GB2301890A

GB2301890A - Ultrasonic liquid interface detector

Info

Publication number: GB2301890A
Application number: GB9611346A
Authority: GB
Inventors: Matti Nissinen
Original assignee: Acutest Oy
Current assignee: Acutest Oy
Priority date: 1995-06-07
Filing date: 1996-05-31
Publication date: 1996-12-18
Also published as: NO962274D0; FR2735228A1; GB9611346D0; NO962274L; FI952784A0; FI97829B; FR2735228B1; FI97829C

Abstract

An elongated stop 1 reaching across the interfaces is mounted vertically inside tank 2. A transceiver 3 is mounted against the tank body and moved parallel to the stop. Discrete changes in the propagation speed of sound between the transceiver and the stop are used to locate the interfaces. Alternative arrangements include: replacing the movable transceiver with a number of fixed transceivers (fig 2); mounting a stop and a transceiver in the same movable construction (8, fig 2) to be immersed in the tank; and fixing a vertical pipe (10, fig 2) to the outside of the tank with transceiver device placed against the pipe and one wall of the pipe acting as a stop.

Description

Measuring method and apparatus for detecting interfaces The invention relates to a measuring method which provides a safe and reliable detecting of interfaces when various substances form overlap ping layers which arrange themselves by density. The invention further relates to an apparatus for implementing the method.

In the methods used in the prior art, for instance interfaces of various liquids are found in liquid reservoirs by running off liquid through outlets located in different levels and determining visually the moment when the liquid changes its colour or turns into gas. To find for instance hydrofluoric acid, a wooden stick can be used, the colour of which tums dark in the acid. The detection of interface used in this way is inaccurate and when liquids are emitted into the open air, harm to the environment and actual material losses are effected. Further, the risk of accident is always present when the measuring relates to toxic and flammable materials, such as hydrofluoric acid or corresponding materials. The person conducting the measuring has to wear an appropriate protecting clothing manufactured of neoprene and a closed hood, inside which respiratory air is led by a hose.In addition, protective gloves and shoes have to be worn. Furthermore, several outlets have to be mounted in the tanks only to detect interfaces. This will lead to unnecessary production and maintaining costs and deteriorate the construction of the pressure vessel.

The use of ultrasound for detecting surfaces of liquids in closed tanks and containers is discussed in various patent publications. For example, publication GB-2050604 discloses measuring of resonance to detect liquid level, and it involves the use of a resonant frequency of the container itself. Publication GB-2100430 discusses basic technology of reflection. Here the transducer is placed on the bottom of a tank. The distances to the interfaces of the above liquids remain long. Publication GB-2126342 introduces the possibility to examine the height position of a liquid surface by using for example an echo obtained from the opposite wall of the tank. The device detects whether the echo is present or not Because a sufficient echo will not be obtained above the liquid surface from the opposite wall, the detector device functions on the on/off basis.The detector can be used for detectinc e quid level, but not the position of the interfaces between various liquid The purpose of the invention is to present a novel method which can be used for detecting interfaces of various substances, provided that the substances in the container have different speeds of sound propagation. The method can be particularly used in chemical and petrochemical industry in connection with processing and storing of different substances within closed containers which have walls not permitting visual inspection.

To implement what is discussed above, the method according to the invention is characterized in what is presented in the characterizing part of the accompanying claim 1. The method is employing a particular stop which is separate from the tank wall and located at a constant distance from a ultrasonic transducer device in opposed position thereto. From this stop the ultrasound reflects back to the transducer device. The length of the reflection path is smaller than the horizontal reflection path between the transducer and the opposed wall of the container otherwise would be if there were no stop surface.

The apparatus of the invention is in its turn characterized in what is pre sented in the characterizing part of the accompanying claim 5. The apparatus is easy to mount into existing tanks, and it has various possible embodiments.

The main advantages provided by the invention include the following: The invention can be used to accurately detect interfaces. By using the method, there is no need to discharge the material from the tank. Consequently, no harm to the environment and no material losses occur.

Measuring can be conducted in a safe manner without immediate danger to touch the material, because it is not necessary to run the material out from the tank. In addition, the construction of pressure vessels is simplified because no exhaust outlets are required. This will save costs in manufacturing and maintenance.

Further, the detecting method in accordance with the invention can be applied in a manner that the control of the interfaces can be imple mented from a control room without any need for the operator to measure on the spot.

As opposed to other methods employing the ultrasound, the present method provides the advantage that the measuring can be conducted from the side walls of the tank, which are easy to reach unlike the space below the bottom, as presented in the method of publication GB2100430. The short reflection distance enables a sufficiently strong signal also in tanks of large diameters, typically over 1 m, and of considerable height.

In the following description, the invention is described in more detail with reference to the accompanying drawings, where Fig. 1 shows the measuring situation schematically in a tank containing various materials, the tank being shown in a cross section, and a displaying unit of the measuring device showing the interfaces, and Fig. 2 shows some further embodiments of this invention.

Fig. 1 illustrates schematically a tank containing three different substances assembled in layers according to gravity. The cross section of the tank shows substances a, b and c. To find the interfaces between the substances, an elongate stop 1 reaching across the interfaces has been mounted in the tank in vertical position by fixing it in the tank body 2 inside the tank at a constant distance from the inner surface of the vertical side wall of sufficiently rigid construction to retain the volume of the substances. In the lower part the side wall is joined to substantially horizontal bottom and on the top it is closed by a cover. The tank can be of ordinary construction commonly employed in process industry and it can contain well-known outlets and inlets for connecting it to the process.The horizontal distance between the stop and the side wall is about 10 cm and in any case smaller, preferably at least two times smaller than the "average radius" R of the large tank, which is the radius of a circle having an area equal to the area of the horizontal cross-section of the tank. In tanks of circuiar cross-section this average radius equals the actual radius, whereas in tanks of oval or polygonal cross-section this radius is merely an average distance from the center to the inner wall.

The interface in a tank equipped in this manner can be found by employing ultrasound and usual instrumentation used in NDT (non-destructive testing). A ultrasonic transducer device 3 arranged to transmit ultrasonic bursts and to receive reflected bursts is moved vertically paralielly to the stop in contact with the outer surface of the tank wall across the assumed interfaces of the substances. The ultrasonic signal transmitted by the transducer device 3 rebounds from the solid stop 1 extending parallelly with the moving direction. The propagation speed of sound is shown in the display unit, like in the screen 4 of the display device. The retuming echo of the reflected ultrasound is also shown in the screen as an echo 5. The time span between the initial signal 6 and the returning echo is seen in the axis A as a variable X.

When the transducer device is moved in the interface between substances having different sonic speeds, the interface is seen as a quick change in the propagation speed of sound, which can be detected as a changed time span Xl of a returning echo 7. Thus, the transducer device 3 can be used to reliably determine the interfaces of the substances without having to remove the substance from the tank, the pv sition of the transducer device 3 at the moment of the change indicating the exact vertical position of the interface. According to the respective locations of the transducer device 3, the interfaces of the substances are registered and even the quantities of different substances can be defined relatively accurately by multiplying the measured height difference of the interfaces with the known cross-sectional area of the tank. It is possible that the sonic speeds in two adjacent liquid substances do not differ much, but since the reflection time can be observed continuously, it is easy to observe even when a small permanent change occurs, indicating the existence of the interface. Of course, it is possible to detect with the transducer device also the upper level of all liquid substances, that is, liquid level in the tank, as well as a level of a solid substance on the bottom such as a precipitate.

It is particularly to be noted that in the method in accordance with the invention, the moving of the transducer device on the surface of the tank can be implemented manually or mechanically, wherein it can also be automated. Any known transducer device and associated instrumentation known in the art can be used, like for example that shown in pub lication GB 2050604. The transducer device 3 is typically a movable head containing piezoelectric material and connected through cable to control means, which include means for exciting the piezoelectric material and a display unit for continuous observation.

Furthermore, several transducer devices 3 can be mounted in the tank fixedly and spaced vertically at regular distances, as shown in Fig. 2.

The interfaces of the substances are in this case under constant control according to the locations of the transducer devices, and the relative movement of the transducer device 3 and the interface is caused by vertical shift of the interface. Each transducer device 3 is connected to a display unit 4 allowing to selectively monitor each transducer separately. Further ways of implementing the measuring method according to the invention include a fixed stop 1 mounted in the same movable construction 8 with the transducer device 3, wherein the ultrasound transmitted by the transducer device rebounds from the fixed stop located at a constant distance.The construction 8 is immersed in the tank to be surveyed containing various substances, wherein by moving the transducer device vertically by rod 9 or other suspension means guiding also the cable to the tank the interfaces of the substances can be detected on the basis of the change in the propagation speed of the ultrasound. The vertical location of the transducer device in the tank to be surveyed indicates where the interfaces of the substances are situated. The suspension means can be equipped with a scale or other measuring means for indicating the height of the construction 8 at each moment.

The stop which is mounted in the tank in a fixed manner can also be located outside the side wall of the tank. It is possible that outside the tank there exists a separate vertical pipe 10 having a small diameter and lateral connections to the tank, so that the interfaces in the pipe 10 are situated at the same level as in the tank. This embodiment is indicated by broken lines in Fig. 2. When the transducer device is placed against the pipe, the opposite inner wall of the pipe 10 operates as a particular stop. One movable transducer device 3 or several fixed transducer devices can be used in conjunction of this pipe on the same principle as with the stop 1 placed within the voiume of the tank.

Any material causing sufficient reflection of sound can be used as material for the stop 1, many metals and alloys fulfilling these requirements. It is also clear that any frequency in the ultrasonic range above 20 kHz apparent for a man skilled in the art can be used, the frequencies from 0,1 to 10 MHz being the most common in ultrasonic survey. An interface between any two substances causing different propagations of sound therein can be detected by means of the apparatus.

The invention is presented above with reference to some examples of application. However, this is not by any means to restrict the invention to concern only these examples, but several modifications are possible within the scope of the inventive idea defined by the following claims.

Claims

Claims:

1. A measuring method for detecting interfaces, which method based on the propagation speed of sound is used when searching for interfaces of various substances in a tank among the substances having different propagation speeds of sound, wherein the method employs a sound transmitting and receiving transducer device (3), characterized in that the propagation speed of sound transmitted and received by the transducer device (3) in the substance contained in the tank between the transducer device (3) and a stop (1) located at a constant distance from the transducer device (3) and separately from the body (2) of the tank is monitored in order to distinguish between substances having different sonic speeds.

2. A measuring method as set forth in claim 1 characterized in that the stop (1) is mounted into the tank to reach across the interfaces at a constant distance from the place the transducer device is placed, wherein the transducer device (3) is moved along the place it is placed parallelly to the stop (1), or several transducer devices are fixedly mounted on the tank at the stop (1) along a line parallel to the stop.

3. A measuring method as set forth in claim 1, characterized in that the stop (1) is mounted in the same movable construction (8) with the transducer device (3) at a constant distance from it, wherein the construction (8) is immersed in the tank to detect various interfaces.

4. A measuring method as set forth in claims 1, 2 or 3, characterized in that the propagation speed is monitored on a display unit (4), where the time (X) between a sound signal (6) transmitted by the transducer device (3) and the received echo (5) can be detected visually on a time-axis (A), and when the transducer device (3) and the interface move in relation to each other in a manner that a substance having a different sonic speed enters the range of action of the sound signal, the propagation speed of the sound signal coming from the transducer device (3) changes according to the properties of the substances, and on the basis of the time difference between the returning echo (7) and the echo (5) of the previous substance it is possible to note the interface where the substances are connected to each other.

5. An apparatus for detecting interfaces in a tank, comprising a sound transmitting and receiving transducer device (3), characterized in that a stop (1) separate from the body (2) of the tank is situated at a constant distance from the transducer device (3).

6. An apparatus as set forth in claim 5, characterized in that the stop (1) is mounted in the tank to reach across the interfaces at a constant distance from the place the transducer device (3) is placed.

7. An apparatus as set forth in claim 5, characterized in that the stop (1) is mounted in the same movable construction (8) with the transducer device (3) at a constant distance from it, and the construction (8) is connected to suspension means (9) for immersing it into the tank.

8. An apparatus as set forth in any of the preceding claims 5 to 7, characterized in that the transducer device (3) is connected to a display unit (4).

9. A measuring method for detecting interfaces, sl.bst,ntia s hereinbefore described.

10. An apparatus for detecting interfaces in a tank, substantially as hereinbefore described, with reference to the acconpanying drawings.