Method and apparatus to balance a coriolis mass flow meter adding balancing weights by determining reaction forces

A Coriolis flow meter comprises a first flow tube having a first end and a second end. The first end comprises a first reaction force, and the second end comprises a second reaction force. A second flow tube is operably connected to the first flow tube. The second flow tube comprises a first end and a second end. The first end comprises a third reaction force, and the second end comprises, a fourth reaction force. A drive system is operably connected to the first and second flow tubes. At least one balance mass is operably attached the first flow tube or the second flow tube. The one balance mass is sized and positioned to minimize one or more of the first reaction force, the second reaction force, the third reaction force, and the fourth reaction force.

BACKGROUND

A Coriolis mass flow meter is a device that measures the mass flow rate of a fluid passing therethrough generally by employing one or more vibrating tubes that carry the fluid to be measured. The moving mass of the flowing fluid, in combination with the vibration of the tubes causes Coriolis forces to bare on those tubes, which alters their vibration pattern in a way that is measurable. The magnitude of this altered vibration pattern is proportionally related to the mass flow rate of the fluid.

There are many difficulties that arise in the measurement of these altered vibration patterns due primarily to their magnitude which is exceedingly small. The magnitude of the Coriolis induced motion can range from 0.01 mm down to a nanometer or less depending on the design of the flow meter and the magnitude of mass flow rate going therethrough. The smallest measurable motions—sometimes called the resolution or the zero stability is an important factor since that is the minimum flow rate that can be detected by the meter. Any errors in the measurement that are larger than this zero stability value appear as false flow signals on the output of the meter's electronics and therefore are counted as flow when no flow exists.

While there are many causes for errors in the Coriolis deflection measurement, one of the most prevalent errors is due to imbalance in the vibrating tube system. Historically, the importance of good balance was not originally known, so early tube systems were poorly balanced if at all.

Accuracy ratings in a laboratory are important benchmarks, however, in actual field applications many additional factors come into play to cause errors including pipeline vibrations from pumps, pressure and fluid pulsations in the pipe, motor vibrations, and other vibrating equipment that may be mounted nearby the flow meter. These ambient vibrations that are transferred into the Coriolis metering tube system can cause zero-stability errors that cannot be properly corrected. In addition to ambient vibrations, a poorly balanced tube system creates its own vibrations which bear onto the meter body and pipeline system to which it is attached, which cause further measurement errors. All these errors are greatly exacerbated if the vibrating tube system is not properly balanced. A balanced tube system is far more immune to the influence of ambient vibrations and far more immune to generating its own vibrations than a poorly balanced tube system. Therefore, it would be a great advancement in Coriolis flow measurement technology to disclose a method and apparatus to improve the balance of a Coriolis flow meter tube system that is immune to ambient vibrations.

SUMMARY

A solution is disclosed for balancing a Coriolis mass flow meter involving the use of one or more balancing weights that may be sized and positioned to minimize or eliminate susceptibility to errors due to pipeline vibrations and changes in mounting conditions. Size and positioning of the one or more balancing weights may be determined by reaction forces at the base of a first flow tube and a second flow tube.

A Coriolis flow meter may comprise a first flow tube having a first end and a second end. The first end may have a first reaction force, and the second end may have having a second reaction force. A second flow tube may be operably connected to the first flow tube. The second flow tube may have a first end and a second end. The first end may have a third reaction force, and the second end may have a fourth reaction force. A drive system may be operably connected to the first flow tube and the second flow tube. At least one balance mass may be operably attached the first flow tube or the second flow tube. The one balance mass may be sized and positioned to minimize one or more of the first reaction force, the second reaction force, the third reaction force, and the fourth reaction force.

A method to balance a Coriolis mass flow meter tube structure, may comprise the following steps:

determining reaction forces in the x, y, and z direction at a base of a first flow tube and a second flow tube;

combining the reaction forces;

determining a first reaction force cancellation value;

adding at least a first balance mass to the first flow tube at a first location and a second balance mass to the second flow tube at a second location;

determining a second reaction force cancellation value;

determining a change between the first reaction force cancellation value and the second force reaction cancellation value; and,

calculating an optimized location and mass for the at least first balance mass and the at least second balance mass so that the change between the first reaction force cancellation value and the second force cancellation value is substantially zero.

DETAILED DESCRIPTION

One implementation discloses both a method and apparatus to achieve improved balance for a Coriolis mass flow meter. This improved balance directly improves the accuracy of mass flow measurement especially in adverse field conditions where problems of ambient vibration of appurtenant pipes, structures, and equipment are present.

The advantages of one implementation are achieved by first using a method to determine a specific mass value and one or more specified locations for that mass to be attached to the vibrating tube structure. This addition to the tube system is designed to minimize reaction forces at the connection point between the tube system and the body of the meter from both the normal driven vibration of the tubes, and by the altered vibration shape that is caused by the Coriolis forces. By minimizing or eliminating these reaction forces, and without resorting to use of momentum calculations between the tubes, the vibrating tube system becomes highly immune from ambient vibrations entering the tube vibration system and causing measurement errors. Similarly, by minimizing or eliminating these reaction forces, less vibration energy is lost to the appurtenant structures and pipelines, which further reduces the meter's susceptibility to zero or near zero stability errors.

The flow tubes of Coriolis flow meters have taken many shapes and sizes. A simple U-shaped tube or tubes were used. Many shapes and sizes may be utilized including without limitation, straight tubes, S-shaped tubes, B-shaped tubes, omega shaped tubes, slightly bent tubes, and others. Regarding size, Coriolis flow meters have employed tubes or pipes ranging in size from less than 1 mm in diameter to more than 12-inch diameter. The apparatus described herein has application to all of these tube shapes and sizes.

The process of implementing one implementation will now be described. Finite element analysis (FEA) is an engineering software program used by Coriolis flow meter engineers to analyze the vibrations on a Coriolis tube meter structure100. FEA can determine parameters such as tube frequency, deflection of any point on the tube, stress and strain along the tube, and reaction forces where the tubes are attached to the body of the meter, among others. As an example, a pair of tubes, each bent into a general U-shape are shown inFIG. 1. The flow tubes101and102may be oriented in parallel, next to each other, and may be anchored to a heavier structure, typically a manifold casting (not shown), where the fluid may be split into two parallel streams, and conveyed into the tubes on the inlet side of the meter, and out of the tubes on the outlet side. Above the base of the tubes are connections between the tubes101,102, which may be brace-bars103,104in one implementation. The brace-bars103,104may link the two tubes101,102together with a spring like connection to help them vibrate out of phase with each other in a balanced manner in a desired mode of vibration. A drive system200may be operably connected to the first flow tube101and the second flow tube102. In another example implementation, the drive system200may comprise a motion driving magnet105and motion driving coil106. The motion driving magnet105may be operably connected to the first flow tube101, and the driving coil106may be operably connected to the second flow tube102. In another example implementation, the motion driving magnet105and the driving coil106may be operably connected to the second flow tube102and the first flow tube101, respectively. In one example implementation, the motion driving magnet105and the motion driving coil106may be located at the top of the tubes near the center. The motion driving magnet105and the motion driving coil106are used to cause the requisite vibration by applying oscillatory forces on the tubes at a specified frequency to cause deflections in a specific mode or shape of vibration. Also shown attached to the flow tubes101,102is a motion sensing magnet107and a motion sensing coil108on the inlet side of the meter, and motion sensing magnet109and motion sensing coil110on the outlet side of the meter. The drive system200and its components may be positioned at the top of the flow tubes101,102as shown inFIG. 1. In another nonlimiting implementation, the drive system200may be disposed any place along the flow tubes101,102chosen with sound engineering judgment, for example, the bottom of the tubes101,102. In yet another example implementation, the flow tubes101,102may be disposed below the manifold, and the drive system200and its components may be operably coupled to the flow tubes.

In other implementations, additional structures may be included onto a tube structure design such as additional motion drivers, additional motion sensors, temperature sensors, accelerometers, and others. All these structures, which may be operably attached to the flow tubes101,102, may have an effect on the resulting vibratory motion of the tubes101,102, and on the balance and the reaction forces at the base of the tubes.

In one implementation, the first flow tube101may have a first end101aand a second end101b. The second flow tube may have a first end102aand a second end102b. InFIG. 2, reaction forces116,117, and118are shown as forces at the base of one end of the tube101in the X, Y, and Z directions respectively. Similar reaction forces are also present at the base of all four tube ends. In one example, reaction forces116a,117a, and118acan be measured proximate the base of the first end101aof the first flow tube101. In another example, reaction forces116b,117b, and118bcan be measured proximate the base of the first end102aof the second flow tube102. The reaction forces116c,117c, and118cmay be measured proximate the base of the second end101bof the first flow tube101. Reaction forces116d,117d, and118dmay be measured proximate the base of the second end102bof the second flow tube102. By minimizing or balancing these reaction forces, a balanced condition will result which minimizes, as close to zero as possible, stability errors and susceptibility to errors due to ambient vibrations. In one nonlimiting implementation a first reaction force may be the resultant force of the reaction forces116a,117a, and118ameasured proximate the base of the first end101aof the first flow tube. A second reaction force may be the resultant force of the reaction forces116b,117b, and118bthat may be measured proximate the base of the first end102aof the second flow tube102. A third reaction force may be the resultant force of the reaction forces116c,117c, and118cmeasured proximate the base of the second end101bof the first flow tube101. A fourth reaction force may be the resultant force of the reaction forces116d,117d, and118dmeasured proximate the base of the second end102bof the second flow tube102. It should be understood that the first reaction force, second reaction force, third reaction force and fourth reaction force may be defined any one of the locations proximate the base of the first flow tube101and the second flow tube102. For example, the first reaction force may be defined proximate the base of one of the flow tubes101,102other than the first end101aof the first flow tube101. Likewise the second reaction force may be measured proximate the base of one of the flow tubes101,102other than the first end102aof the second flow tube102.

In some implementations, the tubes101,102may have one or more of the masses111,112,113,114attached thereto, and it may be helpful to match the masses of oppositely positioned structures on opposite tubes to improve balance. For example, it may be beneficial to match the mass of the motion driving magnet105to the mass of the motion driving coil106. Similarly, it may be helpful to match the mass of the motion sensing magnets107,109to the masses of their respective motion sensing coils108,110. However, this matching process may not result in an optimized balanced condition. The method of finding the optimized balanced condition will now be described with reference toFIG. 3.

FIG. 3shows the tube structure100, which also includes balance masses111,112,113, and114. The mass magnitude and the location of the balance masses111,112,113,114have been determined to minimize reaction forces116,117, and118, or cancel these reaction forces with those from the other tube ends in the following way. First the tube structure100may be designed to meet normal design specifications such as sensitivity to flow rate, size, weight, frequency, and other parameters. Once the tube structure100has met these example design parameters, it can be evaluated for proper balance by comparing the reaction forces116,117,118to the reaction forces on the other three tube ends to see if these forces cancel each other. By using finite element analysis, the reaction forces at the end of each tube can be determined, and mathematically added together to determine the degree of force cancellation. Normally, there will be found a resultant reaction force or forces that cause imbalance. This imbalance is difficult to avoid in the design process due to the complexity of the tube structure100and all of its appurtenant masses as previously described.

Masses111,112,113, and114are then added to the FEA model of the tube structure100at specified locations symmetrically about the XY plane, and the YZ plane115as shown. Reaction forces at the tube ends are then analyzed for cancellation of the reaction forces116,117,118with the forces from the other tube ends. By moving balance masses111,112,113,114to different positions along the tubes101,102in this symmetrical manner, a list of reaction forces as a function of balance mass magnitude and balance mass location can be determined and analyzed for an optimized balanced condition. The optimized balance condition may be achieved when all the specified reaction forces cancel for a specified mode of vibration or a specified deflected tube shape. Normally two modes of vibration may be included in these analyses, which are the driven mode of vibration, often called the drive mode, and the Coriolis reaction mode shape, often called the twist mode. Both of these deflected shapes cause their own set of reaction forces116,117,118. The optimized balanced condition results when all the reaction forces cancel for both deflected shapes.

By using both the drive mode and the twist mode, dynamic balancing is used to size and position the balance masses111,112,113,114to directly minimize the measured resulting reaction forces using computational mechanics through FEA. The combined reaction force in the X, Y, and Z directions may be directly minimized by analyzing both the drive mode and the twist mode. Of note, computations utilizing the center of gravity of the tubes or the overall Coriolis flow meter is not used for purposes of this dynamic balancing to minimize or eliminate the combined reaction forces.

Any combination of reaction forces may be utilized for the dynamic balance method described herein. For example, the first reaction force may be the only force needed for the calculation to minimize reaction forces. In another example, the first reaction force and the fourth reaction force may be used in the calculation.

FIG. 4Ais a graph showing one example implementation where the reaction forces from the application of four symmetrically disposed test masses of 25 grams each as a function of test mass locations along the tube. The center of the graph at position zero is where the four test masses may be located at the top center of the tube where X-direction is zero according to the orientation triad115. The graph shows finite reaction forces for four reactions, which are forces in the X-direction from the drive mode and the twist mode, and forces in the Y-direction from the drive mode and the twist mode. These four reaction forces do not converge to a zero result at any of the positions shown on the graph ofFIG. 4A. Since a test mass of 25 grams was used to determine the graph ofFIG. 4A, and since the reaction forces are scalable, an algorithm can be applied to scale the curves to reflect larger or smaller test masses.FIG. 4Bis the result of using an algorithm to scale the results of the test masses up to find the mass that minimizes the reaction forces.FIG. 4Bshows that at a scaled mass of 67 grams, the four reactions nearly converge to a best balance solution by placing 67 gram masses at locations +15 and −15 along the tube symmetrically from center.FIG. 3shows approximately the solution masses and their positions along the tubes.

FIG. 4Ais a graph of reaction forces compared to test mass location for a given balance analysis using FEA results. Four reaction forces are shown not to converge to a best balance solution point.

FIG. 4Bis a graph of reaction forces compared to scaled test mass location for a given balance analysis using FEA results. Four reaction forces are shown which nearly converge to a balanced solution point.

In a method to balance a Coriolis mass flow meter tube structure, the method may comprise the following steps. First, the first flow tube101and the second flow tube102may be vibrated in opposition to each other. Reaction forces116a,117a, and118ain the x, y, and z direction proximate the base of the first end101aand the reaction forces116c,117c, and118cproximate the base of the second end101bof the first flow tube101are determined. Reaction forces116b,117b,118bproximate the base of the first end102aof the second flow tube102and the reaction forces116d,117d,118dproximate the base of the second end102bof the second flow tube102are determined. A first reaction force cancellation value is determined. In one implementation, this may be a summation of the reaction forces proximate the base of each end of each flow tube. The first balance mass may be positioned to the first flow tube at a first location. The second balance mass may be positioned to the second flow tube at a second location. A second reaction force cancellation value is determined in a similar manner as the first reaction force calculation value. The first reaction force cancellation value and the second force reaction cancellation value are compared. If the difference between the first reaction force calculation and the second reaction force calculation is zero, close to zero, or at an acceptable level or number, the first balance mass and the second balance mass may maintain positions as it indicates a balanced Coriolis mass flow meter with minimal vibration effects. If the difference between the first reaction force calculation and the second reaction force calculation is not acceptable, the first balance mass and the second balance mass are moved. The steps of determining the reaction forces are successively repeated as many times as needed until the difference between the first reaction force and the second reaction force is zero, near zero, or at an acceptable level or number. A driven mode of vibration of the meter may be identified that may comprise the twist mode shape and the drive mode shape. Then the optimized balanced condition for the Coriolis flow meter may be found when all the reaction forces are minimized for the drive mode shape and the twist mode shape.

In one implementation, the method may further comprise the step of positioning a plurality of balance masses on the first flow tube and the second flow tube such that the plurality of balance masses is axially and radially symmetrically positioned on the first flow tube and the second flow tube. By way of example, any number of balance masses may be utilized with sound engineering judgment. In one implementation, two balance masses may be positioned on each fluid flow tube, which may comprise a first balance mass, a second balance mass, a third balance mass, and a fourth balance mass. In another implementation, each balance mass may be positioned and repositioned until the optimized location is found. In another implementation, each of the balance masses may be symmetrically positioned about the XY plane and the YZ plane.