U.S. patent number RE36,376 [Application Number 09/110,015] was granted by the patent office on 1999-11-09 for stability coriolis mass flow meter.
This patent grant is currently assigned to Micro Motion, Inc.. Invention is credited to Donald R. Cage, Timothy J. Cunningham, James R. Ruesch.
United States Patent |
RE36,376 |
Cage , et al. |
November 9, 1999 |
Stability coriolis mass flow meter
Abstract
An optimized Coriolis mass flow meter is disclosed which has
improved stability to excitations caused by external influences. A
primary source of improvement involves determining by modal
analysis of the flow conduit a location for the sensor means that
minimizes the influence of external excitation of one or more of
the first in phase bending mode, the first out of phase bending
mode, the first out of phase twist mode, the second out of phase
twist mode, the second out of phase bending mode and the third out
of phase bending mode.
Inventors: |
Cage; Donald R. (Longmont,
CO), Ruesch; James R. (Boulder, CO), Cunningham; Timothy
J. (Boulder, CO) |
Assignee: |
Micro Motion, Inc. (Boulder,
CO)
|
Family
ID: |
27002312 |
Appl.
No.: |
09/110,015 |
Filed: |
July 2, 1998 |
PCT
Filed: |
June 08, 1990 |
PCT No.: |
PCT/US90/03284 |
371
Date: |
January 16, 1992 |
102(e)
Date: |
January 16, 1992 |
PCT
Pub. No.: |
WO90/15310 |
PCT
Pub. Date: |
December 12, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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364032 |
Jun 9, 1989 |
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Reissue of: |
820648 |
Jun 8, 1990 |
05301557 |
Apr 12, 1994 |
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Current U.S.
Class: |
73/861.355 |
Current CPC
Class: |
G01F
1/8409 (20130101); G01F 1/8477 (20130101); G01F
1/8413 (20130101) |
Current International
Class: |
G01F
1/84 (20060101); G01F 1/76 (20060101); G01F
001/84 () |
Field of
Search: |
;73/861.355,861.356 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 212 782 |
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Mar 1987 |
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EP |
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0 250 706 |
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Jan 1988 |
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EP |
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1337668A1 |
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Sep 1987 |
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SU |
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8706691 |
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Nov 1987 |
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WO |
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Other References
PP. Kremlevsky, Flowmeters and Quantity Meters, 1989, pp.
348-351..
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Primary Examiner: Noori; Max H.
Assistant Examiner: Patel; Harshad
Attorney, Agent or Firm: Hahn; Thomas S.
Parent Case Text
BACKGROUND OF THE INVENTION
This application is a continuation-in-part of our U.S. patent
application Ser. No. 364,032 filed Jun. 9, 1989 now abandoned.
Claims
We claim:
1. A mass flow meter for flowable materials wherein mass flow rates
for flowable materials are determined based on at least one
measured effect of Coriolis forces, said flow meter comprising: a
support means; at least one continuous flow conduit which is free
of pressure sensitive joints or sections, each of said conduits
being solidly mounted to said support means at inlet and outlet
ends for said conduits; driver means for oscillating each of said
conduits about bending axes adjacent each of said solid mountings;
a pair of sensor means mounted on each of said conduits for
monitoring motion of said conduits while flowable materials are
flowing therethrough and said conduits being oscillated by said
driver means about said bending axes, monitored motion including
motion caused by Coriolis forces about twist axes for each of said
conduits, said sensor means generating signals related to all
motions of said conduits; and signal processing means to detect and
convert said signals to mass flow rate values; in which the
improvement comprises:
fixed mounting of each of said pair of sensor means on each of said
conduits to monitor motions of said conduits including motions
about said twist axes, where each sensor means is mounted between
.Iadd.and as close as possible to two proximate .Iaddend.nodes of a
pair of vibration modes for said conduit, said pair of vibration
modes being selected from a pairing of the first in phase bending
mode, first out of phase bending mode, first out of phase twist
mode, second out of phase twist mode, second out of phase bending
mode, or third out of phase bending mode.
2. A mass flow meter according to claim 1 in which said continuous
flow conduit has an essentially straight inlet leg and an
essentially straight outlet leg which converge toward one another
at said support and are interconnected opposite said support by the
remainder of said continuous conduit.
3. A mass flow meter according to claim 2 in which said inlet and
outlet legs are interconnected opposite said support by an
essentially straight portion of said conduit which curves at either
end to meet each of the essentially straight leg portions.
4. A mass flow meter according to either of claim 2 or claim 3 in
which members of said sensor pairs are placed between nodes of the
second out of phase twist mode and the second out of phase bending
mode on the respective inlet and outlet sides of each flow
conduit.
5. A flow meter according to claim 1 having at least two flow
conduits clamped together by brace bars at points along the inlet
and outlet legs which are spaced from the support, said points
having been determined by modal analysis to be those points which
provide optimum separation between the frequencies of the first in
phase bending mode and the first out of phase bending mode so as to
minimize effects of the first in phase bending mode on meter
sensitivity.
6. A flow meter according to claim 5 in which said continuous flow
conduit has an essentially straight inlet leg and an essentially
straight outlet leg which converge toward one another at said
support and are interconnected opposite said support by the
remainder of said continuous conduit.
7. A flow meter according to claim 5 in which the race bars include
nipple shaped mounting sleeve means.
8. A flow meter according to either of claim 2 or claim 3 having at
least two flow conduits, wherein the support comprises a flangeless
inlet and outlet plenum which further comprises two separated flow
chambers, one for flow separation on the inlet side and one for
flow recombination on the outlet side.
9. A flow meter according to claim 1 further comprising a pressure
tight case of essentially the same geometric configuration as said
flow conduit which encases all of said conduit, said sensor means
and said driver, and is welded to said support.
10. A flow meter according to either of claim 2 or claim 3 further
comprising a pressure tight case of essentially the same geometric
configuration as said flow conduit which encases all of said
conduit, said sensor means and said driver, and is welded to said
support.
Description
In the art of measuring mass flow rates of flowing substances it is
known that flowing a fluid through an oscillating flow conduit
induces Coriolis forces which tend to twist the conduit in a
direction essentially transverse to the direction of fluid flow and
also to the axis about which oscillation occurs. It is also known
that the magnitude of such Coriolis forces is related to both the
mass flow rate of the fluid passing through the conduit and the
angular velocity at which the conduit is oscillated.
One of the major technical problems historically associated with
efforts to design and make Coriolis mass flow rate instruments was
the necessity either to measure accurately or control precisely the
angular velocity of an oscillated flow conduit so that the mass
flow rate of the fluid flowing through the flow conduits could be
calculated using measurements of effects caused by Coriolis forces.
Even if the angular velocity of a flow conduit could be accurately
determined or controlled, precise measurement of the magnitude of
effects caused by Coriolis forces raised another severe technical
problem. This problem arose in part because the magnitude of
generated Coriolis forces is very small in comparison to other
forces such as inertia and damping, therefore resulting Coriolis
force-induced effects are minute. Further, because of the small
magnitude of the Coriolis forces, effects resulting from external
sources such as vibrations induced, for example, by neighboring
machinery or pressure surges in fluid lines, may cause erroneous
determinations of mass flow rates. Such error sources as
discontinuities in the flow tubes, unstable mounting of the tubes
use of tubes lacking mechanically reproducible bending behavior,
etc., often completely masked the effects caused by generated
Coriolis forces, greatly diminishing the practical use of a mass
flow meter.
A mechanical structure and measurement technique which, among other
advantages: (a) avoided the need to measure or control the
magnitude of the angular velocity of a Coriolis mass flow rate
instrument's oscillating flow conduit; (b) concurrently provided
requisite sensitivity and accuracy for the measurement of effects
caused by Coriolis forces; and (c) minimized susceptibility to many
of the errors experienced in earlier experimental mass flow meters,
is taught in U.S. Pat. Nos. Re 31,450, entitled "Method and
Structure for Flow Measurement" and issued Nov. 29, 1983; 4,422,338
entitled "Method and Apparatus for Mass Flow Measurement" and
issued Dec. 27, 1983; and 4,491,025 entitled "Parallel Path
Coriolis Mass Flow Rate Meter" and issued Jan. 1, 1985. The
mechanical arrangements disclosed in these patents incorporate
curved continuous flow conduits that are free of pressure sensitive
joints or sections, such as bellows, rubber connectors or other
pressure deformable portions. These flow conduits are solidly
mounted at their inlet and outlet ends, with their curved portions
cantilevered from the support. For example, in flow meters made
according-to any of the aforementioned patents, the flow conduits
are welded or brazed to the support, so that they are oscillated in
spring-like fashion about axes which are located essentially
contiguous with the solid mounting points of the-flow conduits or,
as disclosed in U.S. Pat. No. 4,491,025, essentially at the
locations of solidly attached brace bar devices designed to clamp
two or more conduits rigidly at points located forward of the
mounting points.
By so fashioning the flow conduits, a mechanical situation arises
whereby, under flow conditions, the forces opposing generated
Coriolis forces in the oscillating flow conduits are essentially
linear spring forces. The Coriolis forces, opposed by essentially
liner spring forces, deflect or twist the oscillating flow conduits
containing flowing fluid about axes located between and essentially
equidistant from the portions of those flow conduits in which the
Coriolis forces manifest themselves. T-he magnitude of the
deflections is a function of the magnitude of the generated
Coriolis forces and the linear spring forces opposing the generated
Coriolis forces. Additionally these solidly mounted, continuous
flow conduits are designed so that they have resonant frequencies
about the oscillation axes (located essentially at the locations of
the mountings or brace bars) that are different from, and
preferably lower than, the resonant frequencies about the axes
relative to which Coriolis forces act.
Various specific shapes of solidly mounted curved flow conduits are
disclosed in the prior art. Included among these are generally
U-shaped conduits "which have legs which converge, diverge or are
skewed substantially" (Re 31,450, col. 5, lines 10-11). Also
disclosed in the art are straight, solidly mounted flow conduits
which work on the same general principles as the curved
conduits.
As stated above, the Coriolis forces are generated when fluid is
flowed through the flow conduits while they are driven to
oscillate. Accordingly, under flow conditions, one portion of each
flow conduit on which the Coriolis forces act will be deflected
(i.e. will twist) so as to move ahead, in the direction in which
the flow conduit is moving, of the other portion of the flow
conduit on which Coriolis forces are acting. The time or phase
relationship between when the first portion of the oscillating flow
conduit deflected by Coriolis forces has passed a preselected point
in the oscillation pathway of the flow conduit to the instant when
the second portion of that conduit passes a corresponding
preselected point in that pathway is a function of the mass flow
rate of the fluid passing through the flow conduit. This time
difference measurement may be made by various kinds of sensors,
including optical sensors as specifically exemplified in U.S. Pat.
No. Re. 31,450, electromagnetic velocity sensors as specifically
exemplified in U.S. Pat. Nos. 4,422,338 and 4,491,025, or position
or acceleration sensors are also disclosed in U.S. Pat. No.
4,422,338. A parallel path double flow conduit embodiment with
sensors for making the preferred time difference measurements is
described in U.S. Pat. No. 4,491,025. This embodiment provides a
Coriolis mass flow rate meter structure which is operated in the
turning fork-like manner earlier described in U.S. Pat. No. Re.
31,450. Detailed discussion of methods and means for combining
motion sensor signals to determine mass flow rate appears in U.S.
Pat. Nos. Re. 31, 450 and 4,442,338 and in application
PCT/US88/02360, published as WO89/00679.
In the aforementioned meter designs, the sensors are-typically
placed at symmetrically located positions along the inlet and
outlet portions of the flow conduit which provide acceptable
sensitivity to enable the selected sensors to make measurements
yielding a mass flow rate that is accurate within +/- 0.2
percent.
On the order of about 100,000 Coriolis mass flow meters have been
built using the invention of one or more of U.S. Pat. Nos. Re
31,450, 4,422,338 and 4,491,025 and these meters have been
extensive commercial use. More than ten years' experience in the
commercial application of these meters to mass flow rate
measurement with a variety of diverse fluid products has shown that
in general, the end users are satisfied with the sensitivity and
accuracy of their performance but desire that the meters be
improved in overall stability, including zero stability, thus
reducing plant maintenance related to these meters, including meter
recalibration. Meter instability, in general, results from
susceptibility of the meters to the unwanted transfer of mechanical
energy from sources external to such meters. Such forces can also
affect the zero (i.e., measured value at no flow) stability of the
flow meters.
While commercial experience as described above has shown
essentially no problem in practical use with fatigue failure of the
flow conduits, it is recognized that potential improvements in
conduit life span by reducing possible sources of fatigue failure
represent a forward step. Similarly, providing a sealed
pressure-tight case increases the suitability of the meters for
hazardous materials applications at significant pressures which may
range up to 1,000 psi and even higher. Even when achievable
pressure rating is balanced against cost considerations involved in
fabricating the case, the use of a case as herein described affords
a pressure rating for the meter at least as high as 300 psi for
flow tube outside diameter sizes up to about 21/2 inches and as
high as 150 psi for larger sized flow tubes.
SUMMARY OF THE INVENTION
The present invention provides an improved mass flow meter with
considerably increased overall stability, including reduced
susceptibility to external forces and increased zero stability,
reduced pressure drop characteristics and better resistance to
fluid pressures. A number of design changes to the Coriolis mass
flow meters manufactured in accordance with one or more of the
previously cited patents have resulted in optimizing their already
successful features and operating characteristics.
The present invention relates to Coriolis mass flow rate meters
that include one or more flow conduits which are driven to
oscillate at the resonant frequency of the flow conduit containing
fluid flowing therethrough. The drive frequency is maintained at
this resonance by a feedback system, heretofore described, which
detects a change in the resonant behavior of the fluid-filled
conduit as a result of the fluid mass change due to changes in
fluid density. The flow conduits of these Coriolis mass flow rate
meters are mounted to oscillate about an oscillation axis located
substantially at the mounting points or at the location of the
brace bars. The resonant frequency of oscillation is that
associated with the oscillation axis. The flow conduit also deforms
(twists) about a second axis which is that axis about which the
flow conduit deflects or twists in response to Coriolis forces
generated as a result of the flow of fluid through the oscillating
flow conduit. This latter axis associated with Coriolis-caused
deflections is substantially transverse to the oscillation axis.
The present invention provides in improved flow meter with enhanced
stability having reduced susceptibility to the influence of outside
forces, primarily because of optimized sensor placement as
explained more fully hereinafter. Other improvements which
contribute to overall stability of the improved meter include
reducing by at least fourfold the mass of the sensors and
driver.
In a preferred embodiment, a modified U-shaped flow conduit design
is provided, having two essentially straight inlet and outlet legs
which converge towards each other at the process line manifold, and
bends, at two symmetrical locations along the length of the
conduit, separated by an essentially straight middle portion. It is
also contemplated that some modified U-shape flow conduits will
have convergent inlet and outlet legs which are separated by a
continuously curved middle portion, rather than a straight middle
portion and that others will have substantially parallel inlet and
outlet legs in accordance with current commercial embodiments.
Attached to each flow conduit at symmetrical locations are two
motion sensors, so located that the susceptibility to external
forces of the signals which they detect and transmit to the meter
electronics is dramatically reduced over that of previously known
commercial mass flow meters. This is accomplished in one preferred
embodiment by locating the motion sensors between but as close as
possible to the nodes on each side of the conduit of the second out
of phase twist mode and the third out of phase bending mode of the
flow tube and placing the driver equidistant between these sensors.
The masses-of the motion sensors plus their mountings and of the
driver plug its mounting are substantially reduced in relation to
the corresponding parts of the mass flow meters heretofore in
commercial use. The susceptibility of the flow conduit to fatigue
failure may optionally be reduced by providing novel brace bars
having a novel nipple shaped sleeve, which serve to define the axis
about which each flow conduit oscillates, but conventional brace
bars may alternatively be used and in some embodiments, brace bars
are omitted. In one embodiment, advantage may be taken of the
convergent U-shape to provide a wafer configuration manifold
structure, without flanges, for connecting to the process line to
be monitored. A special sealed pressure tight case is provided
which encloses the flow conduit, motion sensors, driver and
associated electrical connectors. Several embodiments of such a
case are disclosed herein, among which the embodiment of FIGS. 8
and 9 is preferred .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an optimized Coriolis mass flow meter of this
invention partially within a case
FIG. 1A illustrates the location of the motion sensors as a result
of modal analysis for the FIG. 1 embodiment.
FIGS. 2A and 2B illustrates an option level brace bar
configuration.
FIG. 3 illustrates an optimized Coriolis mass flow meter of this
invention with a wafer manifold structure partially within a case
as shown by FIG. 4;
FIG. 4 illustrates an optional high pressure case design; and
FIGS. 5A-7I illustrate shaker table stability test results.
FIG. 8 illustrates another optional high pressure case design which
is preferred based on cost and ease of fabrication.
FIG. 9 gives further detail regarding the FIG. 8 case design.
FIGS. 10-10F and 11A-11L illustrate further test results as
hereinafter described.
DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE DRAWINGS
The major feature of the present invention the minimization of the
influence of external forces upon meter stability--is achieved
through optional placement of sensors and, to a limited extent, of
brace bars. It has been found that there are essentially six modes
of vibration, within the frequency range of 0 to 2000 Hz,
excitation of which is likely to result in loss of meter stability.
They are identified as (1) the first in phase bending mode (of
lower frequency than the drive frequency), (2) the first out of
phase bending mode, which corresponds to the fundamental drive
frequency, except that the drive frequency is the natural frequency
of the fluid--filled tube (whereas modal analysis is conducted on
the empty tube), (3) the first out of phase twist (also called
torsion or deflection) mode, (4) the second out of phase twist
mode, (5) the second out of phase bending mode and (6) the third
out of phase bending mode. Optimal placement of sensors is achieved
by conducting a modal analysis of the flow tube to locate the two
nodes for each of the six modes on that tube and to determine,
based upon tube geometry, size and material, those node points to
which the sensors should be placed most closely proximate. For a
flow tube of the geometric configuration shown in FIG. 1, for
example, modal analysis has shown the sensors should optimally be
placed intermediate the second out of phase twist node and the
third out of phase bending node on either side of the flow tube, as
close to each of these nodes as possible. Those skilled in the art
will appreciate that, depending upon the geometry and other
characteristics of the flow tube, node points for the six
enumerated modes may be located differently in relation to one
another. In some flow tube shapes two or more node points of
different modes may actually coincide, making it possible to locate
sensors at the location of coincidence and thereby enhancing flow
tube sensitivity. The present invention embraces the discovery, as
a rule of thumb, that meter stability is enhanced by locating the
sensors as close as possible on each side of the tube to at least
two node points, each of which is a node point for a different one
of the modes above, and especially of those designated (3) and (6)
above. The invention further embraces the discovery that the
influence of mode 1, the first in phase bending mode, may be
minimized or eliminated by a placement of the brace bars which
separates harmonics of mode 1 from those of mode 2.
FIG. 1 illustrates a preferred embodiment of a Coriolis mass flow
meter of optimized modified U-shape and motion sensor positioning
and mounting. As in the current commercial meters, the flow
conduits 112 are solidly mounted to the manifold 130 at points 131.
Brace bars 122, are solidly mounted to the flow conduits 112,
thereby defining oscillation axes B' when the flow conduits 112 are
driven in tuning fork fashion by driver 114. When flowable material
flows through conduits 112, the Coriolis forces cause the conduits
to deflect about deflection axes A'. Electrical connectors 125 from
the driver, 126 from motion sensors 118, and 127 from motion
sensors 116 may be connected and supported in stable stress
minimizing fashion to bracket 128 as shown, or alternatively the
bracket may be dispensed with as discussed hereinafter, in favor of
printed circuit boards mounted to the case. The connectors shown in
FIG. 1 are individual wires or ribbon-shaped flexible connectors
with embedded wires, which are mounted in a stable, stress
minimizing half-loop shape. It is contemplated that flexible
connectors, as described in U.S. patent application Ser. No.
865,715, filed May 22, 1986, now abandoned; its continuation U.S.
patent application Ser. No. 272,209, filed Nov. 17, 1988, now
abandoned, and its continuation U.S. patent application Ser. No.
337,324, fled Jul. 10, 1989, may be used. Such flexible connectors
also provide a stable, stress minimizing half-loop shape. In FIG.
1, experimental motion sensors 116 are located at the ends of the
inlet or outlet leg 117 just before the bend. Motion sensor 118 are
located at a position determined by modal analysis according to
this invention, which effectively minimizes the influence of
external forces. In a commercial meter embodiment, only motion
sensors 118 would be present and motion sensors 116, used
experimentally for comparison purposes, would be eliminated. The
meter of FIG. 1 includes case 140 which encloses the flow conduit
and associated attachments and is fixed to the manifold 130, but it
is contemplated that a meter case as shown in FIG. 4 or FIG. 8 and
described below might preferably be used.
Another feature of the flow meter design for the embodiment of FIG.
1 which minimizes the effects of external forces involves balancing
of the flow conduits and their attachments and employing reduced
mass sensor and drive components.
In Coriolis mass flow meters, the flow conduits serve as springs,
with the spring force acting predominantly on the inlet and outlet
legs. The flow conduits of the FIG. 1 embodiment are of modified
U-shape having no permanent deformation from bending in the inlet
and outlet legs. No bending during the fabrication process results
in the absence of permanent deformation in the four regions in
which spring forces act in the double conduit meters. As a result,
all four regions display essentially the same response to spring
forces if similar materials of essentially the same dimensions are
used. This improves the ability of the flow meter manufacturer to
balance the flow conduits. Balance of the flow conduits is further
enhanced by decreasing the masses of the motion sensors and driver
by using light mass magnets and coils and reducing the size of
their mountings.
The motion sensors and driven are comprised, as in current
commercial meters, of a magnet and a coil. The sensors are of the
type disclosed in U.S. Pat. No. 4,422,338 which linearly track the
entire movement of the conduit throughout its oscillation pathway.
In the FIG. 1 embodiment and in other embodiments of this
invention, the total mass of these sensors and of the driver are
reduced over their total mass in current commercial meter
embodiments by a factor of at least about 4 and preferably by a
factor of 5 to 6 or more. This reduction in mass is accomplished by
use of a lightweight bobbin having molded pin connections and by
winding the coil with 50 gauge wire while continuing to use the
same mass magnets. The lightweight sensors and driver are mounted
directly on the flow conduits, thus eliminating mounting brackets.
The lightweight coil used, e.g., in a meter embodiment of the size
and shape of that in FIG. 1 with a flow tube of approximately 0.25
inches outside diameter, has a mass of approximately 300
milligrams. Prior coils used with comparable sized mass flow meters
made pursuant to U.S. Pat. Nos. Re 31,450, 4,422,338 and 4,491,025
and having approximately the same flow tube outside diameter size
had a mass of approximately 963 milligrams. In the same sized meter
embodiment according to FIG. 1, the new assemblies add a total of
approximately 3.9 grams to the mass flow meter (1 driver coil and 2
sensors coils at 300 grams each, and 3 magnets at 1 gram each). By
contrast, in comparable earlier commercial embodiments, the
corresponding assemblies added 22.2 grams (1 driver coil, 2 sensor
coils, 1 coil bracket, 1 magnet bracket and 3 magnets) to the mass
flow meter.
For the FIG. 1 embodiment, a representative modal analysis was
performed. As one result thereof, the second out of phase twist
mode node and the third out of phase bending mode node were located
on a flow conduit having the dimensions and properties shown in
Table 1 below and on FIG. 1A:
TABLE I ______________________________________ Conduit material:
316L stainless steel Conduit length: inches Conduit outer diameter:
inch5 Conduit wall thickness: inch10 Inlet leg: inches 3 Outlet
leg: inches 3 Middle Section: inches 5 Bend radius: inches 1.25
______________________________________ The resulting sensor
location which is midway between the aforenamed mode nodes, was
placed at 22.5.degree. measured from the horizontal area extending
from the bend random centerpost.
This resulting sensor location is not only between, but in the
closest possible proximity to each of the two node points, on each
side of the conduit. As those skilled in the art will readily
appreciate, by performing modal analyses on flow conduits of other
precise shapes, dimensions and materials, each of the node points
for all of the modes enumerated above can be located and resulting
sensor locations can readily be optimized.
In some embodiments of the improved meters of this invention, the
fundamental driving frequency (the first out of phase bending mode)
is increased relative to current commercially available flow meters
made by applicants' assignee, thereby increasing the values of its
harmonics. This results in better separation of the individual
harmonics for the drive mode from that of other modes. In the FIG.
1 embodiment of the size stated, for example, the harmonics of the
other five modes of interest are each separated from harmonics of
the driving frequency by at least 20 Hz, for all frequencies below
2000 Hz.
In the FIG. 1 embodiment, the placement of the brace bars has the
effect of separating the first in phase bending frequency from the
fundamental driving frequency and thereby eliminating possible
effects of excitation of the first in phase bending frequency. The
effects of external forces operating at frequencies corresponding
to the remaining four modes of interest are in part minimized by
the balanced flow conduit design. In addition, the effects of the
second out of phase twist mode and the third out of phase bending
mode are also minimized in this embodiment by locating the motion
sensors between, but in close proximity to the nodes of both these
two modes, which nodes happen to be located close together. It is
contemplated that other location selections can be made to minimize
the effects of those modes that most affect stability of any
particular conduit, taking into account through modal analysis its
size, shape and material.
Testing of the FIG. 1 embodiment of the current commercial Model D
meters manufactured by applicants' assignee and of current
commercial Coriolis mass flow meters manufactured by others at
varying fluid pressures ranging from less than 10 psi up to about
1000 psi established that at fluid pressures approaching 1000 psi,
variations in drive frequency and twist frequency are induced which
adversely affect the accuracy of mass flow measurements. To date,
it has been determined that these effects of high fluid pressure
are minimized by increasing the flow tube wall thickness by
approximately 20% and by enclosing the flow tube assembly in a
specially designed fluid-pressure-insensitive case, as discussed
below.
Applying the wall thickness increase to the meter embodiment of
FIG. 1, for example, for a tube of outside diameter 0.230 inches,
the wall thickness is increased from about 0.010 inches to about
0.012 inches in order to minimize instabilities caused by high
fluid pressure.
FIG. 2 shows an optional brace bar design according to this
invention. Each brace bar 122 is formed, as by punching a piece of
metal (e.g., 316L or 304L stainless steel) or other suitable
material, to provide two sleeves with nippled transitions 121 from
holes having the outer diameter of flow conduit (hole 124), to
larger holes 120. These brace bars are contemplated to be brazed or
welded to the flow conduits in order to reduce stress
concentrations at the point of attachment 123, the primary locus
about which the conduit is oscillated. It is within the scope of
the present invention, however, to utilize conventional brace bars
as earlier disclosed in the art.
FIG. 3 shows an optimized Coriolis meter as in FIG. 1, with an
exploded view of the process line attachment. Instead of the
typical prior art flanged manifold, a novel wafer flangeless
structure 230 is provided for which the ends 232 can be bolted
between the existing flanges in a manufacturing or other commercial
process line, by means of threaded connectors 134 passing through
flange holes 238 and held in place by nuts 235.
FIG. 4 illustrates one form of case which minimizes pressure
effects. This form can be used to enclose the entire flow conduit
and sensor attachment assembly. It comprises a pipe 350 of
sufficient diameter to enclose the flow conduits 312, driver,
motion sensors and associated wire attachments (not shown). The
pipe is bent in the shape of the flow conduit. It is then cut
longitudinally into two essentially equal halves. The flow conduits
312 are fitted into it along with the associated driver, motion
sensors and wiring. The other half is fitted over this combined
assembly and welded along the two longitudinal seams and at the
connections to the manifold. Thus, a pressure tight case is
provided which is suitable for applications involving hazardous
fluid containment and able to withstand significant pressures on
the order of at least 300 psi and up to 500 pounds per square inch
or more. For some embodiments, a printed circuit board may be
attached to the inside of the case, with flexible connections
running from the driver and motion sensors to the circuit board. A
junction box may then be attached to the case and connected to the
circuit board by wires which can be run through pressure tight
fittings at the top of the case. The junction box is in turn
connected to means for processing electronically the signals from
the sensors to give mass flow readout values and, optionally fluid
density readout values.
An alternative case, preferred for ease of fabrication, is shown in
FIG. 8 and a section thereof is shown in FIG. 9. This form of case
is made from stamped steel pieces of half-circular cross section as
shown in FIG. 9, (which depicts piece 2 or 4 from FIG. 8) welded
together to form the case. As specifically applied to the
embodiment of FIG. 1, this case is formed of ten pieces labelled
1-10 on FIG. 8 which are assembled in the following manner:
Five pieces (1, 2, 3, 4 and 5 as labelled on FIG. 8) comprising one
half of the case--i.e., when assembled covering one half the outer
circumference of the flow tube--are welded to the support
comprising the inlet-outlet manifold of the flow tube. Printed
circuit boards, not shown in any of the figures, are affixed to the
case at locations as near as possible to the placement of the
pick-off coil portions of the sensors on the flow tube and the
pick-off coil terminals are connected to these printed circuit
boards by flexures containing wires of the type referred to
hereinabove or by individual half-loop shape wires. Wires are then
run along the case to the center straight section of the case (i.e.
section 3 which encloses the straight flow conduit section 112 of
FIG. 1) where the wiring feed-through to the meter electronics is
located. This feed-through, which is not shown in FIGS. 8 and 9,
may comprise posts to which the wires are directly connected or may
comprise a third printed circuit board to which the wires are
connected and which is, in turn, connected to feed-through posts
and then to a junction box, not shown, positioned on section 3 of
the case at its midsection. After the wiring is completed, the
remaining five piece (not shown in FIG. 8, which is a plan view of
the case) are welded in place to one another, to the support and to
the previously assembled and welded portion, preferably by means of
automated welding. These latter five pieces comprise one half of
the case. The welds between pieces are as shown by the lines on
FIG. 8. In addition, welds are made along the inner and outer
periphery of the case at seam lines which are not shown, but which
connect the top and bottom halves of the case both inside the
enclosure formed by the meter tube and support and outside that
enclosure.
The case embodiments of FIGS. 4, and 8 are illustrative only. Those
skilled in the art will readily recognize that similar cases can
readily be fashioned to any size and shape of curved or straight
tube Coriolis mass flow meter and that, depending upon the precise
shape involved, the embodiment of FIGS. 8 and 9 may advantageously
be made with other numbers of stamped steel half-circumferential
pieces. As is also readily apparent, other wiring arrangements may
be readily devised by those skilled in the art without departing
from the essential principles of this invention.
Shaker table tests were performed to test the influence of external
vibration forces and process line noise in exciting the flow
conduit with its associated attachments. Such external forces are
frequently present during plant operations. FIGS. 5A through 5F
show experimental shaker table test results for a current
commercial Micro Motion, Inc. Model D25 Coriolis mass flow meter.
FIGS. 6A through 6F show results for similar tests for a current
commercial Micro Motion, Inc. Model D40 meter. The Model D25 has a
0.172 inch inner diameter flow conduit; the D40 has a 0.230 inch
inner diameter. FIGS. 7A through 7I show experimental test results
for similar tests for a Coriolis mass flow meter similar to that
shown in FIG. 1.
In each of FIGS. 5A through 7I, the x-axis is the axis through the
meter flanges (i.e., parallel to the oscillation axis B'--B'), the
y-axis is parallel to the plane of the flow conduits (i.e.,
parallel to the deformation axis A'--A'). The z-axis is
perpendicular to the plane of the flow conduits. The parameters of
interest are summarized in Table 2:
TABLE 2 ______________________________________ Vert. Vert. Full
Scale Scale Freq. Electronics Scale % of Flow Rate Sweep output
Motionale FIG. (Ibm/min) (Hz) (mA) Axis
______________________________________ 5A 1.04 4.20 10 z 5B 10.4
1000 z 5C -- 15-2000 -- -- z 5D 1.04 100 x 5E 10.4 1000 x 5F --
15-2000 -- -- x 6A 1.04 100 z 6B 10.4 1000 z 6C -- 15-2000 -- -- z
6D 1.04 100 x 6E 10.4 1000 x 6F -- 15-2000 -- -- x 7A 1.04 100 x 7B
10.4 1000 x 7C -- 15-2000 -- -- x 7D 1.04 100 z 7E 10.4 1000 z 7F
-- 15-2000 -- -- z 7G 1.04 100 y 7H 10.4 1000 y 7I -- 15-2000 -- --
y ______________________________________
These shaker table experiments were performed on complete mass flow
meter assemblies without cases. The output indicated on the strip
chart recordings of FIGS. 5A through 7I are of motion sensor
readings in response to the corresponding external vibration. The
sequence shown in each series of charts is the meter's response to
a linear frequency ramp ranging from 15 Hz to 2 KHz and then
vibration inputs at random frequencies (FIGS. 5C, 5F, 6C, 7C, 7F,
7I). The frequency ramp occurs over a ten minute period and random
vibrations occur over an approximately five minute period. FIGS.
5A, 5B, 5D, 5E, 6A and 6B each indicate the influences of external
vibrations of various frequencies in exciting harmonics of the six
modes of motion of the D25 and D40 meters that are discussed
above.
FIGS. 7A through 7F show susceptibility to excitation due to
external vibrations of a meter of this invention of the FIG. 1
embodiment about the x and z axes (the same axes as in FIGS. 5A
through 5F) and are to the same respective scale. FIGS. 7G through
7I are taken about the y-axis. (The random vibrations were
conducted first in FIGS. 7D-7F). It is noted that the optimized
meter shows dramatically reduced influence of external vibrations
in exciting harmonics of the six modes of motion. Thus, the
optimized design is shown effectively to isolate the meters from
effects of external forces.
In addition, tests were performed to test the influence of external
vibrations on zero stability. Such tests provided results for the
stability of the time difference (.DELTA.t) measurement at no flow,
the so-called jitter test. A frequency counter was used to directly
measure the pulse width of an up/down counter prior to any
averaging or filtering of the electronics. The test was performed
on a shaker table using random frequency input over a range of
accelerations in the x, y, and z directions. The results showed
that, as the accelerations were increased, the influence of
external vibrations resulted in pulse width divergence of one or
more multiples of the average value from the average value for both
the D25 and D40 meters. Such divergence for each axis, is markedly
reduced for the optimized meter. Thus, as in the case of the
vibration tests, the jitter tests showed that the optimized meter
design effectively isolates the meters from the effects of external
forces.
FIGS. 10A-10G inclusive are plots of further data obtained with a
flow meter embodiment constructed as in FIG. 1, having conventional
brace bars and a 20% thicker flow tube than comparably sized
current commercial meters, as herein disclosed, with sensors placed
in accordance with the teachings of this invention between, but as
near as possible to the node points of the modes labelled as 4 and
5 herein above. As tested, the flow tube of this flow meter
embodiment was half covered (i.e., one half of the circumference of
the pipe) by a case of the type shown in FIGS. 8 and 9, and a
junction box (not shown in the drawings) was appended to the
outside of the case where the wires feed through the case. The
junction box was conventionally connected to another box (called
the "remote flow transmitter" or "RFT") containing the meter
electronics and having readout panels for mass flow rate and
density values, from which data was collected for FIGS. 10A-10D
inclusive. The inner diameter of the flow tube wall on this meter
embodiment was approximately 0.206 inches.
FIGS. 10A and 10B each represent calibration plots of accuracy
versus flow rate using water at mass flow rates from 0 to 45 pounds
per minute. FIG. 10A differs from FIG. 10B in that FIG. 10A
represents a "22 point" calibration curve with the first measured
points taken at mass flow rates of about 3 to 4 pounds per minute.
FIG. 10B covers a "45 point" calibration curve in which more data
points, especially for mass flow rates below 5 pounds per minute,
(commencing at about 0.5 pound per minute) were collected.
FIG. 10B also shows fluid line pressure drop data as measured for
mass flow rates from 0 to about 45 pounds per minute. FIGS. 10A and
B combine to show that the meter embodiment of this invention
performs well within the published accuracy values of .+-.0.2%
which characterize the current commercial meters of applicants'
assignee, Micro Motion, Inc. FIG. 10B also illustrates the very
acceptable fluid line pressure drop performance of this meter
embodiment.
FIGS. 10C and 10D are, respectively, plots of measured mass flow
rate and density analog drift values against fluid pressure of
water at values from 0 to approximately 2000 psi. In each instance,
a comparison appears on the plot of average historical standard
deviation measurements for commercial mass flow meters of the
D-series sold by Micro Motion, Inc. In FIG. 10C, flow rate analog
drift and flow rate standard deviation data points are shown in
units of seconds. In FIG. 10D, density analog drift and density
standard deviation are shown in units of grams per cubic
centimeter. In both cases, the data show the meter built in
accordance with this invention to perform well within the measured
standard deviation data.
FIGS 10E, F and G are plots of shaker table test data obtained
similarly to the data depicted in FIGS. 5A to 7I, but presented as
plots of shaker table frequency (in Hertz) against analog output
(i.e. mass flow rate) disturbance in seconds, whereas FIGS. 5A to
7I are reproductions of strip charts plotting shaker table
frequency in Hertz against motion sensor readings per se. In
addition, the data in FIGS. 10E, F and G were collected on the same
meter assembly with half case attachment as that to which FIGS.
10A-D inclusive apply. For comparison purposes, similar plots for
two different D25 flow conduit units, each without case attachment,
are presented in FIGS. 11A-11L inclusive. In each instance, the x,
y and z - axes are as defined above, the shaker table vertical
scale frequency sweep is from 15 to 2000 Hertz, the remote
frequency transmitter from which analog output data were obtained
had a span of 5 grams per second and a calibration factor of 1.
Other parameters of interest are summarized in Table 3:
TABLE 3 ______________________________________ Sweep of the
Vibration Motion FIG. Table Axis Unit
______________________________________ 10E log x Embodiment of this
invention 10F " y Embodiment of this invention 10G " z Embodiment
of this invention 11A linear x Model D25 Unit 1 11B log x Model D25
Unit 1 11C linear y Model D25 Unit 1 11D log y Model D25 Unit 1 11E
linear z Model D25 Unit 1 11F log z Model D25 Unit 1 11G linear x
Moded D25 Unit 2 11H log x Moded D25 Unit 2 11I linear y Moded D25
Unit 2 11J log y Moded D25 Unit 2 11K linear y Moded D25 Unit 2 11L
log y Moded D25 Unit 2 ______________________________________
A linear sweep of the vibration table expends the same amount of
time in moving through each 100 Hertz vibration interval so that,
e.g. a sweep of 15 to 115 Hertz occurs in the same time interval as
e.g. 1000 to 1100 Hertz. In log sweep, an amplified time interval
is consumed at low frequencies, e.g., from 15 to 400 Hertz and a
shortened (or speeded up) time interval is consumed at the higher
frequency end. In both instances, the total sweep time is the same.
As can be seen, log sweep clearly points out the frequencies at
which external excitations have given rise to harmonic disturbances
in current commercial Model D meters. FIGS. 10E, F and G show the
meters of this invention to be markedly less susceptible to such
influences than the D25 flow conduits.
Although the preferred embodiment is illustrated for a dual flow
conduit mass flow meter, it is contemplated that the invention
described herein can be embodied in a Coriolis mass flow meter
having only one flow conduit, either in conjunction with a member
such as leaf spring, or a dummy conduit, that forms a tuning fork
with the flow conduit or under circumstances where the single flow
conduit is of very small mass and is mounted to a base of
relatively very large mass.
While the foregoing detailed discussion focuses, for exemplary
purposes, upon one size and shape of flow tube, numerous changes
and modifications in the actual implementation of the invention
described herein will be readily apparent to those of ordinary
skill in the art, and it is contemplated that such changes and
modifications may be made without departing from the scope of the
invention as defined by the following claims.
* * * * *