Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
1999-02-10
2001-11-13
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
active
06314820
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to Coriolis flowmeters and, more particularly, to apparatus and a method for altering the lateral mode flow tube vibration frequencies of a Coriolis flowmeter. This invention further pertains to apparatus and a method for increasing the frequency separation between the flow tube resonant drive frequency and the flow tube lateral mode vibration frequencies.
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 positioned at different points on the flow tube generate sinusoidal signals that have the same phase when no material flows and have a phase difference between them when material flows. This phase difference is due to 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 substantially 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 the material flow.
In operation, the flow tube is driven out of phase with an adjacent parallel member, such as a balance bar for a straight tube Coriolis flowmeter. The drive force is generated by an electro mechanical driver which generates out of phase vibrations of the flow tube and the balance bar at their combined resonant frequency. For discussion purposes, the balance bar and flow tube may be said to be driven in a vertical plane by the driver. These vertical vibrations are relatively large since they are at the first out of phase bending mode of the flow tube and balance bar and they are driven at their resonant frequency.
The Coriolis deflections of the vibrating flow tube with material flow also occur in the same vertical plane as the drive vibrations. The Coriolis deflections occur at the drive frequency but the tube deflection has the shape of a bending mode with a higher frequency. Therefore the amplitude of the Coriolis deflections is considerably less than the amplitude of the flow tube drive frequency vibrations. Even though the amplitude of the Coriolis response is relatively small, it is the Coriolis response that generates the pick off output signals that are processed by meter electronics apparatus to generate the desired mass flow rate and other information pertaining to the flowing material. The output error of a Coriolis flowmeter may typically be 0.15% or less. In order to achieve this accuracy, it is necessary that the Coriolis pick off signals be free as possible from noise and unwanted signals that can impair processing of the Coriolis pick off signals.
In the operation of a Coriolis flowmeter, the signals induced in the pick offs comprise not only the desired small amplitude Coriolis response signals, but also comprise unwanted signals that are applied to the processing circuitry along with the desired Coriolis response signals. The receipt of these unwanted signals impairs the ability of the processing circuitry to generate output signals meeting the objective of less than 0.15% error.
The unwanted pick off signals may be caused by ambient noise from the surrounding environment. Ambient noise may be due to near by machinery and vibrations caused by nearby automobile and railroad traffic. It may also be caused by vibrations in the pipeline to which the Coriolis flowmeter is connected. Ambient noise can be overcome by proper mounting of the flowmeter to isolate it from the vibrations caused by machinery and traffic. The noise from connected pipeline vibrations can be overcome by appropriate isolation of the flowmeter from the pipeline.
Another source for the unwanted signals is unwanted vibrations in the flowmeter. These unwanted vibrations are more difficult to overcome and can be minimized, but not eliminated, by improved flowmeter design.
All Coriolis flowmeters have mode shapes that result from driving the flow tube at its resonant frequency. A typical flowmeter has vibrational modes that are characterized by their shape as follows:
In phase bend (IPB)
In-phase lateral (IPL)
Out-of phase bend (Drive)
Out-of phase lateral (OPL)
Balance bar second (bend)
The out of phase bend is the desired drive mode; the rest are unwanted modes. All of these modes are inherent to any Coriolis flowmeter; but good design can minimize, but not eliminate, the unwanted modes. Also, the frequency of these modes changes with the density of the flowing material. When a mode changes frequency, there is a potential for interaction between neighboring modes that may cause the flowmeter to become unstable and produce incorrect output date. As mentioned above, the mode that is desired and used to generate the desired output information of the flowmeter is the out of phase bend drive mode. It is this mode that generates the Coriolis forces. The resulting Coriolis response is detected by the pick offs which generate the signals that are used to provide the flowmeter output information.
The in phase lateral and the out of phase lateral vibrations are a problem that must be address in the design of any flowmeter. It is an objective of good Coriolis flowmeter design to ensure that the drive frequency is separated from the lower in-phase lateral frequency and the higher out-of phase lateral frequency by an amount sufficient to avoid or minimize the adverse effects of the two different lateral frequencies on the processing of the Coriolis separate response signal. This is necessary so that the electronic processing circuitry can generate output signals having the required error of less than 0.15%.
The unwanted lateral vibrations can be excited by the drive frequency due to asymmetry of the various flowmeter parts as well as by the ambient noise. The existence of the lateral vibrations is tolerable as long as the frequencies are separated from the drive frequency by sufficient amount. If this separation is inadequate, the proximity of the lateral frequencies to the drive signal frequency can produce beat frequencies and interactions that produce unwanted pick-offs signals that are applied to the electronic processing circuitry along with the desired Coriolis response signal. If these undesired lateral mode signals have an excessive amplitude and/or are close to the frequency of the Coriolis response signal, the electronic processing circuitry may be unable to process the Coriolis signal to generate output information having the desired accuracy.
It may be seen from the above that it is a problem in the design and operation of Coriolis flowmeters to minimize the adverse impact of signals generated by undesired modes of vibration so that the processing of the Coriolis response signal and the output accuracy of the output signal of the flowmeter is not compromised.
SOLUTION
The above problem is solved and an advance in the art is achieved by the present invention which comprises apparatus and a method for altering the vibrational characteristics of Coriolis flowmeter flow tubes so that the drive and Coriolis response frequency is separated by an adequate amount from both the in-phase and out-of phase lateral vibration frequencies. This increased separation enhances the ability of the associated meter electronics to process the Coriolis response signal so that it can achieve the desired output accuracy.
As above described, the unwanted signals that can interfere with the processing of the Coriolis response signals are due to two causes. The first is caused by the presence of ambient noise. The second is due to unwanted vibrations in the structure of the flowmeter. Both of these causes can produce unwanted signals which can impair signal processing of the Coriolis response signal.
Ambient noise can be reduced to an acceptable level by improved mechanical isol
Chrisman, Bynum & Johnson P. C.
Micro Motion Inc.
Patel Harshad
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