Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
1999-08-13
2002-04-02
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
active
06363794
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a single tube Coriolis flowmeter and in particular, to a method and apparatus for a Coriolis flowmeter having a balance bar that enhances the accuracy of the flowmeter by increasing the flow sensitivity, reducing the meter shaking with flow, and making the meter flow sensitivity independent of material density.
Problem
Single tube Coriolis flowmeters are desirable because they eliminate the expense and the problems of flow splitting manifolds of dual tube Coriolis flowmeters. Single tube Coriolis flowmeters have the disadvantage that their flow sensitivity is lower than that of dual tube Coriolis flowmeters. The flow sensitivity is lower for two reasons. The first is that a single tube flowmeter must have a larger diameter flow tube for a given flow rate. This makes it stiffer in bending and less responsive to Coriolis forces. The second reason has to do with the details of how the mass flow rate is determined and the fact that the balance bar does not experience Coriolis force.
In dual tube Coriolis flowmeters, the flow tubes are vibrated out of phase with each other. The dual flow tubes counterbalance each other to create a dynamically balanced structure. Velocity sensors (pick offs) are located at two locations along the flow tubes to sense the relative velocity between the flow tubes. The pick offs are usually located equal distances upstream and downstream from the tubes' midpoints. Each pickoff consists of a magnet fastened to one flow tube and a coil fastened to the other. The relative motion of the coil through the magnetic field produces a voltage. The sinusoidal motion of the vibrating flow tubes produces a sinusoidal voltage in each pickoff. When there is no material flow, the voltages from the two pick offs are in-phase with each other. With material flow, the vibrating tubes are distorted by the Coriolis force of the moving material to cause a phase difference between the two pickoff voltages. The mass flow rate is proportional to this phase difference. It is important to note that both flow tubes are distorted equally (for an equal division of flow) and each flow tube has the same phase shift as the other at corresponding locations. The upstream pickoff magnet velocity has the same phase as the upstream coil velocity and both have the same phase as the voltage generated by the magnet-coil pickoff pair. The downstream pickoff has a different phase than the upstream pickoff; but the phase of the magnet velocity, the coil velocity, and the output voltage of the downstream pickoff are equal to each other.
In most single tube flowmeters, the vibrating flow tube is counterbalanced by a balance bar rather than another flow tube. The exception is Coriolis flowmeters in which a flow tube is driven in the second bending mode with the flow tube being counterbalanced by a pendulum attached to the flow tube. EP 0 908 705 A2 discloses a Coriolis flowmeter that uses a single pendulum as a counterbalance. A Coriolis flowmeter using a pair of pendulums affixed to a flow tube as counterbalances is shown in Japanese document PHD11-44564.
In Coriolis flowmeters driven in the first bending mode and counterbalanced by a balance bar, the pickoff magnets (or coils) are mounted to the balance bar as though it were the second flow tube described above. However, since material does not flow through the balance bar, it does not experience any Coriolis force. The balance bar does experience some torque from the Coriolis induced deflection of the flow tube, but the resultant small deflection of the balance bar results in a small phase shift at each pickoff location on the balance bar that is of the opposite sign as the phase shift at the pickoff locations on the flow tube. The pick offs sense the relative velocity between the phase shifted flow tube and the oppositely phase shifted balance bar.
To determine the phase of the output signal, the flow tube and balance bar velocities at each pickoff are represented by velocity vectors having phase angle and amplitude. The relative velocity (and voltage out of each pickoff) can be determined by adding the two velocity vectors. The flow tube velocity vector has a phase shift due to material flow. The balance bar velocity vector has a small phase shift of opposite sign. Adding these vectors gives the net phase shift with flow of the pickoff. The net phase shift and output voltage of each pickoff is reduced by the oppositely phase shifted balance bar. This net phase shift reduction equates to a reduction in the flow sensitivity of the flowmeter.
All straight tube Coriolis flowmeters have a problem in that the flow tube geometry is inherently stiff and cannot be bent or deflected along its longitudinal axis with the same ease as can a conventional u-tube flowmeter. Single straight tube meters have an additional problem in that the single tube diameter must be increased over the diameter of the dual tubes in order to pass the same flow with the same pressure drop through the meter. The increased tube diameter stiffens the flow tube further. As a result, the flow tube of a single straight tube meter is inherently insensitive to Coriolis forces because of its stiffness. The reduction in flowmeter sensitivity due to the opposite phase shift of the balance bar combined with the reduction in sensitivity due to the larger (single) flow tube diameter results in a combined flowmeter sensitivity so low as to impair the accuracy and commercial acceptance of single tube flowmeters for some applications.
It is a further problem that existing single tube Coriolis flowmeters use balance bars to counter balance the vibrating mass of the flow tube. In order to maintain a dynamic balance over a range of material densities, the ratio of the flow tube vibration amplitude to the balance bar vibrational amplitude changes as material density changes. As the material density increases, the flow tube vibration amplitude decreases and the balance bar vibration amplitude increases so as to maintain equality in the momentum of the two vibrating members. The change in amplitude ratio between the flow tube and the balance bar changes the phase difference between the two pickoff signals in a manner that is best understood by using a vector diagram to predict the output of each pickoff. The net output signal out of the pickoff is a result of the vector addition of the phase shifted velocity of the flow tube and the oppositely phase shifted velocity of the balance bar. As the amplitude ratio changes with increasing material density, the length of the flow tube velocity vector decreases and the length of the balance bar velocity vector increases. The sum of these two vectors, which is proportional to the pickoff output decreases in-phase shift as the length and thus importance of the oppositely phase shifted balance bar grows. This decrease in-phase shift of the pickoff output results in a decrease in meter sensitivity with material density.
It is known to decrease flow sensitivity with the increase in material density. In the prior art method the balance bar has its stiffness reduced in a region on either side of the driver. The stiffness reduction causes the distortion of the balance bar in response to the Coriolis deformation of the flow tube to be greatly increased. It also causes the resonant frequency of the second bending mode of the balance bar to be reduced so that it is closer to, but still above, the drive frequency. (The second bending mode has the same deformed shape as the balance bar takes in response to the Coriolis deformation of the flow tube.) By properly sizing the frequency separation between the drive frequency and the resonant frequency of the second bending mode, a change in flow sensitivity with density cancels the change in flow sensitivity caused by the change in amplitude ratio with density. The problem with this method, however, is that the balance bar deformation due to the flow tube Coriolis deformation is much larger. Because the balance bar deformation results in a phase shift that is opposite to th
Faegre & Benson LLP
Fuller Benjamin R.
Micro Motion Inc.
Thompson Jewel V.
LandOfFree
Method and apparatus for Coriolis flowmeter having an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for Coriolis flowmeter having an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for Coriolis flowmeter having an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2914951