Method for monitoring medium flow by means of Coriolis mass flowmeter and differential pressure measuring device

Abstract

A method (100) for monitoring the flow of a medium by means of a pressure difference measuring device and a Coriolis mass flowmeter, the Coriolis mass flowmeter having two oscillators, each comprising a curved measuring tube pair, which are arranged superposed on one another and parallel to one another in terms of flow between two pressure measuring points of the pressure difference measuring device, the method comprising the following steps: capturing (110) a pressure difference between a first pressure measuring point and a second pressure measuring point; capturing (122) a first density measurement value based on a first oscillation frequency of at least the first oscillator; capturing (124) a second density measurement value based on a second oscillation frequency of at least the second oscillator; and determining (150) a flow measurement value based on the pressure difference when the difference between the first density measurement value and the second density measurement value is less than a density difference limit value.

Description

Background Art

[0001] The invention relates to a method for monitoring the flow of a medium including at least one liquid phase by means of a Coriolis mass flowmeter with a curved measuring tube and a pressure difference measuring device, the pressure difference measuring device being suitable for determining the pressure difference between a first pressure measuring point and a second pressure measuring point of a pipeline, wherein the Coriolis mass flowmeter is arranged between the first pressure measuring point and the second pressure measuring point of the pipeline, wherein the Coriolis mass flowmeter serves as a differential pressure generator, wherein the first pressure measuring point is arranged between a pressure source and the Coriolis mass flowmeter, wherein the second measuring point is arranged between the Coriolis mass flowmeter and a pressure tank. In the case of the described measuring arrangement, a mass flow stop can occur when the bend of the measuring tube is asymmetrically filled with liquid medium, wherein the gas pressure of the pipeline stabilizes this asymmetrical filling. Due to the asymmetrical liquid column in the tube bend, this results in a pressure difference between the two pressure measuring points. This pressure difference indicates flow even if the medium is not actually moving.

[0002] Therefore, it is known to connect a throttled bypass capillary line between the two pressure measurement points in parallel with the pipeline, which should lead to pressure balance between the two pressure measurement points within a reasonable amount of time in the case of non-moving media. This works in principle, but requires a lot of maintenance because the capillary can easily become clogged. Moreover, in the operating mode in which the flow measurement is carried out by means of a Coriolis mass flowmeter, the flow through the capillary line is not recorded by the Coriolis mass flowmeter. This leads to measurement errors.

[0003] Patent application DE102019125682.8, which was not published as of the earliest filing date of the present application, discloses another method in which medium properties such as the density or speed of sound are compared at two measuring points before and after the Coriolis mass flowmeter. In the case of agreement, the pressure difference will be assessed as being caused by the flow. In the case of a persistent significant deviation, it will be assumed that the medium is not moving and that the pressure difference is caused by a still water column. This method works reliably, but is associated with the additional cost of two sensors.

[0004] It is therefore an object of the present invention to provide a method which makes it possible to reliably distinguish between stationary and flowing media with little effort. According to the invention, this object is achieved by a method according to claim 1 . Summary of the invention

[0005] The method of the invention serves to monitor the flow of a medium comprising at least one liquid phase by means of a Coriolis mass flowmeter and a pressure difference measuring device, the pressure difference measuring device being suitable for determining a pressure difference between a first pressure measuring point and a second pressure measuring point of a pipeline, wherein the Coriolis mass flowmeter is arranged between the first pressure measuring point and the second pressure measuring point of the pipeline, wherein the Coriolis mass flowmeter serves as a differential pressure generator, wherein the first pressure measuring point is arranged between a pressure source and the Coriolis mass flowmeter, wherein the second pressure measuring point is arranged between the Coriolis mass flowmeter and a pressure tank; wherein the Coriolis mass flowmeter comprises at least two oscillators with at least one bend in the measuring tube, wherein the bend in the measuring tube comprises at least one oscillator. The tube has in each case a low point which is arranged at the deepest point of the measuring tube relative to the local gravitational field, wherein the measuring tubes of the oscillators are arranged between two shunt tubes so that the measuring tubes are connected for parallel flow, wherein the first low point of the measuring tube of the first oscillator is arranged above the second low point of the measuring tube of the second oscillator, wherein the method comprises the steps of: recording a pressure difference between a first pressure measurement point and a second pressure measurement point; recording a first density measurement value based on a first oscillation frequency of at least the first oscillator; recording a second density measurement value based on a second oscillation frequency of at least the second oscillator; and determining a flow measurement value based on the pressure difference when the difference between the first density measurement value and the second density measurement value is less than a density difference limit value.

[0006] In a further development of the invention, the density difference limit value comprises a function of the density of the medium.

[0007] In a further development of the invention, the density difference limit value comprises a function of the aeration of the medium.

[0008] In a further development of the invention, the filling is detected based on density fluctuations and / or damping of measuring tube oscillations or fluctuations in the damping.

[0009] In a further development of the invention, the first measuring tube and the second measuring tube have in each case a measuring tube center line, wherein the density difference limit value is a function of the vertical spacing of the measuring tube center lines at the measuring tube low point.

[0010] In a further development of the invention, determining the flow measurement value based on the pressure difference comprises checking whether the pressure difference exceeds a threshold value, wherein a zero flow is assumed to be below the threshold value.

[0011] In a further development of the invention, the threshold value is not greater than 30 mbar, in particular not greater than 25 mbar.

[0012] In a further development of the invention, a valve is arranged between the second pressure measuring point and the pressure tank. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will now be explained based on examples of embodiments presented in the accompanying drawings, which are as follows:

[0014] Figure 1 It is a longitudinal cross-sectional view of the Coriolis mass flowmeter when the medium is not moving;

[0015] Figure 2 is a graph showing the relationship between density measurement and pressure difference at the measurement point;

[0016] Figure 3 is a graph showing the relationship between apparent density measurements and differential damping of measuring tube oscillations; and

[0017] Figure 4 is a flow chart of an example of an embodiment of the method of the present invention. DETAILED DESCRIPTION

[0018] based on Figure 1 Explain the problem involved in the present invention.

[0019] When the liquid flow through the Coriolis mass flowmeter 200 is suspended at a temporally unfavorable point, it can occur that the measuring tube bends 210, 220 are only partially filled with liquid 230, wherein the liquid remains moving with the gas 240 at an inlet end pressure p1, which is a sum of the base pressure p0 and the hydrostatic pressure p of the moving liquid column. l In contrast, the outlet side pressure p2 is substantially equal to the base pressure p0. Insofar as the Coriolis mass flowmeter 200 is used as a differential pressure generator between two measuring points of the inlet side pressure and the outlet side pressure, even if the liquid is practically at rest, in order to determine the flow rate between the two measuring points based on the pressure difference, the hydrostatic pressure p of the moving liquid column is l It looks like there is flow. The purpose of the present invention is to distinguish this apparent flow from actual flow.

[0020] To this end, the present invention at least in part utilizes Figure 2 and Figure 3 , in which case a Coriolis mass flowmeter 200 is involved, having four curved measuring tubes, which are arranged in each case to form two pairs of parallel measuring tubes 210, 220 of the same type, wherein the low points 212, 222 of the measuring tubes of a pair of measuring tubes are in each case located at the same height and differ between the measuring tube pairs.

[0021] Figure 2 and Figure 3 The data in this paper come from experiments in which a liquid is loaded with a slowly increasing pressure of a gas. Figure 1The measuring tube of the shown Coriolis mass flowmeter 200 is in fact steadily pushed. The relationships shown here for a certain point in time can be those that actually govern a given situation when the flow is stopped under actual operating conditions.

[0022] Figure 2 It is shown that in the case of pushing out the liquid, the pressure difference first rises without affecting the density measurement. This is because the interface 234 between the liquid 230 and the gas 240 has not initially reached the internal coupler 214, 224 of the measuring tube pair of the Coriolis mass flowmeter 200, limiting the oscillation part of the measuring tube 210, 220 that is effective for measurement. When the interface 234 finally passes below the coupler 214, 224, the liquid 230 missing in the measuring tube 210, 220 acts in the form of a seemingly reduced density. This effect is more obvious in the case of the internal or upper measuring tube pair 210, because the relative importance of the lost liquid is greater than that of the external or lower measuring tube pair 220. Therefore, when the density measurement deviation between the measuring tube pair 210, 220 is detected in the measuring operation, it is regarded as an indication of partially filling the measuring tube when the liquid is not flowing. Accordingly, the associated pressure difference is not interpreted as being caused by flow.

[0023] Figure 3 Instead of the pressure difference curve, a curve for the difference in damping of the measuring tube oscillations is shown. It is clear that in the case of completely filled measuring tubes 210, 220, the damping of the measuring tube oscillations of the different measuring tube pairs is essentially equal and the differences are therefore negligible. However, as the push of the liquid 230 increases, the damping characteristics of the measuring tube pairs differ, since the relative importance of the interface position is greater in the case of the inner measuring tube pair 210.

[0024] As a result, it is therefore possible to determine, based on the difference between the density deviation and the damping of the measuring tube oscillations, whether the pressure difference between the measuring points indicates flow or no flow.

[0025] Actual density differences can also occur when the aerated liquid undergoes stratification in the pipeline, wherein in this case the density is expected to be lower in the upper measuring tube pair. However, in this case, corresponding density fluctuations should be detected as the aeration flows. Furthermore, the aeration can be characterized using so-called multi-frequency techniques. In the absence of these features, it can be assumed that the liquid is not flowing.

[0026] When the above principles are applied, the method of the present invention can Figure 4 The embodiments shown are implemented in examples.

[0027] The method 100 begins in step 110 by recording a pressure difference between a first pressure measurement point and a second pressure measurement point, wherein Figure 1A Coriolis mass flowmeter is arranged between the pressure measurement points. A first density measurement value is then recorded in step 122 based on at least a first oscillation frequency of the first oscillator, and a second density measurement value is recorded in step 124 based on at least a second oscillation frequency of the second oscillator. A density difference is then formed in step 130. It is then tested in step 140 whether the density difference is less than a density difference limit value. If this is true, a flow measurement value is then determined based on the pressure difference in step 150. In this case, it is preferably also checked whether the pressure difference exceeds a threshold value of, for example, 25 mbar. If the test result is false, a zero flow can be associated with the pressure difference.

Claims

1. A method (100) for monitoring the flow of a medium comprising at least one liquid phase by means of a Coriolis mass flowmeter and a pressure difference measuring device, the pressure difference measuring device being adapted to determine a pressure difference between a first pressure measurement point and a second pressure measurement point of a pipeline, wherein the Coriolis mass flowmeter is arranged between the first pressure measurement point and the second pressure measurement point of the pipeline, wherein the Coriolis mass flowmeter acts as a differential pressure generator, wherein the first pressure measurement point is arranged between a pressure source and the Coriolis mass flowmeter, wherein the second pressure measurement point is arranged between the Coriolis mass flowmeter and a pressure tank; in, The Coriolis mass flowmeter comprises at least two oscillators with in each case at least one curved measuring tube, wherein the curved measuring tubes have in each case a low point which is arranged at the deepest point of the measuring tube relative to the local gravitational field, wherein the measuring tubes of the oscillators are arranged between two flow diverter tubes so that the measuring tubes are connected for parallel flow, wherein a first low point of the measuring tube of a first oscillator is arranged above a second low point of the measuring tube of a second oscillator, The method (100) comprises the following steps: recording (110) a pressure difference between the first pressure measurement point and the second pressure measurement point; recording a first density measurement based on at least a first oscillation frequency of the first oscillator (122); recording a second density measurement based on at least a second oscillation frequency of the second oscillator (124); and When the difference between the first density measurement and the second density measurement is less than a density difference limit, a flow measurement is determined based on the pressure difference (150).

2. The method according to claim 1, wherein: The density difference limit is a function of the density of the medium.

3. The method according to claim 1, wherein: The density difference limit value is a function of the aeration of the medium.

4. The method according to claim 3, wherein: The filling is detected based on density fluctuations and / or damping of measuring tube oscillations or fluctuations in the damping.

5. The method according to claim 1, wherein: The measuring tube of the first oscillator and the measuring tube of the second oscillator have in each case a measuring tube center line, wherein the density difference limit value is a function of the vertical spacing of the measuring tube center lines at the low point of the measuring tubes.

6. The method according to claim 1, wherein: Determining a flow measurement based on the pressure difference includes checking whether the pressure difference exceeds a threshold, wherein zero flow is assumed to be below the threshold.

7. The method according to claim 6, wherein: The threshold value is not greater than 30 mbar.

8. The method according to claim 7, wherein: The threshold value is not greater than 25 mbar.

9. The method according to claim 1, wherein: A valve is arranged between the second pressure measurement point and the pressure tank.

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