Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
2001-11-26
2004-08-17
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Mass flow by imparting angular or transverse momentum to the...
Reexamination Certificate
active
06776053
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a Coriolis flowmeter that measures a flow of process material having an ultra high level of purity.
PROBLEM
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information pertaining to materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. Flowmeters have one or more flow tubes of a straight, curved or irregular configuration. Each flow tube has a set of natural vibration modes which may be of a simple bending, torsional, or twisting type. Each material filled flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes are defined in part by the combined mass of the flow tubes and the material within the flow tubes. If desired, a flowmeter need not be driven at a natural mode.
Material flows into the flowmeter from a connected material source on the inlet side. The material passes through the flow tube or flow tubes and exits the outlet side of the flowmeter.
A driver applies force to oscillate the flow tube. When there is no material flow, all points along a flow tube oscillate with an identical phase in the first bending mode of the flow tube. Coriolis accelerations cause each point on the flow tube to have a different phase with respect to other points on the flow tube. The phase on the inlet side of the flow tube lags the driver; the phase on the outlet side leads the driver. Pickoffs are placed on the flow tube to produce sinusoidal signals representative of the motion of the flow tube. The phase difference between two sensor signals is divided by the frequency of oscillation to obtain a delay which is o proportional to the mass flow rate of the material flow.
It is known to use flowmeters having different flow tube configurations. Among these configurations are single tube, dual tube, straight tube, curved tube, and flow tubes of irregular configuration. Most of the flowmeters are made of metal such as aluminum, steel, stainless steel and titanium. Glass flow tubes are also known.
The positive attributes of titanium in flowmeters are its high strength and low coefficient of thermal expansion (CTE). The negative attributes of titanium are its metallic properties and cost of manufacturing. In semiconductor wafer processing, metal ions are a contaminant. Metal ions in contact with the wafer areas of an integrated circuit can cause a short circuit and ruin the device. Also, a Titanium flowmeter is difficult and expensive to produce.
The prior art also suggests plastic flow tubes and plastic flowmeters. This includes prior art in which the entirety of the flowmeter is plastic as well as that in which only the flow tube is formed of plastic. Much of this prior art merely contains an assertion that a flowmeter may be made of various materials such as steel, stainless steel, titanium or plastic. This prior art is not instructive in so far as concerns the disclosure of a plastic Coriolis flowmeter that can accurately output information over a range in operating conditions including temperature.
The mere substitution of a plastic flow tube for a metal flow tube will produce a structure that looks like a flowmeter. However, the structure will not function as a flowmeter to generate accurate output information over a useful range of operating conditions. The mere assertion that a flowmeter could be made out of plastic is nothing more than the abstraction that plastic can be substituted for metal. It does not teach how a plastic flowmeter can be manufactured to generate accurate information over a useful range of operating conditions.
It is a problem in some applications that the typical Coriolis flowmeter may contaminate the process material. This is undesirable for systems in which material of an ultra high level of purity must be delivered by the flowmeter to a user application. This is the case in the fabrication of semi-conductor wafers which requires the use of a process material that is free of contaminants including ions migrating from the tubes of the process material flow path. In such applications, the flow tube can be a source of contaminants. The metal walls of a flow tube can release ions into the process material flow. The released ions can cause the chips on a semi-conductor wafer to be defective. The same is true for a glass flow tube which can release the lead ions from the glass into the process material flow. The same is also true for the flow tubes formed of conventional plastics.
A plastic termed PFA is free from this objection since the material of which it is composed does not release deleterious ions into the material flow. The use of PFA for a flow tube is suggested in U.S. Pat. No. 5,918,285 to Vanderpol. This suggestion is incidental to the Vanderpol disclosure since the patent discloses no information regarding how a flowmeter having a PFA flow tube could be manufactured to generate accurate flow information.
Flow tubes lined with PFA, as disclosed in U.S. Pat. No. 5,403,533 to Dieter Meier, attempted to combine the positive attributes of both metal and plastic technologies but encountered new challenges that could not be solved until the present invention. Metal flow tubes lined with PFA allow metal ions to migrate through the thin PFA coating layer and into the flow stream to cause contamination. Also, the metal flow tube material and the PFA liner have different thermal properties. This caused the PFA liner to disengage from the flow tube to create leaks and performance problems. The manufacturing process for lining the metal flow tubes with PFA is also extremely costly.
SOLUTION
The above and other problems are solved and an advance of the art is achieved by the present invention which discloses a Coriolis flowmeter having at least one flow tube formed of perfluoroalkoxy copolymer (PFA) plastic. The flow tube is coupled to a driver and to at least one pickoff sensor to enable the PFA flow tube to function as part of Coriolis flowmeter that can provide accurate output information over range of operating conditions for a process material flow of ultra high purity suitable for use in applications such as semi-conductor fabrication and the like which require the material flow to be free of contaminants down to the ionic level.
A flow path constructed entirely of PFA has many of the benefits of Titanium and PFA lined flow tubes without the drawbacks. PFA is a fluoropolymer with superior chemical resistance, little metal ion release, low particle generation, and is manufacturable without expending large amounts of capital. PFA material is strong and can be extruded into high quality thin wall tubing. Thin-walled PFA tubing has low flexural stiffness enabling a higher sensitivity to mass flow rate and improved immunity to elastic dynamic interaction between the flow tube and the process pipeline. The material and physical properties of PFA allow larger tube vibration amplitudes at lower stress levels and result in near infinite fatigue life span. Also, the higher vibration amplitude allows the use of small low-mass transducers, which in turn improves density sensitivity and immunity to mount variation.
A first preferred exemplary embodiment of the invention comprises a flowmeter having a single PFA plastic flow tube coupled to a massive metal base which balances the end node vibration of the flow tube. The base is U-shaped and the plastic flow tube extends through coaxial holes in the two legs of the U. The plastic flow tube is affixed to the holes in the base by means of an appropriate adhesive such as cyanoacrylate also termed Loctite
420
. The longitudinal center of the flow tube is affixed to an electromagnetic driver which receives a drive signal from a meter electronics to vibrate the flow tube transversely to the longitudinal axis of the flow tube. This vibration may be at the first bending resonant frequency of the flow tube. The flow tube is coupled to pickoffs which detect the Coriolis response of the vibratin
Bell Mark James
McNulty Daniel Patrick
Pawlas Gary Edward
Schlosser Martin Andrew
Wheeler Matthew Glen
Duft Setter Ollila & Bornsen LLC
Emerson Electric, Inc.
Lefkowitz Edward
Thompson Jewwl V.
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