mirror images of the first path ( 30 a ) and the second path ( 30 d ) with respect to a vertical reference plane ( 28 ) defined by the central longitudinal axis ( 22 );

second evaluation means ( 38 b ) to determine a third flow velocity value from the third path ( 30 c ) by comparing the ultrasonic transit times along the third path ( 30 c ) with and against the fluid flow and a fourth flow velocity value from the fourth path ( 30 d ) by comparing the ultrasonic transit times along the fourth path ( 30 d ) with and against the fluid flow;

second integration means ( 40 b ) to determine the flow velocity from the third flow velocity value and the fourth flow velocity value; and

correction means ( 44 ) to compare the flow velocities of the first two-path measurement system and the second two-path measurement system, or to correct the flow velocity based on the flow velocities determined from both two-path measurement systems.

12. A method for determining a flow velocity of a fluid in a conduit having a central longitudinal axis ( 22 ) and a cross section ( 24 ) with a radius (R), the central longitudinal axis ( 22 ) defining a horizontal reference plane ( 26 ), the method comprising the steps of:

sending ultrasonic signals along a two-path measurement system comprising a first path ( 30 a ) and a second path ( 30 b ) in a direction with and against the fluid flow, each path ( 30 a - b ) oriented parallel to the horizontal reference plane ( 26 ) with a distance to the horizontal reference plane ( 26 ) while having a component transverse to the central longitudinal axis ( 22 );

determining a first flow velocity value from the first path ( 30 a ) by comparing the ultrasonic transit times along the first path ( 30 a ) with and against the fluid flow and a second flow velocity value from the second path ( 30 b ) by comparing the ultrasonic transit times along the second path ( 30 b ) with and against the fluid flow; and

determining the flow velocity by approximation of the flow through the cross section ( 24 ) based on the first flow velocity value and the second flow velocity value, wherein the ultrasonic signals are sent along paths ( 30 a - b ) with the distance to the horizontal reference plane ( 26 ) being greater than half the radius (R).

13. The method according to claim 12 , wherein the ultrasonic signals are sent along paths ( 30 a - b ) with the distance to the horizontal reference plane ( 26 ) being smaller than 0.7, in particular being within the interval [0.55, 0.65] or being essentially 0.6 in units of the radius (R).

14. The method according to claim 12 , wherein an unreliable path ( 30 a - b ) or breakdown of a path ( 30 a - b ) is detected by comparison of the first flow velocity value and the second flow velocity with a default value or with each other.

15. The method according to claim 14 , wherein an alternate flow velocity value is used instead of the flow velocity value when a lack of reliability or a breakdown of a path ( 30 a - b ) is detected, the alternate flow velocity value being a default value or the rescaled flow velocity value of the other path ( 30 a - b ).