Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
2000-09-22
2003-10-21
Williams, Hezron (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
active
06634241
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a connecting ring for Coriolis flowmeter and in particular to a method and apparatus that enables the bonding of Coriolis flowmeter elements having different thermal coefficients of expansion.
Problem
Single tube Coriolis flowmeters typically have a balance bar surrounding a flow tube and intermediate annular connecting rings that couple each end of the balance bar to the flow tube. The connecting rings are often affixed to the balance bar and the flow tube by a brazing process in order to provide a rigid and permanent connection. The integrity of the braze joints is important because, in operation, the balance bar and the material filled flow tube are vibrated in phase opposition. The flow tube vibration is necessary to produce the Coriolis acceleration on the flowing material and the balance bar vibration is necessary to counterbalance the vibrating flow tube. The connecting rings and their braze joints ensure that the flow tube, the connecting rings and the balance bar define an integral dynamically balanced structure. If the joints are not of high and consistent integrity, the balance of the vibrating structure can be impaired along with the accuracy of the flowmeter.
Flaws in the braze joints can also reduce the life of a flowmeter. The location of the joints between the oppositely vibrating balance bar and material filled flow tube puts the braze joints in a region of high stress. Furthermore, the stress is cyclic and reverses sign with every vibration cycle. Flawed or incomplete braze joints tend to have geometries which concentrate and increase the cyclic stress. The stress can even be elevated to the point where it causes fatigue cracking and failure of the meter. It can thus be seen that the connecting rings and the brazes constitute critical elements in the successful operation of a Coriolis flowmeter.
Prior art meters have traditionally been brazed by an operation in which the cylindrical balance bar is placed over the flow tube and then the annular connecting rings are placed over the flow tube and into the ends of the balance bar. Braze material is applied to the surfaces that couple the connecting ring to the balance bar and flow tube. The structure is then placed in a oven and heated to approximately 800° C. The braze material melts and flows by capillary attraction into the small clearances separating the flow tube, connecting ring, and balance bar. The structure is then cooled and braze material solidifies to form an integral structure comprising the balance bar, connecting ring, and flow tube.
The brazing process is well suited for applications in which similar material is used for the flow tube, connecting ring, and balance bar. These elements are machined prior to the brazing operation so that an optimum clearance (gap) of approximately 0.005 cm exist between the surfaces to be bonded. This gap is sufficiently small that the capillary attraction overcomes the force of gravity and sucks the liquid braze material into the gap rather than allowing it to run down the flow tube and balance bar. Upon cooling an integral solid structure is formed.
The braze process using the prior art component design, however, is not well suited for the bonding of materials having different thermal coefficients of expansion. This is a problem because it is necessary to make the flow tube of titanium for performance reasons. Titanium is very expensive and difficult to weld and fabricate. Therefore, for reasons of economy, a stainless steel balance bar is preferred. Stainless steel has a thermal expansion coefficient that is approximately twice that of titanium. When the components are heated in the brazing furnace the stainless steel balance bar expands twice as much as the titanium flow tube and connection rings. At brazing temperature this differential expansion opens the gaps between the titanium and stainless steel parts so that the capillary attraction is no longer sufficient to hold the braze material in the gaps.
For an example, let it be assumed that the parts are machined to have a 0.005 cm gap at room temperature. At brazing temperature (800° C.) the gap between the outside of the titanium flow tube and the inside of the titanium connecting ring does not change significantly because they both expand the same amount. However, the gap between the outside of the titanium connecting ring and the inside of the stainless steel balance bar increases at brazing temperature. Titanium expands at approximately 7.2×10
−6
cm/cm/° C. while stainless steel expands at approximately 16.2×10
−6
cm/cm/° C. The difference in expansion rate is thus 9×10
6
cm/cm/° C. Assuming that the cylindrical surfaces to be brazed have a diameter of 2.54 cm, when the structure is heated to the brazing temperature of 800° C, the inside surface of the balance bar expands 0.0177 cm more than the outside surface of the connecting ring. The gap produced by the differential expansion is added to the original clearance of 0.005 cm to produce a gap of 0.023 cm at brazing temperature. This 0.023 cm gap is not suitable for a successful brazing operation since the capillary attraction is not sufficiently strong to prevent the liquid braze material from running out of the joint. Furthermore, if the parts are not fixtured with extreme precision, the gap is likely to become 0.046 cm on one side and zero on the other as the connecting ring moves to one side in the balance bar bore. This lack of concentricity can result in a braze that extends only partly around the circumference of the intended braze surface. The result is a defective braze joint between the flow tube and connecting ring when the structure is cooled.
Attempts have been made in the prior art to overcome the problem of brazing a titanium flow tube and to a non-titanium balance bar. These efforts include the use of threaded braze surfaces to couple the elements together. This is not satisfactory since threading of the parts is expensive and the materials still expand at different rates so that outer threads on the connecting ring would not be tightly coupled to inner threads on the balance bar resulting in loss of concentricity and the possibility of partial brazes.
For the above and other reasons it is a problem in the art of Coriolis flowmeter construction to reliably and inexpensively braze materials having different thermal expansion coefficients. In particular it is difficult to provide an integral structure wherein non-titanium balance bars are reliably brazed to titanium flow tubes and titanium connecting rings. In the above discussion it is assumed that the flow tube and connecting rings are titanium and that the balance bar is made of material such as stainless steel or other material having a higher thermal coefficient of expansion. A similar problem arises when the flow tube is made of titanium and the connecting rings and balance bar are made of stainless steel or when any of the parts to be brazed has a thermal expansion coefficient different than any other of the parts.
Solution
The above and other problems are solved and an advance in the art is achieved by a method and apparatus provided by the present invention. The present invention relates to a Coriolis flowmeter that has a geometry such that the connecting ring can be inexpensively and reliably brazed to a flow tube and balance bar of dissimilar materials. In a typical application of the apparatus and method of the present invention, a titanium connecting ring is brazed to a balance bar formed out of material having a much higher coefficient of expansion such as stainless steel.
The titanium connecting ring has a radially inner braze joint with the titanium flow tube. The connecting ring surface of this joint is axially parallel to the outer surface of the flow tube. Put simply, the inner braze surface of the connecting ring and the outer braze surface of the flow tube are cylindrical as in the prior art. Since the flow tube and the connecting ring are both formed of titanium, they have the same expansio
Dickens C
Duft Setter Ollila & Bornsen LLC
Micro Motion Inc.
Williams Hezron
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