Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
2002-03-11
2004-07-20
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
C073S861355, C073S861357
Reexamination Certificate
active
06763730
ABSTRACT:
The present invention relates to meters which derive a measurement by vibrating a metering tube, particularly Coriolis-type flow meters, such as are described in U.S. Pat. No. 4,422,338, U.S. Pat. No. 5,423,221, U.S. Pat. No. 4,856,346, U.S. Pat. No. 5,394,758, U.S. Pat. No. 4,192,184 and U.S. Re. Pat. No. 31,450, the disclosures of each of which are herein incorporated by reference. The invention is also applicable to other meters, for example densitometers which operate by vibrating a metering tube.
Coriolis meters may be used to obtain a measure of mass flow rate, from the phase difference between sensor outputs and also a measure of density, from the resonant frequency. Pursuant to the invention, we have found that density measurements in conventional meters may be inaccurate, particularly when the stress in the metering tube varies, for example when the meter is subjected to a change in temperature, either ambient temperature or temperature of process fluid. The problem is particularly acute in straight, or nearly straight, tube meters, where we have found that uncompensated errors of tens of percent and even compensated errors of several percent may arise in the event of a change in process fluid temperature in a meter which is intended to have an accuracy of the order of 0.1%.
Our investigations have shown that, at least in the case of a straight tube meter, the errors arise primarily due to changes in tension in the metering tube. It is known that frequency of vibration is dependent on tension in the tube (as well as fluid density) and it is known to take a single measurement of tension and store this. However as mentioned, the tension is liable to change, particularly with temperature fluctuations, and also with aging of the tube.
To enable tension measurements to be made in situ, it is known to mount one or more strain gauges on the flow meter tube and to obtain a measure of strain from the strain gauges. Pursuant to the invention it is has been appreciated that provision of strain gauges may not give accurate results, as they only provide a local indication of strain. The strain gauges may also require calibration and temperature compensation.
Changes in tension are generally less of a problem in meters which have a higher compliance, for example where the length of the metering tube is large in comparison to the distance between fixing points and incorporates one or more bends, an example being a B-tube meter. Nonetheless, other factors which may affect stress in the tube, for example pressure, may affect density measurements or other measurements such as flow.
EP-A-848234 and U.S. Pat. No. 5,734,112 both disclose coriolis flow meters having two parallel bent tubes with sensors mounted close to first and second nodes of vibration and wherein the tubes are excited in two modes of vibration.
EP-A-701 107 discloses an arrangement in which the resonant frequencies of two vibration modes of a straight tube meter are measured and in which it is demonstrated that the ratio of the two frequencies is a linear function of tension. Thus, from the ratio of frequencies, a measure of tension can be calculated. We have investigated the techniques and assumptions proposed in that disclosure, as discussed further below and have discovered that, whilst the reasoning and results presented in that disclosure are useful and may provide a useful improvement on previous strain-gauge methods, there is room for improvement and the technique cannot produce highly accurate meters. Specifically, analysis pursuant to the invention reveals that, surprisingly, the relationship is not truly linear, nor can it be readily corrected, and better results can be achieved by a different approach.
FIG. 15
shows the error in the stress estimate based on a technique as disclosed.
In a general aspect, the invention proposes use of measurements of resonant frequencies for two or more independent vibrational modes of a metering tube to obtain a measure of density of fluid in the metering tube compensated for variation of stress in the metering tube or to obtain a measure of stress in the metering tube wherein the ratio of said resonant frequencies is dependent on density or wherein the stress is determined as a non-linear function of the ratio of said resonant frequencies.
In a first method aspect, the invention provides a method of obtaining a measure of stress in a fluid metering tube, the fluid having a density, the method comprising inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density; obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and deriving said measure of stress from said first and second resonant frequencies based on modelling the fluid density as a first function of stress and the first resonant frequency and modelling the fluid density as a second function of stress and the second resonant frequency and solving to determine stress as a function of said frequencies.
In a second method aspect, the invention provides a method of obtaining a measure of stress in a fluid metering tube, the fluid having a density, the method comprising inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density; obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and deriving said measure of stress from said first and second resonant frequencies based on determining possible pairs of values of stress and density corresponding to one of the first and second resonant frequency and selecting a pair of values based on the other of the first and second resonant frequencies.
In a third method aspect the invention provides a method of obtaining a measure of density of fluid in a metering tube, the tube being subjected to a stress, the method comprising inducing first and second vibration modes in the tube and obtaining a first resonant frequency of the first vibration mode which is a first function of stress and density; obtaining a second resonant frequency of the second vibration mode which is a second function of stress and density; and deriving said measure of density from said first and second resonant frequencies based on modelling the fluid density as a first function of stress and the first resonant frequency and modelling the fluid density as a second function of stress and the second resonant frequency and solving to eliminate stress.
As will become apparent as the description proceeds, all of the above aspects stem from a common and novel approach to determination of density or stress in a metering tube which not only may provide better results than prior art techniques but may be simpler to implement. It will be apparent that the above techniques are not limited to implementations where the ratio of the two frequencies is independent of density; the methods may be used where the ratio of the first and second frequencies varies with density and/or where the stress is not a linear function of this ratio (whether or not this ratio is explicitly determined (unlike the prior art, there is no need to determine this ratio)). Whilst the invention works where such constraints are not met, it of course equally works if they are met; indeed such constraints are not directly relevant to the invention.
Preferred features are set out in the dependent claims and other preferable and optional features and the advantages thereof will be apparent from the following.
The invention extends to apparatus arranged to perform a method according to any method aspect, which may comprise a signal processor of a flowmeter and may include said metering tube.
In a first apparatus aspect, the invention provides apparatus for obtaining a measure of stress in a fluid metering tube, the fluid having a density, the apparatus comprising:
exciter means for inducing first and second vibration
ABB Limited
Ellington Alandra N.
Hunt, Jr. Ross F.
Lefkowitz Edward
Stites & Harbison PLLC
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