Coriolis mass flowmeter

Details Coriolis mass flowmeter Coriolis mass flowmeter

1998-10-05

2001-01-09

Patel, Harshad (Department: 2855)

Measuring and testing

Volume or rate of flow

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

Reexamination Certificate

active

06170339

ABSTRACT:

The invention concerns a mass flowmeter that operates according to the Coriolis principle, with an essentially straight Coriolis tube that conducts a flowing medium, with at least one oscillation driver, assigned to the Coriolis tube and which excites the Coriolis tube, with at least one detector, assigned to the Coriolis tube and which registers Coriolis forces or Coriolis oscillations caused by Coriolis forces, and with a cylindrical shell that encloses the Coriolis tube.
BACKGROUND OF THE INVENTION
It is specified at the outset that the mass flowmeter under discussion comprises, among other items, at least one oscillation driver “assigned” to the Coriolis tube and at least one detector “assigned” to the Coriolis tube. As a rule, the oscillation driver or oscillation drivers (or in any case a part of the oscillation driver or oscillation drivers), and the detector or detectors (or in any case a part of the detector or detectors), are connected to the Coriolis tube. Since this is not necessary, however, the expression “assigned” is used instead of the expression “connected”.
In the case of mass flowmeters that operate according to the Coriolis principle, one makes a basic distinction between those in which, on the one hand, the Coriolis tube is at least essentially straight and those in which, on the other hand, the Coriolis tube is loop-shaped. In addition, one distinguishes in the case of mass flowmeters like the one under discussion between those that have, on the one hand, only one Coriolis tube and those that have, on the other hand, two Coriolis tubes. In the constructions with two Coriolis tubes, they can lie in series or in parallel with each other, in a hydrodynamic sense.
In recent times, mass flowmeters with only one essentially straight Coriolis tube have found increasing acceptance. Mass flowmeters with straight Coriolis tubes are mechanically simple to construct and therefore relatively inexpensive to produce. The inner surfaces of their Coriolis tubes are easy to work on (e.g., to polish), they show a small pressure drop, and they are self-emptying.
Despite all their advantages, mass flowmeters with only one straight Coriolis tube are problematic from a number of aspects.
First, because of the thermally-caused expansions and/or stresses associated with a straight Coriolis tube, the accuracy of its measurements depends on the temperature of the flowing medium. In the extreme case, the thermally-caused stresses can even lead to mechanical damage, specifically to stress cracks in the Coriolis tube. This problem is discussed for example in DE 41 24 295 A1 and DE 196 01 342 A1.
Furthermore, the coupling of external perturbations into a mass flowmeter with only one Coriolis tube is stronger than in the case of mass flowmeters in which two parallel Coriolis tubes are operated in the manner of a tuning fork, because in the case of mass flowmeters with only one Coriolis tube, the center of gravity of the mass flowmeter is not fixed. DE 44 23 168 A1 and DE 196 932 500 A1 deal with minimization of the coupling in of external perturbations, i.e., vibrations in the surrounding tubing system.
Finally, DE 197 32 605 A1 deals with the difficulty that arises in measuring the extremely small phase differences between the two signals delivered by the detectors, which are proportional to the mass flow, in the case of a mass flowmeter with a straight Coriolis tube. The usual industrial requirements for accuracy of the mass flowmeter demand a resolution of phase differences of the order of 1×10
−5
degrees.
Over and above the problems mentioned above, for which solutions have been approached already in the quoted publications, there is a further problem in mass flowmeters with a straight Coriolis tube, in that the total length of the mass flowmeter is always large in comparison with the nominal length of the Coriolis tube. The large length of the mass flowmeter, relative to the nominal length of the Coriolis tube, results from the necessity, at a given Coriolis tube diameter, to provide a free length of the Coriolis tube over which the Coriolis tube can oscillate with the necessary amplitude. If the Coriolis tube is too stiff, oscillation of the Coriolis tube can not take place with sufficient amplitude to guarantee a reasonable measurement accuracy. The resulting large total length of a mass flowmeter with a straight Coriolis tube is obviously not desirable, since the space available for installation in, for example, the chemical industry and the food preparation industry, where these mass flowmeters are often used, is limited.
SUMMARY OF THE INVENTION
The object of the invention is, therefore, to make available a mass flowmeter with a straight Coriolis tube that has a smaller total length, relative to the nominal length of the Coriolis tube, than the previously used mass flowmeters of the type under discussion. Furthermore, it is to be ensured that thermally-caused expansions and/or stresses in the Coriolis tube do not lead to mechanical damage of the mass flowmeter.
The object as derived and demonstrated to this point is solved according to the invention, first of all, by mounting the Coriolis tube flexibly to an inlet and an outlet of a shell, preferably a cylindrical shell. For this purpose the inlet and outlet are formed preferably either as conically tapered diaphragms, or formed in such a way (with an arched shape, for example) that both the inlet and outlet can expand slightly without causing large stresses in the inlet or outlet or in the Coriolis tube.
Because the Coriolis tube of a mass flowmeter operating by the Coriolis principle is, in accordance with the invention, mounted flexibly or floating, it is not necessary to deform the Coriolis tube, as in mass flowmeters of the prior art. Also, a flexibly mounted Coriolis tube of “almost unlimited” stiffness can be excited in such a way that Coriolis forces are generated in combination with a medium flowing through the Coriolis tube. Since a deformation of the Coriolis tube is not necessary, the Coriolis tube can be made with a shorter length, relative to its nominal length, and therefore the total length of the mass flowmeter constructed according to the invention can also be relatively small. This is true especially if the inlet and outlet that serve as flexible mounts for the Coriolis tube are short in length. With optimal dimensioning of the Coriolis tube, the cylindrical shell, and the components that serve as flexible mounts for the Coriolis tube, a mass flowmeter can be realized, for example, that has a total length of only 20 cm or at most 30 cm for a maximum flow of 300 kg/min.
It is also advantageous, in mass flowmeters designed according to the invention, that the frequency of oscillation of the Coriolis tube is practically independent of temperature, since the flexibly mounted Coriolis tube is not subjected to either bending or torsion.
An especially preferred embodiment of a mass flowmeter according to the invention is one in which the cylindrical shell forms the meter housing of the mass flowmeter, and the cylindrical shell is preferably made as a relatively heavy metal block, hollowed out to accept the Coriolis tube, oscillation driver and detector. This embodiment of a mass flowmeter according to the invention can be clamped in directly between the flanges of adjoining tubing, and a non-alignment of the Coriolis tube with the adjoining tubing—naturally within limits—does not lead to problems.
A mass flowmeter made according to the invention can also have, however, in addition to the cylindrical shell, a special meter housing fitted with flanges on both ends as needed, as is actually usual in current engineering practice. In this embodiment, there are two connecting tubes between the Coriolis tube and the meter housing. The connecting tubes are attached preferably to the inlet and outlet, somewhere near the middle of the tapered region. It is helpful in achieving the desired “limitless” movability of the Coriolis tube if the above-mentioned connecting tubes have the form of diaphragms.
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