Andersen, Poul Arndt (Flaktveitveien 386 Flaktveit, N-5091, NO)
Hansen, Rolf Kahrs (Kolåshøyden 56 Flaktveit, N-5091, NO)
Andersen, Poul Arndt (Flaktveitveien 386 Flaktveit, N-5091, NO)
| 1. | Acoustic instrument for measuring fluid levels in a tank or a container, the instrument comprising coupling means for coupling the instrument near the exterior bottom surface of the tank and at least one piezoelectric transducer for emitting and receiving acoustic energy to and from the tank, and control means coupled to the transducer means for generating and receiving signals to and from the transducer, c h a r a c t e r i z e d in that at least one of the piezoelectric transducers includes a substantially horizontal, twodimensional matrix of essentially identical rodshaped, essentially vertically oriented piezoelectric elements, having lengths being substantially longer than their cross sections. |
| 2. | Acoustic instrument according to claim 1, c h a r a c t e r i z e d in that said piezoelectric elements are contained in a matrix of a polymer material. |
| 3. | Acoustic instrument according to claim 1, c h a r a c t e r i z e d in that the at least one transducer comprises at least 100 piezoelectric elements, said elements being connect in parallel to the control means. |
| 4. | Acoustic instrument according to claim 1, c h a r a c t e r i z e d in that it includes at least one piezoelectric transducer capable of emitting and receiving acoustic signals. |
| 5. | Acoustic instrument according to claim 4, c h a r a c t e r i z e d in that the piezoelectric elements in said transducer are interconnected to substantially simultaneously generate an acoustic signal as a response to a signal generated from the control means. |
| 6. | Use of an acoustic instrument according to one of claims 15 to control the fluid separation in a separation process, wherein one or more instruments are positioned along the length of an elongated separation tank, through which a fluid mixture is moving at a chosen velocity. |
| 7. | Method for measuring fluid levels in a tank or a container containing one or more fluids, comprising the emitting of an acoustic signal from a piezoelectric transducer acoustically coupled to the exterior of the bottom of the tank or container, c h a r a c t e r i z e d in comprising the steps of: receiving acoustic signals reflected in the tank at a substantially horizontal matrix of essentially identical, rodshaped piezoelectric elements, essentially vertically oriented, having lengths being substantially longer than their cross section, positioned at essentially the same position as said emitting transducer, finding the time lapsed from said emitting of a signal to said receiving of a corresponding reflected signal, and locating the fluid level or levels from said time lapse. |
| 8. | Method according to claim 7, c h a r a c t e r i z e d in that said matrix includes piezoelectric elements being shaped as rods having a length being substantially longer their cross sections, said rods having an essentially vertical orientation, and being contained in a matrix of a polymer material. |
| 9. | Method according to claim 7 or 8, c h a r a c t e r i z e d in that said generated signal includes acoustical pulses, and that said received signal includes pulses reflected from the transition zones between the fluids. |
| 10. | Method according to claim 9, c h a r a c t e r i z e d in that it includes measuring of the width, and possibly the amplitude, of said received pulses and generating on this basis a signal indicating the thickness of said transition zones. |
| 11. | Method according to any one of the claims 710, c h a r a c t e r i z e d in that the signal is emitted and received from the same acoustic transducer. |
In fluid tanks, such as fuel tanks, beverages oil/gas- tanks etc, there is a need for instruments being capable of measuring the liquid level without having a sensor posi- tioned inside the tank. This may be because the entering point of the sensor weakens the tank, because it contains fluids which may destroy the sensors, or simply because it is facilitates easy installation, service and maintenance.
In oil/gas-production there may be several different layers of fluids, having different densities. Thus a number of fluid-levels must be measured using the same instrument.
Because of the difference in acoustic impedance between gases and liquids, and thus the large reflectivity on the surface separating these phases, it is difficult to measure several fluid levels using a transducer positioned on the top of the tank, as the reflections from surfaces between different liquids will be much weaker. Therefore it is preferred to position the transducers under the tank, emitting acoustic energy vertically to obtain maximum reflection from the different layers.
US patent 4,901,245 describes a non-intrusive sensor system using a piezo-electric transceiver mounted on the bottom of a tank and directs pulses of sound energy verti- cally towards a liquid surface. The system is adapted to find the time difference between the emission of the pulse and the receipt of the reflected pulse, and to find the liquid level based on the sound velocity of the liquid.
A problem with this type of sensor systems is false echoes and ringing effects in the tank and the sensor system, reducing the precision of the measurements and thus making simultaneous measurements of more than one fluid level difficult. The US patent removes unwanted reflections
from the signal by disregarding signals indicating source- to-surface distances outside a given range. This, however, does not really solve the problems associated with the precise measurements of several layers of fluids.
Another problem relating to the solution described in the US patent is related to the fact that the transition zones between the fluids may not be a well defined surface, but for example includes foam. Because of the lack of precision this type of surface is not measured correctly.
In GB 2.251.687 a two-dimensional, vertically oriented array of six piezoelectric elements is described for use in navigation through shallow waters. The matrix is being used to form a wide, but low, horizontal acoustic beam so as to avoid reflections from the bottom. The piezoelectric elements are rod-shaped and have a vertical orientation and the acoustic waves are generated oscillating the thickness of the elongated elements. The shape of the beam is provided by the interference between the signals from the different piezo-electric elements. Similarly the direction of the detected signals are found by correlating the phase signals from the different elements. This system is, however, not suitable for exact detection of signals received within small time periods from one direction, as the elements, connected to the electronic circuits individually or in pairs, will have the same limitations as the detector described in the abovementioned US patent.
It is an object of this invention to provide an acoustic instrument being capable of measuring several different fluid levels in a tank, container or the like.
This is obtained using an instrument as defined above and characterized in that at least one of the piezoelectric transducers includes a substantially horizontal, two- dimensional matrix of essentially identical rod-shaped, essentially vertically oriented piezoelectric elements, having lengths being substantially longer than their cross sections.
It is also an object of this invention to provide an acoustical instrument which may be positioned in one or more positions along an elongated separator tank to control the separation of the fluids in the tank.
It is also an object of this invention to provide a method for measuring of fluid levels in a tank or a container containing one or more fluids. The method according to the invention comprising the steps of -emitting an acoustic signal from a piezoelectric trans- ducer acoustically coupled to the exterior of the bottom of the tank or container, -receiving acoustic signals reflected in the tank at a substantially horizontal matrix of piezoelectric elements positioned at essentially the same position as said emitting transducer, -finding the time lapsed from said emitting of a signal to said receiving of a corresponding reflected signal, and -locating the fluid level or levels from said time lapse.
In a preferred embodiment of the invention the piezo- electric elements are shaped as rods having a length being substantially longer their cross sections, said rods having an essentially vertical orientation being contained in a matrix of a polymer material, and are adapted to oscillate in the longitudinal direction.
The thin rods will have a well known resonance frequency in the longitudinal direction, and, being contained in a polymer material having good damping characteristics, the transversal oscillations are reduced or almost removed completely. This reduces the ringing effects in the transducer and thus improves the sensitivity of the transducer. Also, as the piezoelectric rods are inter- connected, the sensitivity to acoustic waves having a direction other than vertical, will be reduced.
In this specific use the piezoelectric crystals are excited using an electric pulse creating an oscillation in the crystal. The material surrounding the piezoelectric crystals quickly dampens the oscillation and thus decreases the relaxation time of the crystals. The crystals are thereby able to receive the reflected signals much sooner than ordinary piezoelectric crystals, having a significantly longer relaxation time. This reduces the minimum measurable distance between the transducer and the surface.
In the publication"Manufacturing of 1-3 piezocomposite SonoPanelTM transducers"by R. Gentilman, D. Fiore, Hong Pham, W. Serwatka and L. Bowen, SPIE proceeding series, volume 2447, page 274-281, a composite material as described above is shown, being produced by Material Systems Inc.
The invention will be described in detail below, refer- ring the accompanying drawings, which show examples of pos- sible embodiments of the invention.
Figure 1 shows a cross section of a tank containing four different fluids, and illustrating a measurement being performed according to the invention.
Figure 2 illustrates a perspective view of a piezo-electric transducer used according to the invention.
In figure 1 a tank 1 is shown containing four different fluids, and thus having three different fluid levels 3a, 3b, 3c. Three different types of transition zones or interfaces between the fluids are shown, the first 3a being a steady, calm surface, the second transition zone 3b being diffuse, containing bubbles or the like, and the third 3c containing waves.
A transducer 2 is positioned beneath the tank 1 trans- mitting acoustic energy, preferably in pulses, into the tank in a substantially vertical direction and receiving the sig- nal. The bottom of the tank is made of a acoustically con- ductive material, and does not contain inner structures or large quantities of particles and the like on the bottom scattering the acoustic energy. The transducer comprises coupling means (not shown) for acoustically coupling the transducer to the tank. The piezoelectric material is preferably placed in a hermetically sealed housing, and the housing is fastened to the tank using an adhesive. The transducer may be held in position using a spring loaded device (not shown).
In the preferred embodiment the same transducer 2 is used for generating and receiving the acoustic signal. It is, however, possible to use a dedicated transmitter for generating the signal.
The first reflected acoustic energy 4a from the first interface 3a is well behaved and gives a sharply reflected signal. An acoustic signal transmitted into the tank is
reflected substantially without any distortion or scattering of the reflected signal. This fluid level may, at least in a tank containing two fluids, be measured using known equipment, such as described in the abovementioned US patent.
The second reflected signal 4b is reflected and scattered from an uneven transition zone 3b, possibly con- taining a partial mixture and bubbles of the two fluids.
Since the reflecting zone has a thickness the signal is reflected from a volume instead of a surface, which distorts the phase of the reflected signal, dampens and/or broadens the reflected pulses. Analysing the received signal, using commercially available equipment, thus gives a possibility to characterize the reflecting zone. For example a pulse reflected from a surface comprising foam may consist of a leading, slightly distorted wavefront followed by a decreas- ing amplitude making the reflected pulse broader than the emitted pulse.
This analysis may be used in the controlling of separator tanks using a number of transducers along an elongated tank. The width of the reflected pulse will indicate the degree of separation at each measured point, and thus provide an opportunity for controlling the different parameters of the process in order to maximize the result.
The third reflected signal 4c will have variations in the time lapse between the emitting and receiving of the signal. Detecting the phase of the signal received at each element also provides a possibility for detecting the direc- tion, and variations in the direction, in which the signal is reflected.
The fourth reflected signal 4d is reflected from the top of the tank, and is easily removed, as the distance is known.
Figure 2 shows an example of a transducer 2 comprising a two-dimensional array of rod-shaped piezo-electric elements 5. Preferably the number of piezo-electric elements is more than 100. As the elements have an essent- ially one-dimensional shape their vibrational character- istics are well defined.
The piezo-electric elements are embedded in a polymer material 6 being able to suppress transversal vibrations in the rods, and are preferably adapted to oscillate mainly in the longitudinal direction. Thus the only vibrations left in the rods are a well defined vibrational mode in the longitudinal direction of the rods. Thus unwanted vibrations may easily be filtered out of the measured signal using standard equipment.
In addition the piezoelectric elements are connected to each other in parallel, making it strongly directional and thus making it unsuitable for detecting or directing signals to or from other directions. The array of piezoelectric elements will thus be operated as a single piezoelectric element oscillating in the thickness mode, but without unwanted oscillations or distorting signals having other directions than the vertical.
When using the same transducer to emit and receive the acoustic energy the vibrations produced when emitting the signal are suppressed or may be filtered out providing a possibility for detecting fluid levels much closer to the transducer then the conventional equipment. Also, the structure of this transducer used according to the invention provides a more accurate measurement than the known instru- ments for measuring of fluid levels in tanks, since there are less vibrations distorting the measured signal.
If the transducer is provided with means for selectively controlling the timing of each piezo-electric element, the emitted signal may also be focused at a chosen level, as the relative delay of the emitted signals from each element will add up to a wavefront which may be focused or directed at a chosen position. This technique is described in the article of Sverre Holm in IEEE TRANS. UFFC, September 25,1997,"Bessel and conical beams and approximations with annular arrays". As the direction of the emitted beam may be changed by delaying the signals emitted from each element, the beam may be adjusted electronically to obtain a vertical beam, even if the transducer is not in an exact horizontal orientation.
Focusing may also be done by slightly altering the shape of the transducer or providing an acoustic lens on the surface
of the transducer.
The analysis of the signal is based on assuming that the signal is a periodic sinus-signal, and may thus be written as #=Acos(#t+#) where A is the amplitude, which may be time dependant, # is the frequency, t is time and # is the phase of the signal.
As sinx=cos (x-n/2) the periodic signal may be characterized completely by measuring the signal at a time tl and then at a time t2 corresponding to a phase shift of 90°. Thus <BR> <BR> <BR> <BR> <BR> #1=Acos(#t1+#)=Asin(#t2+#)<BR> <BR> <BR> ¢2=Acos (St2+ç)<BR> <BR> <BR> <BR> <BR> <BR> <BR> in which (D, and ¢2 are called the quadratic components of the signal. Thus the amplitude and phase of the signal are given by (#12+#22)1/2=a ans #1/#2=sin#/cos#=tan#
where the time dependant part A is neglected.
The received signal is analysed by finding the quadra- tic components of the received signal. This is done by first assuming that the received signal may be described as ¢=A (t) cos (#t) <BR> <BR> <BR> in which A (t) is the time dependant amplitude and w is the frequency. This signal is multiplied by a reference signal <BR> <BR> <BR> Bcos (t+a) with a constant amplitude B and a phase shift a in relation to the signal, and the reference signal shifted <BR> <BR> <BR> 90°, Bcos (#t+α-II/2) :<BR> cos(2#t+α) + cosα<BR> <BR> <BR> <BR> <BR> <BR> #1#(t)Bcos(#t+α)=A(t)B= <BR> <BR> 2 and
sin (2#t+α) + sinα<BR> <BR> <BR> #2 (t) =# (t) =A(t)B(#t+α-II/2) 2 We see that Vl and #2 both contain time dependant parts being periodic with a frequency 2u, as well as parts only depending on time and phase. The phase difference between the reference signal and the measured signal may also be time dependant, giving a=a (t). As the variations in A (t) and a (t) is limited in bandwidth the periodic signal with the frequency 2z may be removed using a low-pass filter. <BR> <BR> <BR> <P>Thus we have:<BR> α(t)<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> #1(t)=LP{#1(t)}=A(t)B cos<BR> <BR> <BR> α(t)<BR> <BR> <BR> <BR> <BR> <BR> <BR> #2(t)=LP{#2(t)}=A(t)Bsin<BR> <BR> 2 Since B is known the amplitude and phase of the signal in relation to the reference signal may be found.
Having generated l and ¢2 they may be digitalized at a much lower sampling rate than original signal. After being digitized the amplitude is
which defines the demodulated signal.
Defining the transit time from the transducer to the transition zone and back as #=2(#t+#f) in which Tt is the transit time through the material in the tank, and if is the transit time through the fluid below the transition zone. Of course, if may be the sum of transition times through different layers of fluids.
The fluid level sf above the bottom inside the tank is therefore sf=(#/2-st/ct)cf in which ct and cf is the sound velocity through the tank material and the fluid, respectively. The sound velocity through the fluid cf is found as a function of pressure,
temperature and the fluid characteristics, and ct is known.
By defining r as the time t when the value of C (t) exceeds a chosen threshold the level Sf may be found. Adding repeated measurements will improve the signal to noise ratio. A number of different fluid levels in the same tank will of course provide a number of different values of sf.
Measuring the duration, as well as the shape, of the received pulse and comparing this to the duration and shape of the emitted pulse may, as stated above, be used to char- acterize the reflecting surface.
Used in an separator tank, e. g. in the oil industry, a number of transducers may, as mentioned above, be used, positioned at different locations along an elongated tank, to control the quality of the separating process. As the fluids move slowly in the tank they separate, making it possible to take out e. g. water, oil and gas at different levels. The success of this method depends on a number of parameters, such as temperature, pressure, the time the fluids use through the tank and the addition of substances aiding the separation. These parameters may be adjusted according to measured quality of the surfaces, and thus to improve the result of the process.