CN2784885Y - Tube-wall differential pressure type vortex frequency detector for vortex flowmeter - Google Patents

Abstract

The utility model discloses a pipe-wall differential pressure type vortex frequency detector for a vortex street flowmeter. Two pressure ports symmetric to a vortex generating body are arranged on the pipe wall at the down stream of the vortex generating body of the vortex street flowmeter with the distance of 1/5D-D (D is the internal diameter of a pipeline) with an incident flow end face of the generating body; the differential pressure of the two points is measured by a differential pressure transducer; the output differential pressure signals of the pipe wall can change with the generation and shedding of vortex in the pipeline, wherein the signals comprise vortex frequency information; thus, the vortex frequency can be obtained through measuring the differential pressure signals of the pipe wall and volume flow value can be obtained further. In the methods suggested by the utility model, the transducer system is independent of the vortex generating body and is positioned at the outside of the pipeline, the flow of medium in the pipe can not be affected, at the time of maintenance and replacement, pipe flow does not need to be cut off, the vortex generating body can not be disassembled, so that on-line maintenance and replacement of the sensor can be realized. The utility model has the advantages of strong interference immunity, low lower limit measurable flow capacity, convenient operation, easy realization and reliable operation.

Description

The pipe wall differential pressure type vortex frequency detecting device of vortex shedding flow meter

Technical field

The utility model relates to a kind of pipe wall differential pressure type vortex frequency detecting device of vortex shedding flow meter.

Background technology

Vortex shedding flow meter belongs to the emerging product that is used for flow measurement of a class, and it has, and Applicable media is wide, reliability is high, the more high outstanding advantage of accuracy of measurement, has therefore all obtained using widely in industrial circle.The detection of vortex frequency is a gordian technique of vortex shedding flow meter, and the detection method of existing vortex shedding flow meter vortex frequency mainly contains: heated filament detection method, temperature-sensitive detection method, capacitance detecting method, piezoelectric crystal detection method and the ultrasonic Detection Method that occurs recently.By contrast, piezoelectric crystal detection method general effect is best, and also commonly used, other method is used seldom.But there are two serious problems in the piezoelectric crystal detection method, and promptly piezoelectric crystal is responsive to Analysis of Pipe Vibrations, when near fluid or pipeline and the pipeline vibration being arranged, will produce very big error, even can't use; It two is the long-term less stable used of piezoelectric crystal, particularly can't at high temperature measure.In order to address the above problem, the researchist has carried out research extensively and profoundly from the version of sensor and the analyzing and processing equal angles of flow signal, has obtained the achievement of some, but all is difficult to fundamentally be solved.

Summary of the invention

The purpose of this utility model provides a kind of pipe wall differential pressure type vortex frequency detecting device of vortex shedding flow meter.

The pipe wall differential pressure type vortex frequency detecting device of vortex shedding flow meter has the measurement pipeline section, is provided with the vortex generation body of vortex shedding flow meter in measuring pipeline section, meets the stream end face in the downstream of the vortex generation body of vortex shedding flow meter apart from vortex generation body

Between the measurement pipeline section on have two and be symmetrical in the pressure ports of measuring the pipeline section axis, be symmetrical between the pressure port of measuring the pipeline section axis at two and be connected to differential pressure pick-up, be connected to amplifier after the differential pressure pick-up, be connected to the shaping detecting circuit after the amplifier.

In the method that the utility model proposes, sensing system is independent of vortex generation body and is positioned at the pipeline outside, to the not influence of flowing of medium in managing, do not need to cut off pipe stream when keeping in repair and changing and dismantle vortex generation body, can realize sensor on-line maintenance and replacing; Has stronger anti-interference; Measurable flow amount lower limit is lower; Easy and simple to handle, be easy to realize reliable operation.

Description of drawings

Fig. 1 (a) is the pipe wall differential pressure type vortex frequency detecting device structural representation of vortex shedding flow meter;

Fig. 1 (b) is the pipe wall differential pressure type vortex frequency detecting device structure side view of vortex shedding flow meter;

Fig. 2 is the vortex generation body cross sectional representation of the utility model vortex shedding flow meter;

Fig. 3 is the distribution schematic diagram of the utility model experiment pressure port:

Fig. 4 (a) is the tube wall differential pressure signal synoptic diagram of the utility model when the air measuring media;

Fig. 4 (b) is the tube wall differential pressure signal synoptic diagram of the utility model when the water gaging medium;

Fig. 5 (a) is Strouhal number St and the reynolds number Re of the utility model when the air measuring media _DConcern synoptic diagram;

Fig. 5 (b) is Strouhal number St and the reynolds number Re of the utility model when the water gaging medium _DConcern synoptic diagram;

Fig. 6 (a) is tube wall differential pressure amplitude and the discharge relation synoptic diagram of the utility model when the air measuring media:

Fig. 6 (b) is tube wall differential pressure amplitude and the discharge relation synoptic diagram of the utility model when the water gaging medium.

Embodiment

According to fluid mechanics principle, in vortex shedding flow meter, must there be the variation of pressure in the place that has vortex to produce, the variation of rule appearred in the pressure in flow field near alternately the vortex that produces will inevitably cause, the frequency of its variation is corresponding one by one with the frequency of vortex, so can measure vortex frequency by detecting the fluctuation differential pressure that certain point-to-point transmission of determining in the body wake flow takes place, thereby realize the measurement of volumetric flow rate.Equate with frequency owing to phase differential on the body symmetria bilateralis point takes place be 180 ° and Oscillation Amplitude, so the symmetric points that differential pressure pressure position is chosen at vortex generation body on the tube wall are more conducive to detection.

As scheme shown in the l, the pipe wall differential pressure type vortex frequency detecting device of vortex shedding flow meter has measures pipeline section 1, is provided with the vortex generation body 4 of vortex shedding flow meter in measuring pipeline section, meets the stream end face in the downstream of the vortex generation body of vortex shedding flow meter apart from vortex generation body

Have two pressure ports 2 that are symmetrical in vortex generation body on the measurement pipeline section 1 between (D is the pipeline interior diameter), be symmetrical between the pressure port of measuring the pipeline section axis at two and be connected to differential pressure pick-up 3, be connected to amplifier 5 after the differential pressure pick-up 3, be connected to shaping detecting circuit 6 after the amplifier 5.The shape of cross section of vortex generation body 4 is trapezoidal, and the fluoran stream surface width of vortex generation body 4 is 0.20～0.33 with the ratio of measuring the pipeline section internal diameter.Measuring the pipeline section internal diameter is 25～250mm.For 50mm inner diameter measurement pipeline section, the fluoran stream surface width of vortex generation body 4 is 12～16mm, and the lee side width is 2～5mm, and total length is 15～20mm, and side hypotenuse length is 14～25mm.

Differential pressure pick-up 3 adopts the 24PCB type differential pressure pick-up of Honeywell company, its performance parameter is: 10.0 ± 0.01 VDC power supply, range 5psi (34KPa), repeatability 0.15%, reaction time 1.0ms, input impedance 5.0k Ω, output impedance 5.0k Ω, weight 2g, operating ambient temperature-40 ℃～85 ℃.

What amplifier 5 adopted is the AD620 chip, and its performance parameter is: gain margin 1～1000, and ± 2.3V～± the 18V power supply, low power consumption 1.3mA, temperature drift 0.6 μ V/ ℃.

The effect of shaping detecting circuit 6 is the square voltage pulse signals that the sine wave input are transformed into certain load capacity.

Fast Fourier transform (FFT) is a kind of spectral analysis method, and it is transformed into frequency domain with time-domain signal, conveniently to extract the vortex street frequency values.Its principle is as follows: if time history x (t) be limited in time interval (0, T) in, the original of the power spectral density function at frequency f place is estimated as

$G_x\left(f\right)=\frac{2}{T}{\left|X_n(f,T)\right|}^2,$ T＝NΔt

If limited discrete Fourier transform (DFT) is

$X(f,T)=\mathrm{\&Delta;t}\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{x}_k\exp (-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="Y20052010178100081.png,Y20052010178100093.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;fk\&Delta;t}),$ x _k＝x(kτ)，k＝0，1，…，N-1。

FFT discrete frequency value is

$f_n=\frac{n}{\mathrm{N\&Delta;t}},$ n＝0，1，2，…，N-1

Then power spectrum is

$G_x\left(f\right)={\left|\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{k}_k\exp (-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="Y20052010178100081.png,Y20052010178100093.png"\; state="\{\{state\}\}">j</figure-callout>}\frac{2\mathrm{\&pi;nk}}{N})\right|}^2,$ n＝0，1，2，…，N/2。

Flow media is respectively under the situation of water and air and all tests in pipe, the interior diameter D=50mm of pipeline, and the xsect of vortex generation body is approximately trapezoidal, fluoran stream surface width d=14mm, concrete shape and other parameters are as shown in Figure 2.The pressure port of tube wall differential pressure is chosen in three couples of diverse location a, b, the c behind the generation body, and they lay respectively at apart from the body fluoran stream surface takes place The downstream of D, as shown in Figure 3.The diameter of pressure port is 2mm, and the length of two pressure guiding pipes is 20cm, and differential pressure pick-up is the 24PC type of Honeywell company, and what amplifier adopted is the AD620 type.The differential pressure frequency is tried to achieve by fast fourier transform.Fig. 4 has provided under the different flow The original signal of place's tube wall differential pressure, they are along with the generation of vortex and come off and fluctuate.

1. Strouhal number

The St and the Re of various flows moving medium, different pressures position _DCorresponding relation as shown in Figure 5.The St of various situations remains constant basically, and their value is all equal, is about 0.253.

2. instrument coefficient

The instrument coefficient of various flows moving medium, different pressures position is listed in table 1, and the maximum relative error between them is less than 1%.

Table 1 instrument coefficient (Hzhm ^-3)

Flow media	Position 1	Position 2	Position 3
				Water	2.5559	2.5540	2.5630
Air	2.5674	2.5679	2.5686

3. minimum measurable flow speed

The minimum measurable flow speed of various situations and the measurement lower limit of conventional vortex shedding flow meter are listed in table 2, in the certain limit behind vortex generation body, the closer to body takes place, the curl of vortex is strong more, the sensitivity of measuring is high more, no matter be water or air, minimum measurable flow amount is all with moving behind the pressure position and increase.When measuring water, the measurement lower limit of position a only is 52% of a conventional table; When measuring air, the measurement lower limit of position a is 75% of a conventional table, therefore adopts the tube wall differential pressure method can expand the measurement lower limit of vortex shedding flow meter effectively; And the pressure location-appropriate near taking place, physical efficiency is further reduced measurement lower limit.

The minimum measurable flow speed of table 2 (ms ^-1)

Flow media	Position 1	Position 2	Position 3	Conventional table *
					Water	0.22	0.35	0.49	0.42
Air	2.78	2.92	4.32	3.70

^*Conventional table data draw from " Liang Guowei, Cai Wuchang. flow measurement technology and instrument. Beijing: China Machine Press, 2002 "

4. the influence of pressure guiding pipe to measuring

Definition tube wall differential pressure average amplitude p _MaxFor

${\stackrel{\&OverBar;}{p}}_{\max }=\frac{\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{p}_{\max }\left(i\right)-\underset{i=1}{\overset{N}{\mathrm{\&Sigma;}}}{p}_{\min }\left(i\right)}{2N}$

P in the formula _Max(i), p _Min(i) be respectively the maximal value and the minimum value of i vortex cycle inner tubal wall differential pressure; The total periodicity of N for detecting.p _MaxValue has reflected the size of vortex intensity, should increase with the increase of flow.

Experiment p _MaxWith the relation of q as shown in Figure 6, what provide among Fig. 6 (a) is that flow media is when being air, from the p of three pairs of different pressure position measurements _MaxWith the distribution situation of q, visible three curves are the monotone increasing with the increase of q all, has met theoretical prediction preferably; Shown in Fig. 6 (b) is the situation of flow media when being water, and the shape of three curves is similar, as q＜83m ³H ^-1, p _MaxIncrease with flow increases, at q=83m ³H ^-1Near obtain maximum value, as q＞83m ³H ^-1, p _MaxNo longer the increase with flow increases, but sharply descends until q＞120m ³H ^-1After increase progressively gently gradually.

This is because lower pressure guiding pipe natural frequency has hindered the response of pressure measuring system to dynamic tube wall differential pressure, thereby causes bigger measuring error, is not inconsistent with theory relation.But according to the analysis of front as can be seen, the error of tube wall differential pressure amplitude does not influence the measurement of frequency, and is very little to the influence of St and instrument coefficient, is that the measurement of flow hardly can be influenced yet;

According to above principle and experimental result, can measure flow value accurately and conveniently by this method as can be known.

Claims (4)

1, a kind of pipe wall differential pressure type vortex frequency detecting device of vortex shedding flow meter, it is characterized in that: it has measurement pipeline section (1), in measuring pipeline section, be provided with the vortex generation body (4) of vortex shedding flow meter, have two on meeting measurement pipeline section (1) between stream end face ÷ D～D apart from vortex generation body in the downstream of the vortex generation body of vortex shedding flow meter and be symmetrical in the pressure ports (2) of measuring the pipeline section axis, be symmetrical between the pressure port of measuring the pipeline section axis at two and be connected to differential pressure pick-up (3), differential pressure pick-up (3) is connected to amplifier (5) afterwards, and amplifier (5) is connected to shaping detecting circuit (6) afterwards.

2, the pipe wall differential pressure type vortex frequency detecting device of a kind of vortex shedding flow meter according to claim 1, it is characterized in that: the shape of cross section of described vortex generation body (4) is trapezoidal, and the fluoran stream surface width of vortex generation body (4) is 0.20～0.33 with the ratio of measuring the pipeline section internal diameter.

3, the pipe wall differential pressure type vortex frequency detecting device of a kind of vortex shedding flow meter according to claim 1 is characterized in that: described measurement pipeline section internal diameter is 25～250mm.

4, the pipe wall differential pressure type vortex frequency detecting device of a kind of vortex shedding flow meter according to claim 1, it is characterized in that: for the measurement pipeline section of 50mm internal diameter, the fluoran stream surface width of described vortex generation body (4) is 12～16mm, the lee side width is 2～5mm, total length is 15～20mm, and side hypotenuse length is 14～25mm.

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