FI76885C - Acoustic flow measurement method and device for application thereof. - Google Patents

Description

76885 1 Acoustic flow measurement method and applicable device Acoustic flow measurement method and application for heating up to 5

The invention relates to an acoustic flow measurement method for measuring the flow rate, volume flow and / or mass flow of gases, liquids and / or multiphase suspensions in a tube or a corresponding waveguide using sound waves propagating upstream and downstream.

The invention further relates to a device for carrying out the method of the invention, comprising a measuring tube or a similar waveguide in which the measured current flows, which device further comprises sound signal transmitting sensors such as loudspeakers connected at a distance from the measuring tube or the like and comprising a sound receiving sensor such as microphones. .

20 Many different measurement methods have been developed for gas flow measurements based on differential pressure (venturl pipes), oscillation (vortex) or turbine measurement. Em. the main drawback of the known measurement methods is that the methods depend on the flow profile of the installation site. In this case, the meter inspection el 25 carried out in the tilting plants no longer corresponds exactly to the flow conditions at the meter location. In addition, the calibration measurement may stop the operation of the process line piping.

The following is a description of the prior art related to the invention.

U.S. Pat. No. 4,445,389 discloses a flow velocity, volume flow and mass flow measuring device for a fluid based on the tao wave mode of a pipe. The device disclosed in the U.S. patent uses two constant frequencies from which it is electrically controlled. As a result of the superposition of the forward and countercurrent sound wave 35, the measuring device obtains a standing wave in the tube, the phase difference of which is compared with the reference situation. The false difference is detected by two microphones, from which the measuring electronics 2 76885 1 calculate the false difference caused by the flow on the basis of the amplitude of the signal obtained. The false difference obtained from the measurement is proportional to the flow rate. However, the brightness of the method is to accurately measure the lie difference in strong noise, eg in an industrial environment, by means of amplitude. The U.S. patent discloses the use of Erlkols-5 Steep Filters and the isolation of a special measuring tube from other piping to reduce the above. However, these factors complicate or even prevent the use of the method in an industrial environment.

In addition, the measuring device of the aforementioned U.S. patent requires highly electrically symmetrical broadband (A f to 200 kHz) and relatively expensive microphones to operate to prevent phase difference distortions. The method also requires precise positioning of the microphones to reduce reflections and other sources of interference. Eliminating these factors in wages where temperature changes and other disturbances are significant is laborious. The measuring device is an interference-prone and carefully handled unit in an industrial environment.

Hubert Lechner has presented in his article by L. H. Lechner: "Ultrasonic flow metering based on transit time differentials which are insensitive 20 to flow profile", J. Acouet. Soc. Am. 74 (3) Sep. 1983, a method for using ultrasound in a waveguide upstream and downstream. In the method, however, the flow rate remains dependent on the speed of sound in the liquid under study. In this case, changes in the composition of the liquid, temperature, density, pressure greatly affect the accuracy of the measurement. Em. the method described in the article is not suitable for measuring gas flows, because the ultrasound is rapidly attenuated in several gases and the acoustic fitting of the ultrasonic sensor to the gases is difficult to obtain a sufficiently large signal.

In older ultrasound-based (L. C. Lymmworth: "Ultra-30 Sonic flowmeters," Physical Acoustics, Academic Press, vol. 14.

pp. 407-525, 1979) solutions, the sound is conducted upstream and downstream obliquely through the diameter of the pipe. However, this results in a comparison profile dependence, thus influencing the performance of accurate measurements.

In addition, measurement methods using ultrasonic technology and correlation technology are already known, for which reference is made, for example, to FI patent 67627 (corresponding U.S. patent 4,484,478).

3 76885 1 It is a general object of the present invention to obviate the drawbacks of the prior art.

It is a particular object of the invention to provide a method and apparatus for measuring flow that is better suited to known industrial environments and is less sensitive to disturbances such as noise and the like.

It is a further object of the invention to improve the measurement accuracy of previously known devices.

10

It is also an object of the invention to provide a device which is less sensitive than known devices to changes in the pressure, density or the like of the medium to be measured.

It is also an object of the invention to provide a method and apparatus for measuring the flow rate that integrates a flow rate profile.

In order to achieve the above and later objects, the method of the invention is mainly characterized in that a wideband audio signal from sound sources is passed in a measuring tube or similar waveguide in both the upstream and upstream plane wave modes and and on the basis of the distance between the measuring points, the flow rate is determined and further, if necessary, in a known manner by calculating the volume flow and / or the ground flow in the gases, liquids and multiphase suspensions.

The device according to the invention, in turn, is mainly characterized in that said sound receiving sensors are arranged at a certain known distance from each other and said sound signal transmitting sensors are located at a distance from the first at a distance downstream of said second sound receiving sensor, that the device comprises a signal generator and an amplifier for supplying wideband audio signals to said first and second sound transmitting sensors and that the device further comprises a correlator to which the signals from the first and second sound receiving sensors are fed.

The essential advantage of the invention is e.g. in that variations in the flow rate profile are taken into account (Integrated) both in the flow direction 15 and in the transverse direction.

In the following, the invention will be described in detail with reference to some application examples of the invention shown in the figures of the accompanying drawing, to the details of which the invention is in no way narrowly limited.

Figure 1 illustrates an acoustic planar wave underlying the invention in a measuring tube in which the current to be measured moves.

Figure 2 shows the general principle of correlation flow measurement.

Figure 3 shows the functions f1 (t) and f2 (t) and their cross-correlation function Rj 2 * 30. Figure 4 shows the method and apparatus according to the invention schematically and partly in block diagram form.

Figure 5 is a schematic block diagram of a polarity correlator used in the invention.

Figure 6 shows a multiplexer alternative of the invention.

Figure 5 shows a switch diagram of a polarity correlator according to the invention.

Figure 8 shows a correlation function at a given gas flow rate.

5

Figure 9 shows the correlation between an acoustic measurement according to the invention and a known vortex measurement.

Figure 10 shows the flow rate Q as a function of the motor frequency (fm) of the fan providing the flow to be measured.

First of all, the basic background of the measurement method according to the invention will be described.

The flow measurement method of the invention utilizes relatively wideband sound waves which propagate in the measuring tube 10a upstream and downstream of the gas flow e to be measured. The maximum frequency of the sound spectrum is selected for the cut-off frequency of the plane wave of the tube 10a according to Equation (1).

20 fcoto £ f "1,7 · D 0> where 25 c = speed of sound in the free gas in question D * diameter of the pipe.

In the invention, the flow rate is measured by real-time correlation (Figure 1) using a continuous wideband sound wave propagating in a tube 10a or a similar flow channel 3Q, the frequency range of which is below the cut-off geometry (Formula 1) determined by the Helm-Helz wave equation. Em. below the frequency fcutoff, only a planar wave propagates, which integrates the flow profile over the pipe density between the receiving sensors. Thus, the measurement is independent of temperature, density and flow profile (B. Robertson: "Flow and temperature profile Independence of flow measurements using long acoustic wawes", J. of Fluids Engineering, March 1984, vol. T06 / T9; B. Robertson : "Effect of 6 76885 1 arbitrary temperature and flow profiles on the speed of sound in a pipe", J. Acoust. Soc. Am. Vol. 62, No. 4, Oct 1977). The measurement also does not cause pressure differences in the measuring instrument.

In the comparison measurement method according to the invention, a real-time correlator is applied, most preferably a polar index correlator 100 developed in VTT's reactor laboratory. The structural example and operation of the correlator 100 will be described in more detail below with reference to Figs.

Most preferably, the polarity correlator 100 is used in the invention to determine the cross-correlation function of the two signals from the comparator 110. The second input signal, f ^ (t), is in real time and the second f ^ Ct-f) is delayed by the filter repeater. When f ^ - f ^. talk about the autocorrelation function. The word "polarity" means that if the functions f j and f2 are continuous, their value is determined with only one bit precision, te. deciding whether the function is positive or negative compared to a predetermined "zero" reference level. If the possible values of the signals are +1 (positive) and -1 (negative), the normalized correlation function can be determined 20

ITI

R12 (x) s - / - (f l (t) f2 (t-x) + l) dt.

T 0 2 (la)

When there is a strong positive correlation between the signals, the function R ^ 2 25 8aa is close to 1, and when there is a strong negative correlation, the value is close to zero. When there is no correlation at all, a value of 0.5 is obtained.

If the positive value of the signal f1 or f2 is defined as a logic value of 30 it while the negative value corresponds to a logic of 0, a logic function that has equal values integrandlna I (t, -c) »- (f. (T) f (tx) + l db) 35 2 can be implemented with an exclusive NOR gate.

^ 76885 1 The polarity correlator 100 samples the integrand, equation (Ib), with a delay 'f over a period of time. The integration is performed by killing the sampling results into calculators with a measurement period T. Each individual value X, designated a channel, has its own calculator. The maximum sampling frequency 5 can be equal to the clock frequency of the shift register. A schematic diagram of the polarity correlator is shown in Figure 5. In Figure 5, the sampling frequency is equal to the clock frequency of the shift register.

Sampling must take place with a certain phase shift with respect to the clock pulses of the shift register 10 to ensure that sampling takes place in the shift register continuity state.

In Fig. 5, the polarity correlator microcircuit is indicated by reference numeral 200 and delimited by a dashed line. Circuit 200 is implemented using 15 semi-custom design techniques based on Micronas Oy Inc.'s 800-port CMOS port array 7800. Half of the gates in the gate array 7800 are arranged to form 80 resettable D-flip-flops with the other ports forming 160 ULA elements (Universal Logic Array) consisting of five nMOS-pMOS transistor pairs.

20 A four-channel polarity correlator interleaver containing 16-bit wave counters is implemented for one 7800 gate array piece.

Figure 6 is a circuit diagram of a MAS-7808 chip using pre-wired D-flip-flops. Of these, 64 are used with four 16-bit wave counters (F2 ... F17, F20 ... F35, F38 ... F53 and F56 ... F71). Four flip-flops 25 (F18, F36, F54 and F72) store wave counter overflows and four (F1, F19, F37 and F55) are used for the shift register chain.

Em. the page can be operated in two substantially different ways depending on the SHIFT / COUNTER (S / C) control output. SHIFT / COUNTER - LOW 30 corresponds to the normal (COUNTER) mode, in which the wave counters calculate the coincidences of the signals F1 and F2 sampled by the falling edge of the WRITE signal. The overflow switch is set each time the most significant bit of the calculator changes from HIGH to LOW. Setting the overflow bit on one of the four channels sets the open drain OFL output to LOW.

35 The reading of the contents of the calculators shall be performed using the simultaneously available mode of operation of the transfer register, a special mode of operation shall be used for reading. In this method, 16 wave counter phases are combined 8 76885 1 to form 16-bit shift registers that are timed at the falling edge of the WRITE signal. Each WRITE pulse1 moves the transfer register forward by one step, allowing the contents of the counters to be read starting with the most significant bits.

5

In the correlator 100, the measurement signals can be compared in time based on their polarity. The maximum / minimum of the cross-correlation function ^ obtained from the signals is used to calculate the time delay of the signals. The measurement principle is shown in Figure 4.

10

When measuring the travel time of sound with the correlator 100 upstream and downstream, the gas flow rate v. Is obtained by the equation

KoaoU

15 Vk “8U’ 5 (ΐ ’ΐ) (W

where ^ = travel time of the downstream sound wave 2Q Ί2 “travel time of the countercurrent sound wave L = distance between the measuring points and M ^.

The volume flow Q and the mass flow M can be calculated by equations (3) and (4): 25 «v-gas' A <3) where A is the average cross-sectional area with a measurement interval of 30 v-gas * gas flow rate, and with an ideal gas M can be calculated as follows:

VP

35 M - Q ·? (4) 9 76885 ^ where Ϋ adiabatic gas constant P pressure in the pipe 5 c “speed of sound in the gas *» L / 2 (1+ l / * ^).

In the measuring arrangement shown in Fig. A, the sound wave in the measuring tube 10a propagates upstream and downstream at the velocities c + vjac-v (c = velocity of sound in the substance in question and v * flow velocity). By measuring the corresponding travel times' f and ^ with the correlator 10 100 and using their inverse values (Equation (2)), the speed of sound in the substance to be measured is eliminated. Since the measurement can be made simultaneously upstream and downstream, the flow rate v is independent of the speed of sound, pressure, temperature and density in question. in matter.

15

The polarlete correlator 100 used in the method considers only the phase of the signal, but the amplitude. By comparing a sufficiently large number of signal average exceedances (T) and undershoot (Q) with each other, the mutual correlation of the two measurement signals can be determined. The correlation-20 function tells the time delay at which the same binary sequence occurs at two measurement points. Therefore, the accuracy of the amplitude from the sensors does not affect the measurement when the signal-to-noise ratio is considered sufficient by the transmission signal (the acoustic power transmitted in the noise environment is adjusted to a suitably high level). Method el requires special filtrations for the measurement. In addition, the scope of the measuring range is wider than the reference meters on the market.

In the following, some advantageous detailed embodiments of the measuring method and device according to the invention and the experimental measurements performed with them will be described.

The measurement arrangement shown schematically in block diagram in Fig. 4 comprises a signal generator 14 which supplies electrical signals having a suitable frequency range to the speakers 13a 33 and 13b at the points K1 and K1 in connection with the measuring tube 10a. On the receiving side, the measuring arrangement comprises microphones 14a and 14b at points and in connection with the measuring tube 10a, which are the said measuring interval L

10 76885 1 apart. The electrical signals received by said microphones 14a and 14b are fed to a comparator 110 which compares the signals and supplies them, suitably modified, to a correlator 100. From the correlator 100, a measurement signal is applied to a unit 120 which contains e.g.

5 a data processing device which calculates the flow rate v to be measured according to formulas (2), (3) or (4), the flow rate Q to be measured, if necessary, or the mass flow M to be measured. Unit 120 also includes suitable devices for printing or displaying the measurement result.

Figure 7 schematically shows a preferred embodiment of the invention in which one correlator unit 100 and several measuring tubes 10 ^, 10 ^, 10 ^ ... 10 ^ are used (there are N measuring tubes). The measurement signals from the different measuring tubes are fed to a multiplexer unit 130, which in turn connects the measuring signals of each measuring tube to the correlator-15 unit 100. The measuring times of the different measuring tubes 10p..10 100, but a certain or certain measuring tubes are connected less frequently than others according to the requirements of the measuring need.

Measurements according to the method of the invention were made with the gas flow piping of VTT's reactor laboratory, which was arranged according to Figure 4. As the gas to be measured, air was used, the flow rate v of which could be controlled by a fan 11 equipped with an inverter-driven motor 11a connected to the piping 10. A 2 mm walled and 88.9 mm diameter steel pipe 10a was used as the measuring pipe.

The sound waves were conducted to the piping through the T-branches 12a and 12b by means of speakers 13a and 13b. The measurement was performed by directing sinusoidal frequency sweeps alternately upstream and downstream in the range of 0.6 to 2.4 kHz to speakers 13a or 13b and measuring audio signals with microphones 14a and 14b (Sennhelser KE4-211 0 4.75 mm, length 4.2 mm). Frequently, the frequency sweep may generally be in the range of 0.1 to 10 kHz, taking into account formula 35 (i). The frequency sweep time (5.0 s) was set to be the same as the measurement time of the correlator 100. The signals of the microphones 14a and 14b were converted by a comparator 110 into a TTL level pulse train. As a result of the comparison of the pulse Π 76885 1 sequences at the gas flow rate v «12 m / s from the correlator 100, a correlation function was obtained, which is shown in Fig. 8.

The measured values el do not take into account possible temperature changes 5 between the upstream and downstream measurements. The measurement results are shown in the following Table 1, in which the average of the ten measurements and the standard deviation for the flow rate at each frequency f of the motor 11a are calculated.

10 15 20 25 30 35 12 7 6 8 8 5 9 m a s

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032 - • mtnoNCSootnomomOLn ih ^ ^ h— »<n en ^ ^ mm 13 76885 1 The errors in Table 1 are calculated according to the law of accumulation of the mean error as follows: - V .V (L - i-) 2 (Ä, 2 + iiiOi + uor, ( 5) τ1 τ2 L τ, 4 τ24 5 where v - gas flow rate 10 rrr2 * sound travel times upstream and downstream A 1 »Δ 2" standard deviation of travel times Δ ^ measurement interval error (* 0.002 m) L * measurement interval length (* 6.29 m).

15 Λ /, Δν 2 ^ (6) ÄQ s Q · V (-) 2 + (^ -) # where 20 v flow rate ^ v flow rate error A - average cross-sectional area over the measurement range ^ A / A average cross-sectional area error = 2 · / ^ R / rr * inner radius of the tube («= 42, A5 mm) 25 ^ r - error of the inner radius of the tube (+0.1 mm).

The measurement results were compared with the FL0WTEC DMV6330 vortex flowmeter 20 manufactured by Endress + Hauser, the accuracy of which is reported by manufacturer 3 as 1% of the final value (250 m / h) or the measured value.

30

Figure 9 shows the correlation of acoustic flow measurement to vortex flow measurement.

A common parameter in the measurement is the supply frequency 55 of the motor 11a of the fan 11, fQ. Using this frequency, Fig. 10 compares the dependence of the flow rate on the rotational frequency f of the motor 11.

14 76885 1 Acoustic flow measurement and vortex flow measurement gave compatible results within the error limits. The correlation 2 of the methods (r * 0.9994, Fig. 8) and the response of the methods to changes in motor frequency f 2 ° (r * 0.9999) show that the acoustic flow measurement 5 according to the invention is a very linear and reliable way to measure flow rate and flow rate.

The acoustic measurement according to the invention also appears to be insensitive to changes in temperature (compensation), pressure, density and flow profile 10 during the measurement interval. Thanks to the correlation technique, accurate time measurement can be achieved at relatively low frequencies. The best relative initial resolution for the equipment was (v 0 m / s) At / t 0.06 ps / 18271.6 us 3Ί0 ^ for the measurement of five consecutive sound travel times (measurement time 5.0 s / measurement time).

15

The shortest possible measurement time of the equipment described above is about 0.1 s, which provides a quick way to study the change in flow velocity over a wide (0 - 100 m / s) measuring range.

20 Possible changes in temperature between upstream and downstream measurements may also have an effect on measurement errors, as a 0.1 degree change causes an additional error in the flow rate of approx. + 0.03 m / s. This can be eliminated by measuring the temperature or by measuring the flow time of the sound upstream and downstream simultaneously.

25

In the following, the claims are set forth within the scope of the inventive idea defined by which the various details of the invention may vary and differ from those set forth above by way of example only.

30 35

Claims (10)

15 76885 An acoustic flow measurement method for measuring the flow rate, volume flow and / tal mass flow of gases, liquids and / or multi-phase suspension in a tube (10a) or a corresponding waveguide using upstream and downstream sound waves, characterized in that the sound sources (13a) the wideband audio signal is passed through a measuring tube (10a) or a corresponding waveguide in both 10 upstream and downstream planar wave modes and that the maximum and / or minimum of the correlation functions (R ^ 2) obtained for the upstream and downstream audio signals (f ^ (t), f2 (t)) the flow rate is determined on the basis of the distance (L) between the measuring points (j ^, ^ 2) 8 * kä of the sound travel points (j ^, ^ 2) and, if necessary, by calculating the volume flow and / or mass flow in gases, liquids and multiphase suspensions. 1 Claims Method according to claim 1, characterized in that the flow rate (v) to be measured is calculated on the basis of the distance (L) between the measuring points (M 1, M 2) and the sound travel times (* Γ ^ and Ί ^) obtained from said correlation function upstream and downstream from the formula 25 1 Λ A V'H ^ 'V and / tal that the volume flow in the gases, liquids and multiphase suspension is calculated using the average 30 internal cross-sectional area (A) of the measuring tube (10a) Method according to claim 1 or 2, characterized in that the method uses a polarity correlator (100) which is substantially independent of amplitude. 16 76885 Method according to one of Claims 1 to 3, characterized in that the method uses a frequency sweep with a most sinusoidal waveform as the broadband audio signal, and that the upper limit frequency of the frequency sweep is generally in the range from 0.1 to 10 kHz, taking into account formula (1). Method according to one of Claims 1 to 4, characterized in that the transmission time of the broadband audio signal, such as the frequency scan time, is set to the same as the measurement start time of the correlator (100). Method according to one of Claims 1 to 5, characterized in that a plurality of measuring tubes (10 to 10) are applied in the method, each of which is connected in turn via a multiplexer unit (130) to the same correlator unit (100). Apparatus for carrying out the method according to any one of claims 1 to 6, comprising a measuring tube (10a) or a similar waveguide in which the flow to be measured flows, which apparatus further comprises an audio signal transmission sensor such as loudspeakers (13a, 13b) spaced apart. to each other in connection with a measuring tube (10a) or the like and that the device comprises a sound receiving sensor such as ml-25 phones (14a, 14b), characterized in that said sound receiving sensor (14a, 14b) is arranged at a certain known distance (L) from each other and said audio signal the first (13a) of the transmitting sensor (13a, 13b) is located at a distance upstream of the first sound receiving sensor (14a) sufficient for the formation of the planar wave mode 30 and the second of the audio signal transmitting sensors (13b) is located at a distance downstream of said second sound receiving sensor the device comprises a signal generator (14) and an amplifier (15) for supplying wideband audio signals to said first and second audio transmission sensors (13a, 13b) and 1776885 1, the device further comprising a correlator (100) to which signals from the first and second audio reception sensors (14a, 14b) are input. Device according to claim 7, characterized in that said correlator is a polarlethel correlator (100). Device according to claim 8, characterized in that the electrical signals obtained from said first and second sound receiving sensors (14a, 14b), preferably microphones, are fed to a comparator (110) where they are compared and the reference output signals are fed to said polarlet correlator (100). Device according to one of Claims 7 to 9, characterized in that the device comprises a plurality (N) of separate measuring tubes 15 (10 ... in a suitable measurement sequence in turn for the same correlator unit (100). 20 25 30 35 18 768 8 5 Ί Patentkrav