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Common fault problems of flowmeter Posted by : admin / Posted on : Apr 03, 2020

On-site verification of the flow measurement system generally includes zero calibration and indication calibration other than zero. Normally, it seems that the zero calibration is performed first. After the zero is normal, the indication of other points other than zero will be performed if conditions permit. Value verification. If the zero point is abnormal, you should generally find the cause first, and then check the indication of other points after making it normal. Calibration of the zero point of the flow measurement system When using the field to calibrate the zero point of the flow measurement system is no different from the calibration method in the laboratory, it is to make the flow through the flow meter to zero, and then read the flow meter Indication. It is just that the conditions for using the site are not ideal for the laboratory, and there are many disadvantages. Experienced engineering supervisors or acceptance personnel should check the zero-point indication of the flowmeter before checking and accepting the flowmeter, because this verification is the easiest to implement and the most important. During the calibration, the flowmeter can neither exist out of nothing nor point to a negative value.
Pay attention to the following points when zeroing:
  ① Ensure that the fluid flow through the flowmeter is indeed zero. This is the basis for zeroing the flowmeter. There are not many people who can achieve no leakage after the shut-off valve is used on site for a period of time. Therefore, when proofreading the zero point, you need to confirm this to avoid self-defeating.
  ② The measuring channel of the flowmeter must be filled with the measured medium. This is particularly important for electromagnetic flow meters. Because most electromagnetic flowmeters will point to the degree value when the tube is empty, this is due to the open circuit between the electrodes when the measuring tube is empty, so that the indicated value exceeds full scale.
  ③ Small signal removal problem. For a flowmeter output with an analog signal, the analog circuit inevitably drifts slightly, resulting in a slight offset in the zero point. It is usually solved by the method of trumpet cutoff. This method also has disadvantages, because the small flow signal below the cutoff point is also cut off together, so the cutoff point cannot be set too high. After the flow meter is universally programmable, the cut-off point can be set arbitrarily as needed, providing an effective means for solving this difficulty. However, it should be noted that some transmitters (such as differential pressure transmitters) have a certain inclination due to installation, or are subjected to mechanical stress, resulting in zero drift, which cannot be solved by the method of small signal cut-off, only zero calibration Method to solve. The vortex flowmeter measures the liquid. The tube is filled with liquid and surrounds the sensor with good damping. If the measuring tube is filled with gas, because the density and viscosity of the gas are much smaller than the liquid, the damping characteristics are poor, the vibration of the pipe or plant, or even the strong vibration of the surrounding air, will cause the value expressed by the instrument to be "out of nothing." .
  ④ The effect of vibration on the zero point of the vortex flowmeter. When the measuring tube is filled with the measured medium, if the zero point indication value is too high, that is, there is a phenomenon of "nothing occurs". Generally, it can be adjusted by noise balance (NB) and trigger level adjustment (TLA). The output returns to zero. However, if the vibration at the installation site is severe, it is often impossible to solve the problem with the method of instrument adjustment, because adjusting the trigger level too high, or adjusting the amplifier gain (GAIN) too small, will inevitably lead to an increase in the measurable minimum flow value. Even when the flow is large, the signal generated by the vortex street is still lower than the trigger threshold, and is filtered as noise. At this time, you have to find another way, such as reducing vibration, replacing with a better vibration resistance instrument.
  ⑤ Vortex flowmeter is easy to introduce interference at zero flow. The main reason why the vortex flowmeter is easy to accept external interference during zero calibration is because of the variable gain characteristics of its sensor front amplifier. Taking a piezoelectric sensor as an example, since the output amplitude of the sensor is proportional to the square of the flow velocity flowing through the measuring tube, the higher the flow velocity, the larger the output amplitude of the sensor; otherwise, the output amplitude is smaller. The weak signal is sent to the front amplifier of the vortex flowmeter. This amplifier uses a variable gain amplifier to amplify the frequency signal with a large amplitude difference to a signal with approximately equal amplitude. That is, when the flow rate is high, the input frequency is high and the gain is small. When the flow rate is low, the input frequency is low and the gain is large. Of course, when the input frequency is zero, the gain is the largest. At this time, various interferences are amplified by many times without discrimination, and higher than the threshold value of the trigger, and finally sent as a signal To the output.