Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
1998-12-28
2001-07-10
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
C073S861240
Reexamination Certificate
active
06257071
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to apparatus and methods for measuring flow rates of fluids. In particular, the invention relates to an improved vortex flowmeter for ultra-pure applications. Such applications require that the flow-meter not introduce contaminants to the fluid flow being measured.
Vortex flowmeters measure the rate of flow of a fluid, termed a process fluid, by measuring the frequency of artificially induced vortices in the fluid. Such flowmeters are known in the art, and include those marketed by The Foxboro Company, Foxboro, Mass., U.S.A (“Foxboro”), for example, under the trade designation 83. Another example of a vortex flow meter known in the art can be found in U.S. Pat. No. 4,220,046, herein incorporated by reference. Vortex flowmeters are popular because of their relatively high accuracy and wide dynamic range. In addition, many vortex flow meters can operate in extreme temperatures, for example up to 800° F. The term “ultrapure applications” herein refers to applications in which the process fluid has a purity of typically in the order of one part per trillion (“PPT”).
Vortex flowmeters typically have a tubular passage, such as a pipe, for guiding the process fluid, and have a vortex shedder, also termed a bluff body, interposed in the path of fluid flow. The vortex-shedder creates a series of spaced vortices downstream in the flowing fluid. Under certain conditions, the vortex shedder creates two nearly-parallel rows of spaced vortices on opposite sides of the shedder. These vortices are known in the art as a Von Karman vortex street. The vortices in one row are staggered with respect to the vortices in the other row. It is understood that the frequency of these generated vortices is linearly proportional to the average flow velocity of the fluid. Thus, a measurement of the frequency of the vortices provides a measure of the average flow velocity. A vortex-responsive sensor detects the pressure fluctuations associated with the passage of the vortices and drives an electronic unit that determines the frequency of the vortices, to determine the flow velocity of the fluid.
The process fluid contacts many exposed surfaces of the vortex flow meter, such as surfaces of the tubular passage, of the vortex shedder, and of the sensor, as it passes through the instrument. In applications where the process fluid is ultra-pure, there is a risk of contamination of the process fluid as a result of such contact of the process fluid with fluid accessible surfaces of the flow meter.
It is known in the art to polish the fluid accessible surfaces of a vortex flowmeter to a surface smoothness typically of at least a No. 4 mill finish. Such smoothness of the fluid accessible surfaces diminishes entrapment of the process fluid in the surfaces, and diminishes contamination of the fluid as a result of the fluid removing particles from surfaces of the instrument. In addition to the described degree of smoothness of the surfaces, traditionally all inside angles between the various fluid accessible surfaces of a vortex flow meter are configured to be either greater than 135 degrees or rounded with a radius of curvature of at least 0.25 inches (0.635 cm).
It is costly and time-consuming to attain these stringent requirements regarding the smoothness of the surfaces and regarding the angles between the surfaces of a traditional vortex flowmeter for sanitary applications. Another disadvantage of polishing the fluid accessible surfaces of a vortex flow meter to the degree necessary for ultra-pure applications is that such a polishing typically results in some distortion of the contours of the vortex shedder. Such distortions typically degrade the accuracy of the measurement of the flow velocity of the fluid.
It is thus desirable to provide a vortex flow meter that meets the requirements for ultra-pure applications, and that is easier and less costly to produce than present vortex flow meters for ultra-pure applications.
It is another object of the invention to provide a vortex flow meter for ultra-pure applications that is at least equally as accurate as vortex flow-meters for other applications.
It is another object of the invention to provide a vortex flowmeter for ultra-pure applications that does not require a smoothness of its fluid accessible surfaces to the degree presently deemed necessary.
It is yet another object of the invention to provide a flowmeter whose fluid accessible surfaces do not entrap the process fluid.
SUMMARY OF THE INVENTION
The invention attains the foregoing and other objects by providing a vortex flowmeter that has a continuous and smooth fluorocarbon layer coating on a number of its fluid accessible surfaces. This coating diminishes, if not prevents, contamination of the process fluid due to contact with the instrument surfaces, and yet preserves accurate flow-measuring operations of the flow-meter. A flowmeter according to the invention has a pipe having a tubular passage for containing a fluid flow to be measured, and has a flow obstruction element disposed at least partly in the tubular passage for producing two streams of spaced vortices. A sensor element, disposed downstream from the induced vortices, produces a set of electrical signals in response to passage of the vortices. An electronic module, which can be separate from the flow meter and connected to it electronically, receives these electrical signals to calculate the frequency of the vortices, for measuring the flow velocity of the fluid. The obstruction element, the tubular passage, and the sensor element, have fluid accessible surfaces. The term fluid-accessible surfaces refers to those surfaces of the flow meter that are in contact with the process fluid in absence of the fluorocarbon layer coating. The polymer coating which the invention provides covers the fluid-accessible surfaces of the obstruction element, and of the tubular passage.
The vortex-type flowmeter of the invention meets the exacting purity requirements for use in ultra-pure applications, and thus is suited for such applications. For example, the flowmeter of the invention can be utilized in a variety of semiconductor applications for measuring the flow rate of ultra-pure water without contaminating the water. The invention prevents or minimizes contact between the process fluid and the fluid accessible surfaces of the tubular passage and of the shedder of the vortex flowmeter by coating all such surfaces with the fluorocarbon polymer layer. The coating of these fluid accessible surfaces allows the flowmeter to measure the flow rate of a process fluid having a level of purity of typically one part per trillion (“PPT”) without contaminating the fluid.
The fluorocarbon polymers that the invention employs for coating the fluid accessible surfaces of the vortex flowmeter are known in the art for other uses. For example, U.S. Pat. No. 5,093,403, herein incorporated by reference, describes a variety of polymers that are suited for the practice of the present invention. The invention preferably employs perfluoroalkoxy tetrafluoro ethylene copolymer resin (“PFA”) admixed with an additive selected from the group consisting of a nitride, an oxide, a diboride, and a carbide of silicon, of zirconium, of tungsten, of boron, or of a polyether. Perfluoroalkoxy tetrafluoro ethylene copolymer resin is commercially available, such as the TEFLON-P 532-5012 PFA powder resin available from E. I. Dupont de Numours & Company, Inc. of Wilmington, Del., U.S.A. Further, the invention preferably admixes the PFA with silicon carbide (“SiC”) to obtain the desired coating composition. One preferred resin for the practice of the invention is a PFA-based composite sold by Edlon Products, Inc., Pennsylvania, U.S.A., under the designation SC-7005.
One aspect of the invention relates to manufacturing a vortex flowmeter that requires less, if any, polishing of its fluid accessible surfaces before application of the fluorocarbon coating. In contrast, present-day vortex flowmeters utilized for measuring flow rates of ultra-pure p
Dessert Wayne A.
Keusseyan Khoren
Foley Hoag & Eliot LLP
Foxboro Company
Liepmann W. Hugo
Oliver Kevin A.
Patel Harshad
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