Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
2000-03-16
2001-10-09
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
C073S861240
Reexamination Certificate
active
06298734
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved vortex shedding flowmeter, and more particularly to an improved sensor arrangement in a vortex shedding flowmeter.
BACKGROUND OF THE INVENTION
Vortex flowmeters are known in the art. They can be used to measure the mass flow rate (typically used for gaseous delivery conduits) or volumetric flow rate (typically used for liquid delivery conduits) of a fluid flowing through a conduit. The mass flow rate is equal to the density of the fluid flowing in the conduit times the velocity of the fluid times the cross-sectional area of the conduit. The density can be treated as a constant with a value of the density provided in advance for the calculation, in the cases where density is a known constant, such as for liquid delivery systems. Alternatively, the density can be calculated in a known fashion from an equation of state such as the ideal gas law, if the pressure and temperature of the gas can be sensed. Accordingly, some vortex flowmeters also include sensors for sensing the pressure and temperature of the fluid in the conduit. The volumetric flow rate is equal to the velocity of the fluid times the cross-sectional area of the conduit.
As can be seen, the calculation of mass flow rate and volumetric flow rate each require a determination of the fluid velocity. Fluid velocity is typically measured in vortex flowmeters by inserting a bluff body, or shedder bar, into the flow of turbulent fluid and counting the frequency of the vortices produced thereby, since the frequency of the vortices is proportional to the fluid velocity for well-designed flowmeters. As shown in
FIG. 1
, a conduit
20
with fluid flowing therethrough in a direction shown by an arrow
22
will form vortices after passing a bluff body
24
placed in the conduit
20
. The vortices will be alternately created in wake
26
or wake
28
formed on opposite sides of the bluff body
24
. Each of the wakes
26
and
28
are composed of a series of vortices
30
. The vortices
30
of wake
26
rotate counterclockwise, while the vortices
30
of wake
28
rotate clockwise, as seen in FIG.
1
. The vortices
30
are generated one at a time, alternating between the opposite sides of the bluff body
24
. The vortices
30
interact with their surrounding space by overpowering every other nearby swirl on the verge of development. It is known in the art that the distance (or wavelength) between successive vortices is constant, within a given distance downstream of the bluff body
24
. Since the distance between successive vortices is constant and the inside diameter of the flow conduit is constant, the three-dimensional volume of fluid between the vortices is also constant. By sensing the number of vortices passing by the sensor in a given time, the vortex flowmeter can compute the total volume of fluid which is passed through the conduit in that same given amount of time.
It is important for the fluid flow through the conduit to be turbulent rather than laminar. Turbulent flow is determined using the well known dimensionless number called the Reynolds Number:
Re
=
ρ
VD
μ
where
Re=Reynolds Number
&rgr;=mass density of the fluid being measured
V=velocity of the fluid being measured
D=internal diameter of the fluid conduit
&mgr;=viscosity of the fluid being measured
The Strouhal Number is the other dimensionless number that quantifies the vortex phenomenon. The Strouhal Number is defined as:
St
=
fd
V
where
St=Strouhal Number
f=frequency
d=equals width of the bluff body
V=equals fluid velocity
Well-designed vortex flowmeters exhibit a constant Strouhal Number across a large range of Reynolds Numbers, indicating a consistent linear output over a wide range of flows and fluid types. Below this linear range, intelligent electronics automatically correct for the variation in the Strouhal Number with a Reynolds Number. Known smart electronics correct for this non-linearity by calculating the Reynolds Number based on either constant values of the fluid's density and viscosity stored in the instrument's memory or measured values of pressure and temperature in an equation of state (such as the ideal gas law) for density in an equation to predict viscosity.
Vortex flowmeters can be used in either in-line or insertion applications. In in-line applications, the flowmeter body includes a section of fluid conduit which may have flange connections at opposite ends for connection to opposed ends of an existing fluid conduit. Insertion type vortex flowmeters may include a shroud which houses a bluff body and a sensor that can be inserted into an existing fluid conduit or pipeline via an opening on the radial wall of the fluid conduit.
While there are many vortex flowmeter products available on the market today and disclosed in the patent literature, it is believed that there are none that optimally fit the following requirements for a vortex flowmeter: high sensitivity (expressed as the ratio between the maximum and the minimum measurable flow rates, also known as the turndown ratio), rugged, able to withstand pressure fluctuations, substantially free from sensitivity to vibration, inexpensive, reliable, and compact. In particular, it is desirable to design a vortex flowmeter with improved signal-to-noise ratios, a decrease in the sensitivity to vibration, and with a design that can be easily and cost-effectively manufactured. For example, there are other vortex flowmeters available today that may satisfy certain of these parameters but may be relatively difficult to manufacture. For example, some vortex flowmeters require sensors that are difficult to mount, wires that are difficult to connect to the sensors, and designs that require the potting of the sensor into a sensor tube.
It is against this background and the desire to solve the problems of the prior art that the present invention has been developed.
SUMMARY OF THE INVENTION
The present invention is related to a vortex flowmeter for sensing characteristics of fluid flow through a conduit. The flowmeter includes a housing affixed to the conduit, an elongated, flexible sleeve affixed to the housing at one end and having a sensor tab defined at an opposite end, and an elongated sensor affixed to the housing at one end. The sensor has an elongated finger defined at an opposite end, the elongated finger having a longitudinal axis. The finger is slidably received within the flexible sleeve, the sensor having a sensing element therein that senses forces normal to the longitudinal axis. The housing is attachable to the fluid conduit in a manner to allow at least a portion of the flexible sleeve, with the sensor slidably received therein, to extend into the conduit so that the sensor tab can be located in the vicinity of vortices in the flow of fluid through the conduit. The flexible sleeve flexes in response to the vortices in the vicinity of the sensor tab, the sensor sensing the normal forces imparted by the flexible sleeve.
The housing may include a shroud extending into the fluid conduit in the vicinity of the sensor tab. The sensor tab may be located within a cylindrical section of the shroud. The shroud may include a bluff body mounted thereto upstream of the sensor tab. The bluff body may have a downstream-protruding wall generally aligned with and underneath the sensor tab. The sensor tab may have a downstream edge that is generally aligned with a downstream edge of the downstream-producing wall.
The elongated sensor may include a pair of spaced-apart, disc-shaped piezoelectric crystals having radial centers generally aligned with the longitudinal axis of the sensor, and the sensor also includes an electrode arrangement located between and contacting the pair of piezoelectric crystals. The electrode arrangement may include a pair of generally C-shaped electrodes, and the pair of piezoelectric crystals each include a pair of generally C-shaped electrode pads on opposed faces of the pair of crystals, and a generally circular
Storer William James A.
Tuck Sheldon
Holland & Hart LLP
Patel Harshad
Sirr F. A.
VorTek Instruments LLC
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