Method and apparatus for measuring a gas flow velocity

1. A method of measuring a gas flow velocity using an ultrasonic gas flow meter ( 10 ),

wherein said ultrasonic gas flow meter ( 10 ) includes a horizontal piping ( 12 ) through which gas ( 14 ) can flow in a flow direction (A) and which has a central axis ( 36 ), at least one pair of measurement paths ( 30 ) of equal length spaced apart, extending parallel to one another in separate horizontal planes and inclined to said flow direction (A),

wherein each horizontal plane of a pair of measurement paths ( 30 ) is vertically offset by an equal predefined distance from said central axis ( 36 ),

wherein an ultrasonic transducer ( 16 , 18 ) is arranged at each end of said measurement path ( 30 ) and each ultrasonic transducer ( 16 , 18 ) is adapted to selectively act as an ultrasonic transmitter ( 22 ) and an ultrasonic receiver ( 24 ), the method comprising the steps of measuring a flow velocity of said gas ( 14 ) in a measurement path ( 30 ) arranged in an upper region of said piping ( 12 ), measuring a flow velocity of said gas ( 14 ) in a measurement path ( 14 ) arranged in a lower region of said piping ( 12 ) and using the results of these two measurements to predict if a liquid ( 24 ) is present in said piping ( 12 ) during said measurement,

wherein said measurement of the respective flow velocities is repeated at a predetermined frequency and for a predetermined time period, to form a mean flow velocity and a statistical spread for each measurement path ( 30 ), wherein said statistical spread is correlated to a turbulence present in said measurement path ( 30 ),

wherein a turbulence ratio is formed by dividing a sum of said turbulences measured for each measurement path ( 30 ) arranged in said lower region by a sum of said turbulences measured for each measurement path ( 30 ) arranged in said upper region,

and wherein a velocity ratio is formed by dividing a sum of said mean flow velocities measured for each measurement path ( 30 ) arranged in said upper region by a sum of said mean flow velocities measured for each measurement path ( 30 ) arranged in said lower region, and wherein a liquid factor (S 2 ; S 4 ; S 6 ) is calculated by multiplying said velocity ratio by said turbulence ratio.

2. A method in accordance with claim 1 , wherein said ultrasonic gas flow meter ( 10 ) measures a flow velocity of said gas ( 14 ) flowing through said piping ( 14 ) on the basis of differences between propagation times (T 1 , T 2 ) of ultrasonic signals transmitted and received between each pair of transducers ( 16 , 18 ) and propagating in opposite directions along each measurement path ( 30 ).

3. A method in accordance with claim 1 , wherein a comparison of said flow velocity from said upper region to said flow velocity from said lower region is carried out to predict if liquid is present in said piping ( 12 ).

4. A method in accordance with claim 1 , wherein a comparison between said turbulence present in said measurement paths ( 30 ) is carried out to predict if a liquid ( 26 ) is present in said piping ( 12 ) during said measurement.

5. A method in accordance with claim 1 , wherein if said turbulence ratio is approximately equal to _b 1 , said piping ( 12 ) is considered to contain virtually no liquid and if said turbulence ratio is greater than 1, said piping ( 12 ) is considered to contain an amount of liquid.

6. A method in accordance with claim 1 , wherein said liquid factor (S 2 ; S 4 ; S 6 ) is compared to a predetermined liquid factor stored in a memory of said ultrasonic gas flow meter ( 10 ), wherein said predetermined liquid factor is selected to correlate to an amount of liquid ( 26 ) present in said piping ( 12 ) for a predefined flow velocity and wherein said comparison is used to determine said amount of liquid ( 26 ) present in said piping ( 12 ) for said measured flow velocity.

7. A method in accordance with claim 6 , wherein a set of predefined liquid factors are stored in said memory which correlate to different amounts of liquid present at different flow velocities for different types of gases and different types of liquids.

8. A method in accordance with claim 1 , wherein a plurality of said liquid factors (S 2 ; S 4 ; S 6 ) is obtained and compared to said predefined liquid factors.

9. A method in accordance with claim 1 , wherein said method outputs at least one of said flow velocity, said mean flow velocity, said turbulence, said gas flow velocity, said turbulence ratio, said velocity ratio, said liquid factor (S 2 ; S 4 ; S 6 ) and said amount of liquid ( 26 ) present in said piping ( 12 ).

10. A method in accordance with claim 1 , wherein one pair of measurement paths ( 30 ) is present in said ultrasonic gas flow meter ( 10 ).

11. A method in accordance with claim 1 , wherein two pairs of measurement paths ( 30 ) are present in said ultrasonic gas flow meter ( 10 ).

12. A method in accordance with claim 1 , wherein three pairs of measurement paths ( 30 ) are present in said ultrasonic gas flow meter ( 10 ).

13. A method in accordance with claim 1 , wherein four pairs of measurement paths ( 30 ) are present in said ultrasonic gas flow meter ( 10 ).

14. An ultrasonic gas flow meter ( 10 ) including a horizontal piping ( 12 ) through which gas ( 14 ) can flow in a flow direction (A) and which has a central axis ( 36 ), at least one pair of measurement paths ( 30 ) of equal length spaced apart, extending parallel to one another in separate horizontal planes and inclined to said flow direction (A),

wherein each horizontal plane of a pair of measurement paths ( 30 ) is vertically offset by an equal predefined distance from said central axis ( 36 ),

wherein an ultrasonic transducer ( 16 , 18 ) is arranged at each end of said measurement path ( 30 ) and each ultrasonic transducer ( 16 , 18 ) is adapted to selectively act as an ultrasonic transmitter ( 22 ) and an ultrasonic receiver ( 24 ), the ultrasonic gas flow meter ( 10 ) also includes an evaluation unit ( 32 ) which is adapted to initiate a measurement of a flow velocity of said gas ( 14 ) in a measurement path ( 30 ) arranged in an upper region of said piping ( 12 ), a measurement of a flow velocity of said gas ( 14 ) in a measurement path ( 30 ) arranged in a lower region of said piping ( 12 ) and to use the results of these two measurements to predict if a liquid ( 24 )is present in said piping ( 12 ) during said measurement,

wherein said measurement of the respective flow velocities is repeated at a predetermined frequency and for a predetermined time period, to form a mean flow velocity and a statistical spread for each measurement path ( 30 ), wherein said statistical spread is correlated to a turbulence present in said measurement path ( 30 ),

wherein a turbulence ratio is formed by dividing a sum of said turbulences measured for each measurement path ( 30 ) arranged in said lower region by a sum of said turbulences measured for each measurement path ( 30 ) arranged in said upper region,

and wherein a velocity ratio is formed by dividing a sum of said mean flow velocities measured for each measurement path ( 30 ) arranged in said upper region by a sum of said mean flow velocities measured for each measurement path ( 30 ) arranged in said lower region, and wherein a liquid factor (S 2 ; S 4 ; S 6 ) is calculated by multiplying said velocity ratio by said turbulence ratio.

15. A gas transport system ( 40 ), including at least one horizontal transport piping ( 38 ), a gas input ( 42 ), a gas output ( 44 ) and at least one ultrasonic gas flow meter ( 10 ) including a horizontal piping ( 12 ) through which gas ( 14 ) can flow in a flow direction (A) and which has a central axis ( 36 ), at least one pair of measurement paths ( 30 ) of equal length spaced apart, extending parallel to one another in separate horizontal planes and inclined to said flow direction (A),

wherein each horizontal plane of a pair of measurement paths ( 30 ) is vertically offset by an equal predefined distance from said central axis ( 36 ),

wherein an ultrasonic transducer ( 16 , 18 ) is arranged at each end of said measurement path ( 30 ) and each ultrasonic transducer ( 16 , 18 ) is adapted to selectively act as an ultrasonic transmitter ( 22 ) and an ultrasonic receiver ( 24 ), the ultrasonic gas flow meter ( 10 ) also includes an evaluation unit ( 32 ) which is adapted to initiate a measurement of a flow velocity of said gas ( 14 ) in a measurement path ( 30 ) arranged in an upper region of said piping ( 12 ), a measurement of a flow velocity of said gas ( 14 ) in a measurement path ( 30 ) arranged in a lower region of said piping ( 12 ) and to use the results of these two measurements to predict if a liquid ( 24 )is present in said piping ( 12 ) during said measurement,

wherein said measurement of the respective flow velocities is repeated at a predetermined frequency and for a predetermined time period, to form a mean flow velocity and a statistical spread for each measurement path ( 30 ), wherein said statistical spread is correlated to a turbulence present in said measurement path ( 30 ),

wherein a turbulence ratio is formed by dividing a sum of said turbulences measured for each measurement path ( 30 ) arranged in said lower region by a sum of said turbulences measured for each measurement path ( 30 ) arranged in said upper region,

and wherein a velocity ratio is formed by dividing a sum of said mean flow velocities measured for each measurement path ( 30 ) arranged in said upper region by a sum of said mean flow velocities measured for each measurement path ( 30 ) arranged in said lower region, and wherein a liquid factor (S 2 ; S 4 ; S 6 ) is calculated by multiplying said velocity ratio by said turbulence ratio.