DE102019108985A1 - Magnetic-inductive flow meter - Google Patents

Abstract

The invention relates to a magneto-inductive flowmeter that has a measuring tube, at least one magnetic field generating device for generating a magnetic field in the medium that is essentially perpendicular to the longitudinal direction, the magnetic field generating device comprising a pole piece or a saddle coil, the magnetic field generating device in a cross section of the measuring tube encompasses the measuring tube at a maximum central angle β, an electrode system with at least two electrode pairs which are set up to detect a voltage induced in the medium perpendicular to the magnetic field and to the longitudinal direction, a first pair of electrodes being arranged in a first cross-sectional plane perpendicular to the measuring tube axis, wherein a second pair of electrodes is arranged in a second cross-sectional plane lying perpendicular to the measuring tube axis, wherein the first cross-sectional plane is spaced apart from the second cross-sectional plane and which is thereby marked It is shown that a central angle α in an orthogonal projection of the cross-sectional planes spans a minimal sector of a circle in which the electrodes located on one side of the measuring tube and projected onto the orthogonal projection are distributed, and that the central angles α and β are coordinated so that the The flow meter is insensitive to deviations in a rotationally symmetrical flow.

Description

The invention relates to a magnetic-inductive flow meter.

Electromagnetic flowmeters are used to determine the flow rate and volume flow of a medium in a measuring tube. An electromagnetic flowmeter consists of a magnet system that generates a magnetic field perpendicular to the direction of flow of the medium. Single or multiple coils are usually used for this. In order to achieve a predominantly homogeneous magnetic field, pole pieces are additionally shaped and attached in such a way that the magnetic field lines run essentially perpendicular to the measuring tube axis over the entire pipe cross-section. A pair of electrodes attached to the outer surface of the measuring tube picks up an electrical voltage that is perpendicular to the direction of flow and to the magnetic field, which is generated when a conductive medium flows in the direction of flow when a magnetic field is applied. Since, according to Faraday's law of induction, the tapped voltage depends on the speed of the flowing medium, the flow rate u and, with the addition of a known pipe cross section, the volume flow rate V̇ can be determined from the voltage.

Electromagnetic flowmeters are sensitive to the flow profile of the medium. Depending on the pipe system and measuring device, measurement errors of several percent can occur. A straight tube, the length of which corresponds to at least five to ten times the nominal diameter of the measuring tube, is therefore usually installed on the inlet-side end face. However, there are known applications in which this minimum distance, the so-called inlet section, cannot be maintained. This is the case, for example, when a pipe system is in a confined space. The in DE 10 2014 113 408 A1 disclosed invention in which a narrowing of the pipe diameter leads to the conditioning of the flow, whereby the influence of the flow profile is minimized, so that a 0-DN inlet section can be used.

The disadvantage of this embodiment, however, is that although a lower sensitivity to rotationally asymmetrical flow profiles can be achieved, a pressure loss must be accepted. In addition, this configuration is limited to pipe systems with DN <350.

The sensitivity of the flow measurement to a rotationally asymmetrical flow profile depends on the geometry of the measuring tube and the electrodes. Therefore, the influences of the tube and electrode geometry must be taken into account for the correct description of the speed-dependent induction voltage. The two influences mentioned are described mathematically by a weight function GF.

wobei für die Bestimmung der Spannung U(x), die Strömungsgeschwindigkeit v(x') und die Gewichtsfunktion GF(x',x) über das Volumen des Messrohres integriert werden. Dabei wird die Gewichtsfunktion GF anhand GF(x',x) = B × ∇G(x',x), mit dem Magnetfeld B(x') und einer Greenschen Funktion G, die von den elektrischen Randbedingungen gegeben wird, beschrieben. Das Ziel eines Optimierungsverfahrens ist es nun die Geometrie des Aufbaus dahingehend zu optimieren, dass im gesamten Strömungsprofil ∇×GF = 0 gilt. Dies ist jedoch für ein Rohr mit einem einzelnen punktförmigen Elektrodenpaar nicht möglich. Einen möglichen Lösungsansatz liefert die Anpassung der Elektrodenform. Dies ist jedoch nicht praktisch und verursacht neue Schwierigkeiten. Ein weiterer Lösungsansatz besteht darin, mehrere Elektrodenpaaren zu verwenden.The influence of the geometry on the flow can best be illustrated by the following context: $$U\left(x\right)={\displaystyle \underset{V}{\int }v\left(x\text{'}\right)}G\mathrm{F.}\left(x\text{'},x\right)dV$$

where for the determination of the voltage U (x), the flow velocity v (x ') and the weight function GF (x', x) are integrated over the volume of the measuring tube. The weight function GF is described using GF (x ', x) = B × ∇G (x', x), with the magnetic field B (x ') and a Green function G, which is given by the electrical boundary conditions. The goal of an optimization process is to optimize the geometry of the structure in such a way that ∇ × GF = 0 applies in the entire flow profile. However, this is not possible for a pipe with a single point-shaped pair of electrodes. One possible solution is to adapt the shape of the electrodes. However, this is not practical and creates new difficulties. Another approach is to use multiple pairs of electrodes.

For example, from the CN 101294832 A a magneto-inductive flowmeter is known which has two pairs of electrodes which are arranged axially symmetrically in a pipe cross-section in order to minimize the influence of the flow profile on the determination of the volume flow. The two electrode axes defined by the respective electrode pairs span an angle of approx. 40 ° in the cross section of the measuring tube.

Another implementation is in DE 10 2015 113 390 A1 shown, in which a second and third pair of electrodes are arranged on defined electrode axes which are arranged at an angle of less than or equal to ± 45 ° with respect to a first electrode axis oriented perpendicular to the magnetic field.

The EP 0878694 A1 also discloses a magnetic-inductive flowmeter which, based on the prior art, improves the measurement accuracy in the area by using two additional pairs of electrodes, the electrode axes of which each form an angle of approximately 45 ° to the electrode axis of the conventional electrode pair to the measuring tube axis realized by measuring errors below 1%. This is achieved in particular in that the on the electrodes pending potential differences are recorded and weighted individually.

The WO 2017 025 314 A1 teaches an electromagnetic flowmeter with a first pair of electrodes in a first cross-sectional plane. In addition to the first pair of electrodes, the flowmeter is provided with up to four additional pairs of electrodes, which are distributed over a second and a third cross-sectional plane, each of which is spaced from the first cross-sectional plane. Using the additional measurement voltages recorded before and after the first cross-sectional plane, a statement can be made about the flow profile, which leads to a reduction in the measurement deviation from the actual flow.

However, it is a disadvantage of these configurations that, although the measurement accuracy is optimized for small diameters, it does not achieve the desired reduction in measurement errors with commercially available measuring tubes with a large nominal diameter. It is also disadvantageous that a weighting factor has to be taken into account for each pair of electrodes, although it is not clear from the outset how this has to be selected as a function of the pipe system or the rotationally asymmetrical flow profile.

The invention is based on the object of providing a magnetic-inductive flow measuring device which minimizes the influences of a rotationally asymmetrical flow profile when determining the flow rate and the volume flow.

The object is achieved by the electromagnetic flow measuring device according to claim 1.

• - ein Messrohr, zum Führen des Mediums in eine durch eine Messrohrachse definierte Längsrichtung, wobei das Messrohr eine einlaufseitige Stirnfläche und eine auslaufseitige Stirnfläche aufweist, welche das Messrohr in Längsrichtung abgrenzen;
• - mindestens eine magnetfelderzeugende Vorrichtung zum Erzeugen eines im Wesentlichen senkrecht zur Längsrichtung stehenden Magnetfeldes im Medium, wobei die magnetfelderzeugende Vorrichtung einen Polschuh oder eine Sattelspule umfasst, wobei die magnetfelderzeugende Vorrichtung in einem Querschnitt des Messrohres das Messrohr in einem maximalen Mittelpunktswinkel β umgreift,
• - ein Elektrodensystem mit mindestens zwei Elektrodenpaaren, die dazu eingerichtet sind, eine senkrecht zum Magnetfeld und zur Längsrichtung induzierte Spannung im Medium zu erfassen, wobei eine vertikale Messrohr-Längsebene das Messrohr in eine erste Seite und in eine zweite Seite einteilt, wobei sich jeweils eine erste Elektrode des Elektrodenpaares auf der ersten Seite des Messrohres befindet, wobei sich jeweils eine zweite Elektrode des Elektrodenpaares auf der zweiten Seite befindet, wobei ein erstes Elektrodenpaar in einer ersten senkrecht zur Messrohrachse liegenden Querschnittsebene angeordnet ist, wobei ein zweites Elektrodenpaar in einer zweiten senkrecht zur Messrohrachse liegenden Querschnittsebene angeordnet ist, wobei die erste Querschnittsebene zur zweiten Querschnittsebene beabstandet ist, dadurch gekennzeichnet, dass ein Mittelpunktswinkel α in einer Orthogonalprojektion der Querschnittsebenen einen minimalen Kreissektor aufspannt, in dem sich die auf jeweils einer Seite des Messrohres befindlichen und auf die Orthogonalprojektion projizierten Elektroden verteilen, und dass die Mittelpunktswinkel α und β so aufeinander abgestimmt sind, dass das Durchflussmessgerät in dem Maß unempfindlich gegenüber Abweichungen einer rotationssymmetrischen Strömung ist, dass das magnetisch-induktive Durchflussmessgerät bei einer Testmessung einen Messfehler der Durchflussgeschwindigkeit ${\mathrm{\Delta }}_u=\left|\frac{u_{va}-u_S}{u_{va}}\right|$
  
  und/oder einen Messfehler des Volumendurchflusses ${\mathrm{\Delta }}_{\dot{V}}=\left|\frac{{\dot{V}}_{va}-{\dot{V}}_S}{{\dot{V}}_{va}}\right|$
  
  kleiner 1,0%, insbesondere kleiner 0,5% und bevorzugt kleiner 0,2% aufweist, wobei eine Durchflussgeschwindigkeit u_va und/oder ein Volumendurchfluss V̇_va im Falle einer Strömung mit voll ausgebildeten Strömungsprofil bestimmt werden, wobei eine Durchflussgeschwindigkeit u_s und/oder ein Volumendurchfluss V̇_s im Falle einer rotationsunsymmetrischen Strömung bestimmt werden.

An electromagnetic flowmeter according to the invention for measuring the flow rate u or the volume flow V̇ of a medium comprises:

• a measuring tube for guiding the medium in a longitudinal direction defined by a measuring tube axis, the measuring tube having an inlet-side end face and an outlet-side end face which delimit the measuring tube in the longitudinal direction;
• - At least one magnetic field generating device for generating a magnetic field in the medium which is essentially perpendicular to the longitudinal direction, the magnetic field generating device comprising a pole piece or a saddle coil, the magnetic field generating device encompassing the measuring tube at a maximum central angle β in a cross section of the measuring tube,
• - An electrode system with at least two pairs of electrodes which are set up to detect a voltage induced in the medium perpendicular to the magnetic field and to the longitudinal direction, a vertical longitudinal plane of the measuring tube dividing the measuring tube into a first side and a second side, one in each case The first electrode of the electrode pair is located on the first side of the measuring tube, a second electrode of the electrode pair being located on the second side, a first electrode pair being arranged in a first cross-sectional plane perpendicular to the measuring tube axis, with a second electrode pair being arranged in a second perpendicular to the Measuring tube axis lying cross-sectional plane is arranged, wherein the first cross-sectional plane is spaced apart from the second cross-sectional plane, characterized in that a central angle α spans a minimal circular sector in an orthogonal projection of the cross-sectional planes, in which the on each one Be te of the measuring tube and projected onto the orthogonal projection, and that the central angles α and β are matched to one another in such a way that the flow meter is insensitive to deviations in a rotationally symmetrical flow to the extent that the magnetic-inductive flow meter shows a measurement error in a test measurement Flow rate ${\mathrm{\Delta }}_u=\left|\frac{u_{va}-u_{\mathrm{S.}}}{u_{va}}\right|$
  
  and / or a measurement error of the volume flow ${\mathrm{\Delta }}_{\dot{V}}=\left|\frac{{\dot{V}}_{va}-{\dot{V}}_{\mathrm{S.}}}{{\dot{V}}_{va}}\right|$
  
  less than 1.0%, in particular less than 0.5% and preferably less than 0.2%, a flow rate u _va and / or a volume flow rate V̇ _{va being determined} in the case of a flow with a fully developed flow profile, a flow rate u _s and / or a volume flow rate V̇ _{s can be determined} in the case of a rotationally asymmetrical flow.

An electromagnetic flowmeter that is insensitive to a rotationally asymmetrical flow profile is ideally suited for monitoring pipe systems in which an inlet section has Length corresponds to a multiple of the nominal diameter of the measuring tube, cannot be realized.

After disturbances, depending on the distance and type of disturbance, measurement errors occur due to a non-ideal flow profile, since a magnetic-inductive flow meter normally assumes and has been optimized to the effect that a fully developed, rotationally symmetrical flow profile is present. A fully developed, rotationally symmetrical flow profile is to be understood as the flow profile that no longer changes in the flow direction. Such a flow profile is formed, for example, in a measuring tube with an inlet section corresponding to 30 times the nominal diameter of the measuring tube and a medium speed of 2 m / s.

In the prior art, magnetic-inductive flowmeters with at least two pairs of electrodes that strike a central angle γ in a cross section of the measuring tube are already known. Adjacent electrodes usually have a central angle δ with a fixed angular value of approx. 180 ° / (N + 1), the natural number N corresponding to the number of electrode pairs. With a weighting of the potential differences occurring separately at the electrodes, measurement errors of less than 1% can be realized for rotationally asymmetrical flow profiles. It has turned out to be astonishing that the measurement error due to a rotationally asymmetrical flow profile can be further reduced significantly by modifying the central angles α and β, the central angle α not appearing in a cross section in the measuring tube, as is known from the prior art but is opened in an orthogonal projection of several individual cross-sectional planes through the electrodes projected there.

The central angle β serves as a parameter for the magnetic field generating device and indicates the extent to which the measuring tube is encompassed in the cross section by the segment of the magnetic field generating device coupling the magnetic field into the medium.

While a small angle β ensures that the magnetic field lines are bundled exclusively in the center of the measuring tube, the use of a large central angle β creates a magnetic field that is almost homogeneous over the entire cross section of the measuring tube. The central angle β is characterized by two straight lines which meet in the center of the tube and which each intersect one of the two ends of the pole piece.

Devices that generate magnetic fields are known which comprise a guide plate in the outer region and at least one shielding element between a pole piece and the guide plate and / or above the guide plate and the electromagnet. These segments fulfill the task of reducing interference or stray fields and are not responsible for coupling the magnetic field into the medium.

The measuring tube is designed to be electrically insulating on its inside which is in contact with the medium, on the one hand e.g. in that the measuring tube itself consists entirely of an insulating material, in particular of sintered ceramic, preferably of aluminum oxide ceramic, or of a plastic. On the other hand, the measuring tube can also be implemented in that a non-ferromagnetic metal tube, especially a stainless steel tube, is lined on the inside with an insulating layer made of a suitable plastic, especially hard rubber, soft rubber or polyfluoroethylene, preferably polytetrafluoroethylene.

The magnetic field generating device is arranged outside of the measuring tube and is attached completely adjacent, partially adjacent or at a fixed distance from the measuring tube. However, magnetic-inductive flowmeters are also known which have magnetic field generating devices cast in the liner or in the wall. The electrodes, however, are not necessarily embedded, but can also be used subsequently as pin electrodes. However, in many cases electrodes with electrode heads, e.g. so-called mushroom head electrodes, preferred for use in a magnetic-inductive flow meter. In the context of the present invention, these can advantageously also be encapsulated during manufacture when the material of the wall is formed.

The measuring tube is designed to be electrically insulating on its inside which is in contact with the medium, on the one hand e.g. in that the measuring tube itself consists entirely of an insulating material, in particular of sintered ceramic, preferably of aluminum oxide ceramic, or of a plastic. On the other hand, the insulation can also be implemented in that a non-ferromagnetic metal tube, especially a stainless steel tube, is lined on the inside with an insulating layer made of a suitable plastic, especially hard rubber, polyurethane, soft rubber or polyfluoroethylene, preferably polytetrafluoroethylene.

A flow meter based on the Coriolis principle and with a measuring accuracy of 0.1% is used to determine the reference value. This is built into a pipe system and serves as a reference system for the electromagnetic flowmeter according to the invention.

Advantageous refinements of the invention are the subject of the subclaims.

According to one embodiment, a rotationally asymmetrical flow is generated for the test measurement by a disturbance set up on the inlet-side end face and comprising at least one disturbance source.

The test measurement can contain many different sources of interference, all of which can assume any installation angle. By using sufficiently different disturbances, the central angle α and β can be optimized so that the measurement error of a special disturbance takes a value less than 0.05% and the maximum measurement error of any disturbance takes a value less than 0.5%.

It has been found that the use of two sufficiently different sources of interference, in particular a diaphragm and a
90 ° pipe bend, an already sufficiently good midpoint angle pair α and β is determined for a magnetic-inductive flow meter, which has a maximum measurement error of 0.5% for any other type of disturbance. By including additional sources of interference in the test measurement, the optimized parameters change only marginally, which means that the resulting measurement error changes only slightly.

According to one embodiment, the source of interference comprises a diaphragm or a 90 ° pipe bend,
whereby 10% of the cross section of the measuring tube is covered by the diaphragm,
wherein the diaphragm has a chord which limits the diaphragm towards the tube,
wherein the diaphragm assumes a first diaphragm orientation or a second diaphragm orientation,
whereby in the first diaphragm orientation the circular chord is oriented perpendicular to the magnetic field and in the second diaphragm orientation the circular chord is oriented parallel to the magnetic field,
whereby the 90 ° pipe bend assumes a first pipe bend orientation or a second pipe bend orientation,
wherein the first pipe bend orientation is characterized by a pipe axis running perpendicular to the magnetic field and to the longitudinal direction of the measuring tube and the second pipe bend orientation is characterized by a pipe axis running parallel to the magnetic field and perpendicular to the longitudinal direction of the measuring pipe.

According to one embodiment, the disturbance is set up at a distance of 0-DN from the inlet-side end face.

According to one embodiment, insensitivity to a rotationally asymmetrical flow profile is given when the Reynolds number of the medium in the measuring tube is greater than or equal to 100,000, in particular greater than or equal to 50,000 and preferably greater than or equal to 10,000.

According to one embodiment, the flow measuring device has three pairs of electrodes.

According to one embodiment, the third pair of electrodes is arranged in a third cross-sectional plane lying perpendicular to the measuring tube axis.

According to one embodiment, the second and the third cross-sectional plane are at the same distance from the first cross-sectional plane.

According to one embodiment, the third pair of electrodes is arranged in the first or second cross-sectional plane.

According to one embodiment, at least two electrodes, in particular all electrodes located on one side of the measuring tube with respect to the vertical longitudinal plane of the measuring tube, are short-circuited.

The technical success of this embodiment of the invention is that it has been found that by adapting the angles α and β, sampling of the individual potential differences with the addition of empirically determined weighting factors can be dispensed with and the voltage applied across all electrodes less in the event of a fault deviates by more than 0.5% from a measured value determined on the basis of a fully developed flow profile. It is therefore not necessary to weight the individual voltage values, which means that the evaluation unit for determining the applied voltage and the resulting flow velocity can be significantly simplified. It is now sufficient to convert the measured voltage value into a flow velocity or a volume flow by means of a calibration.
The electrodes are short-circuited to one another in particular by cables and preferably by a conductive sheet metal part. This offers a simple and stable installation and also provides an inexpensive alternative to known solutions. The electrodes are connected to a control and evaluation unit that uses the voltage induced in the electrodes to provide information about the flow rate and the volume flow in the measuring tube.

According to one embodiment, it applies to the central angle α that 30 ° α 60 ° and in particular that 40 ° α 50 °.

According to one embodiment, it applies to the central angle β that 50 ° β 90 ° and in particular that 70 ° β 80 °.

According to one embodiment, the electrodes are arranged axially symmetrical to the vertical longitudinal plane of the measuring tube.

According to one embodiment, two each stretch on one side of the measuring tube ( 1 ) and in the orthogonal projection of the cross-sectional planes adjacent electrodes have a central angle δ = α / (N − 1), where a natural number N corresponds to the number of electrode pairs.

The adjustment of the central angles α and β is carried out with a simulation program or on the basis of a test setup. A test environment is defined or set up and the center point angles of the flow meter are adjusted until the measurement error for the set test environments is minimal.

According to one embodiment, a rotationally asymmetrical flow is generated for the test measurement by a disturbance set up on the inlet-side end face and comprising at least one disturbance source.

The test measurement can also be used to coordinate the optimal central angles α and β and is then carried out in advance, so that a flow profile-independent magnetic-inductive flow meter is implemented taking into account the optimized central angle pair α and β.

The test measurement can contain many different sources of interference, all of which can assume any installation angle. By using sufficiently different disturbances, the central angles α and β can be optimized in such a way that the measurement error of a special disturbance takes a value less than 0.05% and the maximum measurement error of any disturbance takes a value less than 0.5%.

It has been found that by using two sufficiently different sources of interference, in particular a diaphragm and a 90 ° pipe bend, an already sufficiently good midpoint angle pair α and β can be determined for an electromagnetic flowmeter, so that any other type of interference can result in a leads to a maximum measurement error of 0.5%. By including additional sources of interference in the test measurement, the optimized parameters change only marginally, which means that the resulting measurement error changes only slightly.

• 1: einen Querschnitt eines magnetisch-induktiven Durchflussmessgerätes nach dem Stand der Technik;
• 2: eine Orthogonalprojektion einer Ausgestaltung des erfindungsgemäßen magnetisch-induktiven Durchflussmessgerätes mit zwei Elektrodenpaaren;
• 3: eine perspektivische Darstellung eines erfindungsgemäßen magnetisch-induktiven Durchflussmessgerätes mit drei Elektrodenpaaren und einer Orthogonalprojektion;
• 4: eine Längsansicht einer zweiten Ausgestaltung des erfindungsgemäßen magnetisch-induktiven Durchflussmessgerätes; und
• 5: eine Darstellung zweier Blenden und zweier Rohrbogenorientierungen, für die Testmessungen zur Ermittlung des Messfehlers.

The invention is explained in more detail with reference to the following figures. It shows:

• 1 : a cross section of a magneto-inductive flow measuring device according to the prior art;
• 2 : an orthogonal projection of an embodiment of the magnetic-inductive flow measuring device according to the invention with two pairs of electrodes;
• 3 : a perspective view of a magnetic-inductive flow measuring device according to the invention with three pairs of electrodes and an orthogonal projection;
• 4th : a longitudinal view of a second embodiment of the magnetic-inductive flow measuring device according to the invention; and
• 5 : a representation of two orifices and two pipe elbow orientations for the test measurements to determine the measurement error.

The 1 shows a magneto-inductive flow measuring device known from the prior art. The structure and the measuring principle of a magnetic-inductive flow meter are basically known. Through a measuring tube ( 1 ) a medium is passed that has electrical conductivity. A magnetic field generating device ( 7th ) is attached so that the magnetic field lines are oriented perpendicular to a longitudinal direction defined by the measuring tube axis. As a magnetic field generating device ( 7th ) a saddle coil or a pole piece with an attached coil and coil core is preferably suitable. When a magnetic field is applied, ( 1 ) a flow-dependent potential distribution, which with two on the inner wall of the measuring tube ( 1 ) attached electrodes ( 3 , 4th ) is tapped. As a rule, these are arranged diametrically and form an electrode axis which runs perpendicular to the magnetic field lines and the longitudinal axis of the pipe. On the basis of the measured voltage, taking into account the magnetic flux density, the flow rate and, taking into account the pipe cross-sectional area, the volume flow of the medium can be determined. In order to derive the on the first and second electrodes ( 3 , 4th ) applied measurement voltage across the pipe ( 8th ), the inner wall is covered with an insulating material, for example a plastic liner ( 2 ) lined. The magnetic field built up by a magnetic field generating device, for example an electromagnet, is generated by a direct current of alternating polarity clocked by an operating unit. This ensures a stable zero point and makes the measurement insensitive to the effects of electrochemical interference. A measuring unit reads the data on the electrodes ( 3 , 4th ) and outputs the applied voltage the flow rate and / or the volume flow of the medium calculated by means of an evaluation unit. Commercially available electromagnetic flowmeters have, in addition to the electrodes ( 3 , 4th ) two more electrodes ( 5 , 6th ) on. On the one hand, it is ideally used at the highest point in the pipe ( 8th ) attached level monitoring electrode ( 5 ) to partially fill the measuring tube ( 1 ) to detect, to forward this information to the user and / or to take the level into account when determining the volume flow. A reference electrode is also used ( 6th ), which are usually diametrically opposed to the level monitoring electrode ( 5 ) is appropriate to ensure sufficient grounding of the medium.

2 shows an orthogonal projection ( 36 ) an embodiment of an electromagnetic flowmeter according to the invention. A medium that has electrical conductivity is passed through a measuring tube. A magnetic field generating device is attached in such a way that the magnetic field lines are oriented perpendicular to a longitudinal direction defined by the measuring tube axis. A saddle coil or as in FIG. 2 is preferably suitable as a magnetic field generating device 2 shown is a projection of a pole piece ( 14th ). When a magnetic field is applied, a potential distribution is created in the measuring tube, which is determined by two electrodes attached to the inner wall of the measuring tube ( 18th , 19th ) at least two pairs of electrodes can be tapped. As a rule, these electrodes, which form an electrode pair, are arranged diametrically and form an electrode axis or an abscissa axis ( 13 ), which are essentially perpendicular to the vertical longitudinal plane of the measuring tube ( 12 ) and running lengthways. Based on the measured voltage, taking into account the magnetic flux density, the flow rate of the medium u and, taking into account the pipe cross-sectional area, the volume flow rate V can be determined. To prevent the voltage applied to the electrodes from being discharged through the pipeline, the inner wall is covered with an insulating material. The magnetic field built up by an electromagnet, for example, is generated by a clocked direct current of alternating polarity. This ensures a stable zero point and makes the measurement insensitive to the effects of multiphase substances, inhomogeneities in the liquid or low conductivity. According to the invention, at least two pairs of electrodes are used to determine the volume flow rate V̇. In the schematic representation 2 , an orthogonal projection of a magnetic-inductive flow meter with three pairs of electrodes is shown as an example. In addition to the electrodes for tapping a potential difference, additional electrodes in the form of measuring medium monitoring or grounding electrodes are often built into the measuring tube, which are used to measure an electrical reference potential or to detect partially filled measuring tubes or to record the temperature of the medium using a built-in temperature sensor. These are not taken into account in the schematic representation. There is always a first electrode ( 18th ) of the electrode pair on the first side (I) of the measuring tube and a second electrode ( 19th ) of the pair of electrodes on the second side (II). The external electrodes on one side span in the orthogonal projection ( 36 ) has a central angle α. The other electrodes are distributed within the open circle segment, preferably on the inner wall of the measuring tube.

The device generating the magnetic field is usually designed in such a way that the magnetic field lines are distributed as homogeneously as possible over a cross section of the measuring tube. This means that measurement errors of less than 0.2% can be achieved, especially for fully developed flow profiles. In the case of a rotationally asymmetrical flow profile, a homogeneous magnetic field can have a disadvantageous effect on the measurement accuracy. This problem can be solved according to the invention by adapting the device generating the magnetic field, in particular by adapting the center angle β.

By varying the central angle β, which describes the extent to which a segment of the magnetic field generating device attached to the measuring tube encompasses the measuring tube, a further degree of freedom is obtained for reducing the measuring error. A segment coupling the magnetic field into the medium can be a pole piece ( 14th ), which has two legs adjoining a flat surface or two circular arcs attached to its flat surface. Alternatively, a pole piece ( 14th ) also take the form of an arc of a circle. In general, a segment coupling the magnetic field into the medium can assume any contour, consisting of at least one further sub-segment. To determine the maximum center angle β, the subsegments are taken into account that are essentially responsible for coupling the magnetic field into the medium.

The in 2 the orthogonal projection shown, the electrodes are in direct contact with the medium; however, as mentioned above, the coupling can also take place capacitively.

The 3 shows a perspective view of a measuring tube according to the invention ( 1 ) with three pairs of electrodes and an orthogonal projection ( 36 ). A first pair of electrodes ( 15th ) is in a first cross-sectional plane ( 37 ) arranged. The first pair of electrodes ( 15th ) diametrically arranged, as it is known in electromagnetic flowmeters according to the prior art. A second pair of electrodes ( 16 ) is in a second cross-sectional plane ( 38 ) arranged between the inlet-side face ( 10 ) of the measuring tube and the first cross-sectional plane ( 37 ) is arranged. A third pair of electrodes ( 17th ) is in a third cross-sectional plane ( 39 ) arranged between the outlet-side end face ( 11 ) of the measuring tube and the first cross-sectional plane ( 37 ) is located. The cross-sectional planes are each spaced apart. The second and third pair of electrodes ( 16 , 17th ) are not arranged diametrically in this embodiment. Instead, the two pairs of electrodes ( 16 , 17th ) each on an electrode axis that is parallel to the electrode axis of the first pair of electrodes ( 15th ) runs. On the orthogonal projection ( 36 ) the external electrodes on one side form a central angle α with the center of the measuring tube. Furthermore, the 3 a pole piece ( 14th ), which has a cross-section that coincides with the orthogonal projection ( 36 ) is projected. The projection of the pole piece ( 14th ) is characterized by a central angle β.

An electromagnetic flowmeter comprises an inlet-side face ( 10 ) and an outlet-side face ( 11 ). A to the inlet-side face ( 10 ) mounted 90 ° pipe bend or an orifice affect the flow profile of the medium, so that in the measuring pipe ( 1 ) there is a rotationally asymmetrical flow profile.

bzw. ${\mathrm{\Delta }}_{\dot{V}}=\left|\frac{{\dot{V}}_{va}-{\dot{V}}_S}{{\dot{V}}_{va}}\right|,$

wobei die Durchflussgeschwindigkeit u_va und der Volumendurchfluss V̇_va im Falle einer Strömung mit voll ausgebildeten Strömungsprofil bestimmt werden, und die Durchflussgeschwindigkeit u_s und der Volumendurchfluss V̇_s im Falle eines rotationsunsymmetrischen Strömungsprofils bestimmt werden. Dabei ist der reale Volumendurchfluss V̇_real in beiden Fällen identisch und im Falle des voll ausgebildeten Strömungsprofils optimalerweise gleich dem gemessenen Volumendurchfluss V̇_va.The measurement errors of the flow velocity u and the volume flow V̇ are ${\mathrm{\Delta }}_u=\left|\frac{u_{va}-u_{\mathrm{S.}}}{u_{va}}\right|$

or. ${\mathrm{\Delta }}_{\dot{V}}=\left|\frac{{\dot{V}}_{va}-{\dot{V}}_{\mathrm{S.}}}{{\dot{V}}_{va}}\right|,$

whereby the flow rate u _va and the volume flow rate V̇ _{va are determined} in the case of a flow with a fully developed flow profile, and the flow rate u _s and the volume flow rate V̇ _{s are determined} in the case of a rotationally asymmetrical flow profile. The real volume flow rate V̇ _{real is} identical in both cases and, in the case of the fully developed flow profile, optimally equal to the measured volume flow rate V̇ _va .

In the 4th is shown the longitudinal view of an embodiment of the flow measuring device according to the invention. It has a first and a second cross-sectional plane ( 37 , 38 ), in which two pairs of electrodes are attached. The electrode pairs each lie on an electrode axis which run parallel to one another. In an orthogonal projection (not shown) of the two cross-sectional planes there is always an electrode of the first cross-sectional plane ( 37 ) and an electrode of the second cross-sectional plane ( 38 ) one above the other. Such an electrode arrangement is also claimed. Furthermore, a magnetic field generating device ( 7th ) and a schematic magnetic field distribution ( 44 ) shown in longitudinal section. In this case the magnetic field distribution is ( 44 ) shown in Gaussian shape. The two cross-sectional planes ( 37 , 38 ) are arranged in such a way that the magnetic field strength in the respective cross-sectional plane has at least half and preferably at least ¾ of the maximum magnetic field strength in the longitudinal cross-section.

• Im ersten Schritt werden die Mittelpunktswinkel α und β so angepasst, dass der Messfehler der Durchflussgeschwindigkeit in Testmessungen mit einer einzelnen Störung minimal wird. Dabei wird die Störung durch eine Blende (B) oder einen 90°Rohrbogen (90°R) generiert.

The 5 shows apertures (B) with different aperture orientations ( B1 , B2 ) and 90 ° pipe bends ( 90 ° R ) when installed and with different pipe bend orientations ( R1 , R2 ). These components serve as the cause of failure in test measurements or simulations. In the simulations, an electromagnetic flowmeter with at least two pairs of electrodes forms the basis for calculating the optimal parameters. The area of the electrodes is larger than punctiform, but finitely large. The optimization of the central angles α and β takes place in the following steps:

• In the first step, the central angles α and β are adjusted so that the measurement error of the flow velocity in test measurements with a single disturbance is minimal. The interference is caused by an orifice plate (B) or a 90 ° pipe bend ( 90 ° R ) generated.

The diaphragm (B) covers 10% of the pipe cross-section and has a chord that limits the diaphragm towards the pipe. It takes a first aperture orientation ( B1 ) or a second aperture orientation ( B2 ), which are rotated in particular by 90 ° to each other. The chord is at the first aperture orientation ( B1 ) perpendicular to the magnetic field and with the second aperture orientation ( B2 ) oriented parallel to the magnetic field. The first aperture orientation ( B1 ) and the second aperture orientation ( B2 ) of a diaphragm (B) is shown schematically in 5 shown. The black-filled circle segment represents the area that blocks part of the cross-sectional area of the measuring tube. In the test measurement, the orifice (B) is attached at a distance of 0-DN to the front face on the inlet side. Alternatively, a 90 ° elbow ( 90 ° R ) at the entrance to the inlet end face at a distance of 0-DN, whereby the 90 ° pipe bend ( 90 ° R ) a first pipe bend orientation ( R1 ) or a second elbow orientation ( R2 ), which are in particular rotated by 90 ° to each other. The first elbow orientation ( R1 ) and the second elbow orientation ( R2 ) a 90 ° elbow ( 90 ° R ) is schematically in 5 shown. At the first pipe bend orientation ( R1 ) runs the pipe axis ( 42 ) parallel to the abscissa axis ( 41 ) of the electromagnetic flowmeter ( 43 ). The adjustment of the central angles α and β is preferably carried out for the two disorders with both orientations.

In the second step, the center angle pair is determined, the maximum measurement error of which is minimal for all test measurements carried out.

List of reference symbols

1

Measuring tube

2

Liner

3

first electrode

4th

second electrode

5

Level monitoring electrode

6th

Reference electrode

7th

Magnetic field generating device

8th

pipe

9

Measuring, operating and / or evaluation unit

10

inlet end face

11

end face on the outlet side

12

vertical measuring tube longitudinal plane

13

Abscissa axis

14th

Pole piece

15th

first pair of electrodes

16

second pair of electrodes

17th

third pair of electrodes

18th

first electrode of a pair of electrodes

19th

second electrode of a pair of electrodes

36

Orthogonal projection

37

first cross-sectional plane

38

second cross-sectional plane

39

third cross-sectional plane

40

vertical measuring tube longitudinal plane

41

Abscissa axis

42

Pipe axis

43

magnetic-inductive flow meter

44

Magnetic field distribution

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102014113408 A1 [0003]
• CN 101294832 A [0007]
• DE 102015113390 A1 [0008]
• EP 0878694 A1 [0009]
• WO 2017025314 A1 [0010]

Claims (14)

Magnetic-inductive flow meter for measuring the flow rate u or the volume flow V of a medium, comprising: a measuring tube (1) for guiding the medium in a longitudinal direction defined by a measuring tube axis, the measuring tube having an inlet-side end face (10) and an outlet-side end face (11) which delimit the measuring tube (1) in the longitudinal direction; - At least one magnetic field generating device (7) for generating a magnetic field in the medium which is essentially perpendicular to the longitudinal direction, the magnetic field generating device (7) comprising a pole shoe or a saddle coil, the magnetic field generating device (7) in a cross section of the measuring tube (1) encompasses the measuring tube (1) at a maximum central angle β, - an electrode system with at least two pairs of electrodes which are set up to detect a voltage induced in the medium perpendicular to the magnetic field and to the longitudinal direction, with a vertical measuring tube longitudinal plane (12) being the measuring tube (1) divided into a first side (I) and a second side (II), with a first electrode (18) of the electrode pair being located on the first side (I) of the measuring tube, with a second electrode (19 ) of the pair of electrodes is located on the second side (II), a first pair of electrodes (15) in a first perpendicular to the Mes cross-sectional plane (37) lying on the tube axis, a second pair of electrodes (16) being arranged in a second cross-sectional plane (38) lying perpendicular to the measuring tube axis, the first cross-sectional plane (37) being spaced from the second cross-sectional plane (38), characterized in that a central angle α in an orthogonal projection (36) of the cross-sectional planes spans a minimal circular sector in which the electrodes located on one side of the measuring tube and projected onto the orthogonal projection are distributed, and that the central angles α and β are matched to one another so that the flow meter is insensitive to deviations in a rotationally symmetrical flow to the extent that the magnetic-inductive flowmeter shows a measurement error in the flow rate during a test measurement ${\mathrm{\Delta }}_u=\left|\frac{u_{va}-u_{\mathrm{S.}}}{u_{va}}\right|$

and / or a measurement error of the volume flow ${\mathrm{\Delta }}_{\dot{V}}=\left|\frac{{\dot{V}}_{va}-{\dot{V}}_{\mathrm{S.}}}{{\dot{V}}_{va}}\right|$

less than 1.0%, in particular less than 0.5% and preferably less than 0.2%, a flow rate u _va and / or a volume flow rate V̇ _{va being determined} in the case of a flow with a fully developed flow profile, a flow rate u _s and / or a volume flow rate V̇ _{s can be determined} in the case of a rotationally asymmetrical flow. Electromagnetic flowmeter according to Claim 1 , wherein for the test measurement a rotationally asymmetrical flow is generated by a disturbance which is set up on the inlet-side end face (10) and includes at least one source of disturbance. Electromagnetic flowmeter according to Claim 2 , the source of interference comprising a diaphragm (B) or a 90 ° pipe bend (90 ° R), the diaphragm (B) covering 10% of the cross-section of the measuring tube (1), the diaphragm (B) having a chord, which delimits the diaphragm towards the pipe, whereby the diaphragm (B) assumes a first diaphragm orientation (B1) or a second diaphragm orientation (B2), whereby in the first diaphragm orientation (B1) the circular chord is oriented perpendicular to the magnetic field and in the second diaphragm orientation ( B2) the circular chord is oriented parallel to the magnetic field, with the 90 ° pipe bend (90 ° R) assuming a first pipe bend orientation (R1) or a second pipe bend orientation (R2), the first pipe bend orientation (R1) being perpendicular to the magnetic field and to the The pipe axis (11) running in the longitudinal direction of the measuring pipe (1) is characterized and the second pipe bend orientation (R2) is characterized by a pipe axis (1) running parallel to the magnetic field and perpendicular to the longitudinal direction (4) of the measuring pipe (1) 1). Magnetic-inductive flow measuring device according to one of the preceding claims, wherein the disturbance is set up at a distance of 0-DN from the inlet-side end face (10). Electromagnetic flowmeter according to one of the preceding claims, insensitivity to a rotationally asymmetrical flow profile with a Reynolds number of the medium in the measuring tube (1) greater than or equal to 100,000, in particular greater than or equal to 50,000 and preferably greater than or equal to 10,000. Electromagnetic flow meter according to one of the Claims 1 to 5 , wherein the flow meter has three pairs of electrodes. Electromagnetic flowmeter according to Claim 6 , the third pair of electrodes (17) being arranged in a third cross-sectional plane (39) lying perpendicular to the measuring tube axis. Electromagnetic flowmeter according to Claim 7 wherein the second and the third cross-sectional plane (38, 39) are at the same distance from the first cross-sectional plane (37). Electromagnetic flowmeter according to Claim 6 wherein the third pair of electrodes (17) is arranged in the first or second cross-sectional plane (37, 38). Electromagnetic flowmeter according to one of the preceding claims, wherein at least two electrodes, in particular all electrodes located on one side of the measuring tube with respect to the vertical measuring tube longitudinal plane (12), are short-circuited. Electromagnetic flowmeter according to one of the preceding claims, wherein for the central angle α it applies that 30 ° ≤ α 60 ° and in particular that 40 ° α 50 °. Electromagnetic flowmeter according to one of the preceding claims, where for the central angle β it applies that 50 ° β 90 ° and in particular that 70 ° β 80 °. Magnetic-inductive flow measuring device according to one of the preceding claims, wherein the electrodes are arranged axially symmetrically to the vertical longitudinal plane (12) of the measuring tube. Electromagnetic flowmeter according to one of the preceding claims, wherein two electrodes on one side of the measuring tube (1) and adjacent electrodes in the orthogonal projection of the cross-sectional planes span a central angle δ = α / (N - 1), with a natural number N being the number of Corresponds to electrode pairs.

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