Working Principle and Maintenance of Coriolis Mass Flowmeter

my country's trade measurement is settled by quality. Tianchen Borui's mass flowmeter can directly measure the quality of fluids, which has a great advantage in foreign trade. Especially for oil product measurement, it is necessary to master the performance of the mass flowmeter and the method of daily use, so that the role of the mass flowmeter can be maximized in the future use process.

1. Working Principle of Coriolis Mass Flow Meter

The measuring part of the Coriolis mass flowmeter is mainly composed of an elastic measuring tube, an excitation unit, a vibration pickup unit, a closed-loop self-excitation amplifier unit, etc. The working principle is: the elastic excitation unit maintains the sensitive structure, which is mainly composed of the elastic measuring tube, in a resonant state, and the measuring tube performs a "bending main vibration"; when the mass flow flows through the vibrating measuring tube, the "Coriolis effect" generated causes the measuring tube to produce a "torsion secondary vibration" directly related to the "mass flow" flowing through on the basis of the above-mentioned "main vibration"; by detecting the "compound vibration" of the measuring tube, the mass flow of the fluid can be directly obtained. Obviously, the key to the stable and reliable operation of the Coriolis mass flowmeter is to ensure that it is in an ideal vibration state, that is, to suppress or minimize its influence as much as possible in practical applications.

2. Main factors affecting measurement performance and solutions

2.1 Installation stress

During the installation of the mass flowmeter, if the sensor flange of the flowmeter is not aligned with the central axis of the pipeline (that is, the sensor flange is not parallel to the pipeline flange) or the pipeline temperature changes, the stress generated by the pipeline will form pressure, torque, and tension on the measuring tube of the mass flowmeter, causing asymmetry or deformation of the detection probe, thereby causing zero drift and metering errors.

Solution:

(1) Strictly follow the specifications when installing the flow meter.

(2) After the flow meter is installed, call up the "Zero Adjustment Menu" and record the factory zero preset value. After zero adjustment is completed, observe the zero value at this time. If the difference between the two values ​​is large (the two values ​​must be guaranteed to be in the same order of magnitude), it means that the installation stress is large and it should be reinstalled.

2.2 Environmental vibration and electromagnetic interference

When the mass flow meter is working normally, the measuring tube is in a vibrating state and is very sensitive to external vibration. If there are other vibration sources on the same support platform or in the nearby area, the vibration frequency of the vibration source will interact with the working vibration frequency of the mass flow meter measuring tube, causing abnormal vibration and zero drift of the flow meter, resulting in metering errors, and in severe cases, the flow meter will not work; at the same time, because the sensor makes the measuring tube vibrate through the excitation coil, if there is a large magnetic field interference near the flow meter, it will also have a great impact on the measurement results.

Solution: At present, with the continuous improvement of the manufacturing process and technology of mass flowmeters, for example, the application of DSP digital signal processing technology and MVD technology of companies such as Micro Motion and Beijing Tianchen Borui, compared with previous analog equipment, the digital processing of the front end greatly reduces the signal noise and optimizes the measurement signal. When selecting instruments, flowmeters with the above functions should be considered as much as possible. However, this cannot fundamentally eliminate interference. Therefore, when designing and installing mass flowmeters, they should be kept away from large transformers, motors and other equipment that generate large magnetic fields to prevent interference with their excitation magnetic fields. When vibration interference cannot be avoided, isolation measures such as connecting the vibration pipe with a flexible pipe and isolating the vibration support frame can be taken to isolate the flowmeter from the vibration interference source.

2.3 Influence of measuring medium pressure

When the operating pressure differs greatly from the calibration pressure, the change in the measuring medium pressure will affect the tightness of the measuring tube and the degree of the Bouden effect, destroying the symmetry of the measuring tube, thereby causing changes in the measurement sensitivity of the sensor flow and density, which cannot be ignored for high-precision measurement.

Solution: We can eliminate or reduce this effect by performing pressure compensation and pressure zero adjustment on the mass flow meter. There are two methods for configuring pressure compensation:

(1) If the operating pressure is a known fixed value, the external pressure value can be entered into the mass flow meter transmitter for compensation.

（2）如果操作压力变化较为显著，可以进行质量流量计变送器组态，来轮询一个外部压力测量设备，通过此外部压力测量设备获得实时动态压力值进行补偿。 注意：在组态压力补偿时，必须提供流量检定压力。

2.4 Two-phase flow problem

Since the current flow meter manufacturing technology can only accurately measure single-phase flow, in the actual measurement process, when the operating conditions change, the liquid medium will vaporize and form a two-phase flow, thus affecting normal measurement.

Solution: Improve the working conditions of the fluid medium so that the bubbles in the process fluid are distributed as evenly as possible to meet the normal measurement requirements of the flow meter. The specific measures are as follows:

(1) Straight pipe laying. The vortex caused by the bends in the pipeline can cause bubbles to enter the sensor tube unevenly, resulting in measurement errors.

(2) Increase the flow rate. The purpose of increasing the flow rate is to make the speed at which bubbles in the two-phase flow pass through the measuring tube the same as the speed at which they enter the measuring tube, thereby offsetting the buoyancy of bubbles and the effect of low-viscosity fluids (bubbles in low-viscosity fluids are not easy to disperse and tend to gather into large air masses). When using a Micro Motion flowmeter, it is recommended that the flow rate be no less than 1/5 of the full scale.

(3) Choose to install it in a vertical pipe with an upward flow direction. At low flow rates, bubbles will gather in the upper half of the measuring tube; after the vertical pipe is laid, the buoyancy of the bubbles and the flowing medium can easily and evenly discharge the bubbles.

(4) Use a rectifier to help distribute bubbles in the fluid. It will be more effective if used with an eliminator. 2.5 Influence of density and viscosity of the measuring medium

The change of the density of the measured medium will directly affect the flow measurement system, thus changing the balance of the flow sensor and causing zero offset; while the viscosity of the medium will change the damping characteristics of the system and cause zero offset. Solution: Try to use it to measure a single or several media with small density differences.

2.6 Measuring tube damage

During the use of mass flowmeter, due to the corrosion of fluid, external stress, foreign matter entering, etc., the measuring tube is directly damaged, affecting the performance of the measuring tube and causing inaccurate measurement. Solution: It is recommended to install a corresponding filter at the front end of the flowmeter to prevent foreign matter from entering; try to reduce the installation stress during installation.

3. Daily use and maintenance

Through the above analysis, after our continuous practice and exploration, we have formed a relatively complete set of methods in the operation, monitoring, maintenance and troubleshooting of mass flow meters, and achieved good results. Let's introduce it to you.

3.1 Establishment of equipment files

The establishment of files for mass flow meters is essential, which can facilitate the daily maintenance and management of mass flow meters. The equipment files should include the installation location of the mass flow meter, the name of the measured medium, the working parameters of the medium, the diameter of the pipeline, the internal setting parameters of the mass flow meter, the normal flow rate, the maximum flow rate, etc., as well as the certificate of each calibration and the daily inspection and maintenance records.

3.2 Zero point calibration

Before the mass flow meter is put into operation, it is necessary to perform zero adjustment on the flow meter with full pipe under pressure. The steps are as follows:

(1) After the flow meter is installed, power on and preheat the mass flow meter for about 30 minutes. During this period, the flow meter parameters can be checked, the output circuit can be inspected, and the network data acquisition can be debugged.

(2) Pipeline operation: Open the valves before and after the meter in sequence, close the bypass valve, and allow the measured medium to flow normally in the flow meter. After the flow rate stabilizes, cut off the valve after the meter and ensure there is no leakage. At this time, make sure that the mass flow meter is filled with medium.

(3) Observe whether the "live zero point" of the flow meter is stable. Zero point adjustment can only be performed after the "live zero point" is stable and the small signal cutoff value is set to zero.

(4) After the zero point adjustment is completed, open the valve after the meter to restore the normal operation of the pipeline.

3.3 Realize network data monitoring of mass flow meters The internal measurement data acquisition network of Sinopec system adopts distributed acquisition workstations and server/client data sharing structure. Data acquisition is divided into three levels: instrument layer, data acquisition layer and data management layer. The operating status of field instruments is monitored in real time through the data acquisition network. Once an abnormality occurs, it can be discovered in the first time and analyzed and processed in time.