A method for increasing stability of a Coriolis mass flow meter
by relieving residual and internal stress within the flow tube and
related structures of the meter. The method including the steps
of bending the flow tube in the desired form, fixing related flow
tube structures onto the flow tube, clamping the flow tube so as
to maintain the desired form during subsequent processes, annealing
the flow tube, and cryogenically cooling the flow tube. An apparatus
to be utilized along with the method of the present invention fixes
the flow tube during the annealing and cryogenic cooling treatment
processes to maintain the desired shape of the flow tube. The invention
also contemplates the flow tube made in accordance with the method
of the present invention.
1. A method of treating structures of a Coriolis mass flow meter,
including a flow tube comprising the steps of: bending the flow
tube of the Coriolis mass flow meter into a desired shape, fixing
related flow meter elements onto the flow tube, annealing the bent
flow tube and cryogenically cooling the bent and annealed flow tube,
thereby relieving internal and residual stress within the flow tube
2. The method as claimed in claim 1 further comprising the step
of supporting the bent flow tube in the desired shape during the
annealing and cooling.
3. The method as claimed in claim 1 wherein the flow tube is bent
by a cold working process.
4. The method as claimed in claim 1 wherein the annealing process
includes raising the temperature of the flow tube to approximately
5. The method as claimed in claim 1 wherein at least a portion
of the cryogenic cooling process id performed at approximately -320.degree.
6. The method as claimed in claim 1 further comprising the step
of fixing a frame structure onto the flow tube to define a sensing
portion within the length of the flow tube.
7. The method as claimed in claim 1 wherein the fixing step includes
a vacuum brazing process.
8. The method as claimed in claim 7 wherein the fixing step and
the annealing step are performed simultaneously.
9. A method of treating structures of a Coriolis mass flow meter,
including a flow tube comprising the steps of: forming the flow
tube of the Coriolis mass flow meter into a desired shape, supporting
the formed flow tube for subsequent processing, annealing the supported
flow tube, cryogenically cooling the annealed and supported flow
tube, returning the cooled flow tube to ambient temperature, and
assembling the flow meter incorporating the formed, annealed, and
cryogenically treated flow tube.
10. The method as claimed in claim 9 wherein the forming step includes
bending the flow tube into the desired shape.
11. The method as claimed in claim 10 further comprising the step
of attaching related flow meter elements to the flow tube substantially
simultaneously the annealing step.
12. A method of treating structures of a Coriolis mass flow meter
including flow tube, comprising the steps of:
bending the flow tube of the Coriolis mass flow meter into a desired
fixing a frame structure onto the flow tube to define a sensing
portion within the length of the flow tube,
annealing the bent flow tube, and
cryogenically cooling then bent and annealed flow tube,
thereby relieving internal and residual stress within the flow
13. The method as claimed in claim 12 further comprising the step
of supporting the bent flow tube in the desired shape during the
annealing and cooling process.
14. The method as claimed in claim 13 wherein the fixing step includes
a vacuum brazing process.
15. The method as claimed in claim 13 wherein the fixing step and
the annealing step are performed simultaneously
16. The method as claimed in claim 12 wherein the flow tube is
bent by a cold working process.
17. The method as claimed in claim 12 wherein the annealing process
includes raising the temperature of the flow tube to approximately
18. The method as claimed in claim 12 wherein at least a portion
of the cryogenic cooling process is performed at approximately -320.degree.
FIELD OF THE INVENTION
The present invention relates to a method for improving the stability
and accuracy of Coriolis-type mass flow meters. Particularly, the
invention relates to a method for the treatment of the flow tube
and related structures of a Coriolis type mass flow meter by the
combination of an annealing process and a cryogenic cooling process.
BACKGROUND OF THE INVENTION
Coriolis-type mass flow meters operate on the principal that a
fluent material passing through a flow tube, when exposed to a deflection
or oscillation transverse to the direction of flow through the tubing,
will react with a measurable force (the Coriolis force) on the walls
of the tubing. The Coriolis reaction is generated by the fluent
material moving at an instantaneously changing curvalinear path.
The Coriolis reaction acts with a force directly proportional to
the mass of the material in the tubing.
Sipin Pat. No. 3355944 discloses that a flow tube including a
distortion or deflection of the flow from a straight path will substantially
increase the measurability of the Coriolis reaction. Commonly assigned,
co-pending U.S. application Ser. No. 912893 filed Sept. 26 1986
discloses a stable flow tube structure having a centralized center
of gravity which is less susceptible to sensor signal contamination
due to external noise or other vibrational influences. Further,
the signal to noise ratio of the flow meter is increased by oscillating
the flow tube at a resonant frequency which is higher than its fundamental
resonance. Commonly assigned application U.S. Ser. No. 249805
filed Sept. 27 1988 and its continuation-in-part U.S. application
Ser. No. 404919 Filed Sept. 8 1989 disclose that the tuning
of the vibrational wave pattern, induced along the flow tube length
by the oscillation of the driver, to the resonance of the Coriolis
reaction will significantly increase the measurability of the Coriolis
reaction and, thus, increase the ability of the flow meter to accurately
determine the mass flow.
Many of the commercial Coriolis mass flow meters that are available
include a curved flow tube. The curvature imparted to the tubing
is the result plastic deformation, i.e. bending, of the flow tube
by, typically, cold working processes. These type processes for
flow tube formation create residual internal stress within the tubing
material. Further, localized residual stress is created within the
tubing and related flow meter elements by welding, brazing or otherwise
fixing these elements onto the flow tube during assembly of the
During normal operation, flow tubes within a Coriolis mass flow
meter, whether or not having a curvature, are continuously vibrationally
or rotationally oscillated at a relatively high rate of speed for
extended periods of time. The non-uniformity of the flow tube meter
structural elements may cause significant errors within the mass
flow determination. Further, the normal operation of the flow meter
may cause the flow meter structure to relax or fail due to the presence
of the residual stress within the flow tube material. Variations
over time in the characteristics of the flow tube and related elements
may significantly effect the accuracy of the Coriolis reaction measurement
and, thus, the mass flow determination.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for treatment
of Coriolis mass flow meter structures to relieve residual internal
stress within those structures and to stabilize the operation of
the flow meter over its useful life.
The method of the present invention contemplates forming the flow
tube in the desired shape for use within the Coriolis mass flow
meter. This formation may include the bending of the flow tube or
other cold working processes. The flow tube is annealed to relieve
internal and residual stress created by the forming process(es).
The flow tube is then cryogenically cooled for an extended period
to stabilize the material of the flow tube.
The method of the present invention may also include the attachment
of fixed elements onto the flow tube. These related flow meter structures,
such as sensors, clamping pins or the like, may be attached to the
flow tube in any convenient matter, such as by welding, brazing
or the like prior to the cryogenic cooling step.
The apparatus of the present invention includes a flow tube bending
form for maintaining the form of the flow tube during both the annealing
and cryogenic cooling steps.
The present invention further contemplates a Coriolis mass flow
meter including a flow tube and related structures which have been
formed in accordance with the method contemplated by the present
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of illustrating the present invention, there is
shown in the drawings a form which is presently preferred; it being
understood, however, that this invention is not limited to the precise
arrangements and instrumentalities shown.
FIG. 1 shows a top plan view of an apparatus in accordance with
the present invention.
FIG. 2 shows an exploded view of the apparatus shown in FIG. 1.
FIG. 3 shows a Coriolis mass flow meter structure for use along
with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method for treatment of Coriolis
mass flow meter structures. In the figures, where like numerals
illustrate like elements, there is shown a form of the apparatus
in accordance with the present invention. This apparatus generally
includes a flow tube bending form 10 and a clamping block 12 which
serves to fix a flow tube 14 during the method contemplated by the
The flow tube 14 as illustrated in FIGS. 1 and 2 is formed generally
in accordance with the teachings of commonly assigned, co-pending
U.S. application Serial No. 912893 filed Sept. 26 1986 and particularly
in accordance with U.S. application Serial No. 404919 filed Sept.
8 1989 and its parent U.S. application Ser. No. 249805 filed
Sept. 27 1988. These commonly assigned applications are herein
incorporated by reference. The flow tube 14 generally includes an
inlet end 16 and an outlet end 18 mounting segments 20 22 and
a sensing portion 24. The sensing portion 24 within a Coriolis mass
flow meter is generally defined as the unsupported flow tube length
positioned between fixed ends when assembled within the flow meter.
The sensing portion 24 of the flow tube 14 as formed in the manner
shown surrounds its center of gravity 26. The mounting segments
20 22 are those portions of the flow tube 14 which fix the position
of the tubing in the flow meter mounting structure (not shown).
The mounting segments 20 22 generally extend between the sensing
portion 24 and the inlet end 16 and outlet end 18 respectively.
Adjacent the mounting segments 20 22 is a curvature in the tubing
which may be provided as a stress relief for forces acting along
the flow line communicating with the flow tube 14.
The bending form 10 shown in FIGS. 1 and 2 includes a series of
turn blocks 28 30 32 and 34. A series of alignment pins 36 are
positioned proximal to the turn blocks 28-34. A series of dual alignment
pins 38 are positioned between adjacent turn blocks 28 and 30 32
and 34. The space between alignment pins 36 and their associated
turn blocks 28-34 as well as the space between dual alignment pins
38 substantially conforms to the outside diameter of the flow tube
14. The pattern of the pins and blocks generally define a discontinuous
channel for placement of the flow tube 14. A series of projections
40 are provided on the bending form 10. These projections engage
within corresponding key holes 42 in clamping block 12. Slots 44
in clamping block 12 receive the stress relief bends in the flow
tube 14 between the mounting portions 20 22 and the inlet and outlet
ends 16 18 respectively.
The flow tube 14 is bent into the desired form prior to the flow
meter assembly. It is contemplated that the bending of the flow
tube will be a cold working or similar process, as generally known
in the art. This bending of the flow tube 14 creates internal residual
stress within the tubing material. In order to relieve this residual
stress, an annealing process is applied to the flow tube 14. This
annealing process includes heating the flow tube 14 such as in
a vacuum furnace, to a temperature in the range of 2100.degree.
F. for approximately ten minutes.
The fixing of the flow tube 14 between the bending form 10 and
clamp block 12 is to maintain the shape of the flow tube 14 during
the annealing and subsequent treatment processes. The formed flow
tube 14 is placed within the space between pins 36 and blocks 28-34
and the space between dual pins 38. Upon positioning the flow tube
14 on bending form 10 clamping block 12 is attached to the bending
form 10 over the flow tube 14. Thus, upon assembly, the bending
form 10 and clamping block 12 generally form a sandwich with the
flow tube 14 being positioned therebetween. The channel defined
by the blocks 28-34 pins 36 and dual pins 38 prevent the flow
tube from relaxing when the temperature of the tubing is elevated
during the annealing process.
The working of the flow tube 14 into its desired form generally
changes the structure of its material. The bent tubing includes
long and short crystalline structures. The annealing process relieves
the residual stress created within the flow tube 14 by recrystallizing
the tubing material. For example, if the flow tube is made of stainless
steel, the annealing will cause the carbon to return into the alloy
It has been found that upon annealing the flow tube, further stress
relief can be achieved by performing a deep cryogenic cooling process
on the flow tube and related elements. The cryogenic cooling process
may be performed in any number of methods. One method is described
by Smith, U.S. Pat. No. 4739622 which is herein incorporated
The cryogenic cooling process described in Smith reduces the temperature
in stages to approximately -320.degree. F. Initial cooling of the
flow tube is performed by evaporating vapors from a cryogenic liquid
pool. Thereafter, the cooling is performed by partial or substantial
submersion or soaking of the flow tube 14 along with the bending
form 10 and the clamping block 12 in the cryogenic liquid. After
the soaking period, the temperature within the cooling chamber is
raised to ambient temperature by a controlled evaporation of the
cryogenic liquid. The cryogenic cooling process generally takes
a period in excess of 24 hours. The length of the process is typically
dependent upon the mass of the load within the cooling chamber.
Upon emergence from the cooling chamber at ambient temperature,
the flow tube 14 is removed from form 10 and block 12 and then placed
into the flow meter assembly.
The annealing process results in a recrystallization of the material.
However, the annealed flow tube 14 includes portions where the crystalline
structure remains non-homogenious. Such irregularities within the
tubing may detrimentally effect the operation of the flow meter.
The cryogenic cooling process further relieves residual stress by
providing a more homogeneous crystalline structure. It is believed
that the cryogenic cooling realigns the molecular structure within
the tubing which has distorted during the various working processes
and which has not been fully recrystallized by the annealing process.
Generally, for stainless steel tubing the structure of the material
resulting from the cryogenic cooling process includes more of the
carbon in solution and, thus, becomes more martensitic. The cooling
process removes the austenite within the material and provides slightly
smaller grains therein.
The effect of the realignment and recrystallization caused by the
cryogenic cooling process is an increase in the density of the material,
likely by a reduction of the space between the grains. Further,
the spring constant of the tubing is slightly higher since the material
is relatively stiffer. This is generally opposite of the annealing
process which softens the material to reduce residual stress.
The method of the present invention generally provides the advantage
of preventing failure of the tubing material due to peak stress
locations. Peak stress is the sum of the residual and operating
stresses within the tubing. Further, the reduction of residual stress
within the flow tube prevents the tubing from relaxing during extended
operation. Moreover, the reformation of the tubing material stabilizes
the spring constant of the flow tube over the useful life. Since
flow tubes within Coriolis mass flow meters are oscillated at a
relatively high rate of speed for extended periods of time, this
stability provides an accuracy factor within the flow meter operation.
The following chart represents the stability and calibration of
a flow meter including a stainless steel (316L) flow tube having
an 0.062 inch outside diameter and an 0.008 inch wall thickness.
The stability and calibration for the flow tube has been determined
as a function of the treatment processes performed.
______________________________________ Calibration Stability Process
Factor (lbs/min) ______________________________________ Bending
only 65 .001 to .005 Bending and Annealing 15 .0002 to .0007 Bending,
Annealing and 22 .0001 to .0003 Cryogenic cooling ______________________________________
The calibration factor may be determined by measuring the signal
produced from a flow meter with respect to a known flow rate. The
calibration factor is a constant which may be multiplied by the
measured phase difference of the meter to produce a mass flow reading.
(M=mass flow; C.sub.f =calibration factor; and .phi.=net phase
The stability of a calibrated flow tube is determined by operating
the flow meter for a period of time without flow moving therethrough.
The output of the meter is logged over the period. Even though there
is no flow moving through the meter, all meters typically do not
produce a zero flow result due to instability of the tubing structure
and other portions of the flow meter, and due to the effects of
outside noise on the flow meter. The resulting mass flow signal
produced during this testing is a measure of the accuracy of the
flow meter in determining the mass flow rate.
As can be seen by this chart, the annealed and cryogenically cooled
flow tube has a higher calibration factor over that which is only
annealed. This is likely due to the increase in the spring constant
of the tubing. The annealed and cryogenically cooled flow tube has
a substantially greater stability than that generally achieved by
the untreated flow tube or tubing that has been processed only by
annealing. For a flow meter that has a high range of instability,
a high value for the stability factor, each mass flow determination
becomes less exact. Thus, if the mass flow rate is 1 lb/min with
a stability factor of 0.0002 lb/min, the ultimate mass flow rate
determination is much more accurate than one that includes a variation
in the mass flow determination at a rate of 0.001 lb/min.
The stability of the flow meter which has been annealed and cryogenically
cooled is contemplated to continue over its operational life. This
is due to the fact that the flow tube and its attached elements
will not relax over time during normal operation. Further, after
the flow meter has been operated in the field, recalibration may
be performed by repeating the calibration procedure, described above,
with the flow meter providing a substantially constant calibration
Residual stress is also created within the flow tube by the assembly
process and can be removed by the method contemplated by the present
invention. For example, typically, a pin, clamp, bracket or other
means is provided adjacent the ends of the flow tube so as to fix
the resonant frequency of the sensing portion. Such structures are
often attached directly onto the flow tube material. These attachment
processes may also create residual stress within the tubing. Upon
annealing and cryogenic cooling of the flow tube, including the
joint between the attached elements and the tubing, the residual
and internal stress at the joint are substantially removed.
FIG. 3 shows a fixing structure for use along with a flow tube
treated by the method of the present invention. A fixing frame or
flange 46 is attached directly onto the flow tube 14 at the desired
position for defining the preferred resonant frequency of the sensing
portion 24. The flange 46 may be attached to the outside of the
tubing by a vacuum brazing, welding or similar process. A resonant
pin 48 is used to fix flange 46 to define the sensing portion 24
of the flow tube 14. The pin 48 includes a base 50 having two upstanding
projections 52 forming an opening 54 therebetween. The projections
52 include a slot 56 facing inwardly towards the opening 54 and
aligned therewith so as to receive flange 46. A clamp 58 is attached
to the top of the projections 52 by means of screws 60.
The brazing of flow meter elements onto the flow tube may be performed
simultaneously with the annealing process. As shown in FIG. 1 the
forming blocks 30 and 32 include slots 80 for receipt of the attached
flanges 46. Prior to the annealing step, flange 46 is fixed to the
tubing at the desired position and the brazing material applied
to the joint. The slots 80 hold the flanges 46 in the desired position
on the flow tube 14. Since the annealing process is preferably performed
at around 2100.degree. F., the brazing material is contemplated
to be a pure copper or a boron and nickel combination. Other materials
for brazing will have too low of a melting temperature. However,
some of the brazing material will flow during the annealing/brazing
process. Channels or openings (not shown) may be provided in the
bending form 10 and clamping block 12 adjacent the slots 80 to permit
the material to flow away from these fixtures. Thus, the brazing
material, will attach the flanges 46 or other structures, to the
flow tube 12 and not attach the flow tube to the treatment fixtures.
It should be noted that, since the annealing/ brazing step is preferably
performed in a vacuum, the brazing material may flow in either direction
from the joint. Thus, the channels may be required on both the bending
form 10 and the clamping block 12. Other methods of directing the
flow of the brazing material from the joint to prevent attachment
to the fixtures may also be used.
Further attachment to the flow tube of structural elements of the
flow meter is also contemplated. Sensors or portions thereof may
be fixed to the flow tube. Also, the brace structure used along
with the flow meter of copending application Ser. No. 404919 filed
Sept. 8 1989 (referred to above), may be attached at the central
position on the flow tube 14 along with the driver. This brace and
portions of the driver may also be fixed to the flow tube in any
convenient manner. Further, dual tube type Coriolis mass flow meters
may be treated in the manner contemplated by the present invention.
Such dual tube which are attached to the flow tube ends for dividing
the inlet flow equally between the parallel tubes. Other flow meter
structures may also be processed by the method of the present invention
as will be apparent to those skilled in the art.
The present invention may be embodied in other specific forms without
departing from the spirit or essential attributes thereof and, accordingly,
reference should be made to the appended claims, rather than to
the foregoing specification, as indicating the scope of the invention.