Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
2002-03-29
2003-07-29
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
active
06598489
ABSTRACT:
FIELDS OF THE INVENTION
This invention relates to Coriolis flowmeter and, in particular, to Coriolis flowmeters having a single flow tube connected to a surrounding balance bar. This invention further relates to a straight flow tube Coriolis flowmeter having a balance bar whose design and construction reduce the residual thermal stress on the flow tube due to manufacturing operations involving temperatures exceeding those encountered during normal operating conditions of the flowmeter.
PROBLEM
Coriolis flowmeters having a single straight flow tube surrounded in part by a balance bar are known. The balance bar counterbalances the vibrating flow tube so that the balance bar and flow tube together form a dynamically balanced structure. The outer axial ends of the balance bar are connected by connecting rings to the outer surface of the flow tube. These connections are accomplished by a high temperature bonding operation such as by brazing or soldering.
The thermal stresses and corrosive fluids to which flow tubes are subjected in normal operation require that they be formed of titanium. A titanium flow tube is readily brazed to a titanium balance bar and, since both have the same thermal expansion coefficient, the cooled assembly has very little residual thermal stress. Unfortunately, titanium balance bars are very costly. Balance bars could be made of less expensive steel except for the problem of high temperature brazing. Steel has a thermal expansion coefficient that is close enough to titanium so that under normal flowmeter operating temperatures neither the flow tube nor the balance bar are over-stressed due to differential thermal expansion. However, the high temperature brazing operation used to join the parts (more than 746° Centigrade) expands the steel balance bar significantly more than the titanium flow tube. As these parts begin to cool, the braze material solidifies at a temperature well above the normal operating temperature range of the flowmeter. Continued cooling shrinks the steel balance bar axially more than the titanium flow tube. This differential shrinkage can amount to a strain level of more than 2.3 mm per meter of balance bar length. A shrinkage of this magnitude puts the titanium flow tube in compression with a stress of 0.23 GPa and can impair the performance of the flowmeter or can even exceed the yield strength of the flow tube under certain conditions.
Attempts have been made to overcome the differential thermal expansion/contraction problems associated with the use of a titanium flow tube with a balance bar formed of a different material such as steel. However, these attempts have not been wholly successful and have resulted in the creation of other problems. One such attempt involves the use of a steel balance bar having two half sections joined by bellows as a center portion. This solution is not ideal since bellows are axially symmetric and make difficult the controlling of the vibrational modes of the balance bar. Another solution, is the joining of separate balance bar sections by means of leaf springs. This solution also results in problems in the vibrational characteristics of the balance bar.
It can therefore be seen in view of the above that it is a problem to provide a Coriolis flowmeter having a titanium flow tube and a steel balance bar that are not susceptible to structural damage during the manufacturing operations involving the use of high temperature bonding operations.
SOLUTION
The residual thermal stress problem is solved by separating the balance bar structure into two independent halves. These halves are separated from each other in the central portion so that, in the braze furnace, the balance bar halves can expand and contract without contacting each other and without stressing the flow tube. At this time, the balance bar halves are brazed to the flow tube via connecting rings at their axially outer ends.
A significant aspect of this invention involves the method in which the balance bar halves are connected to each other after the brazing step. This connection is necessary for several reasons. First, it is necessary for the tuning of the modes of vibration of the balance bar-flow tube assembly. This assembly is a distributed mass-stiffness system and therefore has an infinite number of modes of vibration. One of the important modes of vibration is the drive mode in which the flow tube and the balance bar vibrate out-of-phase with each other in the drive plane direction in their first bending modes. There is another vibration mode which looks like the drive mode except that it occurs in a direction perpendicular to the drive mode. This mode is called the lateral mode. If the lateral mode frequency is too near the drive mode frequency, the accuracy of the meter is reduced. The prior art bellows connection means has equal bending stiffnesses in both the drive and lateral directions resulting in nearly equal resonant frequencies. The present invention separates these two frequencies by making the connection means between the two balance bar halves stiffer in bending in the drive direction than in the lateral direction. This raises the drive mode frequency above the lateral mode frequency.
Connecting the two balance bar halves also enables both halves to be driven in the drive mode by a single driver. This is important because two drivers would have to be precisely matched in order to avoid deforming the flow tube in a shape that looks like a response to fluid flow. Such deformation would give erroneous flow readings.
The present invention overcomes the above discussed and other problems resulting from high temperature induced stress on the flow tube during the brazing of the balance bar to the outer walls of the flow tube via connecting rings. This new process includes the steps of providing a balance bar having separate halves; bonding the outer axial ends of the balance bar halves via connecting rings to the outer walls of the flow tube; connecting balance bar halves to each other by side channel members which are positioned parallel to the longitudinal axis of the balance bar halves and are bisected by the neutral surface of the assembly in the drive mode. (The neutral surface in the drive mode is defined as the theoretical surface in a bending member that experiences neither compressive nor tensile stresses. In the single tube flowmeter it is defined by the tube axis and a line intersecting it that extends in the lateral direction.) Furthermore, the side channels are radially spaced apart from the outer surface of the balance bar. The side channels are connected to the balance bar by means of pegs. The pegs are inserted into holes in the channel members and holes in the balance bar halves.
The pair of cylindrical balance bar halves has a combined axial length somewhat less than that of a conventional integral balance bar. The inner axial ends of the balance bar halves are separated a desired amount to form a center section that separates the balance bar halves. The flow tube is inserted within the interior of the balance bar halves. The outer axial ends of the balance bar halves are aligned with a connecting ring having a center opening through which the flow tube is extended. The outer periphery of the connecting ring is then brazed to the axial outer end of each balance bar half and the inner surface of the connecting ring is simultaneously brazed to the outer surface of the flow tube.
During the same brazing operation, the pegs are brazed into holes in the walls of each balance bar half. The hole centers are located on the neutral surface. At this time, the fabrication of the flowmeter has proceeded to the point where the outer ends of the balance bar halves are brazed to the flow tube via the connecting rings and the pegs are brazed into the holes in the balance bar walls.
Following the brazing operation the pegs are inserted into holes in the side channels and welded. Each balance bar half is connected by a welding operation to each side channel using a pair of pegs for each side channel. A greater number of pegs such a
Bertchie Theodore L.
Van Cleve Craig Brainerd
Dickens Charlene
Duft Setter Ollila & Bornsen LLC
Micro Motion Inc.
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