Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
1999-06-30
2002-02-05
Noori, Max (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
active
06343517
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a Coriolis flowmeter and in particular to a single tube Coriolis flowmeter structure for connecting the vibrating elements of the Coriolis flowmeter to the flowmeter case. The invention further relates to a Coriolis flowmeter that can be manufactured, tested, balanced, and stored prior to the attachment of process connection flanges.
PROBLEM
It is a problem to provide a single flow tube Coriolis flowmeter that operates satisfactorily over a wide range of variations in the operating parameters of the flowmeter. These parameters include operating temperature, the density of the material flow as well as the material pressure and flow rate. A change in material density, thermally induced stress in the flow tube, or pressure stress on the flow tube can each result in an unbalanced condition which effects the accuracy of the flowmeter. Changes in these parameters degrade the static and dynamic isolation of the vibratory elements of the flowmeter. The problem is to keep the vibratory elements immune from the effects of changes in operating parameters. The accelerations that result from the change in operating parameters impair flowmeter accuracy by adding to or subtracting from the Coriolis acceleration of the material. The unwanted accelerations cannot be compensated for because they vary with the mounting conditions of the flowmeter. In addition, the mounting conditions often change with time and temperature in unknown ways.
Even though a variation of the material parameters and mounting conditions is to be expected, it is desired that the flowmeter remain operational and produce accurate output information. It is also desired that the structural integrity of the flowmeter elements be maintained as these parameters vary. It is a goal to design a Coriolis flowmeter so that it operates with suitable accuracy and does not destroy itself as the flowmeter elements are subject to varying operating temperatures. Flowmeter designers also desire that the flowmeter calibration will remain constant and flat over a reasonably wide range of material densities.
In order to achieve these design objectives, a Coriolis flowmeter must have a dynamically balanced vibrating structure that operates in a controlled and predictable manner over a range of operating parameter variations. The flowmeter elements external to the vibrating system should not vibrate or communicate vibration to the vibratory system. A Coriolis flowmeter often comprises a single straight flow tube surrounded by a balance bar and brace bars coupling the balance bar ends to the flow tube. In operation, vibration nodes (regions of no vibration) occur between the flow tube and the balance bar. The nodes define the length of the flow tube that is subject to Coriolis force. The vibration nodes of the flow tube and the surrounding balance bar should remain in the brace bars over the range of parameters for which the flowmeter is designed. Since the balance bar, brace bar, and flow tube comprise a dynamically balanced system, the vibrating mass times the vibration velocity of the balance bar should equal the vibrating mass times the vibration velocity of the flow tube. As long as these conditions are met, and no other unbalanced forces or torques are applied to the non-vibratory elements of the flowmeter, the vibration nodes remain in the brace bars and the other flow meter elements remain free from vibration. However, prior art attempts have all fallen short of meeting these conditions.
A prior art attempt to minimize node movement and the communication of vibration from the case to the vibratory system is shown in the U.S. Pat. No. 5,473,949 to Cage. This patent discloses a straight tube Coriolis flowmeter having a flow tube and a surrounding balance bar coupled by brace bars. The design is unique in that each brace bar also comprises a portion of each case end of the flowmeter. This geometry uses the mass of the case to keep the vibration nodes near the brace bar. For instance, a high density material in the flow tube causes the vibration nodes to move very slightly into the active portion of the flow tube with the result that the brace bars and case ends (and case) move in phase with the balance bar. Momentum is conserved since the mass times velocity of the case plus the mass times the velocity of the balance bar equals the mass times velocity of the flow tube. A low density material causes the node to move slightly into the balance bar with the result that the case moves in phase with the flow tube and momentum is once again conserved. The problem with the Cage design is that momentum is conserved by the case moving with the light member. The vibration is of small amplitude because the case is massive, but it is still large enough that different mounting conditions can effect the accuracy of the meter.
Another example of a prior art flowmeter that attempts to minimize unwanted accelerations is shown in U.S. Pat. No. 5,365,794 to Krohne. This patent discloses a flow tube surrounded by a concentric balance bar and distinct brace bars that couple the balance bar ends to the flow tube. In this design the balance bar ends are not connected to the case as in the Cage design. Furthermore, the inactive portion of the flow tube, external to the brace bar regions, is not connected to any of the support structure except by the tube ends that are connected to the flange faces. This structure operates satisfactorily as long the ratio of the vibration amplitude of the flow tube divided by the vibration amplitude of the balance bar does not vary from its design point. At the design point, the torque applied to the brace bars by the flow tubes is equal and opposite to the torque applied to the brace bars by the balance bar. The result is that the inactive portions of the flow tube are indeed inactive and remain on the axis of the meter. The problem arises when the material density changes. A high density material causes the vibration amplitude ratio to change. To conserve momentum the vibration amplitude of the heavy flow tube decreases while the vibration amplitude of the balance bar increases. The change in amplitude ratio causes the torques at the brace bar to become mismatched. The higher amplitude balance bar applies more torque to the junction than the lower amplitude flow tube. The inactive portion of the flow tube makes up the torque difference and bends as a result. Unfortunately, the translation increases the amplitude of the heavy flow tube and makes the balance worse. Ultimately, the flow tube (containing the high density material) ends up vibrating in phase with the case and the vibration nodes move far from their balanced locations and the meter accuracy suffers.
EPO patent 0,759,542 by Oval (FIGS. 8A and 8B of EPO patent 0,759,542) provides a Coriolis flowmeter having a straight flow tube surrounded by a concentric balance bar whose ends are coupled by case connect links to the inner wall of a case. The flow tube ends are coupled to end flanges. This structure provides dual connection points at each end of the flowmeter between the balance bar/flow tube and the case structure including the end flanges. The case connect link design uses the mass of the case to help reduce the movement of the end nodes (as in the Cage design). However, large changes in the amplitude ratio cause torque unbalance at the brace bars (like the Krohne design) and bending in the inactive regions of the flow tube. While the vibration is less than with the other prior art, it is still sufficient to degrade the meter performance.
It can therefore be seen from the above that it is a problem of prior art to provide a Coriolis flowmeter structure for which a shift of the material density does not degrade the static and dynamic isolation of the flowmeter's vibratory system and the corresponding reduction in the accuracy of the meter.
It is also a problem in the manufacture, balancing and testing of a Coriolis flowmeter to minimize the number of Coriolis flowmeters of a given model that must be maintained in inv
Lanham Gregory Treat
Lister Ernest Dale
Ollila Curtis John
Van Cleve Craig Brainerd
Chrisman Bynum & Johnson, P.C.
Micro Motion Inc.
Noori Max
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