There are many factors that affect the accuracy of the metal tube variable area flow meter. In addition to the design and structure of the instrument itself, the flow characteristics and working conditions of the fluid will affect the measurement accuracy of the flow meter. The factors that affect the accuracy of the rotameter can be roughly divided into the following categories:
Processing accuracy of tapered tube and float
The metal tube variable cross-section flow meter is to verify the height of the flow meter float, and then display the scale on the panel through the principle of magnetic coupling, so the uniformity of the taper of the tapered tube will affect the accuracy. When the uniformity of the conical tube is poor, increasing the number of real points can improve accuracy.
Flow coefficient
The flow coefficient of the metal rotameter is measured from experiments, so there are natural experimental errors. The accuracy of the C-Re curve has a great influence on the accuracy of the rotameter. Especially when the Reynolds number is low, or when the measurement is in the transition zone between laminar flow and turbulent flow, the accuracy will be affected to a certain extent. At this time, it is best to use real flow verification.
Standard accuracy
The uncertainty of the measurement standard itself affects the accuracy of the flow meter scale during the mass transmission test.
The influence of physical conditions
The measurement of fluid density, viscosity, temperature, and pressure in the field of use will also affect the measurement accuracy of the flow meter, especially in the case of gas flow measurement, it is obvious that the viscosity change of high-viscosity liquid due to temperature changes affects the measurement accuracy. Factors such as the high center of gravity and eccentricity of the float will cause left and right swings during work, and the rapid rotation of the float off-axis in place will affect the measurement accuracy. (At this time, the flow rate is often too large, that is, the float shakes to make the flow area smaller).
Whether the verticality of the installation meets the standard
If the metal tube variable area flow meter is installed incorrectly, errors will be introduced, so it should be installed vertically or horizontally as required.
Human error
Verification by skilled personnel can control the error to a minimum. For example, the soap film tube is used to verify the flow meter, the chronograph stopwatch, and the volume accuracy of the soap film tube has been calibrated, but the judgment ability and proficiency of the personnel will affect the indication value of the flow meter.
The influence of air bubbles and attachments
In the part where air bubbles enter, the influence on the measurement is obvious. In the case of small flow, especially when bubbles are attached to the float and conical tube, it will affect the buoyancy and also affect the flow area, so special attention should be paid at this time.
How to ensure the measurement accuracy of the metal tube variable area flow meter?
In order to ensure the normal operation of the metal tube variable area flow meter and achieve the required measurement accuracy, the following points should generally be noted:
- The metal tube variable cross-section flow meter must be installed vertically on a non-vibrating pipeline. The fluid flows through the flow meter from bottom to top, and the verticality is better than 2°. When installed horizontally, the horizontal angle is better than 2°. In order to facilitate maintenance and Replace the flow meter, clean the measuring pipe, and add a bypass and a bypass valve to the metal tube rotameter installed on the processing pipeline.
- There should be a straight pipe section at least 5 times the pipe diameter at the inlet of the metal pipe variable area flow meter. There should be a 250mm straight pipe section at the outlet.
- If the medium contains ferromagnetic substances, a magnetic filter should be installed; if the medium contains solid impurities, it should be considered to install a filter between the valve and the straight pipe section.
- When used for gas measurement, it should be ensured that the pipeline pressure is not less than 5 times the pressure loss of the flow meter, so that the floater can work stably.
- When heat insulation protection measures are required, if the temperature of the measured medium is higher than 220°C or the fluid temperature is too low, it is easy to crystallize. The jacket type should be selected for cooling or heat preservation.
- Pipe flanges, fasteners, gaskets, and flow meter flanges must be of the same standard to ensure normal installation and operation of the meter.
- The failure of the metal tube variable area flow meter generally occurs when the device is just started. Because the pipe is not purged cleanly, the floater is stuck by solid particles. At this time, the pointer of the indicator stops at a certain position. At this time, the valves on both sides of the flow meter should be closed first, then the upper flange should be removed, the float should be taken out for cleaning, and then reinstalled. Note that the nuts on the upper flange should be tightened in a balanced manner, and the washers should be filled.
- The flange of the processing pipeline must be coaxial with the flange of the flow meter and parallel to each other, in order to avoid the deformation of the flow meter caused by the pipeline. Properly support the pipeline to avoid pipeline vibration and reduce the axial load of the flow meter. The control valve in the measurement system should be installed downstream of the flow meter.
- At least 10cm around the installation, since the instrument transmits signals through magnetic coupling, in order to ensure the performance of the instrument, no ferromagnetic substances are allowed. A meter that measures a gas calibrated at a specific pressure if the gas is vented directly to the atmosphere at the outlet of the meter. There will be an air pressure drop at the float and distorting the data. If this is the working condition, a valve should be installed at the outlet of the meter.
- Special care must be taken when installing PTFE-lined gauges due to uneven pressure. PTFE will deform, so the flange nuts should be tightened evenly and symmetrically.
- When installing the metal tube variable area flow meter, try to avoid direct sunlight on the display and the instrument with a liquid crystal display. So as not to reduce the life of the LCD.
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