Abstrict A double-tube type Coriolis flow meter is provided for measuring
mass flow. The flow meter includes a straight flow tube (4) through
which the fluid being measured flows, a hollow outer tube (5) secured
at both ends thereof to act as a counterbalance, a drive unit (7),
and a pair of sensors. The flow tube (4) is caused to vibrate by
the drive unit (7) so that the mass flow of the fluid is measured
by detecting a phase difference proportional to Coriolis force acting
on the flow tube (4) by the sensors. Sheet members (coupling plates
6) secure the flow tube (4) to both ends of the outer tube (5) and
integrally form the coupling plate (6) with tube-assembly supporting
leaf springs (14) in the double-tube type Coriolis flow meter. By
connecting the flow tube to the outer tube via the sheet members
in this manner, stresses produced between the tubes are absorbed
and a uniform temperature distribution is maintained in the outer
tube and manufacturing costs are reduced.
Claims What is claimed is:
1. A double-tube type Coriolis flow meter comprising:
a straight flow tube defining a space through which the fluid being
measured flows;
a hollow outer tube disposed concentrically outside said flow tube,
said hollow outer tube having a first end and a second end;
a first sheet member securing said flow tube to said first end
of said outer tube;
a second sheet member securing said flow tube to said second end
of said outer tube such that with said first end and said second
end secured to said flow tube, said outer tube functions as a counterbalance,
each of said first sheet member and said second sheet member having
a resiliency so as to absorb a difference in elongation between
said flow tube and said outer tube which is caused due to thermal
stress;
a drive unit; and
a pair of sensors, said flow tube being caused to vibrate by said
drive unit so that the mass flow of said fluid being measured is
measured by detecting with said sensors a phase difference proportional
to Coriolis force acting on said flow tube as a result of the vibration.
2. A double-tube type Coriolis flow meter as set forth in claim
1 wherein said sheet member is formed into a disc shape closing
the gap between said concentrically disposed flow tube and said
outer tube, and fixedly fitted to the outer periphery of said flow
tube and the inner periphery of said outer tube.
3. A double-tube type Coriolis flow meter as set forth in claim
1 further comprising an outer housing, each of said sheet members
respectively symmetrically extending outwardly of said outer tube
in a radial direction to form leaf springs, with the ends of each
of said leaf springs respectively fixedly fitted to the outer housing.
4. A double-tube type Coriolis flow meter as set forth in claim
2 further comprising an outer housing, each of said sheet members
respectively symmetrically extending outwardly of said outer tube
in a radial direction to form leaf springs, with the ends of each
of said leaf springs respectively fixedly fitted to the outer housing.
5. A double-tube type Coriolis flow meter comprising:
an outer housing;
a straight flow tube defining a space through which the fluid being
measured flows;
a hollow outer tube disposed concentrically outside said flow tube,
said hollow outer tube having a first end and a second end;
a first sheet member securing said flow tube to said first end
of said outer tube, said first sheet member having a first sheet
radial extension with radial extension ends extending symmetrically
radially outwardly of said hollow outer tube to form leaf springs,
said radial extension ends being respectively fixedly fitted to
said outer housing;
a second sheet member securing said flow tube to said second end
of said outer tube such that with said first end and said second
end secured to said flow tube, said outer tube functions as a counterbalance,
said second sheet member having a second sheet radial extension
with second sheet radial extension ends extending symmetrically
radially outwardly of said hollow outer tube to form leaf springs,
said second sheet radial extension ends being respectively fixedly
fitted to said outer housing, each of said first sheet member and
said second sheet member having a resiliency so as to absorb a difference
in elongation between said flow tube and said outer tube which is
caused due to thermal stress;
a drive unit and thermal stress;
a pair of sensors, said flow tube being caused to vibrate by said
drive unit so that the mass flow of said fluid being measured is
measured by detecting with said sensors a phase difference proportional
to Coriolis force acting on said flow tube as a result of the vibration.
6. A double-tube type Coriolis flow meter comprising:
an outer housing;
a straight flow tube defining a space through which the fluid being
measured flows;
a hollow outer tube disposed concentrically outside said flow tube,
said hollow outer tube having a first end and a second end;
a first sheet member securing said flow tube to said first end
of said outer tube, said first sheet member having an annular disc
shape closing the gap between said concentrically disposed flow
tube and said outer tube, and fixedly fitted to the outer periphery
of said flow tube and the inner periphery of said outer tube and
having a first sheet radial extension with radial extension ends
extending symmetrically radially outwardly of said hollow outer
tube to form leaf springs, said radial extension ends being respectively
fixedly fitted to said outer housing;
a second sheet member securing said flow tube to said second end
of said outer tube, said second sheet member having an annular disc
shape closing the gap between said concentrically disposed flow
tube and said outer tube, and fixedly fitted to the outer periphery
of said flow tube and the inner periphery of said outer tube such
that with said first end and said second end secured to said flow
tube, said outer tube functions as a counterbalance, said second
sheet member having a second sheet radial extension with second
sheet radial extension ends extending symmetrically radially outwardly
of said hollow outer tube to form leaf springs, said second sheet
radial extension ends being respectively fixedly fitted to said
outer housing, each of said first sheet member and said second sheet
member having a resiliency so as to absorb a difference in elongation
between said flow tube and said outer tube which is caused due to
thermal stress;
a drive unit; and
a pair of sensors, said flow tube being caused to vibrate by said
drive unit so that the mass flow of said fluid being measured is
measured by detecting with said sensors a phase difference proportional
to Coriolis force acting on said flow tube as a result of the vibration.
Description FIELD OF THE INVENTION
The present invention relates to a double-tube type Coriolis flow
meter with counterbalancing outer tube.
BACKGROUND OF THE INVENTION
The Coriolis flow meter is designed to measure the mass flow of
a flowing medium being measured taking advantage of the fact that
when a measuring tube supported at both ends thereof, through which
the medium being measured flows, is caused to vibrate, Coriolis
force acting on the measuring tube is proportional to the mass flow
of the medium being measured.
Now, a conventional double-tube type Coriolis flow meter will be
described, referring to FIG. 4. The Coriolis flow meter is of a
double-tube type having a hollow cylindrical outer housing 1 with
connecting flanges 2 at both ends, in which a double straight-tube
assembly comprising a flow tube 4 and an outer tube 5 is disposed
coaxially with the outer housing 1. A fluid being measured flows
in the straight flow tube 4 and a counterweight 10 is mounted at
the center of the hollow straight outer tube 5. Both the flow tube
4 and the outer tube 5 are coaxially secured at both ends thereof
to each other via coupling blocks 12 which are rigid bodies. The
weight of the counterweight 10 is adjusted so that the natural frequency
of the flow tube 4 with the coupling blocks 12 at both ends serving
as supporting parts, becomes equal to the natural frequency of the
outer tube 5.
At the center of the flow tube 4 and the outer tube 5 provided
is a drive unit 7 for causing the flow tube 4 and the outer tube
5 to resonate with each other in opposite phases. A pair of sensors
8 are provided at symmetrical locations on both sides of the drive
unit 7 to detect a phase difference produced in the flow tube by
the Coriolis force.
This double-tube type Coriolis flow meter is of a simple and compact
construction and capable of stably detecting a mass flow proportional
to a phase difference produced by the Coriolis force.
In the double-tube type Coriolis flow meter, however, when a change
in the temperature of the fluid being measured happens to cause
a large temperature difference between the flow tube and the outer
tube, thermal stress may be produced in the longitudinal direction
of the tubes, causing the spring constant and accordingly. The natural
frequency of the tubes to change. This could deteriorate the energy
balance, making the resonance of the tubes difficult.
The Coriolis flow meter is usually associated with an error, called
the instrumental error, between the true value, namely, the actual
flow of a fluid, and the measured value as the result of measurement
by an instrument. A commonly practiced method of efficiently correcting
the instrumental error in the double-tube type Coriolis flow meter
is to measure temperature or stress at a given point on the outer
tube and make corrections based on the measurements. But if temperature
distribution in the outer tube becomes uneven, accurate error correction
cannot be accomplished. The use of coupling blocks 12 that are rigid
bodies as used in the conventional type tends to cause an increase
in local thermal conduction through the coupling blocks 12 at both
ends, rather than even temperature distribution over the overall
length, leading to an uneven temperature distribution in the outer
tube.
Furthermore, although the vibration supporting ends are generally
connected to the tubes by brazing, a large difference between the
thermal capacities of the supporting ends and the tubes could make
brazing procedures complex, leading to increased cost.
This invention is, therefore intended to solve these problems,
and it is an object of this invention to provide a double-tube type
Coriolis flow meter wherein the flow tube and the outer tube are
connected to one another with sheet members to absorb stresses produced
between tubes and limit local thermal conduction to ensure uniform
temperature in the outer tube, and the thickness of the sheet members
is made closer to that of the tubes to simplify brazing procedures
and attain good brazing performances.
DISCLOSURE OF THE INVENTION
This invention was contrived under the aforementioned circumstances,
and the double-tube type Coriolis flow meter according to this invention
comprises a straight flow tube 4 in which the fluid being measured
flows, a hollow outer tube 5 that is disposed concentrically outside
the flow tube 4 secured at both ends to act as a counterbalance,
a drive unit 7 and a pair of sensors 8. As the flow tube 4 is caused
to vibrate by the drive unit 7 a phase difference proportional
to the Coriolis force acting on the flow tube 4 is produced by the
vibration. The phase difference is detected by the sensors 8 at
both ends of the flow tube 4 to measure mass flow. This invention
is characterized by a double-tube type Coriolis flow meter in which
the flow tube 4 is fixedly fitted to both ends of the outer tube
5 via sheet members.
In this invention, resonance is stably maintained without changing
the spring constant due to thermal stress and changing the natural
frequency of the tubes since the stress produced between the tubes
is absorbed by connecting the flow tube and the outer tube at both
ends via sheet members. Furthermore, the use of sheet members helps
reduce thermal conduction due to its thin thickness, increasing
the ratio of uniform thermal conduction in the longitudinal direction
through a space between the flow tube and the outer tube. This contributes
to making the temperature of the outer tube uniform. As a result,
it is possible to correct the instrumental errors more accurately
since temperature measurement at a point on the outer tube to correct
instrumental errors becomes more accurate. In addition, by making
the thickness of the sheet members closer to that of the tubes,
the thermal capacities of them can be made almost equal, leading
to simplified brazing procedures and improved brazing performance.
The double-tube type Coriolis flow meter according to this invention
is characterized by its construction where the sheet members are
formed into a disc shape so as to close the gap between the concentrically
disposed flow tube 4 and outer tube 5 with the outer periphery
of the flow tube 4 brazed to the inner periphery of the outer tube
5. By forming the sheet members into a disc shape, this invention
makes it possible to realize positive brazing with sufficient mechanical
strength while maintaining a thin thickness enough to make thermal
conduction through the sheet members difficult.
Furthermore, the double-tube type Coriolis flow meter according
to this invention is characterized by its construction where the
sheet members symmetrically extended toward the outside of the outer
tube 5 in the radial direction to form leaf springs 14 with the
ends thereof fixedly fitted to the outer housing 1. With this construction,
leaf springs 14 of a simple construction can be provided to ensure
stable vibration. Forming the coupling plates and the leaf springs
into an integral structure helps reduce the number of parts, leading
to reduced manufactured cost. Complete agreement of the supporting
points of the flow tube and the outer tube with the supporting points
of the entire tube assembly ensures stable vibration, leading to
a high-performance Coriolis flow meter manufactured at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an example of a double-tube type Coriolis flow meter
to which this invention is applied.
FIG. 2 is an enlarged detailed view of the connecting part of the
flow tube and the outer tube encircled and marked with A in FIG.
1.
FIG. 3 is a partially enlarged view of the connecting part of the
flow tube and the outer tube, representing another example of the
Coriolis flow meter to which this invention is applied.
FIG. 4 shows a double-tube type Coriolis flow meter of a prior
art.
BEST MODE FOR CARRYING OUT THE INVENTION
In the following, this invention will be described in detail, referring
to the accompanying drawings. FIG. 1 shows an example of a double-tube
type Coriolis flow meter to which this invention is applied. The
double-tube type Coriolis flow meter has a hollow cylindrical outer
housing 1 having connecting flanges 2 at both ends. Inside the outer
housing 1 disposed is a straight flow tube 4 through which the fluid
being measured flows, made of stainless steel, Hastelloy, titanium
alloy, for example. Outside the flow tube 4 concentrically secured
is a hollow outer tube 5 at both ends thereof via sheet members
(coupling plates 6) in such a manner as to form a concentric dual
tube assembly except for both ends in the longitudinal direction,
as will be described in detail later. The locations at which the
flow tube 4 and the outer tube 5 are connected serve as supporting
points. The outer tube 5 is made of a resilient material, such as
stainless steel, Hastelloy and titanium alloy, and has at the middle
thereof a counterweight 10 having a counterbalancing function.
Vibration of the flow tube 4 and detection of the phase difference
produced by the Coriolis force caused by the vibration are achieved
in the normal manner. That is, a drive unit 7 mounted on the outer
tube 5 causes the flow tube 4 to vibrate in the primary mode of
the natural frequency thereof. As the fluid flows in the flow tube
4 Coriolis forces are generated in opposite directions in the inflow
and the outflow sides, with the central portion, at which the vibration
speed becomes maximum, as the boundary. This results in deflection
of the flow tube in an undulated manner. This undulated deflection
is called the secondary-mode component. The flow tube is thus subjected
to displacement as a result of superposition of the primary-mode
vibration due to the vibration caused by the drive unit and the
secondary-mode vibration due to the Coriolis force. A pair of sensors
8 are installed on the outer tube 5 at positions on both sides of
the drive unit 7 at which the secondary-mode component becomes maximum,
so that the phase difference of the flow tube 4 due to the Coriolis
force is detected to determine the mass flow of the fluid being
measured.
FIG. 2(a) is an enlarged detailed view of the connecting part,
or the supporting point, of the flow tube 4 and the outer tube 5
encircled and marked with A in FIG. 1. In this invention, a sheet
member, as exemplified by the coupling plate 6 is used to connect
the flow tube 4 and the outer tube 5. Whereas the thinner the sheet
member the more favorable from the viewpoint of heat conduction,
as will be described later, it should preferably have a thickness
similar to that of both tubes to ensure a strong brazing bond from
the viewpoint of mechanical strength necessary to support the outer
tube. Similarly, from the viewpoint of mechanical strength and strong
brazing bond, the coupling plate 6 should preferably be of a disc
shape closing the gap between the flow tube 4 and the outer tube
5 disposed concentrically with one another.
Similarly to FIG. 2(a), FIG. 2(b) is an enlarged detailed view
of the connecting part, which serves as a supporting point, of the
flow tube 4 and the outer tube 5. The part of the coupling plate
6 to which the flow tube 4 is connected can be pressed into a shape
as shown in the figure to obtain stable brazing strength.
This coupling plate 6 is made of stainless steel, Hastelloy, titanium
alloy, etc. As shown in the figure, the flow tube 4 and the outer
tube 5 are integrally secured at both ends thereof to form a vibration
supporting point via the coupling plate 6 by brazing the coupling
plate 6 to the outer periphery of the flow tube 4 and to the inner
periphery of the outer tube 5 respectively.
For the brazing material, gold, nickel, silver, vanadium, etc.
can be used. Although an example where the inner periphery of the
outer tube 5 is brazed to the outer periphery of the coupling plate
6 is shown in the figure, the outer periphery of the coupling plate
6 can be made contact with, and brazed to, the butt end of the outer
tube 5. Furthermore, welding, adhesive bonding or other appropriate
means may be used in place of brazing.
According to this invention, thermal stress can be absorbed by
connecting the flow tube 4 and the outer tube 5 using sheet members.
If the temperature of the fluid changes, the flow tube 4 in which
the fluid flows immediately follows that temperature change, while
a delay is caused in the temperature response of the outer tube
5 which is outside the flow tube 4. That is, a temperature difference
occurs between the flow tube 4 and the outer tube 5 and thermal
stress is caused due to the difference in elongation between the
flow tube 4 and the outer tube 5 resulting from the temperature
difference. According to this invention, this thermal stress can
be absorbed by the resiliency of the aforementioned coupling plates
6. Thus, resonance can be stably maintained without changes in the
spring constant due to thermal stress and in the natural frequency
of the tubes.
According to this invention, uniform temperature distribution in
the outer tube 5 can be maintained by connecting the tubes using
the coupling plates 6. As described above, if the fluid temperature
changes, the temperature of the flow tube 4 that also changes according
to the change in the fluid temperature is transmitted to the outer
tube 5 via a space (air that exists there and radiation) between
the concentrically disposed two tubes, and through the heat conduction
of the coupling plates 6. Whereas the heat conduction through the
space between both tubes is uniform in the longitudinal direction,
the heat conduction via the coupling plates 6 is realized only from
both ends. This may cause uneven temperature distribution. In this
invention, the use of sheet members as the coupling plates 6 in
place of rigid blocks used in the prior art, can reduce the heat
conduction via the coupling plates 6 which might have caused uneven
temperature distribution, compared with the uniform heat conduction
through the space between both tubes. Thus, temperature distribution
in the outer tube as a whole can be made uniform, and a more accurate
value for the temperature measured at a point on the outer tube
to correct instrumental errors can be obtained. As a result, instrumental
errors can be corrected more accurately.
FIG. 3 is a partially enlarged view of the connecting part of the
flow tube and the outer tube shown in another example of a Coriolis
flow meter to which this invention is applied. In the figure, numeral
14 refers to a leaf spring integrally formed with the coupling plate
6.
In a double-tube type, it is known that supporting the vibration
fulcrums by the outer housing 1 via a leaf spring 14 can reduce
the effects of changes in the characteristics of the supporting
part, leading to stable vibration. In the example shown, the leaf
spring 14 is formed by extending the coupling plate 6 symmetrically
toward the right and left sides up to the outside of the outer tube
5 in the radial direction. In other words, a piece of sheet metal
is commonly used for both the leaf spring 14 and the coupling plate
6. By doing so, it is possible to form a sheet-metal part of a simple
construction having both the functions of the coupling plate 6 and
the leaf spring 14. With this sheet-metal part, brazing can be performed
easily. As shown in the figure, the left and right parts of the
outer tube 5 are cut away, with the upper and lower parts left uncut
as protruded parts 13. Brazing is carried out by fitting the integrally
formed coupling plate 6 and leaf spring 14 into the cut-away portions.
The other ends of the leaf spring 14 are fixedly fitted to the outer
housing 1 with appropriate means, such as brazing.
In the example shown, it is assumed that vibration is caused in
the vertical direction in the figure, and accordingly the leaf spring
14 is extended in the horizontal direction perpendicular to the
direction of vibration. The direction in which the leaf spring 14
is extended, however, may be in the same direction as that of vibration,
namely in the vertical direction, or in four radial directions.
INDUSTRIAL APPLICABILITY
As described above, the double-tube type Coriolis flow meter according
to this invention having a counterbalancing outer tube can absorb
the stress generated between the flow tube and the outer tube and
make the temperature of the outer tube uniform by limiting local
thermal conduction. |