Coriolis mass flowmeter

Details Coriolis mass flowmeter Coriolis mass flowmeter

2002-12-16

2004-08-31

Lefkowitz, Edward (Department: 2855)

Measuring and testing

Volume or rate of flow

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

Reexamination Certificate

active

06782764

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Coriolis mass flowmeter having im disposed on a straight line which contains said median point and proved stability, measuring accuracy, and vibration-proof performance.
2. Description of the Related Art
In order to facilitate understanding of the present invention, a description will be given of problems encountered with known arts, with reference to
FIGS. 74
to
78
.
FIG. 74
shows the structure of a conventional apparatus disclosed, for example, in Japanese Unexamined Patent Application Publication No. 6-109512.
Referring to this Figure, a vibration tube
1
has both ends connected to flanges
2
which are used for connecting the vibration tube
1
between conduits. An ocillator
3
is fixed to a median point of the vibration tube
1
.
Vibration sensors
4
and
5
are positioned near both ends of the vibration tube
1
. The vibration tube
1
is fixed at its both ends to a housing
6
.
With this arrangement, a fluid as a measuring object is caused to flow through the vibration tube
1
while the ocillator
3
is activated.
A Coriolis force expressed by the following equation is generated, where the angular velocity of the vibration caused by the ocillator
3
is expressed by [&ohgr;] and the flow velocity of the fluid is expressed by [V], with each symbol in brackets indicating a vector quantity. The mass flow rate of the fluid can be determined by measuring the vibration which is proportional to the Coriolis force.
Fc=−
2
m[&ohgr;]×[V]
FIG. 75
shows the structure of another conventional apparatus disclosed, for example, in Japanese Unexamined Patent Application Publication No. 11-108723.
A vibration tube
11
performs a simple harmonic oscillation or a circular motion on a circle which is at a predetermined radial distance from each point on a reference axis
14
defined as a straight line interconnecting an upstream fixed end
12
and a downstream fixed end
13
of the vibration tube
11
.
An ocillator
15
is provided on the median point of the vibration tube
11
. Vibration sensors
16
and
17
are disposed near both ends of the vibration tube
11
.
FIG. 76
is a cross-sectional view of the vibration tube
11
taken along the line b—b of FIG.
75
.
FIG. 77
is a cross-sectional view of the vibration tube
11
taken along the line a—a or c—c of FIG.
75
.
FIG. 78
is a perspective view illustrating the manner in which the vibration tube
11
vibrates.
Referring to
FIGS. 76 and 77
, the vibration tube
11
when not ocillated is held near a position indicated by “A”.
When the vibration tube
11
is ocillated, the center of the vibration tube
11
moves on a circle of a radius R(x) from the reference axis
14
.
At the position of the cross-section b—b, the center of the vibration tube
11
oscillates on an arc or a part of a circle having a radius R(b) from the reference axis
14
. At the position of the cross-section a—a or c—c, the center of the vibration tube
11
oscillates on an arc or a part of a circle of a radius R(a) or R(c) from the reference axis
14
, from a position A to a position B and from the position B to the position A, and from the position A to a position C and then again to the position A, and repeats this operation.
Symbols “A”, “B” and “C” in
FIG. 78
respectively correspond to the positions of the vibration tube
11
indicated by the same symbols in
FIGS. 76 and 77
. Numerals
12
and
13
denote fixed ends of the vibration tube
11
, and
14
denotes the reference axis which is the straight line interconnecting these fixed ends.
Since each point on the vibration tube
11
oscillates only on an arc or a part of circle which is at a constant distance from the reference axis
14
, the length of the vibration tube
11
is held constant regardless of the angular position of the vibration tube
11
.
In the conventional apparatus of the type described, the vibration tube
1
is fixed at its both ends. However, when the size of the flowmeter is limited, it is extremely difficult to perfectly fix both ends of the vibration tube so as to completely isolate the tube from vibration.
Two major problems are encountered with the conventional apparatus.
One of these problems is that the flowmeter is susceptible to external conditions.
More specifically, the housing of the flowmeter by itself cannot fully accommodate the influence of any vibration or stress of external piping, so that such external vibration or stress is transmitted to the internal vibration tube
11
to cause a change in the mode of vibration of the tube
11
, resulting in fluctuation of the output and errors such as shifting of zero point.
The other problem is that the vibration of the internal vibration tube is propagated externally of the flowmeter.
External propagation of the vibration and insufficient isolation from external vibration cause the following drawbacks.
(1) Internal vibration is rendered unstable due to low Q value, enhancing susceptibility to vibration noise other than intentionally ocillated vibration.
(2) Electrical power consumption is increased due to large energy used for ocillatation.
(3) External propagation of vibration is significantly affected by external factors such as the manner of installation, stress in the piping and change in ambient conditions such as temperature, with the result that the mode of vibration of the vibration tube
11
is varied to allow easy change of the zero point and the span.
In the arrangement shown in
FIG. 75
, each point on the vibration tube
11
performs simple harmonic oscillation along an arc or a part of a circle of a predetermined radial distance from the reference axis
14
which is defined as being the straight line interconnecting the upstream end
12
and the downstream end
13
of the vibration tube
11
.
The force acting on each fixed end of the vibration tube under the described ocillated vibration is mainly composed of torque or rotational component acting about the reference axis. This offers more effective isolation from vibration than in the arrangement shown in FIG.
74
. However, the position of the center of gravity of the whole vibration system is shifted due to the change in the position of the vibration tube caused by the ocillated vibration.
Shift of the gravity center allows easy external propagation of vibration from the flowmeter, so that the problem in regard to the isolation from vibration still remains unsolved.
FIG. 29
is a plan view of a critical portion of a known Coriolis mass flowmeter of the type disclosed in Japanese Unexamined Patent Application Publication No. 61-189417.
FIG. 30
is a side elevational view of the structure shown in FIG.
29
.
FIGS. 31 and 32
are illustrations of the operation of the known flowmeter shown in FIG.
29
.
Referring to these Figures, a vibration tube has a first branch tube
218
and a second branch tube
219
which are in parallel with each other and which are supported by support plates
241
and
242
at their both ends.
A pair of vibration sensors
223
and
224
and an ocillator
221
are connected between these two branch tubes
218
and
219
, so that these branch tubes are ocillated to constantly vibrate at their resonance frequency.
In most cases, the branch tubes perform ocillated vibrations in a basic resonance mode as illustrated in FIG.
31
. More specifically, the first branch tube
218
vibrates to change its position from A to B, from B to A, from A to C and back again to A and then again to B and so on. In the meantime, the second branch tube
219
vibrates to change its position from A′ to B′, from B′ to A′, from A′ to C′ and back again to A′ and then again to B′ and so on. These two branch tubes vibrate in opposite phases in symmetry with each other.
It is also possible to arrange such that the branch tubes
218
and
219
vibrate in a high-order resonance mode as illustrated in FIG.
32
. In this case also, the first branch tube
218
vibrates to

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