Patent Description:
Vortex flowmeters are used in the industrial process measurement and control field for measuring a flow rate of a fluid. Vortex flowmeters are typically inserted in a flow pipe that carries the fluid to be measured. Industry applications include petroleum, chemical, pulp and paper, mining and materials, oil and gas. The operating principle of a vortex flowmeter is based on a vortex shedding phenomenon known as the von Karman effect. As fluid passes a bluff body, it separates and generates vortices that are shed alternately along and behind each side of the bluff body. These vortices cause areas of fluctuating pressure that are detected by a sensor mounted in the bluff body or downstream of the bluff body. While the frequency of vortex generation is essentially proportional to fluid velocity, this proportionality varies with the conduit Reynolds number. The conduit Reynolds number is a function of the fluid density, the fluid velocity, the fluid losses, and the inside diameter of the conduit.

In piping systems, there is a desire to keep the Reynolds number low in order to reduce power loss in the piping system. This desire to reduce power loss leads to use of larger pipe sizes and lower ranges of fluid flow velocity in the piping system.

In vortex flowmeters, there are limits to the range of flows that are measurable. When flow velocities are below a lower limit of measurable flow, the vortex flowmeter cannot be relied upon to provide an accurate indication of flow.

For these reasons, reducer vortex flowmeters have been developed in which the flowmeter is connected by flanges to a flanged piping system. The diameter of the measurement section of the bore of the flowmeter is smaller than the inner diameter of the pipe section to which the flowmeter is connected. At the upstream or inlet end, the flow passage is reduced or constricted from the piping system diameter to the bore diameter of the flowmeter. At the downstream, or outlet end the diameter of the passage must increase to match the diameter of the piping system downstream of the flowmeter. This is often achieved with a tapering reducer section at the upstream end and a tapering expander section at the downstream end of the vortex flowmeter.

Patent Document <CIT> relates to a vortex flowmeter comprising a vortex generating bluff body disposed across a flow passage, a first vortex sensing planar member disposed downstream of and parallel to the bluff body across a first half of the cross section of the flow passage, and a second vortex sensing planar member disposed downstream of and parallel to the bluff body across a second half of the cross section of the flow passage opposite to the first half thereof, wherein the leading edges of the first and second vortex sensing planar members are offset from one another by a distance generally equal to a half wave length of the sinuating streamlines created by the vortices generated by the bluff body; wherein two fluctuating electrical signals respectively generated by the first and second vortex sensing planar members are combined in such a way that the noise generated by the mechanical vibration of the flowmeter body is cancelled therebetween and an alternating output electrical signal representing the vortices is obtained.

Patent Document <CIT> relates to a reduced bore vortex flowmeter and flowmeter body including a fluid inlet couplable in series to an upstream portion of a fluid flow conduit. The inlet fairs into a central bore having a transverse cross-sectional dimension less than that of the conduit, and which houses a shedder. The central bore is communicably coupled to a fluid outlet couplable to a downstream portion of the conduit. The inlet has a stepped or structured inner wall, including a first wall portion disposed at first angle to the downstream direction, and a second wall portion disposed at a second angle to the downstream direction. The second angle is greater than the first angle, so that the first and second wall portions form a substantially concave axial cross-section. The stepped intake improves linearity of flow measurements by reducing velocity profile errors and/or extending contracted flow to the shedder over a relatively wide flow range.

Patent Document <CIT> relates to a multiple flow metering system including first and second vortex flowmeters for measuring a flow of a fluid. The first vortex flowmeter includes a first bluff body having a leading edge and a shape for inducing a first stream of vortices in the fluid as a function of the flow at a wavelength λ. The first vortex flowmeter also includes a first vortex sensor which generates a first output based on a frequency of the first stream of vortices. The second vortex flowmeter includes a second bluff body having a leading edge and a shape for inducing a second stream of vortices in the fluid as a function of the flow at the wavelength λ. The second vortex flowmeter also includes a second vortex sensor which generates a second output based on the frequency of the second stream of vortices. The leading edge of the second body is spaced from the leading edge of the first body by <NUM>. 15λ to <NUM>.

Patent Document <CIT> relates to a flow rate detector for measuring the flow rate of a fluid which is introduced into a fluid passage.

The present invention relates to a vortex flowmeter comprising: a flow tube body having a flow passage extending between an upstream end and a downstream end, the flow passage having a measurement section of a diameter DM and a reducer section between the upstream end and the measurement section in which the diameter reduces from a larger diameter DP at the upstream end to the diameter DM of the measurement section, wherein the reducer section is comprised in a flange reducer, and the measurement section extends from the reducer section; and only one bluff body positioned in the measurement section to produce vortices that alternate at a frequency related to flow rate of fluid through the flow passage; wherein a length LM from a location of inception of vortex shedding at the bluff body to a first expansion in diameter of the flow passage downstream of the bluff body is greater than a vortex wavelength λ of the vortices produced by the bluff body. The flowmeter flow passage preferably has a measurement section with a diameter of greater than <NUM> inches (<NUM>).

<FIG> shows prior art reducer vortex flowmeter <NUM> mounted between pipeline sections <NUM> and <NUM>. Flowmeter <NUM> includes flow tube <NUM> and transmitter <NUM>. Flow tube <NUM> is formed by upstream flange reducer <NUM>, measurement section <NUM>, and downstream flange expander <NUM>. Bluff body <NUM> and sensor <NUM> are located within measurement section <NUM> of flow tube <NUM>. Pipeline sections <NUM> and <NUM> have an inner diameter DP that is larger than the bore diameter of DM of measurement section <NUM>.

Flange reducer <NUM> is attached to pipe section <NUM> by fasteners, such as bolts and nuts (not shown). Flange reducer <NUM> includes tapered bore section <NUM>, which reduces in diameter from pipe diameter DP to measurement bore diameter DM. Constant diameter section <NUM> has diameter DM to match measurement section <NUM>.

Flange expander <NUM> is located between measurement section <NUM> and pipe section <NUM>. Flange expander <NUM> includes constant diameter section <NUM> located downstream of measurement section <NUM> and expander section <NUM>, which is tapered to increase from diameter DM of section <NUM> to diameter DP of pipe <NUM>. Flange expander <NUM> is mounted to pipe <NUM> by connectors such as bolts and nuts (not shown).

In <FIG>, process fluid flows from pipe <NUM> through flange reducer <NUM> into measurement section <NUM>. As the process fluid passes through flange reducer <NUM>, the diameter of the flow passage reduces from DP to DM, and the fluid slowly increases in velocity.

When the process fluid encounters bluff body <NUM>, Karman effect vortex shedding occurs. The process fluid separates and vortices that are shed alternately along and behind each side of bluff body <NUM>. These vortices cause areas of fluctuating pressure that are detected by sensor <NUM>. Transmitter <NUM> receives the sensor signal from sensor <NUM> and produces a flow measurement value based upon the frequency of the vortex generation. The output of transmitter <NUM> is a flow value that is transmitted in either analog or digital form. For example, the flow value may be represented by an analog current over a two wire loop that ranges from <NUM> to <NUM> mA. Alternatively, the flow value may be transmitted in a digital form over a two wire loop using the HART digital protocol, over a communication bus using a digital protocol such as Foundation fieldbus, or by wireless transmission using a wireless protocol such as WirelessHART (IEC <NUM>).

In <FIG>, sensor <NUM> is shown as being mounted in bluff body <NUM>. In other embodiments, sensor <NUM> may be positioned separate from bluff body <NUM> at a location or locations downstream of the front face of bluff body <NUM>.

Flow of the process fluid continues from measurement section <NUM> into flange expander <NUM>. Diameter DM does not change within flange expander <NUM> until process fluid reaches expander section <NUM>. At that point, the inner diameter of expander <NUM> increases until flange expander <NUM> meets pipe <NUM>. Expander section <NUM> provides a smooth transition in diameter from DM to DP.

As shown in <FIG>, length LM is a distance from the front face of bluff body <NUM> (where fluid first encounters bluff body <NUM>) to expander section <NUM> (where the diameter first begins to expand from measurement section diameter DM to pipe diameter DP).

Vortex flowmeters are designed for installation in flanged piping systems that include different pressure ranges and a range of sizes in standardized steps of nominal pipe diameters. These steps typically include ½ inch (<NUM>), <NUM> inch (<NUM>), <NUM>½ inch (<NUM>), <NUM> inch (<NUM>), <NUM> inch (<NUM>), <NUM> inch (<NUM>), <NUM> inch (<NUM>), <NUM> inch (<NUM>), <NUM> inch (<NUM>), and <NUM> inch (<NUM>). A reducer vortex flowmeter like flowmeter <NUM> typically has a measurement bore diameter DM that is under sized by one pipe size step from the pipeline diameter DP of the process piping system in which it is installed.

It has been discovered that certain reducer vortex flowmeters can exhibit less accuracy and less stable signals then their standard "straight through" counterpart (which do not include a reducer and expander). The reduced accuracy and less stable signals tend to be exhibited by the larger diameter reducer vortex flowmeters, where the overall length of the reducer flow meter, and particularly the length LM from the front face of the bluff body to the first expansion is shorter relative to measurement bore diameter DM.

Test results show that an expansion downstream of bluff body <NUM> can adversely affect repeatability of the vortex shedding signal. <FIG> shows an analog <NUM>-20mA signal representing the measured flow value versus time from an <NUM> inch (<NUM>) reducer vortex flowmeter (i.e., DP = <NUM> inches (<NUM>), DM = <NUM> inches (<NUM>)) having the configuration shown in <FIG>. Signals are shown for flow rates ranging from 1ft/s (<NUM>/s) up to 21ft/s (<NUM>/s). <FIG> shows sporadic dips in the <NUM>-20mA signal.

<FIG> shows oscilloscope data from the same <NUM> inch (<NUM>) reducer vortex flowmeter used to produce the graphs of <FIG>. <FIG> shows both an analog signal from the sensor, and a digital pulse generated based on the analog signal. The oscilloscope data shows that instability of the vortex shedding signal can cause missed pulses, which results in fluctuations in the flow value measured.

Expansions in diameter that are too abrupt for the fluid to follow will cause boundary layer separation and transient pressure gradients. These pressure gradients can negatively affect the upstream dynamic vortex shedding within a vortex flowmeter.

<FIG> shows reducer vortex flowmeter 10A, which is an embodiment of the present invention. Reducer vortex flowmeter 10A is similar to prior art flowmeter <NUM> of <FIG>, and similar reference numbers are used to identify similar parts. Flow tube 16A of <FIG> differs from flow tube <NUM> shown in <FIG> by the use of straight flange 24A. As shown in <FIG>, flange 24A contains only constant diameter section 34A that extends from measurement section <NUM> all the way to pipe <NUM>. Bore diameter DF is constant over entire length LM from the forward edge of bluff body <NUM> to the first expansion which occurs at the interface between flange 24A and pipe <NUM>.

The present invention is based upon the discovery that length LM could be related to the wavelength of the vortex produced by bluff body <NUM>. The wavelength λ of the vortex is constant for all velocities within the calibration range of flowmeter 10A. If distance LM from the inception of vortex shedding at the front face of bluff body <NUM> to the first expansion in diameter downstream of bluff body <NUM> equals or exceeds the vortex wavelength, then each vortex should properly develop. If LM is less than λ, then instability in the vortices can occur.

With an in-house state of the art example of a flowmeter according to prior art reducer flowmeter <NUM>, instability can occur when DM is <NUM> inches (<NUM>) or larger because the ratio of LM to λ is less than <NUM>. In other words, for DM of <NUM> inch (<NUM>) or larger, LM of in-house state of the art reducer vortex flowmeter <NUM> is less than the vortex wavelength λ of the vortices that are produced by bluff body <NUM>.

To determine the value of LM required for reducer vortex flowmeter 10A, vortex wavelength λ must be determined. The following equations show how λ can be determined for a particular vortex shedding flowmeter. <MAT> where λ equals vortex wavelength, v equals flow velocity, and f equals vortex frequency. <MAT> where K is the K factor characteristic of a vortex flowmeter in pulse (pul) per unit volume (vol). <MAT> where t equals time. <MAT> where A equals cross-sectional area. <MAT> <MAT> <MAT>.

As shown in Eq. <NUM>, the vortex wavelength can be determined if the K factor and the cross-sectional area of metering section <NUM> is known. Each vortex shedding flowmeter has a K factor that is typically determined by calibration at the factory. For purposes of the following calculations, nominal K factor values for vortex shedding flowmeters of different sizes have been used. Table <NUM> shows wavelength λ for different reducer flowmeters. The pipeline diameters DP range from <NUM> inch to <NUM> inches (<NUM> to <NUM>), and the corresponding flowmeter bore diameters DM range from <NUM> inch to <NUM> inches (<NUM> to <NUM>), respectively.

Based upon the wavelengths shown in Table <NUM>, the in-house state of the art reducer vortex flowmeter <NUM> has a value of LM that is greater than the vortex wavelength for DM up to <NUM> inches (<NUM>). However, for prior art reducer flowmeters of <FIG> having a measurement bore diameter DM of <NUM> inches (<NUM>) or larger, vortex wavelength λ exceeds length LM.

<FIG> shows a graph comparing standard deviation of a <NUM>-20mA signal produced by an <NUM> inch (<NUM>) in-house state of the art reducer vortex flowmeter of the prior art design of flowmeter <NUM> shown in <FIG>, and an <NUM> inch (<NUM>) reducer vortex flowmeter having the configuration of flowmeter 10A in <FIG>. For each of the flow rates from <NUM> ft/s (<NUM>/s) down to <NUM> ft/s (<NUM>/s), standard deviation of the signals produced by each flowmeter are shown. <FIG> shows that the standard deviation in the signal produced by flowmeter 10A was consistently less than the standard deviation in the signal produced by the in-house state of the art flowmeter <NUM>. The vortex shedding signal produced by flowmeter 10A was up to <NUM> percent less noisy than flowmeter 10A.

<FIG> shows the <NUM>-20mA output verses time for reducer vortex flowmeter 10A of <FIG>. A comparison of <FIG> shows that the dips present in the signals shown in <FIG> are not shown in the signals shown in <FIG>.

<FIG> is a chart showing the ratio of distance LM to vortex wavelength λ for in-house state of the art flowmeter <NUM> and flowmeter 10A with different pipeline sizes DP ranging from <NUM> inch (<NUM>) to <NUM> inch (<NUM>) nominal pipe size. As shown in <FIG>, the in-house state of the art flowmeter <NUM> has a ratio greater than <NUM> only up to a pipeline diameter DP of <NUM> inches (<NUM>) (and DM of <NUM> inches (<NUM>)). The reducer flowmeters used with pipeline diameters DP of <NUM> inches (<NUM>) or greater (and DM of <NUM> inches (<NUM>) or greater) all have a ratio of LM to λ of less than <NUM>, and therefore have the potential for instability of the vortex shedding that can cause missed pulses.

In contrast, flowmeter 10A in the present invention has a ratio of LM to λ that is greater than or equal to <NUM> all the way to pipeline diameter of <NUM> inches (<NUM>). Enhanced stability at the <NUM> inch (<NUM>) and <NUM> inch (<NUM>) (and higher) pipe sizes can be achieved with flowmeter 10A by a small increase in LM. This can be achieved by increasing the length of flange 24A by about <NUM> inch (<NUM>) for the <NUM> inch (<NUM>) pipeline diameter and about <NUM> inches (<NUM>) for the <NUM> inch (<NUM>) pipeline diameter.

Although section 34A shown in <FIG> has been shown as a constant diameter section, the diameter of section 34A can have a very slight taper without presenting an expansion in diameter that is too abrupt for the fluid to follow. In other words, the diameter of section 34A is substantially constant out to LM greater than λ At the inlet end of flowmeter 10A, reducer section <NUM> is shown as having a straight taper between pipe <NUM> and constant diameter section <NUM>. Other profiles that provide a smooth transition from pipe diameter DP to measurement diameter DM could be employed in flange reducer <NUM>.

Claim 1:
A vortex flowmeter (10A) comprising:
a flow tube body (16A) having a flow passage extending between an upstream end and a downstream end, the flow passage having a measurement section (<NUM>) of a diameter DM and a reducer section (<NUM>) between the upstream end and the measurement section (<NUM>) in which the diameter reduces from a larger diameter DP at the upstream end to the diameter DM of the measurement section (<NUM>), wherein the reducer section (<NUM>) is comprised in a flange reducer (<NUM>), and the measurement section (<NUM>) extends from the reducer section; and
only one bluff body (<NUM>) positioned in the measurement section (<NUM>) to produce vortices that alternate at a frequency related to flow rate of fluid through the flow passage;
and characterised in that
a length LM from a location of inception of vortex shedding at the bluff body (<NUM>) to a first expansion in diameter of the flow passage downstream of the bluff body (<NUM>) is greater than a vortex wavelength λ of the vortices produced by the bluff body (<NUM>).