Mass flowmeter

Details Mass flowmeter Mass flowmeter

2000-02-07

2002-06-04

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Volume or rate of flow

Mass flow by imparting angular or transverse momentum to the...

C073S861000

Reexamination Certificate

active

06397685

ABSTRACT:

This invention relates to a mass flowmeter designed to operate by the Coriolis principle, incorporating an essentially straight fluid-conducting Coriolis measuring tube, at least one oscillator associated with and exciting the Coriolis measuring tube, at least one detector associated with the Coriolis measuring tube and capturing the Coriolis forces and/or the Coriolis oscillations generated by Coriolis forces, and a compensating cylinder in which the Coriolis measuring tube is mounted by way of a mechanical connection between the Coriolis measuring tube and the compensating cylinder.
The above states that the mass flowmeter in question incorporates at least one oscillator “associated” with the Coriolis measuring tube, and at least one detector “associated” with the Coriolis measuring tube. Typically, the oscillator or oscillators, or at least part of the oscillator(s), and the detector or detectors or at least part of the detector(s), are connected to the Coriolis measuring tube. However, since such connection is not a must, the term used herein is “associated” rather than “connected”.
BACKGROUND OF THE INVENTION
There are two basic types of mass flowmeters operating by the Coriolis principle, one employing a more or less straight Coriolis measuring tube, the other a looped Coriolis measuring tube. As another differentiating feature, there are mass flowmeters with only one Coriolis measuring tube and those with two Coriolis measuring tubes, in the latter case permitting either parallel or in-line flow of the fluid.
In recent times, mass flowmeters employing only one essentially straight Coriolis measuring tube have increasingly gained in popularity. Compared to mass flowmeters using either two straight Coriolis measuring tubes or one looped Coriolis measuring tube, Coriolis-type mass flowmeters with only one straight measuring tube offer significant advantages. The advantage over mass flowmeters with two straight Coriolis measuring tubes lies primarily in the fact that, in contrast to dual Coriolis measuring tubes, single-tube designs do not require a flow divider or flow combiner. Compared to single or dual looped Coriolis measuring tubes, the main advantage of the straight Coriolis tube design is that it is easier to manufacture than a looped Coriolis measuring tube, that there is less of a pressure drop in a straight Coriolis measuring tube than in a looped Coriolis measuring tube, and that a straight Coriolis measuring tube is easier to clean than a looped Coriolis measuring tube.
Still, all these advantages notwithstanding, mass flowmeters with only one straight Coriolis measuring tube present problems in a variety of ways.
First of all, in a straight Coriolis measuring tube, thermal expansion and stress cause variations in the measuring accuracy as a function of the temperature of the moving fluid. In extreme cases, thermal stress can even lead to mechanical defects such as stress-induced fissures in the Coriolis measuring tube.
The above-mentioned problems with mass flowmeters employing straight Coriolis measuring tubes have already been addressed by the industry (reference is made in particular to German patent 41 24 295, German patent disclosure 41 43 361 and the German patent 42 24 379). The problems have been largely solved, on the one hand, by connecting the Coriolis measuring tube with the compensating cylinder in such fashion that any relative movement in the axial direction is inhibited whereby the axial distance of the connecting point between the Coriolis measuring tube and the compensating cylinder defines the length of oscillation of the Coriolis measuring tube, and, on the other hand, by positioning the Coriolis measuring tube in the compensating cylinder in tensile-prestressed condition (German patent 41 24 295), and/or by producing the Coriolis measuring tube and the compensating cylinder from materials having identical or nearly identical coefficients of thermal expansion (German patent disclosure 41 43 361), and/or by providing a length-variation sensor capable of detecting changes in the oscillation length of the Coriolis measuring tube and of correcting the measurements for varying oscillation-length and stress factors (German patent 42 24 379). Most notably, it has been possible to produce a Coriolis-type mass flowmeter employing a single Coriolis measuring tube with a measuring accuracy of within about 0.1% (ref. Prospectus for the “Zulassung des Corimass G-Gerates zum elchpflichtigen Verkehr” issued by KROHNE Messtechnik GmbH & Co. KG).
However, mass flowmeters operating by the Coriolis principle and employing a straight Coriolis measuring tube do have one inherent drawback (ref. European patent disclosure 0 521 439):
It is necessary for the Coriolis measuring tube or tubes in mass flowmeters operating by the Coriolis principle to oscillate under the action of at least one oscillator; after all, it is the oscillation of the Coriolis measuring tube or tubes and the flow of mass through the Coriolis measuring tube or tubes that produces the Coriolis forces or Coriolis oscillations.
In mass flowmeters employing two straight Coriolis measuring tubes or one or two looped Coriolis measuring tube(s), the Coriolis measuring tubes or the active oscillating sections of the looped Coriolis measuring tubes are identical in design and so positioned and excited, that they oscillate in mutually opposite directions. As a desirable result, the overall oscillating structure has no external oscillating effect. The center of inertia remains stationary, compensating for any forces encountered. It follows that no oscillations are introduced into a pipeline system in which this type of mass flowmeter is installed, so that no pipeline vibrations will affect the accuracy of the measurements.
Of course, Coriolis-type mass flowmeters employing only one straight Coriolis measuring tube do not offer the benefit of counter-oscillating measuring tubes. The center of mass does not remain stationary and there is no compensation for spurious forces. As a result, a mass flowmeter of this type when installed in a pipeline will transfer vibrations into the pipe which, in turn, can affect the measuring accuracy. The industry has already addressed the task of minimizing the introduction of extraneous interferences, i.e. vibrations in the surrounding pipeline structure (ref. German patent disclosures 44 23 168 and 196 32 500).
To control the aforementioned problems which are peculiar to Coriolis-type mass flowmeters employing only one straight Coriolis measuring tube, the pipeline system in which the mass flowmeter is installed is often provided with additional clamping devices. As a rule, the pipe through which the fluid flows to the mass flowmeter and the pipe through which the fluid is carried away from the mass flowmeter are clamped down at spatial intervals corresponding to ten to fifteen times the pipe diameter.
Another proposed approach to the aforementioned problems which are peculiar to mass flowmeters operating by the Coriolis principle and employing only one straight Coriolis measuring tube, has been to install so-called antiresonators at the point where the Coriolis measuring tube is mounted, which antiresonators should have a resonant spectrum of a bandwidth that matches at least one intrinsic, natural vibration of the Coriolis measuring tube (ref. European patent disclosure 0 521 439). It has been found, however, that in the case of mass flowmeters which are very accurate to begin with, this approach offers no further improvement in terms of measuring accuracy or error reduction.
Another approach has been, especially in the case of a mass flowmeter employing only one straight Coriolis measuring tube, to mount on the compensating cylinder an equalizing unit of a symmetrical design and positioned in a symmetrical relation to the center of the Coriolis measuring tube (German patent disclosure 197 10 806). That equalizing unit must be so designed that the oscillation amplitude of the compensating cylinder is minimal and preferably close to zero.
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