Method for determining the gas portion in the medium flowing through a Coriolis mass flowmeter

Abstract

A method for determining the gas portion in the medium flowing through a Coriolis mass flowmeter, wherein the Coriolis mass flowmeter has at least one measuring tube, at least one oscillation generator, at least two oscillation sensors and at least one control and evaluation unit, wherein the method is characterized in that the density value ρ.sub.100 of the gas-free medium is determined in a ρ.sub.100 step, that the density value ρ.sub.mess of the medium flowing through the measuring tube is measured in a ρ.sub.mess step, that a quantity GVQ for the gas portion of the medium flowing through the measuring tube is calculated in a GVQ step with the density value ρ.sub.100 and the density value ρ.sub.mess, and that the quantity GVQ is output for the gas portion of the medium flowing through the measuring tube.

Claims

1. A method for determining the gas portion in the medium flowing through a Coriolis mass flowmeter, wherein the Coriolis mass flowmeter has at least one measuring tube, at least one oscillation generator, at least two oscillation sensors and at least one control and evaluation unit, the method comprising: determining a first density value ρ.sub.100 of the medium by measuring the density of the medium flowing through the at least one measuring tube when an indicator variable I.sub.2-phase for the presence of gas inclusions indicates that the medium flowing through the at least one measuring tube is free of gas inclusions; determining a second density value ρ.sub.mess of the medium flowing through the at least one measuring tube; calculating a quantity GVQ for the gas portion of the medium flowing through the at least one measuring tube with the first density value ρ.sub.100 and the second density value ρ.sub.mess; and outputting the quantity GVQ for the gas portion of the medium flowing through the at least one measuring tube.

2. The method according to claim 1, further comprising continuously determining the indicator variable I.sub.2-phase.

3. The method according to claim 1, wherein the step of determining the second density value ρ.sub.mess of the medium flowing through the at least one measuring tube is carried out when the indicator variable I.sub.2-phase indicates that the medium flowing through the at least one measuring tube has gas inclusions.

4. The method according to claim 1, wherein the gas-volume ratio GVR or the gas-volume fraction GVF is calculated as a quantity GVQ for the gas portion of the medium flowing through the at least one measuring tube.

5. The method according to claim 1, wherein the quantity GVQ of the gas portion of the medium flowing through the at least one measuring tube is calculated in the GVQ step under the assumption that the density of the gas portion is zero.

6. The method according to claim 1, wherein the medium pressure p.sub.m of the medium flowing through the at least one measuring tube is measured and a corrected density value ρ.sub.mess (p.sub.m) is determined with the measured medium pressure p.sub.m and the measured second density value ρ.sub.mess of the medium flowing through the at least one measuring tube, wherein the corrected density value ρ.sub.mess (p.sub.m) is used as the measured second density value ρ.sub.mess.

7. The method according to claim 1, wherein the calculated quantity GVQ for the gas portion of the medium flowing through the at least one measuring tube is compared to a limit value GVQ.sub.limit for the gas portion of the medium flowing through the at least one measuring tube and a status message is output in the event of a defined deviation of the calculated quantity GVQ from the limit value GVQ.sub.limit.

8. The method according to claim 1, wherein the step of calculating the quantity GVQ for the gas portion of the medium flowing through the at least one measuring tube involves use of a last determined first density value ρ.sub.100 of the medium.

9. A coriolis mass flowmeter, comprising: at least one measuring tube through which a medium can flow; at least one oscillation generator; at least two oscillation sensors; and at least one control and evaluation unit; wherein the at least one control and evaluation unit is designed such that, during operation of the Coriolis mass flowmeter, the at least one control and evaluation unit determines a first density value ρ.sub.100 of the medium by measuring the density of the medium flowing through the at least one measuring tube when an indicator variable I.sub.2-phase for the presence of gas inclusions indicates that the medium flowing through the at least one measuring tube is free of gas inclusions; wherein the at least one control and evaluation unit measures a second density value ρ.sub.mess of the medium flowing through the measuring tube; wherein the at least one control and evaluation unit calculates a quantity GVQ for the gas portion of the medium flowing through the measuring tube with the first density value ρ.sub.100 and the second density value ρ.sub.mess; and wherein the at least one control and evaluation unit outputs the quantity GVQ for the gas portion of the medium flowing through the measuring tube.

10. The coriolis mass flowmeter according to claim 9, wherein the at least one control and evaluation unit is designed to determine the indicator variable I.sub.2-phase.

11. The coriolis mass flowmeter according to claim 9, wherein the at least one control and evaluation unit is designed such that the following process step is carried out: determining the second density value ρ.sub.mess of the medium flowing through the at least one measuring tube when the indicator variable I.sub.2-phase indicates that the medium flowing through the at least one measuring tube has gas inclusions.

12. The coriolis mass flowmeter according to claim 9, wherein the at least one control and evaluation unit is designed such that the following process step is carried out: calculating the gas-volume ratio GVR or the gas-volume fraction GVF as a quantity GVQ for the gas portion of the medium flowing through the at least one measuring tube.

13. The coriolis mass flowmeter according to claim 9, wherein the at least one control and evaluation unit is designed such that the following process step is carried out: calculating the quantity GVQ of the gas portion of the medium flowing through the at least one measuring tube under the assumption that the density of the gas portion is zero.

14. The coriolis mass flowmeter according to claim 9, wherein the at least one control and evaluation unit is designed such that the following process step is carried out: measuring the medium pressure p.sub.m of the medium flowing through the at least one measuring tube and determines a corrected density value ρ.sub.mess (p.sub.m) with the measured medium pressure p.sub.m and the measured second density value ρ.sub.mess, wherein the corrected density value ρ.sub.mess (p.sub.m) is used as the measured second density value ρ.sub.mess.

15. The coriolis mass flowmeter according to claim 9, wherein the at least one control and evaluation unit is designed such that the following process step is carried out: comparing the calculated quantity GVQ for the gas portion of the medium flowing through the at least one measuring tube to a limit value GVQ.sub.limit for the gas portion of the medium flowing through the at least one measuring tube, and outputs a status message in the event of a defined deviation of the calculated quantity GVQ from the limit value GVQ.sub.limit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In detail, there are now a number of possibilities for designing and further developing the described method for determining the gas portion in the medium flowing through a Coriolis mass flowmeter and the corresponding Coriolis mass flowmeter. This is described using the following figures.

(2) FIG. 1 schematically illustrates a Coriolis mass flowmeter in which a method for determining the gas portion in the medium flowing through its measuring tubes is implemented.

(3) FIG. 2a schematically illustrates a method for determining the gas portion in the medium flowing through the measuring tubes of the Coriolis mass flowmeter.

(4) FIG. 2b schematically illustrates a further implementation of the method for determining the gas portion in the medium flowing through a Coriolis mass flowmeter.

(5) FIG. 3 schematically illustrates the method for determining the gas portion using an indicator variable for a two-phase flow.

(6) FIG. 4 illustrates a further variation of the method for determining the gas portion using an indicator variable for a two-phase flow.

(7) FIG. 5 illustrates a further development of the aforementioned methods using the medium pressure.

(8) FIG. 6 illustrates an implementation of the method using a threshold value for the quantity GVQ for the gas portion of the medium flowing through the measuring tube.

DETAILED DESCRIPTION

(9) FIGS. 1 to 6 show various aspects of a method 1 for determining the gas portion in the medium 3 flowing through a Coriolis mass flowmeter 2. The Coriolis mass flowmeter 2 has two curved measuring tubes 4, an oscillation generator 5, two oscillation sensors 6 and a control and evaluation unit 7. In the view in FIG. 1 only one measuring tube 4 can be seen, the second measuring tube is covered by the first measuring tube 4 in this view. In principle, it is not important in this invention whether the Coriolis mass flowmeter has straight or curved measuring tubes, nor does the number of measuring tubes play any role with regard to the fundamental implementation of the method 1 of interest here.

(10) During operation, the measuring tubes 4 with the oscillation generator 5 are excited to a harmonic oscillation in a fundamental oscillation mode. In the presence of a flow of the medium 3 through the measuring tubes 4, opposite Coriolis forces act on the measuring tubes 4 on the inlet and outlet side, whereby a superimposed higher oscillation mode is generated. The phase difference between the superimposed oscillation of the measuring tubes 4 on the inlet and outlet side is a measure of the mass flow through the measuring tubes 4.

(11) As mentioned above, the occurrence of a two-phase flow, i.e. a flow with a gas component and a liquid and/or solid component, is problematic because damping and noise effects impair the measurement. In addition, variable gas portions also cause a change in the density of the medium 3 and, thus, also a change in the resonance frequency of the element capable of oscillation consisting of the measuring tubes 4 and the medium 3 flowing in the measuring tubes 4. Using various control technology measures, it is possible to track the operating point of the Coriolis mass flowmeter 2 by tracking the excitation frequency of the oscillation generator 5 to the variable resonance frequency.

(12) In the embodiment shown in FIG. 1, the control and evaluation unit 7 comprises a digital signal processor and corresponding I/O interfaces for controlling the oscillation generator 5 and for reading the oscillation signals of the oscillation sensors 6. The control technology measures mentioned are implemented on the digital signal processor by means of programming.

(13) It is known in the state of the art to detect the occurrence of a two-phase flow by evaluating state variables of the Coriolis mass flowmeter 2 and to indicate the occurrence with a corresponding indicator variable, whereby such an indicator variable does not contain a reliable quantitative statement about the gas portion in the medium 3, but rather makes a binary statement about whether medium 3 has a gas portion or not.

(14) FIGS. 2a and 2b show a method 1 with which the gas portion in the medium 3 flowing through the Coriolis mass flowmeter 2 can be determined. For this, it is provided that, in a ρ.sub.100 step, 8 the density value ρ.sub.100 of the gas-free medium is first determined. In a ρ.sub.mess step 9, the density value ρ.sub.mess of the medium 3 flowing through the measuring tubes 4 is measured. The measurement is carried out by determining the resonance frequency of the system. Furthermore, in a GVQ step 10, a quantity GVQ is calculated for the gas portion of the medium 3 flowing through the measuring tubes 4 with the density value ρ.sub.100 and the density value ρ.sub.mess. Finally, the quantity GVQ is output for the gas portion of the medium 3 flowing through the measuring tubes 4. The differences in FIGS. 2a and 2b are intended to illustrate that the order in which the ρ.sub.100 step 8 and the ρ.sub.mess step 9 are carried out is irrelevant. Both values, i.e. the density value ρ.sub.100 and the density value ρ.sub.mess, are required in order to determine the quantity GVQ for the gas portion in the medium 3.

(15) In the embodiment according to FIG. 2a, the density value ρ.sub.100 of the gas-free medium is simply given as a fixed value, so the density value ρ.sub.100 is known in this case. This makes sense if it can be practically ruled out that the density value ρ.sub.100 of the gas-free medium will change. In other cases, however, the density value ρ.sub.100 of the gas-free medium can also be determined, for example, by a measurement, which makes sense if it is expected that the density value ρ.sub.100 of the gas-free medium can also change during operation.

(16) FIGS. 3 and 4 show that an indicator variable I.sub.2-phase is used for the presence of gas inclusions in the medium flowing through the measuring tubes 4. A common feature of the embodiments is that the indicator variable I.sub.2-phase for the presence of gas inclusions in the medium 3 flowing through the measuring tubes 4 is first determined in an indicator step 11. In the examples shown, this indicator variable I.sub.2-phase is determined continuously during operation of the Coriolis mass flowmeter 2. The indicator variable I.sub.2-phase only provides information about whether a gas phase is present in the medium 3 or not. It is therefore only a binary signal. In any case, the indicator variable I.sub.2-phase for the presence of gas inclusions in the medium 3 flowing through the measuring tubes 4 is determined in an indicator step 11. In the example shown in FIG. 3, the method 1 is designed in such a manner that the ρ.sub.100 step 8 is carried out by measuring the density ρ of the medium 3 flowing through the measuring tubes 4 only if the indicator variable I.sub.2-phase indicates that the medium 3 flowing through the measuring tubes 4 is free of gas inclusions. The ρ.sub.100 step 8 is therefore only triggered if the indicator variable I.sub.2-phase indicates this, otherwise the ρ.sub.mess step 9 for determining the density value ρ.sub.mess is calculated if gas inclusions are present. In the GVQ step 10, the last determined value ρ.sub.100 of the gas-free medium 3 is always used.

(17) In the embodiment according to FIG. 4, the density value ρ of the medium 3 is determined in a general step 8, 9. It is obvious that this density value ρ is the density value ρ.sub.100 of the gas-free medium 3 if the indicator variable I.sub.2-phase indicates that the medium 3 has no gas portion. In the event that the medium 3 has a gas portion and this is therefore indicated by the indicator variable I.sub.2-phase the density value ρ corresponds to the density value ρ.sub.mess of the medium 3 flowing through the measuring tube 4. In the combined step 8, 9 the density ρ of the medium 3 is therefore always measured in general and stored as density value ρ.sub.100 or as density value ρ.sub.mess depending on the indicator variable I.sub.2-phase.

(18) The density values shown in FIG. 5 for determining the gas portion in the medium 3 flowing through the Coriolis mass flowmeter 2 is characterized in that the medium pressure p.sub.m of the medium flowing through the measuring tubes 4 is measured 12 and a corrected density value ρ.sub.mess(p.sub.m) is determined with the measured medium pressure p.sub.m and the measured density value ρ.sub.mess of the medium 3 flowing through the measuring tube 4, wherein the corrected density value ρ.sub.mess(p.sub.m) is used as the basis for the further method as the measured density value ρ.sub.mess, i.e. in particular in the GVQ step 10 for determining the gas portion.

(19) Finally, FIG. 6 shows that the calculated quantity GVQ for the gas portion of the medium 3 flowing through the measuring tubes 4 is compared with a limit value GVQ.sub.limit for the gas portion of the medium 3 flowing through the measuring tube 4 and that a status message is output in the event of a specified deviation of the calculated quantity GVQ from the limit value GVQ.sub.limit.

(20) FIG. 1 shows schematically that the control and evaluation unit 7 is connected to a display unit 13. The quantity GVQ for the gas portion in the medium 3 can be shown here directly on the display unit 13. However, it is also possible to output the quantity GVQ for the gas portion via a fieldbus interface 14, for example to a higher-level control room. In another configuration, the quantity GVQ is simply output to the control and evaluation unit 7 and stored there. The value for the quantity GVQ can then also be used internally, for example for control or diagnostic purposes.