Selection and maintenance of vortex flowmeter
/div> & nbsp;& nbsp; preface:
& nbsp;& nbsp; In the process of steam measurement, our company has used a variety of measuring instruments, but due to various reasons can not meet the requirements of accurate measurement, are abolished unused. In 2004, after investigation and comparison, our company used stress vortex flowmeter to measure steam. Because it is a new type of imported instrument, I don't know much about its performance, characteristics, requirements and principles, and many problems have appeared in the use and maintenance. Now I will summarize some problems and solutions encountered in my work and study, and learn from you together.
& nbsp;& nbsp; 1. Principle and circuit structure of vortex flowmeter.
& nbsp;& nbsp; 1.1 working principle: the basic principle of vortex street flow sensor is Carmen vortex street principle. When an object is vertically inserted into the measuring tube, regular vortices will be generated alternately when the fluid passes through both sides of the object, which is called Karman vortex street. The release frequency of Karman vortex street, the velocity of fluid and the width of column can be expressed by the following formula:
& nbsp;& nbsp; f=St?V/d
& nbsp;& nbsp; Where f -- release frequency of Carmen vortex street (Hz)
& nbsp;& nbsp; St -- Strouhal number
& nbsp;& nbsp; V -- fluid velocity (M / s)
& nbsp;& nbsp; D -- width of upstream surface of column (m)
& nbsp;& nbsp; Because st is a constant which depends on the cylinder section switch and has nothing to do with the liquid properties and flow rate, ST is between 0.17 and 0.18, so the release frequency of vortex is directly proportional to the flow rate. As long as the detection frequency f, the flow rate V can be obtained, and the liquid volume flow can be obtained from V.
& nbsp;& nbsp; When the fluid flows through the cylinder, the vortices are separated alternately, and the fluctuating pressure is produced in the wake on both sides of the cylinder and behind the cylinder. The detection probe behind the cylinder is affected by the micro pulsating pressure, so that the piezoelectric crystal element in the probe is subjected to alternating stress and produces the corresponding alternating charge signal. After the charge signal is processed by the detection amplifier, the pulse signal which is proportional to the flow rate is output to the integrator for display and accumulation.
& nbsp;& nbsp; 1.2 circuit composition: as shown in Figure 1
& nbsp;& nbsp; Charge converter: converts the alternating charge signal output from the piezoelectric element into a voltage signal proportional to the amount of charge.
& nbsp;& nbsp; Amplifier, low-pass filter: signal amplification, noise elimination.
& nbsp;& nbsp; Schmidt shaper: the voltage of vortex frequency is converted into a certain amplitude pulse signal. At the same time, the circuit has a lag effect on the input and output signals, so it can prevent the oscillation caused by noise.
& nbsp;& nbsp; Pulse output amplifier: amplify the pulse signal output by Schmidt shaping circuit.
& nbsp;& nbsp; 2. Installation of vortex flowmeter and connection between measuring systems:
& nbsp;& nbsp; 2.1 installation:
& nbsp;& nbsp; It is very important to choose the installation point correctly and install the sensor correctly. If the installation is wrong, the measurement accuracy will be affected, the service life of the sensor will be affected, and even the sensor will be damaged.
& nbsp;& nbsp; 2.1.1 selection of installation site:
& nbsp;& nbsp; 2.1.1.1 environmental temperature: - 40 ℃ ≤ t ≤ 55 ℃. If the thermal radiation of production equipment is affected, thermal insulation and ventilation measures shall be taken.
& nbsp;& nbsp; 2.1.1.2 ambient air: the flowmeter should not be installed in the environment containing corrosive gas. If it must be installed, ventilation measures must be taken.
& nbsp;& nbsp; 2.1.1.3 mechanical vibration and impact: vibration will produce interference signal. If the vibration and impact on the pipeline are strong and the medium flow rate is both low, the interference signal may be greater than the flow signal, resulting in metering error. Therefore, the flowmeter should be installed in the place with small vibration and impact, and the installation position should be at the vibration frequency of 5 ~ 20Hz, and the vibration acceleration should not be greater than 1G, otherwise vibration reduction measures should be taken.
& nbsp;& nbsp; 2.1.2 pipeline installation and requirements:
& nbsp;& nbsp; 2.1.2.1 the upstream and downstream sides of the sensor shall have enough straight pipe sections. The length of straight pipe section shall conform to table 1.
& nbsp;& nbsp; 2.1.2.2 within the specified straight pipe length, the inlet and outlet sections of the pipeline shall be straight by visual inspection, and the sensor shall not be installed downstream of the regulating valve and semi open gate valve as far as possible. Generally, the sensor shall not be installed after expanding the pipe.
& nbsp;& nbsp; 2.1.2.3 the sensor can be installed vertically, horizontally or at any other angle, but when the sensor is installed vertically or obliquely, the flow direction of the fluid should be from bottom to top.
& nbsp;& nbsp; 2.1.2.4 for the setting of pressure taking point and measuring point, the pressure taking point shall be outside 10d of the sensor and the temperature measuring point shall be outside 5D.
& nbsp;& nbsp; 2.2 wiring:
& nbsp;& nbsp; 2.2.1 the signal connection between vortex flowmeter measurement systems is shown in Figure 2
& nbsp;& nbsp; 1. 2 is the signal input terminal of the amplifier board, the signal is provided by the piezoelectric crystal probe, terminal C is the power supply terminal, connected to 12V + or 24V + of the integrator, B is the signal output terminal of the amplifier board, corresponding to the integrator F +, and a is the 0V terminal of the amplifier board, corresponding to the integrator F -. G is the grounding terminal of amplifier board. The amplifying board and the totalizer are connected by three core shielded wires.
& nbsp;& nbsp; 2.2.2 the pipes and the shell of the amplifying plate for installing the sensor shall be well grounded（ Grounding resistance R ≤ Ω） & nbsp;& nbsp;& nbsp; 3 maintenance of vortex flow sensor:
& nbsp;& nbsp; 3.1 probe maintenance:
& nbsp;& nbsp; three point one. 1 probe structure: as shown in Figure 3. The probe consists of two pieces of piezoelectric crystal, one for anti-seismic and the other for detection. The probe leads a and B are the two leads of the test piece.
& nbsp;& nbsp; 3.1.2 probe detection:
& nbsp;& nbsp; 3.1.2.1 test method of multimeter: 200m Ω Measure the resistance of a and B, a and shell, B and shell respectively, and the resistance value is ∞. If there is resistance value, the probe will be damaged. Measure the capacitance of a and B with a multimeter at 2n, and the capacitance value shall not be changed. When there is no load, the capacitance shall be constant between 0.15nf and 0.35nf. When there is external vibration or fluid flow, the capacitance value changes and is larger than 0.15nf ~ 0.35nf, then the probe is normal.
& nbsp;& nbsp; 3.1.2.2 oscillograph detection method: connect the probe with the ultra-low oscillograph as shown in Figure 4, tap the head of the probe, if there is irregular sine wave on the oscillograph screen (as shown in Figure 5), it means that the probe is in good condition, and if there is no similar waveform, the probe is damaged< BR>
& nbsp;& nbsp; 3.1.2.3 overall calibration detection method: on the premise that the amplifier board and totalizer are good, connect the probe, amplifier board and totalizer according to normal use, tap the head of the probe with an object, if the frequency or instantaneous flow of the totalizer changes, the probe is good, if there is no change, the probe is damaged.
& nbsp;& nbsp; The probe can not be repaired after it is determined to be damaged.
& nbsp;& nbsp; 3.2 maintenance of amplification board.
& nbsp;& nbsp; 3.2.1 schematic diagram of enlarged plate: (as shown in Figure 6)
& nbsp;& nbsp; The circuit consists of charge converter, voltage amplifier, active low-pass filter, trigger shaper, power supply neutral point circuit and voltage pulse conversion circuit.
& nbsp;& nbsp; 3.2.1.1 charge converter: composed of operational amplifier i1-1, R20, R11, R12, C11 ~ C14, it is a charge / voltage converter with two terminal input. The alternating charge signal output by voltage probe transistor is input by S1 and S2, and the voltage signal output by the converter is proportional to the amount of charge.
& nbsp;& nbsp; 3.2.1.2 voltage amplifier: it is composed of operational amplifier I1-2, R21, R22 and potentiometer W1. It is a voltage amplifier with adjustable gain.
& nbsp;& nbsp; 3.2.1.3 low pass filter: it is composed of operational amplifier i1-3, r31-r34, c31-c33 and d31, D32. R31, R32, C33 determine the cutoff point of the filter. Different aperture sensors need different cutoff points, which can be realized by changing C32, C33.
& nbsp;& nbsp; 3.2.1.4 trigger shaping unit: including trigger (i2-1, R41, R42, C02, C41, W2) and shaper (IC3).
& nbsp;& nbsp; 3.2.1.5 power supply neutral point: the detection amplifier is powered by a single power supply + 24 V, and the power supply neutral point circuit is composed of operational amplifiers i2-2, R01, R02 and C01 to provide + 12 V neutral point potential for the chairman amplifier.
& nbsp;& nbsp; 3.2.1.6 voltage pulse / current pulse electric appliance: voltage stabilizing block iC4 and resistors R03 and r04 constitute constant current voltage stabilizing circuit, triode T01 and resistor R05 constitute pulse transformation amplification circuit, which converts voltage pulse into current pulse.
& nbsp;& nbsp; 3.2.2 amplification plate test:
& nbsp;& nbsp; In the schematic diagram, there are four detection points: TP1, TP2, TP3, TP4. When the amplifier board is normal, its output waveform is as follows: (Fig. 7) < br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br >
& nbsp;& nbsp; When there is a problem with the amplifier board, the oscillograph is used to monitor the waveforms of the above four test points to judge where the fault occurs, and then the components are checked according to the sequence of capacitor, diode, regulator, integrated block and resistance to judge whether they are good or bad, and the components are replaced to repair the circuit.
& nbsp;& nbsp; In addition, there is a very simple way to judge whether the enlarged board is good or bad. If it is known that the secondary meter is good, connect the amplifier board with the totalizer according to normal use, touch the input end of the amplifier board by hand, and the observation window displays the output of the instrument. If the display frequency of the display instrument is 50 Hz or 100 Hz, it means that the amplifier board is intact. If there is no output or other frequencies, it means that the amplifier board is broken. It can be repaired according to the above method.
& nbsp;& nbsp; 4 selection of vortex flowmeter:
& nbsp;& nbsp; Instrument selection is an important work in instrument application. Many faults of instruments in practical application are related to the selection and wrong use of instruments. After more than two years of use of vortex flowmeter, we have learned a lot from its selection and use, which are very important for the selection of vortex flowmeter and the increase of measurement accuracy in the future
& nbsp;& nbsp; 4.1 the diameter of the flowmeter shall be selected according to the actual situation of the process. Generally, the diameter of the selected instrument shall be such that the minimum gas flow rate shall not be less than 7m / s and the liquid flow rate shall not be less than 0.7m/s.
& nbsp;& nbsp; 4.2 try to avoid that the frequency of normal flow is about 50 Hz, because it is not easy to detect the power frequency interference into the instrument, which will cause measurement error.
& nbsp;& nbsp; four. 3 in actual production, because the designed pipeline is generally too large, the actual fluid flow will be too small. If the vortex diameter is selected according to the original pipeline, the measurement will be inaccurate or even impossible. Therefore, it is very important to determine the lower limit of vortex flowmeter. The most suitable vortex street can be selected only when the lower limit flow rate is determined.
& nbsp;& nbsp; 5 Conclusion:
& nbsp;& nbsp; Through the description of the principle of vortex flowmeter, the analysis of the circuit, and the introduction of some methods and experience in the maintenance process. I believe you have a certain understanding of vortex flowmeter. I hope these maintenance methods and experience can be helpful to peers.
& nbsp;& nbsp; In the process of steam measurement, our company has used a variety of measuring instruments, but due to various reasons can not meet the requirements of accurate measurement, are abolished unused. In 2004, after investigation and comparison, our company used stress vortex flowmeter to measure steam. Because it is a new type of imported instrument, I don't know much about its performance, characteristics, requirements and principles, and many problems have appeared in the use and maintenance. Now I will summarize some problems and solutions encountered in my work and study, and learn from you together.
& nbsp;& nbsp; 1. Principle and circuit structure of vortex flowmeter.
& nbsp;& nbsp; 1.1 working principle: the basic principle of vortex street flow sensor is Carmen vortex street principle. When an object is vertically inserted into the measuring tube, regular vortices will be generated alternately when the fluid passes through both sides of the object, which is called Karman vortex street. The release frequency of Karman vortex street, the velocity of fluid and the width of column can be expressed by the following formula:
& nbsp;& nbsp; f=St?V/d
& nbsp;& nbsp; Where f -- release frequency of Carmen vortex street (Hz)
& nbsp;& nbsp; St -- Strouhal number
& nbsp;& nbsp; V -- fluid velocity (M / s)
& nbsp;& nbsp; D -- width of upstream surface of column (m)
& nbsp;& nbsp; Because st is a constant which depends on the cylinder section switch and has nothing to do with the liquid properties and flow rate, ST is between 0.17 and 0.18, so the release frequency of vortex is directly proportional to the flow rate. As long as the detection frequency f, the flow rate V can be obtained, and the liquid volume flow can be obtained from V.
& nbsp;& nbsp; When the fluid flows through the cylinder, the vortices are separated alternately, and the fluctuating pressure is produced in the wake on both sides of the cylinder and behind the cylinder. The detection probe behind the cylinder is affected by the micro pulsating pressure, so that the piezoelectric crystal element in the probe is subjected to alternating stress and produces the corresponding alternating charge signal. After the charge signal is processed by the detection amplifier, the pulse signal which is proportional to the flow rate is output to the integrator for display and accumulation.
& nbsp;& nbsp; 1.2 circuit composition: as shown in Figure 1
& nbsp;& nbsp; Charge converter: converts the alternating charge signal output from the piezoelectric element into a voltage signal proportional to the amount of charge.
& nbsp;& nbsp; Amplifier, low-pass filter: signal amplification, noise elimination.
& nbsp;& nbsp; Schmidt shaper: the voltage of vortex frequency is converted into a certain amplitude pulse signal. At the same time, the circuit has a lag effect on the input and output signals, so it can prevent the oscillation caused by noise.
& nbsp;& nbsp; Pulse output amplifier: amplify the pulse signal output by Schmidt shaping circuit.
& nbsp;& nbsp; 2. Installation of vortex flowmeter and connection between measuring systems:
& nbsp;& nbsp; 2.1 installation:
& nbsp;& nbsp; It is very important to choose the installation point correctly and install the sensor correctly. If the installation is wrong, the measurement accuracy will be affected, the service life of the sensor will be affected, and even the sensor will be damaged.
& nbsp;& nbsp; 2.1.1 selection of installation site:
& nbsp;& nbsp; 2.1.1.1 environmental temperature: - 40 ℃ ≤ t ≤ 55 ℃. If the thermal radiation of production equipment is affected, thermal insulation and ventilation measures shall be taken.
& nbsp;& nbsp; 2.1.1.2 ambient air: the flowmeter should not be installed in the environment containing corrosive gas. If it must be installed, ventilation measures must be taken.
& nbsp;& nbsp; 2.1.1.3 mechanical vibration and impact: vibration will produce interference signal. If the vibration and impact on the pipeline are strong and the medium flow rate is both low, the interference signal may be greater than the flow signal, resulting in metering error. Therefore, the flowmeter should be installed in the place with small vibration and impact, and the installation position should be at the vibration frequency of 5 ~ 20Hz, and the vibration acceleration should not be greater than 1G, otherwise vibration reduction measures should be taken.
& nbsp;& nbsp; 2.1.2 pipeline installation and requirements:
& nbsp;& nbsp; 2.1.2.1 the upstream and downstream sides of the sensor shall have enough straight pipe sections. The length of straight pipe section shall conform to table 1.
& nbsp;& nbsp; 2.1.2.2 within the specified straight pipe length, the inlet and outlet sections of the pipeline shall be straight by visual inspection, and the sensor shall not be installed downstream of the regulating valve and semi open gate valve as far as possible. Generally, the sensor shall not be installed after expanding the pipe.
& nbsp;& nbsp; 2.1.2.3 the sensor can be installed vertically, horizontally or at any other angle, but when the sensor is installed vertically or obliquely, the flow direction of the fluid should be from bottom to top.
& nbsp;& nbsp; 2.1.2.4 for the setting of pressure taking point and measuring point, the pressure taking point shall be outside 10d of the sensor and the temperature measuring point shall be outside 5D.
& nbsp;& nbsp; 2.2 wiring:
& nbsp;& nbsp; 2.2.1 the signal connection between vortex flowmeter measurement systems is shown in Figure 2
& nbsp;& nbsp; 1. 2 is the signal input terminal of the amplifier board, the signal is provided by the piezoelectric crystal probe, terminal C is the power supply terminal, connected to 12V + or 24V + of the integrator, B is the signal output terminal of the amplifier board, corresponding to the integrator F +, and a is the 0V terminal of the amplifier board, corresponding to the integrator F -. G is the grounding terminal of amplifier board. The amplifying board and the totalizer are connected by three core shielded wires.
& nbsp;& nbsp; 2.2.2 the pipes and the shell of the amplifying plate for installing the sensor shall be well grounded（ Grounding resistance R ≤ Ω） & nbsp;& nbsp;& nbsp; 3 maintenance of vortex flow sensor:
& nbsp;& nbsp; 3.1 probe maintenance:
& nbsp;& nbsp; three point one. 1 probe structure: as shown in Figure 3. The probe consists of two pieces of piezoelectric crystal, one for anti-seismic and the other for detection. The probe leads a and B are the two leads of the test piece.
& nbsp;& nbsp; 3.1.2 probe detection:
& nbsp;& nbsp; 3.1.2.1 test method of multimeter: 200m Ω Measure the resistance of a and B, a and shell, B and shell respectively, and the resistance value is ∞. If there is resistance value, the probe will be damaged. Measure the capacitance of a and B with a multimeter at 2n, and the capacitance value shall not be changed. When there is no load, the capacitance shall be constant between 0.15nf and 0.35nf. When there is external vibration or fluid flow, the capacitance value changes and is larger than 0.15nf ~ 0.35nf, then the probe is normal.
& nbsp;& nbsp; 3.1.2.2 oscillograph detection method: connect the probe with the ultra-low oscillograph as shown in Figure 4, tap the head of the probe, if there is irregular sine wave on the oscillograph screen (as shown in Figure 5), it means that the probe is in good condition, and if there is no similar waveform, the probe is damaged< BR>
& nbsp;& nbsp; 3.1.2.3 overall calibration detection method: on the premise that the amplifier board and totalizer are good, connect the probe, amplifier board and totalizer according to normal use, tap the head of the probe with an object, if the frequency or instantaneous flow of the totalizer changes, the probe is good, if there is no change, the probe is damaged.
& nbsp;& nbsp; The probe can not be repaired after it is determined to be damaged.
& nbsp;& nbsp; 3.2 maintenance of amplification board.
& nbsp;& nbsp; 3.2.1 schematic diagram of enlarged plate: (as shown in Figure 6)
& nbsp;& nbsp; The circuit consists of charge converter, voltage amplifier, active low-pass filter, trigger shaper, power supply neutral point circuit and voltage pulse conversion circuit.
& nbsp;& nbsp; 3.2.1.1 charge converter: composed of operational amplifier i1-1, R20, R11, R12, C11 ~ C14, it is a charge / voltage converter with two terminal input. The alternating charge signal output by voltage probe transistor is input by S1 and S2, and the voltage signal output by the converter is proportional to the amount of charge.
& nbsp;& nbsp; 3.2.1.2 voltage amplifier: it is composed of operational amplifier I1-2, R21, R22 and potentiometer W1. It is a voltage amplifier with adjustable gain.
& nbsp;& nbsp; 3.2.1.3 low pass filter: it is composed of operational amplifier i1-3, r31-r34, c31-c33 and d31, D32. R31, R32, C33 determine the cutoff point of the filter. Different aperture sensors need different cutoff points, which can be realized by changing C32, C33.
& nbsp;& nbsp; 3.2.1.4 trigger shaping unit: including trigger (i2-1, R41, R42, C02, C41, W2) and shaper (IC3).
& nbsp;& nbsp; 3.2.1.5 power supply neutral point: the detection amplifier is powered by a single power supply + 24 V, and the power supply neutral point circuit is composed of operational amplifiers i2-2, R01, R02 and C01 to provide + 12 V neutral point potential for the chairman amplifier.
& nbsp;& nbsp; 3.2.1.6 voltage pulse / current pulse electric appliance: voltage stabilizing block iC4 and resistors R03 and r04 constitute constant current voltage stabilizing circuit, triode T01 and resistor R05 constitute pulse transformation amplification circuit, which converts voltage pulse into current pulse.
& nbsp;& nbsp; 3.2.2 amplification plate test:
& nbsp;& nbsp; In the schematic diagram, there are four detection points: TP1, TP2, TP3, TP4. When the amplifier board is normal, its output waveform is as follows: (Fig. 7) < br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br > br >
& nbsp;& nbsp; When there is a problem with the amplifier board, the oscillograph is used to monitor the waveforms of the above four test points to judge where the fault occurs, and then the components are checked according to the sequence of capacitor, diode, regulator, integrated block and resistance to judge whether they are good or bad, and the components are replaced to repair the circuit.
& nbsp;& nbsp; In addition, there is a very simple way to judge whether the enlarged board is good or bad. If it is known that the secondary meter is good, connect the amplifier board with the totalizer according to normal use, touch the input end of the amplifier board by hand, and the observation window displays the output of the instrument. If the display frequency of the display instrument is 50 Hz or 100 Hz, it means that the amplifier board is intact. If there is no output or other frequencies, it means that the amplifier board is broken. It can be repaired according to the above method.
& nbsp;& nbsp; 4 selection of vortex flowmeter:
& nbsp;& nbsp; Instrument selection is an important work in instrument application. Many faults of instruments in practical application are related to the selection and wrong use of instruments. After more than two years of use of vortex flowmeter, we have learned a lot from its selection and use, which are very important for the selection of vortex flowmeter and the increase of measurement accuracy in the future
& nbsp;& nbsp; 4.1 the diameter of the flowmeter shall be selected according to the actual situation of the process. Generally, the diameter of the selected instrument shall be such that the minimum gas flow rate shall not be less than 7m / s and the liquid flow rate shall not be less than 0.7m/s.
& nbsp;& nbsp; 4.2 try to avoid that the frequency of normal flow is about 50 Hz, because it is not easy to detect the power frequency interference into the instrument, which will cause measurement error.
& nbsp;& nbsp; four. 3 in actual production, because the designed pipeline is generally too large, the actual fluid flow will be too small. If the vortex diameter is selected according to the original pipeline, the measurement will be inaccurate or even impossible. Therefore, it is very important to determine the lower limit of vortex flowmeter. The most suitable vortex street can be selected only when the lower limit flow rate is determined.
& nbsp;& nbsp; 5 Conclusion:
& nbsp;& nbsp; Through the description of the principle of vortex flowmeter, the analysis of the circuit, and the introduction of some methods and experience in the maintenance process. I believe you have a certain understanding of vortex flowmeter. I hope these maintenance methods and experience can be helpful to peers.
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