The choice of electromagnetic flowmeter is mainly the correct choice of the transmitter, and the converter only needs to be matched with it.
Choice of caliber and range
The diameter of the transmitter is usually the same as that of the piping system. If the piping system is to be designed, the diameter can be selected according to the flow range and flow rate. For electromagnetic flowmeters, the flow rate is suitable for 2 - 4m / s. In special cases, such as solid particles in the liquid, in consideration of wear, the optional flow rate is ≤ 3m / s, for easy to manage the fluid. Available flow rate ≥ 2m / s. After the flow rate is determined, the transmitter diameter can be determined according to qv= D2.
The range of the transmitter can be selected according to two principles: one is that the full scale of the meter is greater than the expected maximum flow value; the other is that the normal flow is greater than 50% of the full scale of the meter to ensure a certain measurement accuracy.
Temperature and pressure selection
There are certain restrictions on the fluid pressure and temperature that the electromagnetic flowmeter can measure. When used, the pressure used must be lower than the working pressure specified by the flowmeter. At present, the working pressure specifications of domestically produced electromagnetic flowmeters are:
Less than 50mm caliber, working pressure is 1.6MPa; 900 mm caliber, working pressure is 1 MPa;More than 1000mm caliber, working pressure is 0.6MPa.
If there are special requirements on the pressure resistance of the transmitter, you can negotiate with the manufacturer.
The operating temperature of the electromagnetic flowmeter depends on the lining material used, which is generally 5 - 70 ° C. Such as special treatment, can exceed the above range, such as the wear-resistant corrosion-resistant electromagnetic flowmeter produced by Tianjin Automation Instrument No.3. The transmitter allows the measured medium temperature to be -40 to 130 °C.
Selection of lining material and electrode tree material
The lining material and electrode material of the transmitter must be correctly selected according to the physical and chemical properties of the medium. Otherwise, the instrument will be damaged quickly due to the corrosion of the lining and the electrode, and the corrosive medium will easily cause an accident if it leaks. Therefore, the electrode and the lining material must be carefully selected according to the specific measurement medium in the production process.
Several problems of solutions
Recalculating the differential pressure scale
Temperature and pressure compensation can only reduce the measurement error, not only can not solve the problem fundamentally, but also the measurement signal exceeds 20mA, resulting in steam leakage measurement. The transmitter measurement signal exceeds 20 mA, indicating that the actual measured differential pressure signal ΔP exceeds the design differential pressure value.
Increase temperature and pressure compensation
When the temperature and pressure of the steam change, the density of the steam changes, and the steam flow measurement produces an error. Measurement error can be reduced by temperature and pressure compensation. Since the temperature of the saturated steam is a single-valued function of the pressure, the temperature and pressure compensation of the saturated steam can be pressure compensated or temperature compensated. Because the pressure signal detection is sensitive and the compensation accuracy is high, it is compensated by pressure and realized by DCS.
Conclusion
Steam is a special medium. As the pressure and temperature change, the density of steam changes. Therefore, it is necessary to compensate for temperature and pressure. When the pressure and temperature fluctuation of the steam are not large, that is, when the operating condition parameters deviate from the design parameters and the influence on the measurement is small, the temperature and pressure compensation measures can achieve the purpose of accurate measurement. However, when the operating parameters deviate too much from the design parameters or the operating parameters fluctuate frequently and are too large, even with the temperature and pressure compensation, it is difficult to meet the measurement accuracy requirements. At this point, only differential pressure or flow can be recalculated for a particular throttling element.
Ultrasonic flowmeter common problems (2)
5, the external clip type ultrasonic flowmeter signal is low?
This depends on the technical content of the instrument itself. After a large number of test cases on the spot, it proves that the long-term pipeline, serious scaling and large diameter, the Aitulel AFTU-2W series is better than other external-clip ultrasonic flowmeters. The signal is very fast and the signal is very stable. .
Solution: For large diameter and serious scaling, it is recommended to use a good quality external clamp type ultrasonic flowmeter. The pipe at the probe installation should be cleaned, and the air between the probe and the workpiece surface should be removed with a coupling agent or coupling piece to make the ultrasonic wave. It can be effectively introduced into the pipeline to ensure sufficient sound intensity transmission on the detection surface.
6. Occasionally, there are bubbles in the measurement medium. Does the ultrasonic flowmeter with the time difference method have any influence?
Today's Aitulel AFTU-2W series external clip type ultrasonic flowmeter has dual mode. When there is air bubble, it can be automatically transferred to Doppler mode to measure. When the bubble disappears, it will automatically turn into the time difference method.
7. The meter cannot be used under strong interference on the spot?
On-site inverter or high voltage cable field strong electromagnetic interference
Recommendation: Keep away from the inverter or high voltage cable field electromagnetic interference
8. What is the minimum diameter of the externally clamped ultrasonic flowmeter on the market? What is the highest temperature?
Answer: Now only the minimum diameter of the AFTU-2W series in the world can be measured to 6mm, the temperature can be measured at 550 °C, such as the measurement of molten salt and heat transfer oil, which is not possible with other brands of flowmeters. of.
9. How to choose a suitable ultrasonic flowmeter?
Answer: Pipe material, pipe wall thickness and pipe diameter; fluid type, contain impurities, air bubbles and full pipe; fluid temperature, flow meter type, portable or fixed online.