DEVICE FOR DETERMINING THE FLOW VELOCITY OF A MEDIUM IN A CYLINDRICAL PIPE

Abstract: Flow velocity measuring device with a first (6, 8) and a second (106, 108) pair and transducers, for respectively emitting acoustic waves through a medium conduit (1 and receiving same after multiple reflections in the conduit wall, the transducer (6) of the first pair emitting at an angle (α) to the central longitudinal plane (14, 114) of said conduit and the transducer (106) of the second pair emitting at an angle (-α) with respect to said plane (14, 114).

Description: The invention relates to a device such as is described in the preamble of Claim 1. Such a device is known per se from US-A-4,103,551. Although the flow velocity can be determined accurately with said known device, said device has the drawback that the obtained measurement result is incorrect when "swirls" occur in the flow. The direction and magnitude of such swirls has a noticeable effect on the measurement result, which is all the more objectionable because it is never possible to determine with certainty with what intensity swirl occurs. Whatever the cause to which it is due, swirl decays only very slowly and in practice always occurs after disturbances such as bends, shut-off valves and the like, as a result of which the measurement is incorrect. The object of the invention is to eliminate this drawback. According to the invention, this is achieved by the measures as described in the characteristic of Claim 1. A swirl directed in the direction in which the first transducer emits will result in that the measurementsignal, obtained with the first and second transducers, is too large by a certain amount. In the other system, however, in which the third an fourth transducers co-operate, this same swirl will result in a too low a rate being measured. The average of the two measurementsignals represents the required flow velocity. The difference between the results of the two respective measurements is a measure of the intensity of the swirl which can be used as a compensation by the conversion of the measured to the average flow velocity. Advantageous embodiments are described in the subclaims. It is observed that DE-A-25 52 072 describes a device for measuring the flow velocity in a cylindrical pipe based on a different principle, namely using two pairs of ultrasonic transducers, which do not alternate function as transmitter and receiver respectively and without reflection of the acoustic waves on the pipe walls in which the harmfull side-effects of the "swirls" are eliminated by mounting the transducers at very specific positions on the pipe wall. The invention is explained by reference to the drawing. Figure 1 is a diagrammatic representation of an embodiment of the device according to the invention. Figures 2 and 3 are sections through the conduit. Figure 4 is a diagrammatic representation of a transducer to be used in a second embodiment. In figure 1, the reference numeral 1 indicates a conduit in which a medium is flowing in the direction of the arrow 2. With the device according to the invention it is intended to accurately determine the flow velocity of said medium, and in particular, in such a manner that swirl occurring in the medium, as indicated diagrammatically with the broken lines 4 in Figures 2 and 3, has no effect on the measurement result. For this purpose, the device according to the invention comprises a first measurement system of the type as known from US-A-4,103,551 consisting of a first ultrasonic transducer 6 fitted in the position pos. 1 and a second ultrasonic transducer 8 fitted in the positioned pos. 2, both connected to a measuring and energizing device 10 which is also of a type known per se. The transducer 6 emits an ultrasonic wave in the direction of the arrow 12 at an angle + α to the central longitudinal plane 14 of the conduit and an angle β to the plane 16 perpendicular to said central longitudinal plane. After two reflections at the conduit walls, namely at the points 18 and 20, said ultrasonic wave reaches the transducer 8 in pos. 2. The measuring device 10 measures the transit time of the acoustic wave between the transducers 6 and 8. Then the transducer 8 emits an acoustic wave along the same path back to the transducer 6 and the transit time of the second wave is again measured with the measuring device 10. From the two measurement signals thus obtained, a quantity is derived which is a measure of the flow velocity of the medium in the direction of the arrow 2 and which with good accuracy represents said velocity if no swirls occurs in the flow. If swirl does in fact occur, direction and magnitude of said swirl have a noticeable effect on the measurement result. The invention eliminates this effect by providing at some distance from the transducers 6 and 8, a second set of transducers 106 and 108, both connected to the controlling and measuring device 110; this second set of transducers operates in the same manner as the transducers 6 and 8, with however, the difference that the transducer 106 now emits along the same path in a direction with respect to the central longitudinal plane 114 which is opposite to the direction in which the transducer 6 emits; the angle to the said central longitudinal plane is indicated in Figure 3 by - α. After reflection at the points 118 and 120 of the pipe inside wall, the emitted wave reaches the transducer 108, after which the transducer 108 emits back an acoustic wave to the transducer 106; in the manner as described above, the measuring device 110 is used to obtain a second quantity representing the flow velocity which, if no swirl is present in the flow, will of course correspond to the first quantity obtained with the measuring device 10. When, however, swirls in fact occur, then this second quantity representing the velocity will differ from the first one. The two obtained values are fed to the electronic processing device 22 which determines the average of the two values; said average is a correct measure of the actual flow velocity. It is also possible to derive from the two measured values a signal representing the swirl intensity which can also be introduced as an additional correction factor. Within the scope of the invention, various modifications are possible. Thus, it is possible to make use of a system with only two measuring positions if a transducer such as the transducer 24 represented diagrammatically in figure 4 is used at each measuring position. Said transducer is of double construction and has two emitting (and receiving) surfaces 26 and 28 respectively, the main directions of which include an angle of respectively + α and - β with the central longitudinal plane 30 of the pipe. Such a transducer must, however, inevitably have a fairly large diameter and is consequently more difficult to fit in a conduit.

Claims: Device for determining the flow velocity of a medium in a cylindrical conduit (1), comprising a first transducer (6) mounted on said conduit (1) for emitting an incoming acoustic wave from a first position (pos 1) located at a point within a central longitudinal plane (14) of the conduit (1) at a first angle α to said central longitudinal plane (14) and at a second angle β to a plane (16) perpendicular thereto and a second transducer (8), mounted on said conduit (1) for detecting at a point located at a second position (pos 2) situated on said conduit (1) and at a distance from said first transducer (6) said incoming acoustic wave travelling along a first path and resulting from multiple reflection of said incoming acoustic wave at points (18, 20) at the inside wall of said conduit (1) and for emitting back, from said second position (pos 2) in a direction opposite to that of the incoming wave an acoustic wave which arrives at said first transducer (6) after traversing said first path in the opposite direction, and with means (10) connected to said first (6) and second (8) transducers for deriving from the respective transit times of the acoustic waves between said transducers (6, 8) a first flow velocity value, characterized by a third transducer (106) for emitting from a third position (pos 3) at an angle -α to said central longitudinal plane (114) opposite to said first angled α and over a second path similar to said first path an acoustic wave to a fourth transducer (108) in a fourth position (pos 4), whereafter said fourth transducer (108) emits an acoustic wave back to said third transducer (106), means (110) for deriving a second flow velocity value from the transit time values obtained from said third (106) and fourth (108) transducers, and means (22) for deriving an average flow velocity value from said first and second flow velocity values. Device according to Claim 1, characterized by means for deriving a value representing the swirl velocity from the first and second flow velocity values. Device according to Claims 1-2, characterized in that the third (pos. 3) and fourth (pos. 4) positions are at a distance from the second position (pos. 2). Device according to Claims 1-2, characterized in that the third position coincides with the first, and the fourth position coincides with the second position. Device according to Claims 1-4, characterized in that the path of the acoustic wave consists of three mutually equal sections and all four positions lie on the same generatrix of the conduit.

Inventors: DRENTHEN, JAN GERT
Applicants: SERVEX B.V.
Application year: 1987
Main classification: G01F1
IPC codes: G01F1