Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
1999-07-08
2001-08-21
Noori, Max (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
Reexamination Certificate
active
06276218
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to flow meters and, more particularly, to a vortex detector with enhanced sensitivity and signal processing for sensing and measuring vortex frequencies at very low flow rates.
2. Description of the Prior Art
Vortex shedding flow meters have been used for many years for a wide variety of applications and have proven to be quite popular because of their ability to measure the flow rates of a wide range of fluids accurately and reliably, including steam, liquids, and gases. A vortex shedding flow meter operates on the principle that a bluff body, when placed in a moving fluid, produces an alternating series of vortices, called a vortex street, at a frequency that is directly related to the velocity of the moving fluid. The amplitude of the each vortex is proportional to the square of the frequency of the vortex street. Some vortex shedding flow meters detect the frequency of the shed vortices, thus the flow rate, by having a vane positioned downstream from the bluff body. As the vortices in the vortex street pass over the vane, alternating lateral forces deflect the vane one way and then the other in much the same way that a flag furls in the wind in response to the vortices shed from the flag pole. The deflections of the vane can be detected and measured. The strengths of the vortices in the vortex street are related to the density of the fluid and its velocity. Therefore, high density, high velocity fluids produce strong vortices, while the vortices produced in low density, low velocity fluids are relatively weak.
One of the primary advantages of vortex shedding flow meters is that they have no moving parts, other than the flexure of the vane, bluff body, or other structure used as the transducer, and their inherent ruggedness makes them ideally suited for applications that involve extreme temperatures and pressures. However, one of the most serious disadvantages of vortex shedding flow meters is their inability to detect vortices in gases or other low density fluids very accurately as well as their inability to detect and measure vortices in fluids flowing at very low flow rates accurately. It has been very difficult, if not practically impossible, to detect in an accurate and dependable manner the very small vane deflections that result from the weak vortices produced in low speed flows of low density fluids, including liquids such as water.
Another disadvantage associated with currently available vortex shedding flow meters is that their signal to noise ratios are relatively low. Since transducers are typically used to detect the mechanical reaction of the vane to the passing vortices in the vortex street, they also pick up the other mechanical movements of the vane as well as vibrations and other noise in the fluid and in the pipe in which the flow meters are mounted, which can include the structural vibrations of pipe lines, low frequency acoustical noises penetrating the pipe wall, noises associated with flow fluctuations unrelated to the vortex street, and the like. The adverse effect of a low signal to noise ratio becomes particularly serious when trying to measure low speed flows of fluids, especially low density fluids, since the vortices themselves are quite weak. Therefore, the correspondingly wealk signals produced by the vane deflection transducers may be lost or undetectable in the background noise.
One solution to the vortex detection problem associated with low density fluids has been to use ultrasound to detect the frequency of the vortices in the vortex street. Unfortunately, however, such ultrasonic vortex detection is not without its own drawbacks, including the errors introduced by bubbles and particles suspended in the fluid, as well as a general lack of ruggedness and durability, which makes them undesirable for use in high temperature, high pressure flow conditions.
The patent issued to Lew el al., U.S. Pat. No. 4,699,012, solves some of the shortcomings of the prior art vortex shedding flow meters by teaching the use of piezo-electric transducers to measure the deflection of the vane. Lew also achieves an improvement in the signal to noise ratio by mounting the vane on a thin diaphragm-like structure to increase the magnitude of the vane deflection, thus also increasing the magnitude of the output signal from the transducers. While Lew's vortex mieter does achieve an improvement in signal to noise ratio over the prior art, additional improvements to signal to noise ratio would further enhance the usefulness of vortex shedding flow meters, particularly in the measurement of low velocity and low density fluids.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of this invention to provide a vortex detector that has an improved signal to noise ratio.
Another object of this invention is to provide a vortex detector of high sensitivity and high accuracy.
A further object of this invention is to provide a more sensitive vortex detector for use in flow meters to enable the capability of measuring flow velocities of fluids having a wide range of densities, including lower densities.
Still another object is to provide a vortex detector of simple and rugged construction that is easy to manufacture and provides dependable and consistent performance.
A more specific object of this invention is to provide a more sensitive vortex detector that is capable of measuring low speed flows of fluids and flows of low density fluids.
Another specific object of this invention is to provide improved signal processing to enhance signal detection as well as to provide more accurate and reliable measurements.
Still another object of this invention is to provide an improved signal processing algorithm for more robust readouts that follow flow changes more rapidly and more accurately, yet do not fluctuate so much as to be impractical to use.
Additional objects, advantages, and novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, the improved vortex shedding flow meter according to this invention may comprise a generally cylindrical housing or body that defines a flow passage extending from an upstream end to a downstream end. A vortex generator diametrically disposed across a first cross-section of the flow passage and oriented substantially perpendicular to the central axis of the flow passage generates a vortex street of vortices in a fluid flowing in the flow passage. A vane having a relatively low modulus of elasticity extends inward into the flow passage from the housing or body such that it is substantially parallel to the vortex generator, substantially perpendicular to the central axis of the flow passage, and positioned in the vortex street. One end of the vane is attached to the housing or body via an area of reduced thickness that acts as a fulcrum so that the vane is canthevered. In the first embodiment of the present invention, the opposed end of the vane is left unattached to the housing or body. The vortices in the vortex street produce alternating forces on the vane resulting in corresponding alternating deflections of the vane. Strain gauge transducers mounted on a printed circuit card and adjacent the vane detect the deflections of the vane and produce an electrical signal having an amplitude that is related to the strengths of the passing vortices and a frequency that is substantially identical to the vortex shedding frequency. An optional inlet nozzle attached to the upstream end of the housing or body increases the flow velocity of the fluid in the flow
Chrisman Bynum & Johnson
Engineering Measurements Company
Noori Max
Thompson Jewel V.
Young James R.
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