A brief analysis of fault handling for mass flow meters

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I came across a document that I think can be helpful for learning about mass flow meters, so I’m posting it here to share it. Statement: It is not original, as there was no time to write a systematic and expository piece like this. 1 Introduction A mass flow meter is a flow measurement instrument with high measurement accuracy, high technical complexity, and stable and reliable operation; it can directly measure the mass flow rate of fluids. With the development of the socio-economy and technology, mass flow meters are being used more and more widely in on-site measurement and trade settlement. At present, a large number of high-precision mass flowmeters of a certain type have been introduced and used in various applications within the Tuhua Oil Field, such as single-well metering, handover metering, and liquefied gas metering. The use of these flowmeters has significantly improved the level of metering in the oil field, but it has also given rise to numerous problems. The most important issue is the lack of reasonable and effective methods and measures to handle failures. How to deal with faults in flow meters is an issue; especially, general technical maintenance personnel often don’t know where to start when faced with simple problems (minor faults) or how to resolve them. This requires us to analyze the problems carefully, draw lessons from experience, and eliminate the faults. To enable mass flow meters to be used more effectively in actual production, it is also necessary to carry out proper testing and maintenance of these instruments to ensure their measurement accuracy. 2. Troubleshooting Common Faults in Mass Flow Meters: We have summarized the problems that arise during the calibration and use of mass flow meters. It has been found that these faults can range from minor to severe, but none of them should be ignored, as otherwise significant measurement errors may occur. Here, we present the analysis and resolution processes for various faults for your reference. The following will illustrate this with specific examples: Example 1: In one facility, a Coriolis mass flow meter was used as a metering device for quantitative control during oil transfer to tank trucks. During the process of loading the trucks, the flow rate varied irregularly, yet the calibration results of the flow meter showed it to be in good condition. After careful analysis on site, it was found that the rapid closure of the solenoid valve located downstream of the flow meter caused intense hydraulic shock to the fluid, which significantly affected the operation of the sensor. By reversing the positions of the flow meter and the solenoid valve (i.e., placing the solenoid valve ahead and the flow meter behind), the impact of the solenoid valve’s closure on the flow meter was eliminated, allowing the flow meter to function normally again. Example 2: A Coriolis mass flow meter at a oil production plant often exhibited false measurement readings; that is, when there was no flow in the pipeline, the cumulative value displayed by the flow meter would change. Upon careful inspection on site, it was found that the mounting supports at both ends of the sensor did not meet the requirements: first, the supports at the two ends were not at equal distances from each other; second, the bottom of the supports was suspended above the ground, with no firm connection in place. After taking measures to ensure that the mounting at both ends of the sensor met the requirements, the flow meter’s measurement function returned to normal. Example 3: A factory uses a Coriolis mass flow meter to measure liquefied petroleum gas, whose normal density ranges from (0.152~0.156) g/cm3. However, during measurement, the density reading often falls below 0.15 g/cm3. At this time, even if no fluid is flowing through the pipe, the flow meter may exhibit virtual flow values in positive and negative directions, resulting in fluctuations in the cumulative value. The main reason for this phenomenon is that the flow meter is not under sufficient pressure, causing the liquefied gas to liquefy; as a result, the ratio of gas to liquid in the medium exceeds the specified range, which renders it unsuitable for proper operation of the flow meter. There are generally two solutions. One is to increase the pressure in the process pipeline and the pressure acting on the flow meter, in order to prevent the process medium from vaporizing. The other solution is to use a function that allows for measurement at low flow rates and low densities; that is, the flow meter does not record data when the flow rate is below a specified level. Alternatively, coriolis mass flow meters can be utilized, as they have the capability to select the appropriate measurement method based on the density range of the medium being measured, so that no measurement is taken when the medium’s density is below a certain threshold. Example 4: During operation, a Coriolis mass flow meter in a factory showed a significant discrepancy between its measured values and those obtained from tank level measurements. Upon inspection, it was found that the sensor was installed too close to the pump room; when both pumps were in operation, the vibration was excessive, which affected the stable operation of the sensor. The situation returned to normal after the sensor was moved to a location farther away from the source of vibration. Example 5: A Coriolis mass flow meter in a factory is used to measure refined oil. When the flow rate exceeds 20 t/h, the density value displayed by the meter is (1–3) g/cm3, and error messages such as “Density out of range” or “Sensor failure” appear; once the flow rate decreases, everything returns to normal. After a thorough on-site inspection, it was determined that there were no issues with the installation of the sensor or the operating conditions at the site. After removing the flow meter, cleaning it prior to calibration revealed that two pebbles were stuck at the inlet of the sensor’s \"Y\"-shaped flow distributor. Once these pebbles were removed, the flow meter passed the calibration test and performed well in actual use. 3 Detection and Maintenance of Mass Flow Meters 3.1 Zero Point Check (Zero Calibration) Zero point drift is a common problem encountered in the practical operation of Coriolis mass flow meters. There are many factors that can cause zero point drift, such as installation stress on the sensor, asymmetrical structure of the measurement tube, and changes in the physical properties of the fluid being measured. Especially when measuring small flow rates, zero drift has a significant impact on measurement accuracy. Therefore, it is very necessary to carry out zero-point inspections and adjustments regularly. The zero-point check should be carried out at least once every three months; where production conditions permit, the interval between zero-point checks for Coriolis mass flow meters installed at critical monitoring points should be reduced accordingly. Whether the zero calibration is completed successfully is a key factor that directly determines whether the flow meter operates within its accuracy range. 3.2 Checking of operating parameters: During use, it is necessary to regularly check whether the set operating parameters have changed, and whether the displayed values for flow rate, density, and temperature are normal. If there are significant discrepancies from the actual values, zero-flow calibration can be performed again using the methods described in the user manual. If things still don’t seem normal after completing the above steps, check whether the various operating parameters set inside the transmitter are correct. 3.3 Regularly check the fault indicators of the flow meter. Depending on the model, specifications, and manufacturer of the flow meter, the ways in which faults are displayed and the details shown vary. For different fault alarm indications, refer to the product user manual to determine the cause of the fault and take appropriate action. 3.4 Regular comprehensive inspections and maintenance: For flowmeters in use, regular comprehensive inspections should be carried out. These inspections should cover various aspects such as the appearance of the sensor, the degree of its secure installation, vibrations in the process pipelines, as well as the readings displayed by the transmitter and indicator instruments; any issues identified must be addressed promptly. 3.5 Maintaining a record of the flow meter: To ensure the long-term reliable operation of the flow meter, users should keep a record of it, documenting in detail each inspection, maintenance, and calibration performed, so as to facilitate better maintenance and use in the future. 3.6 Maintenance of flowmeters used for measuring fouling-prone fluids When measuring fluids that tend to foul (such as those prone to scaling or wax formation), it is necessary to regularly check the operation of the flowmeter. If abnormal performance or significant deviations are detected, fouling inside the sensor should be considered as a possible cause; in such cases, the sensor should be removed and treated using appropriate methods such as purging or cleaning. 3.7 Regular calibration: Regular calibration is carried out according to the application scenario of the flow meter and in compliance with the requirements of the standard system. 4 Conclusion In summary, proper installation, a reasonable configuration of process pipelines, and favorable operating environment conditions are all extremely important for the proper functioning of mass flow meters. The regular inspection and maintenance of mass flow meters are also essential; only in this way can their advantages such as high accuracy and strong stability be fully utilized to ensure reliable operation in actual production.