JPS6110718A - Karman vortex flowmeter - Google Patents

Abstract

PURPOSE:To eliminate the instrumental error of a flowmeter by a low-cost means, by arranging a columnar member in at least one flow path of two or more flow paths provided in a pipe, and making the cross sectional area of at least one remaining flow path variable. CONSTITUTION:A flow straightening grid 4 is provided in the upstream of a pipe 1. At a constricted part 5 at the downstream of the pipe 1, the pipe is branched into a main flow path 6 and a bypass 7, which are combined at an expanded part 8. Karman vortex, which is yield by a vortex yielding body 2 that is inserted in the flow path 6, is detected by vortex detecting part 3. The opening area of the bypass 7 is varied by an adjusting screw 9, which is provided in the bypass 7 through an O ring 10. The flow rates of the pipe 1, the flow path 6 and the bypass are made to be QT, QM and QB. The flow rate QT is computed by using the expressions I and II based on the vortex frequency (f) obtained by the detecting part 3. The dispersions in proportional constants K and K' based on the instrumental error of a flowmeter are corrected by a ratio QB/QM.

Description

Detailed Description of the Invention [Technical field to which the invention pertains] The present invention detects the frequency of the Karman vortex street generated on both downstream sides of a columnar object inserted into a fluid flow, and determines the fluid flow velocity or Related to Karman vortex flowmeter for measuring flow rate.

[Prior art and its problems]

In this type of flow needle, in a certain range of fluid flow speed, the flow speed V (m/s) and the vortex frequency f generated from the columnar object are
(Hz) It is widely known that a proportional relationship holds true between Hz and Hz. That is, if the representative length of the columnar object inserted into the flow is d, this relationship is expressed by the following equation.

f=st □ ・・・・・・(1)
Furthermore, if the cross-sectional area of the pipe through which the fluid flows is A Cm") and the flow rate is Q (m"/sec), the above equation (1) can be expressed as the following equation (V=Q/A) .

Here, SL is a dimensionless number called Strouhal number,
This is a constant that is approximately determined by the shape of the columnar body. Therefore, the following relationship holds true. That is, it can be seen that the flow rate Q can be found by multiplying the vortex frequency f by the proportionality constant K.

However, the representative length d of the columnar body and the cross-sectional area A of the flow path included in this proportionality constant K vary within the dimensional tolerance during manufacturing, so the proportionality constant also differs for each flowmeter. . Therefore, the flow rate must be calculated using different coefficients for each individual flowmeter, which poses a problem in practicality. □So,
As a method to reduce the instrumental error of this proportionality constant, it is possible to increase the manufacturing accuracy of the representative length d of the columnar body and the cross-sectional area A of the flow path.
%), which has the disadvantage of significantly increasing costs.

[Purpose of the invention]

The present invention has been made under these circumstances, and instead of increasing the accuracy of component parts as in the conventional method, it is an object of the present invention to eliminate the instrumental error of this type of flowmeter by a cheaper means.

[Key points of the invention]

The present invention forms two channels, a main channel through which most of the fluid flows, and a bypass channel through which a portion of the fluid flows, in a conduit for flowing the measurement fluid, and a Karman vortex generator is installed in the main channel. At the same time, the cross-sectional area of the bypass flow path is varied to control the flow velocity of the main flow path, making it possible to easily adjust the proportionality constant between the flow rate of the measured fluid and the vortex frequency for each individual flowmeter. This is what I did.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

Fig. 1 is an overall configuration diagram showing an embodiment of the present invention, Fig. 1A is a sectional view along A-AwA in Fig. 1, Fig. 2 is a characteristic diagram showing flow characteristics, and Fig. 3 is a flow rate and vortex frequency. FIG. In FIG. 1, 1 is a pipe for flowing the measurement fluid, 2 is a vortex generator for generating a Karman vortex,
3 is a vortex detection section. Reference numeral 4 denotes a rectifying grid, which is provided at the entrance of the pipe to eliminate disturbances and imbalances in the flow upstream of the pipe 1. Further downstream of the reduced portion 5 provided downstream of the rectifying grid 4, the pipe line 1 is divided into a main flow passage 6 having a substantially rectangular cross section and into which the vortex generator 2 is inserted, and a bypass flow passage 7. The two channels are branched and meet at the enlarged portion 8. Note that the cross-sectional shapes of the main flow path 6 and the bypass flow path 7 are shown in FIG. 1A, and here,
Reference numeral 9 is an adjustment screw for changing the opening area of the bypass passage 7, and reference numeral 10 is an O-ring for eliminating fluid flow from the threaded portion 9.

Next, the operation will be explained. As described above, the measurement fluid flowing into the pipe line 1 is divided into the main flow path 6 and the bypass flow path 7. Here, the inflow flow rate to the pipe line 1 is Qa, the main flow passage 6. The flow rates of bypass channel 7 are QM and Q, respectively.
When s is assumed, the following equation holds true.

QT = QM + QB...
(4) On the other hand, the vortex frequency f detected by the vortex detector 3 is actually proportional to the flow rate QM of the main flow path 6, so if this proportionality constant is denoted by ', then f='Q,・・・・・・(
5) holds true. Therefore, f = K'QM = K' (Qt Qi) is obtained from equation (4) and (51), and therefore, equation (5) is as follows:
7). Note that the proportionality constant in equation (7) is expressed as QM (8). From the above, if Qll/Qi is constant regardless of the flow rate, the vortex frequency f is proportional to the total flow rate QT flowing into pipe 1, so changing the ratio of Ql/QI4 will change the proportionality constant of the flowmeter. It turns out that K can be changed.

By the way, the flow rate flowing through this type of fluid flow channel is determined by the pressure difference between the inlet and outlet, and the following relationship, which is known from fluid dynamics, holds true. In other words, if the pressure difference is ΔP and the flow rate is Q, then, ξ is called a pressure loss coefficient, and is a constant determined by the shape, surface roughness, etc. of the flow path.

The present inventors have confirmed through experiments that this relationship substantially holds true even when there is an obstacle such as a columnar body in the flow path.

The relationship between this pressure difference and flow rate (hereinafter referred to as flow rate characteristics)
is as shown in FIG. As is clear from FIG. 1, the pressure difference ΔP at each inlet and outlet of the main flow path 6 and the bypass flow path 7 is the same, so the flow rate flowing into each flow path is determined by a straight line with a constant ΔP. The flow rate Q, determined by the intersection with the flow rate characteristic curve of the flow path. I QIO, and as is clear from Figure 2, the flow rate ratio QIIO/QM
. becomes constant. Since the pressure difference between the inlets and outlets is constant between the main flow path 6 and the bypass flow path 7 regardless of the flow rate flowing into the pipe line 1, the flow rate ratio QBO/QMO is also constant regardless of the flow rate, and the vortex frequency f is as shown in Fig. 3. It can be seen that it is proportional to the total flow rate QT. If the opening area of the bypass flow path 7 is narrowed down, for example, by the adjustment screw 9, the flow rate characteristic of the bypass flow path changes from curve B to curve B' shown in FIG. 2, and therefore the flow rate becomes Q. From Glg.

decreases to That is, Qm/Q in the above equation (8)
Since the ratio of M changes and the opening area of the bypass flow path changes, even if there is variation in the value, it is possible to set the value to a constant value. Furthermore, Figure 3 shows the relationship between the total flow rate and the vortex frequency. By changing the area of the bypass flow path, the vortex frequency f and the total flow rate Q (QT) can be changed while maintaining their proportionality. It can be seen that the value of the proportionality constant changes.

As described above, the adjustment method according to the present invention is very easy to adjust because it is only necessary to move the adjustment screw so that the flow rate at any one point has a predetermined vortex frequency. It becomes possible to eliminate instrumental differences.

〔Effect of the invention〕

According to the present invention, as a method of eliminating instrumental differences in the proportionality constant between vortex frequency and flow rate due to dimensional tolerances of individual flowmeter components, the fluid flowing into the pipe line is The flow is divided into a main flow path into which most of the fluid flows and a bypass flow path into which a portion of the fluid flows.For example, by changing the cross section (opening) area of the bypass flow path, the flow rate flowing therein is adjusted. As a result, the proportionality constant between the vortex frequency and the flow rate is adjusted, and even if there are variations in the dimensional accuracy of the parts, it can be easily and inexpensively adjusted, and therefore costs can be significantly reduced. This provides the advantage of reducing the

[Brief explanation of drawings]

Fig. 1 is an overall configuration diagram showing an embodiment of the present invention, Fig. 1A is a sectional view taken along line A-A in Fig. 1, Fig. 2 is a characteristic diagram showing flow characteristics, and Fig. 3 is a flow rate and vortex. FIG. 3 is a characteristic diagram showing the relationship with frequency. Code explanation 1... Pipeline, 2... Vortex generator (column-shaped object), 3...
・Vortex detection section, 4... Rectification grating, 5... Reduction section, 6.
...Main flow path, 7...Bypass flow path, 8...Enlarged portion, 9...Adjustment screw, 10...0 ring. Agent Patent Attorney Akio Namiki Agent Patent Attorney Kiyoshi Matsuzaki 111il tJEIA Map

Claims (1)

[Claims] At least two or more channels are formed in the pipe for flowing the measurement fluid, and a columnar member for generating a Karman vortex is disposed in at least one of the channels, while at least one of the remaining channels is One is a Karman vortex flowmeter characterized by comprising a variable member that changes the cross-sectional area of the flow path, and an adjustment member that adjusts the variable member from outside the pipe.