Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
2001-07-27
2002-05-14
Noori, Max (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
C029S888090
Reexamination Certificate
active
06386048
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to a Coriolis flowmeter and in particular to a balance bar for a Coriolis flowmeter. The present invention also pertains to a method of fabricating a balance bar of a single tube Coriolis flowmeter.
Problem
Coriolis flowmeters are characterized by a flow tube through which material flows while the flow tube is caused to vibrate at its resonant frequency. When material is not flowing, every point on the flow tube vibrates in phase with every other point on the flow tube. Two pick off devices (usually velocity sensors) positioned at different points on the flow tube generate sinusoidal signals that have the same phase when no material flows and that have a phase difference between them when material flows. This phase difference is caused by Coriolis forces generated by material flow through the vibrating flow tube. The magnitude of the phase difference between any two points along the length of the flow tube is proportional to the mass flow rate of the material flow. Coriolis mass flowmeters employ signal processing that determines this phase difference and produces an output signal indicating the mass flow rate along with other information pertaining to the material flow.
Coriolis flowmeters may have either a single flow tube and an associated balance bar or a plurality of flow tubes. It is important that the vibrating structure of a Coriolis flowmeter comprise a dynamically balanced system. In Coriolis flowmeters having a pair of flow tubes, the flow tubes are vibrated in phase opposition to form a dynamically balance system. In single flow tube flowmeters, the flow tube is vibrated in phase opposition with an associated balance bar to form a dynamically balanced system.
The vibrating system of a Coriolis flowmeter is operated at the resonant frequency of the vibrating elements including the material filled flow tube. This requirement is of no problem in dual flow tube meters since the two flow tubes are identical to each other, and they both contain the flowing material and therefore have the same resonant frequency. However, it is a problem for single tube flowmeters to meet this requirement. The flow tube and the surrounding balance bar are different structures with different physical and vibrational characteristics. The flow tube is a cylindrical element that has a relatively small diameter compared to the balance bar. In prior art meters, the balance bar is a larger concentric cylindrical tube. For cylinders of the same length, as the diameter increases, the stiffness increases at a faster rate than the mass. The balance bar (without added mass) therefore has a higher resonant frequency than that of the material filled flow tube. Since it is desirable that the balance bar and the material filled flow tube have the same resonant frequency, prior art flowmeters have used expedients such as the attachment of weights to the balance bar to lower its resonant frequency to that of the flow tube. Such arrangements are shown in U.S. Pat. Nos. 5,691,485 and 5,796,012. Although such prior art arrangements are effective to match the resonant frequency of the balance bar to that of the flow tube, the use of mechanical expedients, such as added weights, results in a cumbersome and expensive structure. Furthermore, if the density of the measured fluid is especially high or low, special weights are required to maintain a balanced structure.
Another problem with prior art single tube flowmeters is that their use of a cylindrical member for the balance bar results in the generation of undesired frequencies that are close to the frequency of the Coriolis deflections signals. The Coriolis signals have a frequency equal to the first bending mode (drive frequency) of the flowmeter. It is desirable for efficient signal processing that the Coriolis deflection signals be of a large amplitude and separated in frequency from the unwanted vibrations. This enables the signal processing circuitry to process the Coriolis deflection signals without interference from undesired signals. The use of cylindrical balance bar is a problem since a cylinder is a symmetrical structure having equal vibration frequencies in all planes of vibration. The cylindrical balance bar can have undesired lateral vibrations (perpendicular to the drive plane) that are equal in frequency to the Coriolis deflection signals.
It is a problem that prior art flowmeters use elements such as weights attached to the cylindrical balance bar. Weights can lower the resonant frequency of the balance bar but they do nothing to separate the desired and undesired vibration frequencies. The use of weights is costly and undesirable and limits the density range of the flowmeter.
It is a further problem in the prior art single tube flowmeters to mount the pick offs and a driver to a cylindrical balance bar. Pick offs and drivers comprise a magnet and coil combination with the magnet being mounted on the flow tube and the coil on the balance bar. The mounting of the coil structure to the balance bar requires special machining operations so that the coil hardware can be affixed to the balance bar. Flats are machined on the balance bar because of the difficulty in mounting to a cylindrical surface. Holes need to be drilled and tapped for mounting screws. A problem arises here in that after machining the flats there may not be sufficient wall thickness remaining for enough threads. Finally, a large hole needs to be machined into the center of the flat so that the magnet can protrude through the balance bar wall and into the coil center. This is a cumbersome time consuming and expensive process. Coils must then be made to fit on the surface of the cylindrical balance bar. As a result, each different flowmeter then requires different coils.
A further problem of prior art straight tube Coriolis flowmeters using a cylindrical balance bar is that a separate element, termed a brace bar, must be affixed to the balance bar ends. The brace bar is a ring like element having a plane perpendicular to the longitudinal axis of the balance bar. The outer circumference of each brace bar is affixed to the inner wall of the balance bar at each of its ends. Each brace bar has a center opening for receiving the flow tube which projects through the brace bars and terminates in end flanges. The brace bars are traditionally brazed or welded to the balance bar on their outer circumference and to the flow tube on their inner circumference. The brace bar provides a path that permits the brace bar and flow tube to be connected into a single vibrating structure. The integrity of the joints between the brace bars and the other components are critical. If any of the four brazed or welded joints are incomplete or otherwise flawed, the performance and the reliability of the meter is degraded. It is therefore a problem that there are four joints in a critical region.
It can therefore be seen from the above that the use of a cylindrical member as a balance bar in single tube Coriolis flowmeters creates problems in the lowering of the resonant frequency of the balance bar, reducing lateral vibrations of the balance bar, in the mounting of a driver and pick offs to the balance bar, and in the need for a separate brace bar to connect the balance bar ends with the flow tube.
Solution
The above and other problems are solved and an advance in the art is achieved by the present invention which comprises a method of fabrication and apparatus for the provision of a balance bar that overcomes the above problems of prior art cylindrical balance bars. The balance bar of the present invention comprises a hollow elongated element having an axial center section, voids on each side of the center section and cylindrical elements on each end. The balance bar also includes integral side ribs extending the length of the balance bar. The balance bar is advantageously manufactured by a casting process that provides flat surfaces with holes for accommodating the mounting of a driver and pick off elements.
The provision of a void on each side of the center sec
Cohen Rachael Esther
Garnett Robert Barclay
Lanham Gregory Treat
Ollila Curtis John
Van Cleve Craig Brainerd
Faegre & Benson LLP
Micro Motion Inc.
Noori Max
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