Vortex shedding flow meters are mainly used for measuring industrial pipeline gas, liquid, steam and other media. They are characterized by small pressure loss, large measuring range, high accuracy, and are almost unaffected by fluid density, pressure, temperature, viscosity and other parameters when measuring working volume flow. There are no movable mechanical parts, so the reliability is high, the maintenance is small, and the instrument parameters can be stable for a long time. Our company uses vortex flow meters mainly for flow measurement of steam, water, nitrogen, and raw materials.
The measurement principle of Vortex shedding flow meters
The vortex flow meter is based on the measurement principle of Karman vortex street. A barrier is installed in the pipeline, and Karman vortex street is generated behind it, and alternating pressure is generated on both sides of the barrier. The frequency is proportional to the flow rate of the medium. By detecting this frequency value, it is converted and displayed by an electronic converter and outputs a standard current signal.
Selection of installation location for Vortex shedding flow meters
(1) Strong vibration of pipelines will have a certain impact on the measurement of Vortex shedding flow meters. Therefore, when selecting the installation location of vortex flow meters, it should be avoided to install them on pipelines with strong vibrations to avoid affecting the measurement accuracy. When the pipeline vibrates, vibration reduction measures must be taken.
(2) The existence of power frequency interference signals will also have a very large impact on the measurement of Vortex shedding flow meters. The power frequency signals will be superimposed on the measurement signals. Therefore, vortex flow meters should be avoided as much as possible in environments where high-power motors exist. In this environment, it is necessary to take measures such as grounding the instrument, selecting shielded cables, and using DC signals for signal transmission to eliminate power frequency interference.
(3) The frontal surface of the vortex generator of Vortex shedding flow meters must face the direction of fluid flow. Special attention should be paid during installation, otherwise it will cause very large deviations.
(4) In a system with flow regulation of Vortex shedding flow meters, even if the straight pipe section requirements are met, the vortex flow meter must be installed in front of the regulating valve. Otherwise, the jet generated by the regulating valve will affect the measurement of the vortex flowmeter, and the flow rate will increase when the valve is opened smaller.
Common fault diagnosis and troubleshooting
Based on the previous use and maintenance of vortex flowmeters, some common fault phenomena and analysis and troubleshooting methods of vortex flowmeters are classified and summarized as follows.
Fault phenomenon: There is flow in the pipeline, but the vortex shedding flow meters have no signal output
(1) Cause analysis ① The vortex flowmeter power supply is incorrect; ② The vortex flowmeter amplifier board wiring is virtual or broken; ③ The amplifier is damaged.
(2) Troubleshooting ① Check whether the wiring is correct and whether the power supply is normal; ② Deal with the virtual connection or broken wire of the vortex flowmeter amplifier board wiring; ③ Replace the amplifier.
Fault phenomenon: There is no flow in the pipeline, but the sensor has signal output
(1) Cause analysis ① Poor grounding, introducing interference; ② Large pipeline vibration; ③ Excessive sensor gain.
(2) Troubleshooting ① Check the grounding resistance, which should be no greater than 10Ω, otherwise deal with the grounding; ② Perform shock absorption treatment on the process pipeline; ③ Adjust the gain potentiometer to an appropriate value.
Fault phenomenon: There is normal flow in the pipeline, but the sensor output frequency is 50Hz
(1) Cause analysis: There is a high-power motor or a relatively strong electric field near the flowmeter.
(2) Troubleshooting: Strengthen the shielding of the signal line and change the original pulse signal to a standard current signal.
Fault phenomenon: The sensor output signal is unstable at normal flow
(1) Cause analysis ① Use an oscilloscope to check whether the output signal is missing pulses or multiple pulses. The former indicates that the gain is too small; the latter indicates that the gain is too large; ② The flow meter is not installed correctly, and the flow field vibrates.
(2) Troubleshooting ① Adjust the gain potentiometer to an appropriate value; ② Install it according to the requirements of the straight pipe section before and after the flow meter.
Fault phenomenon: When adjusting the flow, the flow rate is inversely proportional to the valve opening.
(1) Cause analysis: The regulating valve is installed in front of the flow meter, and a jet is generated at a small flow rate.
(2) Troubleshooting: Modify and install the regulating valve behind the flow meter.
Maintenance of Vortex shedding flow meters
The tasks of daily maintenance are: pay close attention to the low battery indicator on the display panel of the vortex flow meter. If there is a flashing phenomenon, the internal battery should be replaced as soon as possible; regularly check the shielding and grounding conditions to eliminate external interference. Sometimes the indication problem is caused by interference; regularly clean the probe of the vortex flow meter. If the probe detection hole is blocked by dirt, it will affect the normal measurement.
