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Hi, this is the Industrial Control Assistant. In the previous episode, we discussed how to troubleshoot problems with mass flow meters. Today, we’re sharing some knowledge about the working principle and daily maintenance of Coriolis mass flow meters. Most mass flow meters available on the market operate based on the Coriolis principle, so let’s take a look at exactly how that principle works. The measuring part of a Coriolis mass flow meter mainly consists of an elastic measuring tube, an excitation unit, a detection unit, and a closed-loop self-oscillation amplification unit. The working principle is as follows: the elastic excitation unit keeps the sensitive structure, which mainly consists of the elastic measuring tube, in a resonant state, causing the measuring tube to undergo \"primary bending vibrations.\" When a mass flow passes through this vibrating measuring tube, the \"Coriolis effect\" results in \"secondary torsional vibrations\" that are directly related to the mass flow rate. By detecting these \"combined vibrations\" of the measuring tube, it is possible to determine the mass flow rate of the fluid directly. Obviously, for a Coriolis mass flow meter to operate stably and reliably, it is essential to maintain an ideal vibration state; in practical applications, the effects of such vibrations must be suppressed or reduced as much as possible. During the installation of a mass flow meter, if the sensor flange of the flow meter is not aligned with the central axis of the pipe (that is, the sensor flange is not parallel to the pipe flange), or if the temperature of the pipe changes, the stresses generated in the pipe will exert pressure, torque, and tensile forces on the measuring tube of the mass flow meter. This can lead to asymmetry or deformation of the sensing element, resulting in zero-point drift and measurement errors. Therefore, we need to pay attention to the following two aspects: (1) Strictly follow the specifications when installing the flow meter. (2) After the flow meter is installed, bring up the “zero adjustment menu” and record the factory-set zero value. After zeroing, check this zero value; if there is a large difference between the two values (the values should be within the same order of magnitude), it indicates that significant installation stress was applied, and the meter should be reinstalled. The maintenance of mass flow meters is also a set of methods developed through continuous practice and experimentation. 1. Establishment of equipment records: It is essential to keep records for mass flow meters, as this facilitates their daily maintenance and management. The equipment documentation should include, as much as possible, information such as the installation location of the mass flow meter, the name of the medium being measured, the operating parameters of that medium, the diameter of the pipes, the internal settings of the mass flow meter, the normal flow rate and the maximum flow rate. It should also contain the certification certificates for each calibration, as well as records of routine inspections and maintenance. 2. Zero calibration: Before putting the mass flow meter into operation, it is necessary to perform zero calibration with the meter under pressure. The steps are as follows: (1) After the flow meter is installed, power it on and let it warm up for about 30 minutes. During this time, it is possible to verify the parameters inside the meter, check the output circuit, and adjust the network data acquisition settings. (2) Pipeline operation: Open the valve before the meter and the valve after the meter in sequence, and close the bypass valve to allow the measuring medium to flow properly through the flow meter. Once the flow rate stabilizes, close the valve after the meter and ensure there are no leaks; at this point, it is essential to ensure that the mass flow meter is filled with the medium. (3) Observe whether the “live zero point” of the flow meter is stable. Zero adjustment can only be carried out after the “live zero point” is stable and the value for rejecting small signals is set to zero. (4) After the zero-point calibration is completed, open the back valve of the gauge to restore normal operation of the pipeline. 3. For implementing network data monitoring of mass flow meters, the Sinopec system’s internal metering data acquisition network utilizes a distributed acquisition workstation and a server/client data sharing architecture. Data acquisition is divided into three levels: the instrument level, the data acquisition level, and the data management level. The operation status of field instruments is monitored in real time through this data acquisition network; any abnormalities can be detected within a short time frame, allowing for prompt analysis and handling. 4. Zero-point tracking: Zero-point stability is one of the important technical parameters of flowmeters; therefore, close attention should be paid to any changes in the zero point during routine maintenance. (1) It is generally necessary to conduct regular inspections on a quarterly basis; (2) during normal use, if there are significant changes in the operating conditions such as changes in the medium, pressure, or temperature, it is necessary to immediately check for any changes in the zero point. If these changes exceed the limits specified by the manufacturer, the zero point must be adjusted again. 5. General steps for troubleshooting mass flow meters: (1) Check whether the power supply and signal connections are correct. (2) Verify that the process variables and internal parameter settings are accurate. (3) Use the transmitter’s status indicators, status alarms, and self-diagnosis results to determine the likely cause of the fault. (4) Inspect the resistance of the sensor coils, RTDs, and the core processor. (5) Check for any vibrations or electromagnetic interference in the area where the flow meter is installed. (6) Determine whether there have been any changes in the physical properties of the fluid being measured, such as density, viscosity, presence of agglomerates, bubbles, or impurities. (7) Activate the bypass circuit to disconnect the flow meter, remove it, and check whether it is clogged or damaged, as well as whether the pipelines are unobstructed. (8) Re-calibrate the flow meter to determine if it is functioning properly. If the above steps still cannot resolve the issue, contact the manufacturer. That’s all for this issue. If you have any questions about industrial control knowledge, feel free to leave a message. New content will be added regularly, so please stay tuned.