Analysis of inaccurate measurement of turbine flowmeter during use
Turbine flowmeter is a new type of intelligent instrument that integrates turbine flow sensor and display totalizer, which is developed by using advanced ultra-low power single-chip microcomputer technology. It has obvious advantages such as interference from external power supply, lightning resistance and low cost.
It can maintain high-precision measurement during use. The performance of the turbine flowmeter is superior, the service life is very long, and it has a strong anti-interference ability. The turbine flowmeter may encounter various electromagnetic interference. The anti-interference ability is not good enough, which may affect the measurement accuracy of the flowmeter.
Since the turbine flowmeter is sensitive to the conditions of the site, the user should meet its ideal requirements under the condition of maximum satisfaction, so as to maintain the accuracy of the factory instrument and obtain ideal measurement data. When the temperature and pressure of the fluid medium are quite different from the reference conditions, the volume change of the sensor housing can be calculated according to the material and temperature and pressure of the sensor, and the meter reading can be corrected. When the viscosity of the fluid medium in the working state is quite different from the viscosity of the calibration fluid, it is necessary to correct the parameters of the viscosity correction curve provided in the installation and operating instructions.
Turbine flowmeters have certain requirements on the length of the upstream and downstream straight pipe sections in the measurement, which must be met according to the installation requirements, especially when the user requires high precision. The meter displays the volume flow under the working conditions of the medium. To know the standard volume flow or mass flow, density compensation is also required.
The accuracy of the turbine flowmeter is determined under reference conditions (or standard working conditions). For turbine flowmeters, the reference conditions generally include: environmental conditions (ambient temperature 15℃～35℃, relative humidity 45%～ 75%, atmospheric pressure 86kPa, ~ 108kPa, no electric field, magnetic field interference, no vibration); power source condition (220V±10% or 110V, 50HZ±1, or 24VDC); fluid condition (when calibrating with fluid, calibrate the pipeline It is an industrial round tube, single-phase Newtonian fluid, fully full tube, fully developed turbulent flow velocity axis symmetrical distribution, no vortex, no disturbance, steady flow. The fluid used for calibration is usually water, oil, air, selected according to needs. In the turbine There are long enough straight pipe sections upstream and downstream of the flowmeter. The fluid temperature is generally room temperature and the fluid pressure is generally below 0.25MPa). Obviously, the actual working conditions of the turbine flowmeter are usually different from the reference conditions, which will cause the accuracy of the turbine flowmeter to change.
Then the accuracy of the turbine flowmeter decreases during use. It may be due to the following possible factors that cause the measurement to be unstable. How to effectively avoid it:
1. The fibrous or viscous impurities in the fluid are attached to the rotating part of the flowmeter, which increases the rotating resistance, reduces the indication value of the meter, and causes negative errors, which is unfavorable to the fluid supplier.
2. Changes in fluid temperature and pressure may cause the liquid in the pipeline to escape the air contained in the pipeline, or because the pressure in the pipeline is lower than the saturated vapor pressure of the fluid, part of the liquid will become steam, or the negative pressure of the medium may cause the external air. The gas is sucked into the pipeline, and these gases flow with the liquid to be measured, causing the indication value of the instrument to increase, resulting in a positive error, which is unfavorable to the demand side of the fluid.
3. There are generally some impurities in the medium, which will cause wear and tear on the bearings and shafts, which will increase the gap between the two, the dynamic balance of the moving parts will be destroyed, the speed will drop, or the dirt will enter the gap, which will increase the movement resistance. , the speed drops. All of these reasons cause the display value of the instrument to decrease and negative errors to appear, which is unfavorable to the fluid supplier.
4. The harsh working environment will also have a great impact on the measurement results, such as electromagnetic field interference, dust, high temperature, vibration, humidity, etc., which may cause the malfunction or failure of the turbine flow sensor, which directly causes the indication error of the turbine flowmeter. , the error may be positive or negative, it may not be obvious, or it may fail completely. For the above phenomenon, when it is more serious, problems can be found from the comparison of the working status of the process, and it is easy to take corresponding measures. However, in the early stages of the problem, the problem cannot be detected without taking special measures.
Turbine flowmeter is a common measuring instrument in flow, which can measure various gases and liquids. It is a kind of turbine that is supported by bearings at both ends in the center of the measuring pipe. When the fluid flows through the pipe, it impacts the turbine blades and drives the turbine, so that the turbine can overcome the fluid resistance torque and friction torque to generate rotation. In a certain flow range, for a certain fluid medium viscosity, the rotational angular velocity of the turbine is proportional to the fluid velocity. Therefore, the fluid flow rate can be obtained from the rotational angular velocity of the turbine, and the fluid flow through the pipeline can be finally obtained by calculation.
