HK1026021B - Coriolis flowmeter having axially compliant case ends - Google Patents

Description

Coriolis flowmeter having axially compliant case ends

Technical Field

The present invention relates to coriolis flowmeters and, more particularly, to coriolis flowmeters closed by a shroud having axially compliant ends.

Background

Straight tube coriolis mass flowmeters are known in the art. They may include a flowmeter having a single straight flow tube, a cylindrical balance tube surrounding the flow tube, and a larger cylindrical housing enclosing both the flow tube and the balance tube. Such a flow meter is shown in us patent 5,398,554. The balance tube is rigidly secured at both ends to the flow tube by tie rods. The flow tube is secured to the end of the cap including the thick end plate. The flow tube extends across both ends of the cover and is connected to a conduit. The primary purpose of the meter cover is to provide physical protection for the meter components enclosed within the cover. These components may include sensitive devices such as drivers, sensors and associated electronic components. It is desirable that these components be physically protected from the environment in which the flow meter is operated. This protection is provided by the cover, which is advantageously made of a strong material with sufficient thickness.

In operation, the flow tubes are subjected to electro-mechanical vibrations out of phase with respect to the balance tube, which is provided to reduce vibrations associated with a single unbalanced flow tube. This vibration imparts coriolis acceleration to the material flowing through the flow tube. The reaction force to the coriolis acceleration slightly deforms the flow tube in the vibration mode shape. This deformation is useful as measured by a sensor connected to or associated with the flow tube. These sensors may be velocity or displacement type. The material flow rate in the flow tube is proportional to the time or phase delay between the signals produced by two such sensors disposed along the length of a straight flow tube. The output signals of these sensors are applied to electronics that generate the desired information, such as mass flow rate, for the material in the flow tube.

Dual straight tube coriolis flowmeters are also known. They are similar to single straight tube flow meters except that they have a second flow tube parallel to the first flow tube. The second flow tube replaces the balance bar of the single flow tube embodiment. The two flow tubes are connected at their ends to a flow manifold which distributes the material to the two flow tubes. The dual flow tube meter may or may not have a drawbar connecting the flow tubes to each other. The flow tubes of a dual flow tube coriolis mass flowmeter vibrate out of phase with each other rather than with a balance bar. Otherwise, it operates the same as a single straight tube flow meter.

Mass flow measurement in both types of straight tube coriolis flowmeters depends on the deformation or bending of the flow tube caused by coriolis forces generated by material flow or by simultaneous electro-mechanical vibration experienced by the flow tube. It is often desirable that the accuracy of a coriolis mass flowmeter be up to 0.1% of the reading. The deformation of the flow tube must therefore be solely dependent on the coriolis forces generated and not affected by external forces and stresses including those generated by operating temperature differences between different portions of the flowmeter. These thermal stresses can create undesirable axial tensile or compressive forces in the flow tube.

Axial tension tends to stiffen the flow tubes making them less responsive to the generated coriolis forces. This results in a decrease in the sensitivity of the flow meter and a decrease in the amount of real flow information reported from coriolis forces. Also, the axial compressive force softens the flow tube, resulting in excessive coriolis flow information reports. Traditionally, the manufacture of straight tube coriolis flowmeters has made the ends of the case extremely rigid, and therefore the forces generated by externally applied loads from the connected conduits are transmitted by the rigid case ends to the case rather than to the flow tube. This successfully isolates the flow tube from external loads, but the stiffness of the case ends and case create problems caused by thermal expansion/contraction of the flow tube and temperature differences between the flow tube and the meter case.

In a straight tube coriolis flowmeter, the temperature differential that often exists between the material in the flow tube and the air outside the flowmeter case can cause the flow tube to have a different temperature than the case. This results in a different amount of thermal expansion for the flow tube than for the cap. These rigid cap ends resist this differential expansion and create an axial force that axially compresses (or stretches) the flow tube, resulting in high axial stresses in the flow tube and errors in the indicated flow rate.

The temperature differential between the flow tube and its case causes axial stress on the flow tube, either in axial compression or in axial tension. In addition to affecting meter accuracy, these stresses can exceed the yield stress of the material making up the flow tube. An axial tensile stress may tear the flow tube end away from the end of the cap or may tear the flow tube itself. Such stresses can also permanently deform the flow tube, thereby permanently changing its calibration factor and rendering it useless. For example, if a stainless steel flow tube is 20 inches (50.8cm) long and 200 ° F (93.3 ℃) hotter than the shroud, it will tend to expand 0.036 inches (0.091cm) more than the shroud. If the cap and cap end are fairly rigid, a compressive stress of about 50,000 pounds per square inch (7.25 newtons per square meter) will develop in the flow tube. The stress may be sufficiently high to cause the flow tube to permanently yield or deform. A similar situation exists when the flow tube is cooler than the cap, but the stress is a tensile force rather than a compressive force.

Two approaches have traditionally been used to reduce thermally induced stresses. The most common method is to fabricate the flow tube from a material having a lower coefficient of thermal expansion than the material from which the cover is fabricated. Titanium is commonly used to make flow tubes because of its low coefficient of expansion and good corrosion resistance. Stainless steel, which has a coefficient of thermal expansion about twice that of titanium, is then used as the cap. The temperature of the enclosure is determined by the amount of heat flowing from the hotter (in this example) flow tube and the amount of heat lost to the cooler atmosphere. By proper design of the conduction path from the flow tube to the case, the flow tube is designed so that the equilibrium temperature of the case is halfway between the flowing material temperature and the ambient air temperature of the case. Because the expansion coefficient of the case is twice that of the flow tube, the resulting flow tube axial stress is independent of the fluid temperature. Second, because titanium has a low coefficient of expansion and a low modulus of elasticity, the likelihood of damage to the flowmeter by thermal stress is greatly reduced.

This design has several problems. The most serious problem is that it can only work in a state of pressure-heat equilibrium. If the temperature of the material in the flow tube changes abruptly, the flow tube temperature changes almost instantaneously, and it takes time for the temperature of the cap to follow the change. During this transient time, the flow tube is subjected to pressure in the axial direction, resulting in measurement errors.

Another problem when using materials of different temperature coefficients to reduce thermal stress in the tube is that the flow tube is only unstressed at a single ambient temperature regardless of the fluid temperature. This is because the equilibrium temperature of the enclosure is half-way between the tube temperature and the ambient temperature. Since only one case temperature for each flow temperature will result in a stress-free flow tube, then only one ambient temperature will result in a stress-free flow tube. This is easily exemplified by the simple case where the temperature of the fluid and the environment (and the tubing and housing) are the same. If the flow tube is unstressed when the tube and cover are at 70F (21.1C), then the tube is in tension when the temperature of the tube and cover is 100F (37.7C) because the steel cover tends to expand more than the titanium tube. On the other hand, if the tube and cap are 40F (4.4C), the cap shrinks more than the flow tube and the flow tube is in compression.

A third serious problem with making the cover and flow tube of different materials is the cost of manufacture. Titanium is expensive and difficult to manufacture. It cannot be welded to stainless steel by conventional techniques and can only be brazed to stainless steel with difficulty.

Another widely used practice and method to reduce thermally induced tube stresses is to design a geometric strain relief in the flow tube. Elbow flowmeters fall into this category. This includes all other flow tubes whose flow tubes are U-shaped, V-shaped, and non-straight, irregular shapes. For straight tube flow meters, strain relief is conventionally located between the shroud end and a drawbar member near the shroud end. In this position, the flow tube is kinematically inactive, so the strain relief properties do not affect the dynamics of the vibrating portion of the flow tube. Among the various strain relief designs used are O-rings, slip joints, metal bellows, and reduced flow tube diameters that act as perforated diaphragms. These strain relief methods operate sufficiently to accomplish their intended function, but they have their own unique problems.

A major problem with bellows and slip joint designs is that they are not easily cleaned. This is a serious problem because cleanability is one of the most common reasons customers choose a straight tube flow meter. A disadvantage of flowmeters using a reduced flowtube diameter near the end of the tube to relieve strain is the high fluid pressure drop. Other geometric designs exist, but they all have drawbacks such as cleanability, pressure drop or drainage.

Attempts have been made to address this problem by using compliant structures to couple the flow tube to the cap or balance tube. This allows the flow tube to more easily expand/contract with thermal stresses. EP- ｃA-0759542 and FR- ｃA-2598801 utilize leaf springs as compliance members. EP-A-0448913 and EP-A-0261435 utilize baffles for this purpose. However, the structure shown in this technique is completed to allow only limited axial movement of the flow tube.

The problems associated with the thermal stress relationship between the flow tube and the surrounding cap are discussed above. In a single tube flowmeter having a balance tube attached to a flow tube, the relationship between the balance tube and the flow tube is the same as the relationship between the cap and the flow tube with respect to temperature differences and thermal stresses. The balance tube is typically rigidly secured to the flow tube by its ends. Thus, the expansion problem between the flow tube and the balance tube is the same as that described above.

It should also be appreciated that while there are various techniques for minimizing the problem of flow tube expansion/contraction for flowmeters having thick non-compliant cap ends, none is without disadvantages. In particular, the problems of thermal transients and varying ambient temperatures remain unsolved.

Disclosure of Invention

The present invention overcomes the above-identified problems and advances the art in providing a flow meter with a shroud, wherein a flow meter comprises: a cylindrical housing having two ends; a substantially flat flow tube means disposed within said housing substantially parallel to the longitudinal axis of said housing; a balance bar having an end coupled to the flow tube mechanism; the method is characterized in that: a disc-like spacer mechanism forming at least one end of said cap; the periphery of the partition mechanism is fixed on the inner wall part of the cover; flow tube means extending axially through said diaphragm means from within the housing at a substantially constant diameter to a terminal end external to said housing, said terminal end being coupled to a source of material and a material container; the flow tube mechanism is fixed on the partition plate mechanism; the spacer plate mechanism includes at least one spacer plate having a transverse dimension substantially greater than its thickness and sufficient axial compliance to allow the flow tube mechanism to grow or shorten in length without permanent deformation as it thermally changes relative to the cap.

The flow tube is attached to an axially compliant diaphragm that functions as a cap end and a balance bar end. The axial compliance of the diaphragm (hereinafter referred to as the diaphragm), including the case end and/or the balance bar end, enables the flow tube to contract and expand relative to the case and balance bar, resulting in reduced axial stresses on the flow tube. This allows the flow tube, cap and balance bar to be made of the same material. The present invention also does not require measuring the temperature of the cap and balance bar, nor does it require compliant devices such as bellows to be part of the flow tube, which can create cleanability and dynamics problems.

The hood end baffle of the present invention may advantageously be comprised of a thin sheet of material such as stainless steel. The baffles are oriented perpendicular to the longitudinal axis of the flow tube. The periphery of the diaphragm is attached to the cap and the central portion of the diaphragm is secured to the flow tube which is attached to an external conduit by its projection. Because the diaphragm is relatively thin compared to its radial or transverse dimension, its central portion can be easily moved axially. The ratio of the radial or transverse dimension to the thickness dimension is at least 16 to 1. Because the flow tube is attached to the central portion of the diaphragm, the flow tube can be easily moved axially relative to the case. The flow tube is restricted from moving in the radial direction by the baffle.

A single partition or pair of partitions may be provided at one or both ends of the enclosure. The use of a single end baffle will allow the bending or pivot torque of a connecting tube to be transmitted to the flow tube within the flow meter housing. This may damage the flow meter or may affect its accuracy. Therefore, it is preferable to use a double partition plate at the end of the hood. Due to its radial stiffness and the physical spacing between them, the use of the dual diaphragm enables the flow tube to resist bending moments and prevent external bending loads from being transferred to the flow tube, while still allowing axial movement between the cap and the flow tube due to thermal expansion.

Next, according to the present invention, the spacer connects each end of the balance tube to the flow tube for the same reason as described above. The balance tube baffle may include a single one or two aperture plates at each end of the balance tube, spaced a suitable distance from each other. As with the lip end baffle, a double baffle is advantageously used on the balance tube to prevent bending moments from being transmitted through the end of the balance tube. However, in the case of a balance bar, the associated bending moment is generated due to vibration of the flow tube. Preventing any flow tube bending moment from extending beyond the balance tube prevents rocking of the entire flow tube, which can lead to meter inaccuracies and increased power required to drive the vibrating flow tube. The dual baffle of the balance tube allows differential expansion between the flow tube and the balance tube in a manner similar to the cap end baffle.

The use of compliant cap end baffles and compliant balance tube end baffles allows for differential thermal expansion between different flowmeter components with relatively little resulting flow tube stress. The compliant diaphragm, however, subjects the flow tube to the axial loads applied by the conduit. Good pipe connections can reduce these external loads to a level that does not significantly affect the performance of the flowmeter. For applications requiring extremely high precision, a strain gauge or similar load or displacement measuring device may be used to measure the axial load imposed by the pipe. Strain or load measurements can be used to compensate for any changes in meter sensitivity caused by axial loads. The strain gauge may be placed on the flow tube, on the pipe adjacent the flow meter or anywhere sensitive to axial loads.

Description of the drawings

The above and other aspects of the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

FIG. 1 discloses a prior art straight tube Coriolis flowmeter;

FIG. 2 discloses a single straight tube Coriolis flowmeter of the present invention;

FIG. 3 discloses a case where the flow meter of FIG. 2 can be coupled to a pipe;

FIG. 4 discloses a Coriolis flowmeter having a pair of straight tubes;

FIG. 5 illustrates bending moments occurring on a flow tube capped with a single end member and resulting tube deflections;

FIG. 6 discloses bending moments and drag on a single flow tube capped with a double end piece;

FIG. 7 discloses tube deflection resulting from differential expansion of the flow tube of the embodiment of FIG. 1;

FIG. 8 discloses deflection of the cap end member resulting from differential expansion of the flow tube of the embodiment of FIG. 4;

FIG. 9 discloses two possible deflections of the planar orifice plate terminal member when there is a temperature gradient such that the central portion is at a higher temperature than the peripheral region;

FIG. 10 discloses one possible embodiment of the cap end component of the flow meter of FIG. 2 with displacement due to the flow tube being hotter and colder than the cap;

fig. 11 discloses a flowmeter shroud constructed according to the shroud end of fig. 10 with a pair of shroud end members at each shroud end.

Detailed Description

Fig. 1 shows a typical prior art straight tube coriolis flowmeter 100 having flanges 103 at both ends thereof that enable it to be coupled to a pipe using bolts inserted through flange holes 112. Flowmeter 100 has a single flow tube 104, flow tube 104 being enclosed within a cover 102, cover 102 having a wall 101 and a cover end piece 109, the latter being attached to flange 103 by piece 111. Flow tube 104 is closed by balance tube 116, balance tube 116 having balance end piece 108 coupled to flow tube 104 and coupled to case end piece 109 with piece 113. The flow tube 104 has an inlet 105 and an outlet 106.

Driver D and left sensor S1 and right sensor S2 are positioned in space 115, space 115 being between wall 107 of balance tube 116 and wall 110 of flow tube 104. As is well known in the art, driver D, which may comprise a magnet and coil combination, is actuated with electronic circuitry (not shown) to oscillate flow tube wall 110 transversely with respect to its longitudinal axis. The sensors S1 and S2 detect these vibrations and coriolis induced tube deformation, which are caused by the material flow through the flow tube 104 and the simultaneous lateral vibration. The output signals from the oscillations produced by the sensors S1 and S2 are applied to associated circuitry (not shown) which determines the phase or time difference between the output signals of the sensors S1 and S2 and derives information from that information about the flowing material, including its mass flow rate.

As described above, the mass flow measurement of a Coriolis flowmeter relies on the bending of the flow tube 104 that is generated in response to the Coriolis forces to which the flow tube is subjected. To achieve the desired accuracy of 0.1% readings, the deformation of the flow tube must be solely dependent on the coriolis force generated, independent of axial stresses generated by other factors including operating temperature differences between different portions of the flowmeter. For the flowmeter of fig. 1, it is common practice to make the cap end 109 extremely steep so that external forces do not affect the flow tube 104. While this technique successfully isolates the flow tube 104 from external forces, the rigidity of the case end 109 creates problems with operating temperature differences between the flow tube 104 and the case 102, including the case wall 101 and the case end 109. These temperature differences can create high axial stresses in the flow tube 104. If the material in the flow tube 104 is more sufficiently warmer than the temperature of the case, the flow tube tends to expand more than the case 102, including the rigid case end 109. The rigid cap end resists such expansion that is attempted to be produced by the flow tube 104 and produces an axial force that compresses the flow tube 104. This temperature difference may cause the flow tube 104 to bend as shown in fig. 7. This can permanently deform the flow tube 104 and destroy its accuracy in detecting coriolis forces. In fig. 7, the wall 110 of the flow tube 104 represents the normal or undeformed state of the flow tube wall 110 of fig. 1. The dashed line 110a represents the deformed state of the flow tube wall 110 due to the temperature of the flow tube 104, which is significantly higher than the temperature of the case 102, including its outer wall 101 and its case end 109. The thickness of the cap end 109 prevents the flow tube wall 110 from attempting to expand axially, and in so doing, causes the wall 110 to assume a deformed, curved position indicated by the dashed line 110 a.

Conversely, if the temperature of the flow tube 104 is significantly lower than the temperature of the cap 102, the cap will attempt to expand more than the flow tube. In doing so, the expansion of the case attempts to axially stretch the flow tube 104 and make the flow tube inflexible. If the temperature difference is large enough, the axial tension can tear the flow tube 104 from the ends 109 of the case and render the meter useless.

As can be seen from the above, the prior art attempts to balance thermal expansion and contraction of the flow tube with the block cover material including the block ends are unsatisfactory, and in some cases where the temperature differential is significant, can result in breakage of the flow tube itself or loss of accuracy in the input information generated by the sensor attached to the flow tube.

Description of FIGS. 2 and 3

Fig. 2 illustrates a first possible exemplary embodiment of the present invention, which includes a flowmeter 200 having a single flow tube 104, the flow tube 104 being located in a housing 102, the housing 102 having a wall 101 and housing end diaphragms 202 and 209 at both ends, the diaphragms being coupled to a flange 103 by members 207, and the flange 103 being coupled to conduits 211 and 212 by a flange 208. Flow tube 104 has a wall 110, similar to flow tube 104 of FIG. 1, with wall 110 surrounded by a cylindrical balance bar 116 having a wall 107. The balance bar wall 107 is secured at both ends to the wall 110 of the flow tube 104 by a pair of spacers 118 and 218. The driver D1 and the sensors S1 and S2 are located within the opening 115, the opening 115 being between the wall 107 of the balance bar 116 and the wall 110 of the flow tube 104.

The primary difference between the embodiment of FIG. 1 and the embodiment of FIG. 2 is that the embodiment of FIG. 2 has a pair of diaphragms 202 and 209 at either end of the cap 102 that couple the cap 102 to the flow tube 104. The diaphragms 202 and 209 are compliant and relatively thin in the axial direction as compared to the rigid cap end 109 alone of fig. 1. The change in the length of the flow tube 104 due to thermal changes causes the compliant baffles 209 and 202 to flex and minimize axial stresses on the flow tube 104.

The flow tube 104 axially contracts or expands due to thermal changes. These axial changes are transmitted through flow tube member 207 to flange 103 and flange 208, which are coupled to conduits 211 and 212. Conduits 211 and 212 are supported by members 308 and 307 and may be provided with elbow members 313 and 314 of fig. 3 that flex in response to changes in the length of flow tube 104. Elbows 313 and 314 absorb these length changes of flow tube 104 and prevent them from being transferred through components 305 and 300 to conduit 306. The flanges 304, 303 may be compared to the flanges 208, 103 in fig. 2. The elbows 313 and 314 also prevent the length of the conduit 306 from changing due to axial stresses applied to the flow tube 104 within the upper housing 102 of fig. 3.

Description of FIG. 4

Fig. 4 discloses another possible exemplary embodiment of the present invention, which includes a dual straight tube flow meter 400 disposed within the housing 102. The cover 102 of fig. 4 is similar to the cover 102 of fig. 2 in that it has a cylindrical outer wall 101, a pair of baffles 209 and 202 at the right end of the cover 102, a thick rigid cover end 109 at the left end of the cover 102, and a member 407 extending from the flow manifold 406 through the rigid left and right cover end baffles 209 and 202 to the flanges 103 at both ends.

The embodiment of fig. 4 differs from the embodiment of fig. 2 in that the embodiment of fig. 4 has a pair of flow tubes 404 and 405 instead of a single flow tube 104 and a surrounding balance tube 116. A driver D, which is a combination of magnets and coils, causes the flow tubes 404 and 405 to vibrate out of phase with each other in response to a drive signal applied to the driver D. The sensors S1 and S2 detect coriolis accelerations and displacements of the two flow tubes.

Both ends 408 of the flow tubes 404 and 405 are connected to a manifold/flow splitter 406. In operation, flowing material coming to the left portion 407 of the flow tube encounters the flow diverter 406, causing the flowing material to diverge between flow tubes 404 and 405. When the material reaches the right end of the flow tubes 405 and 404, the material meets at 406, allowing all fluid to enter the right flow tube section 407, which is attached to the flange 103.

The flow tubes 404 and 405 are made of the same material and are physically the same so that they axially expand and contract uniformly with respect to each other in response to thermal changes. Axial expansion/contraction of the flow tube is resisted by the left end piece 109 but is transmitted to the compliant diaphragms 209 and 202. This allows them to accommodate changes in the length of the flow tubes 404 and 405 without imparting significant stress to the flow tubes. Because the diaphragms 209 and 202 are compliant, they flex outwardly when the flow tube is expanded as shown in FIG. 8. The compliance of the diaphragms 202 and 209 also allows them to flex inward as the flow tubes 404 and 405 contract due to the decrease in temperature.

Description of FIGS. 5 and 6

The flowmeter of fig. 2 and 4 is provided with a double baffle for reasons that will be apparent with reference to fig. 5 and 6. Fig. 5 discloses a cover 102 with a single baffle 209. Fig. 6 has a pair of baffles 209 and 202 located at the left end of the housing 102. The double ended diaphragm of FIG. 6 protects the flow tube 104 from bending moments generated by applied bending loads on the tube. Such loads are common and result from pipe vibration and misalignment of the pipe supports. In fig. 5, it can be seen that application of a force F to the tube assembly 207 causes the single diaphragm 209 to flex outwardly at its upper portion and inwardly at its lower portion as the flow tube 104 pivots about the plane defined by the surface of the diaphragm 209. This deflection is undesirable because it can impart a displacement to the flow tube 104. This displacement of the flow tube 104 due to external forces is undesirable because it can permanently deform the flow tube 104 and change its response to coriolis forces, which is used to determine the flow rate of the material in the flow tube 104.

The flow meter of fig. 6 has a pair of cover components 202 and 209 at the left end of the cover 102 for the flow tube 104 to be protected from externally induced forces present on the flow tube component 207. The pair of baffles 202 and 209 are spaced apart from each other a sufficient distance to prevent the flow tube 104 from pivoting about the baffles as the flow tube 104 of FIG. 5. While the baffles 202 and 209 are compliant in the axial direction, they are sufficiently strong in the transverse plane to prevent the flow tube from moving up and down in FIG. 6. They supply the flow tube 104 at the junction with sufficient force to make the diaphragm resist bending torque.

Description of FIG. 8

The embodiment of FIG. 8 is similar to that of FIG. 4 except that the right end housing baffles 209 and 202 are shown in their outwardly bowed positions relative to the housing 102 due to axial expansion of the flow tubes 404 and 405. FIG. 8 also shows that sensors S1 and S2 and driver D are connected to control circuit 801 by circuit paths 802, 803 and 804. The control circuit 801 applies a signal on path 804 that causes driver D to vibrate flow tubes 404 and 405 out of phase with each other. The control circuit receives the output signals of sensors S1 and S2 on paths 802 and 803, which signals are representative of the oscillations of flow tubes 404 and 405 due to the oscillations induced by driver D and the coriolis oscillations due to the material flow through flow tubes 404 and 405.

Fig. 8 also discloses a strain gauge 806 connected to the control circuit 801 by path 805. The control circuit 801 receives a signal via path 805 indicative of the axial stress encountered by the flow tube assembly 809. The strain gauge 806 may be a component whose resistivity varies with the axial stress to which it is subjected. The strain gauge 806 may be fixedly attached to the flow tube assembly 809. The stress information provided by the strain gauge 806 is used by the control circuitry 801 for flow meter applications requiring very high output information and accuracy.

Also shown in fig. 8 is a temperature sensor 808 that is attached to the outer wall of the flow tube 404. Temperature sensor 808 is connected to control circuit 801 by conductor path 807 and communicates information about the temperature of flow tube 404 to control circuit 801. The control circuit 801 receives temperature information from the sensor 808 and strain gauge information from the component 806 and uses this information to correct the accuracy of the output information generated by the flow meter, including the volume flow rate and the mass flow rate. The temperature of the flow tube changes its modulus of elasticity, which in turn determines the stiffness of the flow tube. The stiffness of the flow tube in turn changes the sensitivity of the flowmeter because the more rigid tube is less compliant than the less rigid tube. The strain gauge information is also used to correct and improve the accuracy of the flow meter output information by the control circuit 801. The strain gauge information indicates the degree to which the flow tube is under tension. The more tension the flow tube is subjected to, the more rigid and less sensitive it is. Conversely, the lower the tension on the flow tube, the more compliant and sensitive it is.

The control circuit 801 uses the information applied by the temperature gauge 808 and strain gauge 806 in a manner well known in the art to improve the accuracy of the information output by the flow meter beyond relying solely on the accuracy obtained from the sensors S1 and S2.

Description of FIG. 9

Fig. 9 discloses a separate diaphragm 209 of the cover 102, which is relatively thin and compliant in accordance with the teachings of the present invention. The flow tube is not shown through its central portion to illustrate its behavior according to temperature gradients in the absence of any force applied by the flow tube. The temperature of the central portion 904 of the diaphragm 209 is relatively higher than the temperature of the peripheral portion thereof adjacent the flow tube wall 101, whereby stresses applied to the diaphragm 209 cause the diaphragm to flex either inwardly or outwardly, as shown by dashed lines 901 and 902 in FIG. 9. Conversely, if the center portion 904 of the diaphragm 209 is cooler than its peripheral portions, the diaphragm remains flat, taut like the head of a drum. This non-linear behavior with temperature gradients can result in small but unpredictable flow tube stresses if the flow tube end displacement due to thermal expansion does not coincide with the optimum displacement of the diaphragm due to thermal gradients. For example, when the flow tube is cooler than the cap, the tube contracts and the diaphragm needs to remain flat. This causes slight stress along the flow tube with small errors in the measured flow rate.

Description of FIG. 10

Fig. 10 discloses a solution to the problem of non-linear behavior of the planar shield end baffle of fig. 9 due to thermal gradients. The embodiment of fig. 10 includes a flow tube 104 having a surrounding cap 102 and cap wall 101 and a permanently bent diaphragm 1002 attached to the wall 110 of the flow tube 104. The normal position of the diaphragm in the absence of a thermal gradient is indicated by solid line 1002, which is permanently bent outward relative to the cover 102. This outwardly curved shape eliminates the unpredictable and non-linear behavior of the embodiment of fig. 9. A positive thermal gradient (the tube is hotter than the shield) bends the center section further outward as shown at location 1002h, while a negative thermal gradient reduces the amount of bending to location 1002 c. The amount of center shift for a given temperature gradient can be determined from the amount of initial (no gradient) curvature. For small initial bends, the shift is quite large, while for larger initial bends the shift due to the temperature gradient becomes smaller. The optimum amount of initial flex is that which displaces the central portion of the diaphragm by an amount equal to the amount that the flow tube ends are displaced relative to the case. For example, if a 200 degree difference between the flow tube and the cap increases the length of the tube by 0.036 compared to the cap, the initial bend of the diaphragm should be set such that a 200 degree gradient between the cap and the tube displaces the central portion of the diaphragm by 0.036. The flow tube 104 can axially contract or expand in response to changes in the temperature of the material within the flow tube, as shown in fig. 10. At the same time, the baffle responds to thermal gradients if the tube is hotter than the shield. The baffle increases its outward bow and if the tube is cooler than the housing, the baffle decreases its outward bow. Figure 10 shows that for a suitable design, the movement of the diaphragm happens to match the differential expansion of the tube. For this design, the flow tube remains independent of thermal stress. Even rapidly changing fluid temperatures do not create stress in the flow tube. If the fluid temperature suddenly rises 200 degrees, the gradient across the end-of-cap assembly rises as rapidly as the tube temperature, and it expands at the same rate as the tube. As the mask begins to heat up, it expands to reduce the difference in length. However, the heated shroud reduces the gradient through the end, resulting in a suitable reduction in end bulging.

Description of FIG. 11

The embodiment of the invention disclosed in fig. 11 is similar to the embodiment of fig. 10, except that the embodiment of fig. 11 has a pair of cap end baffles 1101 and 1102 at both ends of cap 102, whereas the embodiment of fig. 10 has only a single cap end baffle 1002. The pair of baffles 1101 and 1102 in fig. 11 are permanently bent outwardly and provide all of the advantages discussed in the description of the embodiment of fig. 10. However, the pair of baffles 1101 and 1102 prevent the flow tube 104 from pivoting in response to externally generated bending moments and advantageously isolate the flow tube from these moments for the same reasons discussed in detail in FIG. 6 in the case of the embodiment having a pair of cap end members at both ends of the flow meter cap 102.

The embodiment of fig. 11 including member 1103 and flange 103 can be advantageously coupled to the piping system shown in fig. 3, wherein changes in the axial length of flow tube 104 can be absorbed by right angle members 313 and 314 of the piping, and the flowmeter of fig. 11 can be coupled to these right angle members 313 and 314 using flange 103.

It is to be clearly understood that this invention is not limited to the description of the preferred embodiment, but includes other modifications and variations within the scope and spirit of the inventive concept.

For example, the flow meter cover need not be cylindrical, but could be rectangular, triangular, or irregular if desired. The cover may consist of a ball.

Claims (15)

1. A flow meter, comprising:

a cylindrical cover (102) having two end portions;

a substantially flat flow tube means (104) disposed within said housing substantially parallel to the longitudinal axis of said housing;

a balance bar (107) having ends coupled to the flow tube mechanism;

the method is characterized in that:

a disc-like spacer mechanism forming at least one end of said cap;

the periphery of the partition mechanism is fixed on the inner wall part of the cover;

flow tube means extending axially through said diaphragm means from within the housing to a terminal end (103) external to said housing at a substantially constant diameter, said terminal end (103) being coupled to a source of material and a material container;

the flow tube mechanism is fixed on the partition plate mechanism;

the spacer plate mechanism includes at least one spacer plate having a transverse dimension substantially greater than its thickness and sufficient axial compliance to allow the flow tube mechanism to grow or shorten in length without permanent deformation as it thermally changes relative to the cap.

2. The flowmeter of claim 1 wherein said diaphragm means comprises a diaphragm (202) forming one end of said housing and another diaphragm forming the other end of said housing.

3. The flowmeter of claim 1 wherein said diaphragm mechanism comprises a diaphragm (202) forming one end of said housing, and wherein said flowmeter further comprises a rigid non-compliant member (109) comprising the other end of said housing.

4. The flowmeter of claim 2 wherein said diaphragm mechanism comprises a pair of spaced diaphragms (202, 209) at one end of said housing and another pair of spaced diaphragms at the other end of said housing; each pair of said spaced baffles is effective to prevent external bending moments of the tube to which said flow tube assembly is attached from pivoting said flow tube assembly about a pivot point defined by said pair of spaced baffles.

5. The flowmeter of claim 1 wherein said diaphragm mechanism has an axial compliance sufficient to axially flex relative to said case by an amount substantially equal to a change in length of said flow tube mechanism minus a change in length of said case due to thermal changes in said flow tube mechanism relative to said case.

6. The flowmeter of claim 1 wherein said diaphragm means comprises a diaphragm (1002) having a permanently curved surface with its convex side oriented outwardly from said shroud along the longitudinal axis of said flow tube means.

7. The flowmeter of claim 6, wherein:

said diaphragm (1002) taking such an outwardly curved convex position with a curvature that increases as the portion of said diaphragm adjacent said flow tube means is at a higher temperature than the portion of said diaphragm adjacent said cover;

the diaphragm assumes an outwardly curved convex position with a curvature that decreases as the portion of the diaphragm adjacent the flow tube is cooler than the portion of the diaphragm adjacent the cap.

8. The flowmeter of claim 1 wherein said spacer means is sufficiently thin relative to its transverse dimension such that when said flow tube means undergoes a change in length due to thermal changes, the temperature of the central portion of the spacer means is higher than the temperature of the periphery thereof, causing said spacer means to displace axially when not connected to said flow tube means by an amount substantially equal to the amount by which the central portion of said spacer means displaces axially when connected to said flow tube means.

9. The flowmeter of claim 1, wherein:

said balance bar (107) being substantially parallel to said flow tube means;

the balance bar end includes a balance bar diaphragm mechanism (118, 218) connecting the balance bar to the flow tube mechanism;

said flow tube means being secured to and extending through said balance bar diaphragm means;

the balance bar spacer mechanism has axial compliance sufficient to cause the flow tube mechanism to undergo a length change without permanent deformation as it thermally changes relative to the balance bar.

10. The flowmeter of claim 9 wherein said balance bar is cylindrical and surrounds said flow tube means.

11. The flowmeter of claim 1 wherein said flow tube means (104) comprises a single flow tube.

12. The flowmeter of claim 11 wherein said single flow tube is attached to a balance bar positioned substantially parallel to said flow tube means.

13. The flowmeter of claim 1 wherein said flow tube means comprises a pair of parallel flow tubes (404, 405).

14. The flowmeter of claim 1, wherein:

said disc-shaped diaphragm means having a substantially flat surface;

said periphery comprises the circumference of one of said disc-shaped diaphragm means.

15. The flowmeter of claim 1, further comprising:

a control circuit (801) for generating a driver signal;

a driver (D) coupled to said flow tube mechanism for oscillating said flow tube mechanism in response to generation of said driver signal;

sensor means (S1, S2) on said flow tube means for generating a sensor output signal representative of oscillation of said flow tube means;

means (802, 803) for applying said sensor means output signal to said control circuit;

a temperature detector (808) on said flow tube means for generating an output signal representative of the temperature of said flow tube means;

means (807) for applying said temperature detector output signal to said control circuit;

a strain gauge (806) on said flow tube means for generating output information representative of axial stress on said flow tube means;

means (805) for applying the stress information to the control circuitry;

said control circuit being responsive to receipt of said sensor mechanism output signal for producing a first accuracy output relating to the flow rate of material in said flow meter;

the control is responsive to receipt of the temperature information and the stress information and the sensor mechanism output information for producing an output signal of higher level accuracy regarding the flow rate of material in the flowmeter.