TRANSDUCER DEVICE OPERATING WITH ULTRASOUND FOR THE MEASUREMENT OF THE FLOW VELOCITY OF A FLUID IN A PIPE

Abstract: A transducer device operating with ultrasound for the measurement of the flow velocity of a fluid in a pipe (1) comprising two transducers (2, 3) mounted on their respective sides of the pipe and directed obliquely in relation to the direction of the flow, which transducers alternately transmit and receive sonic pulses to, respectively from each other. The two transducers (2, 3) are designed with their centre axes deviating from the imaginary connecting line between the transducers in a direction against the mentioned flow direction (1).

Description: TRANSDUCER DEVICE OPERATING WITH ULTRA SOUND FOR THE MEA¬ SUREMENT OF THE FLOW VELOCITY OF A FLUID IN A PIPE This invention relates to a transducer device operating with ultra sound for the measurement of the flow velocity of a fluid in a pipe, comprising two transducers mounted on their respective sides of the pipe and directed obliquely in rela¬ tion to the direction of the flow, which transducers alter- nately transmit and receive sonic pulses to, respectively from each other. TECHNICAL FIELD The invention is especially developed in connection with the need for measuring the gas flow to a so-called flare in connection with facilities for exploration and production of hydro carbons, but it is not limited to such a utilization. There is a definite need to be able to cover, with one transducer arrangement, a relatively wide range of flow velocity in an accurate manner, whilst there is also a need for safety, particularly when measuring in so-called danger areas. , BACKGROUND ART The transducer devices known today, of the kind mentioned initially, have the disadvantage that the measurable range of velocity is relatively limited. Also, the accuracy of measurement is not always completely satisfactory. In the case of measuring in dangerous areas (danger of explosion) it is necessary to transmit signals for evaluation to a safety zone and this transmission of signals will often disturb the measurements, because the signals are exposed to electricmagnetic influences from the environment during the necessary transmission through cables. DISCLOSURE OF INVENTION According to the invention -, it is suggested that the two transducers should be arranged with their centre axes devi¬ ating from the imaginary connecting line between the trans¬ ducers in a direction against the mentioned direction of flow of the fluid whose flow velocity is to be measured. A wider range of measurement is thus achieved because the divergent orientation of the transducers compensates for the deviation which will affect the sonic pulses in the fluid flow. According to the invention, in connection wit intended utilization of the new transducer device for measuring of the velocity of a gass flow in a pipe leasing to a flare, it has been found that the imaginary connecting line can advan- tageously form an angle of aprox. 55 in the flow direction, and that the deviation for the respective transducer will be aprox. _7o- n 1 Λ 0o. With such a deviation of orientation of the two transducers, which deviation of orientation is independent of the diame¬ ter of the pipe, a potential measurement range of 0 to aprox. 100 metres/second can be covered with only one trans¬ ducer device according to the invention. Further, according to the invention, it has been found that in a range of low speed, preferentially from 0 to 7 metres/second, a CW-signal (continuous wave signal) can be utilized (a signal with rectangular pulse whose momentary frequency increases linearly throughout the duration of the pulse) , whilst for medium and higher flow-velocities, pref¬ erentially over 7 metres/second and up to over 100 metres/second, a CHIRP-signal with correlative detection is used. B]£-. using two signal systems in this way a broad measurement range is achieved in a very advantageous manner. For high and medium rates of flow velocity, CHIRP-signal with correl¬ ative detection is used in order to overcome high levels of noise, whilst a CW-phase measurement in combination with the CHIRP-measurement is used for the lower velocity. The CHIRP-signals are sent upstream and downstream to roughly determine the flow-velocity value, followed by CW-bursts which increase resolution. The commission of the CW- measurement for flow-velocities over 7 metres/second enables a doubling of the bandwidth for the CHIRP-measurements. According to the invention it can be advantageous to use, as respective transducer, a transducer which is designed as a cup-formed metal body whose base as membrane is assigned an electrode on its inside, which is sealed off against the fluid-flow, and whose cup-wall is designed with an annulus filled with a powerful damping material, for example epoxy, rubber or epoxy mixed with metal particles. The annulus can advantageously be open towards the outside of the cup-base, and in a particularly advantageous embodiment the cup-formed metal body is made of titanium, since such an embodiment makes it especially suitable for use in connection with gas flows. Transducers of this type make it practicable to transmit signals over long distances in air or gas with very low so-called ringing compared with other, commercially avail¬ able ultra-sound transducers. Moreover, a transducer of this type will be innately safe and can therefore be utilized for instance in the case of flow-measurement of explosive gases. An advantageous transducer arrangement according to the invention can comprise a transmitter and a receiver connect- ed to each transducer, where each such transmitter and receiver has an optic receiver part respectively an optic transmitter part. A first multiplexer is connected to the two transmitters through optical fibres, and a second multi- plexer is connected to the two receivers through optical fibres. A CHIRP-generator and a CW-generator are connected to the first multiplexer, a CHIRP-correlator and a CW-detector are connected to the other multiplexer, whilst a clock and sequence-generator are connected to the multiplexers, the CHIRP-generator, the CW-generator, the CHIRP-correlator, the CW-correlator and CW-detector, in that a computer is con- nected to the generator, the CW-detector and the CHIRP- correlator. The use of the optical fibres here is particularly impor¬ tant. Use of this means that "the communication channel" between the point of measurement and the control-room will be immune to interfering signals of the type which would be able to cause interference in a normal electric transmission by cable. BRIEF DESCRIPTION OF DRAWINGS The invention shall be explained more closely with reference to the drawings, where % Fig. 1 shows a schematic longitudinal section through a pipe with two transducers placed diagonally facing each other. Fig. 2 shows a processing system for the transducer device according to the invention, and Fig. 3 shows two possible, advantageous embodiments of the transducers. MODES OF CARRYING OUT THE INVENTION In Fig. 1 a pipe 1 for the transportation of gas is shown. In the pipe wall a transducer is mounted and diagonally facing this, a second transducer 3 is mounted in the pipe wall. The two transducers 2, 3 are orientated in such a way in relation to the direction of flow of the gas in the pipe 1, indicated by the arrow, that their imaginary connection line as mentioned in Fig. 1 forms an angle of 55 with the direction of the flow. The transducer 2, which transmits co-current, forms an angle of 65 in relation to the flow direction, and deviates, then, 10 from the connecting line. The transducer 3, which is the transducer which trans¬ mits counter-current, has a corresponding deviation of 10° from the connecting line and thus forms an angle of 45° co-current. As is indicated in Fig. 1, both directions of deviation are placed on the upstream side of the imaginary connecting line. The values mentioned in Fig. 1 represent an optimisation which will make it possible to be able to cover at least an range of speed from 0-80 metres/second in the pipe 1. As mentioned, very good measurements are achieved if, with the shown orientation of the transducers, a CW-signal is used in combination with a CHIRP-signal in a lower flow- velocity range, preferably from 0-7 metres/second, whilst for medium and higher flow-speeds, up to 77-100 metres/second, a CHIRP-signal with correlation detection is used. A processing system which makes this possible is shown in Fig. 2. In the block diagram shown in Fig. 2, the two transducers 2 and 3 are shown in a so-called danger area (danger of explo¬ sion) , surrounded by a broken line 20, Each transducer 2 , 3 is as shown galvanically isolated and is connected to a respective transmitter and receiver 4, 5 and 6, 7. Each such transmitter and receiver comprises an optic receiver unit respectively optic transmitter unit and is by means of opti¬ cal fibres 8, 9, 10 and 11 connected to respectively a first multiplexer 12 and a second multiplexer 13, The optic fibres 8, 9, 10 and 11 form the optic connection between the dan¬ gerous area where the transducers are placed, and a control room 30 which is situated in a safe area, In addition to the two multiplexers 12, 13, a CHIRP-generator 14, a CW- generator 19, a CW-detector 15 and a CHIRP-correlator 16, together with a clock and sequence generator 17 and a com¬ puter 18, are all to be found in the control room. As can be seen from Fig. 2, the CHIRP-generator 14 and the CW-generator 19 are connected to the first multiplexer 12. The CW-detector is connected to the second multiplexer 13, and the CHIRP-correlator 16 is connected to the second mul¬ tiplexer 13. The clock and sequence generator 17 is as shown connected to both of the multiplexers 12, 13, the CHIRP- generator 14, the CW-generator 19, the CW-detector 15, the CHIRP-correlator 16 and the computer 18, which is also con¬ nected to the CW-detector 15 and the CHIRP-correlator 16. The flow meter provided with the invention is a transit time-difference meter. The principle of measurement is based on the fact that the propagation velocity of an ultra-sound pulse in a moving medium, will be modulated by the velocity of the medium in the direction of propagation. When two ultra-sound transducers are directed towards each other with an" angle of © in relation to the flow direction, the flow velocity of the medium is calculated by means of the follow¬ ing equation U χ = D , fc 3?. " fc ?, sin 29 t 23 * t 32 where U X = the flow velocity along the axis of the pipe D = the pipe diameter © = the connecting lines angle relative to the flow direction or the axis of the pipe -,-. = the transit time from the transducer 2 and the transducer 3 (downstream) and t-. ~ = tthhee ttrraannssiitt ttiimmee ffrroomm ttlhe transducer 3 to the transducer 3 (upstream) The counter propagating ultra sound pulses are advantageously generated with transducers of the type shown in Fig. 3 and 4. These transducers are intended for opera¬ tion with 80 kHz transmitter frequency. The transducers are intended for use in an explosive environment. The energy supply for exitation of the transducer is an electricity source which is available in the vicinity of the measure¬ ment area. The transmission of the modulated signals from the transducers is controlled from the control room, and the communication link between the transducers at the point of measurement and the signal treatment and estimation unit in the control room takes place, as mentioned, by means of optical fibres. Use of optical fibres provides for an opti¬ mal arrangement because the "channel" which the optical fibres forms between the transducers and the control room, will be a communication channel which is immune to any form of electromagnetic noise from the environment. Such electro¬ magnetic noise can be caused of for instance by radio waves, coupling or uncoupling of electricity using appliances and possibly also coupling between other electric leads. The optical connection between the transducers at the point of measurement and the control room function therefore as a block against many interfering signals from the environment. Signal transmission by means of optic fibres also guarantees complete galvanized isolation. By utilizing optic fibres the length of transmission without signal degradation can be up to 1000 metres. The actual transducers are vital elements and as mentioned, the transducers shown in Fig. 3 and 4, which are variations of each other, are very suitable for use in the transducer device according to the present invention. The transducers shown in Fig. 3 and 4 enable signal transmission across long distances in air or gas with very low ringing compared with many commercially available ultra-sound transducers. The transducer 2 shown in Fig. 3 is shown in section and is cup-formed. The base 21 of the cup is machined so that it forms a membrane. On the inside of the cup base or membrane 21 a suitable piezo-electric element 22 is mounted. The actual housing 23 can then serve as the one electrode, while the free end of the piezo-electric element can be used as the other electrode for electric exitation of the transduc¬ er. As shown in Fig. 3 the side wall of the cup 23 is machined so that an annulus 24 is formed. This annulus is so deep that .it extends from the mounting side of the transducer and all the way out until at the same level as the membrane 21. The annulus 24 is filled with a powerful damping material. Such a damping material can be for example epoxy, rubber or epoxy with built-in metal particles. The entire transducer housing 23 can, in a hermetically sealing manner, be coupled to a not shown supporter on the mounting side 25 , so that the electric contacts can be pre¬ vented from being exposed to the environment in which the transducer is to operate. This is naturally particularly advantageous when the transducer is used in an explosive environment. The operational frequency of the transducer can be decided by choosing suitable dimensions for the piezo-electric ele¬ ment 22 and the thickness of the metal membrane 21. The transducer shown in Fig. 3 can then be designed to cover normal sonic frequencies and ultrasonic frequencies. A metal material for the transducer cup 23 is chosen accord¬ ing to the demands which are made of the transducer (envi- ronmental demands) . Titanium, steel, aluminium, carbon fibre and other materials can thus be used for the transducer housing 23. The membrane embodiment which is employed provides a good coupling to a gas medium. There is a relatively large amount of metal mass at the edge of the membrane in order to achieve the desired border conditions and to prevent too strong a coupling to the housing. The coupling to the hous- ing will cause noise and the effect of the noise can be suppressed advantageously by means of the built-in damping material in the annulus 24. The embodiment in Fig. 4 mainly corresponds to the embodi¬ ment in Fig. 3 and the only virtual difference is that the annulus 24 is machined out from the membrane side and not from the mounting side 25. The same reference numbers as in Fig. 3 are therefore used, but with the addition of an index for the reference numbers 23 and 24 which indicate the hous¬ ing and the annulus. The CHIRP-signal which is utilized is a rectangular pulse whose momentary frequency increases linearly during the length of the pulse. The output for the correlator in the receiver will be a .compressed pulse. The maximum amplitude in the correlator output is detected by a top-detector and the transmission time is measured from the end of the trans¬ mission and until the detection of the correlation top. The CW-signal is given in bursts, for example with a fre¬ quency of 67.5676kHz. The duration of such a signal will then be 14.8 sec. A problem is that where there are large variations in the transmission times, the phase detector output will repeat itself at intervals corresponding to the period, that is 14.8 sec. In order to achieve a reliable result it will therefore be necessary to know the number of whole CW-periods in the transit time. The results of the CHIRP-measurement can be used here to determine this number, while the CW-measurement provides the fractions. The mean of the CW-phase measurement is calculated from 128 periods of the signal. For a correct result it will be nec¬ essary for each measurement to be within the same period of the reference signal. To achieve this two detection systems are used. The relative phase for the received signal in relation to the reference signal is detected prior to the measurement and in-phase or out of phase measurement is chosen. The transit time difference t, i.e. t, 2 - t 2 , in the formu¬ la for the flow velocity tj above, is determined in the computer on the basis of the combined CW- and CHIRP- measurements. The flow velocity is so calculated by means of St. and the transit time from the CHIRP-measurements.

Claims: PATENT CLAIMS 1. Transducer device operating with ultrasound for measur- ing the flow velocity of a fluid in a pipe (1) , compris¬ ing two transducers mounted on their respective sides of the pipe and directed obliquely in relation to the direction of the flow (2,3) which alternately transmit and receive sonic pulses to, respectively from, each other, c h a r a c t e r i z e d in that the two transducers (2,3) are arranged with their centre axes deviating from the imaginary connecting line between the transducers directed against the mentioned flow direc¬ tion (1) . 2. Transducer device according to claim 1, c h a r a c ¬ t e r i z e d in that the imaginary connecting line forms an angle of approximately 55 to the co-current direction, and in that the deviation for the respective • transducer (2,3) is approximately 10°. 3. Transducer device according to claim 1 or 2, c h a r ¬ a c t e r i z e d in that in a low range of flow veloc¬ ity, preferentially from 0-7 metres/second, a CW-signal in combination with a CHIRP-signal is used, while in the case of medium and higher ranges of flow velocity, pref¬ erentially over 7 metres/second and up to 77-100 metres/second, a CHIRP-signal with correlation detection is used. 4. Transducer device according to one of the preceding claims, c h a r a c t e r i z e d in that as respec¬ tive transducer, a cup-formed metal body (21) is used whose base (19) as membrane is assigned an electrode (20) on its inside, which is sealed off from the fluid flow, and whose cup-wall is designed with an annulus (22) filled with a powerful damping material, for example epoxy, rubber or epoxy mixed with metal particles. 5. Transducer device according to claim 4, c h a r a c ¬ t e r i z e d in that annulus (22') is open towards the outside of the cup base. 6. Transducer device according to claim 4, c h a r a c ¬ t e r i z e d in that annulus (22) is open towards the top of the cup. 7. Transducer device according to claim 4, 5 or 6, c h a - r a c t e r i z e d in that the cup-formed metal body (21) is made of titanium. 8. Transducer device according to one of the preceding claims, c h a r a c t e r i z e d in that each trans- ducer (2,-3) is connecter to a transmitter (4,5) and a receiver (6,7) .with optic receiver part respectively optic transmitter part, a first multiplexer (12) is connected to two trans¬ mitters (4,5) through optic fibres (8,10), a second multiplexer (13) is connected to the two receivers (6,7) through optic fibres (9,11) , a CHIRP-generator (14) and a generator (19) are connected to the first multiplexer (12) , a CW-detector 15 and CHIRP-correlator (16) are connected to the,second multiplexers (12,13), the CHIRP-generator (14) , the CW-generator (19) , the CW- detector (15) and the CHIRP-correlator (16) , and a com¬ puter (18) are connected to the generator (17) , the CW-detector (15) and the CHIRP-correlator (16) .

Inventors: HAMMER ERLING, MYLVAGANAM KANAGASABA, HAMMER, ERLING, MYLVAGANAM, KANAGASABA
Applicants: NORSKE STATS OLJESELSKAP, DEN NORSKE STATS OLJESELSKAP A.S
Application year: 1988
Main classification: G01P5
IPC codes: G01F1, G01P5