When the fluid flow is blocked by a nonlinear cylinder perpendicular to the flow direction, an obvious vortex will be generated on the downstream side of the cylinder, which becomes a Karman vortex street, and the formation of the vortex street is related to the fluid Reynolds number.
By measuring the separation frequency of vortex, the velocity and instantaneous flow rate of fluid can be measured. Strouhal number St is a dimensionless constant that can be determined by experiments. The linear part of the relationship between dimensionless constant St and Reynolds number-like Re function, that is, the linear measurement range of vortex flowmeter, can calculate the velocity of fluid in the pipe by detecting frequency f, and then calculate the volume flow from the velocity. The ratio of the number of pulses output in a period of time to the volume of fluid (the number of pulses flowing through a unit volume of fluid) is called the instrument coefficient (K coefficient).
The frequency of vortex separation used to measure the flow velocity varies with the flow velocity and is not affected by the density and viscosity of the fluid. Pressure pulsation caused by eddy current separation is detected by piezoelectric pressure sensor probe and converted into pulse signal corresponding to eddy current frequency in detection circuit. The signal converter converts the pulse signal into a standard current signal of 4-20mA and outputs it.
Vortex flowmeter consists of sensor and converter. The sensor includes vortex generator (resistance fluid), detection element and surface body, etc. The converter includes preamplifier, filter shaping circuit, output interface circuit of D/A conversion circuit, terminal, bracket and protective cover.
1, the possible causes of this failure and the corresponding solutions are as follows:
The possible causes of (1) failure are power failure or power failure. So check whether the power supply voltage meets the requirements and whether the polarity of the power supply is correct. Such as current output vortex flowmeter, if the polarity of power supply is reversed, the output current is 0mA instead of 4mA.
(2) The possible fault reason is that the connecting cable is broken or wrongly connected. It is necessary to check whether the line is clear. If there is any fault, please reconnect the line and check it.
(3) The possible fault reason is that the ripple coefficient of the power supply is too large. When checking, the filter electrolytic capacitor can be used to test. Generally speaking, a capacitor is needed. 100 microfarads, 50 volts.
(4) The possible fault reason is that a certain stage of the amplifier board is faulty. You can observe the changes of the output by observing the waveforms of the input and output terminals of the amplifier, or injecting artificial induction signals from the input terminals. If the output does not change, it means that the amplifier is not working properly. In actual maintenance, the motherboard can also be used to replace other types of flowmeters for on-site inspection. Make sure that the amplifier board is faulty and should be replaced.
(5) The possible cause of the fault is the damage of the generator. Firstly, the detection method improves the flow rate and replaces the vortex main board with the same type vortex flowmeter. If the motherboard fails, please check the generator.
The damage of vortex generator often occurs in steam measurement system. Because the technician opens the valve too fast, the high-temperature and high-pressure steam with condensed water violently impacts the vortex generator, resulting in its damage, which is commonly called "water hammer" phenomenon. Therefore, it is necessary to open the guide shower to discharge condensed water, and then slowly open the steam valve to avoid damaging the vortex generator.
(6) The cause of the fault may be that the pipeline flow rate is too low, which does not enter the measurement range. You can increase the flow to see the response of the flowmeter. Tap regularly and continuously near the pipeline near the flowmeter, and no matter what type of flowmeter, the flow display will appear. If the medium to be tested is liquid, it is best to test it with an empty tube, because the liquid itself is a vibration absorber. The fault treatment method is to replace the flowmeter with smaller diameter or increase the flow rate when the process conditions permit.
(7) The possible cause of failure is whether the gap between the generator and the shell is stuck by fine particles. This phenomenon usually occurs after normal work, and then stops and starts again. If so, when it is confirmed that the conversion amplifier and generator are intact, especially when the measuring medium is liquid, it can generally be considered as this kind of fault. The treatment method is to turn on the flow as much as possible when the production process conditions permit, and tap vertically on the welding table at the lower part of the flowmeter with a wooden or rubber hammer, so that the negative pressure generated by the high-speed fluid takes away the fine solid particles. If disassembly and cleaning are allowed on site, the generator can be disassembled, but it is required to be familiar with the structure of the flowmeter, especially the gasket and pressure plate screws, and must be installed in strict accordance with the requirements of the instruction manual. At the same time, we should pay attention to the generator reinstalled after disassembly, which may bring some changes to the flow coefficient. Although it has not changed much, it needs to be corrected.
(8) Check whether there is any error in parameter setting according to the instruction manual, and if there is, please correct it.
2. There is no flow after power-on, only output.
(1) Detect whether there is vibration. Most vortex flowmeters have two-dimensional anti-vibration performance. That is, it can resist the external vibration interference in the direction of fluid resistance and vortex lift, but it can not resist the interference in the direction of vortex lateral thrust.
The most direct detection method is to feel the vibration of the pipeline by hand. External mechanical vibration reduction measures can be adopted, such as using rubber soft joints and gaskets to reduce vibration, and fixing pipes with brackets. In addition, many manufacturers also consider the need of vibration prevention when designing vortex flowmeter. Adjusting some parameter settings can also reduce the influence of vibration on the measurement results, but pay attention to the influence of these parameter settings on the upper and lower limits of flowmeter flow.
(2) Whether the detection output is about 50Hz power frequency interference can be detected by a frequency meter. The solution is to use shielded cable to rewire according to regulations.
(3) The detected output frequency is any constant frequency or constant output current value, and the possible fault reason is that the amplifier board is damaged, resulting in self-excitation. The solution is to replace the amplifier plates 28-30.
(4) Check whether there is high-voltage equipment or high-frequency interference near the flowmeter. In particular, the high-frequency harmonics generated by the welding machine will make the flowmeter have no flow and no signal.
(5) Check whether the junction box is flooded. Especially the vortex flowmeter with current output mode, because of the conduction of water, a bypass current is formed between the positive and negative terminals, which may cause the phenomenon of no flow and no signal.
(6) The possible fault reason is that the amplification factor or trigger sensitivity of the amplifier board is too high. The solution is to debug the magnification and sensitivity, observe the influence on the output, and make necessary adjustments.
(7) The possible fault reason is that the pipeline valve is not completely closed and there is leakage. At this time, it is necessary to check the pressure and valve closure on site.
3. The flow output is unstable
(1) The possible cause of the failure is that the flowmeter is not installed concentrically. The detection method is to remove the instrument to check the position of flange and pipeline. If it is caused by eccentric installation, reinstall the instrument.
(2) The possible fault reason is that the straight pipe section is not enough or the inner diameter of the pipe is inconsistent with the inner diameter of the instrument. It is necessary to detect the inner diameter of the pipeline and the inner diameter of the instrument. If necessary, change the installation position of the instrument or lengthen the front and rear straight pipe sections.
(3) The possible fault reason is that the instrument is not properly selected. If the choice is wrong, the ordinary flow is in the nonlinear region, even near the critical dead zone, then the flow fluctuation may occur. If process conditions permit, increase the flow. If the flow rate is stable, replace the flowmeter with smaller diameter.
(4) The possible reason is that there is liquid-gas or liquid-solid or gas-solid two-phase flow in the fluid. Vortex flowmeter can only measure single-phase flow, but not two-phase flow, which is determined by the working principle of vortex flowmeter. Generally speaking, the existence of liquid-gas, liquid-solid and gas-solid two-phase flow is judged by experience or according to the fluid properties, working conditions, pressure and temperature. If the medium is liquid ammonia, light oil and other volatile liquids, it is easy to appear this phenomenon. For liquid, if it is a liquid-solid two-phase flow, a filter can generally be installed at the upstream end of the flowmeter. If it is a liquid-gas two-phase flow, a deaerator can be installed at the upstream end of the flowmeter.
(5) The possible fault reason is that the flowmeter and flow regulation control system produce system oscillation. Turn the valve opening controller of the regulating control system to manual gear. When the valve is opened to a certain value, if the flow display is relatively stable or slightly fluctuating, it indicates that there is system oscillation. The general processing method is to reset PID parameters.
(6) The possible cause of the fault is that the generator is wound with fibers. If the generator is wrapped with fiber, the vortex force of the vortex will be greatly weakened. Because of the vortex, the fiber swings with the fluid, which leads to the fluctuation of vortex and flow display. You can delete the test, clean it, and then reinstall it.
(7) The possible fault reason is that the input channel of the conversion amplifier is damaged all the way. According to the working principle, when there is no signal all the way, the signal-to-noise ratio is greatly reduced, the background noise is not attenuated and enters the post-processing, and the spike pulse will cause the instability of the flow display. The human body induction signal can be injected from the input end and the change of the output end can be observed. In actual maintenance, the motherboard can also be used to replace other types of flowmeters for on-site inspection. Make sure it is faulty and should be replaced.
(8) The possible cause of the failure is that the instrument parameters are set incorrectly. Check all parameters according to the instructions. If there are any errors, please reset them.
4. The flow measurement error is large.
In addition, it may also be caused by the following reasons.
(1) There is no real-time temperature compensation or fixed temperature and pressure compensation for measuring gas or steam. The treatment method is to install temperature and pressure compensation elements or set fixed value compensation.
(2) Saturated steam is not saturated when measuring steam. The treatment method is to detect the process flow and change the process conditions.
(3) Check whether the parameters of the secondary instrument are set correctly. The input mode of the auxiliary secondary instrument must match the flowmeter, otherwise an error will occur.