What are the factors that affect the measurement accuracy of mass flow meters?

The main factors that affect the measurement accuracy of mass flow meters include: zero point, slope, temperature, air content, blockage, wall hanging, mechanical vibration and electromagnetic interference.

1. Zero point impact

1.1 Zero point drift

The vibration tube is the detection unit of the mass flow meter, and most of them adopt a double-tube structure. Under the action of the driving unit, the two tubes of the vibrating tube vibrate. Due to slight differences in the manufacturing process of the two tubes (such as welding or screw connection, etc.), as well as the structural stiffness of the vibrating tube itself, the inconsistency in the resonant frequencies of the two tubes, and the internal attenuation of the material, the vibration tube machinery Asymmetry of vibration. Therefore, when the physical indicators of the medium fluid, such as flow rate and viscosity, change, the imbalance of the pipe structure will cause the zero position of the flow meter to drift.

Affected by the outdoor ambient temperature, the measuring tube and measuring element will undergo thermal expansion and contraction, causing abnormal flow transmission. At the same time, changes in factors such as installation stress caused by artificial installation during the installation process of the flow meter will also cause uncertain phase differences and zero point drift even if the liquid in the measurement tube is in a stationary state.

1.2 Instrument failure

①Input the same signal and check whether the zero position of the transmitter is normal. If the zero position of the transmitter is abnormal, it may be caused by problems with the transmitter itself or electromagnetic interference. ② Start with the drive feedback voltage. If the difference between the two feedbacks is large, and the cable connection is correct, it can basically be confirmed that the sensor is faulty.

1.3 Process issues

①The mass flowmeter requires full pipe measurement. When the flow pipe is not full or the medium has two-phase flow, the flow and density measurement values ​​will fluctuate greatly, and the flowmeter will give an alarm message. This will cause irregular measurement errors. ②The operating conditions of the mass flow meter deviate greatly from the design conditions. Due to changes in process load, problems such as excessive or too small operating flow, insufficient back pressure, and medium vaporization may occur.

2. Slope

The slope of each flow meter sensor is fixed, which is a parameter directly related to the flow rate and a factor that needs to be calibrated separately. On-site replacement of the transmitter or incorrect pairing will cause measurement errors. The slope is changed, sometimes even to 0, which will definitely cause a large flow error or no flow.

3. Process temperature

Changes in process temperature will cause changes in the stiffness of the sensitive tube and change the working status of the sensor and transmitter. In the winter of 2021, a P80 flow meter on a loading station experienced large flow fluctuations. After inspection, it was found that the low temperature caused the transmitter start line to become loose, and it returned to normal after tightening.

4. Gas-containing

The gas content in the process pipeline mainly affects the mass distribution of the sensitive tube, disrupts the normal working state of the sensitive tube, and causes deviations in measurement.

①Reference density value. If the density is obviously low, the possibility of containing gas is relatively high; if the density is obviously high, the possibility of loose transmitter wiring is relatively high. ②Understand the process medium. In summer, when naphtha and gasoline are loaded into a truck, due to the light medium composition and possible leakage at the valves and flanges, gas is entrained in the pipeline. In some crane positions, the pump is not turned on after the loading valve is opened. If there is a small flow rate of 1~2t/h, if it is not cleared in time, it will cause the flow meter to be empty, causing the actual tank truck loading amount to be insufficient.

5. Over range

① Take gasoline loading measurement as an example. The pipe diameter of a single crane on the stack is DN100. When the instantaneous flow rate reaches 150t/h and above, the flow rate calculated based on the density exceeds 7m/s, which is obviously high, resulting in the measurement accuracy and repeatability of the flow meter. Neither is good. ② On the railway loading platform of a petrochemical enterprise, when too many loading cranes are opened or the crane tubes are not lowered to the bottom and virtual volume and small flow are generated, the minimum flow is only 3~4t/h, which is obviously beyond its measurement range, so it causes Measurement error.