Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
2000-03-02
2002-09-17
Noori, Max (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
active
06450042
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus for and a method of fabricating a Coriolis flowmeter and more particularly a Coriolis flowmeter formed primarily of plastic.
Problem
Coriolis flowmeters are in widespread use in applications that require the generation of accurate information regarding material flow. This information includes mass flow rate and material density. Coriolis flowmeters range in size from meters having a flow tube 0.16 centimeters in diameter to those 15 centimeters in diameter. These flowmeters serve wide range of material flows ranging from approximately several drops per minute, such as for use in anesthesiology systems, to several tons a minute, such as for use in oil pipelines or the loading and unloading of oil tankers. Regardless of its size, most of the applications in which Coriolis flowmeters are used require the highest degree of accuracy such as, for example, a maximum error of 0.15 percent. Also, many of the applications in which Coriolis flowmeters are used involve the generation of flow information for material that is hazardous and for which great care must be taken to prevent material leaks into the environment.
It is a problem that these stringent requirements have heretofore resulted in a high cost of manufacture of Coriolis flowmeters. This high cost of manufacture results from the expensive materials that must be used, such as stainless steel and titanium. This high cost of manufacture also results from the complexities of the manufacturing processes currently used to produce high quality Coriolis flowmeters meeting the above discussed requirements. These steps include extensive machining, welding, brazing, and assembly of parts. Another requirement is that flow tubes of curved flow tube meters must have a constant curvature and be free from kinks. These requirements increase the complexity of the machining and bending operations required to fabricate the flow tube.
Another problem is with the brazing operations used to join the various flowmeter elements. Braze joints are typically used to affix the flow tube to the brace bar. Braze joints are also used to join other parts such as driver and pick off brackets and to affix a manifold to the ends of U shaped flow tubes. Considerable care must be taken in the brazing operations to produce braze joints that securely affixes elements to one another and that are free from microscopic cracks. Also, the brazing operation generates thermal stresses in which a brace bar can cool faster than the flow tube or the other elements to which the brace bar is connected. This rapid and uneven cooling generates a permanent stress in the elements to which the brace bar is connected.
Another problem is that Coriolis flowmeters are not devices that are produced in volumes on an assembly line. They are low production quantity devices which are handcrafted and carefully inspected at each stage of the manufacturing process to ensure that each part meets its design specifications and is of the required accuracy before it is joined to another part. This high degree of care is required to ensure that the completed flowmeter meets its design specifications and is free from defects which could impair its output accuracy or cause its failure.
Another problem of Coriolis flowmeters is that they are often required to process corrosive materials. This degrades the life expectancy and reliability of the flowmeters unless they are fabricated using exotic materials such as stainless steel or titanium. These materials are expensive to purchase and are difficult to fabricate. The use of these materials often results in a flowmeter having elements formed of dissimilar materials; such as a flowmeter that has some stainless steel elements that must be joined to a titanium flow tube to provide an all titanium material flow path that is highly resistant to corrosive process materials.
Another problem of Coriolis flowmeters is that metal flow tubes of an acceptable thickness are relatively stiff and resistant to bending. The thicker the flow tube wall—the stiffer the flow tube. This stiffness opposes the Coriolis forces generated by the material flow and reduces the Coriolis deflections of the vibrating flow tube with material flow. This, in turn, reduces the flowmeter sensitivity by reducing the phase difference of the output signals generated by the flow tube pick offs. This is a particular problem in Coriolis flowmeters which must use flow tubes having thick walls for the containment of high pressure materials. Thus, the use of any metal flow tube is a compromise between the wall thickness required by pressure containment requirements and the flow sensitivity required of the flowmeter. U.S. Pat. No. 5,157,975 discloses a Coriolis flowmeter having a glass flow tube. However, it is brittle and does not solve the above mentioned problems of Coriolis flowmeters having metal flow tubes.
Solution
In accordance with the present invention a Coriolis flowmeter is provided that achieves an advance in the art and solves the above problems including the problem of high material costs and difficulty of manufacturing. The flowmeter of the present invention solves these problems by the use of plastic for most of the elements embodying the flowmeter. The flowmeter of the invention solves the above problems using manufacturing techniques which permit many embodiments of the invention to be formed by injection molding. All embodiments of the invention make extensive use of plastic and injection molding. In particular, all embodiments have a dynamically active structure that is formed entirely of plastic by injection molding.
In accordance with a first possible exemplary embodiment, a Coriolis flowmeter is provided having a single straight flow tube, a surrounding plastic balance bar concentric with the flow tube and a plastic brace bar that connects the ends of the balance bar with the flow tube. The entirety of the dynamically active structure (the flow tube, the balance bar and the brace bar) is formed of plastic by injection molding. The flow tube ends may be subsequently coupled to end flanges by appropriate bonding techniques.
In accordance with a second possible embodiment of the invention, the elements of the dynamically active structure as well as the end flanges are formed of plastic by injection molding. This second embodiment provides a plastic wetted flow path that extends through the entirety of the length of the flowmeter with the material flow extending serially from an inlet flange, through the flow tube to an outlet flange. This embodiment is advantageous in that the plastic wetted flow path eliminates problems of corrosion resulting from an interaction between the process material and metal flowmeter elements such as titanium, stainless steel and other metals. With the possible exception of a driver and pick offs, and case, the entirety of the flowmeter is formed of plastic by injection molding.
The above embodiment is formed by an injection molding process that comprises a first step of forming a flow path core mold having a cavity that defines the physical characteristics of the flow path within the flowmeter. The cavity within the flow path core mold is filled with a metal compound of fusible alloys containing bismuth, lead, tin, cadmium and indium. These alloys have a low melting point of approximately 47° Centigrade. The injected metal is then allowed to cool to its solid state at which time the split halves of the mold are separated and the formed metal is removed. This metal defines, with precision, the material flow path of the flowmeter.
The second step of the process involves forming a wrapper mold having a cavity that defines the exterior of the flowmeter elements be formed. The formed low temperature metal flow path core is inserted into the wrapper mold which is then injected with the plastic that is used to form the exterior of the flowmeter elements. The plastic in the wrapper mold is allowed to cool and solidify following which the split halves of the wrapper mold are separated and the formed p
Lanham Gregory Treat
Pankratz Anthony
Faegre & Benson LLP
Micro Motion Inc.
Noori Max
LandOfFree
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