WO2001059356A2

WO2001059356A2 - Device for determining the filling level of liquefied gases in a cryogenic container

Info

Publication number: WO2001059356A2
Application number: PCT/EP2001/001344
Authority: WO
Inventors: Helmut Henrich; Joachim Kruell
Original assignee: Messer Griesheim Gmbh
Priority date: 2000-02-11
Filing date: 2001-02-08
Publication date: 2001-08-16
Also published as: DE10006221A1; WO2001059356A3

Abstract

Known devices for determining the filling level of liquefied gases in a cryogenic container are provided with an array of sensors distributed vertically along the volume to be measured, and with a signal transmitter for producing a signal that can be detected by the sensors, as well as with a level indicator. The aim of the invention is to provide, starting from the prior art, a simple and low-maintenance level meter. To this end, the signal transmitter comprises a magnet (11) linked with a floating body (10). Said magnet can be displaced along an array of magnetic field sensors (5) and produces a magnetic field that is detected depending on the filling level (8) by one or more of the magnetic field sensors (5) and is transmitted to the level indicator (6) in the form of a filling level signal.

Description

Device for determining the level of liquefied gases in a cryogenic container

The invention relates to a device for determining the filling level of liquefied gas in a cryogenic storage dewar comprising an arrangement of a measurement height of distributed sensors, and a signal generator for generating a detectable signal from the sensors ^", and with a level indicator.

Cryogenic containers for holding a liquefied cryogenic gas (such as air, nitrogen, oxygen, hydrogen, noble gas or natural gas) are one

Surround the vacuum envelope. Special devices are required to determine the fill level in these containers.

A generic device for determining the level is described in DE-A 34 21 803. It deals with the determination of the level of low-boiling, liquefied gases in a cryocontainer using electrical, current-carrying sensors, which are arranged inside the cryocontainer. Outside the cryogenic tank, these are connected to a measuring device, by means of which a temperature-dependent electrical property of the measuring sensors, such as the electrical resistance, is continuously recorded. As soon as a probe is immersed in the cryogenic liquid or emerges from it when the liquid level drops, the monitored electrical property changes, so that in connection with the local arrangement of the probe in question in the cryocontainer, the instantaneous fill level can be read directly using the measuring device. In order to determine the fill level over a predetermined measuring height, several such measuring sensors are evenly distributed over the range of the measuring height.

The measuring device must be calibrated before start-up and every time electrical components are replaced, which requires a certain amount of measurement technology. The object of the invention is to provide a simple and low-maintenance device for determining the fill level in a closed container.

Starting from the device mentioned at the outset, this object is achieved according to the invention in that the signal transmitter has a magnet connected to a buoyancy body, which can be moved along an arrangement of magnetic field sensors and which generates a magnetic field which, depending on the fill level, of one or more of the Magnetic sensors detected and fed to the level indicator as a level signal.

The specific density of the buoyancy body is such that it floats on the cryogenic liquid. Accordingly, it follows the liquid level in the manner of a conventional float, so that it moves up and down within the cryocontainer depending on the fill level of the cryogenic liquid.

At least one magnet is firmly connected to the buoyancy body. Through the connection, the magnet follows the movements of the buoyancy body and thus the level of the cryogenic liquid. With falling or rising

Level, the magnet moves at a predetermined distance along the arrangement of the magnetic field sensors. It generates a magnetic field, the distance and the magnetic field strength being matched to one another such that the magnetic field is detected by at least one magnetic field sensor. Instead of a single magnet, several magnets can also be used, taking care that the maxima of the magnetic fields generated in each case lie on approximately the same horizontal plane in order to prevent incorrect information about the fill level.

The arrangement of the magnetic field sensors directly detects the absolute height of the magnet inside the container. Calibration is not necessary for this. The device according to the invention can therefore be implemented with little outlay in terms of apparatus and measurement technology. The magnetic field sensor acts either as a switch or as a measuring device. The detection of the field by a magnetic field sensor acting as a switch leads to a switching process or a pulse being generated from a predetermined minimum field strength, which is fed to the level indicator as a level signal. In the other case, the field strength or a measurement variable that can be correlated therewith is measured and the measurement value is fed to the level indicator.

An embodiment of the device according to the invention is preferred in which the magnetic field sensors are designed as reed contacts and in which the magnet is a permanent magnet. The magnetic field sensors act as switches. Taking into account the field strength generated by the permanent magnet in the liquefied gas or in the gas atmosphere above it, the distance between the arrangement of the reed contacts and the permanent magnet must be set so that only a part of the reed contacts responds to the magnetic field. This ensures that, depending on the fill level, different reed contacts of the arrangement generate a fill level signal. Magnetic field sensors in the form of reed contacts are inexpensive and require no maintenance.

The buoyancy body within the cryocontainer is advantageously loosely surrounded by an open guide sleeve that extends along the arrangement of the magnetic field sensors. The guide sleeve is open at the top and bottom and runs in a vertical orientation along the magnetic field sensors. It makes it easier to maintain a predetermined distance between the arrangement of the magnetic field sensors and the magnet. In the simplest

Embodiment, the guide sleeve has a circular or rectangular cross section and extends parallel to the linear arrangement of the magnetic field sensors. However, the guide sleeve can also be designed in the form of a double tube surrounding the arrangement of the magnetic field sensors, for example in the form of an annular gap which coaxially surrounds the linear arrangement of the magnetic field sensors.

It has proven to be particularly advantageous to set the distance between adjacent magnetic field sensors to one another at least so small that the test signal of an excitation element which is equidistant from both magnetic field sensors can be detected. This embodiment of the device according to the invention is particularly advantageous in the case of switching sensors in the form of reed contacts, because this always leads to a switching overshoot and thus an evaluable fill level signal is generated. Even if the magnet is exactly between two magnetic field sensors. This is because the switching overshoot becomes effective at least for the two magnetic field sensors closest to the magnet.

An embodiment of the invention is particularly useful

Device in which the cryocontainer is a vehicle tank for liquid natural gas. For physical reasons (low geodetic height and low liquid density), the known level indicators are less suitable for this application.

In a preferred embodiment of the device according to the invention, the buoyancy body is designed as a hollow body which surrounds the magnet. The magnet is arranged inside the hollow body so that it cannot slip or fall off. The volume of the hollow body is matched to the buoyancy of the cryogenic liquid and the material is selected so that it does not or does not significantly impair the magnetic field. For example, aluminum can be used.

The invention is explained in more detail below with the aid of exemplary embodiments and a drawing. In the drawing, the following is shown in a schematic representation:

Figure 1 shows an embodiment of the device for

Measurement of the fill level in an vacuum-insulated liquid gas tank in a front view, FIG. 2 shows the fill level measuring device used in the device according to FIG. 1 in an enlarged illustration in a side view, and FIG. 3 shows a circuit diagram for the measuring device according to FIG. 2. FIG Liquid gas tank 1 shown with a vacuum jacket 2. A level measuring device is arranged within the liquid gas tank 1, which is shown in FIG overall, the reference number 3 is assigned and which is described in more detail below with reference to FIGS. 2 and 3. The fill level measuring device 3 comprises a carrier element 4, which is arranged in a vertical orientation within the liquid gas tank 1 and on which a plurality of reed contacts 5 are fastened. The reed contacts 5 are on the

Measuring height "M" evenly distributed. Furthermore, the level measuring device 3 is connected to a display and evaluation unit 6.

The liquid gas tank 1 is filled with liquefied natural gas 6; the liquid level and thus the current fill level is designated by the reference number 8 in FIG. 1. For filling the liquid gas tank 1 and for removing liquid natural gas, a filling and removal device 9 is provided which projects into the liquid gas tank 1 from the outside.

In FIG. 2, the part of the level measuring device 3 arranged inside the liquid gas tank 1 is shown on an enlarged scale. On the carrier 4, which consists of a glass fiber reinforced plastic plate, a total of ten, electrically connected reed contacts 5 are attached at a distance of 10 mm to each other. Two reed contacts 5 are combined to form a switch (S). In the exemplary embodiment according to FIG. 2, the five reed contacts 5 thus result in five switches S1 to S5, which are distributed uniformly over the measuring height “M”.

Extending in direct contact with the arrangement of the reed contacts 5 is a guide sleeve 9 which is open on both sides and also consists of a glass fiber reinforced plastic. The guide sleeve 9 has a circular cross section with an inner diameter of 28 mm. In Figure 2, a distance between the guide sleeve 9 and reed contacts 5 is shown only for the purpose of clearer illustration.

A float in the form of a closed aluminum tube 10 is guided within the guide sleeve 9. The outer diameter of the aluminum tube 10 is slightly smaller than the inner diameter of the guide sleeve 9 and the inner volume is due to the buoyancy of the liquid

Natural gas 7 adjusted so that it always moves parallel to the liquid surface 8.

The distance between the longitudinal axis of the aluminum tube 10 and the

Arrangement of the reed contacts 5 is approximately 10 mm. Centrally within the Aluminum tube 10 st a rod-shaped permanent magnet 11 attached, which is designed so that the magnetic field strength generated in front of him causes a switching operation in the area of an opposing horizontal contact 5. The aluminum tube 10 and thus also the permanent magnet 11 fastened therein follows any change in the fill level 8 within the cryogenic tank 1 by moving up and down within the guide sleeve 9. Depending on the presence of a magnetic field generated by the permanent magnet 11, one or two of the switches S1 to S5 are switched. The switching process is registered by the display and evaluation unit 6 and displayed as information about the current fill level. As can be seen from the circuit diagram in FIG. 3, a light-emitting diode is assigned to each of the switches S1 to S5. The fill level is indicated by the light-emitting diode symbolizing the switches S1 to S5 in question. The distance between the permanent magnet 11 and the arrangement of the reed contacts 5, the distance between the individual reed contacts 5 and the field strength in the area of the reed contacts 5 and their sensitivity are matched to one another such that at least one of the reed contacts 5 is always connected. In the event that the permanent magnet 11 is located in the area between two adjacent reed contacts 5, there is a switching overlap, so that both reed contacts 5 are closed, which accordingly leads to the connection of two adjacent switches S1 to S5 and is indicated by the two corresponding light-emitting diodes becomes. The level signal generated in this way can be further processed without complex evaluation electronics. For example, when the uppermost switch S5 is reached, a signal for switching off the filling process can be generated. When the lowest switch S1 is reached, a reserve lamp can be activated in addition to the lowest LED or instead of this, which indicates that the tank content is nearing its end.

In the circuit diagram of FIG. 3, each of the switches S1 to S5 is formed by one of the pair of reed contacts (see FIG. 2). Two adjacent reed contacts (FIG. 2) are always combined in parallel to form a switch S1 to S5. Each of the switches S1 to S5 is connected to a light-emitting diode LED1 to LED5 via a series resistor R1 to R5. In the switching state shown in Figure 3, the switch S1 is closed and thus the light emitting diode LED1 activated. This means that one of the reed contacts 5 of switch S1 shown in FIG. 2 is switched.

According to the filling level 8 (FIG. 1), the following switching states are possible in this exemplary embodiment: LED1; LED1 + LED2; LED2; LED2 + LED3; LED3; LED3 + LED4: LED4; LED4 + LED5; LED5. The number of reed contacts and the light emitting diodes is increased or decreased depending on the container height and the desired resolution of the level measurement.

Claims

claims

1.Device for determining the fill level of liquefied gases in a cryocontainer, comprising an arrangement of sensors distributed over a measuring height, and a signal transmitter for generating a signal detectable by the sensors, and with a fill level indicator, characterized in that the signal transmitter has a buoyancy body ( 10) connected magnet (11), which can be moved along an arrangement of magnetic field sensors (5) and which generates a magnetic field which, depending on the fill level (8), is detected by one or more of the magnetic field sensors (5) and as a fill level signal Level indicator (6) is supplied.

2. Device according to claim 1, characterized in that the magnetic field sensors (5) are designed as reed contacts and that the magnet (11) is a permanent magnet.

3. Device according to claim 1 or 2, characterized in that the buoyancy body (10) within the cryocontainer (1) is loosely surrounded by an open, extending along the arrangement of the magnetic field sensors (5) guide sleeve (9).

4. Device according to one of the preceding claims, characterized in that the distance between adjacent magnetic field sensors (5) to one another is set at least so small that the test signal of an excitation element (11) equidistant from them can be detected by both magnetic field sensors (5).

5. Device according to one of the preceding claims, characterized in that the cryogenic holder (1) is a cryogenic tank for liquid natural gas.

6. Device according to one of the preceding claims, characterized in that the buoyancy body (10) is designed as a hollow body which surrounds the magnet (11).