The reason for the decrease in accuracy during the measurement of the turbine flowmeter and the corresponding solution. The turbine flowmeter is a turbine that is supported by a bearing at the center of the measuring pipe. When the fluid flows through the pipe, the turbine blade is impacted and the turbine is generated. Driven to cause the turbine to rotate against fluid resistance torque and frictional torque. Within a certain flow range, for a given fluid medium viscosity, the rotational angular velocity of the turbine is proportional to the fluid flow rate. Thus, the fluid flow rate can be obtained by the rotational angular velocity of the turbine, whereby the fluid flow through the conduit is ultimately achieved by calculation.
Since the turbine flowmeter is sensitive to the conditions on site, the user should meet its ideal requirements in the case of large satisfaction, in order to maintain the degree of factory instrumentation and obtain the desired measurement data. When the temperature and pressure of the fluid medium differ greatly from the reference conditions, the volume change of the sensor housing can be calculated according to the material and temperature of the sensor, and the meter reading can be corrected. When the viscosity of the fluid medium is significantly different from the viscosity of the calibration fluid, the viscosity correction curve provided in the installation and operation instructions is corrected.
Moreover, the turbine flowmeter has certain requirements for the length of the straight pipe section upstream and downstream in the measurement, and it is required to meet the installation requirements, especially when the user demands high precision. The meter displays the volumetric flow under media operating conditions. For standard volume flow or mass flow, density compensation is also required.
The degree of the turbine flow meter is determined under the reference conditions (or standard operating conditions). For the turbine flow meter, the reference conditions generally include: environmental conditions (ambient temperature 15 ° C ~ 35 ° C, relative humidity 45% ~ 75) %, atmospheric pressure 86kPa, ~108kPa, no electric field, magnetic field interference, no vibration); power source conditions (220V ± 10% or 110V, 50HZ ± 1, or 24VDC); fluid conditions (when the fluid is calibrated, the calibration pipeline is Industrial round tube, single-phase Newtonian fluid, fully filled, fully developed turbulent velocity, axisymmetric distribution, no vortex, no disturbance, constant flow. The calibration fluid is usually water, oil, air, as needed. There are long straight sections in the upstream and downstream of the meter. The fluid temperature is generally room temperature and the fluid pressure is generally below 0.25 MPa. It is clear that the actual operating conditions of the turbine flow meter are often different from the reference conditions, which can cause a change in the degree of the turbine flow meter.
The turbine flowmeter is degraded during use and may be due to the following reasons:
1. The fibrous or viscous impurities in the fluid are attached to the rotating part of the flowmeter, so that the rotational resistance is increased, causing the indication value of the meter to decrease, and a negative error occurs, which is unfavorable to the supply side of the fluid.
2. Changes in fluid temperature and pressure may cause air in the pipeline to escape from the air contained in the pipeline or because the pressure in the pipeline is lower than the saturated vapor pressure of the fluid, causing some of the liquid to become steam, or may be caused by the negative pressure of the medium. The gas is sucked into the pipeline, and these gases flow with the liquid to be measured, causing the meter to increase in value and a positive error, which is unfavorable to the demand of the fluid.
3, the medium generally has some impurities, the bearing, the shaft should wear, so that the gap between the two increases, the dynamic balance of the moving parts is destroyed, the speed decreases, or the dirt enters the gap, so that the motion resistance increases The speed drops. For these reasons, the display value of the meter is reduced, and a negative error occurs, which is unfavorable to the supplier of the fluid.
4. The harsh working environment will also have a large impact on the measurement results, such as electromagnetic field interference, dust, high temperature, vibration, humidity, etc., which may cause malfunction or malfunction of the turbine flow sensor, directly causing the indication error of the turbine flowmeter. The error is positive or negative, may not be obvious, or may be completely invalid. For the above phenomenon, when it is more serious, problems can be found from the comparison of the working state of the process, and it is easy to take corresponding measures. However, in the early stages of the problem, problems cannot be discovered without special measures.
In summary, according to the principle of turbine flowmeter and years of field work experience, Xiyuan Instrument Technology recommends that users install filters on the upstream of the dirty flow or liquid that may generate gas. Or air eliminator, air collector, strengthen daily maintenance work, regularly clean the filter, remove gas or debris in the air eliminator, collector, and ensure the normal operation of the flow meter. In the initial stage of installation of the turbine flowmeter, a pair of pressure measuring ports are added to the pipe wall of the pipe diameter of the upstream and downstream of the flowmeter. When necessary, it is connected with the differential pressure gauge. In the working state, the flowmeter indication value and difference are recorded. The contrast relationship between the pressure gauge indication values. If everything is normal at this time, use this comparison relationship as the basis for checking whether the flowmeter works normally. This comparison relationship is measured periodically and the record is tracked. If the flowmeter indication value is too small under the same differential pressure gauge indication value, the flowmeter may be suspected to be malfunctioning. The method of checking the flowmeter's sensor can be done by first checking the transmitter. The method is to remove the sender separately, input a known signal, and find the problem by comparing the output values. If the fault cannot be eliminated for a short time, you can replace it with a new one. If it is not the cause, the flowmeter should be removed from the process line and further carefully examined for symptomatic treatment.
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