Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
2000-07-26
2003-09-09
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
C073S861210
Reexamination Certificate
active
06615673
ABSTRACT:
BACKGROUND
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 measuring flow velocity of fluids.
Vortex flowmeters typically have a tubular passage, such as a pipe, for guiding a flowing fluid, herein referred to as process fluid, and have an obstruction element, also termed a vortex shedder, interposed in the path of the flowing fluid. The obstruction element includes a blunt surface facing the fluid flow for creating a series of spaced vortices downstream in the flowing fluid and includes a “tail” section for stabilizing the vortices. Under certain conditions, the vortex shedder creates two nearly-parallel rows of vortices on opposite sides of the shedder. These vortices are known as a Van 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 of the fluid. 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.
Vortex shedders of some conventional flowmeters are integrally cast as one unit with the pipe within which the shedder resides. As the size of the vortex shedder increases, or at very small sizes, such integral casting process is not practical. Thus, in many conventional flowmeters, the pipe and the vortex shedder are manufactured separately, and subsequently assembled together. The manufacture of such a conventional vortex flow meter typically consists of providing a pipe that is machined to accommodate a separate mounting plate, and to include a small locating hole diametrically opposed to the mounting pad for precise positioning of a shedder. In many conventional vortex flowmeters, the obstruction element spans the entire inner diameter of the pipe that contains the process fluid. The manufacturing process typically produces the shedder through precision casting. The cast shedder has curved ends having radii that are machined to conform with the inner curvature of the pipe. The machined shedder typically includes a locating hole on one end and a recess adjacent the other end for accommodating a sensor.
The assembly of such a shedder within the pipe includes the steps of aligning the radial extension of the shedder within the pipe by placing the locating hole of the shedder in register with the locating hole of the pipe, and inserting in the registered holes a set screw or like pin members to maintain the alignment of the shedder. The shedder is then welded to the inner surface of the pipe along both of its curved ends. The mounting plate is located precisely over the shedder and welded in place.
Such conventional flowmeter construction suffers from a number of drawbacks. In particular, the locating steps for alignment purposes can be time-consuming and costly. Further, any flaw in the alignment steps can adversely affect the flowmeter operation. For example, slight changes in the alignment of the shedder within the pipe can adversely impact the linearity of the flow meter. Another disadvantage of the conventional construction is that the process of producing curved ends for the shedder, especially for large shedders that fit within pipes having inner diameters of approximately 6 inches (15 cm) or larger, is cumbersome and timeconsuming.
Shedders are typically dimensioned such that they are not in intimate contact with the inside of the pipe before welding. A weld material, applied around the perimeter of each end of the shedder to within approximately a fraction of an inch of the blunt surface and the tail section of the shedder, holds the shedder in place. The resulting gaps between the blunt surface and the pipe, and between the tail portion and the pipe are spanned by small tack welds. Such tack welds are susceptible to failures, for example due to mechanical stress, that loosen the shedder with a concomitant loss of accuracy.
Another conventional vortex flowmeter structure accommodates thermal expansion of the shedder within the pipe by providing gaps between the wall of the fluid-guiding pipe and the shedder. Nevertheless, thermal expansion of the shedder in such vortex flowmeters can lead to weld cracking, and concomitant loosening of the shedder.
Another disadvantage of conventional designs is that separate shedders are required for pipes having different sizes. For example, the pressure of the flowing fluid can vary from one application to another. As the application pressure increases, the thickness of the wall of the pipe for guiding the fluid needs to increase to accommodate the increased pressure. Industry standards typically hold the outer diameter (“OD”) of the pipe constant, and adjust the inner diameter (“ID”) in order to change the wall thickness. The change in the ID necessitates a different shedder, having a different length, to fit within the pipe.
It is thus an object of the invention to provide a vortex flowmeter that is less costly to manufacture than conventional flowmeters of comparable performance.
It is another object of the invention to provide a vortex shedder that is adaptable to a number of flowtube sizes.
It is another object of the invention to provide a flow meter that is less susceptible to loosening of the shedder as a result of mechanical and/or thermal stress.
It is a further object of the invention to provide a vortex flowmeter that can be configured for use with different types of fluids.
Other general and specific objects of this invention will in part be obvious and in part be evident from the drawings and description which follow.
SUMMARY OF THE INVENTION
The invention attains the foregoing and other objects by providing an improved vortex-sensing apparatus for measuring flow velocity of a fluid. The flowmeter of the invention includes a conduit having a wall that forms a passage for guiding the fluid. The wall includes two opposed openings that allow passage of a vortex-creating obstruction element therethrough. The opposed openings can be positioned along any chord of the pipe cross section. A preferred embodiment places the opposed openings along a central chord, i.e., diameter of the pipe. For illustration purposes, the embodiments of the invention are described herein by reference to diametrically opposed openings. The obstruction element includes an elongate portion, and two mounting pads, each of which is integrally connected to a different end of the elongate portion. The opposed openings allow insertion of the obstruction element into the passage such that each mounting pad partially protrudes externally of the pipe, through the openings. The flow of the process fluid past the obstruction element produces two streams of spaced vortices, with the vortices in one stream staggered or spatially offset with respect to those in the other stream. A sensor element detects the vortices, thereby measuring the flow velocity of the fluid.
One preferred practice of the invention bevels both pipe openings, or otherwise prepares the areas to be welded for accepting the weld deposit, externally to allow a full penetration weld of the obstruction element to the outside surface of the wall of the conduit. The welding of the shedder externally to the conduit maintains the rigid disposition of the shedder within the passage. In addition, the full penetration weld of the shedder to the pipe creates a pressure-tight attachment of the shedder to the pipe, and structurally reinforces the pipe against internal pressure.
As is conventional, in a vortex flowmeter for practice of the invention, the elongate portion of the shedder includes a blunt surface, and a tail portion. The disposition of the shedder within the passage is preferably such that the blunt surface faces the flo
Foley & Hoag LLP
Mack Corey D.
Oliver Kevin A.
The Foxboro Company
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